7.2 PI relay control setpoint
Display: Ax.SP
Range: Any display value
Default Value: 0
The control setpoint is set to the value in displayed engineering units required for the control
process. The controller will attempt to vary the control output to keep the process variable at the
setpoint. Note that the control setpoint value can be reached and adjusted via the “easy access”
mode (see “Explanation of functions” chapter) if the ACCS function is set to EASY. This feature
could be useful if the setpoint is to be frequently changed.
Process above
setpoint
Process below
setpoint
Process at
setpoint
SETPOINT
PROCESS
TIME
7.3 PI relay control span
Display: ctrI SPAN
Range: Any display value
Default Value: 100
The function of the control span is to define the limit to which the PI control values will relate. The
control span value will be common to all control relays i.e. if more than one control relay output is
being used then each of these relays operates from the same control span setting. The span value
defines the range over which the input must change to cause a 100% change in the control output
when the proportional gain is set to 1.000. This function affects the overall gain of the controller
and is normally set to the process value limits that the controller requires for normal operation.
For example if the control setpoint (Ax.SP) is 70 and the ctrI SPAN is 20 and Ax.PG is set
to 1.000 then an error of 20 from the setpoint will cause a 100% change in proportional control
output. For example with Ax.SP at 70, ctrI SPAN at 20, Ax.PG at 1.000 and AxbS at
0.000 a display reading of 50 or lower (Ax.SP minus ctrI SPAN) the control output will be
at 100% i.e. the relay will be on continuously. The control output will then gradually adjust the
on/off time as the display value reaches the setpoint.
For instruments with more than one input where the number of decimal points displayed may vary
the control span will take on the value of the main display and so may or may not match the
decimal points shown in the input being controlled. e.g. a control span of 2.00 will act as a control
span of 20.0 if the input to be controlled is displayed with only 1 decimal point.
46 of 56 PM4IVMAN-2.4-0
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