Monthly Archive: November 2006

Nov
27

Getting Rid of Dead Time – Beam Me Up Scotty

How can we get rid of dead time in our loops so we can be rich and famous by Friday? PID controllers with dead time compensation are reported to eliminate dead time in terms of a controller seeing the effect of changes in its controller output. For set point changes where all the controller needs …

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Nov
27

Saving PID Tuning When Replacing a Control System

When replacing an existing control system, the startup of the new system can often be made much smoother by re-using the PID tuning that has been established over the years with the existing system. If you are lucky, the form of the PID and units of the tuning parameters of the old and new systems …

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Nov
20

PID Design

The PID is by far the most common feedback control technique used in the process industry. Thus, during the development of DeltaV we placed a special emphasis on the features that should be included in the PID block. There were differences in the PID implementation of the two control system manufactured by Emerson at that …

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Nov
20

Without Dead Time and Disturbances I Would be Out of a Job

If the total loop dead time was zero, you could set the controller gain as large and the reset time as small as desired. If there were no disturbances, you could simply sequence the controller outputs for startup, transitions, and shutdown. Process dynamics, controller tuning, and loop performance would be a non issue. I once …

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Nov
13

Tuning in a Distributed Environment

The tuning of a PID control loop should be based on an accurate knowledge of the control parameter response to a change in the associated manipulated process input. If the process response is determined using values communicated from a controller to a workstation, then these communications may impact the identified process response. For example, the …

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Nov
13

Disturbance Location and Speed and Process Delay/Lag Ratio

Last week we discussed the effect of disturbance timing on performance. This week we turn our attention to the location and speed of the upset and the Delay/Lag (dead time to time constant) ratio of the process. Most control text books and papers show a step disturbance on the process output, which is the process …

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Nov
06

Embedding MPC in a Control System

My first exposure to model predictive control, MPC, was in late 1979 when I attended a meeting called by Bob Otto, ISA Fellow. Bob had just returned from the AIChE 86th Annual National Meeting where he sat in on Charlie Cutler and Ramaker’s presentation of their paper Dynamic matrix control-a computer control algorithm. This landmark …

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Nov
06

Communication Interval, Control Execution Time, Analyzer Cycle Time, and Scan Time

We could talk about how important communication is for our society and even more importantly our marriage but let’s stick to something we are more interested in as automation engineers particularly since we essentially have no control over politicians and spouses. So let’s talk about communication intervals, control execution intervals, analyzer cycle times, and input …

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