Mark: I boost the controller output change by half of the dead band whenever the output changes direction. This kicks the valve signal through the dead band, eliminating the dead time and providing the process with the missing flow response.
Greg: One of the best-kept secrets rediscovered periodically is the ability of a valve position controller (VPC) to optimize a process by controlling a valve position through precision manipulation of a process PID (see my blog post, "Valve Position Control," http://tinyurl.com/3mh65zr). The VPC is simply a PID whose process variable and setpoint are the current position and the desired position, respectively, of the process PID that is the constraint (limit) to improving process efficiency or capacity. Since an exact position does not need to be optimized, often the output of the process PID is used instead of a read-back of actual valve position. Of course, this assumes a true throttle valve with a good actuator and positioner are used, so there are no big offsets between the controller output and actual valve position. For example, a VPC reduces energy use by reducing compressor discharge pressure by pushing the furthest open feed valve to the maximum throttle position with just enough valve capacity and valve gain (slope of installed valve characteristic) for the flow loop to deal with disturbances and load changes. The VPC PID will interact with the primary process PID (e.g., compressor pressure) and secondary process PID (e.g., feed flow), so the VPC must be detuned.
The classic solution is an integral only (I-only) controller tuned to be 10x slower than the slowest process PID (e.g., compressor pressure PID). However, the process PID can get into trouble because the VPC is so slow. What do you do to ensure the VPC can help the process PID deal with large upsets, but not cause the VPC to fight with the process PID over control of the process (e.g., compressor) and valves (e.g., feed valves)?
Mark: I use an "enhanced position" for the VPC that is the target process PID output biased by a term indicative of a process PID's ability to do its job. The term is the square of the process PID error (with the sign retained) multiplied by a gain that determines the amount of the enhancement. The squaring of the error provides intelligence to ignore insignificant errors, but take aggressive action to help a process PID in trouble. The enhancement term is then added to or subtracted from the target process PID output depending on the process gain sign. The process PID whose error is used for the enhancement is usually the target PID, whose valve position is being optimized. However, the enhancement could come from any PID whose performance affects the efficacy of the VPC. Another PID may serve this role, and more than one PID may qualify. In these cases multiple terms with individual gains may be used.
Believe it or Don't
- A control valve specification actually stated the valve must respond
- Control valve stick-slip near the seat was the same as vendor tests at 50% open
- A control valve had a resolution and sensitivity better than the measurement
- A PID was tuned using a university text book for process input disturbance rejection
- A plant used all of the PID parameters and options
- A plant actually used all of the power of a Distributed Control System
- A level loop become unstable due to too high of a controller gain
- A level loop had a reset time that was too large
- An operator understood what a matrix predictive controller was doing
- A meeting was cancelled to get work done