I’m lucky enough to be on the email distribution list for the ISA Mentor Program where automation hall of fame legend, Greg McMillan, shares some of the work he continues to offer well into his golden years. Greg and Hunter Vegas started this program in 2011 with the intent to transfer knowledge between seasoned automation veterans and engineers newer in their careers.
I bring all this up because there was a great question asked by one of the program’s protégés/protégées. In the blog post, How to Improve Loop Performance for Dead Time Dominant Systems, Greg wrote:
If the deadtime is due to analyzer cycle time or wireless update rate, we can use an enhanced PID (e.g., PIDPlus) to effectively prevent the PID from responding between updates. If the open loop response is deadtime dominant mostly due to the analyzer or wireless device, the effect of a new error upon update results in a correction proportional to the PID gain multiplied by the open loop error. If the PID gain is set equal to the inverse of the open loop gain for a self-regulating process, the correction is perfect and takes care of the step disturbance in a single execution after an update in the PID process variable.
The protégé asked:
- Could you please indicate what is the open loop error and how to calculate it? What units will it have?
- When you refer to the open loop gain, do you refer to the process gain?
The open loop gain is what people normally call the “process gain” even though more than the process is involved in the computation of the gain. I like the term “open loop gain” to better denote it is the product of the valve or variable speed gain, process gain and measurement gain.
The open loop error is the error for a disturbance if the controller was in manual. You really don’t need to know open loop error. You just need to know the open loop gain and simply set PID gain to be the inverse of the open loop gain if the dead time from the analyzer is much greater than the 63% process response time (process time constant plus process dead time).
I suggest setting PID gain less than this until you are sure you know the open loop gain accurately and there are no special circumstances. I have attached the white paper I wrote on PIDPlus. While the focus is on wireless updates, the conclusions are applicable to analyzer cycle time.
The following Control Talk Blog attempted to improve terminology- Understanding Terminology to Advance Yourself and the Automation Profession
The following article may help as well: Wireless – Overcoming challenges of PID control & analyzer applications
If you have process control and automation experience that you’d like to pass along to motivated folks new to our profession, visit the ISA Mentor Program page for more information.
You can also find a collection of Greg’s blog posts in the Tips for New Process Automation Folks area in the Emerson Exchange 365 community.