On 23 April 2010 04:00, Jon Elson <el...@pico-systems.com> wrote: > > Andy Pugh wrote:
>> ... with closed-loop spindle control you will automatically get the >> spindle speed you ask for in the G-code regardless of which pulley and >> back-gear combination you have selected, > I understand the concept, but I'm not sure I like it. Oh, one other > thing is engaging the back gear REVERSES the spindle on a Bridgeport! > So, unless the logic would read the spindle direction, figure out the > backgear is engaged and reverse the VFD, it won't work anyway. Interesting question, and one that potentially links in to an issue that I have with my spindle speed control (and haven't mentioned as I think it is due to a fault with my HAL setup rather than the underlying concept) Firstly, it works very well, in general. It doesn't "hunt" for speed, it "just works" as far as M3 S1200 giving 1200 rpm at the spindle regardless of the selected gear. Otherwise I have to decide on the gear when writing the G-code and fudge the spindle speed demand. All apart from one quirk: My VFD doesn't go below 5Hz, and the output of the PID hal function runs from positive to negative full-scale and that is wired to the pwmgen function. My VFD direction selector is wired to the spindle-forwards / spindle-reverse pins, not to the direction pin of the pwmgen. This means that if I select a speed lower than the VFD minimum then the PID ramps the PWM down into the negative area, which increases the pwm duty cycle and toggles the direction bit. But toggling the bit has no effect, in fact the increased pwm duty cycle increases spindle speed, so the PID takes the pwm further negative, and so on. The effect is that if I demand a speed below the minimum that the VFD can do, I get max speed instead. I can think of a number of solutions, I just haven't implemented any of them yet. making some use of the pwm direction output seems critical, I probably need to XOR the spindle-forward / spindle-reverse outputs with the pwmgen direction signal. That does mean that closed-loop can, indeed, compensate for a reversed spindle though, in fact it is that capability which indirectly causes my problem. As for speed signal noise, my speed signal is fairly random, (it is a home-made encoder ring with slot-sensors) but using the right PID values seems to filter that effect adequately. Incidentally, because the spindle pwm duty is a constant demand rather than a delta demand, the PID controller is being used in "I-only" mode with P = 0. It perhaps ought to be used in PI mode and added to the spindle speed demand, but the way I have it seems to work. All of which is rather a digression from the original subject, which was encoder installation on a Bridgeport. That looks like a neat installation, invisible and out of the way of swarf is good. I have just thought of another way of doing the same thing that might work, you could potentially make a "gasket with mounting tabs" with the sensors on the (folded down) tabs and the remainder clamped between the castings over the full mating surface. I guess that would depend on how critical the casting separation is, ie whether there is a bearing in the top cover or something else that "cares". Perhaps with a strip of flat-flexy-PCB to get the signals out through the gap. I can almost see it being offered as a product by one of our more commercial members, though possibly not a profitable one. -- atp ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
