Capacitive edge sensor and sensor electronics for segmented mirror telescopes

Abstract

Edge sensors are high precision displacement sensors used in segmented mirror telescope to measure relative displacement between two mirror segments. An edge sensor based on parallel plate capacitor is being used in Keck as well as GTC telescopes and has also been proposed for upcoming Thirty Meter Telescope. In this 9th MTech project an attempt has been made to develop a sensing electronics for the TMT edge sensor by opting a technique which is different from what JPL has used. Our aim is to measure the change in the displacement with resolution in nm. Here, in this sensor, the change in the displacement causes the change in the capacitance of the sensor. For a constant current flowing through a capacitive sensor, change in the capacitance will reflect as a change in voltage across the capacitor. By measuring changes in voltage between two electrodes, one can determine the capacitance and hence gap or height. The above technique is a direct method to measure the capacitance. For small changes in the displacement i.e. in 10’s of nm or single digit micrometer, the change in the voltage across or current through the capacitor would be extremely small but measurable. Whereas, in indirect methods, the change in the capacitance will reflect as change in amplitude, frequency, phase as well as pulse width of a driver signal. The circuit should be sensitive to detect the smallest possible changes in these parameters, so that the output should be responsive to the smallest change in the displacement or capacitance. In the amplitude modulation technique, it is the amplitude of the signal changes, whenever there is change in the capacitance. However, the problem of amplitude modulation technique is that it is subjected to both internal and external noises. Whereas, in the frequency modulation technique, the system is found to be very unstable. It is also difficult to measure the instability to correct the error. Therefore, in this project, we used indirect method called pulse width modulation technique to measure the capacitance or displacement. In the pulse width technique, 555 timer is used in mono stable multi vibrator mode, where the output pulse width of the timer is dependent on the sensor capacitance connected to the timer. Output of the timer will be ‘ON’ while the timer is triggered. Multiple triggers give multiple output pulses. The width of each pulse is proportional to the capacitance of the sensor. By measuring the pulse width, the capacitance can be known. The capacitance is directly proportional to the displacement in the height and indirectly proportional to the gap between the electrodes. Pulse width is measured by using built in 16 bit timer available in the PIC micro-controller. Experiments are conduct to get height as well as gap sensitivity of the TMT sensor by using our own sensing electronics. Attempts have also been made to improve the sensor resolution by applying a novel technique which amplifies the raw pulse width