Friction is a well-known performance limitation for gimbaled EO director systems. While much research study has been directed to bearing friction, the well-known friction models in literature, being represented in time, position, and rate domain, are not amenable to most LOS jitter analysis. Furthermore the type of mission profiles to which large gimbals are subjected have received limited attention in this field of research, so the selection of an appropriate friction model is not obvious. This paper fits popular friction models to experimental data, and studies the models in frequency domain.
In addition to servo control and power amplification, motion control systems for optical tracking pedestals feature capabilities such as electro-optical tracking using an integrated Automatic Video Tracker (AVT) card. An electro-optical system tracking loop is comprised of sensors mounted on a pointing pedestal, an AVT that detects a target in the sensor imagery, and a tracking filter algorithm that commands the pedestal to follow the target. The tracking filter algorithm receives the target boresight error from the AVT and calculates motion demands for the pedestal servo controller. This paper presents a tracking algorithm based on target state estimation using a Kalman filter. The servo demands are based on calculating the Kalman filter state estimate from absolute line-of-sight angles to the target. Simulations are used to compare its performance to tracking loops without tracking filters, and to other tracking filter algorithms, such as rate feedback loops closed around boresight error. Issues such as data latency and sensor alignment error are discussed.
Aeroelastic control of flutter by means of trailing edge surfaces can be a very effective method, providing that
the actuation system is capable of generating suffcient force and displacement over the bandwidth of interest.
This effort describes the mechanical design aspects of a flap actuation system using V-stack piezoelectric
actuator and Q-parameterization technique for identifying the plant at supercritical speeds. A flap actuation
mechanism that takes advantage of the shape of the actuator (V) was designed. In order to validate the
actuation concept the actuator was integrated into a NACA 0015 typical section that was tested in the wind
tunnel at Duke University. An initial nominal controller was designed to stabilize the typical section for a
limited range of speeds above the open-loop flutter boundary. The technique of Q-parameterization was then
used to parameterize the unstable system as a function of stable systems, each derived from the nominal
controller. Operating in closed loop, flutter was suppressed at the speed it occurred in open loop, and the
flutter boundary was extended by more than 50%.
Vibration-induced jitter degrades the pointing and imaging performance of precision optical systems. Practical active jitter reduction is achieved by maintaining beam alignment with mirror-positioning control systems. In the presence of time-varying or uncertain disturbances, jitter control systems using fixed-gain feedback control loops cannot operate without significant limitations on their performance. A feedback control technique called Q-parameterization can adapt to time-varying disturbances by adjusting its parameters in real time to maintain optimal performance. Adaptive feedback jitter control using Q-parameterization is experimentally verified on an optical testbed, increasing jitter reduction compared to an H2-optimal fixed-gain controller.
Optical jitter, the centroid-shifting of a light image, concerns engineers and scientists working with lasers and electro-optical systems. Even micron-level relative motion between individual optical components such as mirrors and lenses causes optical jitter, resulting in pointing inaccuracy, blurred high-resolution images, and poor nanotechnology quality. Typical jitter control technology uses fast-steering mirrors to correct for structural and acoustic disturbances in the beam train. Unknown or time-varying disturbance characteristics necessitate a controller that can adapt its parameters in realtime. The application of one such adaptive feedback controller algorithm has been proposed by the authors. The algorithm uses a technique known as Q-parameterization to structure the controller as a function of plant coprime factors and a free parameter, Q. An inherent property of this structure is the formation of a disturbance estimate based on subtraction of the controller influence from the feedback signal. The free parameter, Q, filters this estimate to form a portion of the control signal. If the controller influence on the feedback signal is estimated from accurately modeled plant dynamics, the disturbance estimate contains no feedback information allowing Q to be designed in an open-loop fashion. A gradient descent Least Mean Squares (LMS) algorithm updates the coefficients of the filter Q in realtime to minimize the frequency-weighted RMS jitter. Experiments on an optical jitter control testbed with Q set to a 200-tap digital finite impulse response (FIR) filter resulted in jitter reductions of 35% - 50%, without requiring prior knowledge of the disturbance spectrum.
An algorithm is presented which uses adaptive Q-parameterized compensators for control of stable or unstable systems. Internal stability is maintained by forming the compensator out of plant-stabilizing coprime factors, and an on-line gradient descent method adapts the free parameter to minimize the mean squared error between the desired and actual output. The adaptation algorithm is derived for a compensator in the form of a finite impulse response (FIR) filter and a lattice infinite impulse response (IIR) filter. Simulations predict good performance for both tonal and broadband disturbances, and a duct noise control experiment results in a 37 dB tonal reduction.
This paper presents the results of experiments involving jitter suppression of optical components. Acoustic disturbances and structurally transmitted vibration contribute to the jitter of optical systems such as lasers. Active and passive methods must be used to suppress jitter from entering the optical train. An experimental test bed is constructed to study the effects of acoustic disturbances on an optical system. A laser source is directed onto a light-detecting target by way of a turning mirror and fast-steering mirror (FSM). The FSM, actuated by three piezoelectric stacks, provides tilt in both the elevation and azimuth axes. Both mirrors are exposed to an acoustic disturbance. The objective is to use knowledge of the acoustic-structural interaction to design a controller that precisely points the laser. To achieve this, several control methodologies are studied. A servo control loop around the FSM is designed using an H2 approach. By feeding back the laser beam position to the FSM, the jitter is reduced by a factor of 2.5. Feedforward methods are also explored using microphones and accelerometers as disturbance sensors. Acoustic noise control is studied as a means of reducing the sound pressure level in the proximity of the optics. Sound pressure sensed by a microphone was fed to a loudspeaker and the loop was closed with an H2 optimal controller.