At 09:10 PM 11/3/2008, you wrote:
>Hi Dave.
>
>Yes, for stepper motor machines, emc creates a pulse for each and every
>incremental motion of the motors -- a typical machine might have 8000
>pulses per inch, and that machine moving at 2 inches per second would
>generate 16000 pulses per second.
>
>After interpreting the gcode, the details of a specific move are sent to
>the realtime trajectory planner. Every 1ms, emc produces a new target
>point which is reachable in the next 1ms from the current point given
>the acceleration and velocity limits of the machine.
>
>At this point, it becomes the role of the hardware-specific code to "do
>something" to get hte motors to the right location. In the case of
>servos, this may involve using the "PID" algorithm to create an
>appropriate analog velocity command. In the case of a stepper system
>attached to the parport, "stepgen" now enters the picture.
>
>"stepgen", which also has an idea of the acceleration and velocity
>constraints of the machine, has several pieces of data: the last
>velocity, the current position, and the new position. Taking all these
>together, it finds a new velocity (which is really a frequency of
>pulses) that will hit the new position or come as close as possible
>given the machine constraints (in a properly-configured system, reaching
>the new position is always possible; on a bad one, it sometimes isn't,
>and the operator will be greated with a "following error" pop-up). It
>stores this new velocity (frequency) command in a particular memory
>location.
>
>The frequency command in that memory location is then read by the "fast
>thread", which is executing typically at a rate between 10kHz to
>50kHz--say, every 25 microseconds. stepgen creates a series of pulses
>at around the commanded frequency by using a "DDS" (direct digital
>synthesis) algorithm; if the required rate is between 75 and 50
>microseconds, then there will be a delay of either 2 or 3 periods of
>25us between pulses.
>
>The value produced by each step generator is then gathered according to
>the HAL configuration and output on the parport.
>
>.. and that's how babies are made^W^W^Wemc2 drives stepper systems.
>
>Jeff
Jeff,
Just curious, what size is the pulse "word" and what info
does it contain? Is there a velocity pulse, or are there separate
pulses for step and direction?
Mark
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