Hardy HI 1756-nDF - Manual (Page 26)

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Chapter 4
Overview of Auto-Preact Function
The auto_preact function predicts the required preact weight to ensure that the weight on
the scale at the end of a fill cycle is equal to the desired target weight, after all the inflight
material has cleared the physical pathways and settled on to the scale. The setpoint
weight is the weight at which the relay is closed, and is calculated by subtracting the
preact weight from the desired target weight.
Setpoint weight = target weight – auto_preact weight
At the beginning of the fill cycle the material flow rate is not linear and the start wait
parameter is used to ignore the flow rate during this time. Once the calculated setpoint
has been reached the relay will close the valve and any in flight material will continue to
accumulate on the scale for a short period of time. The update time parameter is used to
set the predicted time for the inflight material to settle onto the scale so a reliable weight
reading can be taken.
This weight value is used in the auto-preact function to continually modify the setpoint
value so the correct target weight is reached.
The auto-preact function combines a proportional plus integral filter based on historical
differences between the target weight and the weight measured on the scale at the end of
each fill cycle; and a predictive function based on the current flow rate.
The following parameters are used to configure the auto-preact function.
DINT Configuration Parameters used in Auto-Preact
Parameter Default Description
capture
averages 3 Sets the number of fill/dispense cycles to average the target
error values and flow rate values to provide the auto_preact
value.
auto preact
enable 0 Selects between the internally calculated auto-preact value
and the external preact value in the output table.
REAL Configuration Parameters used in Auto-Preact
Parameter Default Description
start wait 2.0 Period of time that the flow rate is ignored in the auto-
preact calculation
update time 2.0 Period of time after the setpoint value has been reached that
a new weight input can be taken to update the auto-preact
function.
stable 1.0 The maximum peak to peak percentage difference in flow
rate used to detect a stable flow rate.