Deadbands

14 Dec 2019 at 20:22 // under Simple Saturdays Process Control

Continuing after last week's discussion about process noise, I will now cover another tool used to help mitigate noise.

Deadbands are useful when the the result is a digital signal, it is either on or off. Most control loops are analogue and have a range of outputs, usually a control valve position from 0 to 100%. Occasionally something will be controlled by turning something on or off. This could be an oil heater to keep some lube oil at the right temperature or it could be a pump that automatically empties a sump once it fills above a certain level.

Another, more common situation is an alarm. Once a reading increases above an alarm setpoint, the alarm is triggered. The alarm is either on or off.

There is a problem with these systems when the reading bounces around about the setpoint. If an alarm is set when a temperature goes above 60°C, it would be very annoying if it keeps bouncing between 60.1, 59.8, 60.2, 59.9. Each time the reading crosses the 60°C mark, it would either trigger a new alarm or clear the previous one. Similarly if you were turning a piece of equipment on and off every second, you would soon break something.

To stop this, these systems are fitted with deadbands. The deadband is the range when the control system ignores the exact reading and just keeps doing what it was doing before.

In the case of the alarm set at 60, the deadband would probably be set to something link 1°C. In this case if the reading bounces around the 60°C setpoint, then the alarm would be triggered, but then stay in alarm until the reading dropped below 59°C. Systems with lots of noise, where the readings bounce around a lot, may need larger deadbands. The deadband needs to be bigger than the noise.

In the case of the automatic pump out, the deadband would be bigger still. The system may turn the pump in at 30%, and have a deadband of 20%. As a result, the pump would stay on until the level drops below 10%.

Overreacting to changes causes problems. The control system ignores any changes within the deadband and only responds once the signal has traveled through it and out the other side.

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