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Motion control in an interrupt.
Parts
This consists of two parts, the planner and the executor.
The planner receives target positions. Each time it receives a target position, it replans so as to reach that position as soon as possible; the output of a plan consists of a set of motion segments, each following a 3rd order polynomial.
The executor processes the current state and decides whether to toggle the step lines of the MCU.
These two processes communicate by means of a command queue.
Executor
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Update a cycle counter
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Evaluates the next output of the position polynomial (3 adds)
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determine whether to toggle a stepper, and do so.
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Determine whether to start the next segment; if so, advance in the command queue.
Command queue
The command queue takes the form of a ring buffer, with each item containing a motion segment. The ring buffer must be large enough to contain a complete motion profile plus a terminating "stop" segment.
A motion profile segment consists of the following values:
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For step computation
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Δx1..Δx3 (initial values)
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start time (in ticks)
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For use by the planner
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v₀: Initial velocity
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a₀: Initial acceleration
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The actual position at the start time (written by executor)
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The following invariants hold for the command queue:
Planner
Aborting
In case of an abort, the fastest stop profile will consist of at most 3 segments: const -jerk to max -a, const -a to to lead-out, const +j to a=v=0.
If we’re already in the last three segments, we’re already in a race to a stop, so there’s no need to handle an abort specially.
In the cv segment, we can simply advance the start times of each leadout segment. In the lead-in, we need to replan; this is, however, easy enough:
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Predict state in $t_plan$ cycles
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Identify how to reach a=0; prepare a segment accordingly
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Feed the end values from said segment to the leadout calculator
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replace the next N commands with this leadout.