QUESTION: What should you watch out for when calling and assigning parameters to the controller functions in function blocks FB41, FB42 and FB43? ANSWER: Function blocks FB41, FB42 and FB43 are contained in the STEP 7 standard library under PID blocks. When using these blocks, please pay attention to the points below. 1. FB 41 "CONT_C" l The controller must be called in a watchdog interrupt OB (e.g. OB35). l The "CYCLE" parameter corresponds to the sampling time. The time between block calls must be parameterized here. l The controller's default setting is manual mode (MAN_ON=true). The control loop is interrupted. l In order to switch from manual mode to automatic mode without any jolts, at least 2 block runs need to be performed in manual mode. l The block should be run once with "COM_RST" = true when the CPU is rebooted. l The "COM_RST" parameter can be used to block the controller. 2. FB 42 "CONT_S" l The controller must be called in a watchdog interrupt OB (e.g. OB35). l The "CYCLE" parameter corresponds to the sampling time. The time between block calls must be parameterized here. l The controller's default setting is manual mode (LMNS_ON = true). l The block should be run once with "COM_RST" = true when the CPU is rebooted. l The "COM_RST" parameter can be used to block the controller. l The "PULSE_TM" and "BREAK_TM" parameters are to be configured to a complete multiple of the "CYCLE" parameter. 3. FB 43 "PULSEGEN" l The pulse generator must be called in a watchdog interrupt OB (e.g. OB35). l The block should be run once with "COM_RST" = true when the CPU is rebooted. l The "CYCLE" parameter corresponds to the sample time. The time between block calls must be parameterized here. l FB 43 can only be used in conjunction with FB 41. l The "PER_TM" parameter must be configured to the sampling time for the controller (FB41). l The call time for the FB41 is reached via a clock distributor. |
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