. .

Threaded View

Previous Post Previous Post   Next Post Next Post
  1. #9
    I think you're mixing up two different control loops. Any servo motor drive with position control uses feedback to maintain the position error below a specified limit. The closed loop stepper system is just another type of servo system, hence both 'standard' servo motors and 'closed loop stepper' systems correct for position error. How well they can correct is dependent on the motor ratings, but with either system if you exceed the motor torque rating the result is the same - loss of position (i.e. position error outside limit for some time).

    Within the motor driver, the position command (step&dir) is converted to a speed reference using a PID (+possible other bits) controller, then into a torque reference using a PID (+other bits) controller, which is the input to the vector current control algorithm that uses a further two PI controllers to set the current within the motor. The speed measured from the encoder, or back-emf sensing, is fed into the PI controller and used to correct the position error. So you actually have several control loops inside one another. Sometimes people implement the position control loop inside the motion controller, and instead send speed&direction commands to the motor drive, but doing that is a separate can of worms. It is these PID controllers that are tuned when a servo motor is set up - the difference for the stepper system is they only come with a limited set of motors, which combined with the different characteristics of stepper motors means they get away without the user tuning the system for 'most applications'.

    Again, this control system is essentially the same (except for the 'other bits') for all the types of motors commonly used for servos, including steppers, and can be implemented without an encoder so long as the motor speed is sufficient.

    Quote Originally Posted by JAZZCNC View Post
    just sticking a Servo motor and drive on a machine using Mach3 makes it closed loop.!!. . . . NOPE. . .Why not.? . . . .Because Closing the loop is the function of the Motion Controller not the Servo drive or Motor.
    The servo motor+drive, or closed loop stepper+drive are themselves closed loop, but the system as a whole arguably isn't as Mach3 doesn't know what the following error is. When the fault pin from the driver is connected to an input to Mach3, then Mach3 knows when the error exceeded the set threshold (i.e. the motor was operated outside its ratings for too long) and can act accordingly. You can set it up this way for servos, stepper based servos and stepper drivers with stall-detection.
    Whether you close the position control loop within the motion controller (e.g. using LinuxCNC) or the driver is a separate debate to the one we're having now about the virtues of steppers and servo motors. I don't think that's worth discussing here, given that due to bandwidth limitations it's not viable using a parallel port which is probably the system most people reading this will use.

    I'm not saying there are no advantages to using the closed-loop stepper system, I'm just confident that they're not tangible for the vast majority of machines made on this forum and in the case where they are, a standard servo motor system is likely offer the same performance at a lower cost.
    Last edited by Jonathan; 22-01-2014 at 01:05 AM. Reason: excessive punctuation
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Controller for 8NM closed loop leadshine drivers
    By Matt81 in forum Motor Drivers & Controllers
    Replies: 5
    Last Post: 19-06-2014, 09:08 AM
  2. Testing Leadshine closed loop stepper system.
    By JAZZCNC in forum Stepper & Servo Motors
    Replies: 10
    Last Post: 13-10-2012, 05:39 PM
  3. Closed Loop Stepper Compared with Open Loop Stepper
    By Leadshine-Erci in forum Stepper & Servo Motors
    Replies: 0
    Last Post: 25-05-2012, 08:11 AM
  4. Closed Loop steppers
    By funkmonkey in forum Stepper & Servo Motors
    Replies: 24
    Last Post: 18-07-2009, 09:56 PM
  5. Droping Steps, closed loop ? HELP !!!
    By bobin in forum Machine Discussion
    Replies: 7
    Last Post: 20-05-2009, 03:56 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •