CEMP-E
TI 811-12
18 August 1998
by a digital output. EP relays are placed in a pneumatic local loop control circuit to apply air pressure to a
device, exhaust air pressure from a device, or transfer control from one device to another. Control air is
obtained from the existing compressed instrument air system.
d. Controllers continuously measure changes in controlled variables and automatically send
appropriate signals to adjust equipment or devices to correct any deviation from the desired setpoint.
(1) Single input Control Point Adjustment (CPA) controllers are used when reset control is
required. The setpoint of the controller must be adjustable over a range of plus or minus ten percent of
the primary sensor span.
(2) Dual input controllers can be used instead of single input CPA controllers when the
adjustable control range needs to exceed more than plus or minus 10 percent of the primary sensor span.
(3) Some electric and electronic controllers have CPA or remote setpoint inputs, which may
require a 4 to 20 mA signal or a varying resistance (rheostat) input to adjust the control loop setpoint. An
example is a centrifugal chiller capacity controller which permits gradual chiller demand limiting by the
UMCS.
e. Current to pneumatic (I/P) transducers are electrically operated by an AO in the smart field
panel. The AO signal is converted into a pneumatic output signal compatible with the local control loop or
actuator. These proportional signals position valves, dampers, and reset local loop control setpoints.
3.
MICROPROCESSOR-BASED CONTROLLERS.
a. Many HVAC, utility and process systems utilize microprocessor-based controllers. One example
is the single-loop digital controller utilized in standard control panels for HVAC control systems. Standard
control panels include interfaces for connection to UMCS. Another example is an application-specific
unitary controller provided as a packaged equipment control system by an equipment or system supplier.
b. Some microprocessor-based controllers may be interfaced with a UMCS smart field panel with a
controller communication port which utilizes a standardized communication interface such as EIA 485. In
this case, up to 32 microprocessor-based controllers may be interfaced on a single communication circuit
to a smart field panel. The designer will investigate existing microprocessor based controllers to
determine if they are equipped with the controller communication port.
c. If the existing microprocessor-based controllers do not include controller communication ports,
the designer will consider two options for interface of the controllers with the UMCS. The first option is
replacement of the existing microprocessor-based controllers with units equipped with the proper
controller communication ports. The second option is to provide CPA interface to the microprocessor-
based controllers through 4 to 20 mA analog outputs if the controllers are equipped to accept a remote
setpoint signal.
5-5
Previous Page
