Fixes for Deadly Deadband

While there are some cases where deadband is helpful, in most applications the effect is extremely detrimental and confusing. Deadband can arise from any sources either intentionally or inadvertently. Deadband creates deadtime and for certain conditions excessive and persistent oscillations.

The increase in loop deadtime is the deadband divided by the rate of change of controller output.  The increase in deadtime can increase the peak error and integrated error from a load disturbance. If there are two or more integrators in the system due to integral action in the valve positioner, variable speed drive, controller(s), or process, a limit cycle will develop.

The biggest and most troublesome source of deadband is backlash from an on-off or isolation valve (tight shutoff valve) posing as a throttling valve. The positioner seeing feedback from the actuator shaft of such rotary valves often does not realize the internal closure member (e.g. ball or disk) is not responding due to backlash from the connections between the shaft, stem and ball or disk or the shaft windup from seal friction. The positioner diagnostics say everything is fine even meeting the requirements set by the ISA-75.25.01 Standard for Measuring Valve Response. Creative story telling develops to explain the oscillations in the process.

An on-off or isolation valve offers a great advantage when used in series with a throttle valve. Besides achieving tight shutoff, the placement of a quickly stroked completely open or closed on-off or isolation valve close-coupled to the connection into the process eliminates the deadtime and any unbalance between ratioed flows during the start and stop of reactant and reagents enabling more precise composition and pH control. The throttle valve is located at a position that is more accessible for better maintenance and with some straight runs upstream and downstream. The throttle valve straight run requirements are rather minimal but can give a more consistent flow relationship between valve position and flow.

For the throttle valve, the best solution is to get rid of the excessive deadband.  Given that you are literally and figuratively, stuck with deadband principally when the source is a big valve, an increase in the PID gain will reduce the peak and integrated absolute error (IAE) by increasing the rate of change of the PID output and thus decreasing the additional deadtime from deadband. If there is a limit cycle, increasing the PID gain reduces the amplitude and period of the limit cycle, decreasing the persistent IAE and increasing the ability of downstream volumes to filter out the oscillations. Open loop step tests don’t reveal the additional deadtime but show a decrease in process gain upon a reversal of direction of step change. A filter time can be judiciously added that is less than 20% of the total loop deadtime seen in the test to prevent changes in the PID output from noise exceeding the deadband of the valve. For more on the effects of backlash see the May 2016 Control article “How to specify control valves that don’t compromise control” and the recording of the YouTube recording to be posted in June on the “ISA Mentor Program Webinar Playlist” of my ISA Mentor WebEx “ ISA-Mentor-Program-WebEx-Best-Control-Valve-Rev0.pdf ”.  The article white paper and presentation also shows that an increase in PID gain eliminates an oscillation from poor positioner sensitivity by making changes in the valve signal larger than sensitivity limit.

A simple algorithm can be configured to increase the change in PID output by an amount slightly less than the deadband when the output changes direction and the change is greater than the noise band seen in the PID output. The kick of the output upon a change in direction eliminates the deadtime and lost motion from backlash. The practical issue is the deadband may vary with valve position, time, operating conditions, and positioner tuning. These algorithms are often used for Model Predictive Control besides PID control.

A lead-lag on the valve signal can reduce the effect of deadband, resolution and positioner sensitivity but the valve movement can quickly become erratic for a lead much larger than the lag time and noise.

Often deadband is a parameter in a Variable Speed Drive (VSD) setup to reduce changes in speed from noise. Often deadband is set too large because of a lack of understanding of the detrimental effect. The deadband should be just slightly larger than the noise band seen in the VSD setpoint.

Dynamic simulation with a backlash-stiction block and a PID with external reset feedback can show this and much more. The virtual plant is my lab to rapidly explore, discover, prototype and test solutions.

I recently went to a Grateful Dead tribute band concert. The “dead heads” were grateful the music of the band was not dead. Keep your control system alive by not succumbing to the deadly deadband.