1. Artificial Feel System using Magneto-Rheological Fluid on Aircraft
Control Stick
Vignesh Manoharan* and Daewon Kim
Department of Aerospace Engineering, Embry-Riddle Aeronautical University, 600 S. Clyde Morris
Blvd., Daytona Beach, FL, 32114
ABSTRACT
The conventional feel systemin aircraft occupies large space in the cockpit and has complicated designs. The primary
objective of this research is to develop an artificial feel force system that can overcome some drawbacks of the current
system.A novel feel systemusing magneto-rheological (MR) fluid is constructed to precisely controlthe shearstress under
the magnetic field. To validate the functionality of the MR artificial feel system,the final systemis fabricated and multiple
tests are performed to acquire force-velocity characteristics that are compared to the mathematical model derived. In
addition, the PID closed loop control algorithm is developed to simulate the dynamic systemmodel. Both experimental
and simulation results are compared to validate the derived systemmodel. The systemresponse time and sampling rates
are evaluated and compared to the conventional systemat the end. It is concluded that the developed artificial feel system
can precisely control and acts as a fail proof systemwhen incorporated with a modern fly-by-wire aircraft system.
Keywords: Artificial feel system, magneto-rheological fluid, haptic device, aircraft control stick
1. INTRODUCTION
During the initial decades of controlled flight, the pilot’s control stick was directly connected to the control surfaces in
aircraft. Therefore, the pilot was able to feel the aerodynamic forces acting on the control surfaces when maneuvering the
aircraft using the control stick. However, as the fly-by-wire control systems began to be implemented in the 1970s, booster
system were used to magnify the force given by the pilot for bigger aircrafts. To improve convenience of the pilot in
controlling the aircraft, artificial feel systems were designed and incorporated into the flight control systems.
An artificial feel force systemfor a control stick was presented by Griffith1 which explains the systemconstruction and its
operation, as shown in Figure 1. This systemincludes a nonlinear linkage apparatus coupled between the stick and a
variable force gradient spring. This was the earliest stage of development in artificial feel force devices. The system,
however, was bulky and had complicated linkages which ultimately required further improvements . A similar system was
patented by Berthet and Bondivenne2 wherein the control stick was moved by an actuatorcontrolled by the onboard flight
computer. The position of the stick was varied using actuators in accordance with the force and position data measured by
sensors.The major drawback in using this system was the possibility of the hydraulic systemcreating back pressure that
could move the control stick automatically even if force was not applied to the stick.
An active control stick assembly developed by Hanlon et al.3 included a housing assembly and a control stick support
body mounted within the housing assembly which allowed for rotation about two orthogonaland co-planar axes. A control
stick was coupled to support the body and rotated froma null position to a plurality of control positions.A spring element
was also coupled to the housing assembly as well as the control stick support body and passively biased the control stick
toward the null position. While recent patents have been filed for different artificial feel force systems,none includes the
use ofan MR fluid damper as the artificial feel device. However, many scholarly journals have been published on modeling
MR fluid properties and also in designing MR fluid dampers.
*manoharv@my.erau.edu; +1-386-679-5635; daytonabeach.erau.edu/college-engineering/aerospace

2. Figure 1. Construction of servo motor based feel system in aircraft1
One recent publication by Ahmadkhanlou et al.4 derived an improved model to accurately simulate MR fluid properties in
actuators and sensors. This model was based on the kinetic energy theory and modeled two MR fluid particles as a
dumbbell systemattached by a spring. It was shown that this model greatly improved on the prediction of MR fluid shear
stress before the yield point over the previous models. Another publication, which was presented at a workshop on using
smart structures forseismic control, discussed fundamentalequations and optimaldesign measures of merits when building
an MR damper.5 The authors explained various strategies followed by developing their MR damper such as electric circuit
design, magneto static modeling, and fabrication. However, based upon our current knowledge, no publication has been
found about using an MR damper as an artificial feel device in aircraft control systems,which provides an opportunity for
research in this field.
MR fluid is a non-Newtonian fluid and contains magnetic particles suspended in a carrier fluid which is mostly vegetable
oil. The fluid has very high viscosity especially when it is exposed to magnetic field and behaves like a viscoelastic solid.
The shearstress ofMR fluid can be varied very precisely by changing the magnetic field intensity.MR fluid has the ability
to resist or to transmit force and can be controlled using an electromagnet which provides solution for several controlbased
applications. In addition, the fluid is not affected by temperature and requires very less current for actuation depending
upon the output force required. The material behavior of the fluid under magnetic fields is shown in Figure 2.
Figure 2. (a) MR fluid behaviour with and without magnetic field 6
and (b) MR Particles alignment under magnetic field.7
MR fluid can generally be operated in three modes, i.e. flow mode, shearmode, and squeeze mode, as shown in Figure 3.
The flow mode is preferred for many commercial applications as this specific mode allows more shear force offered by
the fluid. In the flow mode, the top and bottom surfaces of the device are fixed and the fluid is made to flow in between
by pressure.The application of magnetic field slows down the flow process.In the shear mode, one wall is fixed and the
other wall moves with respect to the fluid in between two surfaces.This movement of upper surface creates shearforce in
the fluid and makes it to flow. In the squeeze mode of operation,the pressure or force is applied on the top surface and the
fluid is squeezed in between two surfaces.
(b)(a)

3. Figure 3. Three modes of operation of MR fluids: (a) flow mode, (b) shear mode, and (c) squeeze mode.8
2. MATHEMATICAL MODEL OF MR FEEL FORCE DEVICE
The smart artificial feel device can be controlled electronically by changing the magnetic field. The MR fluid can be
operated in different modes as explained in the previous section. However, since our haptic device is designed to be
operated specifically in the shear mode, the controlling parameter is the viscosity of the MR fluid. Therefore, the selection
of appropriate model for replicating the mechanical characteristics of the device is required.
Braz Caesar9 clearly portrayed the challenges in modeling the MR fluid device by listing out the uncertainties in the
modeling approach. In their paper, the dynamic and parametric modeling methodology was selected to simplify the
modeling approach and to considerthe mechanical characteristics of smart devices.They also explained different modeling
methodologies for MR devices. They discussed that the Bingham plastic model was the basic model for MR fluid which
included non-linear behavior and controlling characteristic in the numerical model. Based upon their results,the Bingham
model was chosen for this specific feel systemapplication with certain assumptions made based on the characteristics of
the device.
2.1 Bingham plastic model
Bingham model10 includes the non-Newtonian characteristic in the first part and the Newtonian behavior in the second
part of the equation. The shear stress offered by the MR fluid is given by Equation (1).
τ = τy(H) + η𝛾̇ (1)
where 𝜏 𝑦 is the yield stress offered by the fluid, H is the magnetic field intensity, η is the effective bulk viscosity of
composite system, and 𝛾̇ is the shear rate of the fluid. The Bingham mechanical model of the device is shown in Figure 4.
The basic shearstress equation is used to model the MR feel device with approximations. The mechanical Bingham model
contains Coulomb friction element along with the viscous damper element, as shown in Equation (2).
Fd= Fc sgn(ẋ) + C0 ẋ (2)
where Fd is the force offered by the damper, Fc is the Coulomb friction element, ẋ is the piston velocity, and C0 is the
dynamic viscosity constant.
Figure 4. Mechanical Bingham model.9
Several assumptions are made in the Bingham modeling approach based on the mechanical characteristics of the feel force
device, as listed here. First, the MR fluid operating area is considered to be the gap between the sponge and the cylinder
wall. The MR fluid operates in the shear mode within the gap and there is no porous flow of fluid between the cells in the
polyurethane foam. Also, the velocity profile is linear for the MR fluid in the gap as the size of gap is negligible compared
to the width ofcylinder as shown in Figure 5. In addition, although there can be a nonlinear deformation of the polyurethane
foam inside the piston, the device is assumed to provide no hysteresis.
(a) (b) (c)

4. Figure 5. Schematic of cross section of the MR device
The Bingham modeling approach has been followed with the aforementioned assumptions compromising the accuracy of
the modeling behavior of the device. Later, the numerical model is simulated and the results are compared with the
experimental results to validate the modeling approach in the later stage.
2.2 Electrical equation and yield stress
The piston has coils wound up in a steel core and these coils act as an electromagnet. The electromagnetic circuit can be
considered as a solenoid with current and voltage as input parameters. Various magnetic field intensities can be obtained
with different input current values and the wire turns. An electrical circuit model of the solenoid can be derived from the
basic Ohm’s law equation,neglecting the capacitance part and including only the inductance,resistance and current by the
below equation,11
Vt =ItR + L.di/dt, where L is the inductance in the circuit.
The Laplace transform of the above equation is made to derive the state space equation of the electric circuit and the final
equation with voltage in the coil can be given by Equation (3).
H(s) = [kH/(Te.S+1)]V(s) (3)
where kH is z.R constant with the resistance offered by the electric circuit, Te is the electrical time constant (Te=L/R), and
V(s) is the voltage from the coil.
Applying the Laplace transform to the yield stress variable in the first part of Equation (1), we get the yield stress as a
function of the magnetic field in state space equation, where τy is converted into first order transfer function model.
Fc= τy(H)
Fc= [bH/(Tm+1)]H(s) (4)
where bH is the gain coefficient of force generated by the device, Tm is the time constant that is the time taken by MR fluid
to react, and H(s) is the magnetic field.
The force offered by the device Fd is given by Milecki12 that includes Equation (4),
𝐹𝑑 = (
𝑏 𝐻
𝑇 𝑚 𝑆+1
H(s)) sgn[s.y(s)] + 𝑏 𝑣[s.y(s)] (5)
Substituting Equation (3) into Equation (5), we get,
𝐹𝑑 = (
𝑏 𝐻 𝑘 𝐻
(𝑇 𝑚 𝑆+1)(𝑇 𝑒 𝑆+1)
𝑉(s)) sgn [s.y(s)] + 𝑏 𝑣[s.y(s)] (6)

5. The above equation is the final model for the smart device that is to be used in artificial feel system and the value for the
constants kH,bH ,Tm, Te are chosen based on the approximations made from the experiment and they are further used in
the simulation.
3. DESIGN AND FABRICATION OF MR DEVICE
The preliminary design of the device is modeled using CATIA. The purpose of this device is to supply haptic force to the
pilot control stick. Therefore, the force capacity of the device should be very less compared to the traditional MR device.
A new design for less force characteristics using MR fluid soaked within a foam material is discussed by Carlson and
Jolly13. Their work explains the advantages ofusing a foam as MR fluid reservoir and also compares different MR device
design configurations. Therefore, the foam material is narrowed down further to open celled polyurethane in this research
and the MR device is constructed.The polyurethane foam is identified as potential material for this research based on its
cell size and absorption characteristics. The research work by Chrzan and Charson14 portrays the device construction and
MR foam working. The fabrication guidelines from their paper are used in construction of MR device for artificial feel
system.
Finally, the MR device is constructed as a simple piston with a hollow cylinder. The piston and the cylinder are made out
of low carbon steelto reduce the effect of flux leakage from the electrical circuit. The MR device is constructed based on
the space requirements for installing the artificial feel systeminside the aircraft cockpit. The design for the feel systemis
carefully made to deliver the required force and also to remain compact. The polyurethane foam is bonded to the
circumference of the piston using special adhesive called Cyberbond 2025.15 Later the foam is soaked in the MR fluid so
that the sponge absorbs maximum amount of MR fluid. The fabricated piston and the MR device together are shown in
Figure 6.
Table 1. MR device design parameters
No Parameter Value
1. Piston diameter 3.125 in
2. Piston rod length 10 in
3. Cylinder ID 3.5 in
4. Cylinder OD 4 in
5. Cylinder Length 7 in
Table 1 describes the MR device specifications. The piston incorporates the electromagnet and the 0.25 inch sponge that
is completely soaked with MR fluid. The wire is taken out of the hollow cylinder from the top side. The cylinder length is
chosen in such a way that the piston has ± 2 inch stroke length.
Figure 6. MR device piston with (a) side view and (b) top view
(a) (b)

6. 4. EXPERIMENT ON MR FLUID DEVICE
The MR device is fabricated as a simple piston and cylinder assembly. The fabricated device is tested for the force-velocity
characteristics using a tensile/compression testing machine (MTS machine). The testing is performed to check the
compatibility of MR device with the foam design for specific application. The device is mounted on the MTS machine
with the cylinder on top and piston attached to the bottomclamp. The upperend grip of the machine is fixed and stationary,
while the lower end grip is attached to a movable shaft that enables displacement of the MR device piston. The power
supply for the electromagnet is setup and it is remotely controlled using LabVIEW software. A LabVIEW code is
developed to input current to the electromagnet through NI-DAQ. The mounting of MR device onto the MTS machine is
shown in Figure 7. The resultant forces are plotted for velocity and displacement from the MTS machine and are shown
in Figures 8 and 9, respectively.
Different current values are selected and given as input to the device and the corresponding force-velocity characteristics
are recorded. The experiment is started with 2A of current and the force is recorded approximately as 10 N. The maximu m
current that can be supplied to the device is recorded as 17A. However, the required force of 70 N is achieved for 14A
current and therefore 14A is set as current saturation limit for the device. The response time of the device for each change
in input current is also recorded. For 5A current, the device recorded 25 N of force and also from the previous current
values, it is clear that the force relatively increase or decrease based on relative change in current values.
The force vs velocity plot shown in Figure 9 provides sufficient details about the constructed MR device. The pattern of
the plot closely follows the traditional Bingham plastic model as the force range is constant for different velocity values.
However, the device shows hysteresis which is neglected in the model, compromising accuracy for an easier modeling
method. The current values are randomly chosen with the first value being 2.2A and the corresponding forces are plotted
against velocity. For instance,comparing the displacement and velocity plot for the 5A current, the device provides 25 N
of force approximately. The uneven lines in the plot are due to noise and other disturbances in data acquisition and also
errors from the MTS machine itself. With proper noise filtering, the disturbances in data acquisition can be reduced and
better results can be recorded. The experiment is performed in particular number of cycles to validate its performance.
The device recorded a maximum of 70 N in positive direction and 100 N in negative direction for an input current of
approximately 14A. The different range of forces in positive and negative directions were due to the minor misalignment
of piston and cylinder when mounted in the MTS machine. The error is also due to non-uniformity of the cylinder inner
diameter due to minor imperfections. This error can be avoided by using a precisely fabricated steel cylinder which could
demand a high fabrication cost.The maximum force recorded in the positive direction can be considered as the maximu m
force offered by the device. The results prove that the device is capable of providing 70 N of force which also meets the
FAA standards for force on control stick.16
Based upon the future applications and further requirements, different experimentation procedures can be followed to test
the device for various applications. For example, durability test can be performed to test the device for force characteristics
in a particular number of cycles. Failure tests can be performed to estimate possible means of device failure. Saturation
tests can be performed to check the capacity of the device for current input, as the device cannot supply more force after
it reaches the saturation limit for the specific current value.

7. Figure 7. (a) MR device mounted in MTSmachine (b) MTSmachine with LabVIEW code for experiment
Figure 8. Experimental force vs displacement plot of the MR device
(a) (b)
Cy
lin
der
Pi
st
o
n

8. Figure 9. Force vs velocity plot from MTS machine with different current values
5. MR DEVICE MODEL SIMULATION
After testing the functionality of the MR device, validation of the derived mathematical model for the MR device is
performed. The derived transfer function model is a multi-input single output system. The mathematical model is simulated
using LabVIEW software and control simulation loop is used to perform the simulation. Later, the final transfer function
equation is used as input blocks in LabVIEW software. Also different blocks are constructed and combined into the
simulation model using mathematical operator in the software. The coefficients, such as bH, kH, Te and Tm that are used in
the transfer function equation, are experimentally found and also acquired from the work of Milecki.12
In the simulation model, the velocity and voltages are given as inputs and the force is given as output parameter. The first
transfer function block converts the given voltage into current value. The input current is saturated in the next block and
given to the second transferfunction. The given input current is changed into the corresponding shearstress based on the
magnetic field function in this block. The velocity in simulation is given as a constant value and also a constant viscosity
gain of 1 is selected. The output is monitored in a graph and the corresponding values are recorded. The simulation loop
in LabVIEW is shown in Figure 10.
The simulation is performed with the same set of current values used for experimentation so that it will be easy to compare
the experiment with the simulation result. Only the velocity plot is acquired for the simulation as it is sufficient enough to
prove the certainty of the derived mathematical model. The velocity plot also follows the same traditional Bingham plastic
model pattern. The force remains constant for different velocity range. For 2.2A of input current, the simulation model
provides approximately 10 N of force, which proves that the experimental and the simulation results closely match. Since
this is a mathematical model, there are no possible noise errors and the lines are straight on the plots. The hysteresis
parameter is not included in the mathematical model and hence the force lines in the simulation force-velocity plot are
vertically straight. But hysteresis is present in the real time force-velocity experimentation plot and this is one of the result
that can be improved in the future by choosing bettermathematical modeling approach.The accuracy of the model can be
improved by including the hysteresis and also friction characteristics of the MR device in the model. Figure 11 shows that
the experimental and simulation results are in close range for the same current input values.

9. Figure 10. Simulation loop created in LabVIEW with transfer function blocks
Figure 11. Simulation of Force vs velocity plot shows resemblance with experimental results
6. FEEL SYSTEM DEVELOPMENT
The assumptions made for mathematical modeling of the MR device replicates the mechanical characteristics of the device
and it is proved from the simulation. The development of artificial feel systemis carried out after the experimentation and
simulation of the MR model. The joystick is considered as pilot’s control stick and it is interfaced with the MR device
piston and the piston has stroke length of ±2 inch. For the real time data acquisition, a load cell and a laser displacement
sensorare used. The load cell is interfaced between the piston and the joystick. The load cell is first calibrated with dead
weights and corresponding voltage values are recorded for the force values. The load cell in the system is capable of
measuring forces up to 250 N. The load cell is excited using an external power supply. Figure 12 shows the laboratory
setup of the feel system.

10. The laser displacement sensormonitors the velocity of the MR device piston. The load cell measures the force and gives
as input parameter to the LabVIEW program. The input force is monitored and corresponding output voltage values are
calculated and given as output to the MR device. A PID control loop is developed to automatically calculate the feedback
forces based on the user input. The input from the laser displacement sensor is given to the PID control loop. Desired
output force value is given as set point to the PID loop. The NI DAQ hardware is used to interface the sensors with the
LabVIEW program. After interfacing the sensors, the functionality of the entire systemis tested. The response time and
sampling rate of the device are monitored and adjusted based on the user requirement during the experimentation.
Figure 12. Laboratory setup of feel systemwith MR device and sensors interfaced
7. CONCLUSION
The concept of using a MR fluid device in providing the feel force to the aircraft control stick is validated. The novel idea
is modelled, simulated, experimented and analyzed for the specific aerospace application. In every stage of research, the
device is proved to be compatible for feel force application. Significantly, the MR feel system is identified as compact,
simple in construction, easy to interface with flight system, and offers precise control with high durability. These
advantages of the system fulfills the objective of this research for smart feel systemapplication applied to the aircraft
control stick. The device can be further examined based on industry standards for commercialization in the future.
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