Objective: Design proposal for a body-weight support system to overcome current limitations faced by therapists at Stroke Rehabilitation Center, Buffalo General Hospital. The design proposal included extensive work on the operations, mechanical, software, electrical, and control aspects.

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Gait Provision System (GPS)
BE 400/500
Zhaowei Chen
Kritika Lakhotia
Kim McGarrity
Sarah McKinley
Vienna Mott
December 13th, 2015

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ACKNOWLEDGEMENTS
We are immensely grateful to the therapists at the Stroke Rehabilitation Center at Buffalo General
Hospital to have spared their time for explaining the equipments and demonstrating its use. It was an
insightful experience for us and it helped us in designing our device with strategy. We hope that our
design can provide them with ideas for the future.
We would like to thank Dr Filip Stefanovic for giving us the wonderful opportunity of visiting the
center and for giving us this project as well. The ground work that he layed for this project made our work
extremely smooth.
Finally, we would like to thank our fellow classmates for BE400/500 for giving their suggestions
during the presentation to help our design grow.
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TABLE OF CONTENTS
1. Operations Phase
1.1 Operations Scope
1.2 Document Overview
1.3 Objectives
1.4 Operations Overview
1.4.1 Prioritized functional requirements
1.4.2 Prioritized performance requirements
1.4.3 Environmental conditions
1.5 System Overview
1.5.1 System scope
1.5.2 System Architecture
1.6 Functional Flowchart
1.6.1 Phases of use
1.6.2 System stages with phases of use
2. Mechanical Phase
2.1 Mechanical Scope
2.2 Mechanical Overview
2.2.1 System overview
2.2.2 3D view
2.3 Structure Design
2.3.1 Frame and Base
2.3.2 Front and Back Support
2.2.3 Passive Harness and Harness Support
2.2.4 Harness Support
2.4 Motor Unit(s)
​3. Software Phase
3.1 Scope
3.2 Front End Description
3.2.1 Flow chart of application use
3.2.2 Remote interface
3.3 Software Architecture
3.4 Software Requirements
3.5 Mini Specifications
3.6 Software Tasks
3.6.1 Normal operations
3.6.2 Low battery operations
3.6.3 Early versus late training
3.7 Systems Impact
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4. Electrical Phase
4.1 Scope
4.2 Noteable Systems
4.2.1 Notable systems
4.2.2 Force detection and feedback
4.2.3 iPad
4.3 System Architecture
4.3.1 Location of components
4.3.2 Serial communications
4.4 Power Consumption
5. Control Phase
5.1 Scope
5.2 Control Specifications
5.3 Hierarchy
5.4 Systems Model
5.4.1 Lift motor
5.4.2 Vibration motor
​6. Appendix and list of changes
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GENERAL OVERVIEW OF SYSTEMS
Figure 1:​ A basic overview of our design
Operations:
A body­weight support system is a device that helps the user to perform tasks, typically walking,
by providing support and balance. It utilizes a harness to provide stability and reduce the amount of
weight beared on the legs. This enables training of muscles in the walking motion, even for users with
limited muscular and balance capabilities. The body weight support system over a treadmill is a favorable
option for motor learning. This system is said to provide means of recovery by improving the endurance
in speed of walking and the assistance required to walk.
The current support system at Kaleida Health’s Stroke Rehabilitation Center at Buffalo has
certain limitations that need to be addressed and solved, in order to provide better rehabilitation assistance
to the users. These issues include problems related to the imbalance in the user’s motion, monitoring of
the patient by more than one therapist, attachment problems, and tolerance levels of the patient.
Mechanical:
When looking at the mechanical design of a body­weight support system (BWS) the main goals
would be to safely support the load for patients within a specified weight range, allow for unrestricted
natural movement and gait, be easy to use, and provide optimal comfort and security when in use.
The system designed in this report contains four main segments and two stages of use. The
segments include base and frame, front and back supports, passive harness and its supports, and the
motor. Each of these segments are then divided into smaller components and compared with other
trade­offs. The two stages of use, which are described in detail in the system overview, are a standing
assist and walking assist.
The measurements used in each aspect of the design follow anthropometric data found from The
Ergonomics Center. 5th Percentile female data and 95th percentile male data were used as limits for the
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maximum and minimum dimensions and weight. An appendix in the back of the report contains some
data used.
Electrical:
This system includes a patient lift system and a force detection and feedback system. The patient
lift system is activated by a separate remote control, which then signals a motor to lift the patient to a
standing position. Once the patient is ready to begin walking with the device, the force feedback systems
can be activated.
The force detection and feedback section allows the device to detect the force output by the
patient, transmit the information to the visual display via Bluetooth. Physical feedback is also given, via
vibrating motors.
Software:
The main software component of this device is a downloadable application (app) onto an iPad
mini which can be hooked up directly to the device or removed and used separately. This app, named gait
provision system or GPS for short, will play a part in being our main user interface, information
processor, and operating system. Several functions of the GPS include a library of patient data for quick
and easy use, visual feedback for the patient as they are using the device, and an adjustable metronome.
There are four main stages of use which are focused on throughout the paper: caregiver input, patient use,
data output and review, and online access.
There are also two forms of feedback, visual and physical, that will come from force sensors
placed within the side supports. These sensors will be linked directly to the iPad for the visual feedback
and once reached a specified threshold, will initiate small vibrations to the side of the patient letting them
know where most of their weight is leaning.
Control:
The Gait Provision System has two main control components; a lift and vibration motor. The
control requirements for each of these will be described in the first section of the report as well as some
foreseen limitations. The preceding section prioritizes each component and provides flow charts for better
understanding of the autonomous and manual controls. Lastly, the third section will go into some basic
calculations and models of the system behavior.
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1. OPERATIONS PHASE
1.1 OPERATIONS SCOPE
A body­weight support system is a device that helps the user to perform tasks, typically walking,
by providing support and balance. It utilizes a harness to provide stability and reduce the amount of
weight beared on the legs. This enables training of muscles in the walking motion, even for users with
limited muscular and balance capabilities. The body weight support system over a treadmill is a favorable
option for motor learning. This system is said to provide means of recovery by improving the endurance
in speed of walking and the assistance required to walk.
The current support system at Kaleida Health’s Stroke Rehabilitation Center at Buffalo has
certain limitations that need to be addressed and solved, in order to provide better rehabilitation assistance
to the users. These issues include problems related to the imbalance in the user’s motion, monitoring of
the patient by more than one therapist, attachment problems, and tolerance levels of the patient.
1.2 DOCUMENT OVERVIEW
The design intends to overcome most of the limitations mentioned above, and provide maximum
beneficial prospects to the users while maintaining the quality of the device. The design includes optimal
constancy for the patient through a movable side and back support. With the use of a variety of
components, it is desired that the labor in terms of therapist would largely decrease, and also reduce the
associated cost. The quality is an important factor that would be taken into consideration in terms of the
material used for the equipment and will be discussed eventually in the further phases. The design
includes distinct functional requirements with the intention of each requirement reaping favorable
benefits. The performance parameters need to assessed considerably since it is validation of a kind to
know the usefulness of the design that has been created and the parameters been delved into further in the
document.
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1.3 OBJECTIVES
Need & Requirements Rationale
1. Easy attachment The first requirement is that the device should be
easy for attachment. If it’s hard to get the patient
harnessed properly, the further steps become
difficult. This can be worn while the patient is on
the wheelchair.
2. No harm to the patient (safety) If the device isn’t safe, the rehabilitation gait
training loses its purpose. So the second
consideration should be the safety issue. For
example, the device should hold the patient tightly
so that patient does not fall down during the
training.
3. Support
For a BWSS, the weight supporting issue after we
get the patient to fit comfortably and safely is
important. As people have different weight
ranging from around 100 pounds to more than 300
pounds, the system should be able to hold the
weight, or even measure the weight and adjust the
parameters corresponding to it.
4. Keeps patient balanced It is essential to keep the patient balanced during
the training. It is partly a safety issue, but more
importantly, balanced training can get a better
result (especially for patients who have trouble
distributing weight equally to both feet) because
normal people walk stably. The individual
supports (arm supports and front and back
supports) contribute to it.
5. Feedback To know the improvement of the patient with
every passing session, a feedback component is
required in the device. Visual feedback and a
physical feedback are provided.
6. Improve human gait Finally, after all necessary considerations the
patient’s gait should be restored to maximum or
complete normalcy.
Table 1.3­1: ​List of need and requirements for our design with their rationale
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1.4 OPERATIONS OVERVIEW
1.4.1 Prioritized Functional Requirements
Components Implications
1. Upper body supports (under the arms) Necessary for stabilization. Ensures that the
patient remains in an upright position when
mobile. Without this component, the patient
would be susceptible to falling.
2. Movable front/back support Also allows for the stabilization of the individual.
Must be movable for the sake of adjustments for
each person and each use.
3. Hand Grips Method to avoid slipping when the patient is
walking and it allows for extra upper body
support and helps in balancing during gait.
6. Feedback Useful to track progress of the patient with
consecutive sessions. The visual feedback can be
observed on the iPad that is wirelessly linked to
the microcontroller of a force sensor. The physical
feedback would be provided in terms of the
vibrations felt by the patient after a certain force is
reached. Some patients have trouble distributing
weight between both sides of their body during
rehabilitation. This would display the results on
the iPad.
Table 1.4.1­1:​ List of components for our design and their respective implications
1.4.2 Prioritized Performance Requirements:
● Machine Movability/adjustability­ Success can be quantified by the seamlessness of motion.
More specifically, any choppiness found when using the machine would be considered negative
results. Furthermore, the ease of adjustability would also be a measure of performance. Finding
the right settings for an individual should take a minimal amount of time and should be accurate
every time. Range of motion would be dependent on the design construction of the device and in
this one, most components are fairly adjustable. This makes range of motion easier as it will be
suited to the patient’s needs. Normal gait pattern is the final and the most important parameter of
usefulness of the device.
● Ease of use ­ Performance can also be gauged upon the user friendliness for the therapist. This
design should allow use of the machine with only one therapist assisting in the way of settings
and guidance. The patient’s progress is the most effective way of knowing whether the device has
been easy for the patient to use and this can be tracked by the feedback results discussed in detail
in the software phase of Section 3.3 and 3.5.
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● Pressure Sensor Recordings ­ Progress can be tracked based on the recording of the number of
steps taken by each individual, as well as the weight distribution recorded. Perhaps an algorithm
based on weight, height, and other specifications could be used to ensure a percentage of progress
before the patient is able to walk freely.
1.4.3 Environmental Conditions:
The only environmental condition that may need to be addressed is the space requirement necessary to
use this product, but also to store it. Considering the tasks at hand, the size would be the only impeding
factor. However, the design would be no larger than current designs, so implementation in rehabilitation
facilities should not be an issue.
1.5 SYSTEM OVERVIEW
1.5.1 System Scope
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1.5.2 System Architecture
Structure​: A lift assist device that lifts the patient under the arms will be accessible for the patient
without need of a harness, and be able to accommodate more users regardless of size.
Human Interface​: The device will be made with adjustable heights and positions for both the device as a
whole and the supporting pads. The pads will be mechanically adjustable, so that the caregiver can adjust
them specifically to the user, and the device can accommodate as many users as possible.
Control​: The device will be self­propelled by the user, as it will have wheels to enable it to move with the
user.
Actuators​: The device does not need to output any energy to the patient.
Sensors:​ The device will have force transducers the the supporting pads in order to sense the force
exerted on the machine. Analysis of these force outputs can help caregivers to understand the exact nature
of the patient’s balance issues.
Power​: The sensors and any data storage will have to be powered. A battery mounted on the device,
rather than a power cord, is optimal because the device is mobile.
Software Interface​: The data collected by the sensors will need to be processed before it can be of use. In
order for optimal efficiency as well of ease of use, it will be a very simple user interface, that can also
store previous patient records in order to monitor progress.
Modes of operation​: The device can be used by a patient as simply a weight assist device or as a weight
assist device as well as a balance assist. These multiple modes of use increase the number of patients that
can use and benefit from this device.
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1.6 FUNCTIONAL FLOWCHART
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1.6.1 Phases of use:
1. Take out of storage and move in front of seated patient
2. Adjust width of support according to individual needs
3. Place supports under the patient’s arms
4. Raise to standing height ­ lift stage
5. Calibrate pressure sensors, and other components if necessary for the patient
6. Patient use ­ walking stage
7. Return support system to storage
1.6.2 System stages with phases of use
Lift Stage:​ bring patient from sitting to standing
Figure 1.6.2­1​: The process demonstrating the lifting stage of the patient
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Walking Stage:​ device moves with patient to support them
Figure 1.6.2­2:​ The walking stage is demonstrated in the above image
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2. MECHANICAL PHASE
2.1 MECHANICAL SCOPE
When looking at the mechanical design of a body­weight support system (BWS) the main goals
would be to safely support the load for patients within a specified weight range, allow for unrestricted
natural movement and gait, be easy to use, and provide optimal comfort and security when in use.
The system designed in this report contains four main segments and two stages of use. The
segments include base and frame, front and back supports, passive harness and its supports, and the
motor. Each of these segments are then divided into smaller components and compared with other
trade­offs. The two stages of use, which are described in detail in the system overview, are a standing
assist and walking assist.
The measurements used in each aspect of the design follow anthropometric data found from The
Ergonomics Center. 5th Percentile female data and 95th percentile male data were used as limits for the
maximum and minimum dimensions and weight. An appendix in the back of the report contains some
data used.
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2.2 MECHANICAL OVERVIEW
Figure 2.1­1. ​Device overview with different components labeled A­H
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2.2.1 System Overview:
The proposed body­weight support system has two separate stages of use and therefore two
distinct mechanical systems: active standing assist, and walking assist.
Standing Assist​: is a powered system where the patient is moved from a seated position to
standing, assisted by the support system.
Mechanical components:
­ Frame
­ Base
­ Wheels
­ Foot rest
­ Knee rest
­ Harness
­ Attachment cables
­ Lift motor
1) Base wheels are locked
2) Seated patient dons harness
3) Patient’s feet and knees are secured to the feet and knee rests, respectively.
4) The harness is attached to the active lift motor via cords.
5) The force generated by the lift motor lifts the patient to a standing position
Walking Assist​: The patient is using the support system to help balance and walk, while
supported by arm, torso and back supports, as well as the harness which is disconnected from the
lifting assist and secured overhead.
Mechanical Components:
­ Frame
­ Base
­ Torso support
­ Back Support
­ Arm support and grip
­ Harness
­ Overhead attachment cables
­ Overhead attachment clip
1) Once the patient is standing, the back support is attached to the device, and adjusted to lie
securely with his or her back.
2) The arm and torso supports are also adjusted to suit the patient
3) The patient is attached to the overhead harness attachment
4) The wheel locks are released
5) The patient can now move forward using the assist device.
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2.2.2 3D view:
Figure 2.2.2.1 ​3D Device overview
Dimensions:
The dimensions of this device can be broken down into the frame and base, which are not
adjustable, as well as adjustable components: the knee and foot rests, and the arm and back support.
Holistically, the device’s minimum floor space required for storage is determined by the non­collapsible
frame at 3.99 square meters.
Material Modulus of
Elasticity
(10^9 N/m^2)
Ultimate Tensile
Strength (10^6
N/m^2)
Yield
Strength
(10^6
N/m^2)
Price
(USD/kg)
Weight
(kg/m^3)
Stainless Steel
AISI 302
180 860 502 4.53 7916.48
Aluminum 69 110 95 1.58 2698.79
Titanium Alloy 105­120 900 730 18.96 4539.47
Table 2.3.1.3 ​Material Comparison
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Figure 2.2.2.2. ​Main frame and dimensions
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Figure 2.2.2.3. ​Foot and knee supports, standard hinge for rotation
Figure 2.2.2.4.​Forearm platforms and handgrip
Figure 2.2.2.5. ​Top view of the back support, inserts for front support
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Figure 2.2.2.6. ​Top view of the back support, pop pin adjustments are located on the side and within the
curvature is vinyl padding
2.3 STRUCTURE DESIGN
2.3.1Frame and Base
Requirement Criteria
Selective mobility: immobile when using standing
assist attachment and unattaching patient, yet not
impede patient when walking.
Type of wheel and accompanying wheel locks
Load bearing: needs to support weight of an
average patient
Material used for frame and base.
Stability: the structure must not overturn or tilt. Dimensions of base and frame, placement of
loads.
Table 2.3.1.1 ​List of requirements for the devices frame and base and their criteria
Base:
The base will be formed by two parallel beams that the frame will connect. The wheels will be placed at
both ends of the beams, as well as directly under the point of attachment of the frame. The material
selected for the base was stainless steel, based on its ability to bear large loads with minimal deformation,
as well as relative low density to allow for a lighter device. The type of wheels selected were stainless
steel with a 9 cm diameter.
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Figure 2.3.1.2:​ Wheel placement within the base
Trade off: Material Weight versus Strength
A denser material adds weight to the structure that must ultimately be moved by the patient.
While a higher density material may have a higher yield strength and can therefore support higher
loading, a material too dense will lead to a device too heavy to move with the user.
Furthermore, deformation must be taken into account­ a material with a high modulus of elasticity
will deform under a load. Deformation is preferred over total breakage and failure of a more brittle
material, but if the base deform too much, there will no longer be clearance between the ground and the
device to move forward.
Dimensions: space versus stability
It is preferable to have a compact device to enable its user to move unencumbered through a floor
space, as well as easy storage. However, it is important to have a large base of support for the device to
minimize the chance of an applied force, due to either the active standing assist or support while walking,
overturning the device
Considering the horizontal force applied by the patient to the device as her or she walks forward,
overturning of the device is an issue with regards to the moment created where the wheels meet the floor
at the end of each beam. As shown in diagram 1 of figure 9, While the applied force (F​A​), moment arm of
the applied force, and the weight of both the patient and the machine (W​D+P ​)are fixed, the moment arm is
not; therefore a longer base is favorable in order to maximize this counter­ moment.
​ Figure 2.3.1.4​: Overturning moment dependent on 1) base length and 2) Height
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Wheel type: Traction versus Mobility
It is important that a wheel is selected with an small enough rolling friction that the device is
mobile. While there needs to be some resistance to movement as the patient needs to be able to support
themselves with the device, a wheel with too high of a rolling resistance will impede the user's ability to
walk with it.
Frame: ​The frame will be formed by a vertical beam attached to each of the base supports, as well as a
connecting cross bar at the top. The material selected for the frame is once again stainless steel, as the
same requirements of strengths and minimal deformation is required.
Trade offs:
Height: Patient Height versus stability
Once again, the dimensions of the device dictate the magnitude of moments that could potentially
lead to overturning of the device. The height of the device needs to be tall enough to comfortably
accommodate most users, taking into account that the height of patients with altered gait patterns may
fluctuate as he or she moves through the gait cycle. The limitation is that although it is assumed any force
on the device is acting purely vertically. As shown in diagram 2 of figure 9, any deviation from this and
the applied force (F​A​) the horizontal component (F​H ​) then has a large moment arm (H) and therefore a
large moment that could ultimately lead to the device overturning.
2.3.2 Front and Back Support
Requirement Criteria
Support the patient, keeping them in an upright
standing position.
Padded supports from the front and back that fit
tightly around the patient's lumbar region
Adjustable in order to comfortably fit various
body types
Supports can be adjusted vertically and
horizontally depending on the height and width of
the patient
Only one therapist needed to attach and adjust
support
Simple incremental adjustment knobs and pins
are used
Hold body weight of up to 150 kg Materials used have a high strength and are
slightly elastic
Allow patient to direct movement and keep
balance
The forearm platforms attached to the front
support give a place for the patient to rest their
arms and grip the device for balance
Table 2.3.2.1 ​List of requirements for the front and back support and their criteria
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Front Support
The front support, although a separate component, will be attached directly to the frame of the device.
The main body of the front support will be made from Cherry wood due to its easy workability, good
modulus of rupture (84.8 MPa), elasticity(10.3 GPa) , and moderate pricing.[2]
Wood Trade­Offs:
1. Sycamore: Modulus of rupture (69 MPa), Elastic Modulus (9.79 GPa), Moderate Pricing
2. Gaboon Ebony: Although this wood has a higher MOR and elasticity it is very difficult to work
with due to its density [2]
Padding
The padding attached to the front and back support for prime comfort will be made from a vinyl
material. These materials are cleanable, highly durable, and waterproof along with being comfortable [3].
Trade­Offs:
1. Cotton: Although a very soft and comfortable material it is not as durable or as wear resistant
as vinyl
2. Micro­fiber: Can wear quickly and is slightly more expensive
Interval Adjustment
The front and back support adjustability ranges include the 5​th​
percentile female to the 95​th
percentile male in order to account for most of the population. Pop pins will control the vertical
adjustments of the front support and attachment of the back support to the front support while spring pins
will control the grip adjustment on the forearm platforms.
Trade­Offs:
Tension knobs wear over time which could cause slipping of the components when the device
was in use leading to injury.
Forearm Platform
The forearm platforms will be attached directly to the front support via spring pins. They will be
70cm long and 12 cm wide with a rubber grip. Placing the arm supports at the forearm allow for a more
comfortable motion when walking. Since the patient forearms also vary in length the grip will also be
adjustable, moving along the platform via spring pins.
Shape
By cutting out a hemi circle in the front support we are conforming to the patient's lumbar region
providing more security and comfort.
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Trade­Offs :
1. No cut out: Leaving the wood in a rectangular shape can cause discomfort to the patient
since the force of the device when moving will be directed to the center of their body
rather than being distributed.
2. Flips which flip out: would not provide enough force to support patient
2.2.3 Passive Harness and Harness Support
Harness
REQUIREMENTS CRITERIA
To support optimal weight of
the patient
Have a harness that can carry the weight of an average American
– strong and flexible material
Maximum comfort to the
patient
Thick padding to prevent the patient from feeling
scratchy/irritable. The material plays a cardinal role in this case
too and it should ideally be soft yet provide the tensile strength to
handle the weight of the patient.
Easy adjustability and
positioning of the harness
(easy donning and doffing)
Quick­ release fasteners/buckles through the harness make the
process rapid. Also detachability of the harness with respect to
being able to be hung through D­rings from the top pole.
Table 2.2.3.1. ​List of requirements for the harness and their criteria
Material
Lycra, Rayon, Cotton (Selected for the design) – In combination, this is the most preferred one as is used
in the bioness model BWSS harness [4]. The ratio of each material is subjective and a detailed technical
aspect. The
Lycra – Stretch ability and the elasticity aspect wonderfully for adjustability.
Cotton – Material for inner quality linings would provide the thick padding and the comfort to prevent
irritation.
Rayon – For breathability through the material in the sense that it doesn’t interfere with the blood
circulation of the patient, usually due to the tightness of the harness around the patient.
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Table 2.2.3.2​ Material comparision
Trade­off:
Polyester ­ This was the most likely one that was used at the hospital and in the earlier systems. It is a
synthetic and isn’t the softest material available which would most likely result in irritation to the patient
which is certainly undesirable.
Size
For our design, the following specification works well [5]:
Torso Circumference 28" to 50" (71 to 127 cm)
User Capacity For 300 lb (136 kg)
The general harness sizing chart can also be followed for the design to accommodate individual sizing of
the harness according to the height and weight [6].
Type of harness (in terms of the length of the harness)
Harness until the hip/waist (Selected configuration) – As shown in the figure below, this one gels
with the design overview discussed earlier. Advantages of the harness type shown below:
● Enough space for the front and underarm support for our design idea
● D­rings on the harness to make it attach to the top frame and considers the detachability factor.
The patient can wear it on the wheelchair and then as the gait rehabilitation session begins, can
attach the ring to the frame at the top.
● We are going to incorporate the harness until the thighs so that it is detachable just to have an
option of having the patient being pulled in from the wheelchair from the waist. This would
incorporate the addition of extra support for heavy­weighted persons.
● Range of motion wouldn’t be an issue in terms of the joints (which is usually 0 to 360 degrees).
Since the harness has attachment points and adjustable buckles, it can be according to the
patient’s wish and hence, that would be important in determining the range of motion
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Figure 2.3.3.3​ A general overview of the harness that we would require with the attachments labeled [7]
Trade­off:
Harness until the knees – This design has been used for various BWS systems and provides the
advantages in terms of support to the knees and the hip area in order to distribute the force and weight
optimally. This has not been considered for our design purpose since the knee caps are already being
implemented in a different manner as discussed. Also, the complications of having a large harness and too
many attachment issues/points, it’s best to avoid especially if that advantage is already being taken care
of.
Attachment points/fasteners:
There are straps for shoulders and the chest with attachment points in order to provide the
performance and adjustability to the patient in the case of a fall event. The harness has to be snug and not,
overly tight which makes it irritable.
Single attachment point at the back – To aid the distribution of pressure, improving the user’s comfort as
shown in the figure below.
Figure 2.3.3.4​ Attachment point at the back [8]
Velcro fasteners​ – To the attachment straps, it provides an ease of use.
­ Requires low maintenance
­ Strong material ­ can hold the weight required in this design
­ Secure
Buckles:
Quick­connect buckles (for selected configuration)
­ Also called the load­bearing buckles is the strongest one available in the market that don’t open
while under load.
­ Highest degree of safety
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­ Ease of attachment
­ Cost­effective
­ Quick tension adjustment of the strap by simply pulling on the yellow tab whereas it’s slightly
complicated for the others
­ 4000 lbs loading capacity
Figure 2.3.3.5​ Quick­connect buckle configuration [9]
Trade­offs:
1. Pass through buckles ­ Complicated in terms of its working and would increase the time in terms
of buckling/unbuckling
2. Tongue buckles ­ Complicated to adjust tension on the strap. Hence, it would increase time for
the harness wearing for the patient
2.2.4 Harness Support
Requirements Criteria
Standing assistance must not impede harness
and must allow for easy attachment
Size of materials are minimized where
applicable. This applies to thickness and
length.
Materials should be able to support a load
greater than or equal to the 95​th​
percentile male
Material and thickness of cables used to attach
BWS system to harness are sturdy and tensile
strength is large.
Cost shall be minimized without inhibiting
effectiveness
Total cost of each material is as low as possible
while still seeing optimal effects.
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Clip device shall be strong enough to support
opposing forces
Clip material and size are the best proportion
of size to strength.
Table 2.2.4.1. ​List of requirements for the harness support and their criteria
Figure 2.2.4.2.​ Bungee cord segment with a ZipHook hook on either end. Process used to decide on this
material is outlined below.
Cable Material: ​Upon comparison of different materials, sizes, and costs, the optimal material decided
upon was bungee cord. This eliminates the need for a pulley system and can work well with the motors
and the harness D­rings through the use of a hook. The type of hook used will be discussed in the next
section. The bungee cord that will be used will be 1.27 cm in diameter and each has a tensile strength of
about 204 kg. It is made of a rubber fiber core and the outer jacket is a nylon and polyester blend. The
cost needed for this material is $90 for 7.62 meters or $325 for 30.48 meters. Using these calculated costs,
it would be prudent to use the larger amount and split it into segments to use throughout the 3 sections
that require bungee cord on this device. The pieces attaching to the back harness must be 90 cm each.
That allows for excess material to be held by the motors as well as accounting for the fact that the patient
will be assisted in standing from a seated position. While seated in a wheelchair, an average sized man
has a waist height of 68.5 cm and the wheelchair has a distance from front to back of 106.5 cm [28].
Using pythagorean theorem along with these values, the cable must measure 78.82 cm. Using 90 cm
accounts for this and the extra cable needed. The third piece of cable will reach from the top of the device
to the back of the harness upon standing the patient up. Using the patient height previously described and
the height of the device, this cable should measure 65 cm. Once again, this will allow for taller patients to
be able to use this system as well.
Trade­offs: Rope was considered as an alternative to the bungee cord that was ultimately used. The chart
below demonstrates the types of rope that were considered along with their properties. While rope has
some properties that are superior to bungee material such as a greater tensile strength, eliminating the cost
and possible problems surrounding a pulley system seems to outweigh the benefits.
Rope comparison chart:
Material (1.27 cm) Thickness (cm) Minimum Tensile Strength Cost (per m)
Nylon 0.566 6,200 lbs $2.95
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Polyester 0.648 5,085 lbs $5.54
Polypropylene 0.966 4,200 lbs $0.72
Table 2.2.4.3. ​Material comparison table for different types of rope [10, 11, 12]
Bungee cord hook: ​Four ZipHook hooks will be attached to the various cables used in this device. The
two cords that attach to the harness will each have a hook opposite the end with the motor. These will
attach to the D­rings of the harness and assist in lifting the patient. The other hooks are used used in the
cable that attaches the harness to the top of the machine. One hook will be attached to the main frame and
the other will attach to a D­ring on the back of the harness. Other hooks were looked into and the
reasoning for not using them is outlined below.
Trade offs: The ultimate decision to use the ZipHook was based on the criteria outlined below. The other
options were not as heavy duty and that was the overwhelming factor here. Safety was the ultimate goal
and even though the vinyl coated steel hook was less expensive, the safety factor outweighed that cost.
Bungee cord hook comparison chart:
Type of hook Advantage Price (each)
Dichromated Light to medium duty $0.56
High­impact vinyl coated steel Resists rust $0.10
ZipHook High strength, adjustable $0.55
Table 2.2.4.4 ​Compares three different types of hooks [13, 14, 15]
2.4 MOTOR UNITS
Requirements Criteria
Should be able to support the
maximal body weight.
The motors should be able to lift the patient’s body, based on
motor specs.
Slow and safe movement Motor will have high torque and low rpm
Small and light enough to fit
into the machine
Our smart device will have a small size and be lightweight. We
want to fit the motors into the frame of our device, so the
dimension of the motors need to be small enough to fit well
inside the frame.
Table 2.4.1 ​List of requirements for the motor unit
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We need a motor with high torque and low rpm. Around 10 cm/s could be a reasonable speed to
lift the patient. For a man weighing 100kg (about 1000N), and with a working distance for the motor of 5
cm, we need a motor with 50Nm at 20 rpm. We have two sides that require these motors, so we need a 25
Nm motor at 20 rpm on each side. Below are some of the suitable motors we found available online.
Motor
number
Torque Speed Price Dimension link
A 80 Nm 25 rpm $240 235.8 x 79 x 131.3 mm [16]
B 10Nm 65 rpm $82 177.5 x 48 x 88.8 mm [17]
C 5 Nm 40 rpm $111 178 x 60 x 100.6 mm [18]
D 50 Nm 3.3rpm $53.5 310​x​235​x​250mm [19]
E 11.5 Nm 50 RPM $74 [20]
F 100Nm 25/35 rpm $179 307 x 171 x 120 mm [21]
G 20 Nm 100 rpm $169 178 x 60 x 100.6 mm [22]
H 2.6 Nm 10 rpm $57 157​ x 82 x 67mm [23]
I 1 Nm 10 rpm $ 79 152​ x 93 x 67mm [24]
Table 2.4.2 ​List of available motors online that suit for our design
We want a small motor to fit into our device. Regarding both the price and size, we think the
motor number B could be a good choice, as we may need 4 of them. This motor could be able to fit into
the frame.
Figure 2.4.3​ Motor structure
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3. SOFTWARE PHASE
3.1 SCOPE
The main software component of this device is a downloadable application (app) onto an iPad
mini which can be hooked up directly to the device or removed and used separately. This app, named gait
provision system or GPS for short, will play a part in being our main user interface, information
processor, and operating system. Several functions of the GPS include a library of patient data for quick
and easy use, visual feedback for the patient as they are using the device, and an adjustable metronome.
There are four main stages of use which are focused on throughout the paper: caregiver input, patient use,
data output and review, and online access.
There are also two forms of feedback, visual and physical, that will come from force sensors
placed within the side supports. These sensors will be linked directly to the iPad for the visual feedback
and once reached a specified threshold, will initiate small vibrations to the side of the patient letting them
know where most of their weight is leaning.
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3.2 FRONT END DESCRIPTION
3.2.1 Flow Chart of Application Use
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Figure 3.2.1­1: ​iPad screen general design
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3.2.2 Remote Interface
The second point of user interface would consist of the “IN” and “OUT” buttons on the DC motor
remote. After the therapist has attached the cables from the motor to the patient's harness the “IN” button
is pressed in order to slowly and gently lift the patient from a seated position to standing position. After
the rehabilitation session is complete, the “OUT” button is pressed to gradually return the patient to their
original seated position. A preview of the remote is provided in the picture below. Due to the simplicity of
the remote, this paper will mainly focus on the iPad mini and the Gait Provision System App.
Figure 3.2.2­1. ​Basic design of the DC motor remote control
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3.3 SOFTWARE ARCHITECTURE
Figure 3.3­1: ​Overview of software architecture
3.4 SOFTWARE REQUIREMENTS
General requirement specifications for Apple iPad Mini 2​ [29]:
● Storage capacity chosen: 32GB, Internal RAM: 1 GB
○ Each file is expected to require 1.5GB. After subtracting data that comes on the iPad, it
is estimated that 17 files could be stored on this iPad. 30MB is the maximum space that
the app can hold before crashing, due to research on the product. The iPad comes with
some data already downloaded. Therefore, the maximum space on the iPad is below
32GB. Each patient would theoretically require data use of 50MB for the metronome,
2.5MB per photo taken (if any are taken), and 12.28MB for display resolution. These are
worst case scenarios, so there may be more space available, but the estimates give a
lowest possible result. In a better case with the estimates being on the lower end, up to 50
data files can be stored. Once again, the data is also saved to the cloud, so great values of
space available are not necessary.
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● Operating system: iOS 9 (with updates when new operating systems are released)
● Display: 7.9­inch (diagonal) LED­backlit Multi­Touch with IPS technology
● Resolution: 1536 x 2048 pixels (~324 ppi pixel density)
● Built­in 24.3­watt­hour rechargeable lithium­polymer battery, 6470 mAh (up to 10 h capacity
after browsing, and media usage)
● Sensors: Three­axis gyro, accelerometer, ambient light sensor
Advantages of chosen device:
● Most accessible in terms of the memory required
● User­friendly design for the therapists and patients to use
● Large and easy touch­interface
​Patient data storage:
● 32 GB internal storage
● Alternatives available for extreme cases – online storage through apps – Google Drive/Dropbox
● iCloud for backup and saving device space. Also allows other devices connected to the cloud to
access this information.
Viewing sensor information/feedback:
● Power requirement & Battery – optimal as per specifications
● Data monitored through iCloud – no installation of software required – easy and cost­effective
● iOS is updated – prevent hindrance to the working of the app
● Communication requirement between the sensor element and the iOS hardware’s microcontroller
system (that contains user­defined touch screen interface)
● Display resolution – 1024X768 – 32­bit color depth – 12,28 MB
Metronome:
● Audio features: Noise ­93.8dB / Crosstalk ­82.9dB & stereo­speakers [29]
● Display resolution as above
● Alternative to the in­built in our app design – App Ludwig Metronome or ​TempoPerfect
Metronome Software​ could be used alongside – App space is around 10 MB [30]
Motor:
● For resolution – Max voltage = 20V span – Effective number of bits required = 16 bits
­ Smallest possible increment that can be detected at 16­bits = 2​16​
, 20V divided by the
increment gives us 305µV per count
­ The precision values chosen allow for precision of the motor speed. If the motor
moves too quickly, the patient may be lifted at an unsafe speed. A high level of
precision is necessary alongside the low rpm for optimal speed and safety.
­ Smallest theoretical change that can be detected is 305µV – precision that can be
acquired
­ Works for our system since it doesn’t require a large motor load, nor is a complex
functioning required – hence, works for the speed required by our motor
● For motor control – waveform of 8 bits – each of which can be “on” or “off” mode [31]
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­ 8­bits represents the control property of the power and speed of the motor. Specificity and
accuracy is necessary for this reason.
3.5 MINI­SPECIFICATIONS
Adjust settings:
Figure 3.5­1: ​Outline of settings process and psuedocode to go along with this step
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Force sensor:
Figure 3.5­2: ​Outline of force sensor process and psuedocode to go along with this step
Practice screen functions:
Figure 3.5­3: ​Outline of practice screen processes and psuedocode to go along with this step
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Vibration:
The pseudocode is a simplified representation of the fact that
vibration is caused by sensor activation. When the force output
exceeds a threshold, the sensor sends a signal to the motor that
activates a 5 second physical feedback.
Figure 3.5­4: ​Outline of psuedocode to
go along with the vibration processes
3.6 SOFTWARE TASKS
3.6.1 Normal operations:
­The app is opened and a prompt is generated to collect
patient data based on whether the patient is new or
returning. This is recorded and saved to the iPad and the
cloud for future access. Walking data is also uploaded to
this database and can be accessed at any time using
another device connected to the iCloud.
­Movement outside of the targeted range of motion is
calculated by a force percentage that is exerted on a
particular side. This will cause a vibration sensation
around the patient’s ribcage area where the harness is
attached. The patient will be able to visually see their gait
pattern by looking at the iPad that displays sensor results
on the screen. Force indications will be separated by
color and diameter of each dot. The dots represent a
relative unit of force.
­When the app is being used, there will be an option to
start the metronome. Tempo is chosen by the therapist
based on patient independence and ability to maintain
speed. This will continue until the therapist manually
stops it. If the patient is more advanced and prefers
music, the therapist can open the apple music app and
play music while this app is running. The iPad speakers
will be sufficient for volume, but the option for wireless
speakers is not closed off.
Figure 3.6.1­1: ​Steps needed to input patient
information. (Image displayed to the right).
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Figure 3.6.1­2: ​Steps needed to activate and use the
metronome option
​Figure 3.6.1­1: ​Steps used in the force
sensing and feedback segments of use
3.6.2 Low Battery Operation:
­Since the iPad will be charged separately, low battery operations must be considered. While the battery
should last so long as the iPad is being charged nightly, there will be a low battery mode. At this point,
the vibration will yield to preserve battery. The metronome or music will also pause. For this reason, it
would be recommended that the hospital has more than one iPad that can access the cloud or the iPad is
within reach of a charging station when the battery is running low.
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3.6.3 Early vs. Late Training:
Early training: Initially, the parameters based on the patient will need to be stored. This will allow
for estimated settings of the body weight support system. The patient will use the sensors to guide their
return to healthy gait in the early stages. They will also use the resources contained in the app such as the
metronome and music. Lastly, the vibration mechanism will be used alongside the sensors to reiterate
improper motion to the patient.
Late training: Later in use, the patient will rely less on the sensors and the vibration caused by
improper gait and more on feeling. They will eventually be able to follow a rhythm without the use of a
metronome of musical beat. This device can be placed over a treadmill or be used on its own as the
patient becomes more comfortable with walking.
Figure 3.6.3­1: ​Early and late training similarities and differences
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3.7 SYSTEMS IMPACT
● Thermal ­ The iPad will not have any direct thermal impact on the system because it is charged
outside of the system.
● Electrical ­ The electrical component will be hooked up to the iPad in order to provide the sensory
and visual feedback based on patient use. This includes the sensors that will be recording and
conveying patient leaning information, as well as the vibration mechanism that is a result of the
former. These wires will run through the frame into the side harnesses and send signals to the
iPad.
● Mechanical ­ The iPad will be attached via gooseneck hose. This means that the mechanics of the
device must include an adjustable means of attachment for the iPad. Since the iPad will be
charged outside of the device, it will be necessary to make the attachment secure during use.
There will also be an option of adding wireless speakers at a later date, so space will need to be
left for that adaptation. However, this should be an easy adaptation due to the previous design
model having space in that area.
● Controls ­The iPad will be touchscreen and thus controlled solely in this manner. The wires that
connect to the vibrating feedback component will cause vibration when the sensors measure force
out of the recommended range. The sensor view will be a direct view of the force output on the
torso portion of the harness. Lastly, the metronome or music will be accessible on the iPad
through an application. Any adjustments in software will be done on the iPad touchscreen, but
can also be accessed via any device connected to the iCloud system.
● Operations ­ The bodyweight support system is still autonomous with a manual addition. The
iPad is not used to drive the system, but rather for data storage and feedback. Therefore, the
device can be used without these functions, even though they are useful.
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4. ELECTRICAL PHASE
4.1 SCOPE
This system includes a patient lift system and a force detection and feedback system. The
patient lift system is activated by a separate remote control, which then signals a motor to lift the
patient to a standing position. Once the patient is ready to begin walking with the device, the
force feedback systems can be activated.
The force detection and feedback section allows the device to detect the force output by
the patient, transmit the information to the visual display via Bluetooth. Physical feedback is also
given, via vibrating motors.
4.2 NOTEABLE SYSTEMS
4.2.1 Notable Systems
The lift system is comprised of a motor to lift the patient that can be activated remotely
by the caregiver. The remote control signals a receiver which processes the signal and activates
the lift motor.
Two types of remote control communication were considered. Both infrared and radio
frequency signals are common forms of remote communication, with various drawbacks and
benefits, as shown in table 1. Infrared signaling was ultimately selected, as it had a lower current
draw, and any benefits of radio frequency, such as the greater range, would not be beneficial
when it is assumed that the caregiver will never be farther than 10 meters from the device while
it is in use.
Range
(m)
Light of Sight
Required
Current Draw
(mA)
Voltage Draw
(V)
Infrared 10 Yes 50 1.4
Radio
Frequency
30 No 60 3
Table 1: ​Remote Control Communication [1,2]
Two DC motors are used to lift the patient. In order to ensure the patient’s safety, a motor
capable of lifting a load much greater than an average patient was selected. Therefore, the motors
have a base torque rating of 20 N­m, and will overdrive to a maximum 80 N­m. Although this
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motor requires a higher current input, the safety of the patient is of the upmost importance, so it
was only a small trade off.
Table 2:​ Lift Motor
4.2.2.​ ​Force Detection and Feedback
The force sensors (piezoelectric sensors) located on the back support will be designed to
output a voltage to the vibration feedback once maximum force input is detected. These sensors
will also be linked to the iPad wirelessly through Bluetooth. When one sensor is triggered the
other sensor will turn off in order to signal to the patient which side they are leaning (applying
more force) to. Once the patient has corrected their position the force sensors will return to
neutral state and stop producing a voltage.
For the force sensors, the least sensitive model was selected. The force data must be
transmitted wirelessly to the iPad; limiting the sensitivity and therefore the amount of data that
must be transmitted allows for quicker communication.
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Model Sensitivity (mV/lb) Current Draw (mA) Voltage Draw (V) Weight (oz)
208C01 500 2­20 18­30 0.80
208C03 10 2­20 20­30 0.80
208C04 5 2­20 20­30 0.80
Table 3: ​General Purpose Quartz Force Sensors [3]
A vibration motor combining a 1.5­6V DC motor and a weight totaling one gram proved
to be a strong option to act alongside the force sensors. ​The output force measured by the sensor
will dictate when vibration feedback will be reinforced to the patient. When the force exceeds a
certain threshold, it will be indicated on the screen for the patient and the vibration mechanism
will begin. It will cease when the aforementioned force sensors compute a leaning force below
the threshold. These sensors will be placed on either side so the feedback is directly related to the
side that is receiving too much force.
4.2.3. iPad
An IPad will provide visual feedback to the patient while the system is in use, as well as
serve as the caregiver’s interface for analyzing force output data after a therapy session.
The iPad will not have an external power supply while in use with the walking assist
device. Since charging cords and additional battery packs can be restrictive and easy to misplace,
only the iPad’s internal battery will be used as a power supply. The iPad battery can power it
uninterrupted for up to six hours; this should be adequate for multiple therapy sessions
throughout a day.
● Power consumption (display on) : 2.69W
● Charging Time :10W USB power adaptor – Requirement 2.1 A at a min voltage of 4.97V
● 4 – 6 hours Battery life
● Battery:–24.3W lithium­polymer battery
Communication requirement –
● Via Bluetooth­based wireless communication system – between the microcontroller
element in the sensor and iPad’s Bluetooth 4.0 Wireless system
● Distance between the devices – factor for communication – required would be 10 – 15 m
– not an issue as it’ll be within the system
● Accuracy in detecting the correct device
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● Peak current consumption ­ <15 mA, Max voltage = 0.034 V
Trade­offs:
Wired sensor and its communication –
● Energy consumption is less as compared to using Bluetooth for communication
● Wired – could impede in the system – in terms of hindrance to other components
● Complex in terms of adjustments since a harness wearing is already being done – patient
intolerance with it
● With Bluetooth – only the click of a button
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4.3 SYSTEM ARCHITECTURE
4.3.1 Location of Components
Figure 4.3.1.1. On Frame Components: ​This diagram shows the electrical components located directly
on the support frame; this includes the iPad, IR receiver, and lift motor.
Figure 4.3.1.2. Harness Components: ​This diagram shows the electrical components location
within the harness; this includes the force sensors, vibrating motors, Bluetooth communication,
and battery feedback.
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Figure 4.3.1.3. Force Detection/Feedback: ​This diagram shows the architecture for only one
force sensor. The Bluetooth system is not entirely shown due to its many features and
complications. The inputs are “Force Sensor Input” and “IPad” and the outputs are to
“Physical Feedback” and “IPad”
4.3.2. Serial Communications
RS­232 is the most common serial interface; it’s used as a standard component for most
computers. RS­232 only allow for one transmitter and one receiver on each line. And most of
this device transmission speed is limited to 20 kb/s at 12m. Those properties are not suitable for
our device. Regarding the requirement of our system, RS­422 could be the best choice.
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Table 4.3.2.1. ​Comparison of serial communications: RS232, RS422, RS485
4.4 Power Consumption
Mode 100 V 115V 230V
Sleep 0.17W 0.18W 0.18W
Idle – Display on 2.7W 2.69W 2.80W
Power adapter – no load 0.09W 0.09W 0.09W
Power Adapter efficiency 80% 81% 80%
Table 4.3.2.2. ​Power consumption for iPad mini 2
iPad­
​For a 24.3 W­h Li­Po battery,
● Using a power adaptor: At 4.9V and 2.1A for 4960 mAh, will work for 2.36 hours
● When display­on (continuously) & a fully charged battery at 4.2V, 0.66A, 6470 mAh, will work
for 9.8 hours – works because a session wouldn’t last for this long. Can probably be used for two
sessions without charging
[The number of hours considered for further calculations]
Force Sensor communication –
● 3V DC with Max current rating – 90mA
● Rated power consumption = 195mW, 1.12 Wh and 373 mAh
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● Can be used for – 4.14 hours continuously with the above power consumption with respect to the
iPad use as well
● Energy consumed per day – 0.8073 Wh/day
Bluetooth communication –
● For Bluetooth Low Energy (BLE): Peak current consumption = 15mA and Max Voltage = 0.034
V, Output Power – 10mW and use for average 9.8 hours/day
● Energy consumed per day = 0.098 Wh/day– clearly low!
● General reviews – that the battery drain is not visibly faster for an iPad when the Bluetooth is on
Motor communication –
● For the motors
● Current rating – 5 A, Supply voltage – 24V DC
● Rated power consumption = 40 W
● Energy consumed per day = 0.392 kWh/day
As seen in the figure below, the Bluetooth and the force sensor hardly take up energy consumption as
compared to the motor and the display.
Figure 4.3.2.3.​ Summation of the energy consumption for different communications
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5. CONTROL PHASE
5.1 SCOPE
The Gait Provision System has two main control components; a lift and vibration motor. The
control requirements for each of these will be described in the first section of the report as well as some
foreseen limitations. The preceding section prioritizes each component and provides flow charts for better
understanding of the autonomous and manual controls. Lastly, the third section will go into some basic
calculations and models of the system behavior.
5.2 SPECIFICATIONS
There are two systems considered for control: the lift motor, and the vibrating feedback. Both
have unique restrictions that dictate the need requirements for each system.
Lift Motor:
Damping Ratio:​ When considering a lift motor, the damping ratio should typically be as close to
1 as possible; an underdamped system could potentially lead to lifting the patient too far or too quickly.
Steady State Error: ​The error of the system to be dictated by the reaction time of the therapist
using the lift motor; typically, this correlated to a very low percentage, between 1­5%
Rise Time: ​The speed of the lift motor’s response to input is once again limited by the need to
avoid lifting the patient for too long, and potentially pulling the patient too close to the supports.
Therefore, is it necessary that the rise time is very short, in order to cease motor output as quickly as
possible when necessary.
Overshoot: ​Once again, it is critical to avoid overshoot to limit the potential over injuring the
patient by lifting him or her too quickly.
Vibrating Feedback:
Damping Ratio: ​It is important the the patient can feel the vibrating feedback promptly.
Additionally, the maximum force output to the patient by the vibrating motor is very small, so an
underdamped system, rather than an overdamped system, is preferable.
Steady State Error: ​Once again, error is dictated by the reaction time of the user, and should be
kept very low to ensure prompt, effective vibrating feedback
Rise Time: ​As the vibrating feedback is meant to help the patient correct his or her balance, it is
important that the system provides prompt feedback­ however, it is important that the system first allow
the patient to first correct themselves. Therefore, rise time should not fall below the limitation of human
response time.
Overshoot: ​Since the maximum force output of the motor is small enough to not harm the patient,
overshoot is not a huge concern. However, is should be limited as to not to overpower the motor and
cause burnout.
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5.3 HIERARCHY
The lift motor and the vibration motor work independently of each other and at different stages of
use of the device. Hence, the control hierarchy would be individually described for them.
5.3.1 Lift motor
The hier​archy is as described below in the figure. The patient lifting is activated by a remote
control that activates the motor to actually lift the patient. The control is solely in the hands of the
therapist and hence is of the highest priority. The motor is another component that is the result of
controlling the highest prioritized component. The microcontroller is the middleman which transfers the
signal from the receiver and eventually the motor is powered for the final output – torque to lift the
patient.
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Figure 5.3.1.1​. Control hierarchy of the lift motor for GPS
5.3.2 Vibration Feedback
The hierarchy determined is based around the vibrating operating system by the DC motor and
sensor information entering the system. When the output exceeds a normal distribution of weight, the
vibrating feedback is enabled. This lasts for five seconds. These are shown below the main block in the
figure below. Leading into the previously referenced DC motor and sensor are the force output based on
the patient using the GPS. Lastly, when the sensor picks up signal, it is also represented as visual
feedback.
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Figure 5.3.2.1.​ Control hierarchy of the vibration feedback for GPS
5.4 SYSTEMS MODEL
5.4.1 Lift Motor
The output speed of the motor is 25 rpm. And the maximum torque is 80 N for one motor. We
can change the torque regarding the patient’s weight data in the system by current control. Both of current
and voltage control should work for our motor, we can choose current control.
Figure 5.4.1.1. ​DC motor circuit diagram and torque directions [39]
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Figure 5.4.1.2. ​Closed loop control block diagram for a single lift motor
Figure 5.4.1.3. ​Mathematical model of the DC motor [39]
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5.4.2 Vibration Motor
The typical natural frequency of a linear resonant actuator vibration motor is between 150Hz and
205Hz [1]. For the purpose of our calculations we will say n= 150Hz. The damping ratio is the
exponential decay frequency divided by the natural frequency. When < 1 the system is underdamped,
when > 1 it is overdamped, and when = 1 it is critically damped [38].
Figure 5.4.2.1. ​Closed loop control block diagram for a single vibration motor
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Figure 5 visually shows the possibly time responses of the vibration motor, it is understood that it
is a second order response. Some possible causes of error include motor orientation, position of the device
on the body, changes in vibration frequency, and environmental factors. 250ms with 2%steady state error
was chosen as the settling time because that is the average human reaction time. The overshoot was
calculated using = .02 and equation:
Giving us a .939% overshoot and a max amplitude of 3.878 with 2v being our steady state value.
Figure 5.4.2.2. ​Three possible time response curves of the vibration motor: Under­damped, Critically
Damped, and Overdamped.
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5. http://www.biodex.com/physical­medicine/products/harnesses/standard­support­harness
6. http://www.wernerco.com/us/support/fall­protection/know­your­harness
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8. http://www.wernerco.com/us/support/fall­protection/know­your­harness
9. http://api.capitalsafety.com/api/assets/download/1/6514545
10. http://www.webriggingsupply.com/pages/catalog/rope/3strand­twisted­premium.html
11. http://www.engineeringtoolbox.com/polyester­rope­strength­d_1514.html
12. http://www.webriggingsupply.com/pages/catalog/rope/rope­3strand­manila.html#B
13. http://www.versalestore.com/c272/Dichromated­Hooks.htm
14. http://www.alibaba.com/product­detail/­EZ­LOAD­1­Inch­25mm_60228606606.html
15. http://www.homedepot.com/p/Keeper­Adjustable­ZipHook­10­Pack­06479/203882701
16. http://uk.rs­online.com/web/p/dc­geared­motors/6685534/
17. http://uk.rs­online.com/web/p/dc­geared­motors/7736781/
18. http://uk.rs­online.com/web/p/dc­geared­motors/3808633/
19. http://www.aliexpress.com/store/product/high­torque­low­speed­60mm­diameter­planetary­gear­
motor­PG60ZY58123000­714K/912389_670945409.html
20. http://www.ebay.com/itm/Electric­Gear­Motor­12v­Low­Speed­50­RPM­Gearmotor­DC­10mm­
Shaft­Coupling­/371233515334?hash=item566f3de746:g:614AAOSwk5FUsfrT
21. https://www.motiondynamics.com.au/worm­drive­motor­12v­180w­30­40­rpm­20­100nm­torque
.html
22. http://za.rs­online.com/web/p/dc­geared­motors/3636674/
23. http://www.aliexpress.com/store/product/DC­24V­10rpm­100kg­cm­High­torque­BBQ­worm­ge
ar­motor­free­shipping/807097_468609934.html
24. http://www.aliexpress.com/item/GW4468­24V­10rpm­1000N­cm­1­2A­15W­Low­speed­High­T
orque­worm­gearbox­motor­Electric/1689316175.html
25. http://www.uline.com/Product/Detail/H­3609/Packing­Tables/Set­of­4­Lockable­Casters?pricode
=WY703&gadtype=pla&id=H­3609&gclid=CPb669ih6cgCFQoXHwoddqMMmQ&gclsrc=aw.d
s
26. Frey, Martin, et al. "A novel mechatronic body weight support system." ​Neural Systems and
Rehabilitation Engineering, IEEE Transactions on​ 14.3 (2006): 311­321.
27. http://www.shockstrap.com/blog/which­tie­down­webbing­is­better­polypropylene­nylon­or­poly
ester
28. http://www.ada.gov/descript/reg3a/figA3ds.ht​m
29. http://www.gsmarena.com/apple_ipad_mini_2­5735.php
30. https://itunes.apple.com/us/app/ludwig­metronome/id413743609?mt=8
31. http://uk.rs­online.com/web/p/dc­geared­motors/6685534/
32. http://www.digikey.com/product­search/en/optoelectronics/infrared­uv­visible­emitters/524328?k
=led
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62.
33. http://www.digikey.com/product­search/en/discrete­semiconductor­products/rf­transistors­bjt/137
6962?k=rf%20emitter
34. http://www.protostack.com/other/vibration­motor­10x3mm
35. https://www.pcb.com/testmeasurement/force/genpur
36. http://www.lammertbies.nl/comm/info/RS­485.html
37. https://www.apple.com/environment/pdf/products/ipad/iPadmini2_PER_sept2015.pdf
38. http://www.precisionmicrodrives.com/application­notes­technical­guides/application­bulletins/ab
­028­vibration­motor­comparison­guide
39. www.researchgate.net/file.PostFileLoader.html?id
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63.
APPENDIX
1. Anthropometric data:
Figure 1​: ​Anthropometric data. All data were taken from 5th percentile female to 95 percentile male.
A: Stature; B: Elbow Rest Height, Standing; C: Buttock Height; D: Elbow­Center of Grip Length; E:
Shoulder­Elbow Length; F: Buttock­Knee Length; G: Buttock­Popliteal Length; H: Knee Height,
Sitting; I: Popliteal Height; J: Forearm­Forearm Breadth;
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64.
Ideas for further exploration:
1. Motorized wheels to help patient walk on the ground.
REQUIREMENTS CRITERIA
To help patient walk on the ground Walking on the ground provides different feelings as
compared to walking on a treadmill. It could be a good
improvement if our device can help the patient walk on
the ground.
Safety The whole device should stay balanced when the
patient is walking. A zero moment arm around the
middle of the device would be indicative of this.
Lock system The device should not be movable before the patient is
ready to walk. The wheels should remain locked before
walking, and unlocked when the patient is prepared to
walk.
Table 8.
We could design 4 casters on the bottom to help the whole device move on the ground. 4 motors are
connected to the casters to generate the force (needs further calculation, for example, if the body weight
of the whole device is G, and the friction factor is u, so the total force the 4 motor can generate should be
set to N=uG ) that is needed to move the whole device, so that when patient is walking, there is no
resistance from the device. The casters need to have a switch to control them and lock the wheels before
actually walking. We found a set of 4 lockable casters available online for only $25. [25]
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65.
2. Counterweight system
REQUIREMENTS CRITERIA
Counterweight The center of body gravity is actually moving when
people walk on the ground. It could be much better if
we can have a Z­direction movable counterweight
system. The system should also be able to change the
counterweight load so that therapist can adjust the
weight to the most comfortable load for the patient.
Safety Adding a counterweight system may introduce extra
weight to the whole device. This will introduce a need
to check for balance forces among the device.
Table 9.
There have been publishings discussing a counterweight segment which is composed of a passive
elastic spring element to take over the main body weight and a controlled electric drive element to
generate accurate extra force which is convenient to adjust. [26] Both of the counterweight elements
would be connected to a polyester rope which would be connected to the patient through roller wheels and
an electric winch. The two roller wheels are exclusively for connection, the electric winch can be helpful
to adjust the rope length to adapt to different
patient size. As for the safety concern, it is
hypothesized that the counterweight
system is not feasible for our device, it
may increase complexity and safety
problems. ​Please look to the right for a
detailed design (figure 15).
Figure 15. ​Passive elastic spring counter
weight system [26]
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66.
Trade offs for the counterweight system:
Figure 17. ​Compares different counter weight systems [26]
Trade offs for the rope:
The table below shows the comparison of three different possible materials: polypropylene, nylon and
polyester. It should be easy to tell that polymer is th​e best choice based on these specs.
Table 10.​ [27]
3. Original design
Below pictures show our original design: 1, using two adjustable beams under patient’s arm and
back­front support to secure the patient in the device; 2, connecting the harness to the counterweight
system ( the counterweight could be adjustable), so that the harness can help lift up the patient from the
wheelchair and function as a controlled counterweight load. However, the counterweight system might be
too complicate, and make the device looks like a big heavy stuff. Also, considering the safety issue, we
don’t think this is good device.
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67.
Figure 18.​ Original design of our device A: The outlook of the device. B: U­shaped base C: underarm
and back­front support system. D:Harness system E: counterweight system.
4. Adjustable seater and elliptical trainer
Adjustable seater ­ This was thought of to address the problem of fatigue. For a patient that is new to
rehabilitation or overweight, fatigue may onset before their session is completed. For this reason, a seat
would be able to be used to let them rest, rather than having to remove them from the harness.
Elliptical ­ This was implemented with the idea to allow for stationary movement while mimicking
natural gait. Similar to an elliptical machine in a gym, there would be the option to change resistance
options.
Both of the above were chalked off from the final design because it was increasing the complexity of
adding more components than required and we did not want to deteriorate the quality of the device.
66

68.
List of changes:
1. Operations phase:
● Revision of Objectives ­ minor changes in accordance to the final design and grammatical fix
(Section 1.3)
● Revision of the prioritized functional requirements ­ removal of elliptical and adjustable seater
(Section 1.4.1)
● Revision of the prioritized performance requirements ­ minor fix in accordance to the suggestions
provided (Section 1.4.2)
● Shift of phases of use to the end of the operations phase with additional diagrams (Section 1.6.1)
2. Mechanical phase:
● Revised the order for material comparison table.
● Revised the pictures for front back support.
● The loading capacity for the buckles is 4000 lbs.
● Move the material comparison for different polymer to the harness support part.
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