S-Curve Motion Profiles:
Command
Acceleration vs. Peak Acceleration
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The XMP's DSP uses a formula
to convert the "commanded acceleration" for an S-Curve
move to a "peak acceleration." The DSP calculates
the trapezoidal profile first, then adjusts the accel/decel
portions of the move based on the "Jerk Percent" parameter,
while keeping the move time at a constant value. For
example, a 0% jerk would look like a trapezoidal velocity
profile (no jerk portion), and a 100% jerk would have
no constant accel/decel (all jerks). See the table
below.
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Changes in Jerk
Percent parameter change the
acceleration-deceleration curve characteristics
The conversion formula is:
where,
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max_accel is the maximum acceleration
(or decel) for a point-to-point profile.
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accel is
the specified acceleration (or decel) from the application
code.
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jerk_percent is the specified jerk percentage
(0 to 100.0) from the application code.
Specifying jerk percent is a handy way to specify jerk in your motion moves. There are some pros and cons that you should be aware of when using jerk percent.
Pros:
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Jerk percent makes it easy to specify the jerk that your move will make. You can also know the change that the jerk will have on your move without having to change the move time.
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Jerk percent is easy to understand from an intuitive point of view.
Cons:
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Jerk percent changes the max acceleration.
A couple of cases:
- With 0% jerk, maximum acceleration is as specified by the user.
- With 50% jerk, maximum acceleration is 133% of the value specified
by the user.
- With 100% jerk, maximum acceleration is 200% of the value specified
by the user.
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In most systems there is a practical limit to acceleration, as it is proportional to current in most motor types.
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As the amount of time spent in acceleration changes, so does the amount of time spent in constant jerk.
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