S-Curve Acceleration and Deceleration

S-curve acceleration and deceleration refers to the shape of the velocity profile of a given move. Without using s-curve acceleration when you load an acceleration, velocity, and position, the motor tries to go from 0 to the specified acceleration instantaneously. When a motor does this, it creates a trapezoidal velocity profile. When the motor is ready to stop, it once again goes from 0 acceleration to a negative acceleration as fast as it can until it is at 0 velocity and then abruptly stops. These abrupt starts and stops create the sharp corners of the trapezoidal profile. The sharp corners translate to a very high jerk. Jerk is the derivative of acceleration and refers to abrupt changes in acceleration.

To explain jerk a little better, if you were driving your car in first gear and suddenly decided to switch into reverse, this would create an extremely large amount of jerk and might destroy your car. Suddenly accelerating and decelerating like that tends to reduce the life of mechanical components. To reduce the jerk, the transition into maximum acceleration needs to occur smoothly by slowly reaching a target acceleration or deceleration. The s-curve is used to slowly reach a certain acceleration or deceleration.

The following image shows the effect that the s-curve has on the velocity profile. Smoothing out the corners of the trapezoid using the s-curve reduces jerk and prolongs the life of mechanical parts.

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You need to define mathematically precisely the parameterization of the S-profile National uses. I.e. does acceleration increase linearly to a peak and then decrease linearly to zero at which point the constant velocity point is reached?
- Alexander Jacobson, PLX Inc.. alex_j@bellsouth.net - Jan 4, 2005