Is Lambda a Bad Tuning Rule?
July 6, 2012
Control Global recently published an article titled: “The Case Against Lambda Tuning”. In this article, controls guru Greg Shinskey makes the argument that Lambda tuning fails miserably for a loop responding to a disturbance. He bases his argument on two loop performance measures:
- Maximum deviation from set point
- Integral of error
He then goes on to describe optimum tuning as a balance between the two performance objectives and provides tuning guidelines for achieving this. A very nice explanation indeed.
Although the article is technically correct, I don’t fully agree with its premise: that all controllers should be tuned to respond fast and reject disturbances.
It’s like arguing against buying a minivan because it does not perform as well as a Corvette. However, minivans are less expensive and can seat several kids. So if economy and seat count are the objectives for your large family’s car, the minivan makes a fine choice. Similarly, if robustness and stability are your control loop performance objectives, Lambda tuning makes a good choice.
I am not saying Shinskey is wrong for promoting fast loop response, but keep in mind that fast response is only one of the possible performance objectives. Stability and robustness are also valid control objectives, and Lambda tuning caters to these needs. It is a fine tuning rule if you want to stabilize control loops in complex, interactive plants. For example, the paper industry makes widespread use of Lambda tuning to keep their highly-interactive paper machines stable. It is also a unique tuning rule that allows you to specify the loop’s speed of response to a set point change, i.e. the closed loop time constant.
You may wonder about the downside of using Lambda tuning. Well, the penalty one pays for increased control loop stability is a slower rejection of disturbances. So, to Greg’s point, Lambda tuning is not well suited for fast disturbance rejection on processes with long time constants. But it sure is a useful tuning rule for improving loop stability or dialing in a specific speed of response.
Once you understand the pros and cons of using Lambda tuning, you can decide if it meets your needs for any particular control loop.
Learn more about Lambda and other tuning rules from the book Process Control for Practitioners.
Then try it out for yourself using the OptiControls Loop Simulator.
Stay Tuned!
Jacques Smuts
Founder and Principal Consultant
OptiControls Inc.
Posted in 4. Controller Tuning
I agree with you.
As you exposed in your minivan vs Corvette example, everything in engineering is a question of use and balance. Our job as engineer is always to find the solution that match with our requirements (about functionality, safety, isolation, endurance, etc) but respecting our constraints (price, weight, dimensions, legislation, etc)
A loop tuned very agressively intending to achieve the fastest response can be very harmful in another part of your plant downstream (think on a very agressive primary controller in a cascade loop, it will “fight” with the secondary controller). Your “performance parameter” for a loop depends on what objective you want to achieve (speed, stability, slow movement, less valve displacement, etc).