On Fri, 18 Apr 2008, Kirk Wallace wrote: > Date: Fri, 18 Apr 2008 21:09:31 -0700 > From: Kirk Wallace <[EMAIL PROTECTED]> > Reply-To: "Enhanced Machine Controller (EMC)" > <emc-users@lists.sourceforge.net> > To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> > Subject: Re: [Emc-users] Bandit Steppers > > On Fri, 2008-04-18 at 22:13 -0500, Jon Elson wrote: > > I read this at Wikipedia: > "A new development in stepper control is to incorporate a rotor position > feedback (eg. an encoder or resolver), so that the commutation can be > made optimal for torque generation according to actual rotor position. > This turns the stepper motor into a high pole count brushless servo > motor, with exceptional low speed torque and position resolution. An > advance on this technique is to normally run the motor in open loop > mode, and only enter closed loop mode if the rotor position error > becomes too large -- this will allow the system to avoid hunting or > oscillating, a common servo problem."
Certainly not new... In our limited experience, for low speeds (normal stepper range) this works quite well, though a multipole 3 phase motor would be better as the drive circuitry would be simpler. We had no problem with oscillation. Encoder feedback requires a high resolution encoder( we used 4096 count), and a quite high sample rate (we were using 40 KHz) SNIP > > > Please correct me if I am wrong, because I am making some of this up. > With single stepping one stator winding acts on the rotor. The rotor and > stator poles match well so there is a tight sweet spot. With half > stepping, some of the steps use one winding an others use two, so their > torque is nearly twice as much except the rotor poles are trying to > compromise between two stator poles so there is a wider sweet spot (?). > I would think micro-stepping is just like this, except you get to try to > set where the compromise is. I don't know what quarter stepping is, but > if it adds a third winding in the mix, I would think the sweet spot > would get wider, so even though the maximum torque is high, the > stiffness at the ideal position is probably not so good. I may be wrong also but other than the detent torque, I dont think the static stiffness is any different with microstepped or single stepped drives SNIP > > But, I don't think you can tune a real servo system down to the last few > encoder counts, or am I wrong? > At low speeds or predictable loads, and a high enough sample rate, +- 1 count is possible, +- 5 or so more likely. Because of the integral term (with its effectively infinite gain), its easy to force the error down to nothing in the static or very slow motion case. At intermediate speeds where the integral time constant slows the response to small profile errors, or any speed with fast load variations, you will get larger uncorrectable errors (because they must be corrected by the P and D terms which have limited gain). I tend to think in electrical terms, so basically the servo system has an extremely low output impedance (high stiffness) at low load frequencies (due to the integral term), A higher impedance at intermediate frequencies (due to the finite gain of the P/D terms), and a high impedance at load frequencies above the servo systems frequency response (but this is counteracted by the mass and friction of the machine) > -- > Kirk Wallace (California, USA > http://www.wallacecompany.com/machine_shop/ > Hardinge HNC/EMC CNC lathe, > Bridgeport mill conversion, doing XY now, > Zubal lathe conversion pending > Craftsman AA 109 restoration > Shizuoka ST-N/EMC CNC) > Peter Wallace Mesa Electronics (\__/) (='.'=) This is Bunny. Copy and paste bunny into your (")_(") signature to help him gain world domination. ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
