THORLABS GVS001, User Manual

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Page 22 18728-D01
Single- and Dual-Axis Scanning Galvo Systems for Small Beam Diameters
Mirror periodically shoots off to one side and then stops
If the mirror suddenly shoot off to one side and then stops it is likely that either the
position sensing circuitry is not functioning correctly or the motor cable is incorrectly
wired. When this happens most likely either the drive electronics will output a constant
drive voltage or the loop feedback will be positive. Consequently the motor jumps to
one extreme and an overposition fault is triggered. Once the drive electronics is
disabled the scanner will bounce freely backwards and come to rest. After a delay the
electronics will attempt to resume operation and the process will repeat.
Galvo mostly behaves normally but periodically becomes unstable
If the galvo driver card is incorrectly tuned it is possible that the galvo system can
appear to be behaving correctly most of the time, but with a brief period where the
system suddenly becomes unstable repetitively occuring. This can be caused if the
maximum error signal value is exceeded. The fault control circuitry responds by
lowering the error gain which may cause the system to behave normally. However,
once the system tries to resume normal operation the system is likely to become
unstable again and the process will repeat.
Oscillation in the galvo motor current
If the galvo system is drawing more current than expected, if the scanners or the
driver cards are overheating, if the scanners are making a hissing noise or if the
position accuracy is less than expected, this may be due to oscillations in the galvo
motor current. This can be identified by viewing the coil current signal J6 pin 4 on an
oscilloscope. The problem will manifest itself as a high frequency (>1kHz) sinusoidal
oscillation in the current, unrelated to the position signal. Normally the scanner will still
appear to be correctly following the command signal, but the oscillation may show up
in the position signal if the effect is very strong.
This effect is normally caused by crosstalk between the position sensing circuitry and
the motor drive current. Repositioning the motor drive cable will normally help to avoid
this problem. If the user replaces the motor cables with their own cables they should
ensure that they keep the wires as short as possible and use separate shielded cables
for the position sensing and motor drive signals.
Cross talk between axes
Cross talk between the two motors will normal show up as a slight movement in one
axis when one motor is moved quickly. This typically occurs if both the motors are run
off a same power supply and the power supply cannot deliver the peak currents
demanded by the galvos. There will then be a drop in the power supply voltage which
will then affect the behaviour of the remaining axis. Choosing a different power supply
with sufficient peak drive current capability should solve this problem.