Kirk Wallace wrote: > On Thu, 2008-04-17 at 18:21 -0400, John Kasunich wrote: >> >> Do you know your motor's rated current and voltage (not supply voltage, >> winding voltage, determined by multiplying the rated winding current by >> the winding resistance). >> > There is no data attached to the motors. I measured 1.1 Ohms across each > coil, with 1.0 Ohms across the leads, so the coils are somewhere > around .1 or .2 Ohms.
That seems rather low, but making accurate resistance measurements under an ohm or two with a regular meter is an exercise in futility anyway. I don't know your general level of electronics expertise, nor the equipment that you have handy, so some of this be be out of place. Anyway, you might want to try a 4-wire resistance measurement. If you have a bench power supply with constant current mode, put a known current (an amp or so) through a winding and measure the voltage drop. Ohms law gives you the resistance, and as long as you make the voltage measurements right at the motor terminals lead resistance doesn't matter. One other note - tell us if you are measuring from one end to the other, or from one end to the center tap. > The 8 Amp figure comes from the driver circuit > board where the current limit potentiometer indicates an 8 Amp max. Is > any of this useful? Is the pot turned all the way up? The motor might be 4 or 6 or 7A. If the motor is indeed a half-ohm or less and can deliver enough torque at 7 amps or less, a Gecko would probably run it quite well - a lot smoother and probably faster than the existing drive. > I scoped the driver board inputs again and corrected my signal diagram > on the schematic. The gray traces are the result of the coil input and > it's inhibit. I confirmed this with scoping the far side of the input > optocoupler. I played with the axis speed and noticed the inhibit gets > smaller and goes away around half speed. The trace leads me to believe > the designers were trying to soften the low speed steps, with a brief > half step, so that the motor would not overshoot and degrade the surface > finish. If this is the case and not for some sort of electrical reason, > I can switch over to EMC2 right away, and not worry about blowing out a > board. Of course Stepgen doesn't have this inhibit signal available, so > I may need to find another way, maybe using selective and adjustable > delays in HAL. If step overshoot is a common stepper problem, is there a > common way to fix it? > So far we're assuming that the existing drive has a current limiter (actually a pair of them) on the lines going to the center taps. The presence of a "current" pot sure points that way. The inhibit is something else - it prevents both ends of a winding from being connected to ground at the same time. That would be bad. Whether it is actually needed for that reason, and how long it needs to be, is hard to tell without knowing a lot more about the drive. For example, the 2N4126 and 2N4124 transistors on your schematic are puny little small-signal transistors. Probably part of the drive circuits for the main switches. It would be more interesting to see what the main power transistors are. MOSFETs are fast, bipolars aren't. The latter are more likely to need an inhibit. Hint: When reverse engineering any power circuit, start with the power components. The big parts will define the basic topology of the circuit, which usually one of a few standard configurations, and is also most of what you need to know. There are a million ways of doing the little stuff like transistor drive, every drive is different and for the most part it doesn't matter, as long as it works. If you could trace from the motor terminals to the power transistors and from the power transistors to ground and/or the main power supply, we would be able to tell what we're dealing with. Good background reading (if you haven't read it already): http://www.geckodrive.com/photos/Step_motor_basics.pdf In my opinion, reading and understanding that paper should be step one for anyone working on a stepper machine. Regards, John Kasunich ------------------------------------------------------------------------- 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
