Sheila, Stefan
Our first version of this fix had a bug: the linear fit below the sqrt limiter meant that there is a non-zero chance to drive the arm in the wrong direction, resulting in a DRMI lock-loss. We addressed this in the next version of the code with a quadratic fit:
- The initial limiter is set at l=1e-4. Above it (x>l), we still simply have sqrt(x) for CARM_TR.
- Below x<-3*l, we have TR_CARM=0
- Between -3*l<x<l we add the 2nd path f=A*(x-x0) + f0, with
- f0=-sqrt(l) = -0.01
- x0=-3 l = -3e-4
- A = 1/(16*l^(3/2)) = 62500
- The sum of the two paths gives a smooth interpolation. We tested this code and verified that the FE code does what it should.
- Next we wanted to optimize the threshold limit l:
- Looking at past locks, the pk2pk during PREP_TR_CARM in TR_CARM is about 0.1 cts
- Thus the following parameters might be even better:
- l=0.01
- f0=-sqrt(l) = -0.1
- x0=-3 l = -3e-2
- A = 1/(16*l^(3/2)) = 62.5
- We installed this as version V2 of this code.
- Due to the earthquake we have not yet tested this yet.