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14.6 Setting Tuning Loop Gains
14.6.1 Basic Iterative Tuning Approach
Because PID parameters are dependent on the process being controlled, there are no
predetermined values that will work. However, a simple iterative process can be used to
find acceptable values for Kp, Ki, and Kd gains.
1. Set all the reference array parameters to 0, then set the CV Upper and CV Lower
Clamps to the highest and lowest CV expected. Set the Sample Period to a value
within the range Tc/10 to Tc/100, where Tc is the estimated process time constant
defined in the previous section,
Determining the Process Characteristics .
2. Put the PID function block in Manual mode and set the Manual Command (word 14
in the reference array) to different values to check if CV can be moved to Upper and
Lower Clamp. Record the PV value at some CV point and load it into SP.
3. Set a small gain, such as 100 * Maximum CV/Maximum PV, into Kp and turn off
Manual mode. Step SP by 2% to 10% of the Maximum PV range and observe PV
response. Increase Kp if PV step response is too slow or reduce Kp if PV overshoots
and oscillates without reaching a steady value.
4. Once a Kp is found, start increasing Ki to get overshooting that dampens out to a
steady value in two to three cycles. This may require reducing Kp. Also try different
SP step sizes and CV operating points.
5. After suitable Kp and Ki gains are found, try adding Kd to get quicker responses to
input changes providing it doesn't cause oscillations. Kd is often not needed and will
not work with noisy PV.
6.
Check gains over different SP operating points and add Dead Band and Minimum
Slew Time if needed. Some Reverse Acting processes may need setting of Config
Word Error Term or Output Polarity bits.
14.6.2 Setting Loop Gains Using the Ziegler and
Nichols Tuning Approach
This approach provides good response to system disturbances with gains producing an
amplitude ratio of 1/4. The amplitude ratio is the ratio of the second peak over the first
peak in the closed loop response.
1.
Determine the three process model parameters, K, Tp and Tc for use in estimating
initial PID loop gains.
2. Calculate the Reaction rate:
R = K/Tc
For Proportional control only, calculate Kp as:
Kp = 1/(R * Tp) = Tc/(K * Tp)
For Proportional and Integral control, use:
Kp = 0.9/(R * Tp) = 0.9 * Tc/(K * Tp) Ki = 0.3 * Kp/Tp
For Proportional, Integral and Derivative control, use:
Kp = G/(R * Tp) where G is from 1.2 to 2.0
Ki = 0.5 * Kp/Tp
Kd = 0.5 * Kp * Tp
3.
Check that the Sample Period is in the range
(Tp + Tc)/10 to (Tp + Tc)/1000
PID Built-in Function Block GFK-1503E User Manual 289
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