----- Original Message ----- From: <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]>; <emc-users@lists.sourceforge.net> Sent: Wednesday, February 13, 2008 5:41 AM Subject: [Emc-users] UK suppliers of stepper motors and drive electronics
> On 12 Feb 2008 at 18:03, [EMAIL PROTECTED] wrote: > >> EMC can do PID just fine. It's steppers that can't. Steppers lose >> torque as the speed increases. There is no way around this, it's just >> the physics of the motor. > > > Did someone rewrite the spec for PID? The P in PID stands for proportional. That means that the signal generated will be proportional to the error. The more steps that are missed, the larger the error, the larger the signal.... > > used to be a way of correcting a system or process in just the same way > that an operator > would, and it certainly didn't require any more torque, just a wait state. As an operator, if the motion is falling behind, I turn the handwheel faster. That requires more torque. > >> PID loop will attempt to correct for a >> lagging motor by requesting more "effort" from the motor. > > When did this become the *only* option PID had, more torque and overspeed? PID is proportional, integral, derivative. The three "options" are to make a correction proportional to the error, proportional to the rate of change of error, or proportional to the integral of the error; or sums of those. > > I'm not trying to be funny here but I've used a lot of these technologies > in the past, and yet, > when it comes to EMC I'm starting to get the impression that some things > are done > differently. If you would be more specific and tell us a particular technology you would like to be able to do within EMC, we might be able to help. > > I'm not quite sure why, I'm not even sure they are, but it is the > impression I'm getting, and I > hope I'm wrong. > > >> Even if the motor just loses a step or two which is >> detected by the scale, you can't get it to catch up - it's already at >> the limit of its power envelope or it wouldn't have fallen behind in the >> first place. > > So, wait state, you surely aren't telling me that EMC will simply carry on > thinking it is > machining a part if the coupling between a motor and leadscrew fails??? Unless you provide a way to detect that, it WILL simply carry on. It the system integrator is concerned about that failure mode, it is the job of the system integrator to provide a way to detect that failure and take approporiate action. Generating an ESTOP might be appropriate. > > >> >> You had an incorrect assumption in your original email: that using >> linear scales will eliminate backlash issues. > > NO, it won't eliminate it, but it will eliminate it from calculations, as > it gives true position, not > estimated position, then add fudge tables. > >> This isn't true at all. >> Backlash is an uncertainty in machine position. If you're climb >> milling, the cutter will tend to pull the table "ahead" of the motor. >> When conventional milling, the cutter will resist motor motion. It's >> not possible for the control to know which type of cutting is taking >> place at any given time, and it may even vary within a move, so there's >> no way to "compensate" for it. > > eh, it is working from a tool path with a defined depth of cut and cutter > overlap from last pass, > direction of beds is also knows so "knowing" whether you are cutting on > the climb or the chip > is as trivial a logic problem as it is for a human operator. EMC interprets gcode. It does not know where the stock is, what stock has been removed, where the table is, or where the clamps are. As far as I know, there is no similar system that knows apriori whether it is climb or conventional milling. > >> Additionally, de-coupling the feedback >> from the motor, especially through a drive with backlash, will make the >> system very hard to tune. The PID integrator will "wind up" as the >> motor starts to spin to take up the backlash, but the feedback won't >> change until the motor is already moving. The motor will slam the table >> into motion, at which time the PID starts to wind up the other way. The >> result is - you guessed it - oscillation. This is very hard to tune >> out. >> >> There has been some discussion recently about using both encoders and >> linear scales, but there isn't any software to do that yet. I think >> this is the "different method of machine control" that Kirk is talking >> about. >> >> As for redundancy, since EMC takes encoder feedback, there isn't really >> any need for a DRO - the EMC display is actual position. > > Listen, I know from experience that my words have a tendency to get > people's backs up, and > I don't want to do this, members of this list have been extremely helpful > and extremely nice. > > But. > > I'm getting an awful suspicion here, and that awful suspicion (and I > dearly hope I'm wrong) is > that EMC is going to suffer the same problems of many open source > projects, it's crap. If you don't want to get people's backs up, I suggest you avoid the use of words like "crap". (And by the way, you are wrong; it is not crap.) > > For example, you've got the gimp, and you've got photoshop. > > It isn't about whether one is free as in beer or one can be modified, it > is about which one is > actually productive for those who wish to edit images only, and have no > interest or talent in > coding. Photoshop creams the gimp. The gimp is only free if my time is > worth nothing, eg > editing images is a hobby, not a job of work and not competing with a job > of work for my time. You haven't given us the rest of the story. Since you are so familiar with these "technolgies", I'd expect you to say something like: "For example, you've got EMC, and you've got XXXXX". Please, tell use what XXXXX is so that those of us who value our time can use it for a machine using screws with backlash, stepper motors, and linear scales and stop wasting our valuable time using EMC. > > I'm starting to suspect that EMC is a project that started out, not to > emulate the commercial > equivalents, but built bit by bit to do various things on the cheap, I'm > starting to suspect that > EMC is not a realtime machine control system, but rather an offline (non > realtime) simulator > that relies on assumption (I sent signals to move X 1.01 mm, therefore I > shall assume it has > moved 1.01 mm) You should spend some time reading about the history of EMC. > > I hope this is not so and I'm wrong, because if not EMC is no use to me. > > Please don't do the "well that's open source buddy and you can always code > your own > solution cos after all it is free software" thing on me, I'm not actually > here with my primary > concern being paying as little as possible or preferably nothing for > software, I'd be quite > prepared to pay for EMC, and as a long lime linux user I dig open source > (can't code myself > but there we go) but at the end of the day when it comes to all forms of > software I'm looking > for a tool to do a job, and I don't mind paying for a good tool. > > For example, you say "As for redundancy, since EMC takes encoder feedback, > there isn't > really > any need for a DRO - the EMC display is actual position." and I'm sort of, > what??? encoder > feedback is only actual position if it is measuring actual position, eg > linear scales... > > Is this simply a case of linear scales used to be frighteningly expensive > so the EMC coder > ignored them and went with the cheap option, and then started fudging > around to try to fix the > problems associated with going the cheap route? > > I'll grant you that rotary encoders on the leadscrews can be pretty > accurate, emphasis on > "can", if you retrofit with class 5 rolled ballscrews and scrape the ways, > but if you're using > trapezoidals fuhgeddabadit... > > Again, I'm not trying to give offence, but I'm wondering what sort of > people are members of > this list, how many are turners, how many are coders, how many are > hobbyists, how many > think that an encoder set up that displays numbers accurate to a few > micron is suddenly > going to allow you to make chips to that same level of "accuracy"? > > Yesterday I bought the DRO and sino scales as I said, the whole package > with everything > (extras) thrown in and a couple of other items was just under 500 quid, an > hour later I > ordered a sheet of 10mm 5083, it cost just about as much as the basic dro > and scales, for > me my time is money and for me materials are money, if EMC isn't going to > save me time > then I'm better off paying money for commercial closed source software, > and if EMC is going Again, I ask: what commercial software will let you use linear scales and stepper motors? If your time is so valuable, why are you using cheap scales with a resolution of only half a thousandth instead of precision components? (Did I miss something? Is this a woodworking machine you are building?) > to try to force me to work wihin arbitrary parameters like you can't use > linear scales and > steppers because of the way the code is written then EMC just did a kcad > and gimp on me, > which is a shame, but this isn't a hobby when you start spending hundreds > of pounds on > materials. > > Again, many thanks to all who have replied to my queries before, much > appreciated. > > cheers > > > ------------------------------------------------------------------------- > This SF.net email is sponsored by: Microsoft > Defy all challenges. Microsoft(R) Visual Studio 2008. > http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > Ken ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
