27
K-Cube Position Aligner
4.5.3
Closed Loop Mode
In ‘Closed Loop’ mode, the signal from the detector is interpreted by the unit, and the
feedback circuit sends position demand signals (XOut and YOut) to the rear panel
XDIFF and YDIFF connectors, which can be used to drive a pair of positio ning
elements (e.g. piezo controllers) in order to position the light beam within the center
of the detector array, see Fig. 4.7. For a detailed diagram of the PID loop, see Fig.
4.8.
When the beam is central, the spot will be in the center of the Target display.
4.5.4
Automatic Open Loop/Closed Loop Switching
The controller is capable of switching automatically between open loop and closed
loop operating modes, depending on whether there is sufficient optical power required
for closed loop operation. Automatic Switching mode can be selected from the top
panel settings menu (Closed Loop with Auto - see Section 4.5.2. ) or by clicking the
Enable Auto Open Loop checkbox on the Control Settings panel - see Section 6.2.2.
If during closed loop operation the SUM signal falls below the minimum specified in
the Trig1 Sum Min parameter, the controller will switch back to open loop mode. If
subsequently the SUM signa l rises above the limit again, the controller will switch
back to closed loop mode.
The automatic switchover works in conjunction with the "Position Demands In Open
Loop Mode" option that defines whether the controller will hold (freeze) the XPOS and
YPOS outputs when switching over to open loop or set them to zero.
Automatic switchover might be advantageous in scenarios where the beam might be
temporarily blocked, for example during experiments involving manual manipulation
of optical components, particularly when the beam path is quite long and the beam
steering actuator can deflect the beam so far that it falls outside the sensor area. In
setups like this and with the controller in closed loop, blocking the beam can result in
the feedback loop ramping the XPOS and/or YPOS outputs to saturation and steering
the beam completely outside the sensor area. When this happens, restoring the beam
will not normally restore the beam alignment as at this point the feedback algorithm
does not even see the beam. However, with automatic switchover the loss of light will
stop the closed loop operation, optionally freeze the last valid beam position and
prevent the outputs ramping up as an unintentional consequence of the loss of
Note
In closed loop mode, with the beam central, the X and Y axis difference
outputs from the photodiode array are zero. However, the position
demand signals on the rear panel XDIFF and YDIFF SMA connectors are
whatever value is necessary to drive the positioning elements to centre
the beam.
Stable, closed loop operation can only be achieved with continuous wave
(CW) lasers. This product is not recommended for use with Pulsed Lasers.
