Drives.pdf

MODULE II
ROBOT DRIVES, CONTROLS AND POWER TRANSMISSION
Contents
1
2.1 Robot drive mechanisms – Hydraulic and Pneumatic
2.2 Robot drive mechanisms – Electric and Mechanical transmission methods
2.3 Electronic and Pneumatic manipulators Construction of Manipulators
2.4 Different Types of Controllers-Proportional, Integral, Differential, PID controllers
2.5 Classification of End effectors
2.6 Drive system for grippers- Mechanical-adhesive-vacuum-magnetic-grippers.
2.7 Active and passive grippers

2
Robot Drive Mechanisms
❏ The robot's capacity to move its body, arm, and wrist is provided by the drive system
used to power the robot.
❏ The drive system determines its speed of operation, load carrying capacity, and its
dynamic performance. To some extent, the drive system determines the kinds of
applications that the robot can accomplish.
❏ Commercially available industrial robots are powered by one of three types of drive
systems. These three systems are
1. Hydraulic drive
2. Electric drive
3. Pneumatic drive

3
❑ Hydraulic drive and electric drive are the two main types of drives used on more
sophisticated robots, while pneumatic drive is used for low load carrying capacity
robots and in cases where oil and electricity cannot be used (fire hazard).
Hydraulic Drive
❑ Hydraulic drive is generally associated with larger robots. The usual advantages of
the hydraulic drive system are that it provides the robot with greater speed and
-strength.
❑ The disadvantages of the hydraulic drive system are that it typically adds to the floor
space required by the robot, and that a hydraulic system is inclined to leak oil which is
a nuisance.

4
❑ Hydraulic drive systems can be designed to actuate either rotational joints or linear
joints.
❑ Rotary vane actuators can be utilized to provide rotary motion, and hydraulic pistons
can be used to accomplish linear motion

5
Basic Components of a Hydraulic System

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Functions of the components
❑ The hydraulic actuator is a device used to convert the fluid power into mechanical
power to do useful work. The actuator may be of the linear type (e.g., hydraulic
cylinder) or rotary type(e.g., hydraulic motor) to provide linear or rotary motion,
respectively.
❑ The hydraulic pump is used to force the fluid from the reservoir to rest of the hydraulic
circuit by converting mechanical energy into hydraulic energy.
❑ Valves are used to control the direction, pressure and flow rate of a fluid flowing
through the circuit.
❑ External power supply (motor) is required to drive the pump.
❑ Reservoir is used to hold the hydraulic liquid, usually hydraulic oil.

7
Cylinder movement is controlled by a three-position change over a control valve.
❑ When the piston of the valve is changed to upper position, the pipe pressure line is
connected to port A and thus the load is raised.
❑ When the position of the valve is changed to lower position, the pipe pressure line is
connected to port B and thus the load is lowered.
❑ When the valve is at center position, it locks the fluid into the cylinder(thereby holding
it in position) and dead-ends the fluid line (causing all the pump output fluid to return
to tank via the pressure relief).

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❑ In industry, a machine designer conveys the design of hydraulic systems using a
circuit diagram. The components of the hydraulic system using symbols. The working
fluid, which is the hydraulic oil, is stored in a reservoir. When the electric motor is
switched ON, it runs a positive displacement pump that draws hydraulic oil through a
filter and delivers at high pressure. The pressurized oil passes through the regulating
valve and does work on actuator. Oil from the other end of the actuator goes back to
the tank via return line. To and fro motion of the cylinder is controlled using directional
control valve.

9

10
The hydraulic system discussed above can be broken down into four
main divisions that are analogous to the four main divisions in an
electrical system.
❑ The power device parallels the electrical generating station.
❑ The control valves parallel the switches, resistors, timers, pressure switches, relays,
etc.
❑ The lines in which the fluid power flows parallel the electrical lines.
❑ The fluid power motor (whether it is a rotating or a non rotating cylinder or a fluid
power motor) parallels the solenoids and electrical motors.

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Pneumatic Drive
❑ Pneumatic drive is generally reserved for smaller robots that possess fewer degrees
of freedom (two- to four-joint motions).
❑ These robots are often limited to simple pick-and-place operations with fast cycles.
❑ These drives have the added advantage of having compliance or ability to absorb
some shock during contact with the environment.
❑ Pneumatic power can be readily adapted to the actuation of piston devices to provide
translational movement of sliding joints.
❑ It can also be used to operate rotary actuators for rotational joints.

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Basic Components of a Pneumatic System
❑ A pneumatic system carries power by employing compressed gas, generally air, as a
fluid for transmitting energy from an energy-generating source to an energy-using
point to accomplish useful work. simple circuit of a pneumatic system with basic
components.

13
Basic Components of a Pneumatic System

14
❑ The pneumatic actuator converts the fluid power into mechanical power to perform
useful work.
❑ The compressor is used to compress the fresh air drawn from the atmosphere.
❑ The storage reservoir is used to store a given volume of compressed air.
❑ The valves are used to control the direction, flow rate and pressure of compressed
air.
❑ External power supply (motor) is used to drive the compressor.
❑ The piping system carries the pressurized air from one location to another.

15
❑ Air is drawn from the atmosphere through an air filter and raised to required pressure
by an air compressor.
❑ As the pressure rises, the temperature also rises; hence, an air cooler is provided to
cool the air with some preliminary treatment to remove the moisture. The treated
pressurized air then needs to get stored to maintain the pressure. With the storage
reservoir, a pressure switch is fitted to start and stop the electric motor when pressure
falls and reaches the required level, respectively.
❑ The three-position change over the valve delivering air to the cylinder operates in a
way similar to its hydraulic circuit.

2.2 Robot drive mechanisms – Electric and Mechanical Transmission
Methods
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Electric Drive
❑ Electric drive systems do not generally provide as much speed or power as hydraulic
systems.
❑ However, the accuracy and repeatability of electric drive robots are usually better.
❑ Consequently, electric robots tend to be smaller, requiring less floor space, and their
applications tend toward more precise work such as assembly.
❑ Electric drive robots are actuated by de stepping motors or de servomotors. These
motors are ideally suited to the actuation of rotational joints through appropriate drive
train and gear systems.

Methods
17
❑ Electric motors can also be used to actuate linear joints (e.g., telescoping arms) by
means of pulley systems or other translational mechanisms.
❑ The economics of the two types of drive systems are also a factor in the decision to
utilize hydraulic drive on large robots and electric drive on smaller robots.
❑ It turns out that the cost of an electric motor is much more proportional to its size,
whereas the cost of a hydraulic drive system is somewhat less dependent on its size.
❑ It should be noted that there is a trend in the design of industrial robots toward all
electric drives, and away from hydraulic robots because of the disadvantages
discussed above.

Methods
18
Electric Drive
❑ These are direct current (DC) or alternating current (AC) servo motors. They are
small in size and are easy to control.
❑ Electric drives are mostly used in position and speed control systems. The motors
can be classified into two groups namely DC motors and AC motors. In this session
we shall study the operation, construction, advantages and limitations of DC and AC
motors

Methods
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Electric Drive

Methods
20
Stepper motor
❑ A stepper motor is a pulse-driven motor that changes the angular position of the rotor
in steps. Due to this nature of a stepper motor, it is widely used in low cost, open loop
position control systems.
❑ Types of stepper motors:
❑ Permanent Magnet- Employ permanent magnet , Low speed, relatively high torque
❑ Variable Reluctance- Does not have permanent magnet, Low torque.

Methods
21
Variable Reluctance Motor
❑ The cylindrical rotor is made of soft steel and has four poles as shown in Fig. It has
four rotor teeth, 90⁰ apart and six stator poles, 60⁰ apart. Electromagnetic field is
produced by activating the stator coils in sequence. It attracts the metal rotor. When
the windings are energized in a reoccurring sequence of 2, 3, 1, and so on, the motor
will rotate in a 30⁰ step angle.
❑ In the non-energized condition, there is no magnetic flux in the air gap, as the stator
is an electromagnet and the rotor is a piece of soft iron; hence, there is no detent
torque. This type of stepper motor is called a variable reluctance stepper.

Methods
22
Variable Reluctance Motor

Methods
23
Permanent magnet (PM) stepper motor
❑ In this type of motor, the rotor is a permanent magnet. Unlike the other stepping
motors, the PM motor rotor has no teeth and is designed to be magnetized at a right
angle to its axis. Figure shows a simple, 90⁰ PM motor with four phases (A-D).
Applying current to each phase in sequence will cause the rotor to rotate by adjusting
to the changing magnetic fields. Although it operates at fairly low speed, the PM
motor has a relatively high torque characteristic. These are low cost motors with
typical step angle ranging between 7.5⁰ to 15⁰.
❑ In the non-energized condition, there is no magnetic flux in the air gap, as the stator
is an electromagnet and the rotor is a piece of soft iron; hence, there is no detent
torque. This type of stepper motor is called a variable reluctance stepper.

Methods
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Permanent magnet (PM) stepper motor

Methods
25
Servomotor
❑ Servomotors are special electromechanical devices that produce precise degrees of
rotation. A servo motor is a DC or AC or brushless DC motor combined with a
position sensing device. Servomotors are also called control motors as they are
involved in controlling a mechanical system. The servomotors are used in a
closed-loop servo system as shown in Figure. A reference input is sent to the servo
amplifier, which controls the speed of the servomotor. A feedback device is mounted
on the machine, which is either an encoder or resolver. This device changes
mechanical motion into electrical signals and is used as a feedback.

Methods
26
Servomotor
❑ This feedback is sent to the error detector, which compares the actual operation with
that of the reference input. If there is an error, that error is fed directly to the amplifier,
which will be used to make necessary corrections in control action. In many servo
systems, both velocity and position are monitored. Servomotors provide accurate
speed, torque, and have ability of direction control.

Methods
27
DC servomotors
❑ DC operated servomotors are usually respond to error signal abruptly and accelerate
the load quickly. A DC servo motor is actually an assembly of four separate
components, namely:
❑ DC motor
❑ gear assembly
❑ position-sensing device
❑ control circuit.

Methods
28
AC servo motor
❑ In this type of motor, the magnetic force is generated by a permanent magnet and
current which further produce the torque. It has no brushes so there is little
noise/vibration. This motor provides high precision control with the help of high
resolution encoder.
❑ The stator is composed of a core and a winding. The rotor part comprises of shaft,
rotor core and a permanent magnet.
❑ Digital encoder can be of optical or magnetic type. It gives digital signals, which are in
proportion of rotation of the shaft.

Methods
29
Mechanical Transmission Methods
❑ Mechanical power transmission refers to the transfer of mechanical energy (physical
motion) from one component to another in machines. Most machines need some form of
mechanical power transmission.
❑ Common examples include electric shavers, water pumps, turbines and automobiles.
The most common mechanical power transmission methods in use in the engineering
industry today are:
1. Shaft couplings
2. Chain drives
3. Gear drives
4. Belt drives

Methods
30
Shaft couplings
❑ Shaft couplings connect two shafts and transmit torque between them. The shafts
may be in line, intersecting but not parallel, or non-intersecting and non-parallel. To
cater to the needs of various applications and environments, many different types and
sizes of couplings are produced.
Advantages
❑ Shaft couplings are low maintenance machine elements
❑ They can absorb shock and vibration
❑ They can handle radial and axial misalignment
❑ They provide heat isolation.

Methods
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Disadvantages
❑ Shaft couplings cannot be used for non-intersecting parallel shafts
❑ Rigid couplings may damage the shaft if misalignment creeps in
❑ Backlash may develop over the service life, putting the couplings, bearings and drive
components under additional stress
❑ Some couplings may loosen over time, damaging the drive components.

Methods
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Belt drives
❑ Belt drives are a fairly common sight in industrial applications. A belt drive system
consists of two pulleys and a belt (or rope). The belt firmly grips both pulleys and transfers
power from the driving shaft to the driven shaft through friction. The belt drive works
equally well for slow and very high speeds and thus finds use in high-speed applications
such as air compressors.
Advantages
❑ Belt drives are more affordable than other drives due to low component cost and high
efficiency
❑ They can transmit power over long distances
❑ Smoother and quieter operation compared to chain drives

Methods
33
Disadvantages
❑ Belt slippage can vary the velocity ratio
❑ Short service life if not maintained well
❑ Finite speed range
❑ They apply a heavy load on the bearings and shafts
❑ To compensate for wear and stretching, they need an idler pulley or some adjustment
of center distance.

Methods
34
Chain drives
❑ Chain drives are used to transmit power between two components that are at a greater
distance. These drives consist of a roller chain and two or more sprockets. The driver
sprocket’s teeth mesh with the roller chain and transfer torque to the driven sprocket.
Chains can be commonly seen in power transmission in bicycles and motorcycles, but
they are also quite common in industrial machines.
Advantages
❑ A chain drive is more compact than a belt drive and can fit into relatively tight spaces
❑ It can transfer torque over long distances
❑ Contrary to belt drives, chain drives do not slip
❑ One chain drive can power multiple shafts at a time

Methods
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Disadvantages
❑ They are noisy and can also cause vibrations
❑ A chain drive cannot work with non-parallel shafts
❑ Some designs require constant lubrication
❑ Misalignment may cause the chain to slip off
❑ A chain drive usually needs an enclosure
❑ It requires an arrangement for chain tensioning in the form of a tightening idler
sprocket.

Methods
36
Gear drives
❑ Gear drives use gears for motion and power transmission from one shaft to another. They
consist of a driving gear (on the input shaft) and a driven gear (on the output shaft). Power
transmission from the power source to the load takes place through the meshing of the
gear teeth. Due to the many available designs, they can work in a number of orientations
and applications.
❑ A gear drive can handle higher loads compared to a chain drive but is only suitable for
short distances, as the gears need to be in direct contact with each other. Using multiple
gears in a gear train makes it possible to change the gear ratio, rotational speed, torque
and direction as needed.

Methods
37
Advantages
❑ Suitable for high mechanical power transmission applications
❑ Gears are sturdy and have long service lives
❑ Compact setup
❑ Gears have high efficiency and do not slip.
Disadvantages
❑ Not suitable when distances between shafts are high, direct connection is needed
❑ Prone to vibration and noise
❑ Metal gears are heavy and increase the weight of the machine
❑ They do not offer any flexibility

Methods
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Power screws
❑ Power screws, also known as lead screws (leadscrews) or translational screws, are
screws that either transmit or receive power. They are different from screw fasteners that
are used to create temporary joints in machines. Power screw consists of a screw and nut
that mesh with each other for power transmission.
❑ In some cases, the nut is stationary while the screw moves for power transmission (screw
jack and vice). In other cases, the nut is the power source and the screw is stationary
(lathe lead screw).

Methods
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Advantages
❑ Power screws are cheap and reliable as they only have a few parts
❑ Some lead screws have self-locking property
❑ It requires little to no maintenance
❑ Capable of lifting heavy loads
❑ Smooth and quiet operation.
Disadvantages
❑ High wear rate compared to other methods of mechanical power transmission
❑ Power screws have poor efficiency
❑ Not suitable for mechanical power transmissions with a very high torque demand

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Construction of manipulators
❑ A robot manipulator is an electronically controlled mechanism, consisting of multiple
segments, that performs tasks by interacting with its environment. They are also
commonly referred to as robotic arms.
❑ Robot manipulators are extensively used in the industrial manufacturing sector and
also have many other specialized applications (for example, the Canadarm was used
on space shuttles to manipulate payloads).
❑ The study of robot manipulators involves dealing with the positions and orientations of
the several segments that make up the manipulators. This module introduces the
basic concepts that are required to describe these positions and orientations of rigid
bodies in space and perform coordinate transformations.

41
❑ Manipulators are composed of an assembly of links and joints. Links are defined as
the rigid sections that make up the mechanism and joints are defined as the
connection between two links. The device attached to the manipulator which interacts
with its environment to perform tasks is called the end-effector.

42

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❑ Planar manipulator: A manipulator is called a planar manipulator if all the moving links
move in planes parallel to one another.
❑ Spherical manipulator: A manipulator is called a spherical manipulator if all the links
perform spherical motions about a common stationary point.
❑ Spatial manipulator: A manipulator is called a spatial manipulator if at least one of the
links of the mechanism possesses a general spatial motion.
❑ Open-loop manipulator (or serial robot): A manipulator is called an open-loop
manipulator if its links form an open-loop chain.

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❑ Parallel manipulator: A manipulator is called a parallel manipulator if it is made up of
a closed-loop chain.
❑ Hybrid manipulator: A manipulator is called a hybrid manipulator if it consists of open
loop and closed loop chains.
Pneumatic actuators system with neat sketch
❑ Pneumatic systems use pressurized air to make things move. Basic pneumatic
system consists of an air generating unit and an air-consuming unit. Air compressed
in compressor is not ready for use as such, air has to be filtered, moisture present in
air has to be dried, and for different applications in plant pressure of air has to be
varied. Several other treatments are given to the air before it reaches finally to the
Actuators. The figure gives an overview of a pneumatic system. Practically some

45
Compressor:
❑ A device, which converts mechanical force and motion into pneumatic fluid power, is
called compressor. Every compressed-air system begins with a compressor, as it is
the source of airflow for all the downstream equipment and processes Electric Motor
Electric motor is used to drive the compressor.

46
Air Receiver:
❑ It is a container in which air is stored under pressure. Pressure Switch. Pressure Switch is
used to maintain the required pressure in the receiver; it adjusts the High Pressure Limit
and Low Pressure Limit in the receiver. The compressor is automatically turned off when
the pressure is about to exceed the high limit and it is also automatically turned on when
the pressure is about to fall below the low limit.
Safety Valve:
❑ The function of the safety valve is to release extra pressure if the pressure inside the
receiver tends to exceed the safe pressure limit of the receiver.
Check Valve:
❑ The valve enables flow in one direction and blocks flow in a counter direction is called
Check Valve. Once compressed air enters the receiver via check valve, it is not allowed to

47
Direction Control Valve:
❑ Directional-control valve are devices used to change the flow direction of fluid within a
Pneumatic/Hydraulic circuit. They control compressed-air flow to cylinders, rotary
actuators, grippers, and other mechanisms in packaging, handling, assembly, and
countless other applications. These valves can be actuated either manually or
electrically.
Pneumatic Actuator:
❑ A device in which power is transferred from one pressurized medium to another
without intensification. Pneumatic actuators are normally used to control processes
requiring quick and accurate response, as they do not require a large amount of
motive force. They may be reciprocating cylinders, rotating motors or may be a robot
end effectors.

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Electronic And Pneumatic Manipulator Control Circuits
❑ Actuators are output devices which convert energy from pressurized hydraulic oil or
compressed air into the required type of action or motion. In general, hydraulic or
pneumatic systems are used for gripping and/or moving operations in industry. These
operations are carried out by using actuators.
Actuators can be classified into three types.
1. Linear actuators: These devices convert hydraulic/pneumatic energy into linear
motion.
2. Rotary actuators: These devices convert hydraulic/pneumatic energy into rotary
motion.
3. Actuators to operate flow control valves: these are used to control the flow and
pressure of fluids such as gases, steam or liquid..

49
❑ The construction of hydraulic and pneumatic linear actuators is similar. However they
differ at their operating pressure ranges. Typical pressure of hydraulic cylinders is
about 100 bar and of pneumatic system is around 10 bar.
1. Single acting cylinder:

50
❑ These cylinders produce work in one direction of motion hence they are named as
single acting cylinders. The compressed air pushes the piston located in the
cylindrical barrel causing the desired motion. The return stroke takes place by the
action of a spring. Generally the spring is provided on the rod side of the cylinder.
2. Double acting cylinder:

51
❑ The main parts of a hydraulic double acting cylinder are: piston, piston rod, cylinder tube,
and end caps. The piston rod is connected to piston head and the other end extends out of
the cylinder. The piston divides the cylinder into two chambers namely the rod end side and
piston end side. The seals prevent the leakage of oil between these two chambers. The
cylindrical tube is fitted with end caps.
❑ The pressurized oil, air enters the cylinder chamber through the ports provided. In the rod
end cover plate, a wiper seal is provided to prevent the leakage of oil and entry of the
contaminants into the cylinder.
❑ The combination of wiper seal, bearing and sealing ring is called as cartridge assembly.
The end caps may be attached to the tube by threaded connection, welded connection or
tie rod connection. The piston seal prevents metal to metal contact and wear of piston head
and the tube. These seals are replaceable. End cushioning is also provided to prevent the

52
3. Cylinder end cushions:

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❑ Double acting cylinders generally contain cylinder cushions at the end of the cylinder to slow
down the movement of the piston near the end of the stroke. Cushioning arrangement avoids
the damage due to the impact occurred when a fast moving piston is stopped by the end
caps.
❑ Deceleration of the piston starts when the tapered plunger enters the opening in the cap and
closes the main fluid exit. This restricts the exhaust flow from the barrel to the port. This
throttling causes the initial speed reduction.
❑ During the last portion of the stroke the oil has to exhaust through an adjustable opening
since main fluid exit closes. Thus the remaining fluid exists through the cushioning valve.
Amount of cushioning can be adjusted by means of cushion screw.
❑ A check valve is provided to achieve fast break away from the end position during retraction
motion. A bleed screw is built into the check valve to remove the air bubbles present in a

54
4. Gear motor: a rotary actuator:

55
❑ Rotary actuators convert energy of pressurized fluid into rotary motion. Rotary actuators
are similar to electric motors but are run on hydraulic or pneumatic power.
❑ It consists of two inter meshing gears inside a housing with one gear attached to the
drive shaft. The air enters from the inlet, causes the rotation of the meshing gear due to
difference in the pressure and produces the torque. The air exists from the exhaust
port. Gear motors tend to leak at low speed, hence are generally used for medium
speed applications.

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5. Vane motor: a rotary actuator
❑ A rotary vane motor consists of a rotor with sliding vanes in the slots provided on the
rotor The rotor is placed eccentrically with the housing. Air enters from the inlet port,
rotates the rotor and thus torque is produced. Air is then released from the exhaust port
(outlet).

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6. Limited rotation actuators
❑ It consists of a single rotating vane connected to output shaft as shown in Fig. It is used
for double acting operation and has a maximum angle of rotation of about 270°. These
are generally used to actuate dampers in robotics and material handling applications.
Other type of limited rotation actuator is a rack and pinion type actuator.

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7. Speed control
❑ For an actuator, the operational speed is determined by the fluid flow rate and the
cylinder actuator area or the motor displacement. The speed can only be controlled by
adjusting the fluid flow to the actuator, because the physical dimension of the actuator
is fixed. Since the air is compressible, flow control is difficult as compared to the
hydraulic system. There are various ways of controlling the fluid flow. One of the
methods is discussed as below.

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❑ the circuit diagram of hydraulic system developed to control the speed of motion of a
piston. Consider a pump which delivers a fluid volume of ‘V’ per minute. The pump has
a fixed displacement. The volume of fluid goes either to the pump or to the actuator.
When the direction control valve moves from its center position the actuator of area ‘A’,
the piston moves with a velocity.

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v = 𝑉/𝐴
❑ If the pump delivery volume ‘V’ can be adjusted by altering swash plate angle of a
piston pump or by using a variable displacement vane pump, no further speed control
will be needed.
❑ Consider a simple operation where a double-acting cylinder is used to transfer parts
from a magazine. The cylinder is to be advanced either by operating a push button or
by a foot pedal. Once the cylinder is fully advanced, it is to be retracted to its initial
position. A 3/2-way roller lever valve is to be used to detect the full extension of the
cylinder. Design a pneumatic circuit for the above-mentioned application

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Working
The pneumatic components which can be used to implement the mentioned task are as
follows:
❑ double acting cylinder
❑ 3/2 push button valve
❑ 3/2 roller valve
❑ shuttle valve
❑ 3/2 foot pedal actuated valve
❑ 5/3 pneumatic actuated direction control valve
❑ compressed air source and connecting piping

62

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❑ As the problem stated, upon actuation of either the push button of valve (S1) or the foot
pedal valve (S2), a signal is generated at 1 or 1(3) side of the shuttle valve. The OR
condition is met and the signal is passed to the control port 14 of the direction control
valve (V2). Due to this signal, the left position of V2 is actuated and the flow of air
starts. Pressure is applied on the piston side of the cylinder (A) and the cylinder
extends. If the push button or pedal valve is released, the signal at the direction control
valve (V2) port is reset. Since DCV (V2) is a double pilot valve, it has a memory
function which doesn’t allow switching of positions. As the piston reaches the rod end
position, the roller valve (S3) is actuated and a signal is applied to port 12 of the DCV
(V2). This causes actuation of right side of DCV (V2). Due to this actuation, the flow
enters at the rod-end side of the cylinder, which pushes the piston towards left and thus
the cylinder retracts.

2.5 Classification of End Effectors
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TYPES OF END EFFECTORS:
❑ There are a wide assortment of end effectors required to perform the variety of different
work functions. The various types can be divided into two major categories:
❑ 1. Grippers
❑ 2. Tools.
Grippers are end effectors used to grasp and hold objects. The objects are generally
work parts that are to be moved by the robot. These part-handling applications include
machine loading and unloading, picking parts from a conveyor, and arranging parts
onto a pallet. In addition to work parts, other objects handled by robot grippers include
cartons, bottles, raw materials, and tools. We tend to think of grippers as mechanical
grasping devices, but there are alternative ways of holding objects involving the use of

65
❑ A robot manipulator is an electronically controlled mechanism, consisting of multiple
segments, that performs tasks by interacting with its environment. They are also
commonly referred to as robotic arms.
❑ Robot manipulators are extensively used in the industrial manufacturing sector and
also have many other specialized applications (for example, the Canadarm was used
on space shuttles to manipulate payloads).
❑ The study of robot manipulators involves dealing with the positions and orientations of
the several segments that make up the manipulators. This module introduces the
basic concepts that are required to describe these positions and orientations of rigid
bodies in space and perform coordinate transformations.

66
❑ Manipulators are composed of an assembly of links and joints. Links are defined as
the rigid sections that make up the mechanism and joints are defined as the
connection between two links. The device attached to the manipulator which interacts
with its environment to perform tasks is called the end-effector.

67

68
❑ Planar manipulator: A manipulator is called a planar manipulator if all the moving links
move in planes parallel to one another.
❑ Spherical manipulator: A manipulator is called a spherical manipulator if all the links
perform spherical motions about a common stationary point.
❑ Spatial manipulator: A manipulator is called a spatial manipulator if at least one of the
links of the mechanism possesses a general spatial motion.
❑ Open-loop manipulator (or serial robot): A manipulator is called an open-loop
manipulator if its links form an open-loop chain.

69
❑ Parallel manipulator: A manipulator is called a parallel manipulator if it is made up of
a closed-loop chain.
❑ Hybrid manipulator: A manipulator is called a hybrid manipulator if it consists of open
loop and closed loop chains.
Pneumatic actuators system with neat sketch
❑ Pneumatic systems use pressurized air to make things move. Basic pneumatic
system consists of an air generating unit and an air-consuming unit. Air compressed
in compressor is not ready for use as such, air has to be filtered, moisture present in
air has to be dried, and for different applications in plant pressure of air has to be
varied. Several other treatments are given to the air before it reaches finally to the
Actuators. The figure gives an overview of a pneumatic system. Practically some

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Compressor:
❑ A device, which converts mechanical force and motion into pneumatic fluid power, is
called compressor. Every compressed-air system begins with a compressor, as it is
the source of airflow for all the downstream equipment and processes Electric Motor
Electric motor is used to drive the compressor.

71
Air Receiver:
❑ It is a container in which air is stored under pressure. Pressure Switch. Pressure Switch is
used to maintain the required pressure in the receiver; it adjusts the High Pressure Limit
and Low Pressure Limit in the receiver. The compressor is automatically turned off when
the pressure is about to exceed the high limit and it is also automatically turned on when
the pressure is about to fall below the low limit.
Safety Valve:
❑ The function of the safety valve is to release extra pressure if the pressure inside the
receiver tends to exceed the safe pressure limit of the receiver.
Check Valve:
❑ The valve enables flow in one direction and blocks flow in a counter direction is called
Check Valve. Once compressed air enters the receiver via check valve, it is not allowed to

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Direction Control Valve:
❑ Directional-control valve are devices used to change the flow direction of fluid within a
Pneumatic/Hydraulic circuit. They control compressed-air flow to cylinders, rotary
actuators, grippers, and other mechanisms in packaging, handling, assembly, and
countless other applications. These valves can be actuated either manually or
electrically.
Pneumatic Actuator:
❑ A device in which power is transferred from one pressurized medium to another
without intensification. Pneumatic actuators are normally used to control processes
requiring quick and accurate response, as they do not require a large amount of
motive force. They may be reciprocating cylinders, rotating motors or may be a robot
end effectors.

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Electronic And Pneumatic Manipulator Control Circuits
❑ Actuators are output devices which convert energy from pressurized hydraulic oil or
compressed air into the required type of action or motion. In general, hydraulic or
pneumatic systems are used for gripping and/or moving operations in industry. These
operations are carried out by using actuators.
Actuators can be classified into three types.
1. Linear actuators: These devices convert hydraulic/pneumatic energy into linear
motion.
2. Rotary actuators: These devices convert hydraulic/pneumatic energy into rotary
motion.
3. Actuators to operate flow control valves: these are used to control the flow and
pressure of fluids such as gases, steam or liquid..

74
❑ The construction of hydraulic and pneumatic linear actuators is similar. However they
differ at their operating pressure ranges. Typical pressure of hydraulic cylinders is
about 100 bar and of pneumatic system is around 10 bar.
1. Single acting cylinder:

75
❑ These cylinders produce work in one direction of motion hence they are named as
single acting cylinders. The compressed air pushes the piston located in the
cylindrical barrel causing the desired motion. The return stroke takes place by the
action of a spring. Generally the spring is provided on the rod side of the cylinder.
2. Double acting cylinder:

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❑ The main parts of a hydraulic double acting cylinder are: piston, piston rod, cylinder tube,
and end caps. The piston rod is connected to piston head and the other end extends out of
the cylinder. The piston divides the cylinder into two chambers namely the rod end side and
piston end side. The seals prevent the leakage of oil between these two chambers. The
cylindrical tube is fitted with end caps.
❑ The pressurized oil, air enters the cylinder chamber through the ports provided. In the rod
end cover plate, a wiper seal is provided to prevent the leakage of oil and entry of the
contaminants into the cylinder.
❑ The combination of wiper seal, bearing and sealing ring is called as cartridge assembly.
The end caps may be attached to the tube by threaded connection, welded connection or
tie rod connection. The piston seal prevents metal to metal contact and wear of piston head
and the tube. These seals are replaceable. End cushioning is also provided to prevent the

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3. Cylinder end cushions:

78
❑ Double acting cylinders generally contain cylinder cushions at the end of the cylinder to slow
down the movement of the piston near the end of the stroke. Cushioning arrangement avoids
the damage due to the impact occurred when a fast moving piston is stopped by the end
caps.
❑ Deceleration of the piston starts when the tapered plunger enters the opening in the cap and
closes the main fluid exit. This restricts the exhaust flow from the barrel to the port. This
throttling causes the initial speed reduction.
❑ During the last portion of the stroke the oil has to exhaust through an adjustable opening
since main fluid exit closes. Thus the remaining fluid exists through the cushioning valve.
Amount of cushioning can be adjusted by means of cushion screw.
❑ A check valve is provided to achieve fast break away from the end position during retraction
motion. A bleed screw is built into the check valve to remove the air bubbles present in a

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4. Gear motor: a rotary actuator:

80
❑ Rotary actuators convert energy of pressurized fluid into rotary motion. Rotary actuators
are similar to electric motors but are run on hydraulic or pneumatic power.
❑ It consists of two inter meshing gears inside a housing with one gear attached to the
drive shaft. The air enters from the inlet, causes the rotation of the meshing gear due to
difference in the pressure and produces the torque. The air exists from the exhaust
port. Gear motors tend to leak at low speed, hence are generally used for medium
speed applications.

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5. Vane motor: a rotary actuator
❑ A rotary vane motor consists of a rotor with sliding vanes in the slots provided on the
rotor The rotor is placed eccentrically with the housing. Air enters from the inlet port,
rotates the rotor and thus torque is produced. Air is then released from the exhaust port
(outlet).

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6. Limited rotation actuators
❑ It consists of a single rotating vane connected to output shaft as shown in Fig. It is used
for double acting operation and has a maximum angle of rotation of about 270°. These
are generally used to actuate dampers in robotics and material handling applications.
Other type of limited rotation actuator is a rack and pinion type actuator.

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7. Speed control
❑ For an actuator, the operational speed is determined by the fluid flow rate and the
cylinder actuator area or the motor displacement. The speed can only be controlled by
adjusting the fluid flow to the actuator, because the physical dimension of the actuator
is fixed. Since the air is compressible, flow control is difficult as compared to the
hydraulic system. There are various ways of controlling the fluid flow. One of the
methods is discussed as below.

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❑ the circuit diagram of hydraulic system developed to control the speed of motion of a
piston. Consider a pump which delivers a fluid volume of ‘V’ per minute. The pump has
a fixed displacement. The volume of fluid goes either to the pump or to the actuator.
When the direction control valve moves from its center position the actuator of area ‘A’,
the piston moves with a velocity.

85
v = 𝑉/𝐴
❑ If the pump delivery volume ‘V’ can be adjusted by altering swash plate angle of a
piston pump or by using a variable displacement vane pump, no further speed control
will be needed.
❑ Consider a simple operation where a double-acting cylinder is used to transfer parts
from a magazine. The cylinder is to be advanced either by operating a push button or
by a foot pedal. Once the cylinder is fully advanced, it is to be retracted to its initial
position. A 3/2-way roller lever valve is to be used to detect the full extension of the
cylinder. Design a pneumatic circuit for the above-mentioned application

86
Working
The pneumatic components which can be used to implement the mentioned task are as
follows:
❑ double acting cylinder
❑ 3/2 push button valve
❑ 3/2 roller valve
❑ shuttle valve
❑ 3/2 foot pedal actuated valve
❑ 5/3 pneumatic actuated direction control valve
❑ compressed air source and connecting piping

87

88
❑ As the problem stated, upon actuation of either the push button of valve (S1) or the foot
pedal valve (S2), a signal is generated at 1 or 1(3) side of the shuttle valve. The OR
condition is met and the signal is passed to the control port 14 of the direction control
valve (V2). Due to this signal, the left position of V2 is actuated and the flow of air
starts. Pressure is applied on the piston side of the cylinder (A) and the cylinder
extends. If the push button or pedal valve is released, the signal at the direction control
valve (V2) port is reset. Since DCV (V2) is a double pilot valve, it has a memory
function which doesn’t allow switching of positions. As the piston reaches the rod end
position, the roller valve (S3) is actuated and a signal is applied to port 12 of the DCV
(V2). This causes actuation of right side of DCV (V2). Due to this actuation, the flow
enters at the rod-end side of the cylinder, which pushes the piston towards left and thus
the cylinder retracts.

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TYPES OF END EFFECTORS:
❑ There are a wide assortment of end effectors required to perform the variety of different
work functions. The various types can be divided into two major categories:
❑ 1. Grippers
❑ 2. Tools.
Grippers are end effectors used to grasp and hold objects. The objects are generally
work parts that are to be moved by the robot. These part-handling applications include
machine loading and unloading, picking parts from a conveyor, and arranging parts
onto a pallet. In addition to work parts, other objects handled by robot grippers include
cartons, bottles, raw materials, and tools. We tend to think of grippers as mechanical
grasping devices, but there are alternative ways of holding objects involving the use of

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❑ Grippers can be classified as single grippers or double grippers although this
classification applies best to mechanical grippers. The single gripper is distinguished by
the fact that only one grasping device is mounted on the robot's wrist. A double gripper
has two gripping devices attached to the wrist and is used to handle two separate
objects. The two gripping devices can be actuated independently. The double gripper is
especially useful in machine loading and unloading applications. To illustrate, suppose
that a particular job calls for a raw workpart to be loaded from a conveyor onto a
machine and the finished part to be unloaded onto another conveyor. With a single
gripper, the robot would have to unload the finished part before picking up the raw part.
This would consume valuable time in the production cycle because the machine would
have to remain open during these handling motions.

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❑ With a double gripper, the robot can pick the part from the incoming conveyor with one of the
gripping devices and have it ready to exchange for the finished part. When the machine cycle
is completed, the robot can reach in for the finished part with the available grasping device,
and insert the raw part into the machine with the other grasping device. The amount of time
that the machine is open is minimized. The term multiple gripper is applied in the case where
two or more grasping mechanisms are fastened to the wrist. Double grippers are a subset of
multiple grippers. The occasions when more than two grippers, would be required are
somewhat rare There is also a cost and reliability penalty which accompanies an increasing
number of gripper devices on one robot arm.
❑ Another way of classifying grippers depends on whether the part is grasped on its exterior
surface or its internal surface, for example, a ring-shaped part. The first type is called an
external gripper and the second type is referred to as an internal gripper.

2.6 Drive system for grippers-
Mechanical-adhesive-vacuum-magnetic-grippers
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MECHANICAL GRIPPERS:
❑ A mechanical gripper is an end effector that uses mechanical fingers actuated by a
mechanism to grasp an object. The fingers, sometimes called jaws, are the
appendages of the gripper that actually make contact with the object. The fingers are
either attached to the mechanism or are an integral part of the mechanism. If the
fingers are of the attachable type, then they can be detached and replaced. The use of
replaceable fingers allows for wear and inter-changeability. Different sets of fingers for
use with the same gripper mechanism can be designed to accommodate different part
models. An example of this interchangeability feature is illustrated in Fig. 5.1, in which
the gripper is designed to accommodate fingers of varying sizes. In most applications,
two fingers are- sufficient to hold the workpart or other object. Grippers with three or
more fingers are less common.

93

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❑ The function of the gripper mechanism is to translate some form of power input into the
grasping action of the fingers against the part. The power input is supplied from the
robot and can be pneumatic, electric, mechanical, or hydraulic. The mechanism must
be able to open and close the fingers and to exert sufficient force against the part when
closed to hold it securely.

95
❑ There are two ways of constraining the part in the gripper. The first is by physical
constriction of the part within the fingers. In this approach, the gripper fingers enclose
the part to some extent, thereby constraining the motion of the part. This is usually
accomplished by designing the contacting surfaces of the fingers to be in the
approximate shape of the part geometry. This method of constraining the part is
illustrated . The second way of holding the part is by friction between the fingers and
the workpart. With this approach, the fingers must apply a force that is sufficient for
friction to retain the part against gravity, acceleration, and any other force that might
arise during the holding portion of the work cycle.

96
❑ The fingers, or the pads attached to the fingers which make contact with the part, are
generally fabricated out of a material that is relatively soft. This tends to increase the
coefficient of friction between the part and the contacting finger surface. It also serves
to protect the part surface from scratching or other damage.The friction method of
holding the part results in a less complicated and therefore less expensive gripper
design, and it tends to be readily adaptable to a greater variety of workparts. However,
there is a problem with the friction method that, is avoided with the physical constriction
method.

97
❑ If a force of sufficient magnitude is applied against the part in a direction parallel to the
friction surfaces of the fingers as shown in Fig. 5.3(a), the part might slip out of the
gripper. To resist this slippage, the gripper must be designed to exert a force that
depends on the weight of the part, the coefficient of friction between the part surface
and the finger surface, the acceleration (or deceleration) of the part, and the orientation
between the direction of motion during acceleration and the direction of the fingers.
Referring to Fig. 5.3(b). the following force equations, Eqs. (5.1) and (5.2), can be used
to determine the required magnitude of the gripper force as a function of these factors.
Equation (5.1) covers the simpler case in which weight alone is the force tending to
cause the part to slip out of the gripper.

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❑ This equation would apply when the force of gravity is directed parallel to the contacting
surfaces. If the force tending to pull the part out of the fingers is greater than the weight
of the object, then Eq. (5.1) would have to be altered. For example, the force of
acceleration would be a significant factor in fast part-handling cycles. Engelberger [3]
suggests that in a high-speed handling operation the acceleration (or deceleration) of
the part could exert a force that is twice the weight of the part. He reduces the problem
to the use of a g factor in a revised version of Eq. (5.1) as follows:.

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❑ where g the g factor. The g factor is supposed to take account of the combined effect of
gravity and acceleration. If the acceleration force is applied in the same direction as the
gravity force, then the g value = 3.0. If the acceleration is applied in the opposite
direction, then the g value 1.0 (2 x the weight of the part due to acceleration minus Ix
the weight of the part due to gravity). If the acceleration is applied in a horizontal
direction, then use g-2.0. The following example will illustrate the use of the equations.

101
Types of Gripper Mechanisms:
❑ There are various ways of classifying mechanical grippers and their actuating mechanisms.
One method is according to the type of finger movement used by the gripper. In this
classification, the grippers can actuate the opening and closing of the fingers by one of the
following motions:1. Pivoting movement 2. Linear or translational movement.
❑ In the pivoting movement, the fingers rotate about fixed pivot points on the gripper to open
and close. The motion is usually accomplished by some kind of linkage mechanism. In the
linear movement, the fingers open and close by moving in parallel to each other. This is
accomplished by means of guide rails so that each finger base slides along a guide rail during
actuation. The translational finger movement might also be accomplished by means of a
linkage which would maintain the fingers in a parallel orientation to each other during
actuation.

102
Types of Gripper Mechanisms:
❑ Mechanical grippers can also be classed according to the type of kinematic device used to
actuate the finger movement. In this classification we have the followingtypes: 1. Linkage
actuation 2. Gear-and-rack actuation 3. Cam actuation 4. Screw actuation 5. Rope-and-pulley
actuation 6. Miscellaneous.
LINKAGE ACTUATION
❑ The linkage category covers a wide range of design possibilities to actuate the opening and
closing of the gripper. A few examples are illustrated in Fig. The design of the linkage
determines how the input force F, to the gripper is converted into the gripping force F applied
by the fingers. The linkage configuration also determines other operational features such as
how wide the gripper fingers will open and how quickly the gripper will actuate.

103
LINKAGE ACTUATION

104
Gear-and-rack actuation
❑ Figure illustrates one method of actuating the gripper fingers using a gear-and-rack
configuration. The rack gear would be attached to a piston or some other mechanism that
would provide a linear motion. Movement of the rack would drive two partial pinion gears, and
these would in turn open and close the fingers.

105
Cam actuation
❑ A cam-and-follower arrangement, often using a spring-loaded follower, can provide the
opening and closing action of the gripper. For example, movement of the cam in one direction
would force the gripper to open, while movement of the cam in the opposite direction would
cause the spring to force the gripper to close. The advantage of this arrangement is that the
spring action would accommodate different sized parts. This might be desirable, for example,
in a machining operation where a single gripper is used to handle the raw work part and the
finished part. The finished part might be significantly smaller after machining.

106
Cam actuation:

107
Screw actuation
❑ An example of the screw-type actuation method is shown in Fig. There screw is turned by a
motor, usually ac companied by a speed reduction mechanism. When the screw is rotated in
one direction, this causes a threaded block to be translated in one direction. When the screw
is rotated in the opposite direction, the threaded block moves in the opposite direction. The
threaded block is, in turn, connected to the gripper fingers to cause the corresponding
opening and closing action.

108
Rope-and-pulley actuation
❑ Rope-and-pulley mechanisms can be designed to open and close a mechanical gripper.
Because of the nature of these mechanisms, some form of tension device must be used to
oppose the motion of the rope or cord in the pulley system. For example, the pulley system
might operate in one direction to open the gripper, and the tension device would take up the
slack in the rope and close the gripper when the pulley system operates in the opposite
direction.
❑ The miscellaneous category is included in our list to allow for gripper-actuating mechanisms
that do not logically fall into one of the above categories. An example might be an expandable
bladder or diaphragm that would he inflated and deflated to actuate the gripper fingers.

109
Gripper Force Analysis
❑ The purpose of the gripper mechanism is to convert input power into the required motion and
force to grasp and hold an object. Let us illustrate the analysis that might be used to
determine the magnitude of the required input power in order to obtain a given gripping force.

110
OTHER TYPES OF GRIPPERS
❑ In addition to mechanical grippers there are a variety of other devices that can be designed
to lift and hold objects. Included among these other types of grippers are the following:1.
Vacuum cups 2. Magnetic grippers 3. Adhesive grippers 4. Hooks, scoops, and other
miscellaneous devices.
Vacuum Cups
❑ Vacuum cups, also called suction cups, can be used as gripper devices for handling
certain types of objects. The usual requirements on the objects to be handled are that
they be flat, smooth, and clean, conditions necessary to form a satisfactory vacuum
between the object and the suction cup. An example of a vacuum cup used to lift flat
glass is pictured in Fig

111
Vacuum Cups
❑ The suction cups used in this type of robot gripper are typically made of elastic material such
as rubber or soft plastic. An exception would be when the object to be handled is composed
of a soft material. In this case, the suction cup would be made of a hard substance. The
shape of the vacuum cup, as shown in the figure, is usually round. Some means of removing
the air between the cup and the part surface to create the vacuum is required. The vacuum
pump and the venturi are two common devices used for this purpose. The vacuum pump is a
piston-operated or vane-driven device powered by an electric motor. It is capable of creating
a relatively high vacuum. The venturi is a simpler device as pictured in Fig. and can be driven
by means of 'shop air pressure. Its initial cost is less than that of a vacuum pump and it is
relatively reliable because of its simplicity. However, the overall reliability of the vacuum
system is dependent on the source of air pressure.

112
Vacuum Cups

113
Vacuum Cups
❑ The lift capacity of the suction cup depends on the effective area of the cup and the negative
air pressure between the cup and the object. The relationship can be summarized in the
following equation: F=PA.
where F = the force or lift capacity, Ib. P = the negative pressure, lb/in². A = the total effective
area of the suction cup(s) used to create the vacuum,
The effective area of the cup during operation is approximately equal to the undeformed area
determined by the diameter of the suction cup. The squashing action of the cup as it presses
against the object would tend to make the effective area slightly larger than the undeformed
area. On the other hand, if the center portion of the cup makes contact against the object during
deformation, this would reduce the effective area over which the vacuum is applied. These two
conditions tend to cancel each other out. The negative air pressure is the pressure differential
between the inside and the outside of the vacuum cup.

114
Magnetic Grippers
❑ Magnetic grippers can be a very feasible means of handling ferrous materials. The stainless
steel plate would not be an appropriate application for a magnetic gripper because 18-8
stainless steel is not attracted by a magnet. Other steels, however, including certain types of
stainless steel, would be suitable candidates for this means of handling, especially when the
materials are handled in sheet or plate form. In general, magnetic grippers offer the following
advantages in robotic-handling applications:
❑ 1. Pick up times are very fast.
❑ 2. Variations in part size can be tolerated. The gripper does not have to designed for one
particular workpart.
❑ 3. They have the ability to handle metal parts with holes (not possible with vacuum grippers).
❑ 4. They require only one surface for gripping..

115
Magnetic Grippers
❑ Disadvantages with magnetic grippers include the residual magnetism remaining in the
workpiece which may cause a problem in subsequent handling, and the possible side
slippage and other errors which limit the precision of this means of handling. Another
potential disadvantage of a magnetic gripper is the problem of picking up only one sheet from
a stack. The magnetic attraction tends to penetrate beyond the top sheet in the stack,
resulting in the possibility that more than a single sheet will be lifted by the magnet. This
problem can be confronted in several ways. First, magnetic grippers can be designed to limit
the effective penetration to the desired depth, which would correspond to the thickness of the
top sheet. Second, the stacking device used to hold the sheets can be designed to separate
the sheets for pick up by the robot. One such type of stacking device is called a 'fanner, and it
makes use of a magnetic field to induce a charge, in the ferrous sheets in the stack.

116
Magnetic Grippers
❑ Each sheet toward the top of the stack is given a magnetic charge, causing them to possess
the same polarity and repel each other. The sheet most affected is the one at the top of the
stack. It tends to rise above the remainder of the stack, thus facilitating pick up by the robot
gripper.
❑ Magnetic grippers can be divided into two categories, those using electromagnets, and those
using permanent magnets. Electromagnetic grippers are easier to control, but require a
source of de power and an appropriate controller unit. As with any other robotic-gripping
device, the pan must be released at the end of the handling cycle. This is easier to
accomplish with an electromagnet than with a permanent magnet. When the part is to be
released, the controller unit reverses the polarity at a reduced power level before switching off
the electromagnet. This procedure acts to cancel the residual magnetism in the workpiece

117
Magnetic Grippers
❑ Permanent magnets have the advantage of not requiring an external power source to operate
the magnet. However, there is a loss of control that accompanies this apparent advantage.
For example, when the part is to be released at the end of the handling cycle, some means of
separating the part from the magnét must be provided. The device which accomplishes this is
called a stripper or stripping device. Its function is to mechanically detach the part from the
magnet. One possible stripper design is illustrated in Fig.

118
Magnetic Grippers
❑ Permanent magnets are often considered for handling tasks in hazardous environments
requiring explosion proof apparatus. The fact that no electrical circuit is needed to operate the
magnet reduces the danger of sparks which might cause ignition in such an environment.
Adhesive Grippers
❑ Gripper designs in which an adhesive substance performs the grasping action can be
used to handle fabrics and other lightweight materials. The requirements on the items
to be handled are that they must be gripped on one side only and that other forms of
grasping such as a vacuum or magnet are not appropriate.

119
Adhesive Grippers
❑ One of the potential limitations of an adhesive gripper is that the adhesive substance
loses its tackiness on repeated usage. Consequently, its reliability as a gripping device
is diminished with each successive operation cycle. To overcome this limitation, the
adhesive material is loaded in the form of a continuous ribbon into a feeding
mechanism that is attached to the robot wrist. The feeding mechanism operates in a
manner similar to a typewriter ribbon mechanism.

120
Hooks, Scoops and other miscellaneous devices:
❑ A variety of other devices can be used to grip parts or materials in robotics applications.
Hooks can be used as end effectors to handle containers of parts and to load and
unload parts hanging from overhead conveyors. Obviously, the items to be handled by
a hook must have some sort of handle to enable the hook to hold it.
❑ Scoops and ladles can be used to handle certain materials in liquid or powder form.
Chemicals in liquid or powder form, food materials, granular substances, and molten
metals are all examples of materials that call be handled by a robot using this method
of holding. One of its limitations is that the amount of material being scooped by the
robot is sometimes difficult to control. Spillage during the handling cycle is also a
problem.

121
❑ Other types of grippers include inflatable devices, in which an inflatable bladder or
diaphragm is expanded to grasp the object. The inflatable bladder is fabricated out of
rubber or other elastic material which makes it appropriate for gripping fragile objects.
The gripper applies a uniform grasping pressure against the surface of the object rather
than a concentrated force typical of a mechanical gripper. An example of the inflatable
bladder type gripper is shown in Fig. Part (a) of the figure shows the bladder fully
expanded. Part (b) shows the bladder used to grasp the inside diameter of a bottle.

122

123
❑ Research and development is being carried out with the objective of designing a
universal gripper capable of grasping and handling a variety of objects with differing
geometries. If such a universal device could be developed and marketed at a relatively
low cost, it would save the time and expense of designing a specific end effector for
each new robot application, Most of the gripper models under consideration are
patterned after the human hand which tums out to possess considerable versatility. A
gripping device with the number of joints and axes of controlled motion as the human
hand is mechanically very complex. Accordingly, these research end effectors typically
have only three fingers rather than five. This reduces the complexity of the hand without
a significant loss of functionality. One possible design of the universal hand is illustrated
in Fig.

124
TOOLS AS END EFFECTORS:
❑ In many applications, the robot is required to manipulate a tool rather than a work part.
In a limited number of these applications, the end effector is a gripper that is designed
to grasp and handle the tool. The reason for using a gripper in these applications is that
there may be more than one tool to be used by the robot in the work cycle. The use of a
gripper permits the tools to be exchanged during the cycle, and thus facilitates this multi
tool handling function.
❑ In most of the robot applications in which a tool is manipulated, the tool is attached
directly to the robot wrist. In these cases the tool is the end effector. Some examples of
tools used as end effectors in robot applications include:

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❑ 1.Spot-welding tools
❑ 2.Arc-welding torch
❑ 3.Spray-painting nozzle
❑ 4.Rotating spindles for operations such as:(a) drilling(b) routing(e) wire brushing(d)
grinding
❑ 5.Liquid cement applicators for assembly
❑ 6.Heating torches
❑ 7 Water jet cutting tool
In each case, the robot must control the actuation of the tool. For example, the robot
must coordinate the actuation of the spot-welding operation as part of its work cycle.

126
❑ This is controlled much in the same manner as the opening and closing of a
mechanical gripper. We will discuss the interface between the robot and its end effector
in the following section:

127
CONSIDERATIONS IN GRIPPER SELECTION AND DESIGN:
❑ Most of this chapter has been concerned with grippers rather than tools as end effectors.
Tools are used for spot welding, arc welding, rotating spindle operations, and other
processing applications. In this section, let us summarize our discussion of grippers by
enumerating some of the considerations in their selection and design. Certainly one of the
considerations deals with determining the grasping requirements for the gripper. Engelberger
[3] defines many of the factors that should be considered in assessing gripping requirements:
❑ 1. The part surface to be grasped must be reachable. For example, it must not be enclosed
within a chuck or other holding fixture.
❑ 2. The size variation of the part must be accounted for, and how this might influence the
accuracy of locating the part. For example, there might be a problem in placing a rough
casting or forging into a chuck for machining operations.

128
❑ 3.The gripper design must accommodate the change in size that occurs between part loading
and unloading. For example, the part size is reduced in machining and forging operations
❑ 4. Consideration must be given to the potential problem of scratching and distorting the part
during gripping, if the part is fragile or has delicate surfaces.
❑ 5. If there is a choice between two different dimensions on a part, the larger dimension should
be selected for grasping Holding the part by its larger surface will provide better control and
stability of the part in positioning.
❑ 6. Gripper fingers can be designed to conform to the part shape by using resilient pads or
self-aligning fingers. The reason for using sell-aligning fingers is to ensure that each finger
makes contact with the part in more than one place. This provides better part control and
physical stability. Use of replaceable fingers will allow for wear and also for interchangeability

129
❑ A related issue is the problem of determining the magnitude of the grasping force that can be
applied to the object by the gripper. The important factors that determine the required grasping
force are The weight of the object Consideration of whether the part can be grasped
consistently about its center of mass. If not, an analysis of the possible moments from off-center
grasping should be considered.
❑ The speed and acceleration with which the robot arm moves (acceleration and deceleration
forces), and the orientational relationship between the direction of movement and the position of
the fingers on the object (whether the movement is parallel or perpendicular to the finger
surface contacting the part).Whether physical constriction or friction is used to hold the part
Coefficient of friction between the object and the gripper fingers.

2.7 Active and Passive grippers
130
Active and Passive Gripper:
❑ The Active Parallel Gripper is a cost-effective and flexible gripping solution for a
variety of objects and features linked jaws, a high gripping force and flexible jaw pad
positioning. The gripper is pneumatic and may be operated with regulated low-pressure
air supply for collaborative applications.
❑ Here, we present a passive, soft robotic gripper that requires power to open and
close but not to maintain a grip, which can be problematic in environments with
limited energy availability (e.g. solar or battery power).
❑ Passive grip, by not requiring power to maintain grip on an object, provides a unique
and safe alternative to energy-limited or energy-scarce environments

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