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Multi-Disciplinary Senior Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: 11565
ITT MIRROR STEERING SYSTEM
Andrew Bishop
Electrical Engineer
Ben Geiger
Electrical Engineer
Matthew Manelis
Mechanical Engineer
Nurkanat Suttibayev
Industrial Engineer
Katherine Hall
Mechanical Engineer
ABSTRACT
NOMENCLATURE
The primary goal of the project is to deliver a device
that redirects laser beams with high precision,
accuracy, and speed using a mirror. The customer,
ITT Geospatial Systems, will use the product on
satellites and other aircrafts for pointing lasers or
tracking targets.
The mirrors used in the system are interchangeable.
However, a 3-inch round mirror is to be used primarily
in the final assembly by ITT. This waffle mirror is
specifically designed to be extremely light in weight
and stable in structure, and is provided by the
customer. This mirror rests in an elevated horizontal
position by an aluminum dowel rod with a flexure
portion that allows for slight movement of the mirror
without material failure. Movement of the mirror is
achieved by placing four current driven force actuators
underneath the mirror. There are two axes of motion
with a range of 5 degrees on each axis. Capacitive
position sensors are each axis for proper feedback.
Through proper design, assembly and testing, this
lightweight mirror pointing assembly will meet as
many of the required performance specifications as
possible.
FEA
FOS
FSM
LIDAR
PID
PCB
RIT
Vdc
Finite Element Analysis
Factor of Safety
Fast Steering Mirror
Light Detection and Ranging
Proportional Integral Derivative
Printed Circuit Board
Rochester Institute of Technology
Direct current voltage
INTRODUCTION
The prototype proposed is a beam steering device
using a rapidly slewed mirror. At this time, a number
of similar optical systems are currently available on
the market by suppliers such as Optics in Motion and
Newport. The FSMs are primarily much smaller in
size (one inch mirror on average), are not designed to
perform in a vacuum. They are also associated with an
exorbitant cost vary on technical specifications. In
fact, a system that closely matches this project’s target
specifications is offered for $15,000. [1] The main
intention of this project is to build a system that can
compete with or outperform similar systems on the
market while maintaining a budget of $2,000. The
final model will have optimal functioning capacity and
will meet preferably all of the high performance
parameters specified by ITT.
Aerospace applications include optical sensing of
scattered light wherein a variety of parameters of
Copyright © 2011 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
interest can be measured from very far distances with
this technology such as temperature, pressure,
position, or vibration. Rapid motion of the mirror is
required in order to collect the massive amount of data
usually needed. Accuracy, precision, and efficient
interaction are the most essential attributes of the
system to optimize. This specification is monitored
through the move time, the settling time and the slew
rate (velocity) of the mirror once it is moved to a
particular position at a specified angle of tilt.
Furthermore, minimal overshoot and oscillation once
the desired position has been attained is essential for
accuracy. Seeing as fast time, quick speed, and
accuracy are so critical, ITT did advise that their slew
rate and settling time specifications were high goal to
reach intentionally.
Once our product is supplied to ITT with our design
specifications, ITT will alter the system adding their
proprietary features for its confidential application.
The product for our purposes is therefore supplied
±24V via a common power supply since it will first be
used in laboratory applications by ITT. The system
will be used to gather information at very long
distances and, consequently, a small tilt range of
movement of the mirror is sufficient, broad coverage.
The entire system including the PCBs, the actuators,
sensors and mirror should be safely contained in a
cylindrical ~ 3-inch diameter enclosure for protection.
All of the specifications fully detailed in Table 1 are
important parameters to consider and aim to reach
when developing the beam steering device.
Table 1: Performance Specifications
In space applications, it is not really practical to have a
system that requires an excess amount of power to run
the unit. An aircraft or satellite, for example, will
have limited power from the engine or power source
that is available for our unit to consume, hence the
power consumption limitations. High voltage across
the actuators has the potential of inducing high energy
arcs in a vacuum, which would be very hazardous.
There is also the issue of heat generated from the unit
that should be monitored.
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Various risks were considered when developing the
architecture. Electrical noise would dilute input and
feedback signals. This significantly reduces accuracy
and is an anticipated very high risk. The signal to
noise ratio should be monitored and minimized, as this
is one way to measure hysteresis, creep, and sensitivity
over life. Staying within the linear range of the voice
coil would reduce this greatly as well as feedback and
reduced friction.
The components that we have with our system are the
mirror, the actuator, and the sensor.
Round shaped mirrors are common for FSM systems
because of its evenly distributed mass and mass center
location. The flexure type often depends on the
application and manufacturer preferences.
Furthermore, systems might have additional mirror
damping features integrated for precision and accuracy
purposes.
Actuators are used to change electrical energy into
mechanical energy. There are various types that could
be used, but the common application is to provide
movement to the mirror with a given controlled
amount of electrical input.
Voice coil and piezoelectric actuators are the most
practical devices considered as options to use to move
the position of the mirror. Voice coil actuators use
wire or coil, a permanent magnet, and electric current
for propulsion, whereas piezoelectric actuation is
based on piezoelectric material that transforms its
shape in linear or rotary directions when an electric
field is applied. Piezoelectric devices, however, are
problematic when supplied high voltage in a vacuum
environment, and voice coils have linear behavior.
Conclusively, it is extremely crucial that the
prototype/device is fast, accurate and precise and
therefore the voice coil design would be the best
actuator device for its attributes.
The sensors are an essential component of the system;
they provide the feedback of the system to the input.
It is important to first note that the system is driven by
constantly monitoring the position of the mirror. The
desired angle of each axis as a function of time in
essence is an input providing the position of the mirror
mount, which is the vertical distance the actuator
moves.
Options of sensors include four main types:
memsgyroscope, inductive, capacitive, and optical
sensors. The memsgyroscope is not accurate when
measuring position, as the integrated signal does not
come out clear. Implementation of the inductive is
difficult to define, and optical sensors are expensive.
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Proceedings of the Multi-Disciplinary Senior Design Conference
Capacitive sensors are used since they are all desired
qualities: accurate, very feasible, and reasonably
priced. Capacitive sensors work by monitoring the
position of the mirror through the voltage difference
between two small conductive plates.
The electrical components include the power supply as
well as two actuators and two sensors per axis. The
circuit boards include the drive PCB which serves
both axes, the PID controller PCB, sensor PCB, and
power regulators per axis.
The most important considerations in the mechanical
realm are the spring constant of the flexure on the
dowel rod holding up the mirror as well as protection
or containment of the mirror and all components.
Figure 1: Upper Half Assembly
DESIGN PROCESS
To clearly define the problem, it was important to first
identify the customer needs and specifications. After
meeting with Michael O’Brien, our representative
member from ITT, a list of required performance
specifications were established, as seen in Table 1.
From the customer interview, it was determined that
rapid movement with minimal overshoot and high
precision and accuracy to each position is essential.
More specifically, the quantitative goal is to obtain a
slew rate greater than 50 degrees/sec, a settling time of
better than 90% in 80 milliseconds, and total power
dissipation of less than 5 watts per axis. The system
includes optical sensors to provide feedback, and four
specifically fabricated actuators to reach all of the
desired parameters. These actuators are to be voice
coil driven, not piezoelectric, as requested by the
customer.
All components of the system will be contained in a 3inch diameter cylinder, including the actuators that
will provide the mechanical force to move the mirror
position. Each axis contains two actuators and two
sensors in series attached to the drive PCB and sensor
PCB.
Once these parameters were established, a house of
quality was created in order to determine which
specifications were most important to consider during
the design process. A number of concepts were then
generated, and a concept selection matrix was used to
determine which design best met the performance
specifications. The final upper half design is shown in
Fig. 1, which includes the mirror, mirror mount,
flexure spring, and actuators.
Each system component was carefully designed in
order to be integrated into the system. Various
computer models were created to simulate each part’s
functionality and performance. This was especially
crucial, as many components of the system rely on its
adjacent counterpart.
The most important components of our assembly are
the actuators, as this is what drives the system.
Therefore, the design process began with this piece.
Two different types of voice coils were considered for
our project. An under hung voice coil is designed so
that the coil is larger than the field area in order to
preserve linearity. The other type is an over hung
voice coil, which has a limited range of motion, but
produces more force per unit of current. Because the
total vertical motion of the voice coil is only about 1
mm, it was decided that an under hung voice coil
would be the best choice, as it would help keep power
consumption to a minimum.
The final design of each actuator is a current
controlled force device. It consists of an iron outer
shell to generate the magnetic field into the right
space, a magnet to produce the magnetic field, and a
coil of wire to produce an opposing magnetic field
which in turn produces a vertical force.
A neodymium magnet and iron core are stacked on top
of each other and placed in a larger iron core bored
cylinder. In this space, coils of wire are wrapped
around the neodymium magnet. As current is induced
through the wire, a magnetic field is produced due to
the strength and proximity of the magnets. A rigid
object is attached between the coils and the mirror
mount to transfer this vertical force. The position of
the mirror can consequently be manipulated by
altering the current through the wire.
Copyright © 2011 Rochester Institute of Technology
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Proceedings of the Multi-Disciplinary Senior Design Conference
The voice coil actuators were modeled using
COMSOL Multiphysics, a computer program that has
the ability to model magnetic fields. Specific
dimensions, such as wall thickness, magnet diameter,
and overall height, were altered in order to create the
greatest magnetic field possible. The final design has
300 windings and a coil radius of 6.5 mm. The
selected design and its resulting magnetic field can be
seen in Fig. 2.
Figure 3 – Force vs. Displacement Plot of Flexure
Spring
Figure 2: Voice Coil Magnetic Strength
From COMSOL, it was determined that a 0.38 Tesla
field exists when a 100 mA current is induced. Using
Eq. (1), where I is the current induced, B is the
magnetic field, N is the number of windings, and Rc is
the voice coil radius, a vertical force of 0.466 N is
calculated. With this information, all remaining system
components could be finalized.
F  2I B NRc
With the flexure spring rate determined, a system of
equations was created to represent our model [2]. This
model simulates how our system will behave when
operating. The mechanical model of the system in one
axis can be seen in Fig. 4. The two springs represent
the left half and right half of the flexure spring, and the
two dampers represent any damping that may be
present in the system. It is later determined that little
damping exists, so it is assumed that these values are
approximately zero. The simplified equations of the
model are documented below (Eq. 2 and 3).
(1)
Once the force output was determined, the flexure
spring could be designed to translate this force into an
angular deflection. A simple flexure design was drawn
in SolidWorks, and with FEA software, the spring rate
was calculated using SolidWorks Simulation. Various
forces were applied to the flexure spring, and the
resulting displacements were documented. An Excel
graph was generated, plotting the displacements on the
x-axis, and the corresponding forces on the y-axis. A
trend line was added to the plot, and using Hooke’s
Law, the spring rate is equal to the slope of this line.
As seen in Fig. 3, the spring rate was calculated to
equal 626.8 N/m.
Figure 4 – Mechanical model of system
mẍ + 2kx = 2F1
(2)
J Ӫ + 2kL22ө = 2F1L1
(3)
The next components designed for our system are the
sensors. Different options were available when
designing these parts, such as capacitive, inductive,
and a MEMs accelerometer.
Ultimately, a capacitive design was selected. The idea
behind this design is to have two plates on the same
axis per voice coil. The plates, which are
approximately 5x7 mm in size, are conductive pieces
through which a change in voltage is measured when
movement occurs. While one plate is stationary, the
Project P11565
Proceedings of the Multi-Disciplinary Senior Design Conference
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other moves with respect to the mirror. Once the
desired position is reached, the sensor will be able to
detect this correct position based on the change in
voltage between the two plates.
The mirror mount was originally designed as a square
piece, but after careful consideration, the shape was
altered to a circular shape. The reason for this change
is because dynamically, its best to move the circular
mirror with a circular piece, and it reduced the
moment of inertia about the pivot point.
The assembly also includes two circular plates. The
bottom base plate supports the PCBs, while the upper
plate holds the actuators and flexure. These plates are
also used for attaching the outer cylinder to the
system, as each plate has four holes drilled into the
side.
Once all components were finalized, a SolidWorks
model was created for each part. An assembly was
generated, and the system was simulated in
SolidWorks Simulation to ensure that the desired
deflection was achieved, while still making sure that
no parts would plastically deform under loading. For
the worst case scenario, all four actuators are
activated, with two pushing up on the mirror, and the
other two pulling down. The bottom plate is also
constrained, as this piece is not expected to move. This
case demonstrates the maximum deflection and
stresses that exist within the system. The results of the
analysis can be seen in Figs. 5 and 6.
Figure 6 – von Mises Stress of System
From the results shown in Figs. 5 and 6, it is
determined that all mechanical components work as
expected, with the maximum vertical deflection being
0.065”. Knowing that the radius of the mirror is 3”, a
simple trigonometry calculation is performed to yield
a tilt degree of 2.5˚. This model represents half of the
range that can be achieved, as the mirror could tilt 2.5˚
downward, producing the desired 5˚ of range. The
maximum stress within the system is 5.75 ksi, which
produces a FOS of about 7. Therefore, no components
will plastically deform.
Electronics:
Three control electronic circuits are needed to move
the system as well as a power regulator circuit to
supply constant voltage.
The control electonics
consist of a controller, a voicecoil driver, and a
position sensor for feedback. The controller was
imagined as a PID, PI, or PD controller durring the
design phase. After determining the equation for
motion in one axis, a model was built in simulink.
Figure 7 – Simulink Model for Total System
Figure 5 – Vertical Displacement of System
Initially, an incorrect equation was used which caused
a delay in the design by about 2 weeks. As a result,
the PCB for the electronics was only partially
completed. Even now, there are issues with the
equation only dealing with idealities. However, even
with the non-idealities not included in the simulink
model, the tuning of it can be used as a starting point
durring testing and assembly. The controller was
tuned and it was then determined that a PD controller
with a pre-amp was needed to attenuate the initial
Copyright © 2011 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
controls by a factor of 10 thousand. This attenuation
is very high because the derivative portion of the
controls is very small. In order to use real capacitance
and resistance values, the preamp had to compensate
for the derivative portion. With a preamp gain of
.0001 V/V, the PD was tuned to a proportional gain of
845 V/V and a derivative gain of 10,000 V/V. The
post-amp was determined to have a gain of 2.5 V/V
from this simulation. The pre- and post-amps also act
as buffer stages between the input and feedback
signals to the PD controller and from the controller to
the driver.
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The power regulator circuit was designed using the
LM337T and LM317T power regulator chips. Three
cirucits are fed an input of -24V to LM337T and +24V
to two LM317Ts. The resistors are used to set the
output power at ±15V and +5V. The capacitors are
used to reduce the noise of the power regulators at
differing frequencies. The +5V is fed back to the
outputs of the elctrical system as well as the sensor.
The ±15V is fed to the other circuits as voltage rails.
The diodes are added as protection diodes for the
system.
Figure 10 – Power Regulation Circuit for 5V, +15V,
and -15V
Figure 8 – PD Controller Circuit
The driver consists of a voltage controlled current
source in the form of a current sourcing opamp with an
input buffer and feedback buffer. The voltage gain of
this circuit was determined to be approximately 1 V/V
(ie, no gain) in PSpice. The load used in simulation in
place of voicecoils was a 100 Ω resistor. The 25 ohm
resistor is in place to set the current flowing to the
voice coil. This produced a current of 100mA through
the load, which was the targed goal for the voice coil
current.
The position sensor is a 33kHz oscillator with a
capacitor bridge, that measures differentially the
voltage across the capacitors in the middle, then
multiplies that with the origional signal, and with no
phase shift in the capacitor bridge this produces a good
phase shift of the signal to ground and up to 66 kHz,
which is then amplified and low pass filtered to get rid
of the higher frequerncy of the final signal. This
circuit required a 5 volt source for the oscillator and
the insturmentation amp. The insturmentation amp
(AD 623) was chosen for its high commonmode
rejection of around 100dB, as well as its high input
impedance so it wouldn’t effect the wave across the
capacitve bridge. Then ±15V was required for all the
op amps used in filtering, and converting of the square
wave of the oscillator to close to a sin wave. The 15V
rails are also required for the AD734 multiplier.
Figure 11 – Basic Schematic for Capacitive Sensor
(single axis)
Figure 9 – Voice Coil Driver Circuit
With the FEA analyses complete, the next phase of the
project is to test each individual component, as well as
the system as a whole, to ensure that as many
performance specifications are met.
Project P11565
Proceedings of the Multi-Disciplinary Senior Design Conference
TESTING
In order to make sure our system meets the required
specifications, numerous tests are performed. Such
tests include the flexure spring constant, power
regulator, voice coil driver circuit, proportional
derivative circuit, position sensor and open loop voice
coil mount, mirror reflectivity, voice coil performance,
sensor accuracy, and the overall system testing.
To test the spring rate of the flexure, a test setup was
constructed that attached to the flexure. The setup
consists of a base piece that the flexure rests on, a top
rectangular piece that sits on top of the flexure, and a
displacement gage that detects the vertical distance
that the flexure travels when a known force is applied.
A series of forces are applied to the top rectangular
plate, and the corresponding displacement is
determined. Much like the FEA modeling, Hooke’s
Law is used to determine the spring rate. A plot of
force vs. displacement is generated, and the slope of
the line passing through each data point is equal to the
spring rate.
When testing the mirror reflectivity, a simple setup
was constructed. A laser beam is pointed at the mirror,
and the resulting image is portrayed a far distance
away. If the image portrayed looks similar to the laser
beam image portrayed at the same location by itself,
then the selected mirror can be used for the assembly.
The same technique was used for reflective mirror
testing used in overall system testing fixture.
Voice coil functionality testing activities included
measuring resistance of the core in the voice coil. The
voice coils were considered operational if the
resistance value of the core was 52 Ω ±1. Furthermore,
10V voltage was applied for each voice coil to
determine the direction of movement.
Page 7
The driver was built on a bread board using 5%
resistors and used the voltage rails created by the
power regulator. The resistances chosen were, again,
as close as possible to the initial design. The voice
coils were represented by a 100 Ω resistor. The 25 Ω
resistor is rated for 3 watts. The circuit was built and
tested. The current through the resistor was 100mA.
The gain on the input buffer was changed slightly to
accommodate a voltage gain of 1V/V.
The driver was retested with the PD controller
connected to the input and with a 10 Vdc input to the
PD, a 100 Ω load, and voltages supplied from the
Power regulators. The current supplied was
approximately 110mA. The gain of the overall
electronics was 1.08V/V there was no phase difference
from input to output at low frequencies. At 1 kHz sin
wave input with a 20 Vpp amplitude, the phase
difference was 200 microseconds.
A mirror was mounted at 45º in order to bounce a laser
directly onto center of our system and back to a wall
located 20 feet behind the laser. DC values and sine
waves offset by 90º were used to obtain the data for
frequency response and deflection of the system.
RESULTS AND DISCUSSION
Based on tests of individual components, most of the
parts are working. Test result showed that PD driver,
sensor circuit, voice coils, mechanical mount system,
and power regulator are all working. However, the
system gets only half a degree deflection while it was
designed for two degrees. Settling time specification is
satisfied by the system performance, but the
movement time is not. The power consumption
specification was not met, which could be improved
by more power efficient components.
Position sensor circuit testing was performed using
oscilloscope, power supply and 2 capacitors analogous
to the one in actual FSM system. The idea behind the
test was to see if swapping the capacitors within the
bridge circuit would result in a value proportional to
the change in capacitance values that we would expect
in the system.
The power regulators were tested on a bread board
using 5% resistors. The resistances and capacitances
chosen were as close to the designed values as
possible (may change this statement). The inputs used
were from a DC power supply, with a current limit of
.3A. The outputs of the three circuits were +14.6V,
+5.3V, and -14.9V. These are acceptable values for a
PCB with 5% resistors.
Figure 12 – DC values for mirror deflection
Copyright © 2011 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
CONCLUSIONS AND RECOMMENDATIONS
Our findings are that system’s deflection specification
was not met due to the voice coils not producing the
force they were designed for. Individual components
worked well, but they were noisy. This noise could be
reduced by integrating PCBs into the system.
For the system as a whole, making the distance from
the top of the voice coil’s metal part to the mount
where the voice coil cores attach should be shortened.
Another recommendation is to look into an inductive
position sensor. Another idea is to redesign the voice
coils for easier, and more accurate construction, as
well as make more pieces then you need to get better
matching. The suggestion for the PD controller is,
once the position sensor is added, tune on the
breadboard to get an idea of where to start, and then
retune on the PCB. Furthermore, testing other chips
for better frequency response is suggested.
For additional recommendations look up individual’s
notes on the project
ACKNOWLEDGMENTS
The team would like to express special thanks to our
customer and sponsor, ITT Geospatial Systems, and
their representative, Mike O’Brien. We would also
like to thank our faculty guide, Alan Raisanen, who
significantly helped our team throughout the project.
Others to thank include the Mechanical Engineering
machine shop members, Mechanical Engineering
professors, and Electrical Engineering professors.
REFERENCES
[1]. Optics In Motion, Standard fast Steering Mirrors.
Optics in Motion, LLC, n.d. Web. 18 Feb. 2011.
<http://www.opticsinmotion.net/
fast_steering_mirrors.html>.
[2]. Inman, Daniel J. Engineering Vibration . 3rd ed.
N.p.: Prentice Hall, 2007. Print.
Project P11565
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