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OpenEtherCATsociety
CSP motor control logic
Goodmorning to all, I'm succesfully using SOEM with an Ethercat Panasonic driver MINAS-A6. So far I've always worked with PWM controlled motors hence I'm not sure about the right approach for ethercat driver motor, for example for accomplishing a trapezoidal movement (acceleration, max velocity and deceleration). Here my PDOs list is reported:
` TxPDOs: 603Fh Error code 6041h Statusword 6061h Modes of operation display 6064h Position actual value 606Ch Velocity actual value 6077h Torque actual value 60B9h Touch probe status 60BAh Touch probe pos1 pos value 60FDh Digital inputs
RxPDOs: 6040h Controlword 6060h Mode of operation 6071h Target Torque 6072h Max Torque 607Ah Target Position 6080h Max motor speed 60B8h Touch Probe function 60FFh Target Velocity 60B0h Position Offset `
Consider that in CSP mode Target Velocity is not used. For moving the motor with a trapezoidal movement I thought two different approaches:
- Keep the Max motor speed constant and modify only the target position. Clearly in the acceleration phase the increment is progressively increased, in the max velocity phase the increment is kept steady and in the deceleration phase is decremented up to the final target position.
- In the second approach, the target position is set equal to the final position from the very start and the several phases (acc, dec, max speed) are controlled using Max motor Speed only.
My questions are:
- What is the most suitable approach?
- can this approach be used for different movimentation types such as following another motor encoder as well?
I know that this topic doesn't concern soem but it'd be very useful for me. Thank you in advanced. Regards, Davidino
In CSP mode the strategy is to deliver a constant stream of position values which the drive should track as closely as possible given the constraints of maximum velocity and maximum torque. The assumption is that the time interval between two successive position target values is so small that the movement is continuous.
This mode works and is used in many solutions. However the downside is that you have very little control over velocity and torque. Because when you limit those values too much the position controller is no longer able to track position. When you set the limits too high the movement becomes less smooth (resembles a stepper motor). If this behaviour is acceptable then you have found your solution.
A mode with full control is CST. The application generates a trajectory (for one or multiple axes) and computes a suitable torque for each motor. A feedback loop stabilizes the velocity and position. It is more work, and needs a better understanding of the physics involved, but has much better performance.
