Controls for Optics: The Role of Dynamic Control Systems in Practical Optics, Study Guides, Projects, Research of Chemistry

Download Controls for Optics: The Role of Dynamic Control Systems in Practical Optics and more Study Guides, Projects, Research Chemistry in PDF only on Docsity! Dynamic Control of Optical Systems Keith Powell Steward Observatory (520) 626-4323 kpowell@as.arizona.edu Topics
Why controls for optics?
Basic concepts of control systems
Example 1: Camera image stabilization
Example 2: MMT Optical Line of Sight control
Summary Basic Closed Loop Control System Components
Control algorithm – Usually software on computer. Many different controller topologies. PID control is by far the most common
System dynamics – Mathematical model of components being controlled (e.g. motion of lens elements, deformable mirror, telescope, etc.) Error Feedback Signal Command Signal Control Algorithm System Dynamics Measurement Dynamics Output Actuator Basic Closed Loop Control System Components II
Measurement device – Provides an estimate of the physical parameters being controlled (rate gyros, accelerometers, position sensors) • Can introduce scaling errors, biases, and noise
Actuation device – The device which provides the corrective motion to the system (piezoelectric actuator, voice coil motors, etc.) • Has limited bandwidth (finite response time) Error Feedback Signal Command Signal Control Algorithm System Dynamics Measurement Dynamics Output Actuator Advantages and Disadvantages of Closed Loop Control Systems
Advantages • Significantly increased accuracy • Reduced sensitivity to external perturbations • Reduction of nonlinear effects such as friction in telescope drives • Increased response bandwidth
Disadvantages • Requires sensing and control devices which can be expensive • Software development and implementation • Can become unstable if not properly designed – MMT pointing controller has significant issues Camera Image Stabilization Issues
The angular velocity sensors have biases which cause lens drift over time. The Nikon VR II system recenters the lens just prior to shutter release to maximize optical quality
Older control algorithms have problems when mounted on tripods due to high frequency vibrations outside the bandwidth of the controller. Software and sensor upgrades have greatly mitigated this effect in current systems
The angular velocity sensors are also being upgraded to have better low frequency response characteristics to allow greater attenuation of image motion Example 2: MMT Optical Line of Sight Control
The MMT has a structural vibration at ~19 Hz which severely degrades the image quality during adaptive optics runs
The vibrational mode causes translation and rotation of the AO secondary with respect to the primary mirror
Movement of the secondary mirror is manifested as image motion (or jitter) of ~40 milli-arcseconds peak at the sensor. This is currently the dominant error source for AO MMT Secondary Motion Translation and Rotation • Translation and rotation each contribute to the overall image motion seen at the sensor. •The calibration factors due to translation and rotation of the secondary are computed in Zemax independently then added together to form the total required correction to the control device, in this case, a deformable mirror αt d αr αr Translation Rotation αtotal = αt + 2αr Simulated Image Motion with Optical LOS Control Off/On
MATLAB simulation created to evaluate various controller algorithms
Initial controller reduced the closed loop image motion by approximately a factor of four
Current algorithm will attenuate image motion by a factor of twenty MMT Optical LOS Issues
Accelerometers typically have low frequency biases which results in drift over time. This can be bad in an open loop controller • Can compensate by filtering low frequency data to remove drift • Can use centroid data from image to recenter the controller command
Finite actuator response time limits the amount of achievable attenuation
WFS noise limits the update rate and amount of gain (or correction) one can put into the system Summary
Future Telescope Applications • Active and adaptive optics systems will be on all new major optical telescopes (MMT,LBT, GMT, etc.) • Large Telescopes will REQUIRE active control of the optical line of sight due to structural vibrations • More sophisticated control algorithms required to integrate individual components (active, adaptive, LOS control, etc.) and improve system performance
Dynamic control of optical systems will become more prevalent in the future • Sensor/actuator cost and size decreasing • Computing power increasing • Maximize the performance of optical systems