Microtech 325 - Standard Control Features

IM 613 / Page 6 (Rev. 7/99)
Actuators
The UVC uses hydraulic, spring return, floating control actuators
with position feedback for valve and damper modulation. These
actuators are controlled using the “Change” and “Step-and-Wait”
control algorithms. All cooling valves are normally closed, and all
heating valves (including “2-pipe” hot/chilled water) are normally
open. Outside air dampers are normally closed, and face and
bypass (F&BP) dampers are normally open to the coil face.
On some units equipped with F&BP dampers, a spring
return, two-position “end-of-cycle” (EOC) valve is used to pre-
vent overheating or overcooling when the damper is in the full
bypass position. Cooling EOC valves are normally closed, and
heating EOC valves (including “2-pipe” hot/chilled water) are nor-
mally open.
The following are standard MicroTech UVC control features as
applicable to the various unit ventilator model types:
• Control Temperature
• Change and Step-&-Wait control algorithms
• Compressor short-cycle protection
• Low ambient lockout
• Random start
• Delayed reversing valve de-energization
• Emergency heat
• Defrost
• Alarm monitoring & controlled response
Control Temperature
All unit ventilators are designed to control the room (or zone)
temperature. In order to maintain more stable room temperature
control, the UVC uses the concept of a “Control Temperature.”
Depending on the unit ventilator model, configuration, and
current mode of operation, the Control Temperature could be
either (1) the actual room temperature or (2) a weighted value
equal to 19/20 room temperature and 1/20 discharge air tem-
perature.
Throughout the remainder of this manual, “room temperature”
and “Control Temperature” will be assumed to be synonymous.
For further information, refer to the sequence of operation
document provided with your unit (see Table 6).
Change and Step-&-Wait Algorithms
The “Change” and “Step-and-Wait” algorithms work together in a
two-stage process to modulate actuators (valves or dampers) in
the unit ventilator. This control process enables the UVC to
maintain tight space temperature control with no overshoot.
There are several Change and Step-and-Wait parameters that
are factory set and adjustable only with a PC equipped with
Monitor software. For most applications, the factory-set
parameters will provide the best control. It is recommended that
these values not be changed.
Following is a brief description of the Change and Step-and-Wait
functions. For additional information, refer to the
MicroTech Unit
Ventilator Controller Monitor Program User’s Manual
.
Change
The Change function changes the valve or damper position
setpoint in response to the deviation of the room temperature
from its setpoint (offset). The amount of actuator-position setpoint
change varies and is dependent on the amount of room
temperature setpoint offset.
The Change algorithm is executed on a factory-set, periodic ba-
sis.
Step-and-Wait
The Step-and-Wait function causes the valve or damper to open
or close as required to maintain the Change function’s calculated
position setpoint. The “step” period is the amount of time the
electric actuator is driven either open or closed, and the “wait”
period is the amount of time the actuator holds its position. The
“step” and “wait” periods vary and are dependent on the amount
of actuator-position setpoint offset.
The Step-and-Wait algorithm is executed on a periodic
basis. This period is a factory-set constant that is equal to the
sum of the “step” and “wait” periods described above.
Communication Ports
The UVC has two communication ports: Comm A and Comm B.
Following are brief descriptions of each communication port’s
function. For further information, refer to the “Service
Information” section of this manual.
Stand-alone and Network Units
Comm B is for MicroTech network communications to the Local
Master Controller using an RS-485 format. (Comm B is configured
this way in a stand-alone unit, but not used.) Comm A is for
communications to an IBM compatible PC using an RS-232 format.
Master/Slave Units
Communications between master and slave UVC’s is done using
an RS-485 format. On the master unit, Comm B is used for
communications to the slaves, and Comm A is not used. On a
slave unit, Comm B is used for communications to the master,
and Comm A is available for communications to an IBM
compatible PC using an RS-232 format.
Temperature Sensors
The MicroTech UVC uses negative temperature coefficient (NTC)
thermistors for temperature sensing. A thermistor chart, which
provides voltage-to-temperature and resistance-to-temperature
conversion data, is included in the “Service Information” section
of this manual (Table 18).
Standard Control Features