Christophe Grellier wrote: > - What is a stepper motor or a servo ? In french, a "moteur" can be a > stepper motor, a handrill motor, or even a car engine. Those are quite > different things. > A stepper motor has fixed positions built into it, and will move to a particular position when commanded. Feeding it more power will just make it hold that position more rigidly. It is normally used with no position sensing device. A servo motor generally moves smoothly when power is applied, and will move faster when more power is applied. It must be used with a position sensing device, as feeding power to the motor gives you no idea how much it has actually moved, due to variations in inertia and friction. > - What are these pulses you are talking about ? > - Why the pulses need to have a good speed and "rythm" ? > A stepper motor responds a bit like a mass with a spring attached. With the winding current in a particular pattern, it will fall into "magnetic lock" every four full step positions. If the loads are excessive, or sudden speed changes are commanded, the motor can jump from one locked position to another. If the step timing is not continuous, it can be hard for the magnetics in the motor to follow the apparent sudden changes in the speed of the electrical poles, and these jumps become more likely. > - Why does Linuxcnc need a realtime kernel, while Mach3 can run on a > stock Windows install ? > Mach uses a realtime driver that attempts to do the same thing, for a very small part of the Mach system. It runs into many of the same problems with interrupt latency. > - What is the difference between software stepgen and hardware stepgen ? > - Is one better than the other ? > Ragged step timing makes it hard for the stepper motors to follow the desired movement. If the timing jitter exceeds some amount depending on mass, stiffness, the motor and the stepper drive, the motor will skip steps of fall out of sync completely, leading to a stalled motor for the rest of the movement. it will pull back into sync when the commanded move comes to a stop, leaving the machine at a different position than commanded. Software step generation has a fundamental limit on the precision of timing of the steps. For instance, the interrupt period on the base thread in a LinuxCNC system might be 20 us. The equivalent time is 100 ns on the Pico Systems Universal Stepper Controller board, or 200 times finer resolution. The 20 us granularity of step timing is not such a great deal at modest speeds, but if you need to produce step pulses at 10,000 per second (rather fast for full-step drives) then the 20 us granularity means that the next faster or slower speed is a 20% jump in speed. So, acceleration and deceleration at 10K steps/second is quite coarse. With our step generator at the same speed, the granularity is only 1 part per thousand, which the motor will never notice.
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