West Control Solutions KS98-1 - Manual (Page 230)

Two-point controller
Switching controller with two switching statuses:
1. Heating switched on; r output Y1 = 1
2. Heating switched off; r output Y1 = 0
E.g. for temperature control with electrical heating (inverse operation) or cooling (direct operation)
Adjust cycle time Tp1 as follows: Tp Tu 1 0 25
<= ·
,
With higher Tp1, oscillations must be expected. Tp1 corresponds to the minimum cycle time (time in seconds) at 50 %
duty cycle.
PD action
(
Tn = 0 = switched off Tn = Î)
The working point is in the middle of proportional band X
p1
at 50 % duty cycle. In order to keep the control variable
constant, a defined quantity of energy dependent of is required. This energy causes a permanent control deviation,
which increases with growing X
p1
.
DPID action
By means of the I action, line-out is without permanent control deviation.
The static characteristic of a two-point controller is identical to the one of a continuous controller, with the difference
that a duty cycle instead of a linearly variable current signal is output (relay contact, logic signal 0/20mA or control
output 0/24V).
Working point Y
0
and cycle time Tp1 at 50% are adjustable.
The shortest switch-on or switch-off time is 100ms.
Controller 9499-040-82711
III-230 Control behaviour
Xn100 Xn0 W0 Weff W100
100%
0%
Y1Â
Xeff
Xp1
actual
working point
Fig.: 55
Static operating principle of
a two-point controller
Output Y1 (heating)
Process value
Setpoint