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7.4
Setpoint Control
As mentioned above, there is one analogue output that is used for command setpoint
control that can be used to operate a mass flow controller or similar. The setpoint
command output has a voltage full scale that matches the input channel configuration.
Example
If the input channel is setup for a 100 slpm 5V full scale device, and the setpoint value is
10.0 (assuming setpoint is in Auto mode and not a slave source), then the output voltage
of the setpoint would be 0.5V.
The setpoint does have some configuration parameters that can be used to alter the
function of the setpoint control and they are detailed as follows:
(a)
Setpoint Mode
The setpoint can be configured in one of three modes – Auto, Open and Closed.
·
In Auto mode the setpoint output is dependant on the setpoint source and value
settings (see below).
·
In Open mode the setpoint outputs a voltage greater than the full scale of the
device. For a setpoint command full scale of 5V or less, the output in this mode
is 7V. For any other setpoint command full scale (e.g. 10V), the output in this
mode is 12V.
·
In Closed mode the setpoint outputs a voltage less than the minimum output
voltage of most devices, the setpoint output voltage is –0.25V.
(b)
Setpoint Source
The setpoint source dictates where the setpoint value comes from, assuming the
mode is set to Auto (see above). This can be one of two possibilities – Internal or
Slave.
·
Internal source – the setpoint uses the value set internally via the ‘
▲
’ & ‘
▼
’
switches or via the externals comms.
·
Slave source – in this case the setpoint uses a percentage of an externally
produced value. The percentage is set via the ‘
▲
’ & ‘
▼
’ switches and the external
value comes from the secondary input channel (as mentioned 7.2 in above)
7.5
User Rezero
Over time it is possible that the input may ‘drift’ slightly due to various conditions
(temperature changes, etc.). As such it may be necessary for the channel to be rezeroed
by the user.
The user rezero function is provided for this task. It simply takes the current reading
(sampled and averaged over 3 seconds) and uses that as an additional offset for the
channel in question, subtracting the value from all subsequent readings. Ensure any
process value to be zeroed, is in fact truly zero before performing this function. This would
mean isolating flow devices or fully pumping a pressure device.
Note that the user rezero via comms also provides the facility for clearing any user rezero
value that may have already been set. This should be used before any input calibration is
performed to ensure that the calculated calibration points are not distorted by the user zero
offset.
