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Functional Blocks
POSPROP (Position Proportional)
Description
Used to pulse two digital output devices (one for raise pulses and another for lower pulses) to drive a process variable
(PV) toward its set point (SP). The only valid output destinations are to Digital Output Channel blocks or the Pulse
Count and Pulse Length blocks.
• Typically used to step a valve open or closed, raise or lower a rotary device, or move the plates of a pulp mill
Function
refiner together or apart.
• Compares the error signal (PV - SP) with an error deadband for the raise and lower directions at an interval based
on the configurable cycle time parameter (CYCLETIME). You can also configure the raise and lower deadband
values that are denoted as the parameters ERRORDBR and ERRORDBL, respectively.
• Generates a raise pulse, when the PV is less than the SP minus the raise error deadband (ERRORDBR); or a lower
pulse, when the PV is greater than the SP plus the lower error deadband (ERRORDBL) to reduce the error.
• The pulse duration determines the magnitude of a pulse - the longer the duration, the bigger the pulse. The
POSPROP block will not issue a raise or lower pulse that is longer than the configured cycle time (CYCLETIME) or
the respective maximum pulse time parameter MAXPULSER or MAXPULSEL, whichever is smaller. The block uses
the following values in its pulse duration calculation.
– Error signal (PV - SP)
– Raise or lower gain setting (KR or KL)
– Raise or lower pulse stroke rate (RAISERATE or LOWERRATE)
– Additional raise or lower pulse time (RAISEDEADTM or LOWERDEADTM) based on stiction compensation
(STICTIONR or STICTIONL), when a motor starts up; or backlash compensation (BACKLASHR or BACKLASHL),
when a motor changes direction.
– Minimum raise or lower pulse time (MINPULSER or MINPULSEL)
Timeout
In cascade mode, performs timeout monitoring on SP. If a good SP value is not received within a predefined time, this
Monitoring
block invokes the following timeout processing.
• Sets the input timeout flag (TMOUTFL)
• Holds the SP value at its last good value.
• Changes the mode to a user-specified TMOUTMODE.
• Requests the input's primary to initialize.
If SP times out and the block sheds to Auto mode, block sets its Cascade Request flag (CASREQFL).
Inputs
The required number of inputs is determined by the mode of the POSPROP block.
• If Mode is CAScade, two inputs are required - PV and SP.
• If Mode is AUTOmatic or MANual, only PV is required.
– SP is an initializable input; PV is non-initializable.
– PV must be pulled from another block; you cannot store to it – typically it is connected to the output of an
auxiliary or data acquisition (DATAACQ) block.
– If Mode is CAScade, SP is pulled from another block; if Mode is AUTOmatic, it may be stored by the operator.
– The POSPROP block may have one primary or none, depending on whether SP is configured or not; there is one
primary per initializable input.
The optional raise and lower flag inputs (RAISELMFL and LOWERLMFL) may be set externally to inhibit raise and
lower pulses, respectively. These optional inputs can be pulled from other function blocks.
• PVEUHI and PVEULO define full range of PV in engineering units. They also define the engineering unit range of SP,
Input Ranges
and Limits
since PV and SP are assumed to have the same range.
– PVEUHI represents the 100% of full scale value.
– PVEULO represents the 0% of full scale value.
• SPHILM and SPLOLM define set point operating limits in engineering units.
Prevents operator from storing SP value outside limits; if primary or user program attempts to store value outside
limits, block clamps it to appropriate limit and sets primary's windup status.
More on following page
Publication 1757-RM810A-EN-P - May 2002
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