Control and optimization of air- conditioning systems - Intelligent Buildings and Building Automation - page 142

Off

Operating
differential

Control
differential

Desired room temperature

On

Time

Figure 8.4 Operation of the two-position control.

operates over a very large range. The flow is then separated into parallel
paths each controlled with a valve.

8.1.4 Two-position control and on/off control

Most building processes are controlled by continuous modulating control.
However, there are still many processes controlled using two-position control
primarily due to the low cost of the control actuation devices. Fan-coil units
and room air-conditioning units (both window unit and split-type unit) often
employ a two-position control. In fact, room air-conditioning units usually
adopt an on/off control, which is a special type of two-position control.
The operation of the 'two-position' control action can be illustrated by
Figure 8.4. The difference between the 'on' and 'off' points of the thermostat
is called the control differential . The difference between the high tempera-
ture and low temperature in the room is called the operating differential .
Generally speaking, the smaller the operating differential of the system, the
better the system.
To improve the control action, most of the sophistication added to this
simple two-position control is for the purpose of reducing the operating
differential of the system. An example of methods to reduce the operating dif-
ferential is to employ additional control actions, such as in the case of timed
two-position control. Another example in HVAC systems is to employ a third
control action: floating action . PID control can also be employed to improve
performance of two-position control, namely two-position PID control or
PID bang-bang control. It is worth noting that two-position control or on/
off control cannot control the process variable with high accuracy.

8.1.5 Temperature controls

Temperature control is the most fundamental function of air-conditioning
systems. Temperature control is often done by a temperature controller called
a thermostat which is set to the desired temperature value or set-point. The
temperature deviation, or offset, from the set-point causes a control signal to
be sent to the controlled device. P, PI or PID control functions are normally