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  1. #11
    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
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  2. #12
    Quote Originally Posted by Jonathan View Post
    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.

    Yes I know all this and fully agree, I also admit I may have miss read what you meant but my initial point was that Servo's offer nothing more other than linear torque than these Closed loop Stepper systems do unless you close the Loop properly at the controller which like you say is another can of worms we both don't want to open.

    Whether or not you can find servo's cheaper I think is a close call and I'd say not unless cheap chinese offerings and certainly not respected brand names Like Leadshine unless second hand. Then you also have the other can of worms of Pulse generation and what's needed to connect too and get the rated speed out of those servos, which makes those potentialy cheaper servo's suddenly not so cheap.!!

    It's here where servo's don't match these drives for ease of use and setup which to your average DIY user with limitied knowledge and time is quite important. . . . They essentially just plug n play and offer just the same accuracy.!! . . . Which was my whole initial point.

  3. #13
    In the spirit of this constructive discussion i want to say something more. When looking for motors after i decided i will go with cheap Chinese servo. Then i saw a Leadshine AC 220v servo at ali axpress. As leadshine seemed at that time to be a better choice i was ready to buy until i read this review: 06/01/2009 - Servo Drive Review and Comparison

    It seems the jump should be straight to good brand servos, as the cheaper servos have some inherent problems and lacking of their own.

    What i wonder after i read the review for the Leadshine servo is about the closed loop stepper and its control. It seems quite good on paper, but i still couldn't find a review from a real life implementation on a machine. I mean, would it stand well to what it promises?

    There is also one possible combination that i believe will be nice, better and maybe even cheaper. Closed loop stepper/ stepper with encoder/ + second hand motion control board with its brand stepper drives , usually in the form of tight fit daughter boards or in the same enclosure. That sounds better to me , if one is ready to jump in the head ache world :-)

  4. #14
    Again, this is a totaly out there question,

    Have been reading of people using servos as spindles, can not quite get my head around the pros and cons. Am wandering if i large one of these closed loop stapper motors, might be usable as a spindle under certan curcumstances.

    What am i missing ?

  5. #15
    Quote Originally Posted by george uk View Post
    What am i missing ?
    Torque and speed.
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  6. #16
    HI Jonathon

    would that be the same problem with a servo as a spindle or just these type steppers. I was considering options for slower cutting speeds for stone, and i can not find a reasonably prices spindle that has the torque at lower speeds,

  7. #17
    Normally servo motors have a constant torque rating up to a particular speed - often 3000rpm. For the stepper motor system that speed will be lower, however you can't really make a decision without comparing the available systems. Work out what torque vs speed curve you require, then just look which type of motors gets it for the least money.
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  8. #18
    A Vector drive will give full torque at zero speed, but with an asynchronous motor not a servo or stepper.
    Have a read, look at the vector drive section.
    http://www05.abb.com/global/scot/sco...14482_RevG.pdf
    Last edited by EddyCurrent; 24-01-2014 at 02:28 PM.

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  10. #19
    Quote Originally Posted by EddyCurrent View Post
    A Vector drive will give full torque at zero speed, but with an asynchronous motor not a servo or stepper.
    Using vector control (also known as, FOC - field oriented control) also gets rated torque to zero speed with synchronous motors. Servo motors are generally permanent magnet synchronous motors and they will use FOC and hence can output rated torque down to zero speed, so long as the motor cooling is sufficient.
    Stepper motors are less well defined in their ratings - with FOC they will output some value of torque between zero speed and some value, however this torque value isn't generally quoted in the datasheet, so it is unwise to make a general statement except that this value will be lower than the holding torque.
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  12. #20
    Some servos i have seen go to 6000 rpm or even more, so 3000 is not the limiting speed, when speaking about servos. But the answer to your question is the people use them on mills, belt driven , not cnc for wood. Some use them belt geared also for bestial rotary axis, where at the same time the axis can perform lathe function.

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