The Parts that Advanced Control Cannot Reach
The traditional approach to advanced control is complex, expensive and slow. This is not an issue for the applicaitons that it is conventionally used for, but means that it cannot be used in a huge number of situations where the underlying concept would be invaluable.
Advanced control (by which I mean model-based predicitve control applicaitons like DMC and RMPCT) has evolved to fit a standard strucutre. This involves the use of a supervisory system (typically at L3 in S95 terms) where the advanced control application reisdes and operates. This supervisory system typically communicates with the DCS using some type of OPC (or CIMIO) interface, transferring all inputs and outputs for every scan of the controller - usually including a variety of watchdogs, verification steps etc.
The effect of this standard architecture is that APC applications typically run slowly (1-2 minutes compared to 1-2 seconds for a PID controller). Also the interface is not sufficiently robust for the controllers to operate directly onto valves, so a shedding logic is required and a fall-back regulatory scheme implemented locally in the DCS.
For large, unit-wide applicaitons this is not normally a problem, and APC applications continue to grow in size and complexity. But, does this miss an opportunity?
Process plants are littered with small, fast, interactive systems. The concept behind MBPC is potentially a fabulous solution for control of these systems, but the way it is traditionally implemetned prohibits its use because it's too slow and too difficult.
A plant that I worked on had a classic example - a plant feed was split across two paralled vaporisers with knock-out pots on their vapour lines. An additional intereaction between the two trains came because the overflow from both knock-out pots was returned to only one of the vaporisers. The system was fast (settling time about 10 seconds) and very interactive. We could not use DMC because the CIMIO/OPC interface could not operate fast enough, and decoupling the process using conventional technologies was difficult to do and harder to maintain.
I am interested to know how common this problem is. I would really like to hear about applicaitons that you wish you could use APC on, but cannot do so currently because of the speed, cost and complexity of traditional approaches to advanced control.
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Consider appropriate model and process time constant
From Russ Rhinehart at Olkahoma State:
"This seems to be a general question, so here is a general answer. I, too, think that as long as the controller can calculate appropriate MV action and the final control element can act faster than the process, then control should be good. But, there are two qualifiers: appropriate and faster.
Appropriate Model: If the missile aerodynamic model is generated for mid-tunnel performance, and does not include near-wall drag, then control may deteriorate near the wall. In the chemical industry, MPC applications deteriorate when the process moves away from the operating conditions that generated the model, and enters a region that changes the nature of influences.
Appropriate Controller Tuning: A nonlinear process would need the controller to be tuned differently for steering in one direction than another as the process industry also experiences processes that move faster in one direction than another, or in one operating condition than in another. Gain scheduling is a common solution. If the model is appropriate, then in MPC, either the move suppression factor or the trajectory damping factor could be adjusted dependent on the conditions.
Faster: A rule of thumb is that there should be 10 control actions within one process time-constant. This could be expanded to include delay or deadtime and the final element dynamics too. The control influence (controller plus final element) should have a response time (delay plus three time constants) that is at least one-tenth of the process response time (delay plus three time constants).
Further, if there are secondary controllers (perhaps positioners), they should have a response time that is at least one-fifth of the primary controller. The rule needs to guide control frequency and final element dynamics in the operating conditions that lead to the fastest process response.
R. Russell Rhinehart, PhD
rrr@okstate.edu
Consider 'smart' PID in the DCS
From Jim Ford of Maverick:
"Fast processes such as the one you describe in your letter can often be handled at the regulatory level using “intelligent” control algorithms implemented in the DCS. These algorithms can execute at the DCS execution frequency and without the need for the complex and slow interface between the DCS and the MPC software.
Just as an example, we have written generic control algorithms for multiple-input, adaptive feedforward, decoupling, and “smart” PID in the TDC 3000 Application Module and other DCS’s. If designed properly, the feedforward and decoupling algorithms provide a form of “model-predictive” control, and the “smart” PID algorithm actually provides feedback control that is SUPERIOR to conventional MPC. [Remember, the only mechanism for FEEDBACK control with conventional MPC is by adjustment of the bias between the predicted and actual values of the control variables at each execution – sort of integral-only control.]
These algorithms can be executed every 1 to 2 seconds, if needed. Version of these algorithms will obviously also run in Foxboro I/A, DeltaV, Experion, Yokogawa, and any other DCS that provides the control engineer with the ability to implement user-written programs of medium complexity.
If you will send me a sketch of the dual vaporizer system showing all the control valves and process measurements, I will be glad to suggest a control solution that could be implemented in the DCS. Assuming you have a DCS that provides some programming capability.
Hope this makes some sense and helps."
Best regards,
Dr. James R. (Jim) Ford, P.E.
www.mavtechglobal.com
Greg Shinskey weighs in
"I don't know why anyone would want to use MPC on a fast process. PID control has a higher performance, and PID with dead time has the highest performance of all controllers. For interacting loops, bilinear decoupling and feedforward using first-principles models are more effective than linear dynamic matrix control as well."
Greg Shinskey
shinskey@metrocast.net
Advanced regulatory control may be best
This from Michel Ruel of Top Control:
"Most MPC (or APC) projects are used to push the process to its limits and handling constraints is key. When MPC is used because of strong interactions, using advanced regulatory control is the best way to do it, particularly if the number of variables is low (e.g. 2-3-4). Hence, after defining needs and objectives, process control engineer will develop appropriate control strategies using override control, feedforward..."
Best regards.
Michel Ruel, P.E.
President of Top Control
Web site: www.topcontrol.com
Use the right tool for the job
More from Bela Liptak and others follows, recreated in the Forum based on email exchange:
"We all know that you need the right tool for each job. It sounds trivial, but you don’t use a hammer when a screwdriver is needed. This is the case with control too, PID is a tool, MBC is a tool and we don’t use MBC just because it is available.
Another truth is that we take a close look at the process before figuring out the right controls for it. We don’t just take the process as a given, but we first analyze how it could be made more “controllable”, how, for example, could it’s dead times be reduced?
Having said that, I would say that as long as the manipulated variable is 10 times faster (including dead time), than the controlled one, the speed of the controlled variable should not be a problem in PID implemented in DCS. When I say this, I am talking about seconds. When the time constants are in milliseconds and less (say missile guidance), DCS is obviously too slow, and we use tunnel control algorithms (making no correction the projectile approaches the wall of the tunnel”, but this is a different subject."
Béla Lipták
I referred this topic thread
I referred this topic thread to Bela Liptak, acknowledged controls expert and our frequent contributors to ControlGlobal.com. He had this to say:
"Most colleagues would say that as long as the manipulated variable is 10 times faster (including dead time), than the controlled one, the speed of the controlled variable is no problem, models will work.
"My experience is a bit different. I had good luck with tunnel control, such as guiding a missile in a tunnel and leaving it alone until it approaches one of it's "walls".
Thanks, Bela!
Keith Larson