7 Setting up the relay PI controller
The Relay Proportional + Integral Controller can be made to operate in either pulse width control
or frequency control mode via the Ax OPEr function. Note that the Ax OPEr function will not
be seen until a value has been set for the low or high alarm e.g. for A1Lo or A1Hi. The best
results are usually achieved by initially configuring as a “Proportional Only” controller and then
introducing the Integral functions when stable results are obtained.
Relay 1 and , if fitted, relay 2 can be set to operate in PI control mode. Any other relays fitted will
only operate in normal, non PI operation. The “x” in the Ax OPEr and other functions indicates
the chosen relay i.e. for relay 1 the display will show A1 OPER, A1.SP etc. The Ax OPEr
function allows three choices of operating mode for the chosen relay, namely Ax.AL, Ax.tP and
Ax.Fr. If Ax.AL is selected the chosen relay will operate as a setpoint relay whose operation is
controlled by the AxHi, AxLo etc. settings and the PI control settings will not be seen. See the
“Explanation of functions” chapter for details of operation when Ax.AL is selected. If Ax.tP is
selected then the chosen relay will operate in pulse width control mode. If Ax.Fr is selected then
the chosen relay will operate in the frequency control mode.
Pulse width control - operates by controlling the on to off time ratio of the relay. In a typical
application this would be used to control the length of time for which a dosing pump is switched
on during a control cycle i.e. the pump or other device will continuously operate for the length of
time the relay is activated and will stop operating when the relay is de-activated.
Frequency control - operates by changing the rate at which the relay switches on and off. In a
typical control application the frequency control operation is particularly suited for use when one
shot dosing is used i.e. the pump or other device puts out a fixed dosing quantity for every pulse
received.
7.1 Relay pulse width modulation control mode
To use pulse width modulation control Ax.tP must be selected at the Ax OPEr function.
Relay
Relay
on
on
off
off
time
time
One control cycle
set by xA dc
Pulse width control
Proportional gain increased
leading to greater on time
for a given error.
Proportional gain
at a given value
PM4IVMAN-2.4-0 45 of 56
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