AMALGAMATED INSTRUMENT RM4-IV, Operation And Instruction Manual

Page: 45 / 49

7.11 Relay frequency modulation control mode
To use pulse width modulation control
A
x.
Fr
must be selected at the
A
x
OPEr
function. In
frequency modulation mode the relay on time is fixed. A minimum relay off time can also be set.
The control program will vary the actual off time to suit the error seen between the setpoint and
the measured temperature at the time. For example if extra dosing is needed to reach the setpoint
then the off time will be reduced resulting in more on pulses per period of time i.e. the frequency
of the pulses is controlled to allow the setpoint to be maintained.
Relay
Relay
on
on
off
off
time
time
Frequency control - pulse frequency varies according to settings and control requirement
Frequency PI control operation has many functions in common with PI pulse width control, refer
to the appropriate sections as shown below for these common functions.
A
x.
SP
(Control setpoint) - refer to section 7.2
ctrI SPAN
(Control span) - refer to section 7.3
A
x.
PG
(Proportional gain) - refer to section 7.4
A
x.
IG
(Integral gain) - refer to section 7.5
A
x.
IL
(Integral control low limit) - refer to section 7.7
A
x.
IH
(Integral control high limit) - refer to section 7.6
A
x.
bs
(PI control bias) - refer to section 7.8
A
x.
dc
(PI control cycle period) - refer to section 7.9 . In frequency mode this function sets the
minimum off time. If set to
0
the relay will be disabled. The control program can extend the off
time to maintain the setpoint but not reduce it. If a 100% error is seen then the pulse rate will
be at its maximum i.e. the off time will equal
A
x.
dc
. If a 50% error is seen there will be a pulse
every 2 times
A
x.
dc
. For a 25% error there will be a pulse every 4 times
A
x.
dc
and for a 10%
error there will be a pulse every 10 times
A
x.
dc
.
RM4IVMAN-2.1-0 45 of 49