[Tomura, Kamiizumi, YamaT]
Abstract
We updated the IMC model to mitigate glitches coming from a timing gap of the BO operation, which makes a lockloss related to IMC and/or CARM controls.
Now, changing gain on MC common mode servo can be done more smoothly same as the test on the standalone system.
Because we cannot output laser today due to another works, we haven't tested this code with locked IMC yet.
Details
When we faced locklosses related to the gain change of IMC servo first time, we doubted the difference on the offset between 16dB and 15dB as a cause of lockloss (see also klog#26657). But we found larger change in offset during a gain change than the offset difference between 16dB and 15dB by some tests on the standalone system as discussed in the thread of klog#29112. Changing gain especially for 16dB->15dB and 8dB -> 7dB by BO makes glitches because delay of photocoupler is different between ON and OFF. BO supported by LIGO RCG has an asymmetric delay as 200us (maximum). So, when we change a gain from 16dB (0b10000) to 15dB (0b01111), the actual change is 16dB (0b10000) -> 31dB (0b11111) -> 15dB (0b01111) within 200us as shown in Fig.1. Finally, we could mitigate this glitch by preparing a new real-time model to mitigate the timing gap of BO on the standalone system.
Because the new real-time model seemed to work fine on the standalone system, we installed it in the IMC model. By mitigating a timing gap of BO operation, a change in an offset voltage at OUT2 is also mitigated from Fig.1 to Fig.2 (Fig.1 was taken in last week because I forgot to save it today...). In the new real-time model, ON and OFF on BO operation can be done almost simultaneously. So we can change a gain from 16dB to 15dB not via 31dB. A situation in the case of 8dB -> 7dB is also improved from Fig.3 to Fig.4.
Latch and intentional delay can be controlled on the MEDM screen (see above IN1GAIN controller in Fig.5). Please keep "ON" for "Use Latch" button if you want to use latch function. When it's in "OFF", latch function is disabled. About a sample number of delay, we confirmed it worked well in N=0~2. Latch channel and gain channel are output from another BO cards each other because of the pin assignment. So this delay is prepared to compensate an individual difference of two BO cards if it's too large. When we will install this model also for CARM common mode servo, we must confirm same things.
