UWE Bristol Engineering showcase 2015

Chris Sandhurst
BEng(Hons) Electrical & Electronic Engineering
Project Supervisor
Nigel Gunton
The Design of a Fault Tolerant Simulated Aircraft Yaw Servo Controller
Introduction
Control loading systems also known as force feedback systems are used in
flight simulators around the world to improve the fidelity of the simulation.
This is achieved by the application of force by the flight model through the
controls into the user replicating the force feel characteristic experienced by
a pilot when flying the real aircraft in order to better simulate the motion
cues, workload and real life stresses and strains experienced by a pilot flying
a real aircraft to produce a higher fidelity simulation. The task here was to
design a control loading computer to control the existing ELU93002 actuator.
Modelling of the Actuator
In order for a controller to be designed, and even more so for analytical
redundancy to be feasible an accurate mathematical model of the actuator,
motor and servo amplifier had to be produced. Initially efforts were made to
produce the model from datasheets using the standard model for armature
controlled DC motors, however this proved problematic. An acceptable
model based on experimental data extracted from the actuator was
eventually used.
A
b
L1
C
c
N
a
L2
B
Servo Control
With a usable model, a controller to improve the performance of the
actuator was devised. Without a tuned controller the performance of the
actuator was slow and inaccurate due to the prominence of a dead band in
the system for small changes in the reference.
Controllers of different types were researched including PID, PI, PD, and
fuzzy. The solution that was settled upon was that of PD control for it’s
simplicity when implemented with the tachometer on the actuator, negating
the need to synthesis a derivative term.
This was implemented using analogue components (LM741 op-amps) and
potentiometers to allow the gains to be varied. Analogue conditioning
circuitry was also produced to prepare the signals generated by the actuators
sensors for use in the controller.
Sine Wave1
0.262
degs >> volts
Plant
0.0901
n1
-0.1639
n2
Estimator
10
Voltage Amp1
l22
8.41
Voltage Amp
Tacho Controller Gain (derivative)
X2'
1
s X2
Integrator2
-90.623
0.139
Tacho Error Trigger
Pot (Degs >> volts)
X2
L21
-1682.1155
tf(5.9058,[0.0154 1])
0.262
X1'
Motor
Tacho Error
l11
>= 1
1
s X1
Integrator1
-181.771
Pot Error Trigger
1
s Motor Posn Rads
Integrator
Tach W >> V
0.0573
Pot Error
>= 1
1.496/(2*pi)
Gain2
1.496/(2*pi*0.0573)
TachoVolts >> degs/sec
Real/Estimator Velocity
2011.905
DC gain velocity
Real/Estimator Position
2011.905
Arm Posn
DC gain position
X2
Est. Arm Posn
1
Constant1
0
Constant
|u|
Abs1
|u|
Abs
>=
Tacho Error Threshold
>=
Posn Error Threshold
Fault Detection
Error State
Real/Estimator Error
Analytical Redundancy
Analytical redundancy was used to devise a method for error detection in
the tachometer and potentiometer sensors. This was achieved through the
use of State Space control theory and the development of a state estimator
in Simulink that could predict the states of one sensor based on the output
of the other. By comparing the state values from the estimator and from the
real sensors, any significant discrepancy could be interpreted as a senor
having failed. With further work faulty sensors could be replaced by
estimated data to maintain function of the actuator.
Force Feedback
A basic force feedback system was designed to allow a basic spring force to
be applied to the servo. This utilized four strain gauges on the actuator arm
in a Wheatstone bridge configuration and a feedforward or open loop type
controller.
State Value (Degrees and Degrees/sec)
60
40
20
0
Error Detection of Disconnected Pot. Signal
Fault Detected
Arm Velocity
-20 Est. Arm Velocity
Arm Position
-40
Est. Arm Position
Error Detected 1
Error Detected 2
-60
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time (Seconds)
Project summary
Control loading systems are used to enhance
simulation experience in modern aircraft flight
simulators by loading the simulated flight controls
with forces that provided a force feedback
characteristic similar to that of a real aircraft. Fault
tolerance in such systems is designed to allow them
to continue to function, at least in a degraded
fashion, in the event of a low level fault such as a
sensor failure. Traditionally fault tolerance in such a
system would use physical redundancy, however this
project seeks to develop fault tolerance through the
use of analytical or model based redundancy.
Project Objectives
This project seeks to design a fault tolerant control
loading computer to provide force feedback using the
Fokker ELU93002 actuator for use in the
AgustaWestland Technology and Simulation
engineering simulator. To achieve this the following
objectives must be met.
• Determine accurate model of the Fokker ELU93002
actuator, servo amplifier and motor.
• Investigate, design and build a controller to control
the positional response of the Servo Actuator.
• Design and build an outer controller to control the
force applied by the actuator.
• Investigate and implement analytical redundancy
to govern the behavior of the actuator in the event
of a fault.
Project Conclusion
An analogue positional servo controller was
successfully design and tested. Force feedback was
also achieved, replicating the feel of a spring using
the analogue controller. Fault detection using
analytical redundancy was demonstrated using the
Simulink model, showing that the use of state
estimators for this purpose is practical and
achievable.