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COVER TOPIC: ADVANCED CONTROL Allan Kern, APC Performance LLC
Feedforward: Not as popular as expected, again Feedforward and feedback: Multivariable control, like single-loop control, can be accomplished primarily with feedback control and selective (not wholesale) use of feedforward.
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ll control engineers learn about feedforward early in their education and careers. It is not complicated and has the potential to reject process disturbances seamlessly, such that there is no impact on the controlled variable. Not even the best-tuned feedback controllers can do that. In feedforward control, a process disturbance is measured and translated into a change in controller output, which is implemented in phase with the disturbance, so the output rejects the disturbance, and the controlled variable continues on unaffected, essentially oblivious to this behind-the-scenes help. Without feedforward, the same process disturbance would upset the controlled variable, resulting in a process deviation (error) from setpoint, and requiring feedback control action to reject the disturbance over time, usually using the well-known proportional-integralderivative (PID) algorithm. This is the inherent limitation of feedback: It KEYWORDS: Feedforward, requires process error to work; feedforfeedback, multivariable control ward has the potential to prevent error Less than 5% of all installed in the first place. control loops throughout industry Feedforward is a powerful tool and take advantage of feedforward capability. a fundamental process control concept Feedforward, for all its virtue, that almost every process controller in adds cost, risk and maintenance the process industry supports. However, to a control loop. less than 5% of all installed control loops Model-based multivariable take advantage of feedforward capabilicontrol (MPC) can be thought ty. Why is such a powerful and available of as feedforward control on tool used so sparingly? steroids – it employs feedforward
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models for every matrix location.
CONSIDER THIS What are your control loops doing right now?
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September 2020
Process industry values reliability: Feedforward adds risk
The experience of feedforward in industry over the past 50 to 75 years tells us that industrial process operation places very high value on reliability. Feedforward, for all its virtue, adds cost, risk and maintenance to a control loop. It adds
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cost because feedforward inputs and models (output translations) have to be provided. It adds risk because performance becomes dependent on the reliability of the additional inputs and models, with models in particular having proven to be generally problematic and unreliable. Feedforward adds maintenance of the inputs and the models, and performance issues become more difficult to troubleshoot and diagnose, compared to a PID loop by itself. Meanwhile, PID feedback control performance, without feedforward, is almost always considered satisfactory despite inherent transient error. The balance of these trade-offs over time has meant feedforward has been adopted sparingly and with great discrimination by industry. To qualify for feedforward, the inputs must be reliable, the models must be reliable and it must add significant value. In the whole process industry, one application routinely meets these criteria: The textbook case of three-element boiler drum level control. In this application, the input is a routine steam flow measurement; the model is eminently robust (it has a fixed gain of 1.0 and instantaneous dynamics, so there is no potential for either gain or dynamic error); and the added value is high, especially as it mitigates the difficult boiler drum “shrink and swell” effects in this critical loop. With feedforward control, the potential risk exists, if the feedforward model dynamics are not correct, to double the disturbance, rather than to cancel it – or worse if the model gain is also not accurate. People often use a reduced feedforward gain to hedge this concern, however, there is no substitute for accurate and reliable model dynamics.
History repeats itself: Model-based multivariable control
The experience of model-based multivariable control (MPC) since the 1980s has emphasized these historical lessons of single-loop feedforward. www.controleng.com
