AB PLC5 PID Manual Ctrl

WhatThe · Dec 11, 2003

working with AB PLC5/40, we have a couple of vfd's that are controlled by flow until additional equipment is running and then control switches to pressure. I also want to be able to control vfd speed manually via HMI. I believe that this can be accomplished using the Set Output Mode (word 0 bit 4) and moving word 16 (CV output) from the active PID into word 10 (set output) of the inactive PID. If manual control is required from HMI then move the HMI manual output value into both the PIDs at word 10.

please let me know if this even in the ball park. I would also appreciate any links or example logic.

rsdoran · Dec 11, 2003

I am not sure I got all of that BUT if the PID's are inactive...ie nothing else can write to the word that sends the command to the VFD...YES

Just verify that when controlling manually nothing else can control the VFD AND that when done in manual that you restore it to PID/auto operation.

firebrd10 · Dec 12, 2003

Depending on your system, You may want to cook up some type of bumpless transfer. Otherwise, when you switch between modes you may get some pretty weird and abrupt movements in your system.

Tim

msinclair · Dec 12, 2003

What I do is to have an intermediate value that is the actual analog ouput or messaged word (called "Actual Output" in the MOV's below) for the device. Then by sequencing my "control logic" something like this, I get the results that I think you are looking for.





    Mode 1 (Flow)

------| |---------------------------+-----------------+

                                    |             MOV |

                                    |   PID1 Ctrl Var |

                                    |   Actual Output |

                                    +-----------------+



    Mode 2 (Pressure)

------| |---------------------------+-----------------+

                                    |             MOV |

                                    |   PID2 Ctrl Var |

                                    |   Actual Output |

                                    +-----------------+



    Manual

------| |---------------------------+-----------------+

                                    |             MOV |

                                    |    Manual Value |

                                    |   Actual Output |

                                    +-----------------+

Obviously you want some conditioning to make sure that there aren't any huge step changes that the device can't handle (and that the values to be sent are within range).

WhatThe · Dec 12, 2003

I guess what really has me confused is the Tieback vs Control Output Mode.

In the past I have just set the bits for Manual and Control Output Mode and then copied my Control Output % from the HMI into the Control Output Word.

Should I also be using the Tieback......please explain.

Thank you.

AB PLC5 PID Manual Ctrl

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