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.
