Endress+Hauser Mycom S CPM 153 Operating Instructions Manual page 55

Mycom S CPM 153
!
!
Endress+Hauser
contact
t
ON
t
OFF
on
off
period T
Fig. 18: left: pulse-width modulation (PWM)
Right: pulse-frequency modulation (PFM)
3. Three PS ("three-point step controller")
With the Mycom S, this type of control is only possible for one process side (acid or
alkali). With two-sided processes, either PWM or PFM must be used for the other pro-
cess side.
This type of actuator controller is intended for actuator drives (e.g. motor-driven valves,
etc.) where a motor must be controlled directly. For this, two relays are required: one
"+relay", which, by picking-up, opens the valve and one "–relay", which closes the valve.
So that the Mycom S can set a actuating variable of, for example, 40% (valve 40%
open), it requires the input of the time that the "+relay" is picked up to completely open a
completely closed valve (= "motor run time").
Note!
If using a driven valve, gate valve or similar, you must determine the motor run time,
before beginning with the menu settings.
4. Analogue (via current output 2, 20 mA)
The current output can be used to output the analogue actuating variable for one or two-
sided processes and cannot be combined with the method described above.
• With one-sided processes, the actuating variable range 0% ... 100%
(or –100% ... 0%) is represented on the selected current range (0 ... 20 mA or 4 ... 20
mA). The output current is proportional to the actuating variable.
• With a two-sided process, the complete actuating variable range from
–100% ... +100% is represented on the given current range. A actuating variable of
0% leads to a current of 10 mA (at 0 ... 20 mA) or 12 mA (at 4 .. 20 mA) (see Fig. 19).
Note!
With a two-sided process, it should be noted that the actuator uses this method (also
known as "split range".
Stroke
A
[%]
100
50
0
4 8 12
Fig. 19: A: Stroke diagram for a control valve
B: Stroke diagram for two contrarotating control valves ("split range")
You can refer to the following selection aids to find the required hardware equipment
level for your process.
on
off
time [s]
mA
16 20
contact
t
ON
Stroke
B
[%]
100
50
0
4 8 12 16 20
6 Start-up
time [s]
mA
55