Optical Linear Encoders

Discussion in 'General' started by jdix, Jan 6, 2016.

  1. jdix

    jdix Member

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    Hi all,

    While waiting for a BigBox to arrive I stumbled across the following youtube vid:

    At ~3mins in it discusses the use of optical linear encoders to keep track of where the inkjet printing heads are rather than using stepper motors.

    As I'm new to 3d printing I was just wondering how come something like this isn't used, and what are the benefits of using stepper motors over linear encoders combined with dc motors? as the film implies they the linear encoders are easier to set up, and possibly more accurate

    Cheers

    James
     
  2. elmoret

    elmoret Administrator

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    DC motors and linear encoders are used in 2D printers largely because they're cheaper in large volume than a stepper + stepper driver. We don't use them in 3D printing because:

    1.) They're more expensive than stepper + stepper driver in smaller quantities
    2.) The closed-loop control algorithm tuning is fairly complex, accounting for all sorts of things from gear reduction backlash to DC motor dynamics. Usually this is done in the factory. As I'm sure you can imagine, that doesn't work as well for open source hobbyist level 3D printing.

    Additionally - though Matthias is an extremely talented woodworker, he is incorrect about the linear encoder. Contrary to what he said, that particular linear encoder won't offer more resolution (or accuracy) than a similar stepper. With a typical 3D printer design and 1/16 micro stepping, you'll see (theoretical) 15 micron resolution. The resolution of that linear encoder pictured is right around 100 microns, based on using his fingerprint ridges for scale. Yes, it is difficult to achieve the theoretical micro stepping resolution in practice due to reduced torque, but 2-3x theoretical is a pretty conservative estimate and that's still better than the linear encoder pictured. At the very least it is equal - certainly not worse.

    One advantage of servos is you can measure load torque. The New Matter MOD-t actually uses this to determine when the nozzle has touched the bed.

    Servos do also offer some advantages that are not of concern here, namely power consumption and mechanical power per motor weight/volume.

    tl;dr: steppers work well and are cheap, servos offer no considerable advantages in this application aside from cost which only comes into play when you're ordering by the containerload.
     
    #2 elmoret, Jan 6, 2016
    Last edited: Jan 6, 2016
    Mike Kelly likes this.
  3. jdix

    jdix Member

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    Hi Elmoret

    Thanks for the explanation. I thought there would be a good reason, but at least having a looked around for a bit nothing leapt out as to why one was used over the other.

    Cheers again
     
  4. UlrichKliegis

    UlrichKliegis Well-Known Member

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    In addition to elmoret's remarks: Steppers are much noisier than servos. Microstepping as implemented in all the cheap controllers of the 8825 etc. family has a slight phase wobble error since the current is divided linearly whereas it should be a more complex trigonometric function.
    Servos can be much faster, but the price goes up exponentially with speed. Steppers lose torque at higher speeds, they have their highest torque when standing still.Servos have a pretty even torque over the whole working speed range.

    A further differentiation is between AC and DC servo motors. DC servos resemble classical (brushed) electric motors while AC servos use a structure similar to the brushless (three phases) motors that model builders, drone makers and high speed mill- and drill spindle makers employ.

    AC servo drivers are highly complex, both in their circuitry and their programming. You have to translate all motion goals into a sequence of three exactly phase shifted currents. More than just switching on and off the current of a stepper coil. Nevertheless, where it is needed and the investment can be justified, they are absolutely superior to DC servos and even tenfold more to steppers.

    Most important for CNC applications: Servos don't miss steps. Steppers do when brought to their (comparatively low) limits.

    Look for the UHU servo controller - there is a lot of information, videos and tutorials around that project.

    [latest edit: 2016-1-08, 23:01 MET (UTC/ZULU +1.00): removed some typos, added maybe others, added some information, corrected wording and typos]
     
    #4 UlrichKliegis, Jan 7, 2016
    Last edited: Jan 8, 2016
    Mike Kelly likes this.

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