﻿ MotionDesigner Reference & User Interface > Motion Laws > Constant Acc and Dec Motion-Law [Parabolic]

# Motion-Law: Constant-Acceleration and-Deceleration

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# Motion-Law: Constant-Acceleration and-Deceleration

### Constant-Acceleration and Constant-Deceleration Cam-Law, Motion-Law

A Traditional Motion-Law. It also has the name: Parabolic motion-law.

#### Segment-Editor AND Blend-Point Editor

CAN specify its:

Start-Position

End-Position

The Start-Position usually flows from the End-Position of the Previous-Segment

CANNOT specify its:

Start-Velocity OR End-Velocity.

Start-Acceleration OR End-Acceleration.

Start-Jerk OR End-Jerk

#### Segment Editor, also

Specify its Segment Range

#### Motion-Law Coefficients

Cv = 2.000

Ca = ±4.000

Cj = ± ∞

Notes:

It has the lowest nominal*

* Nominal: at each discontinuity (Step Change) in its acceleration, there is infinite jerk. Infinite jerk tends to incite vibrations. Thus the real peak acceleration - in a real mechanical system - will be at least 2 times the nominal peak acceleration.

This Motion-Law is often used with servomotors. It is easy to program and even electrical engineers can understand it - the poor things!

We do not recommend this motion-law if the is less than 10, or even 20.

To design a Trapezoidal Velocity segment, [and not the Triangular Velocity segment that this motion-law gives] use the Sine-Constant-Cosine Motion-Law.

#### Application Notes

This Motion-Law has been used frequently in the past because it has the lowest nominal acceleration of the Traditional Motion-Laws. However, it has infinite jerk at three points: the start, end, and at its cross-over. This makes it a very poor choice form a dynamic viewpoint.  Infinite-Jerk incites vibrations in any mechanical system.

Dynamic Performance

The actual acceleration of the load being driven by Constant-Acceleration motion will always be significantly higher than the nominal value, because of induced vibrations.

For this reason, this segment should only be used in applications where inertia effects are small or even insignificant.

Pressure Angle Considerations

This segment produces a relatively large pressure angle - and so might need a large cam for a given lift. The pressure angle for this segment varies quite severely throughout this Motion-Law indicating that it is unsuitable for roller follower applications because of the severe accelerations imposed on the roller that will tend to induce roller slip.

Drive Torques

This law performs badly in terms of drive torque considerations. All of the torque factor curves for this law exhibit a discontinuity, indicating shock loading and noise in operation. Particularly notable is the sudden reversal of the inertia torque factor, and hence of the torsional strain energy, at the cross-over of the motion segment. These reversals will contribute further to noise, shock loading and vibration during operation.