Installation manual

Series C Controller Configuration and Operation Manual
20-2  Athena Controls, Inc. 900M050U00
20.3 Why doesn’t the PV displayed match the value on a
thermometer in the process?
Unless the thermometer and the sensor providing input to the controller are very close to
one another, their readings will not match in some applications, because of temperature
variations within the process. However, if you want the controller to maintain the
setpoint value at the location of the thermometer, instead of at the location of the sensor,
use the input menu’s bias parameter for RTD or thermocouple inputs. Applying bias
allows you to compensate for any difference between sensor temperature and the
location to be measured. The process variable and setpoint will be offset by the value
entered for the bias parameter. (See 5.6.1.)
For example, suppose you want the process to be 150 °F. However, the sensor
providing input to the controller is so close to the heater that it reads 50 degrees higher
than the process at the location of interest to you. Enter -50 as the bias value. Enter the
setpoint of 150. The setpoint 150 will be displayed. However, the controller will use a
setpoint of 200. The process value displayed will also be offset, so that when the
temperature at the sensor location next to the heater is 206 °F, the controller shows 156,
the temperature at the location of interest in the process.
20.4 Why does my compressor cycle so often?
If the controller’s output type parameter is set to PID, the controller attempts to moderate
the rate of change of the PV. PID control is not appropriate for devices such as
compressors that are either on or off. Change the output type to on/off.
• When the on/off output action is configured for reverse action (heating
applications), the controller will apply 100% output if the process temperature is
below the setpoint and 0% if the PV is at the SV.
• When the on/off output action is configured for direct action (cooling
applications), the controller will apply 100% output if the process temperature is
above the setpoint and 0% if the PV is at the SV.
The result of switching from PID to on/off will be a reduction in the cycling of the
compressor. The trade-off is greater oscillation of the process temperature. (If there is
still too much cycling, introduce hysteresis to the on/off control as described in 1.5.3.1.)
20.5 Why doesn’t the controller communicate with the host
computer?
When a controller has been communicating successfully with a MODBUS master or
Multi-Comm host computer, then stops communicating, the cause is most likely damage
to the network wiring. However, before going to search for the fault, take a quick look at
the communication settings on the serial menu. Make sure that the controller ID has not
been changed, and that the other communication settings match those used by the host.
(See 14.5.)