An existing three-channel analog servo loop controller has been redesigned for piezoelectric-transducer-based (PZTbased) etalon control applications to a digital servo loop controller. This change offers several improvements over the previous analog controller, including software control over proportional–integral–derivative (PID) parameters, inclusion of other data of interest such as temperature and pressure in the control laws, improved ability to compensate for PZT hysteresis and mechanical mount fluctuations, ability to provide pre-programmed scanning and stepping routines, improved user interface, expanded data acquisition, and reduced size, weight, and power.
The original analog servo controller only had the ability to correct for a single error term generated by the capacitive gap sensor. This was less than optimal when trying to return to the same gap position due to the hysteresis of the PZT motors and thermal drift in the electronics.
To overcome the limitations of the analog servo loop, it was decided that a control loop could be built around a microcontroller/central processing unit (CPU), i.e., a digital servo loop. The CPU would query various sensors such as a capacitive gap sensor or temperature sensor, among others, then based on re-programmable control laws, provide a driving signal to a high-voltage driver that actuates the PZT motor on the etalon. The system is based on mostly COTS (commercial off-the-shelf) hardware and software.
The design is based around a new generation of direct capacitance to digital converters from Analog Devices, the AD7745. This integrated circuit (IC) allows the measurement of the capacitance of the gap capacitor at up to 90 Hz with resolutions down to 4 aF. This measurement is an absolute value whereas the previous analog design measured capacitance relative to a reference capacitor whose value had some uncertainty. The new design allows one to measure the gap directly, after calibration, thereby greatly improving overall control.
This work was done by Gregory J. Wassick of Michigan Aerospace Corporation for Goddard Space Flight Center. For further information, contact the Goddard Innovative Partnerships Office at (301) 286-5810. GSC-15524-1