Control System

What is output tracking and Setpoint tracking?

What is output tracking?

In a controller manual and automatic operating modes creates a set of potential problems for the PID controller.

A human operator switches the PID controller from automatic to manual mode and then manually adjusts the output to a fresh value. The result would be an immediate “jump” back to the output value calculated by the PID equation in some crude PID controller designs while the controller was in manual. Some controllers, in other words, never stop assessing the PID equation, even while in manual mode

For example, a PD controller (no integral action) operates in automatic mode at a small output value that is too small to reach the required setpoint. The operator switches the controller to manual mode and then increases the output value, enabling the setpoint strategy to the process variable.

The operator switches the controller mode back to automatic when PV is almost equal to SP, expecting the PID equation from this fresh starting point to begin operating again. However, the output would jump back to some lesser value in a crude controller, right where it would have been positioned for these PV and SP circumstances by the PD equation.

A feature designed to overcome this problem – which is so convenient that I consider it an essential feature of any controller with a manual mode – is called output tracking.

With output tracking, the bias value of the controller shifts every time the controller is placed into manual mode and the output value manually changed.

Thus, when the controller is switched from manual mode to automatic mode, the output does not immediately jump to some previously-calculated value, but rather “picks up” from the last manually-set value and begins to control from that point as dictated by the PID equation.

What is setpoint tracking?

From the above explanation, if for some reason the process variable cannot achieve setpoint no matter how far the output signal value is driven by integral action

For example, it could be on a temperature controller where a steam system is the heat source for the process. If the steam system shuts down, the temperature controller will not be able to heat the process up to the setpoint temperature value regardless of how far the steam valve is driven by integral action.

If the steam system is shut down for too long, the outcome is a maximum saturated controller output in a futile effort to warm up the process.

If and when the steam system restarts, the saturated output of the controller will now send the process too much heating steam, causing the process temperature to override setpoint until the essential action pushes the output signal back to a sensible level.

For such situations a similar feature to output tracking – also designed for the convenience of a human operator switching a PID controller between automatic and manual modes – is called setpoint tracking.

The purpose of setpoint tracking is to equalize SP and PV while the controller is in manual mode, so that when the controller gets switched back into automatic mode, it will begin its automatic operation with no error (PV = SP).

This feature is most helpful when starting a process, where the controller may have trouble controlling the process automatically under unusual circumstances.

With setpoint tracking in the controller, for the entire time the controller is in manual mode, the SP value of the controller will be held equal to the PV value (whatever that value happens to be).

Once the operator decides that switching the controller to automatic mode is appropriate, the SP value freezes at the last PV manual mode value, and the controller will continue to control the PV at that SP value.

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