On Friday 30 March 2018 20:34:52 Chris Albertson wrote: > I'm making a stepper motor pulse generator for my own use but others > might find it useful. To help make sure it is useful to others I'm > asking what specs it would need to have to make in "interesting" to > you. > > Here is what it does... You send it via a serial link a point in 6 > or 7 axis (x,y,z,a,b,c) and a time. The pulse generator will step all > the motors such that they all get to the given point at the specified > time and the movement is along a straight line (in joint space). The > units are "steps". Also you can send it one point and then while the > motors are moving you send it the next point and it will go from one > to the next without stopping. This way the serial link timing need > not be perfect. As long as the next point is sent in time, the timing > is near perfect. > > In my use case all of the move commands are sent at a given rate. say > maybe 50 points per second. The output pulses are "standard TTL" > five volts and maybe about 20 ma source/sink > > Whatever sends the commands does all the inverse kinematics, real > world to joint space transform, acceleration control and so on. All > this thing does is handle the real-time pulse generation so no hard > real time work must be done by the sender. my target cost is "dirt > cheap". > > Now for the survey questions: > > 1) what is the fastest pulse rate you would reasonably use. The > DM542T driver I'm using for testing is spec'd to 200KHz but I can't > imagine anyone actually getting close to that. What's a real number?
i have ran them to 340khz laying on the table with a function generator. This corresponds to nominally 3000 rpms. Driver voltage was 60 volts, higher than a 2m542 can tolerate, and the top speed was extremely voltage dependent. I have one machine using 2m542's, on 28 volts and software stepping that may stall its xy drives at anything above 11 or 12 ipm. They are direct drives on some 8mm x whatever pitch is loaded with 1/16" bearings. Certainly less than 5mm/turn screws. The machine on the next table has 38 volts, a 5i25 for much steadier stepper generation, a 2/1 gear between the z motor and the carriage (it a 7x12 lathe, and can move the carriage with a 16mm x 5mm screw at 90 ipm. Some of that is the steadier step generation and some of that is the higher motor voltage. Regardless, its a huge difference. > 2) How much jitter in the pulse timing is acceptable? Yes i know it > depends on the speed. Available torque without a stall will drop like a rock when its more jitter than about 3 or 4 % of the period when up in the nosebleed speeds. > 3) Are people using balanced signals on the step and direction pins. > I kind of doubt it but want to be sure. No. But always feed the logic + to the + terminal on the driver, and the step/dir to the - terminal of the driver as most turn on the input opto with a logic 0 input. This gives more on drive to the opto. And its plumb amazing the difference the bob can make. cnc4pc did make one without any opto's of its own, they aren't needed because all the motor drivers have them, and 2 in series will cut your available motor speeds noticeably. Not to mention they will totally muck up a pwm spindle speed control. My G0704 can go from 25 rpm to 3000 rpm in high gear with a max error of just a percent or so, once I got rid of the opto's. Half that in low gear. Recently the only bob w/o the opto's in the output circuit available on ebay are the SainSmart bob's. They still have opto's in the 5 inputs, and if you have high resolution encoders on the spindle, you may have to bypass the ones for the A/B signals from the encoder else you'll lose spindle control as it speeds up. By high resolution, more than 500 per spindle rev. Since I put mine on the motor, and scaled the head gearing thru a scale multiplier, I'm over 14,000 edges per spindle rev when in backgear. Overkill yes, but thats the price of experience. The fact that the control is unbelievably stiff and stable was a nice side effect. > I don't know if something like this could be used with MK, if so t > could offload all of the hard real-time requirements. I need this > for a robot arm Just one question? Can you build this for less than you can buy it from Mesa? Or Pico? If you time is worth a buck an hour, its a slim chance of yes. Very slim. Otherwise get it from a supplier that supports us. How many motors in your arm? The 7i90HD at less than $70, + 3 7i42TA's at about 45$ ea, about $200, can handle 8 steppers. And have plenty of gpio left to do lots of other things. Limited only by your imagination. -- Cheers, Gene Heskett -- "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> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
