GerardoM and RickS

GUIDANCE FOR A SYSTEM THAT IS ALREADY LOCKED

On the PSL_ISS.adl MEDM screen (see attached image), look at the strip chart in the top-right corner.
The diffracted power level should be about 7%. 
A few percent more or less is OK, but I suggest setting to near 7% at least once per week, say Tuesday during the maintenance period.

To change the diffracted light power, one adjusts the “REFSIGNAL” field in the lower left corner. 
A change in this parameter of 0.01 changes the diffracted power by about 1%, so make small changes.
A larger negative number (say going from -2.00 to -2.01) will decrease the diffracted light level.

This REFSIGNAL field is the DC laser power level (ignoring the minus sign) that the servo compares with
the “Output AC” level on the PD that is selected in the middle-left portion of the screen.

Note that in the screen snapshot the REFSIGNAL is at -2.03 and the PD A Output AC signal is at 2.03.
This indicates that the loop is operating properly; the loop tries to make the PD output be equal to the
reference level (without the minute sign, of course).

Notice that the diffracted light level is varying a bit but is close to 7% on the strip chart. 
At the middle-right edge of the screen the Diffracted Power field indicates 7.38%.  This is the field that is plotted in the strip chart.

GUIDANCE FOR WHEN THE SYSTEM IS NOT LOCKED

In the case that the ISS servo is not locked and you are having difficulty locking it, I suggest the following:

With the loop unlocked (Autolock OFF), observe the PD A AC output level. This may be a bit hard to do if the value is swinging a lot quickly.  
Set the REFSIGNAL level to about ten percent below this observed mean value.
Close the loop (Autolock ON) and observe the diffracted light time series in the strip chart.

If the diffracted light level increases and goes off screen at the top, then your REFSIGNAL setting is too low (absolute value is too small)
so you are not requesting enough light and the servo is trying to diffract a lot of light to give you the low level you requested.
Increase the (absolute value) of the REFSIGNAL field.

If the diffracted light level decreases and goes off screen at the bottom, then your REFSIGNAL setting is too high (absolute value too large)
and you are requesting more light than the servo can give you and still maintain some diffracted light headroom.
Decrease the (absolute value) of the REFSIGNAL field.

Once the system stabilizes, set the diffracted light level to be close to 7% by making small adjustments to the REFSIGNAL value.
Be patient, the time constant is pretty long and small changes make a big difference (on order one or two percent per 0.01 increments in the REFSIGNAL value).

Once the diffracted light level is near 7%, observe a few minutes of the strip chart data. 
The variations should be on the order of what is shown in the attached screenshot.

If all else fails, feel free to call me (Rick) at any hour, any day, and I will try to help over the phone.  My numbers are in the site directory.