Problem with drive speed offset compensation by PLC programme

[toninlg]

Hi,

I'm trying to compensate the speed offset of a drive (when analog command is 0V, speed is not 0) with a PLC programme. I count the number of encoder pulses for x seconds then I calculate the speed and then the voltage to compensate the offset. I try counting for 1 second but that don't work : the motor rotates in the opposite direction, because the calculated voltage is wrong. I try counting for 10 seconds, it seems to work but not all the time. Sometimes the motor also begins to rotate in the opposite direction. Do you have an idea what could be wrong or what I am not doing well?
The encoder is 4096 pulses per rev.

Thanks a lot.

[Al_The_Man]

What PLC? and are you using an analogue module and a counter module for encoder?
Why is the speed not zero when the command is zero?
The balance control on the drive should be adjusted first to achieve zero speed whith zero voltage at analogue input. (jumper input terminals in order to zero).
What type of drive, Velocity (tach F.B.) or Torque Mode?
Are you trying to set up a PID loop? their are dedicated PLC cards for this purpose.
Al.

[toninlg]

It's Beckhoff modules. Analogue module for the drive command (velocity command) and an encoder module that takes A & B encoder signals to count the edges. The speed is not zero, because there is an offset in the drive. There is an internal parameter to compensate it, but I prefer to compensate it by software. There is no PID loop for the moment. I just read the counter (encoder module) to calculate the offset voltage.

[zimbricd]

What type of drive are you using? Which BECKHOFF PLC are you using?

I am an application engineer for BECKHOFF USA and I have just started building my own CNC router for use with TwinCAT NCI. I will be visiting this forum regularly so please feel free to respond with questions. I will have many of my own questions regarding the CNC machine process.

Dave Z

[toninlg]

They are IDC B8000 digital brushless servodrives. I'm using an Ethercat coupler. I'm translating an existing BASIC software that uses PC23 indexer boards (step-dir commands) in order to use Beckhoff modules with analog command to the servodrives. The new PLC software uses directly the count from the encoder module, do some work and outputs the command to the analog module. Perhaps the solution would be to use Twincat NC PTP to do all the regulation? I have been working on other projects for some months and I can't remember all things but I tought there would be a problem doing this : there is a ball and socket joint, so the moves are restricted in two directions so the positions of 3 out of 4 axes are somewhat linked. I can check this before sending a new position to these 3 axes but I have no control on the real output from the positionning NC function blocks so the limit of the ball and socket joint could be gone past?

I hope my explanations are not too confused.

Thank you.