Positioning in Beckhoff compared to Siemens

[aand74]

The system of position control is quite different in Beckhoff compared to Siemens.
In a Beckhoff controller with Beckhoff drives on Ethercat, the CSP profile transmits each cycle the position to the drive. The drive interpolates and generates a velocity from that position signal.
In siemens with drives on Profinet, when DSC is used, a velocity signal is sent to the drive as velocity precontrol and a position error is sent to the drive for positon control in the drive.
Does someone knows the pro's and con's of both system?

 

[Peter Nachtwey]

I am a motion control expert, but I have NO IDEA what CSP or DCS is. I don't keep up with acronyms that other manufacturers make up. I am a purist. I am not a fan of systems that download just the positions every millisecond. This forces the motion controller or drive to calculate the target velocity and acceleration from just the positions. This requires fine position feedback and data that is time stamped or is acquired at CONSTANT intervals. Mathematically they should be equivalent but then there are implementation details.

Downloading velocities every millisecond means one only needs to differentiate once to get the acceleration.

The target velocity and acceleration are necessary for calculating the velocity and acceleration feedforwards.

If I were designing these systems, I would have the master download the jerk each millisecond. The jerk can then be integrated by the drive to generate target acceleration, velocity, and positions.

I thought that Ethercat could download more information at a time since the packet size is no longer to the CAN Open packet size.

 

[Zensequitur]

CSP = Cyclic Synchronous Position

Regards,
Patrick

 

[HTFU]

If you are considering using CSV instead of CSP because newer cards and software default to it, don't. It's quirky and barely works. Get your cards, set them to CSP, compile, download, activate, do it again, because it probably didn't work the first time, and then make sure it's set to CSP. Never look back. Save yourself the grief of wondering what CSV does.
As I understand it, in between communication cycles from the controller PLC to the servo controller card, the controller card takes care of business itself, because it has to update every few microseconds, but the controller won't communicate again for the next x milliseconds. In that interleaving time, the controller card will control on velocity (CSV), position (CSP) or torque(CST). I found no reason to care after configuring maybe 100 servo motors on 50 different machines over 10 years. Maybe you will, I don't know. But I never did. But I do remember that every time I veered away from CSP, "weird things" would happen. It also alters which of the tuning parameters the system ignores, but doesn't make it clear which ones.

Stay on CSP. Unless you really really really strongly suspect you need to not do so. It just seems to work with fewer "wtf"s.