Plate parallelism:  an introduction

The CS100 control system is a three-channel bridge system which uses capacitance micrometers and PZT actuators, incorporated into the TTF, to monitor and correct errors in mirror parallelism and spacing. Two channels (XY) control parallelism and the third (Z) maintains spacing by referencing the cavity length-sensing capacitor to a high stability standard reference capacitor, also located in the TTF. Because this is a closed-loop system, non-linearity and hysteresis in the PZT drive are eliminated entirely, as of course are drifts in cavity parallelism and spacing.

The CS100 servo system works on a nulling method: any imbalance in the capacitance bridge causing an error signal is corrected for by a change in piezo voltage and hence plate spacing until the error is nulled.

Here is an illustration of the electronics that surround the clear optical cavity of the TTF.

When the bridges are balanced before work starts with the TTF, the two sides of each channel X1 and X2, Y1 and Y2, and Z and Cref are balanced. This condition does not mean that the optical cavity is parallel so offsets are introduced by altering the balance of the bridges using the front panel controls to bring the TTF into parallelism. The voltages on the piezos will therefore only be the same if the TTF is perfectly parallel when it is not powered up. This of course will never happen as the piezos suffer from hysteresis and this will show itself as variations in the balance conditions, however, the parallelsim conditions will remain the same. Also the piezo dimensions continue to change after a correction is made due to creep in the lattice structure of the PZT and this will also cause errors that are continually nulled.

The bridges can be unbalanced in several ways. For example, if a partical of dust enters one of the capacitors, as sometimes happens, the capacitance of that capacitor will change causing an error signal that is then nulled changing the parallelism or mean gap of the TTF. Temperature gradients across the TTF will also cause changes in relative piezo lengths etc. which again cause error signals which are nulled by changes in the HV across the relevant PZT. Scanning the cavity spacing is achieved by applying controlled imbalances to the Z, Cref bridge.

The X, Y and Z directions make it easy to operate the TTF and set its parallelism for obvious reasons and each direction has a pair of capacitors which are balanced in the bridge circuitry. These signals undergo a co-ordinate transformation which then changes the voltages on each of the piezos to give the effect of only changing either X, Y or Z.