HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Control Technology, Microelectronics and Nanotechnologyoffset. This is the amount by which the process is off setpoint.The gain of the controller can be increased which narrows thethrottling range of the system and makes the operating line more horizontal.Increasing the gain, can reduce, but not do away with the offset.At high gains the presence of the offset is no longer appreciable and itseems as if the controller is keeping the process on setpoint.Most control loops become unstable if the controller is providedwith a high gain. The exceptions are very slow processes. This limits theuse of plain proportional control to processes which can tolerate highcontroller gains, the narrow proportional bands found in thermostatsand humidostats. The addition of an internal feedback mechanism effectivelyreduces the sensitivity of the device to an acceptable range forproportional control.Integral ModeThis mode is also known as reset mode, since after a load change itreturns the controlled variable to setpoint and resets or eliminates the offsetwhich the proportional controller would cause. This mode is alsoknown as floating control. But, it is most often called integral control sincethe controller output is based on the integral of the controller input.Proportional-plus Integral ControlIn a proportional-plus integral controller the controller output is acombination of the integral and proportional value (Figure 3-3). Theintegral term is divided by the integral time setting of the controller,which is also called the reset time.Figure 3-3. Proportional-plus-integral control.©2001 by The Fairmont Press, Inc. All rights reserved.