CALIBRATION
CONTROLLER TUNING
I-E96-315B
4 - 3
percent to 0 percent cycle repeats if the number of calibration
cycles specified in S7 is more than one. When the calibration
cycles are complete the actuator ramps to the position demand
from the MFP.
Allow the cycle to complete.
Specifications S8 (0 percent) and S9 (100 percent) store the
value of the LVDT differential voltages determined during the
calibration cycle. The new values replace the initial values or
previous calibration values.
CONTROLLER TUNING
Analog Control Tuning Theory
Because there are many aspects of a control loop that affect the
system response, it is impractical to develop a mathematical
model to calculate the optimum controller gain. Control loop
characteristics that affect the system response are:
1. The characteristics of the servo valve, LVDT and hydraulic
actuator such as operating specifications, impedance toler-
ances, line losses, hydraulic system efficiency, actuator spring
tolerance and transformer performance.
2. Lag time inherent in a closed loop (feedback) control sys-
tem.
Response time and system stability are the primary concerns
in determining the controller gain. The objective of analog con-
trol tuning is to have a high frequency response while main-
taining system stability. The hydraulic servo slave is a
proportional plus integral plus derivative controller (PID). PID
controllers readily meet the analog control tuning objectives.
The derivative action of PID controllers increases and stabilizes
the frequency response by compensating for lag time in the
control loop.
PID controllers are complex and tuning them is difficult. The
most common method (90 to 95 percent of the time) of tuning
PID controllers is the trial and error or practical method.
1
The
HSS is designed with the derivative and integral action con-
stant (not tuneable). Tuning is accomplished by changing the
proportional gain. Increasing the proportional gain decreases
system stability and increases frequency response. The practi-
cal method finds the gain that is one level below the gain that
causes instability (oscillation).
1. Ralph K. Johnson, Analog Control Techniques and Tuning, TP89-2 (Wickliffe: Bailey Controls Co., 1989).
