Control Action

• Two-position (on-off) control

• Modulating control

Two-position control systems are always at full capacity or off. Best for systems with slow rate of change for controlled variable. This control is common in low cost systems, and it is relatively imprecise.

Example: Two-position control for steam valve in the steam heating coil.

Differential

Valve closed on

Temp, drop

Valve opens

Valve off as

At Q5°F

100° F reached

Valve open on

Temp, rise

Off

On

95°F

C35°C)

100°F (38°C)

Coil outlet temperature

FIGURE 123

Control differential is difference between “on” and “off’ values of controlled variable. Operating differential is difference between extreme values of controlled variable.

подпись: control differential is difference between “on” and “off’ values of controlled variable. operating differential is difference between extreme values of controlled variable.

Operating differential > Control differential

подпись: operating differential > control differentialTwo-position (on-off) control characteristic for steam valve in Fig. 12.2«.

Control Action

Figure 2-14 Two-position (on-off) control action.

Modulating control systems produce continuously variable output over a range. This is finer control system than two-position system, and it is typical in large HVAC systems.

Control Action

Figure 2-16 Modulating control action.

Throttling range (TR) is a range of input variable over which output varies through its full range. Gain is A output per A input, and it is usually adjustable.

Proportional control is the simplest modulating action for which the controller output is a linear

Function of input:

O

Where Op is the proportional controller output, A is the controller output at zero offset, e is the error (offset), and Kp is the proportional gain constant.

Control Action

Set point Controlled variable (Temperature)

FIGURE 12.4

Proportional control characteristic showing various throttling ranges and the corresponding proportional gains Kp. This characteristic is typical of a heating coil temperature controller.

Smaller TR (larger gain) =>smaller offset. Smaller TR may cause stability problems.

Stability is tendency of a system to find a steady control point after an upset. Instability is tendency for oscillations to grow.

Control Action

Figure 2-18« A stable system under proportional control.

Control Action

Figure 2-186 An unstable system under proportional control.

Proportional plus integral (PI) control is designed to eliminate offset.

Proportional + Factor °c integral of offset

O

Where Opi is the PI controller output, and Ki is the integral gain constant.

Integral term drives offset to zero.

Examples of PI control in buildings include mixed-air control, duct static pressure control, and coil controls.

Proportional plus integral plus derivative (PID) control further speeds up action of PI control May not be suitable for HVAC that usually do not require rapid control response.

Additional control rate of change of error

O

Where Opid is the PID controller output, and Kd is the derivative gain constant. Example of PID application in buildings is duct static pressure control.

Example: Comparison of P, PI, and PID controller response to input step change

Input

C

Load

Change

Error

Time

| Offset

Error

P | Time

Error

Time

P + I

P + I + D Time

FIGURE 12.5

Performance comparison of P, PI, and PID controllers when subjected to a uniform input step change.

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning


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