Absolute encoders


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    Member rick027's Avatar
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    Default Absolute encoders

    i have a question for you guys , i have a cnc router with nema23 hybrid stepper motors . the breakout board is the C10s my drivers are KL-4030's and im also running the C15 relay board...i would like to add absolute encoders to my stepper.. is there any encoders available to do this.

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    Community Moderator Al_The_Man's Avatar
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    Default Re: Absolute encoders

    Why absolute?
    What advantages do you see?
    And to steppers?
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Member rick027's Avatar
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    Default Re: Absolute encoders

    Because I want it to know it's postiin is at all times..



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    Community Moderator Al_The_Man's Avatar
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    Default Re: Absolute encoders

    The only thing you may gain is not having to register zero (home) at power up.
    After this it is the same.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Default Re: Absolute encoders

    There may be a bit of confusion here...

    Absolute encoders output a position value, usually many bits wide (although some may have a serial interface). For the same physical position, they will output the same value; and this value is unique.

    Relative encoders - the most common type - output a movement signal; usually two bits wide (and sometimes, especially for rotary encoders, with an "index" bit that goes active once per rotation). The signal they output is in what is called "quadrature" encoding. This movement signal is kept track of by the electronics that it is connected to; in order to translate the movement signal into a position value. Usually the translation is done in hardware; but if the machine moves slow enough and/or the processor is fast enough, it can be done in software (for example, encoders used for the user interface knobs of many products - like car stereos - are often managed in software).

    Absolute encoders never need to be "homed" - they always output the same position value for where they are. Relative encoders (as the name would indicate) do need this; as the system can tell exactly how far it has moved; but needs to be told where it's starting from. Open loop systems, such as the stepper system you have now; are also completely relative - you need to perform a homing cycle at power up.

    Very few CNC systems have the ability to interface to absolute encoders; those that do tend to be very high end. All the rest are set up to use relative encoders; because as Al_The_Man said, the only functional advantage for a typical CNC is not needing a homing cycle.

    Which control software are you using? If I recall correctly, Mach 3 needed a plugin to do what you want; TurboCNC didn't do it at all; I think LinuxCNC needed hardware assistance (such as a Mesa FPGA card) but as I'm not a LinuxCNC user I'm not sure on that one. At this time, Grbl has no ability to interface directly to encoders; to use them they would have to be interfaced to the motor drivers (like the "closed loop" stepper drivers now available; or something like a G320 from GeckoDrive, which is a servo motor system).





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    Default Re: Absolute encoders

    Quote Originally Posted by __Britt View Post
    There may be a bit of confusion here...

    Absolute encoders output a position value, usually many bits wide (although some may have a serial interface). For the same physical position, they will output the same value; and this value is unique.

    Relative encoders - the most common type - output a movement signal; usually two bits wide (and sometimes, especially for rotary encoders, with an "index" bit that goes active once per rotation). The signal they output is in what is called "quadrature" encoding. This movement signal is kept track of by the electronics that it is connected to; in order to translate the movement signal into a position value. Usually the translation is done in hardware; but if the machine moves slow enough and/or the processor is fast enough, it can be done in software (for example, encoders used for the user interface knobs of many products - like car stereos - are often managed in software).

    Absolute encoders never need to be "homed" - they always output the same position value for where they are. Relative encoders (as the name would indicate) do need this; as the system can tell exactly how far it has moved; but needs to be told where it's starting from. Open loop systems, such as the stepper system you have now; are also completely relative - you need to perform a homing cycle at power up.)
    You have some misconceptions with Encoders

    If an Encoder has many Bits wide positioning, it would be no good for anything ( 1 Bit 1024 ) that's 1024 PPR

    Absolute Encoders you have to Home just the same as Incremental Encoders, there are 2 types of Absolute Encoders one that remembers position for a short while which has a built in power memory storage to keep it active for around 1 hour, and Battery powered which will remember the position when the power goes off

    This does not mean you don't home the machine because it has Absolute Encoders, a good example of this is any Hass mill they use Absolute Encoders, and the machine won't do anything until it is Homed

    Most all CNC Machines use Absolute Encoders

    Any non Hobby CNC system have no problem interfacing with Absolute or Incremental Encoders, on all good systems the Encoder goes first to the Servo Drives then to the control which can output anything to suit the control

    Mactec54


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    Default Re: Absolute encoders

    Quote Originally Posted by rick027 View Post
    Because I want it to know it's postiin is at all times..
    You should of brought stepper system with Encoders already, that close the loop in the stepper drive, or a Ac Servo system

    Mactec54


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    Member Jim Dawson's Avatar
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    Default Re: Absolute encoders

    Quote Originally Posted by rick027 View Post
    i have a question for you guys , i have a cnc router with nema23 hybrid stepper motors . the breakout board is the C10s my drivers are KL-4030's and im also running the C15 relay board...i would like to add absolute encoders to my stepper.. is there any encoders available to do this.

    What do you plan to connect the encoders to? The C10 boards have no encoder inputs, and the drives also have no provision for encoder connection, absolute or otherwise.

    Jim Dawson
    Sandy, Oregon, USA


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    Community Moderator Al_The_Man's Avatar
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    Default Re: Absolute encoders

    Absolute Encoders you have to Home just the same as Incremental Encoders,
    Jeez!! all the robots that I have programmed must have been wrong, all had absolute encoders, which is the industry norm for robots (Fanuc, Mitsubishi etc)
    All continued functioning without homing at power up!
    The vast majority of run of the mill CNC milling-turning machines have historically always used incremental encoders, GE Fanuc, Mitsubishi, Siemens, down to the PC based Galil motion, all these require a homing routine.
    It is only in recent history where the big names have gone to using sine wave co-tangent encoders, and these also do require homing.
    Also it is a misconception that you need a double feedback loop, the drive can be a simple trans conductance amplifier and the PID loop passed to the controller only.
    This has been proved many times.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.


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    Member rick027's Avatar
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    Default Re: Absolute encoders

    Britt ,you hit it right on the button... That's what I want, I want it to know it's postion and also don't wanna keep homing the machine ...I want to start it up and go



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    Member rick027's Avatar
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    Default Re: Absolute encoders

    Well mactec54 at the time I bought these I wasn't thinking about putting encoders on , but know after three years I'm deciding to put some on if I can



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    Member rick027's Avatar
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    Default Re: Absolute encoders

    Well Jim that's why I was asking if I could do it with the stuff I already have. If I can't I can't, I would have to buy new drivers etc.



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