Development of a Fast Steering Mirror of Large Diameter
Byoung Uk Nam*a, Hakin Gimm, Jung-Gon Kim, Gwang Tae Kim, Byung Un Kima
a
5th R&D Institute-1st Directorate, Agency for Defense Development, P.O.Box 35-5, Yuseong-gu,
Daejeon, Republic of Korea, 34186
ABSTRACT
A Fast Steering Mirror (FSM) of large diameter has been designed, built and tested. In order to make continuous
tracking ability without loss of a target image, the FSM should be equipped with a large scale mirror for a wide field of
view and require a high control bandwidth to reduce the tracking error. The design intricacies and trade-offs among
various parameters of the FSM of large diameter to meet the desired goals are discussed. Finally, this device steers the
large aperture mirror about two axes, achieving the operating range of 1mradian and a small-signal closed-loop
bandwidth up to 500Hz which is greater than the structural resonance.
Keywords: Fast steering mirror, large diameter, normal-stress electromagnetic actuator, guide mechanism.
1. INTRODUCTION
The device described in this paper is the FSM which is defined as a mirror mounted to a guide mechanism and
activated by several actuators to direct an optical beam to a target using the reflective optical mirror. The FSM is one of
the major components when pursuing micro-scale movement with rapid steering, for instance, in laser communications, a
high-powered laser device, a high-resolution video image acquisition, space applications and medical systems.1-2 In order
to make a continuous tracking performance without loss of the target image in special application, the FSM should be
equipped with a large scale mirror for a wide field of view and require a high control bandwidth to reduce the tracking
error. There are many available solutions for their application embodying in a relatively small diameter mirror to obtain
the high control bandwidth performance. With the relatively large diameter mirror, lots of additional considerations are
needed to design the mirror fixture, the actuator and the guide mechanism to meet the required performance. In this
paper, we present a special solution that steers the large-aperture optical mirror of 150mm about two axes, through the
operating range of 1.0mradian and a control bandwidth up to 500Hz. Simulation studies have been carried out to
optimize various design parameters of the mirror fixture, the electromagnetic actuator and the guide mechanism.
Performances of the FSM have been studied experimentally and the results are presented.
2. MECHANICAL DESIGN OF FAST STEERING MIRROR
2.1 System Layout
As shown in Fig.1, the FSM is designed with an optical mirror, a mirror fixture, four hinge mechanisms, four
actuators, and four non-contacting sensors. In this illustration, the actuator is realized by a normal-stress electromagnetic
actuator. Although the actuator working on normal stress has a relatively short motion range, such a short stroke is
generally acceptable for the FSM with several milli-radian of motion range. To allow smooth movement of the mirror
assembly in the compliant direction, the special guide mechanism is studied and it can purely rotate the large and heavy
mirror assembly about tip and tilt directions in 3D space. The mirror assembly of 2.2kg consists of optical mirror, mirror
fixture and armature which is a part of the actuator, and is a moving body in this FSM. The angular position of the mirror
assembly is obtained from two pairs of non-contacting eddy-current sensor located underneath the mirror fixture along
the same direction as the actuator pairs. Some of the advantages of using the eddy-current sensor include high bandwidth,
non-contacting, good sensitivity and small volume. Finally, the detected position signals are sent to the control
electronics for servo control.
*nbu@add.re.kr; phone 82 10 6676-3394; fax 82 42 823-3400(Ext. 15527);
Integrated Photonics: Materials, Devices, and Applications IV, edited by Jean-Marc Fédéli and Laurent Vivien,
Proc. of SPIE Vol. 10249, 102490R · © 2017 SPIE · CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2265467
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Base Plate
Position Sensor Holder
Actuator(Stator)
Actuator(Armatu
Mirror Fixture
Optical T
Hinge Mechanism
Position Sensor
(Eddy- Current Sensor)
(a)
(b)
Figure 1. System layout of FSM : (a) Assembly View, (b) Exploded view
2.2 Optical Mirror
The mirror is the key component of the FSM, so special considerations are needed to design the mirror that
compromises neither the mechanical requirements nor the optical requirements of the given FSM application. The optical
requirements are generally expressed in terms of aperture size and wavefront error. The aperture size is governed by
initial beam diameter of the laser source, inherent tilt errors and beam divergence. The wavefront error relates to the
surface quality of the mirror under both gravity and thermal conditions. Although it is tempting to make the mirror
oversized in thickness and aperture to eliminate worry about meeting the optical requirements, the additions to mass and
moments of inertia run counter to the goals for the actuator design and the associated control system. Mechanical
requirement is mainly related to the structural resonance, which is affect the open loop and closed loop bandwidth of the
FSM. Generally, thickness to diameter ratio must be at least 1/10 for reasonable surface flatness of the mirror.3 In our
study, we design our mirror of 150mm diameter and 20mm thickness, which 1/7.5 aspect ratio provides enough stiffness
to obtain a high surface quality.
A selection of the material for the mirror substrate is another important factor which decides crucial parameters of the
FSM such as its first mechanical resonance, surface flatness, machinability, and capability to withstand high thermal
loads. Zerodur is selected for our FSM because of its reasonable stiffness/mass ratio and the exceptional thermal
conductivity performance to resist thermal deformation.
2.3 Mirror Fixture
Mounting of the mirror to the mirror fixture is extremely critical to reduce the wavefront error under the operating
conditions. Titanium is the chosen material for the mirror fixture, for its specific stiffness. Because the materials of the
mirror and mirror fixture are different, it is nature that the thermal expansion mismatch of the mirror fixture force to the
optical mirror to radial direction and it causes the distortion of the mirror surface.
To handle the distortion, the mirror fixture in the shape of the bipod is directly in contact with the mirror. The mirror
fixture may be designed as a flexure or a pure support. This support structure has to manage the optical element from the
mechanical and thermal effects in such a way that these effects on the quality of the optical instruments are minimized.
The mirror and mirror fixture are integrated with three bipods in this paper. Three of radially acting flexures or supports
as seen in Fig. 2(a) has been widely used to accommodate dimensional changes due to temperature variations. 4
The mirror diameter for the current study is given by 150 mm and this size is considered rather bigger than many of
the laser beamed optical system. This mirror assembly of large aperture is going to be operated under a high control
bandwidth over 500 Hz. Accordingly the vibration mode frequencies of the mirror assembly should be far away from the
frequency so that 1200 Hz is set for the minimum requirement of the assembly. It is also understood that the optical
mirror is very sensitive to even the micro scaled motion so that the surface of the mirror should be well managed during
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the design process. This motivation leads to the facts that the bipod should be met for successful mounting of optical
system and should have the sufficient stiffness to maintain the alignment of the optical mirror.
Optical Mirror
Mirror Füture
Bi-pod
Surface
Armature
(b)
(a)
Figure 2. Configuration of mirror assembly : (a) Manufactured picture, (b) Experimental result of wavefront error
We have studied the design approach of the mirror fixture analytically and numerically. Closed-form stiffness
equations of the bipod flexure are derived, and the equations of motion of the mirror assembly are mathematically
formulated to predict the behaviors of the structure. The prediction accuracy of the analytic model for the resonant
frequency is found to be satisfactory as compared with the results of finite-element method.
The goal of the optimization is to minimize the radial stiffness of the bipod as the support structure, providing rather
flexibility at the contact between the mirror and the bipod. And, at the same time, the mirror fixture should have
sufficient structural stiffness that does not affect control bandwidth of the FSM. The optimal design approach has proved
to be worth attempting, satisfying the design requirements regarding the current issues of the radial stiffness and the
frequency for the mirror assembly. Besides, as it is expected, the surface errors are improved based on this approach. Fig.
2(b) shows the experimental result of wavefront error under the mirror assembly, which is less than 0.134λ PV and
0.026λ RMS @ 633nm wavelength. This mirror fixture is made in monolithic structure to minimize the uncertainty
during assembly of components.
2.4 Guide Mechanism
The guide mechanism is composed of four hinge mechanisms, which is shown in Fig.3. The hinge mechanism in the
monolithic structure is mainly consists of a double right-circular notch hinge, a rigid block and a leaf-spring. This flexure
based on elastic deformation at the connection between two rigid bodies are used at the hinge mechanism. Flexure have
many advantages, such as negligible backlash, stick-slip friction, smooth and continuous displacement, an almost linear
displacement relationship between input and output, and inherently infinite resolution. And it doesn’t need the
lubrication. However, the limited range of motion due to elastic limit of flexure material restricts its application area into
the control of fine motion which needs high accuracy motion within small travel range. Generally, the material with
higher strength and fatigue limit and higher elastic modulus is more suitable for fabricating hinge mechanism due to
maximize the bearing capacity with minimum structure size. In this paper, the material of the hinge mechanism was
determined by considering associated manufacturing technologies with its thin shape and required motion range, etc. In
our design case, SUS630 is feasible so long as metal fatigue thresholds are not approached and features having high
stress concentration factors are avoided.
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The benefits provided by the guide mechanism include pure rotation in the desired directions, an adaptable mechanism
for the large and heavy mirror, and adjustable virtual pivot of the moving body allowed by the configuration design.
Mirror Fixture
Optical Mirror
Armature(a part of actuator)
Double Notch Hinge
Rigid Block
Hinge Mechanism
Leaf Spring
(b)
(a)
Figure 3. Configuration of guide mechanism and hinge mechanism : (a) FSM with guide mechanism, (b) Hinge mechanism
Fig.4 shows the rotational behavior concept of the guide mechanism during tip motion which comes from one of the
FEA results. The point of intersection between two opposite hinge mechanisms becomes a virtual pivot that plays the
role of instantaneous rotation center under external force. Therefore, if the initial inclination angle of the hinge
mechanisms is modified, the center of rotation would be altered to the intended point. To realize proper tip movement,
the double right-circular notch hinge and the leaf spring are designed to be thin to provide less stiffness for easy
movement in the tip direction. Since the tip and tilt motions may occur at the same time in 3D space, the payload will
behave in a complex way. Accordingly, the dual right-circular notch hinge is introduced to the mechanism to flexibly
handle the tip and tilt motions of the moving body. On the other hand, the yaw motion that comes from rotational
behavior along the axis normal to the top surface of the payload should be constrained, and accordingly the leaf spring
having an enough stiffness in that direction is used to resist the motion. For the FSM structure in 3D space, all of the
three-axis translational motions and one rotational motion are undesired. These motions can be efficiently constrained by
the guide mechanism, because the shape of guide mechanism is symmetric and parallel which possess inherent
advantages in terms of high load capacity and high stiffness.
.I"'.
Figure 4. Deformation of the guide mechanism in the tip direction and motion of payload
The characteristics and performances of the FSM with the guide mechanism are directly affected by the design of the
shape and dimension of the associated components. Closed-form stiffness equations of the individual flexure hinge are
derived, and the equations of motion of the FSM are mathematically formulated to predict the behaviors of the
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mechanism. The final dimensions of the guide mechanism were determined based on the theoretical model as described
above by the optimization design to minimize the stiffness in the compliant directions and maximizing the stiffness in the
constrained directions. 5
2.5 Actuator
Normal stress electromagnetic actuators have higher force densities than shear-stress electromagnetic actuators
(Lorentz-force type actuators) and offer larger actuating stroke than piezoelectric actuators. However, the nonlinear
behavior of its actuating force, such as being inversely proportional to air gap squared, will complicate control algorithm
significantly. By involving biased flux and designing proper electromagnetic configuration, the actuator has high-force
density similar to a solenoid, but its force output is a linear function of both driving current and armature displacement,
thereby simplifying control algorithm design. So far, flux-biased normal stress electromagnetic actuators have been
successfully employed in a FSM. 6,7
Our actuator is shown in detail by Fig.5. The actuator applies a force to the armature, which is bonded to the edge of
the mirror fixture, in either the positive or negative direction normal to the mirror surface. When the opposite actuators
are activated to the diffenrent direction, rotating torque is developed at the moving body to rotate the mirror assembly in
the actuated axes. The actuator bodies except for the armature are rigidly mounted to the base plate with actuator fixture,
while the mirror assembly including the armature is suspended relative to the base by the guide mechanism.
Core
Core
Excitation
coil winding
Permanent
magnet
Excitation
Air -gap
Armature
coil winding
PM holder
Air -gap
Excitation
coil winding
Excitation
coil winding
Figure 5. Configuration of actuator
Fig.6. shows the magnetic flux density distribution of DC only, AC only and total components. Both the armature
and the core are made of soft magnetic material. The airgaps on upper and lower side of the armature are equally spaced
in initial position. The permanent magnet plus the excitation coils generate flux density B1 on the lower side airgap of the
armature and B2 on the upper side airgap. These flux densities contain both DC component generated by the permanent
magnet and AC component generated by the excitation coil windings. The superposition of DC and AC flux density is
shown in Fig.6(c) which shows the unbalanced flux densities between upper and lower airgap depending on the
magnitude of flowing current. Actuating forces are calculated using Maxwell’s tensor method based on flux analysis.
Finally, we optimized each dimensions of the armature, core, permanent magnet, airgap and coil assembly to maximize
generated force under the fixed volume condition, and this actuator was successfully developed to generate the required
force of 70N at the neutral position. It is not described in detail in this paper.
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(b)
(a)
(c)
Figure 6. Flux density by FEA : (a) DC Flux only, (b) AC Flux only, (c) Total Flux
2.6 Position Sensor
The steered angle detection of the mirror assembly is implemented through four non-contacting displacement
sensors. In this application, the eddy-current sensor is used. This element actually senses the distance from the sensor
probe surface to a target surface and then transduces the measurement into a voltage. This signal is then fed to the FSM
controller. This sensor utilizes a small coil to emit a fluctuating magnetic field generating eddy currents in a conducting
medium. The strength of the eddy current field is used to measure the distance of the target. Some of the advantages of
using the eddy-current sensor include high bandwidth, non-contacting, good sensitivity, and small volume. This
measuring device requires a target plate that furnishes a conducting medium for the eddy currents. The manufacturer
recommends that Eddy-Current displacement sensors use a magnetic field that engulfs the end of the probe. As a result,
the minimum size of the target is about 300% of the probe diameter. To acquire better resolution performance, we choice
a probe of larger diameter and select the target material of titanium which is non-ferrrous metal.
3. PERFORMANCE
3.1 Experimental Setup
The experimental setup of the developed FSM system is presented in Fig.7.
A
:
:`III1
Operating Console
Controller
Position Sensor
[Eddy -current sensor]
FSM
Figure 7. Experimental setup for FSM performance test
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Two normal-stress electromagnetic actuators facing each other are controlled together, and the total FSM system can
be controlled by the simultaneous control of two paires of actuators(four actuators). As mentioned before, the actuator
generate a force based on the Maxwell tensor. To generate a profer rotating torque on the mirror assembly, an accurate
current control is essential. The current is driven by a linear current amplifier, TA115(Trust automation, US) which is
powered by a commercial power supply. The steered angle of the mirror assembly is calculated from forward kinematic
computations by using measured signals at four eddy-current sensor(Lion precision, ECL202 Driver/U8C Probe),
having range of 1mm, the resolution of 55nm rms and bandwidth of 10kHz. The control algorithm is implemented via
DSP controller of DS1005(dSPACE, Germany).
3.2 Open/Closed-Loop Performance
Based on the experimental results, the empirical plant model of the FSM system is obtained through the system
identification technique. Experiments were performed by frequency sweep method and frequency response function for
each drive axis was obtained by using the relationship between the applied torque and the calculated steered angle. And
the plant models were extracted through the least squares estimation technique. Since the anticipated control bandwidth
of the FSM is above 500Hz and there is an unstable structural resonance near 120Hz, a modified lead-lag control
algorithm is used to eliminate the unstabe influence of the structure’s first-order resonance. Based on the plant model, the
controller was designed by using the siso tool of the MATLAB to minimize the position error in the time domain while
realizing the closed-loop control bandwidth of more than 500Hz.
Fig.8 and Fig.9 show the open-loop and closed-loop performance of our FSM system. Stability margins in Fig.8(a)
and Fig.9(a) show that the controlled FSM system has sufficient gain margin and phase margin. And the nominal closedloop control bandwidth at -3dB is 532Hz, which is same at the tip and tilt axis. Fig.10 shows the maximum driving range
of the FSM, which is up to 1.0mrad.
)0
-0
50
na
-50
Ex periment
Si nulation
o
2
-100
10u
o
--
-0 -200
10'
1u-
103
104
Q
10'
102
FrE
1r
103
quency (Hz)
o
Experiment
Simulation
1
-200
Ln3 -400
d
-600
10°
10°
Frequency (Hz)
Experiment
Simulation
m -400
L
d
-600
101
102
103
104
100
Frequency (Hz)
101
102
103
Frequency (Hz)
(a)
(b)
Figure 8. Frequency response results(Tip) : (a) Open-loop : GM(5.5dB), PM(24.5dB), (b) Closed-loop : Bandwidth (532Hz@-3dB
Magnitude)
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100
0]
Experiment
Simulation
50
-20 Experiment
Simulation
D -40 -
0
m -60 -
-50
2
-100
100
03
102
101
104
-80 100
03
102
101
Frequency (Hz)
Frequency (Hz)
(111 /l'
7'
á) 200
Experiment
Simulation
cs)
0
-200
-a
a) -200
Ç -400
Experiment
Simulation
Ç -400 -
0-
0-
-600 r
-600
100
103
102
101
104
100
103
102
101
Frequency (Hz)
Frequency (Hz)
(b)
(a)
Figure 9. Frequency response results(Tilt) : (a) Open-loop : GM(6.2dB), PM(24.9dB), (b) Closed-loop : Bandwidth (532Hz@-3dB
Magnitude)
Maximum Stroke (Tip)
Maximum Stroke (Tilt)
1000
1000
500
500
0
o
-500
-500
-1000
-1000
0
0.5
1 5
2
2.5
3
3.5
4
4.5
5
0
0.5
Time (sec)
1.5
2
2.5
3
3.5
4
4.5
Time (sec)
(a)
(U)
Figure 10. Maximum range of FSM
4. SUMMARY
This paper presents the design, implementation, and experiment of the FSM of the large diameter. The requirements of
the FSM with closed-loop control bandwidth of 500Hz and operating range of 1miliradian was a successfully meeted.
Teh special FSM equipped with the large-diameter optical mirror needs lots of considerations in designing and
manufacturing each components. The proposed system has been successfully developed to satisfy the required
performance through theoretical modeling, numerical analysis and experiments. In the future, the durability performance
will be demonstrated by continuously performing repetitive drive, vibration and impact tests on the guide mechanism,
which is the key component in determining the life cycle of our FSM.
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References
[1] Loney, G. L., “Design of a small-aperture steering mirror for high-bandwidth acquisition and tracking,” Opt.
Eng. 29(11), 1360-1365(1990)
[2] Cho, M., Corredor, A., Dribusch, C., Park, K., Kim, Y. S., Moon, I. K., and Park, W. H., "Development of a fast
steering secondary mirror prototype for the giant Magellan telescope,“ Proc. SPIE 8444, 8444420(2012)
[3] Sweeney, M., Rynkowski, G., Ketabchi, M., and Crowley, R., “Design Consideration for Fast Steering
Mirrors(FSMs),” Proc. SPIE 4773, 63-73(2002)
[4] Tapos, F. M., Edinger, D. J., Hilby, T. R., Ni, M. S., Holmes, B. C., and Stubbs, D. M., "High bandwidth fast
steering mirror," Proc. SPIE 587707, 1-14 (2005).
[5] Nam, B.U., Gimm, H. I, Kang, D. W., and Gweon, D. G., “Design and analysis of a novel tip-tilt guide
mechanism for fast steering of large-scaled mirror,” Optical Engineering 55(10), 106120(2016)
[6] Kluk, D. J., Boulet, M. T., and Trumper, D. L., “A high-bandwidth, high-precision, two-axis steering mirror
with moving iron actuator,” Mechatronics 22, 257-270(2012)
[7] Long, Y., Wei, X., Wang, C., Dai, X., and Wang. S., “Modeling and design of a normal stress electromagnetic
actuator with linear characteristics for fast steering mirror,” Optical Engineering 53(5), 054102(2014)
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