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  1. #21
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    Mariss your description makes perfect sense, you put a step change in load on your stepper and an error exists until the integral component of the PID loop accumulates enough weight to cancel the error. You mention that you can't feel cogging, that is the load on stepper is less than the minimum torque output between steps and the control loop is doing it's job, actually it demonstrates impressive loop response frequency and a well tuned loop. If you can do it with steppers imagine how one of those Kollmorgan servodisc motors will perform AFAIR 0 - 3000 RPM in 60 degrees with zero cogging

    You seem to have got the angular error under control as well.

    A test is to get the stepper driving a viscous load such as a stirrer or better still a cylinder in a pot of syrup or smooth brake and plot the loop error signal, the loop error signal gives the magnitude of the position error of the stepper, as the load increases the cyclic position error will increase slowly until the load exceeds the minimum torque of the stepper then there should be a marked increase in the cyclic position error but the motor will still be able to drive the load at the correct average speed. A purpose built servomotor will have a much smaller cyclic position error because the mimimum and maximum torque of the motor are almost identical. To illustrate this get a stepper and connect all the winding wires together and try to spin it, the steps will be very marked, do the same with a DC or AC servo and it just feels stiff to turn. The stepper still has the cogging, that is inherent in the design but your control loop is fast enough and precise enough to cover it to a large degree. It is very similar to running a DC motor off a poorly filtered supply using a PWM controller with feedback, the feedback loop has an error signal with the supply ripple frequency superimposed but this error signal allows the PWM to filter out most of the ripple.

    The point I was making, was that even with a good feedback loop a stepper will not have the same position accuracy as a motor with little or no inherent cogging, to eliminate the cogging the feedback loop needs an error signal to get this error signal it actually has to have an instantaneous position error.

    Are there any resonance problems with the stepper driving a typical load? Dynamic loads with spring and lost motion tax even the best control loops.



  2. #22
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    I'm nowhere near running the battery of tests you suggest. Of course I will but first I have to have a complete and functional servo. What I've got now is a breadboard that can only run bits and pieces of the actual servo. That is because I'm using an 8-bit processor to emulate what eventually will reside in a CPLD. The processor cannot run the whole servo, it isn't nearly fast enough.

    Step motors do not exhibit cogging. A well designed motor has a torque that is the vector sum of its phase currents. If those currents are sine and cosine then the vector sum is a constant and independent of angular location.

    Mariss



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    Step motors do not exhibit cogging. A well designed motor has a torque that is the vector sum of its phase currents. If those currents are sine and cosine then the vector sum is a constant and independent of angular location.

    Mariss
    An asumption in that analysis is that the magnetic mataterial used in the stepper exhibits constant u vs B at the flux densities in question.

    Next is the difficulty getting a clean sinusoidal current drive of a stepper, I tried last night with a simple R C phase shift circuit fed from a high voltage source to swamp as much of the motor impedance as possible. Result a dismal failure, 4th harmonic voltage distortion was very high, which translated to high current distortion on the phase fed by a capacitor. Not having a current mode audio amplifier I couldn't proceed further. I am sure a current mode 4 quadrant PWM circuit running at 10 or 20x the stepper frequency would be a lot better but I am probably getting off topic.

    Last edited by svenakela; 09-08-2007 at 05:12 AM.


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    Helical Cut,

    A good microstep drive has to drive the motor with accurate sine and cosine phase currents. We keep harmonic distortion under 1%. Once you have an accurate drive then you can test the motor for its linearity. There are very simple and accurate ways of doing that involving only a laser pointer and a mirror sliver. You will find good motors exhibit 1% linearity or better for their electrical to mechanical transfer function.

    Mariss



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    Default Post #14 Very insteresting.....but?

    Quote Originally Posted by Mariss Freimanis View Post
    .....
    I guess what needs explaining first is why even bother selecting a step motor as a servomotor candidate. It certainly is a difficult motor to run closed loop.....

    .......Now let's contrast the two motor types. A NEMA-34 servomotor is rated at 1,000 Watts and a similar size NEMA-34 step motor can deliver only 200 Watts. In the above scenario only 10% of the servomotor's power can be delivered to the load so it is effectively a 100 Watt motor. The step motor can deliver 100% of its rated power. It is effectively twice as powerful in this application.......


    Mariss
    Until I read Post #14 I could not see any valid reason for considering a stepper motor for a closed loop position servo. Mariss, your points are valid and very convincing!

    My only question/concern, is that my experience with servo loops (primarily DC motors*) is that yes, during steady state movements they do run at low utilization of the motor's torque/speed capabilities...BUT in high performance servo loops its usually the acceleration/deceleration performance that separates the boys from the men. That is where the high torque/high speed performance of the DC motor will outperform a stepper in closed loop mode.

    What do you think?

    John

    *I was the Chief engineer and electronics designer of the original SkyCam flying camera system.



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    John,

    I'm honored to make your acquaintance; I'm impressed with your work.

    Of course I pulled the blanket my way a little bit to emphasize a point. Your comment is valid but it also has real limitations that keep it from being as good as it sounds. Please see the attached gif which shows a typical DC servomotor's loadline (speed-torque curve) using the step motor convention of Y-axis for torque.

    A typical DC servomotor has a peak stall torque 5-times the continuous torque rating. Peak stall torque is available at zero RPM only. From there it decreases linearly with RPM until it equals the continuous torque rating at 80% of no-load RPM.

    This torque is usable for transient loads such as accel/decel but at a considerable cost in controller complexity.

    1) Assume a linear accel slope and an intent to reach the rated RPM: The acceleration slope must not exceed the the continuous torque rating of the motor, otherwise a following error will develop as speed approaches rated RPM. Acceleration torque is proportional to the 1st derivative of velocity, which is a constant if accel is a linear ramp. That leaves the motor's "reserve" torque completely unusable.

    2) Assume you want to fashion an acceleration profile that completely utilizes this "reserve" torque. The resulting accel curve would have to be the 1st integral of the motor's loadline, i.e. a parabolic function having a slope of +5 at the origin and a slope of +1 at the rated RPM. Practical implementations of this optimized profile yield a little below 1/2 the time-to-speed as a simple ramp does.

    Mariss

    Attached Thumbnails Attached Thumbnails EMC+closed loop?-dc-motor-loadline-jpg  


  7. #27
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    "A typical DC servomotor has a peak stall torque 5-times the continuous torque rating. Peak stall torque is available at zero RPM only. From there it decreases linearly with RPM until it equals the continuous torque rating at 80% of no-load RPM."

    That must be low end servos, the worst I could find in my datasheets were Baldor M series with about 4.5:1 peak to continuous rated torque, the best ones were kollmorgan U series servodisc motors with 3.8:1 peak to continuous rating. The Kollmorgan motors had no peak torque reduction with increasing RPM. The Yaskawa Minertia series had about 50% peak torque reduction with increasing RPM mostly from losses. That is a long way from 80% torque reduction.

    As for the complexity of driving a DC servo in a manner which can use all it's transient capacity, less than for a stepper, the steppper needs 2 current mode H bridges comprising of 8 power devices, and like mentioned in a previous post, it also needs to provide sine wave output. Contrast that to a DC servo drive which only needs 1 H bridge, preferably current mode as well as well as bus voltage control a total of 5 power devices on a 1 axis machine and no complex PWM sine wave generation. A high performance stepper drive has more power side electronics than an AC servo even.

    The fastest accelleration for the Kollmorgan U series servodisc motors of 0 - 3000 rpm in 1/6th of a rev or 1.8e6 r/sec2 These motors are ironless, they could accellerate even faster if the makers switched to aluminium conductors in the rotor. On further reading of individual motor specs the kollmorgan motors peak to average torque was more like 10:1 I believe that is one way they could get such high accelleration. The peak was specified for 1% duty which IMHO is not a typical CNC cycle, moving down to 4% duty would halve the accelleration quoted. If a stepper could get even 10% of this accelleration it would be competetive with an average DC servo.

    Last edited by HelicalCut; 09-09-2007 at 04:17 AM. Reason: Inaccuracy + spelling mistake


  8. #28
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    If your looking for following error detection (lost steps) then emc can do that right now. (someone finally tried it )

    http://www.cnczone.com/forums/showthread.php?t=43517

    He has set the following error down to .002" so far.

    Quote Originally Posted by grebator View Post
    Hi,guys!

    Does EMC has got closed loops?
    Can i make closed loops with stepper motors?

    EMC+C10-bidirectional breakout board+step drivers+step motors with encoders




  9. #29
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    Most CNC systems will use the motor encoder for a velocity feedback and use the linear glass scales for positioning. Excess backlash or ballscrew wear will cause following error. That is the difference between where the control thinks it should be and where it is......Linear scales give true axis mechanical position and motor mounted encoders only give motor position.



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    ok, I have read though alot of this and have a pretty good understanding of the way things work. I have a few questions about a couple of the nuts and bolts at the heart of the wiring and such.

    please corect me if i make a mistake and fill in that blank stare i have.

    1) emc2 will run closed loop through just a parallel port.
    2) it does this with use of the encoder feedback into the parallal port.
    3) it outputs step and direction to be used in the servo or stepper driver (I have gecko 320's for my servo's)

    in my case this is where i get a little hazy on things...
    the gecko's have to have encoder feedback on the 320's, right? would it have a connection to the encoders that are used to feed ecm2? or would emc2 feed it something?

    I have a large mill withgood old servo dynamics boards in it as well, it would require +15 and -15v signals if my memory serves me correctly, how would i get from point A to a running machine if the output from emc2 is step and direction?

    Where would the Pluto parallel board fall in to all of the works?

    I realize that you guys were talking about steppers but with Mariss somewhat active in this thread and the gecko's being in his area of expertice as well as the emc closed loop questions, hope my quesion is welcome.

    Danny

    Last edited by dannystooblue; 01-04-2008 at 08:44 PM. Reason: adding more


  11. #31
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    if your looking at using gecko servo drives with emc2 - I would look at a couple things from pico systems..
    http://jelinux.pico-systems.com/gecko.html
    this is a gecko interface that works with his universal stepper controller (hardware step generation) http://jelinux.pico-systems.com/univstep.html It hooks the encoders up so that emc knows if there is lost steps
    I will not explain it here because it is pretty self explainitory on his site. This runs from the printer port.

    As far as your other question.. Emc will control servo drives that take +/- 10 volt (+/- 15 seems a bit odd.. you would have to look over emc's supported hardware and see if some of the control cards will do +/- 15v). Then you would use encoders or such back to the interface card for feedback.

    http://wiki.linuxcnc.org/cgi-bin/emc...orted_Hardware


    sam



    Quote Originally Posted by dannystooblue View Post
    ok, I have read though alot of this and have a pretty good understanding of the way things work. I have a few questions about a couple of the nuts and bolts at the heart of the wiring and such.

    please corect me if i make a mistake and fill in that blank stare i have.

    1) emc2 will run closed loop through just a parallel port.
    2) it does this with use of the encoder feedback into the parallal port.
    3) it outputs step and direction to be used in the servo or stepper driver (I have gecko 320's for my servo's)

    in my case this is where i get a little hazy on things...
    the gecko's have to have encoder feedback on the 320's, right? would it have a connection to the encoders that are used to feed ecm2? or would emc2 feed it something?

    I have a large mill withgood old servo dynamics boards in it as well, it would require +15 and -15v signals if my memory serves me correctly, how would i get from point A to a running machine if the output from emc2 is step and direction?

    Where would the Pluto parallel board fall in to all of the works?

    I realize that you guys were talking about steppers but with Mariss somewhat active in this thread and the gecko's being in his area of expertice as well as the emc closed loop questions, hope my quesion is welcome.

    Danny




  12. #32
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    I am trying to figure out how the everything it tied together, can it be done without any extra expensive cards?


    Danny



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    Quote Originally Posted by dannystooblue View Post
    I am trying to figure out how the everything it tied together, can it be done without any extra expensive cards?
    Danny
    A low-performance closed-loop machine that used max. 50kHz step or PWM output and around 50 kHz maximum encoder counting rate could be run from the parallel port.

    For example the m5i20 PCI-card will give you all the performance you need for around $200.



  14. #34
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    Cool Pluto options?

    Quote Originally Posted by dannystooblue View Post
    ok, I have read though alot of this and have a pretty good understanding of the way things work. I have a few questions about a couple of the nuts and bolts at the heart of the wiring and such.

    please corect me if i make a mistake and fill in that blank stare i have.

    1) emc2 will run closed loop through just a parallel port.
    2) it does this with use of the encoder feedback into the parallal port.
    3) it outputs step and direction to be used in the servo or stepper driver (I have gecko 320's for my servo's)

    in my case this is where i get a little hazy on things...
    the gecko's have to have encoder feedback on the 320's, right? would it have a connection to the encoders that are used to feed ecm2? or would emc2 feed it something?

    I have a large mill withgood old servo dynamics boards in it as well, it would require +15 and -15v signals if my memory serves me correctly, how would i get from point A to a running machine if the output from emc2 is step and direction?

    Where would the Pluto parallel board fall in to all of the works?

    I realize that you guys were talking about steppers but with Mariss somewhat active in this thread and the gecko's being in his area of expertice as well as the emc closed loop questions, hope my quesion is welcome.

    Danny
    OK - This is a kinda double edge sword - because Gecko's (320 or 340) are only closed loop between the servo and the gecko amp - position feedback into the PC will require additional hardware. Gecko servo drives act like open loop stepper drives from the PC's point of view.

    One possible option might be using dual Pluto parallel port units on 2 parallel ports - 1 Pluto running a Pluto step config to drive the Gecko's, a second to read the quad from the servo encoders to the PC.

    You might be able to omit the Pluto step unit and run from a para port via BOB if your step rates are not too demanding or you are using a lower count encoder.

    Bottom line is that it is hard to beat the Pico Gecko interface board - without ending up spending as much on a small pile of alternative hardware to get the same effect - and you have to work out all the config - where the Pico unit is basically turnkey.

    I have no direct involvenemt with any of the above named - except that I own Pluto boards, Gecko 320's and 203V's and I plan to get a set of Pico servo amps when my next machine is ready for them.

    ( Lately though I've been on a tooling buying binge, so I have not had much to spend on the retrofit. )



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    Thanks guys, I was almost sure this thread had died a painful death. I have piles of drives, screws, linear rails and such. I just can't seem to find time to put them all together. The mechanical part is easy for me to figure out but the electronics just amazes me.


    Danny



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    Default Into the deep end

    I havejust joined the forum. although it had been a source of information for the past half a year or so.

    On the issue of closed loop soltions; I am using EMC2 single paralele port configured as basic stepper to generate step and direction pulses.

    EMC2 is driving a brushless servo motor linear scale combination.

    The electronics are suplied by a local ( Australian company ) where an external servo loop is established in hardware and EMC provides some supervisory functionality and trajectory path information.

    This approach offers simplicity of stepper motor drive and advantage of direct position measurement of the axes.

    The cost of motors is approx USD 50.00 and the cost of servo amps is approx USD 110.00

    The machine is still in development stage however worst case following error on the heaviest of axes is a count of less than 20 steps ( less than 0.1 mm or 0.004")

    Machine theoretical max velocity is 331mm based on PC latency figures.

    I am nort sure if I ought to mention brand names so let them stay namelss for now.



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    Quote Originally Posted by fenn View Post
    while the previous posts are all quite correct, they leave out some important points that should help clear up some misconceptions around this issue. i'm going to elaborate on unterhaus' "theoretical issue" around the use of step/dir signalling to run a motor in closed loop.

    (skip this paragraph if you know how PID works)
    when a servo motor is commanded to go to a certain position, nothing would happen if it weren't for the PID controller. let's say we are at 0 and want to move to 1 inch. first the motion planner commands "1" and the motor is still. The difference between commanded and actual position (following error) is 1 inch. This is multiplied by the P value, (which was set to say 500 by the machine builder) and then output through a digital-to-analog converter, which usually has somewhere around 10 bits of resolution. This high-resolution signal is amplified and converted into high current to actually put torque on the motor. Now the motor picks up speed and it's cruising along at a good rate. We're at 0.5" now and still have 0.5" to go. P is still adding 250 whatsits to our torque command but we want to slow down before we overshoot the 1" mark - the torque command should be negative! This is where the D term comes in - it slows down the motor by reversing the torque command when we are going too fast. If P and D aren't both set properly, the motor will be sluggish or will oscillate wildly and overshoot the endpoint. Even if P and D are set right, the torque command must faithfully represent their wishes to the motor amplifier, or you get the same problems.

    Now, stepper motors by comparison have a very simple control scheme - if you aren't there yet, take another step. While a microstepping drive can have a position resolution approaching that of a low end servo, its control resolution is still far inferior. Let's say you had a 10x microstepping stepper (2000 steps/rev) with a 250 line encoder (1000 counts/rev) and you are behind by one encoder count, or two steps. The driver's options are: take one step, take two steps, take three steps, etc. If you take one step, you advance the counter one position in the stepper drive, which changes the currents in the coils a little bit, and gives a tiny amount of torque to the motor, which would be enough to move the motor to the desired position eventually, but it won't get there as fast as the motor is capable of doing. If you take two steps it advances the balance-point a little more, and you get a little more torque. Now as soon as you try to take three steps the balance point ends up overshooting the goal point, so you have to take a whole lot of steps backwards all of a sudden once you want to start slowing down. Essentially what you have is a two-bit torque command with a lot of delay built in. This delay is what really hurts the concept.

    It takes a certain amount of time to send those step signals. If you're using cheap (free) parallel port hardware with a maximum step rate of, say 50KHz, (a generous value, and ignoring jitter, latency, and other nasties) then taking 3 steps forward will take 60 microseconds, and taking 6 steps backwards will take another 120 microseconds. We're starting to get into the range of frequencies where our not-so-rapidly changing magnetic fields will actually move the motor backwards and forwards, so there is a limit here in how far you can advance or retard the balance point in order to get more or less torque. This makes for a sluggish control that also tends to overshoot. And we still haven't gotten to the main problem.

    The big problem is that PID doesn't work for steppers. In a servo system, when your 2" endmill hits a large chunk of cast iron, the following error increases and the drive pumps out more current to compensate. This puts more torque on the motor and gets it back on track. If we have a stepper with an encoder, when the endmill hits the chunk of cast iron it puts a torque on the stepper that is higher than the torque that holds it at the balance point and it falls back into the previous "well" a full 10 microsteps in the wrong direction! The following error builds up, and in response the drive commands an extra three steps to increase torque. But a stepper loses torque as it speeds up (due to coil inductance and eddy currents in the iron from the 200 magnetic poles), so the torque pushing back on the motor is already higher than the torque that would cause it to accelerate forward, so it continues on at the same speed. You've just lost a full step but can't do anything about it!

    To be fair, there are some things you could do: 1) slow down the commanded feedrate so it appears to the drive that it's accelerating relative to the commanded position. this should work using something like realtime "adaptive feed" but it has to work fast, your other axes have to decelerate suddenly to stay synchronized, and i've never heard of anyone doing this before 2) add a "boost current" signal to the stepper drive to give the motor more torque for a brief period to recover steps. unfortunately, most drives don't come with this option. also, you might damage the motor's magnets by pushing too much current through it at once, since high performance steppers generally run right on the edge.

    Although closed loop control of steppers may theoretically be possible, all these problems combined with the fact that you will end up needing specialized hardware anyway means that you should just buy the right stuff to begin with.

    The different approach that mariss was talking about is to send a torque or velocity command to a specialized motor driver instead of step/direction. If you go that far, you might as well just design your drive for real AC servo's with a reasonable number of magnetic poles in the first place, and leave the steppers alone.

    I for one would really like to see some inexpensive AC servo drives on the market...
    *glances over at the pile of sanyo denki's*
    Hi,
    I came across to this thread as I was looking for some answer relating to Closed Loop system before I proceed assembling my lathe machine.
    Has anyone tried Mach3 or EMC using AC servos with encoder/linear scale fadded to the table to give table feedback in addition to motor feedback?

    Part list:
    1 x Mitsubishi 3.5KW AC servo + driver to drive Lathe Spindle
    2 x Yaskawa AC servo for Z axis and X axis.
    2 x Heidenhan Linear scale LS 486 + APE 454 (similar to APE 371)
    About the controller board, either Kflop or other boards that can handle feedback.
    Mach 3 or EMC2 or LNC controllers.



  18. #38
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    I am using brushless dc motors, renishaw 20 micron linear scale ( 5 micron resolution) with EMC2 on a gantry type of milling machine



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    Hi Zig,
    Do you have a video of your machine running? can I have a look?
    I was wondering if I still need Linear scale if I use AC servo Motors, what do you reckon?

    Freddy



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    Freddy,

    The choice of ( servo) motors is totaly divorced from choice of encoder.

    You actually have a choice of shaft encoder or linear scales.

    Within linear scale range of products there are a number of options:

    enclosed transmissive glass scales

    open reflective linear scales

    enclosed magnetic linear scales

    open magnetic scales


    In general Your choice of scales will be determined by required resolution, PRICE, integration method ( how You mount the scales onto the machine) and nature of environment You will work in.

    Optical scales will be subject to interference by dirt particles magnetic scales may be more tolerant of dirt but subject to feromagnetic particle interference.

    There is absolutely no reason why You could not use rotary shaft encoders unles You would need to know the position in a more direct form available with a linear scale.

    Rotary shaft encoder would be cheaper however the screw would need to be of a higher grade than a linear encoder.

    Rotary shaft encoder can be used with a timing belt drive linear scale may cause oscillations if used with a sloppy screw drive or belt drive.

    So to round off on Your question. AC servo motors are fine with either type of position measuring system. It really dependes on what process needs.

    On the second point no I do not have a video.. and i do not have a means of generating one.



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