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Thread: Taig CNC Mill, round two

  1. #11
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    I don't know that this has any bearing, but a friend with an old Bridgeport with a Heidenhain controller had a system failure. He ordered a Centroid (not Acorn) complete replacement package. (quite a lot more than an Acorn controller) He said he had it running in less than 24hrs after receiving the kit, and was cutting jobs after some testing a day later. He was doing a running commentary on the CB forums at the time so it seems likely he was not exaggerating.
    *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

  2. #12
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    You had asked this in the closed thread.

    Linuxcnc is the motion controller. It has to run in a realtime environment. External interface cards like mesa lowers the bar a bit for the computer. (latency can be higher - aprox 100us vs if you did printer port software step generation you would want < 20us)

    Linuxcnc has the 'sense, model, act' So 1000 times a second (every ms) linuxcnc
    Reads all inputs (including position feedback) (sense)
    plans trajectory, ladder logic, pid, etc (model)
    writes outputs including torque/velocity commands. (act)

    and the 1000 times a second is settable.. (although works for most machines)

    sam


    Quote Originally Posted by 3 Phase Lightbulb View Post
    You mentioned you needed realtime kernel extensions for your system? Why is that? Doesn't the ethernet based controller decouple any/all real time requirements?

    Edit: Unless the host is reading scales/feedback out-of-band and latency from reading the scales would cause servo/stepper error oscillation/etc?

  3. #13
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    Quote Originally Posted by skunkworks View Post
    You had asked this in the closed thread.

    Linuxcnc is the motion controller. It has to run in a realtime environment. External interface cards like mesa lowers the bar a bit for the computer. (latency can be higher - aprox 100us vs if you did printer port software step generation you would want < 20us)

    Linuxcnc has the 'sense, model, act' So 1000 times a second (every ms) linuxcnc
    Reads all inputs (including position feedback) (sense)
    plans trajectory, ladder logic, pid, etc (model)
    writes outputs including torque/velocity commands. (act)

    and the 1000 times a second is settable.. (although works for most machines)

    sam
    There is an offboard Ethernet card from MESA now. Tormach is using it, although I don't know if they are using exclusively yet. My 1100 controller came with a 5i25.
    *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

  4. #14
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    Quote Originally Posted by skunkworks View Post
    You had asked this in the closed thread.

    Linuxcnc is the motion controller. It has to run in a realtime environment. External interface cards like mesa lowers the bar a bit for the computer. (latency can be higher - aprox 100us vs if you did printer port software step generation you would want < 20us)

    Linuxcnc has the 'sense, model, act' So 1000 times a second (every ms) linuxcnc
    Reads all inputs (including position feedback) (sense)
    plans trajectory, ladder logic, pid, etc (model)
    writes outputs including torque/velocity commands. (act)

    and the 1000 times a second is settable.. (although works for most machines)

    sam
    Ok, I thought positional information was simply sent down to the controller and the controller itself handled the movement, PID correction, etc. It sounds like the computer is doing that and the controller (that you're talking to via Ethernet) just does PWM and doesn't close the loop by itself?
    Work hard play hard

  5. #15
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    Got a meeting. Be back in ~1 hour
    Work hard play hard

  6. #16
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    Many closed loop drivers (all for Mach 3 that I am aware of) close the loop between the motor and the driver. Te driver has an error output only that can be hooked up to an input on the BOB to trigger a fault condition.

    LinuxCNC actually has the capability to close the loop to the control software itself. Skunkworks or one of the others would be better able to answer how that is done.

    The G540 is not capable of closed loop. Its a "package" of a parallel BOB a couple low current relays and 4 relatively low power (3.5amp 48VDC max) open loop stepper drives all in one unit. It may be possible to close the loop directly to the control not going through the G540 with LinuxCNC, but again Skunkworks or one of the other long time LinuxCNC guys could better answer that.
    *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

  7. #17
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    Exactly. The computer handles the 'closing of the loop'

    Quote Originally Posted by 3 Phase Lightbulb View Post
    Ok, I thought positional information was simply sent down to the controller and the controller itself handled the movement, PID correction, etc. It sounds like the computer is doing that and the controller (that you're talking to via Ethernet) just does PWM and doesn't close the loop by itself?

  8. #18
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    Quote Originally Posted by Bob La Londe View Post
    Many closed loop drivers (all for Mach 3 that I am aware of) close the loop between the motor and the driver. Te driver has an error output only that can be hooked up to an input on the BOB to trigger a fault condition.

    LinuxCNC actually has the capability to close the loop to the control software itself. Skunkworks or one of the others would be better able to answer how that is done.

    The G540 is not capable of closed loop. Its a "package" of a parallel BOB a couple low current relays and 4 relatively low power (3.5amp 48VDC max) open loop stepper drives all in one unit. It may be possible to close the loop directly to the control not going through the G540 with LinuxCNC, but again Skunkworks or one of the other long time LinuxCNC guys could better answer that.
    The Acorn has a connector on it that is pin compatible with the G540. I do not know if the Acorn will close the loop, probably not. Open loop I suppose is fine as long as the microstepping is kept reasonble and the steppers have plenty of torque?

  9. #19
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    Quote Originally Posted by RB211 View Post
    The Acorn has a connector on it that is pin compatible with the G540. I do not know if the Acorn will close the loop, probably not. Open loop I suppose is fine as long as the microstepping is kept reasonble and the steppers have plenty of torque?
    Yes. The G540 was originally designed as a direct parallel connection. Its got a DB25 connector and the inputs and outputs correspond directly to a duplex parallel port. Many controllers in order to simplify connections and retrofits have analogous inputs and outputs on the same pins. UC100/300 Smoothstepper and many others. Actually a Smoothstepper has connection analogous to 2 parallel ports and inputs for a third. Even a MESA card can be configured as parallel replacement although its much more flexible than that.

    Open loop is fine if the setup is really good or the settings are conservative. At the moment I am running 5 open loop stepper machines and they cut quite good parts. I only have one closed loop machine and its got massive DC brush servo motors with encoders. Its a beast though. That one throws around over a thousand pounds of saddle and table.

    Like I said I may have drunk the Koolaid on closed loop stepper systems. I do have a couple complete closed loop steppers systems for builds I am working on.
    *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

  10. #20
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    Yes - plenty of machine run this way without an issue. (open loop where the control just assumes it is going where it should)

    Linuxcnc does open loop just fine too.. (using step/dir, quadrature, whatever..) The mesa 7i92 would also plug right into G540. (out of the box it acts like 2 very high speed printer ports)

    Another linuxcnc'ism.. Internally - linuxcnc treats steppers as a closed loop. (that is why if you don't setup linuxcnc correctly - you can actually get following errors with steppers)

    This does allow linuxcnc to use other feedback with steppers (scales, encoders, resolvers... ) You can have it trigger a simple following error - or you could get more fancy.


    Quote Originally Posted by RB211 View Post
    The Acorn has a connector on it that is pin compatible with the G540. I do not know if the Acorn will close the loop, probably not. Open loop I suppose is fine as long as the microstepping is kept reasonble and the steppers have plenty of torque?

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