BTCInstrumentation
9 minutes
Tuning a PID for an Integrating Process (Level Control)
A level or position loop is an integrating process. After a step in output, the measured value does not settle. It keeps ramping, like a tank that fills while inflow beats outflow. So there is no single process gain or time constant to read. Instead you read the ramp rate. The integrating gain K' is the change in the slope of the measured value divided by the change in output. Its units are like percent per minute, per percent of output. The dead time θ is still the delay before the ramp starts. A lambda rule for a PI controller on such a process is Kc = (2λ + θ) / (K' (λ + θ)²) and Ti = 2λ + θ. Here λ is the arrest time you pick: about how long the loop takes to stop a level change. The common mistake is too much integral action. The process already integrates, so a short integral time gives a slow, wide cycle, like a tank sloshing up and down. A sticky valve can make a cycle that looks alike, so check the output shape too. Many level loops run well on P only or with a long integral time. Sometimes you want the level to float, so a surge tank can soak up flow swings. That is averaging level control: low gain, a slow loop, and the level only has to stay inside its limits. After this Concept you can read a ramp rate from a step test and set gains for a level loop.