Jerk
Jerk is the time rate of change of
acceleration/deceleration. Typical units are m/s3
and deg/s3 for linear and rotary applications,
respectively. Jerk is commonly used to minimize backlash
effects, mechanical wear and tear, and to provide a smooth, stable
profile, with clean positioning minimizing overshoot. Jerk is
a function of the mechanical stiffness of an axis, and is difficult
to know prior to properly tuning and benchmarking the actual axis
performance. However, as the number of projects of similar
applications grows, it becomes much easier to closely predict an
appropriate jerk value for an application.
Note: Jerk limitation increases the required
peak torque from the motor and may result in the need for a larger
motor and drive.
Jerk Limited
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Additional acceleration cannot be achieved
without first increasing the jerk value
S-Curving = 100%
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Infinite Jerk
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An ideal/theoretical profile
Enter a jerk value of zero
S-Curving = 0%
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S-Curving
Instead of specifying a specific jerk value,
S-Curving is a percentage of the acceleration time, as per the
following formula.
Notes:
1. S-Curving = 100% is a jerk
limited profile.
2. S-Curving = 0% is an infinite
jerk profile.
Traverse Distance & Position
Window
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Traverse distance
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Distance less than the move
distance
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Traverse time
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Time taken to move through the traverse
distance
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Position window
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Distance from the target position
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Move time
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Time taken to reach the move stroke less the
position window
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Position window time savings
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Time savings if the position window was set to
zero
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Motion
Rules
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1/2 Rule
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t4 = 0
t1 + t2 + t3 = t5 + t6 +
t7
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1/3 Rule
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t1 + t2 + t3 = t4 = t5 + t6 +
t7
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Profile
Types
| Type
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Max Velocity
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Max Accel
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Max Jerk
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Comments
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| Constant accel (1/2 rule, no
jerk)
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2.000 d/t
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4.000 d/t²
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infinite
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Lowest accel, but infinite
jerk
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| Trapezoid accel (1/2 rule, 50%
S-curving)
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2.000 d/t
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5.333 d/t²
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42.7 d/t³
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Good balance of velocity, accel and
low jerk
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| Parabolic
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1.500 d/t
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6.000 d/t²
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12.0 d/t³
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Lowest velocity, but high accel and
rough jerk
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| Sinusoidal
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1.571 d/t
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4.935 d/t²
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infinite
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Low velocity, but infinite
jerk
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| Modified sine
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1.760 d/t
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5.528 d/t²
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69.5 d/t³
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Low velocity and good accel, but
higher jerk
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| Polynomial 5th order
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1.875 d/t
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5.773 d/t²
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60.0 d/t³
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Good balance of velocity, accel and
jerk
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| Modified trapezoidal accel
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2.000 d/t
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4.888 d/t²
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61.4 d/t³
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Low accel, but rough jerk
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| Cycloidal
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2.000 d/t
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6.283 d/t²
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39.5 d/t³
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Smooth accel and jerk, also low
jerk
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| Polynomial 7th order
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2.188 d/t
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7.513 d/t²
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52.5 d/t³
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Smooth jerk, but highest accel and
velocity
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Where:
d = distance
t = time
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