Examples of function block SERVO

Example overview

In this section the use of the function block SERVO is shown in the following examples:

Automatic mode with positional feedback

The example shows the behavior of the function block in automatic mode with positional feedback. If the SEN input is set to 1 (every 4 s in the example), the function block SERVO always takes a new variance value IN-RCPY into account.

The following parameter specifications hold:

Parameter	Specification
t_motor	25 s
t_mini	1 s
sampling period	4 s

Timing diagram (automatic with positional feedback)

Timing diagram for automatic mode with positional feedback

Explanation of the timings

Explanation of the marked positions:

Position No.	Explanation
1	The variance IN-RCPY is 20%: a pulse of length 5 s (=20% of 25 s) was generated at the RAISE output.
2	The variance is still only 10%, a pulse of 2.5 s (= 10% of 25 s) was generated at the RAISE output; the second left over from the previous pulse is not taken into account.
3	The deviation now amounts to -2%, which corresponds to a pulse of 0.5 s on LOWER. Since t_mini is 1 s, no pulse is generated (the duration of 0.5 s is saved however).
4	The variance is still -2%, but the corresponding pulse (0.5 s) is added to the previously stored pulse to make 1 s. The length corresponds to t_mini, so the pulse is locked onto the LOWER output.

Programming example (automatic with positional feedback)

Representation the function plan, part 1

Representation the function plan, part 2

OUT_RCPY

Process value of the valve positional feedback

Example of operating mode automatic without positional feedback in manual mode

The example shows the behavior of the function block in automatic operating mode without positional feedback in manual mode. In this case the INPD value for each execution of the function block SERVO is taken into account, irrespective of the value of the SEN input.

The following parameter specifications hold:

Parameter	Specification
t_motor	25 s
t_mini	1 s

Timing diagram (automatic without positional feedback)

Automatic mode without positional feedback in manual mode

Explanation of the timings

Explanation of the marked positions:

Position No.	Explanation
1	The modification of the PID control output is +20%, in this case the pulse affects the RAISE output and lasts 5 s (= 20% of 25 s).
2	The modification of the PID controller is +2% which corresponds to a pulse duration of 0.5 s. The pulse is less than t_mini (=1 s.) so it does not influence the outputs.
3	At the second modification of +2 %, the function adds this modification to the previous one (which corresponds to a variance which was below the minimum value), which corresponds to a positive total modification of +4 %, i.e. a pulse of 1 s at the RAISE output.
4	For a modification of -24 % the pulse at the LOWER output is 6 s
5	Before the end of the following second a further modification of + 22 % leads to a total system modification of 2 %< modification of t_mini (4 %). The function ends with the minimum pulse of 1 s.

Programming example (automatic without positional feedback)

Representation of the function plan, part 1

Representation of the function plan, part 2

TC2_DEFF

Error output of the process value TC2: If TC2 is faulty, the servoloop is forced into manual mode.