On Wednesday 22 September 2021 15:31:33 Chris Albertson wrote: > SIn waves are there own derivatives and integrals, except for phase. > So a sine wave acceleration works if the gaol is a sine wive velocity > curve. But I assume the goal here is maximum change in velocity. > You get that by running the motor is full reverse. > > I doubt we are seing back EMF because the worm cn not be back driven > by a wheel. > > My guess about the root cause if the power supply current spike is a > shoot-through in the double h-brige because the direction owas > reserved instantly with no though to the gate driver rise and fall > times. (I actualy hope it is this because the fix then is easy and can > be done on software. If I'm wrong th fix is harder. > Not in the same servo cycle as whats presented to the pwmgen comes back out in 4 pulse lumps as its running at 4 kilohertz. To change direction can only be done on the next servo thread. However I have not seen it done in less than 2 servo cycles.
But lets ask Peter since its his pwmgen: Could your fpga pwmgen in the 5i25, running in mode=2, reverse itself in the same servo loop? > On Wed, Sep 22, 2021 at 11:51 AM dave engvall <dengv...@charter.net> wrote: > > Hi, > > > > Fully recognizing that nothing is impossible to the person that > > doesn't have to make it work. With that proviso I will open > > mouth and insert foot. To wit: a sine for acceleration should give a > > more gentle startup and approach to end point. Of course > > implementation is left as an exercise for the student. ;-) > > Now I'll shut up and go away. > > > > Dave > > > > On 9/22/21 11:28 AM, Chris Albertson wrote: > > > So you are driving it with something like this? > > > amazon.com/HiLetgo-BTS7960-Driver... > > > < > > > > https://www.amazon.com/HiLetgo-BTS7960-Driver-Arduino-Current/dp/B00 > >WSN98DC/ref=asc_df_B00WSN98DC/?tag=hyprod-20&linkCode=df0&hvadid=1980 > >90929431&hvpos=&hvnetw=g&hvrand=9099973595926304313&hvpone=&hvptwo=&h > >vqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9031026&hvtargid=pla-40367 > >6040447&psc=1 > > > > > One possible mistake is if even for a short time the both enable > > > inputs > > > > are > > > > > active you will get "shoot through" and short the power supply. > > > Can > > > > you > > > > > verify this NEVER happppens using a scope or logic analyser? Do > > > not bother with a software "hal scope" You need to look at that > > > is actually > > > > on > > > > > the wires with a device that samples the actual metal pins. Real > > > pins > > > > have > > > > > riseand fall times and they can't intersect. > > > > > > I would also just use one PWM generator and connet it to both LPWM > > > and RPWM. A safer why to handle L-EN and R_EN is to make 100% > > > certain that they go through a state where BOTH ARE OFF. Don't > > > flip them instantly as there is a finite rise and decay time. > > > There needs to be soome number of microseconds where both are off. > > > > > > I bet LCNC flips both L-EN and R_EN on eht same software cycle. > > > If so > > > > then > > > > > the supply is shorted via the controller board. > > > > > > There boards are cheap and another way to fix this is to used a > > > boaed > > > > with > > > > > a "forward/Reverse" pin rather then two pins sothere is no chance > > > of enabling both pins. > > > > > > Or using a "smart" controller with a serial interface where the > > > speed and direction are sent as a digital command message. Then > > > the acceletaion > > > > and > > > > > such is handled by the controller. > > > > > > The sounds like an short in the h-bridge to me but I could be > > > wrong. You can test this with a digital scope. on the control > > > pins. > > > > > > On Wed, Sep 22, 2021 at 10:21 AM Gene Heskett > > > <ghesk...@shentel.net> > > > > wrote: > > >> On Wednesday 22 September 2021 11:44:51 John Figie wrote: > > >>> Gene, > > >>> > > >>> I would like to better understand your problem and have been > > >>> thinking about this. I have some questions. > > >>> > > >>>>> I don't think the fact that there is a worm gear matters. The > > >>>>> problem, I bet is the large inertia of the system. > > >>> > > >>> Hmm I am not sure about the inertia. I think if you have a large > > >>> gear reduction then the inertia reflected across the gears > > >>> should appear low from the motors point of view. > > >>> > > >>>>> Aside from proper tuning of the PID gains you could change the > > >>>>> system to use a nested or "cascade" PID. THis allows the > > >>>>> velocity setpoint to be controlled by the position error > > >>>>> see the section "cascade" in the wiki article > > >>>>> https://en.wikipedia.org/wiki/PID_controller > > >>>>> > > >>>>> I think you want a very fast loop i=for the inner PID. LIkely > > >>>>> it wouldbe in external hardwarelike a microcontroller or FPGA. > > >>>>> (does MESA > > >>>> > > >>>> do > > >>>> > > >>>>> the PID algorithm in the FPGA? It should.) > > >>>>> > > >>>>> > > >>>>> > > >>>>> On Sun, Sep 19, 2021 at 3:39 PM Gene Heskett > > >>>>> <ghesk...@shentel.net> > > >>>> > > >>>> wrote: > > >>>>>> Greetings all; > > >>>>>> As most of you know, I built a servo from scratch for a BS-1. > > >>> > > >>> BS-1? What is this? Is this like a Grizzly BS-1 dividing head? > > >>> Did you mount the servo motor to this? > > >> > > >> Not a grizzly, a Chinese clone. > > >> > > >>>>>> But I am not at all happy with its performance. > > >>>>>> It is a motor with a worm output, driving the worm of the > > >>>>>> bs-1. And it has an A/B quad encoder in it. > > >>> > > >>> So you have a 2 worm gear reduction. Where in this system is the > > >>> encoder mounted? > > >> > > >> On the rear of the motor. > > >> > > >>> What are the encoder counts per rev of the encoder? > > >> > > >> DNK, no index in it. So I measured the encoder for 100 turns of > > >> the > > > > BS-1 > > > > >> as verified by the home switch, divided that by 360000 to get a > > >> count per degree scale value. Thats about 666.something per > > >> degree of the BS-1 > > >> > > >>> What are the motor characteristics? > > >> > > >> Brushed PMDC, rated a 100 watts, 24 volts. Intended to run estate > > >> gates by chain drive similar to garage door openers. I assume its > > >> OEM controller has a homing switch, and counts encoder pulses to > > >> open so many pulses when the approaching driver punches his > > >> access button. > > >> > > >>>>>> But I must rather severely limit its run speed because the > > >>>>>> PID doesn't see the null coming near fast enough to slow it > > >>>>>> and stop a couple > > >>> > > >>> When you say null coming what do you mean? Is this the point > > >>> where the desired position is reached according to the motion > > >>> planner in LinuxCNC? > > >> > > >> Yes. I'd assume so. Motion has its own version. > > >> > > >> Null in this context is when the encoder output equals commanded > > >> position. > > >> > > >>>>>> arcseconds early. I can't allow it to use reverse to stop as > > >>>>>> the motor seems to be a near short circuit then, crowbarring > > >>>>>> the power supply, > > >>> > > >>> How do you prevent the PWM from reversing the voltage on the > > >>> motor? Is the motor driven from a PWM in only one polarity? How > > >>> is the PWM and the switches arranged? > > >> > > >> Control is by a LCNC PID, fed by the lcnc encoders position > > >> output feeding the PID feedback, with a pwmgen running in mode 2 > > >> where it has two pulse width modulated outputs, one fwd, one > > >> reverse, which are fed to a BTS 7960 board containing 2 of the > > >> Infineon half bridges for a full bridge control. This IC is rated > > >> at 45 volts, 43 amps. Fed by a 24 volt supply good for nearly 20 > > >> amps, intended for automotive seat position controls, it runs > > >> this motor with zero heating. But if it overshoots the commanded > > >> position and goes into reverse to bring the motor back by more > > >> than a 1 or 2 % drive while the motor is still coasting fwd, it > > >> will crowbar the psu, getting it hot instantly, causeing the psu > > >> to do a shut down until it has cooled for 2 or 3 minutes. Even > > >> then, no heat in the BTS IC's. > > >> > > >> The net result is that I must limit its cuising speed and > > >> accelleration in order for it to coast to a stop using its own > > >> friction while avoiding the use of reverse to stop, usually a > > >> quite small fraction of a degree early. And that is about 10 to > > >> 15% of the speed it can move with the full 24 volts applied. > > >> > > >>> Regards, > > >>> > > >>> John Figie > > >>> > > >>> _______________________________________________ > > >>> Emc-users mailing list > > >>> Emc-users@lists.sourceforge.net > > >>> 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 > > >> Emc-users@lists.sourceforge.net > > >> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
