33
K-Cube Position Aligner
8) Press the MODE button on the position aligner to select ‘Closed Loop’ mode, i.e.
the CLOSED LOOP LED is lit.
The position aligner unit now takes over the alignment, and controls the piezo
units to drive the piezo mirror to direct the laser beam to the central position. The
beam position within the sensor array is indicated by the target display on the top
panel.
9) Depending on the position of other components in the system, it is possible that
the travel range of the piezo actuators driving the mirror is not sufficient to position
the beam on the detector array. In this case, the piezos should be centered and
the actuators on the o ther turning mirrors should be adjusted to reposition the
beam - see Section 4.6.6.
4.6.4
Signs of Instability
The Control loop PID settings allow the system response to be tuned to a particular
application. Higher proportional and integral coefficients decrease the residual error
in the system and also allow the system to respond more quickly to a disturbance (i.e.
the system is able to dampen vibrations at a h igher frequency range). However,
above a certain level, the system becomes unstable and the integral and proportional
coefficients will need to be decreased until the system stabilizes.
Instability in the system may be recognized by the following symptoms:
• Whistling, buzzing or similar noises from the pizo actuators
• Jumping XOut and YOut values (only visible on the GUI panel).
• Spot not stable in the center of the detector.
Once the PID parameters have been adjusted, check that the system is
unconditionally stable. Tap the wo rk surface to check that the system cannot be
provoked into instablity by vibration.
4.6.5
Troubleshooting
In summary, the four most common reasons why the system will not operate correctly
are:
• A piezo controller is connected to the wrong axis - see Section 4.6.3. Item 4 or 6, or
the axes are swapped.
• X-axis or Y-axis Position Demand Sense is set incorrectly - see
Section 6.2.
• PID constants need adjustment - see
Section 6.2.
• The ‘Drive Input Source’ parameter for the Piezo unit is set incorrectly - see
Section
6.2.
Note
The default settings for the PID parameters are correct for the set up
shown in Fig. 4.9. If any of the above signs are apparent, it will be
necessary to connect a PC to adjust the PID settings in the Settings panel
- see Section 6.2.
