wtw CL 298, Operating Manual

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pH 298, LF 298, Oxi 298, Cl 298
Outputs
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The minimum time can be set to adjust to different actuators. It it used to prevent
switching processes that are too quick for the actuator.
If it is undercut and the control parameter is outside the dead band, the actuator
is triggered with the minimum time.
If the switch-off duration is shorter than the minimum time, the relay remains
switched on without interruptions.
5.1.3 PID controller as pulse width controller ( PWM
)
The pulse width controller is linked with the 2 relay outputs (see characteristic
curve). Within the cycle time, a switching pulse is calculated depending on the
desired value deviation. The intervention in the control process takes place with
this switching pulse. The control value is recalculated with the start of every
period duration.
Pulse width output The output of the pulse width is, for example, used to trigger valves.
With pulse width output, the turn-on duration (t
On
) of the output signal is
changed. Depending on the position of the measured value in the proportional
range, the relay is operated for a longer or shorter time period.

If the measured value is at the end of the proportional band ( End value ), the
turn-on duration (t
On
) is long, the turn-off duration is short. The relay is oper-
ated longer.
 If the measured value is at the beginning of the proportional band
figure 5-5 Relay output of the pulse width output
The cycle time (T) consists of the turn-on duration and the turn-off dura-
tion (t
On
, t
Off
) of the relay. While the defined cycle time (T) remains con-
stant, the turn-on duration (t
On
), and thus the pulse width (v), changes
depending on the measured value. The pulse width can be set in the
range 0 % ... 100 %.
v = (t
on
/ T) * 100 %
v = pulse width, T = cycle time, t
On
= turn-on duration
T
Time [s]
Relay
Off
On
t
On
t
Off