How to Configure Feedback for the C-hutch in EPICS:



1. Feedback Preliminary Setup


Before the alignment of the beam first do the following:


1.1  Click Feedback in the main EPICS window to obtain the primary KohzuFeedback panel and under the Feedback line select Off

1.2  From the KohzuFeedback panel choose PID input calc and verify the correct INPUT PV NAMEs

1.3  From the KohzuFeedback panel choose PID limits and verify the correct values for low and high limits (2.7 and 3.0)

1.4  From Kohzu Motors (main panel) choose DAC 1-8, click Kohzu Xtal2. Change the current voltage to 3.0




2. Checking FeedBack Scaler/Trigger


2.1  Select user transforms from UserCalcsB,  choose user transforms 2 (or user transforms 2 (full)) and specify correct INPUT and OUTPUT PV NAMEs in  the PIDscaler Trigger window, then click Enable to  activate:







2.2  Select Scalers C window, specify count time as 0.9 s and check if the counting works (i.e. you see counts every 0.9 s in the Actual count column):







2.3  From the Feedback menu select PID parameters and check that KP=-0.004 (see comment below), KI=1, and KD=0.00 values are as shown below:





(Comment: KP parameter controls how fast the feedback catches the setpoint and it maybe chosen between -0.002 and -0.01)


3. Beam Alignment and Feedback Engagement


3.1 Scan koh2th in SPEC and maximize the beam intensity; tweak DAC (see 1.4) to possibly gain a little more intensity


3.2 (a) From the KohzuFeedback menu select PID input calc

      (b) Check if the Output VALUE is the same as in Reedback (green field in KohzuFeedback).

           Type the calculated Output VALUE as a Setpoint value in KohzuFeedback panel

      (c) Switch Feedback to On:




3.3 From Feedback menu select Plot setpoint and readback, type in the appropriate values for Display maximum/minimum,

      then close and re-open this window to see the updated plot:




  Now Feedback is set!


4. How to Maintain the Correct Feedback Performance


To maintain and control the working Feedback it is advisable to keep open a set of EPICS windows (in the Feedback Workspace), as shown below.


4.1 Normally Feedback correction of the beam is pretty stable and does not need to be watched too closely but over time (it maybe several hours or days) it may slowly drift to low or high limits. In this case, the beam has to be corrected by tweaking Kohzu Theta2 up or down. When doing tweaking, make sure that the total beam intensity stays in the maximum, otherwise switch the Feedback Off and re-align the beam (scan koh2th and koh2chi in SPEC, tweak in EPICS etc., and then re-engage the Feedback).