On 12/18/2010 08:35 AM, emc-users-requ...@lists.sourceforge.net 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. ------------------------------------------------------------------------------ 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
