On 12/18/2010 11:43 PM, cogoman wrote: > On 12/18/2010 08:35 AM, [email protected] wrote: >> I was wondering if u could tell me what this means: Setting PFD, "Adjustable, >> percent fast decay". The manual I got with my stepmaster stepper board says >> this should be adjusted for each axis. It doesn't say what it does or how it >> affects the operation of the stepping motors. Do u know what this is and how >> it would help me tweak the system? >> > With switching current regulation in a stepper driver, the current is > controlled by turning on the switch to one side of the coil connecting > it to the positive coil supply voltage, and connecting the other side to > the negative through the sense resistor. When the current reaches the > predetermined set point, the chip goes into a decay mode. If both > bottom switches are turned on, the current flows through a low voltage, > and consequently the current decays slowly. If the proper low side > switch and high side switch turn on, the current in the coil has to try > to charge the power supply up at its given voltage. High voltage and > high current drains more power out of the coil. > > A micro-stepping controller approximates a sine wave signal on the > coil as it steps. When the sine wave is on the climbing side, it > doesn't matter if the current decays slowly, because each successive > step will require the current to increase. When it's on the descending > side of the sine wave, if the coil's current slowly decays, you have to > wait longer before you take the next step for the coil's current to keep > with the command. > > If you select the fastest decay for everything, on the up-side the > current will have a rather large AC component which could make the > stepper's coils sing, but at least you can step faster. If you select > too slow a decay, you will lose steps at a much lower step rate than > otherwise. > > The Allegro stepper controller chips have variable decay modes. You > can look for a PDF for the A3977 controller chip to find out what they > say. I briefly looked at the ones I have, and didn't find any plots of > decay, but I seem to remember seeing a plot of the different decay modes > for the Allegro chips, and if you find the data sheet that has them, it > should make it clear. > > There may be other issues that I'm not aware of currently, but If I > understand correctly, you have a choice between speed, and quietness. > You will probably choose a balance between the two. > > An additional note about the speed trade-off, I have been considering > a stepper controller to give high speed without high cost. I have seen > stepper motors with inductance ratings between about 1.2 mH to tens of > milli-Henrys. At 12 volts even low inductance stepper motors can take > hundreds of micro-Seconds to achieve their rated full current. At 90 > Volts stepper motors with coils in the range of 2 to 3 mH can start to > tax the speed of EMC2s fast thread driving through the parallel port, > but even at 90 Volts a stepper motor with 12 mH coils will be slow to > respond, and at 12 volts a 12 mH coil will be terribly slow, OK for > tracking stars, but not for fast positioning. > > For (perhaps) more information than you ever wanted to know, search > for "Jones on stepping motors". A generous soul has given a great > tutorial on all aspects of stepping motor operation. > Hi, Cogoman:
Thanks for taking the time to compose this thoughtful response to Cathrine. IMHO the content of your message deserves a place on the LinuxCNC Wiki (which currently considers stepper-motor inductance only from the standpoint of maximum power supply voltage). Perhaps you would consider rewriting it into the "Stepper-specific info" section of "Configuring EMC2" with an appropriate title. Regards, Kent ------------------------------------------------------------------------------ Lotusphere 2011 Register now for Lotusphere 2011 and learn how to connect the dots, take your collaborative environment to the next level, and enter the era of Social Business. http://p.sf.net/sfu/lotusphere-d2d _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
