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  1. #11
    The encoder is 1000 line, this is 4000 PPR.
    Typical ac servo motors have at least 2500 line encoders and that is 10000 PPR.
    While there are incremental encoders with a higher resolution than 2500 line, they are not normally found in the back of an AC servo motors, and if a higher resolution is needed, then you would use a Sine / cosine encoder.
    DC brushless servos are normally on the smaller size. (up to about 200W)
    DC servo are normally brushed and are a thing of the past, but are still used on some applications.
    Quote Originally Posted by Jonathan View Post
    The term 'servo motor' is generally used for DC or brushless DC motors, which tend to have a low pole count compared to a stepper motor, so they are quite different in that respect, however the control systems are similar. The 'smoothness' is greatly dependent on the resolution of the encoder, which is where 'genuine' servo motors are likely to be better since good ones come with much higher resolution encoders - 16-bit and 20-bit are common, compared to only 10-bit (1000ppr) on the stepper motor linked to. To put it simply, with a higher resolution encoder the controller is getting more information about the motor, so is able to compensate for errors, and smaller positional errors.

  2. #12
    Quote Originally Posted by Gary View Post
    The encoder is 1000 line, this is 4000 PPR.
    Fair enough, I misread it. Either way it's a big difference.
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  3. #13
    I see, thanks for the explanation.

  4. #14
    Another difference is that (AFAIK) steppers with encoders are still steppers, so torque drops off fast over a few 100 RPM. Servos (AC at least) tend to go to a few 1000 rpm before torque drops off.

  5. #15
    Quote Originally Posted by Tenson View Post
    Also is the smoothness Jazz mentioned not simply to do with the motor? I wouldn't think the encoder will be responsible for that.
    Encoders just give positional feedback and don't really add anything to the smooth action just the positional accuracy. High resolution encoders just gives better accurecy, provided the motor and drive can resolve to that level.
    This resolving power is where True servo's are better.! These Hybrid systems still use Steppers and although these are 3 phase and decent quality they are still in principle steppers that spring from one step to the next and it's for this reason they don't put very high Res encoders on them.? . . .It would be pointless because the motors can't resolve that high.

    AC or DC Servos don't have this restriction and the drive/motor combination can resolve to much higher degree's. This is also why they tend to spin at least 2x has fast and give linear torque.
    They also cost 5 times the price and are 3 times harder to setup and much more touchy regards tuning, which if not setup correct will give much rougher performance than any stepper.!!

    The reason these Hybrid motors work so smooth is due to combination of 1.2deg 3 phase motors and Digital drives doing a very good job of controlling current/volts and Micro stepping.
    Digital drives alone make for much smoother running motors than analogue anyway so the 1.2deg and 3 phases just add another plus.
    The clever bit is the fact the closed loop is done in the drive not the controller which means it doesn't matter if the controller doesn't support closed loop like Mach3 doesn't(yet) you still have a closed loop system for not a lot of money and none of the hassles of PID tuning etc that comes with servos.

  6. #16
    I was tempted to try these out, while investigating the stepper/encoder combo I found a few other options one of which is to use a motion control card like a kflop or galil.

    Kflop has its own program like mach3 or you can use the kflop as a plugin within mach3. You still need to add your own encoders, but this gives you options to use linear scales or rotary encoders. Nothing beats a mechnically backlash free and super rigid machine but it would be an amazing breakthrough if systems like these could make up for some of the sloppyness in machine builds.
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  7. #17
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 8-9 years. Has a total post count of 1,311. Received thanks 126 times, giving thanks to others 3 times.
    A major advantage of servos is they give far better acceleration compared with a stepper.
    I looked at the leadshine closed loop system, and it's a very cost effective way to get good reliable positioing, but I've personally put that on hold for other reasons.

    And in the encoder count debate, the DMM-tech servos use a 14bit absolute position encoder, and the drive converts it to incremental if you need encoder feedback. However that is the only motor/drive combo I came across in the lower power servos that I looked at, that used an absolute encoder.
    I spent a good bit time a couple weeks ago weighing up the options, and am just waiting to sell a few more bits to buy the first of the servos (not from DMM, as similar power servos/drives can be had over here for similar money)

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