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The most important result is that there have been no statistically significant changes in the tilt variation of the alidade since either the original track surveys during initial construction or the tiltmeter tests in 2013. This conclusion is based on comparisons of tilt results averaged over 2 degree bins in the data. The result confirms that there was no apparent change to system performance.
However, the LMT/GTM is at a particularly difficult site for electromechanical systems. The high altitude has the usual effect of reducing cooling effectiveness for the drives and motors, and the ambient temperature hovers near freezing. Since there is a significant amount of precipitation during some times of the year, there are frequent freeze/thaw cycles. The constant formation and either sublimation or melting of ice, along with the associated high humidity, has been a challenge for the environmental protection of many devices at the LMT/GTM. Because there are a total of 720 primary surface actuators in the system, it is particularly important that the actuators, their local drive control boxes, and their cable connections be able to meet its specifications even under the site conditions.
To confirm the suitability of the actuators, the LMT/GTM procured an initial set of sixteen actuators for testing at the site. After laboratory testing, the actuators were installed into the outer two rings of the telescope and cycled during the early winter months of the 2015–16 scientific observing season. Because of the continuing installation activities in these two rings, they are not illuminated by the receivers, so field testing under actual operational conditions could be conducted without affecting the ongoing scientific observations. This paper presents the characterized performance of the actuators before and after testing, as well as a report on their environmental robustness.
The LMT uses an ”on-the-fly” trajectory generator that receives as input the target location of the telescope and in turn outputs a commanded position to the servo system. The sun avoidance strategy is also implemented ”on-the-fly” where it intercepts the input to the trajectory generator and alters that input to avoid the sun. Two sun avoidance strategies were explored. The first strategy uses a potential field approach where the sun is represented as a high-potential obstacle in the telescope’s workspace and the target location is represented as a low-potential goal. The potential field is repeatedly calculated as the sun and the telescope move and the telescope follows the induced force by this field. The second strategy is based on path planning using visibility graphs where the sun is represented as a polygonal obstacle and the telescope follows the shortest path from its actual position to the target location via the vertices of the sun’s polygon.
The visibility graph approach was chosen as the favorable strategy due to the efficiency of its algorithm and the simplicity of its computation.
A new control system was designed and built within the project to implement an active surface at the LMT. The technical concept for the active surface control system is to provide a set of bus boxes with built-in control and I/O capabilities to run four actuators each. Bus boxes read the LVDT sensor position and limit switch status for each actuator and use this information to drive the actuator’s DC motor, closing the position loop. Each bus box contains a DC power supply for the electronics, a second DC power supply for the motors, an embedded controller with I/O to close the position loop, and a custom printed circuit board to condition the LVDT signals and drive the motors. An interface printed circuit board resides in each actuator providing a single connector access to the LVDT, the motor, and the limit switches. During the fall of 2013, 84 bus boxes were commissioned to control the 336 actuators of the inner three rings of the telescope. The surface correction model was determined using holography measurements and the active surface system has been in regular use during the scientific observation at the LMT.
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