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Objective pre-set values for piezo control also have a defined output limit. This will limit the
distance the piezo can travel. If set to low the piezo system will have a limited travel range,
potentially preventing focus recovery. If set too high the piezo system will repeatedly attempt
to drive over its limits.
Ensure the output limit is set to less than the open loop travel range of the piezo system. Refer
to section 5.2 in order to specify the travel range, which should automatically update the travel
range. For example, a system with an open loop travel range of 450 µm and a closed loop
travel range of 400 µm, an output limit of ~4190000 is set in the pre-set objective values. The
first three digits give the output limit in microns; 400 < 419 < 450. Refer to the manual of your
piezo system to identify these travel ranges.
Bring the sample into focus. The error value should be close to zero, and the piezo focusing
system should be close to its midpoint of travel.
Put the controller into SRV mode by pressing the Servo on/off button on the controller.
If the image is unstable, reduce the P-gain and I-gain by a factor of 10. If the image is very
unstable, switch the controller into MAN mode immediately and reduce the P-gain and I-gain
by a factor of 10, before switching the controller into SRV mode again. If required, repeatedly
reduce the P-gain and I-gain by a factor of two until the image becomes stable.
If focus on the sample cannot be held stably with any parameters, refer to Section 10:
Troubleshooting.
When the image is stable, switch the controller into MAN FOC mode. Bring the sample out of
focus to a positive error value close to the maximum achieved in section 6.3.4.
Switch the controller into SRV mode, paying close attention to the rate at which focus on the
sample is recovered.
If the rate of recovery is fast, and focus is stable, then the current P-gain and I –again values
are appropriate. Both gains can be ad/- 10% to ensure it is optimal. Proceed to the
section section.
If the rate of recovery is slow, and the focus is stable, the the P-gain and I-gain value can be
increased. Move the sample back out of focus (in MAN FOC mode). Increase the value in by
10% and then reactivate SRV mode. Repeat this until the rate of recovery is fast, with the focus
remaining stable.
If the rate of recovery is fast, but focus is unstable, then the P-gain and I-gain must be reduced.
Decrease the P-gain and I-gain in 10% reductions until stable focus can be achieved. Move
the sample back out of focus (in MAN FOC mode) and the reactivate SRV mode to check the
speed of recovery. Find the P-gain and I-gain values that give focus stability at the maximum
rate of focus recovery.
Once suitable P-gain and I-gain have been found, return to MAN FOC mode. Bring the sample
out of focus to a negative error value close to the maximum achieved in section 6.3.4. Switch
the controller into SRV mode to validate the P-and I-gain settings.
