Motion-Law Characteristics / Coefficients
Contents
MOTION LAW CHARACTERISTICS / COEFFICIENTS
A common question is: Which motion-law is best?
When your motion requirements are complex, possibly with many segments that have different position, velocity and acceleration constraints, we will recommend the Flexible Polynomial Motion-Law . It is the law that can satisfy many motion requirements and also have motion-continuity. However, when the motion requirements are simple, it is possible to compare the Traditional Motion-Laws, and decide which is 'probably' the best motion for a particular application.
One way to compare the Traditional Motion-Laws is to compare their Motion-Law Characteristics - also called Motion-Law Coefficients.
MOTION-LAW CHARACTERISTICS
Motion-Law Characteristic Coefficient apply mostly to velocity and acceleration graphs.
A Motion-Law Characteristic Coefficient is the maximum velocity or acceleration when the motion has a stroke of 1 and a period of 1. The coefficients are dimensionless.
For each Motion-Law, the:
| • | Velocity Characteristic Coefficient, Cv = Maximum Velocity ; when its stroke = 1 ; period = 1 second. |
| • | Acceleration Characteristic Coefficient, Ca = Maximum Acceleration ; when its stroke =1 ; period = 1 second. |
Note: Stroke = Displacement [linear or angular] ; Period = Time.
Input Torque Coefficient
Most standard cam mechanisms use a rotating input cam-shaft. The Input Torque Coefficient considers the maximum Torque a cam-shaft must deliver when the input is rotating at a constant angular velocity.
| • | Input Torque Coefficient, Cc = max(vi × ai)/Ca |
Notes
The Torque Coefficient is sometimes called the Power Coefficient - E.g. Heinz-Automation.
Indexer catalogues often use Q [e.g. Sankyo Indexers] rather than Cc . However, later in this section, we use Q for 'inertia ratio'. See 'Cam Mechanism'.
Power Coefficient
Power Coefficient is found from the maximum product of the acceleration and velocity, calculated together at each instant in the motion. Thus,
| • | Power Coefficient, Cp= max(vi × ai) |
Note, Cp ≠ Cv × Ca because maximum values of velocity and acceleration are at different phases of the motion.
How to use the Motion-Law Coefficient to get the actual Velocity and Acceleration
It is easy to use the Motion-Law Coefficients to calculate the actual maximum velocity and the actual maximum acceleration when the stroke and period are not equal to '1':
| • | Actual Maximum Velocity = Cv × Actual Stroke / Actual Period |
| • | Actual Maximum Acceleration = Ca × Actual Stroke / (Actual Period)2 |
Actual Period = Index Angle*N/60
Traditional Motion-Laws
Motion-Law Name |
Non-dimensional Maximum Cv |
Non-dimensional Maximum Acceleration Ca |
Non-dimensional Input Torque Coefficient Cc |
Non-dimensional Power Coefficient Cp |
|---|---|---|---|---|
Constant Acceleration, Parabolic |
2 |
4 |
2 |
8 |
Simple Harmonic |
1.570796 (π/2) |
4.934803 (π2 / 2) |
0.785 |
3.8758 |
Cycloidal |
2 |
6.283185 |
1.298 |
8.1621 |
Modified Trapezoid |
2 |
4.888124 |
1.655 |
8.0894 |
Polynomial 345 |
1.875 |
5.773503 |
1.159 |
6.6925 |
Polynomial 4567 |
2.1875 |
7.5132 |
1.431 |
10.750 |
Modified Sine |
1.759603 |
5.527957 |
0.987 |
5.4575 |
Sine-Constant-Cosine Acceleration + Constant Velocity [SCCA+CV].
Motion Law Name |
Coefficients |
SCCA Segment Parameters |
|||
|---|---|---|---|---|---|
Velocity Coefficient Cv |
Acceleration Coefficient Ca |
a |
b |
c |
|
Modified-Sine |
1.760 |
5.528 |
0.25 |
0 |
0.75 |
Modified-Sine |
1.528 |
5.999 |
0.2 |
0 |
0.6 |
Modified-Sine |
1.404 |
6.616 |
0.1667 |
0 |
0.5 |
Modified-Sine |
1.275 |
8.0127 |
0.125 |
0 |
0.375 |
Modified-Sine |
1.168 |
11.009 |
0.0833 |
0 |
0.25 |
Cycloidal |
1.333 |
8.378 |
0.25 |
0 |
0.25 |
Trapezoidal Velocity |
1.5 |
4.5 |
0 |
0.6667 |
0 |
Three Harmonic Series
Triple Harmonic Motion Law can be nearly equal other Traditional Motion-Laws. Enter the First Harmonic and Second Harmonic in the Segment Editor. MechDesigner calculates the Third Harmonic. |
|||||
|---|---|---|---|---|---|
Motion Law Name |
Coefficients |
Harmonic Segment Parameters |
|||
Velocity Coefficient Cv |
Acceleration Coefficient Ca |
1st |
2nd |
3rd |
|
3-Harmonic |
2.0 |
5.16 |
5.96 |
0 |
0.9696 |
3-Harmonic |
1.72 |
6.07 |
5.1968 |
1.7690 |
0.6057 |
3-Harmonic |
2.0 |
9.42 |
9*Π/4 |
0 |
-3*Π/4 |
MOTION-LAWS COMPARED - for Initial Selection
We can give ratings to the common Traditional Motion-Laws. The ratings can help you select a law when you start a motion and machine design. The ratings range from 1 (relatively bad) to 5 (excellent). The ratings apply to Dwell-Rise Dwell type motions.
If we look at the table, it can be seen that, of the laws listed, that the Modified Sine(MS) is the best for general purposes. Its particular merit is that it is very tolerant of a bad input drive and transmission (elasticity, backlash, wear, low inertia). It is frequently the first choice of cam designers and is almost always used by commercial manufacturers of cam-operated indexing mechanisms.
Additionally, you can look at the Motion-Law Coefficients of the common cam motion-laws. These indicate the relative values of their Velocity Coefficient and Acceleration Coefficient.
Cam Law Designation |
Peak Acceleration |
Output Vibration |
Peak Velocity |
Impact |
Input Torque |
Input Vibration |
Residual Vibration |
Parabolic |
5 |
1 |
2 |
1 |
1 |
1 |
1 |
Simple Harmonic |
3 |
1 |
4 |
4 |
5 |
2 |
1 |
Modified Trapezoid |
3 |
3 |
2 |
2 |
2 |
3 |
3 |
Modified Sinusoid |
2 |
4 |
3 |
4 |
4 |
4 |
4 |
Cycloidal |
1 |
5 |
2 |
3 |
3 |
4 |
5 |
Explanatory Notes
Peak Acceleration This merit rating applies to the nominal maximum output acceleration during the motion period, calculated by the motion-law equation. |
Output Vibration Output vibration is superimposed on the nominal output acceleration, thereby increasing the nominal peak value. The vibration severity depends on the elasticity and operating speed of the mechanism. The merit rating applies to mechanisms of average rigidity running at fairly high speed. |
Peak Velocity Peak Velocity is the nominal maximum output velocity during the motion period, calculated by the motion-law equation. Its value is also increased by superimposed vibration. |
Impact / Backlash Impact forces occur at the locations of backlash in the mechanism when the changeover from acceleration to deceleration occurs. The severity of the impact depends on how gradually the changeover takes place. That is, how low the jerk is at point of impact. Strictly speaking, it is the changeover from positive to negative force or torque that matters, but in most high speed systems, that almost coincides with the acceleration changeover. |
Input Torque The nominal input torque of a mechanism varies throughout the motion period and is a function of the output load profile, and the velocity pattern. The peak acceleration and the peak velocity do not coincide and neither coincides with the peak input torque. Motion-Laws with good, that is low, acceleration do not necessarily have good input torque. |
Input Vibration The elasticity and backlash of the input transmission can cause serious 'over-run' and 'input-vibration'. This is when the sudden reversal of the input torque at the changeover from acceleration to deceleration - or load - causes the cam to jump forwards before it can transmit a decelerating force to the output. The more gradual that the nominal input torque changes sign, the less severe is the overrun and its consequences. |
Residual Vibration Residual Vibration takes place in the dwell period immediately following the motion period in high speed or elastic systems. Its amplitude depends on the vibration generated during the motion period, and the degree of damping present in the output transmission. It is very difficult to add sufficient damping to high speed mechanisms to eliminate residual vibration, so the choice of a motion-law is vital in some cases. |
Motion Laws Compared - another table.
This table includes 'Period Ratio' as a parameter with which you can select a Motion-Law. It is from an ESDU* item.
* Engineering Science Data Unit, published in the UK.
