Hardy HI 1756-nDF - Manual (Page 24)

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Chapter 4
point. The preact value can be positive or negative, to compensate for in flight material,
to ensure the set point operates at the correct absolute net weight value.
The relay’s initial state, OPEN or CLOSED, is determined by mode of operation. The
dynamic state of the relay during a fill cycle depends on the comparison of the current net
weight to the calculated set point value. The set point value is based on three parameters
the target weight, the preact weight, and the mode of operation. These parameters are
defined below:
1. The “target weight”
The target weight is a floating point number indicating the nominal weight value that
should OPEN or CLOSE the relay depending on the mode of operation.
Regardless of mode it is recommended that each fill cycle should start with a TARE
command to set the net weight to zero.
2. The “preact” value
In order to compensate for delays caused by sensor reaction time or in flight material,
there is a preact parameter. The preact value is subtracted from the target weight so
negative preact values will increase the set point weight value with respect to the target
weight value, and positive values will decrease the set point weight value with respect to
the target weight value.
3. The “mode of operation”
In the following description when the relay is OPEN the external valve will be CLOSED,
and when the relay is CLOSED the external valve will be OPEN.
The initial state of each relay and the set point calculation depends upon the mode of
operation. Once a relay has been closed it will not be opened until the next start or restart
of a fill cycle.
The sequential mode is defined as the two relays working sequentially; with relay RA
being active before relay RB. In this mode the two relays cannot be CLOSED at the same
time. This mode could be used to select between two different flow rates into one hopper.
The parallel mode enables each relay to operate independently enabling two feeders at
different speeds to be used to fill a single hopper with the same ingredient.
The different modes of operation, for each weigh scale input, are detailed in the following
table; where the Ref# value refers to the mode value used to select the desired mode of
operation.
Ref# Relay Mode Description of Relay Operation
0 Single Relay
Operation Relay RA is set to the CLOSED state at the beginning
of the fill cycle. Relay RB is held in the OPEN state.
When the absolute net weight value is greater than or
equal to the SPA value relay RA is set to the OPEN
state.
RB is the refill relay.
SPA = target weight TA – preact PA
SPB = 0.0