SERVO function block algorithms
Original instructions
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Algorithm without positional feedback
In this case the SERVO function block assigned to the controller allows astatic control. The algorithm uses the output alteration OUTD rather than the controller’s absolute value output OUT. The output RAISE (or LOWER, depending on the modification sign) is set to 1 for a certain time. This time is proportional to the valve opening time (t_motor) and the modification value OUTD.
The formula enters an initial theoretical value for the length of the pulse (T_IMP) to be sent to the output:
The following still applies for T_IMP (the length of the pulse sent to the output):
If
Then
T_IMP < t_mini
the block does not generate a pulse, but stores the value for the next calculation. This allows correct processing of control applications in which the controller’s output modifications are weak but continuous.
To ensure that pulses which are too short are not generated, the pulses to be sent to the output are limited to a minimum length t_mini.
the PID controller is in manual mode,
T_IMP is calculated continuously at every cycle. The calculation takes into consideration the time periods with a limit of t_motor which have previously been calculated, but not yet assigned. In this way any PID controller output modification can be considered even if the pulse lasts several cycles.
the PID controller is in automatic mode,
the function block SERVO always recalculates the parameter T_IMP if the controller updates its output, i.e. whenever SEN is set to 1. In this operating mode the previously calculated time periods are no longer considered.
Algorithm with positional feedback
The algorithm is very similar to the previous case.
In place of the PID controller output modification the SERVO function block uses the variance between the PID controller absolute value output and the positional feedback (IN - RCPY).
The function block carries out controlling, in which the PID controller output corresponds to the nominal value and the positional feedback RCPY to the actual value.
In contrast to the algorithm without positional feedback, in manual mode the function block stores the time periods, which were calculated previously, but are not yet locked onto the RAISE and LOWER outputs.