On Monday 14 October 2019 13:36:24 John Dammeyer wrote: > > On Sunday 13 October 2019 19:46:55 Kenneth Lerman wrote: > > > Hello All -- especially Gene, > > > > > > I've seen many posts on the lists regarding breakout boards and > > > dealing with issues regarding slow opto-isolators. I have a > > > question: > > > > > > Is anyone using: ISO776x > > > <http://www.ti.com/lit/ds/symlink/iso7760.pdf> devices? > > > > I've seen that pdf before, but in the cases where I've simply > > bypassed them, I had good solid, noise free sources to start with. > > If I use a BoB, its a Saintsmart, simply because there is not any > > opto's in their output path, and most outputs goto the input opto's > > of a stepper driver, which I've found is a bit faster than the > > average bear. A 2M542 for instance can, if the drive is good rail to > > rail stuff, be driven to about 375k steps a second before it > > miss-behaves! That will turn a nema-23 motor rated for 270 oz/in, > > to over 3000 rpms if the supply voltage is pushing the 2M542 at > > around 42 volts. Nearly 2000 revs at 28 volts. > > Hi Gene, > I think you have your units mixed up here. There is no way you will > get a stepper motor to turn at 3000 RPM with a 48V supply. They just > can't do that physically. > Why not? I've done it on the table, driving a 2M542 with a function generator. The point of increased voltage is to overcome the inductance faster to get a usable current thru the coils. That is what the motor steps on, the rotating magnetic field. Give me a driver that hasn't any opto's in its inputs to slow it down, and can tolerate a 150 volt supply voltage, and I'll turn a nema 23 at 10k rpms laying loose on the table. The motor can generally stand several hundred volts, so 150 shouldn't be a problem.
The limits of speed on a stepper are determined by: 1. applied voltage, must be high enough to get a usable current into the coils for working torque in the allocated step durations time between steps. I might add that the more costly 3 phase motors have an advantage in that dept because of the longer time allocated for currant reversals in their coils in the 3 phase design. They are a nice idea, but the cost needs to come down before they'll get any takeup in new designs. but they are very close to a servo with a builtin encoder, except they don't have one. 2. The steadyness of that clocking so the motor isn't being asked to play catch during the current step, then told to slow down on the next one. I've seen some stuff written where a 5% wobble in the step rate can cost you 50% of your torque. This also applies to vfd's. The top speed limit you can run a motor at with a vfd is the motors coil inductance. I can drive this 1hp rated motor at up to about 120hz, effectively getting 2hp out of it. But at 180 hz, 3x its rated speed, the motors coil inductance has limited the coil current to just over an amp, and with bronze bushings the drag is so bad thats as fast as it can turn the spindle. But at the same time I can run it at a 1/4 speed or 15 hz, drawing nearly its nameplate FLA, at no cutting load and do that for long enough to get the job done without its getting so hot I can't touch it. Properly programmed, a vfd is magic, and it can do it with no damage to the motor. Any vfd that takes 30 seconds to get to speed has never been programmed since it came out of the box new. I can turn this 8" chuck at 400 revs, punch in an m4, and just a bit over a second later its turning 400 revs backwards. The drive belts are screaming like Michelin tires, but its doing it. Punch in an m3, and its running fwd with the same sound effects as it made before. Yet its all sequenced in the hal file and the motor is never subjected to more than its nameplate FLA, DC for dynamic braking or AC for running. Never gets anywhere near its LRA, which can destructively heat it pretty fast. And the vfd never gets hot as its staying inside of the motors ratings. Whats not to love? > The Bergerda AC Servos run 3000 RPM and with a 2500 line quadrature > encoder (10,000 edges per second) means a 500,0000 hz step rate is > needed to achieve that. > > But, the Bergerda AC Servo drive has a dual purpose stepping input > that can be wired as RS422 or as opto-isolated up to 24V. According > to the Bergerda motor manual the TTL opto isolated inputs are not > suitable to that high a step rate while the differential RS422 inputs > that are good to that speed. > Makes perfect sense. > So regardless of what sort of PC based step/dir interface you are > using, if you want isolated 500kHz step rates then your BoB needs to > have something like the ISO7760 in front of the RS422 differential > drivers. I don't know if the Bergerda differential inputs are > isolated from the high voltage inside the driver. > > The iso7760 is nice device. The ground on the input side can be > connected to the PC ground and the output ground to the low voltage > CNC machine control ground. A huge advantage, if only for noise isolation, a laudible function. > Note the data sheet also lists that half > the units inside the package (7763) can be reversed so it makes an > ideal single isolator package for multiple RS485 or RS232 circuits or > a BoB where you want to isolate the PC parallel port. Had I known > about this part I'd probably have used it instead of the multiple > devices below. > > What I've used are HCPL2631 and initial testing shows they can handle > the step rates for the Bergerda AC Servo. This module can be > configured with jumpers to interface to either RS422 step/dir/enable > and fault signals for the STMBL Servo Drive or to RS422 step/dir and > opto-isolated enable/fault for the Bergerda AC Servo Drive. > Additionally this little board also creates the 5V for the MESA 7i92H > and two Open Collector signals to reset the HP_UHU DC servo drives in > case of a Fault from one of them. Those are simple, and very slow, > 4N25 devices. > > http://www.autoartisans.com/mill/StepDir2STMBL_SCH.pdf > > I built 3 of them. > http://www.autoartisans.com/mill/StepDir2STMBL.jpg > > Should be installed this week once I make up some cables. Too many > projects. Never enough time. I know that feeling very well. I need to get back to making mahogany sawdust and finish up 3 more blanket chests for my boys. I need to rip off the back porch which was damaged by a flying garden shed in 2010, and make a newer, bigger, better one. Since we own the place free and clear, I can do anything I have time left to do, but first a new aortic valve in my ticker. Otherwise I might not have time left to do all that and this too. :-) In fact, I think the back porch might be first as I am out of furniture assembly room in the garage. Mmmm... > John > > > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users 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) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
