Handbook of air conditioning and refrigeration / Shan K

For a direct-acting pneumatic temperature controller used to control the space temperature
during summer, the branch line pressure may change from 9 to 13 psig (62 to 89 kPag) when the
space temperature increases from 73 to 77°F (22.8 to 25°C). This nozzle-flapper assembly pneumatic
controller operates in proportional control mode. Many other more complicated pneumatic
controllers have been developed to perform other control modes and additional functions.
Electric and Electronic Controllers
Direct Digital Controllers
An electric controller uses switches, relays, and a bridge circuit formed by potentiometers to position
the actuators in on-off, floating, and proportional control modes according to the input signal
from the sensor and the predetermined set point. An electronic controller can provide far more
functions than electric controllers can. It may receive input signals from both the main sensor and
the compensation sensor with amplification and combination.
In the control circuit, an electronic controller basically provides proportional or proportionalintegral
control modes. The output signal from the controller can be used to position an actuator or
to provide the sequencing of actuators, or to change to two-position, floating, or even PID control
modes in conjunction with additional circuits.
A direct digital controller has a microprocessor to implement computer programs to provide various
control functions. In DDC units, there are analog-to-digital (A/D) and digital-to-analog (D/A)
converters to convert analog input to digital signals for processing, or to convert digital signals to
analog for actuators, if necessary. DDC units are stand-alone and microprocessor-based controllers.
Stand-alone means that the controller has sufficient capacity to execute the assigned control
functions alone. Today, there are mainly two types of DDC units: system controllers and unit controllers.
System Controllers. A system controller, also called a stand-alone panel (SAP), as shown in
Fig. 5.13, has the ability to coordinate communications between system controllers, between the
system controller and the personal computer (PC) in the workstation, and between the system controller
and the supported unit controllers. It also has the ability to provide and execute the control
programs for functional control, and to store user databases and trend log values. A system controller
can support 50 to 200 unit controllers on separate unit controller trunk(s).
Unit Controllers. A unit controller, also called a terminal controller, is shown in Fig. 5.14. It usually
has limited capacity to execute factory-loaded computer programs and to provide functional
control for a terminal or a piece of HVAC&R equipment. Unit controllers are often connected in a
separate network and supported by a system controller. The new-generation unit controllers have
greater memory to handle complicated control programs, and they provide time and calendar scheduling,
data storage, and other functions, such as limited programming.
Hardware. Many system controllers have only a single printed-circuit board. Single-board configuration
often offers lower first cost. Its disadvantage is that any component failure requires the
replacement of the complete board. Another approach is that the controller is made from various
modules. The modular approach isolates the component failures and plugs on the single-module
replacement quickly and inexpensively.
Memory. The types of memory included in DDC units are as follows:
ENERGY MANAGEMENT AND CONTROL SYSTEMS 5.23
● Read-only memory (ROM), which stores the software provided by the manufacturer and should
not be modified by the user.