Lambda tuning is a form of internal model control (IMC) that endows a proportional-integral (PI) controller with the ability to generate smooth, nonoscillatory control efforts when responding to changes in the setpoint. Its name derives from the Greek letter lambda (λ), which designates a userspecified performance parameter that dictates how long the controller is allowed to spend on the task of moving the process variable from point A to point B.
Like its more famous cousin, Ziegler-Nichols tuning, lambda tuning involves a set of formulas or tuning rules that dictate the values of the PI parameters required to achieve the desired controller performance. The first step in
applying them is to determine how much and how fast the process responds to the controller’s efforts.
This 9-step test, also known as an open-loop reaction curve test or step test, gives a PI controller everything it needs to know about the behavior of a nonoscillatory process in order to control it:
1. Turn off the controller by switching it to manual mode.
2. Wait until the process variable settles out to a steady-state value.
3. Manually “bump” or “step” the process by forcing the control effort abruptly upwards by B%—whatever it takes to make the process variable move appreciably but not excessively.
4. Record the process variable’s reaction or step response on a trend chart as above, starting at the time when the bump was applied (step 1) and ending when the process variable settles out again.
5. Draw an ascending line tangent to the steepest part of the process variable’s trend line.
6. Draw horizontal lines through the process variable’s initial and final values.
7. Mark where the two horizontal lines intersect the ascending line at points 2 and 3.
8. Record the deadtime D from point 1 to point 2 and the process time constant Tp from point 2 to point 4.
9. Record the change in the process variable from point 3 to point 4 then divide that by B to get the process gain Kp