On Thursday 27 February 2014 06:57:33 Mark Wendt did opine: > On Wed, Feb 26, 2014 at 3:58 PM, Gene Heskett <[email protected]> wrote: > > That is generally a no, unless the stepper is running in idle mode. > > > > The problem is the negative torque curve of the stepper vs its speeds, > > meaning that if it slips a cog, it will be stopped as its in-capable > > of resuming the speed it was moving at when it failed by > > re-accelerating to that speed in a single step. All the feedback > > obtained from the encoder can do is attempt to speed it up to catch > > up, and its not capable of that while under the load that caused it > > to slip that cog in the first place. > > > > The best you can do with the encoders feedback is to use its error > > should the motor slip, to exert the fastest possible e-stop and > > hopefully save the part. If the part isn't damaged, then you'll need > > to rehome the machine and re-start the operation, running at a slower > > rate by sliding the feedrate knob to slower as that portion of the > > operation is being re- approached, hoping it will get thru the hard > > cut the next time. > > > > That said, I think there have been some attempts to use it, and you > > will find some discussion of it in the wiki. I have read it myself > > several times without fully understanding how it was used to > > dynamically rehome the slipped axis to get it back in step even after > > slowing the rest of the machine so it might be able to catch up with > > the rest of the machine now running slower. > > > > To me it seemed like a lot of expense and complexity for a limited CYA > > gain. Far better off to slow the operation to within the steppers > > abilities and/or raise the voltage on the drivers so the torque > > falloff is not as pronounced. > > > > I was nicely amazed by the speeds I could get out of my lathe, whose > > motors are running on around 37 volts, compared to the same motor and > > driver on my mill but running at 28 volts. A finicky 30 ipm on the > > mill, vs a rock solid 60 ipm on the lathe just by going up 9 volts. > > A side effect of the higher voltage on that particular driver, a > > 2M542 from fleabay at around a $50 bill a copy, is that this driver > > runs cooler at the higher voltage. > > > > But look it up on the wiki & make up your own mind. I'm just the > > resident retired old fart. ;-) > > > > > > Cheers, Gene > > Gene pretty much nailed it. If you're going to try an cob together > something that tries to talk like a servo, walk like a servo, and maybe > quack once in a while, you might just as well skip the cobbing and use a > servo. > > Or not worry about it, and use the stepper within it's capabilities. > > Mark
Thanks for the flowers Mark. I should also mention that servos do their magic by exploiting the error budget, limits set before throwing an error in the .INI file. The way we gear steppers, in the distance they move per full step, normally 1/200 of a revolution, will generally translate to an error less than may exist dynamically in the servo system. But that 1/200 is slightly miss- leading, so in the real world, where the motor driver can microstep the motor to what is theoretically angstrom accuracy, the maximum the motor will be out of position (assuming huge flywheels so the incremental velocities aren't a factor) from what the stepgen is telling it to be at, is much closer to 1/400th of a rev. But since its rotational velocity isn't the result of this imaginary, nearly infinite mass flywheel, the real world accuracy before it losses a step and stalls, is far more likely to be in the 1/1000 of a rev area. Using my lathes Z axis as an example for the source of the math, that screw is a 16x5 ball screw, meaning its 16mm in diameter, and moves the carriage 5mm per full revolution. I have a 425 oz triple stack motor, with a browning taperlocked style 40 tooth gear mounted to the motor, made from a set of metal replacement change gears. An 80 tooth gear is similarly mounted to the end of the drive screw by home-made taperlock. The screws end bearing mount has been modified to remove, by preloading its dual bearings, the nominally .007 thou of end play that existed there. So the motor is driving the screw in 2/1 ratio, making the screw effectively a 2.5mm pitch screw, moving 2.5mm per full rev of the motor. Now, assuming we are driving the motor in full steps, and assuming its "settled to the next position" accuracy is that 1/1000 rev, the full step will then move the carriage 0.0025mm. I have a 0.0001 dial indicator that _might_ see that. Cheap Grizzly indicator, rubber in the mount arm, any combination of several more excuses that might cause it to miss that truly tiny amount of motion. And that is to make the point, my drivers are actually running at 8 microsteps. The drivers I am using (2M542's) can do as high as 1/25th step, and the motor and gear slop noise is noticeably reduced, but these poor old atom boards, running at a 23u-s base thread, are all tapped out before the motor is moving anywhere near full speed. The atom's simply cannot issue the steps fast enough to use that fine a microstep. Another consideration when using the drivers microstepping ability to that level with software step generation is that the speed steps get very noticeable and will explore the stall characteristics of the motor quite easily if at maximum speed, you are using more than 25% of the stepgens max rate, because it you use it all, then the next step below wide open is a 50% change in speed if slowing, or a 200% change if accelerating. Either way, any motor I've ever played with will stall instantly. By using the feed slider, listen to the motors, and if at the highest speeds, you are hearing tones that change pitch more than an adjacent key on the piano, you are pushing your luck. Ideally you should hear a simple sliding tone as you adjust the speed, if the steps are really obvious at the higher speeds, its time to re-program the driver for a coarser microstep and adjust the scale value in the .INI file accordingly. It will get noisier. But will be more reliable under work loads too. Or get one of Peters universal stepper boards, which can issue steps at megacycle rates I have been told. FWIW, I have the mixed rubber washer/fender washer style dampers on my motors except for the one on my A/B/C table. I should make another for it, but I find its speed is adequate without it. They are a huge help at soaking up the mid-band resonances that can cause a stall at 1/3rd of full speed. Take them off the mill and I have to reduce the maxvel's in the .INI to about 1/3 or 1/4. That is how important they are! They are visible on the motors in the pix on my web page. There are several other designs extant and I may try the modified skate wheel idea for the next one I make. It would make a better handle, and wouldn't stick out so far. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> NOTICE: Will pay 100 USD for an HP-4815A defective but complete probe assembly. ------------------------------------------------------------------------------ Flow-based real-time traffic analytics software. Cisco certified tool. Monitor traffic, SLAs, QoS, Medianet, WAAS etc. with NetFlow Analyzer Customize your own dashboards, set traffic alerts and generate reports. Network behavioral analysis & security monitoring. All-in-one tool. http://pubads.g.doubleclick.net/gampad/clk?id=126839071&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
