Michael and Yuhang

We reported an issue about not being able to lock GRMC one month ago in elog2713. We asked Pierre for some suggestions of testing the servo. He recommended to take GRMC servo out and use a SR560 instead of the plant in the opto-mechanical control loop. The setup of this test is sketched as the attached figure 1.

The results are briefly summarized in Fig.1 as well. We found that if we don't send signal to TRANSMIS IN and use MAN mode, we can successfully lock control loop. However, using  a positive 0-2V sinusoidal signal with period of 1s can cause the lock/unlock of the loop in AUTO mode. Note that a threshold is chosen at -0.4V. We also found that if we use a positive 0.9-1.1 V sinusoidal signal with period of 1s, we can keep the loop locked in AUTO mode. This test verified that GRMC control servo should work well.

In fact, we succeeded in locking GRMC afterwards without the signal from MZ servo going into GRMC servo's ENABLE IN. Once the ENABLE IN channel is connected to MZ servo, GRMC loop cannot be locked. We checked also that this signal is always zero. Therefore, we suspect the issue comes from MZ servo now. More test will be done to test MZ servo.