Industrial controllers , basic theory

 


Integral Action (Reset)

This function is called Integral Action or reset. The open loop response of the reset mode is shown in

Figure 10, which indicates a step change in the artificial measurement away from the set point at some instant in time. So long as the measurement was at the set point, there is no change in the output due to the reset mode in the controller. However, when any error exists between measurement and set point, the reset will cause the output to begin to change and to continue to change so long as the error exists. This function, then, causes the output to change until the proper output necessary to hold the measurement at the set point at various loads is achieved. This response is added to the proportional response of the controller as shown in figure 11.

The step change in the measurement first causes a proportional response, and then a reset response which is added to the proportional. The more reset action there is in the controller, the more quickly the output changes due to the reset response. The reset adjustment determines how rapidly the output changes as a function of time. Among the various brands of controllers, the amount of reset action is measured in one of two ways; either in minutes per repeat, or the number of repeats per minute. For those controllers measuring reset in minutes per repeat, the reset time is the amount of time necessary for the reset mode to repeat the open loop response caused by proportional mode for a step change in error. Thus, for these controllers, the smaller the reset number, the greater the action of the reset mode. On those controllers which measure reset action in repeats per minute, the adjustment in indicates how many repeats of the proportional action are generated by the reset mode in one minute. Thus, for these controllers the higher the reset number the greater the reset action. Reset time is indicated in figure 11. The proper amount of reset action depends upon how fast the measurement can respond to the additional valve travel it causes.

 

The controller must not drive the valve faster than the dead time in the process , allowing the measurement to respond, or else the valve will go to its limits before the measurement can be brought back to the set point. The valve will then remain in its extreme position until the measurement crosses the set point whereupon the controller will drive the valve to its opposite extreme where will stay until the measurement crosses the set point in the opposite direction. The result will be a reset cycle in which the valve travels from one extreme to another as the measurement oscillates around the set point. When reset is applied in controllers on batch processes where the measurement is away from the set point for long periods between batches, the reset may drive the output to its maximum resulting in "reset wind?up". When the next batch is started, the output will not come off its maximum until the measurement crosses the setpoint, causing large overshoots. This problem can be prevented by including a "batch switch" in the controller.

 

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Function of automatic control .  - The feedback loop. - The measurement .  - The process .  

 The automatic controller .  -  Controlling the process .  -  Selecting controller action - Upsets . 

 Process characteristics and controllability . - Controller responses . - Proportional action . - Integral action (reset ).

 Derivative action . - Conclusion .