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Jan
19

How to Succeed – Part 10

We conclude with a summary on how you can avoid bursts of oscillations, get to setpoint faster, choose the right execution time and filter, coordinate the speed of loops, optimize operations without while protecting equipment, provide a consistent flow response for model predictive control, eliminate limit cycles, and improve analyzer and wireless control loops (all in less than a 1000 words).

A primary controller output can be prevented from going beyond the setpoint limits of a secondary loop driven by the output by obeying setpoint limits in cascade and remote cascade mode. Stopping the output at the setpoint limit allows a more immediate recovery when the output reverses direction. The proper use of the back calculate signal enables a bumpless transfer for mode changes and a responsive transition in override control, and prevents bursts of oscillations for slow secondary loops and slow valves. The use of PV for the back calculate combined with the dynamic reset limit or “external reset feedback”, limits the primary loop output from changing faster than the secondary loop or a slow final control element (control valve or variable frequency drive) can respond preventing a burst of oscillations for large disturbances or setpoint changes. A fast readback of  valve position or drive speed is needed by a dedicated signal or primary HART variable (PV). A secondary HART variable (SV) for valve or speed readback may not be fast enough for the dynamic reset limit.

If the setpoint tracks the PV in the manual mode, then the setpoint change in the auto mode will provide a step in the controller output from the structure “PI on error and D on PV”. If the setpoint is retained at the desired operating point in auto (no setpoint change in auto), the approach to setpoint is extremely slow unless the output is prepositioned by an ROUT mode because the change in controller output to achieve the setpoint is a ramp from integral action instead of a step from the proportional mode. For temperature loops, the integral time setting is large causing a slow rise time. If the setpoint must be retained in the PID loop, setpoint tracking of PV is not used and rise time is sacrificed. For primary loops used in traditional basic control of continuous operations, there are few setpoint changes and controller outputs are prepositioned for startup. When grade transitions, flexible manufacturing, model predictive control and real time optimization result in setpoint changes, the use of setpoint tracking PV in manual enables a smooth  transition to advanced control besides cascade control.

The total of the signal filter time and the PID module execution time should be less than 10% of the smallest integral time to prevent the integrated error from an unmeasured disturbance increasing more than 10%. The filter time should be just large enough to keep fluctuations in the controller output from exceeding the final control element (valve or variable frequency drive) deadband.

Directional velocity limits on the setpoint in an Analog Output (AO) block used in conjunction with a dynamic reset limit and the use of AO PV for the back calculate signal can provide intelligent coordination and regulation of control loop speed without the need for retuning the PID.

Directional AO setpoint velocity limits can provide a slow approach to an optimum and a fast getaway from trouble for valve position control (VPC). See “Don’t Over Look PID in APC” for the many possibilities of VPC.

Directional AO setpoint velocity limits offers quick recovery from surge conditions and a lower chance of re-entry in surge by a fast opening and slow closing of the surge valve. In the old days directional slewing rate was done on the fail open surge valve by quick exhaust valves or boosters with a higher vent rate than pressurization rate. The action of these devices was disruptive and unrepeatable posing operational, maintenance, and tuning problems.

Directional AO setpoint velocity limits offer the opportunity for loops to have the same speed of response despite different tuning and dynamics. The greatest need is commonly seen in coordination of flow loop response. For blending operations, flows are set in ratio to each other. If the setpoints of the flow loops are simultaneous driven and directional velocity limits ensure the speed of the flow response is nearly identical, the blend composition will not be upset by an unbalance in flows. The same strategy is useful for minimizing the upset from load changes and analyzer corrections of ratios for reactor feeds and for using the same model for flow response in model predictive control (MPC), particularly advantageous for minimizing duplicate MPC setup and maintenance in parallel equipment trains.

The limit cycles from deadband (e.g. valve backlash with 2 or more integrators in loops and process) and resolution or threshold sensitivity limits (e.g. valve stiction with one or more integrators in loops or process) can be killed by setting the integral deadband equal to the PV amplitude of the limit cycle. The PV amplitude depends upon operating point and usually gets larger as a valve approaches the closed position or as the product or corrosion builds up on sealing or seating surfaces and stems. The enhanced PID developed for wireless will inherently kill the oscillations if  noise does not trigger an update. A filter or threshold sensitivity setting is used to screen out noise can prevent unnecessary updates.

For wireless and analyzer loops where the time between updates is much larger than the sum of the process time constant and deadtime, the enhanced PID enables the use of a PID gain that is the inverse of the open loop gain. This PID gain provides a single correction for a setpoint change that puts the PV at the setpoint for the next update (see “Wireless – Overcoming challenges of PID control & analyzer applications” for details on the opportunity).

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