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Entry type: FAQ, Entry ID: 18459342, Entry date: 06/23/2008
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What should you watch out for when calling and assigning parameters to the controller functions in function blocks (S)FB41, (S)FB42 and (S)FB43?

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Configuration notes:
The PID Control CONT_C, CONT_S and PULSEGEN control functions are available in two versions but with identical functions:

  • FB 41, FB 42, FB 43
    The loadable controller FBs are regular user blocks that run in all CPUs (S7-300, S7-400).
  • SFB 41, SFB 42, SFB 43
    The controller SFBs that are integrated into the operating system are in all S7-300 CPUs of the types 313C and 314C and in the C7 devices.

The controller blocks are available in the STEP 7 Standard Library (SIMATIC Manager: "File > Open > Libraries > ...") in the "PID Control Blocks" (FBs) folder or "System Function Blocks" (SFBs) folder.

When using these blocks, please watch out for the following points.

  1. FB 41 "CONT_C"
  • To guarantee a constant scanning frequency, the controller should be called in a cyclic interrupt OB without call-up conditions (e.g. OB35).
  • The "CYCLE" parameter corresponds to the scan time. The time between block calls must be configured here  (e.g. time base of the OB35).
  • The default controller setting is manual mode (MAN_ON=true). The control loop is interrupted. In this mode the variable matches the MAN manual value.
  • In order to ensure a smooth switchover from manual to automatic mode, at least two block cycles must be conducted in manual mode.
  • When the CPU is restarted, a block cycle should be conducted with "COM_RST" = true.
  • The "COM_RST" parameter can be used to block the controller.
  1. FB 42 "CONT_S"
  • To guarantee a constant scanning frequency, the controller should be called in a cyclic interrupt OB without call-up conditions (e.g. OB35).
  • The "CYCLE" parameter corresponds to the scan time. The time between block calls must be configured here (e.g. time base of the OB35).
  • The default controller setting is manual mode (LMAN_ON=true).
  • When the CPU is restarted, a block cycle should be conducted with "COM_RST" = true.
  • The "COM_RST" parameter can be used to block the controller.
  • The "PULSE_TM" and "BREAK_TM" parameters are to be set to a whole multiple of the "CYCLE" parameter.
  1. FB 43 "PULSEGEN"
  • To guarantee a constant scanning frequency, the pulse generator should be called in a cyclic interrupt OB without call-up conditions (e.g. OB35).
  • When the CPU is restarted, a block cycle should be conducted with "COM_RST" = true.
  • The "CYCLE" parameter corresponds to the scan time. The time between block calls must be configured here (e.g. time base of the OB35).
  • The FB43 can be operated together with a continuous controller such as FB41, but not with a step-action controller such as the FB42.
  • The "PER_TM" parameter must be set to the scan time value for the controller (FB41).
  • The call time for the FB41 is achieved via a clock distributor (for an example, refer to the Online Help for the FB43).

Note:
Since the OB1 does not guarantee a constant cycle time, no clear value can be assigned to the "CYCLE" parameter. If the "CYCLE" parameter does not match the scan time, then time-dependent control parameters (e.g. TI, TD) seem too fast or too slow.
You can find notes about descriptions and examples for the (S)FBs 41 - 43 in Entry ID: 16532187. The Online Help for the relevant block also contains useful advice.

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