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Tip #89: Identify Deadtime and Ramp Rate

The following tip is from a new book by Greg McMillan and Hunter Vegas titled 101 Tips for a SuccessfulAutomation Career, inspired by the ISA Mentor Program. Today’s Tip #89 is by Greg McMillan.

In Tip #70, we learned that deadtime was the key to loop performance. The Control Talk blog The ABCs ofController Tuning describes how tuning settings can be reduced to a simple function of deadtime. The InTecharticle PID Tuning Rules and the article’s online appendicesdescribe the fundamental relationships between deadtime,performance, and tuning.

Fortunately, deadtime is the easiest aspect of process dynamicsto identify. The deadtime is the delay between a change incontrol output and the beginning of the resulting change in theprocess variable. To ensure that it is a process response and notnoise or coincident unmeasured disturbances, you must ensurethat the response is in the right direction and is sustained.

Once measured and confirmed, the deadtime is used by adeadtime block to create an old PV from a new PV at the blockinput. The ΔPV added to the current PV is a predicted PV ondeadtime in the future, which opens up all sorts of opportunities(Tip #90). Convert the ΔPV to a percent of measurement scale. Ifyou divide the Δ%PV by the deadtime (θo), you have acontinuous train of ramp rates (Δ%PV/Δt) that are updated withevery execution of the block. The ramp rate can be used for asmarter integral mode and feedforward action (Tip #92). If youdivide this ramp rate by the change in controller output, youhave the integrating process gain (Ki) (Equation 1) that can beused for controller tuning, rapid modeling, and adapting dynamicmodels online (Tip #98). If you divide a steady-state open loopgain (Ko) by the integrating process gain, you have the openloop time constant (τo) for a self-regulating process (Equation 2). The open loop gain (Ko) can be approximatedas the ratio of percent process variable (%PV) to the (%CO) controller output at the setpoint. If you haveknowledge of another operating point (%PVo, %COo) or know the %PVo when the controller output is zero(%COo=0), you can subtract these other operating point values from the values at the setpoint to create deviationvariables that give a more accurate open loop gain (Equation 3).

The ramp rate for level can be used to create a rate of change of vessel level or weight for an inferential

measurement of flow. The deadtime block must use a deadtime much larger than the process deadtime so thatthe ΔPV, and consequently the ramp rate, is much larger than noise.

Concept: The identification of deadtime and ramp rate opens up a wide spectrum of opportunities. The use of adeadtime block creates a continuous train of ramp rates as fast as the block executes.

Details: Use auto-tuner software, adaptive tuner software, or the rapidmodeler composite template library block to identify the deadtime, ramp rate,and the integrating process gain or open loop gain, and open loop timeconstant for any large change in controller output or feedforward signal. Use anoise band to screen out insignificant changes. Make sure the identifier islooking for a change in the right direction. Use a time interval much largerthan the deadtime to increase the signal-to-noise ratio when computing theramp rate for the rate of change of level and weight for an inferential flowmeasurement. Also increase the time interval when computing the slope ofbatch processes for batch end point and cycle time optimization (Tip #96). Forinverse response, the deadtime will be increased automatically and the ramprate measured will be based on the response in the right direction. Themethod can be used to identify the dynamics between any process input andprocess output. Use the simple relationship between a true integrating ornear-integrating process and a self-regulating process to convert between anintegrating process gain and steady-state dynamics; that is, open loop gainand open loop time constant. A deadtime block in the identification of the ramprate and the subsequent integrating process gain is essential to improve the signal-to-noise ratio and provide acontinuous train of values. Use the identified dynamics to adapt tieback models (Tip #98). Use deviation variablesto get a more accurate open loop gain. For an integrating process or runaway process models, subtract a loadequal to normal controller output from the current controller output. This load stops the ramp or divergence whenthe current controller output balances out the load.

Watch-Outs: The deadtime and ramp rate should only be identified for setpoint changes and output changes inmanual, remote output, and output tracking mode that are large enough to make noise and unmeasureddisturbances negligible. The deadtime cannot be identified in automatic, cascade, or remote cascade mode ifthere are no setpoint changes or no injection of a known change in controller output.

Exceptions: Dynamics cannot be identified accurately for processes with a deadtime approaching the executiontime of the deadtime block or the identification module.

Insight: The identification of deadtime and ramp rate can provide tuning, rapid modeling, and future values forsmart reset, feedforward, and setpoint responses.

Rule of Thumb: Use a deadtime block to create a continuous train of old PV that when subtracted from thecurrent PV and divided by the deadtime creates a continuous train of ramp rate updates as fast as the deadtimeblock execution time.

Look for another tip next Friday.

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