Uncontrollable changes both inside and outside of a building can impact efforts to maintain a desired temperature. Changes in occupancy, lighting and computer-equipment use, outdoor temperature, and the like can cause the set temperature in a building to fluctuate constantly.

This article discusses three functions of control that can be used to regulate the consistency of an HVAC system while surrounding conditions change: proportional, integral, and derivative.

Proportional, integral, and derivative controls use a closed-loop feedback system to maintain a predetermined process level, or set point. Variation from the set point is referred to as process deviation, or offset error, and requires continuous corrective action for a process output to be maintained.

PROPORTIONAL-INTEGRAL-DERIVATIVE CONTROL

Proportional, integral, and derivative controls work together to maintain a constant temperature:

FIGURE 1: Proportional control

• The proportional control function determines how quickly an output reacts to a process variation. It is a control algorithm in which, in the case of HVAC applications, air or water is moved to a position proportional to the deviation of the value of the controlled variable from the set point. Proportional control's effect on system startup is shown in Figure 1.

• Integral control determines a reaction based on a sum of variations. Also known as reset response, it tends to correct offsets resulting from proportional control. Without integral control, a system may be unable to reach its target value. Proportional-integral (PI) control is a control algorithm combining the proportional and integral control algorithms.

• Derivative control determines reactions to the rates at which process variations change and is very sensitive to measurement noise. It opposes any change and is proportional to the rate of change.

As a controlled variable deviates above or below its set point, PI control corrects the process deviation. The difference between proportional control and PI control is that proportional control is limited to a single control element for each value of a controlled variable, while PI control changes the final control element to accommodate load changes while keeping a control point at or very near its set point.

FIGURE 2: Proportional-integral control

Reset is a proportional-control technique by which a secondary sensor resets the set point of a primary sensor. The reset action of the integral component shifts the proportional function around the set point as the load on the system changes, keeping the control point at the set point by correcting the control signal. Because offset is eliminated, the proportional function usually is set fairly wide to ensure system stability under all operating conditions. PI control's effect on system startup is shown in Figure 2.

Proportional-integral-derivative (PID) control adds the derivative function to PI control. It enhances the PI control algorithm by adding a component that is proportional to the controlled-variable rate of change (derivative), compensating for system dynamics and allowing faster control response.

FIGURE 3: Proportional-integral-derivative control

The farther a control point moves from its set point, the faster the corrective action the derivative function takes to bring it back. The closer a control point moves toward its set point, the slower the corrective action the derivative function takes to reduce the possibility of overshoot. PID control's effect on system startup is shown in Figure 3.

Overshoot is caused by a large offset at startup. Microprocessor-based PID startup may be enhanced-PID (EPID) control. Basic EPID functions are dependent on start value and error ramp time. Start value sets output at a fixed value at startup.

For example, a variable-air-volume-air-handling-system supply fan may have a start value of 20 percent. The start value is high enough to enable the fan to start, allow the fan motor to self-cool, and provide a proof-of-operation signal for a monitoring system. For a heating, cooling, and ventilation air-handling unit (AHU), a suitable start value may be 30 percent because of the heating, ventilation (economizer), and mechanical-cooling demands of the AHU.