We tested a new scheme for IMC locking that is based on fast triggering on the IMC transmitted power. This allows discriminating against higher order modes, without lowering the feed-back gain.

Our plan was to
 - acquire with the fast path (frequency feed-back) only
 - immediately engage the slow path (MC2 feed-back) once above the threshold
 - but avoid a large switching transient
 - and put some slow motion on the optic to make sure we see fringes.

All that was achieved by
 - leaving the the slow path to MC2 on, but
 - have an additional 0.3Hz pole in the slow path. this puts some slow motion on MC2.
 - once triggered, this 0.3Hz pole is simply turned off, engaging the full feed-back without a transient.

To implement this we had to make one minor change to the FILTBANK_TRIGGER.mdl library part:
 - our filter needed to be turned OFF on trigger (not the opposite)
 - we thus added an epics input named "_INVERT" (e.g. H1:LSC-MC_FM_TRIG_INVERT) to the library part
 - if "_INVERT" is 0 (the default), the behavior is unchanged from the previous model.
 - if "_INVERT" is ~0, the trigger fires if the input is below threshold.

With this logic the IMC seems to grablock in less than 1 sec, reproducibly.

The changes (both h1lsc and FILTBANK_TRIGGER.mdl were submitted to the svn (revision 5458).