J
jeremy
11 minutes
Tune the Inner Loop First in Cascade Control
A cascade has two loops. The outer loop watches the value you care about and sets the setpoint of the inner loop. The inner loop drives the actuator. Tune the inner loop first. Put the outer loop in manual so it stops moving the inner setpoint. Then tune the inner loop on its own with setpoint steps and a disturbance, the way you would tune any single loop. It should be fast and firm. It is often P or PI. Once it is done, close it and leave it alone. Now the outer loop sees the inner loop as part of its process. Tune the outer loop with the inner one closed, using the same tests. The inner loop must be clearly faster than the outer one. A common rule of thumb is three to five times faster or more. If the outer loop is tuned first, its gains fit a process that changes as soon as you touch the inner loop, so the work is lost. If the inner loop is too slow, the two loops fight. On the trend both PVs then swing together, and the cycle can look like an outer gain set too high. Check the inner loop's speed before you cut the outer gain. After this Concept you can tune a two-loop cascade in the right order and check that the inner loop is fast enough to hold up the outer one.