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

HVAC Control in the New Millennium3-1. Two-position control system.Loop GainOscillations will result if the control loop gain is greater than 1.0.These oscillations will grow in amplitude until some component in theloop is destroyed. This condition is known as an unstable loop withrunaway oscillations.At a control loop gain of exactly 1.0, the loop will oscillate, but theoscillations will not grow in amplitude. The loop will be in a state ofmarginal stability.Quarter amplitude damping results if the loop gain is 0.5. The loopresponse to an upset will be a damped sinusoid with the second peak at1/4 the amplitude of the first peak. The goal of tuning in most processcontrol loops is to obtain quarter amplitude damping. This will result ifthe product of all the gains in the loop comes to 0.5. It can be achievedby tuning, which adjusts the controller to make the overall gain product0.5.The controller gain, as well as most other gains, consists of asteady-state component, Kc which is the proportional setting. It is unaffectedby the period of oscillation. The dynamic gain, Kd, varies with theperiod of the input (error) sinusoidal wave. The Kd × Kc product is thetotal gain of the controller.Slow processes can be controlled with high-gain controllers, whilecontrollers on fast processes must have low gains. An example of a slow(low-gain) process is space heating, where it takes a long time for theaccumulation of the heat input provided by the manipulated variable tocause a small change in the controlled variable, the space temperature.If the combined gain of that process is 0.01, the controller gain requiredto provide quarter amplitude damping is 50 since 0.01 × 50 = 0.5.©2001 by The Fairmont Press, Inc. All rights reserved.