[Eleonora, Federico, Matteo]

The zero-detect board of the rampeatuo (Pic1) reads a signal (usually the cavity transmission) and compares it with a treshold signal in order to engage the lock. The threshold signal can be manually set with a potentiomenter and goes from -15 V to 15 V. Pierre installed a probe to monitor this threshold some times ago.

In the current configuration the rampeauto sums the transmission signal and the threshold and engages the lock if this sum is > 0. This means that if we don't connect the transmission signal, the cavity gets locked when we set the threshold above 0.  

By connecting the transmission and keeping the threshold below zero we can assure the the lock is engaged only when the transmission is higher than the absolute value of the treshold. This means that we can prevent the servo from locking on HOMs. In the current configuration, the cavity transmission (when it is locked and well aligned) is ~1.5 V. The threshold is set at -0.5 V so that the lock is engaged only when the transmission is higher than 0.5. 

In order to remotely control the lock, we used a stanford to subtract an offset (generated by the recenty installed DAC) to the transmission signal, before the rampeauto. If such offset is 0 the cavity stays locked, if it is larger than 1 V the trasmission signal sent to the rampeauto is lower that 0.5 V and the servo stops the lock. We set the offset at 1.5 V and we verified that the cavity lock can be controlled by adding and removing it.

I added a button on the main MEDM screen to control such offset (pic 2). From now on please try to use remote lock of the filter cavity as much as possible and keep the threshold knob where it is.