Description of the regulation functions
SERVO Function
General The SERVO function supports regulation with a motor type actuator steered by 2 alternating actions (UP and DOWN).
Note: It must be connected in tandem with the analog output of a PID. It cannot be used alone.
When there is a copy of a position, the valve’s position is locked via the INP
(setpoint) and POT (position measurement) inputs.
When the copy does not physically exist, the algorithm no longer uses the PID’s absolute output but the output’s variation. The UP output (or DOWN, according to the variation sign) is put at 1 for a length of time proportional to the actuator opening time and to the variation of the value. Also the notion of minimum pulse time is introduced.
Principle of operating with a position copy
The SERVO function locks the motor’s position according to a setpoint of the INP position from a PID’s output on the [0-10000] format, and to a POT position measurement.
The locking algorithm is a relay with hysteresis.
In this case, the PID, T_MOTOR and T_MINI parameters are not used.
SERVO
PV
UP
OUTP INP
PID
+
SP
-
DOWN
POT HYST
Note: The description of the parameters used is presented in the (See
Programming the SERVO function, p. 374) module.
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TLX DS 37 PL7 xxE
Principle of operating without a position copy
(POT= -10000)
Description of the regulation functions
In this case the SERVO function synchronizes itself with the upstream PID via the bias of the PID parameters table, passed in parameter to the SERVO function.
The algorithm receives in input the PID’s variation output and converts it into pulse period, according to the following formula:
T_IMP (shown in 10
-3
s) = OUT x T_MOTOR / 1000
The acquired period adds itself to the remaining period of the preceding cycles: in fact, what is not "consumed" in a cycle is memorized for the following cycles. This ensures a smooth operation especially with sudden variations of the (ex: PID setpoint level) command and in manual mode.
Note: The description of the parameters used is presented in the (See
Programming the SERVO function, p. 374) module.
TLX DS 37 PL7 xxE
371
Description of the regulation functions
Example The example offered below is done in Ladder language.
+20%
+2%
+2%
+24%
OUT
1 2 3 4 5
+22%
UP
DOWN
5 s 1 s
1 s
T_MOTOR = 25 s
T_MINI = 1 s
Caption:
1.
The PID variation output is of +20% (T_MOTOR pulse =25 s for a 100% variation), in this case the pulse affects the UP output for a period of 5 s,
2.
The PID variation is of +2%, which would correspond to a pulse of 0.5 s. This pulse is less than T_MINI (=1 s.), and it does not affect the outputs,
3.
A second variation of +2% appears and the function holds this variation concurrently with the preceding one (which corresponded to a variation less than the minimal value) for its calculation, which corresponds to a positive global variation of
+4%, and therefore to a pulse of 1 s on the UP output,
4.
A variation of –24% appears and the activated pulse is therefore of 6 s on the
DOWN output,
5.
Before the following second, another variation of +22% brings the system back to a global variation of 2% < to the variation of T_MINI (4%). The function finishes carrying out the minimal pulse of 1 s.
Note 1: The SERVO function does not manage the position limits, these must be managed by the application. If a limit is detected, you must force the output corresponding to 0 (UP for the high limit, DOWN for the low limit).
372
TLX DS 37 PL7 xxE
TLX DS 37 PL7 xxE
Description of the regulation functions
Example: (done in Ladder language)
(* Management of limits *)
Limit_up
Limit_down
OPERATE
SERVO(Outp,%IW3.1,%Q2.1,%Q2.1,%Q2.2,%M>>)
%Q2.1
R
%Q2.2
R
Note 2: Passing to the operating mode with copy to the mode without copy is possible (for example: when the copy has an error, go to mode without copy).
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