Indexing Motion

This tutorial designs an indexing motion.

After this tutorial you should be able to:

delete segments
make sure the Motion-Width is 360­º
change a segment's motion-law
edit the X-axis value of a Blend-Point
edit the Y-axis Position of a Blend-Point

Terminology and Index Motions

Index Motions usually have two segments:

SEGMENT 1: Index Segment.

We must specify its:

Index-Period:  the rotational angle of the input-shaft [X-axis] as the output shaft also rotates
Index-Angle: the rotational angle of the output-shaft [Y-axis] during the Index-Period.
Motion-Law: the mathematical function for the rotational angle of the output-shaft as a function of the rotational angle of the input-shaft .

SEGMENT 2: Dwell Segment.

We must specify its:

Dwell-Period: the number-of-degrees the input-shaft rotates during the dwell segment.
Dwell Period = 360 - Index-Period


When you design a 3D Cam, it is frequently better to use three segments : 1-Dwell – 2-Index – 3-Dwell, to be symmetrical at 180° [360/2].

Period = Time-Period, Duration, or Interval.

Video : Motion Tutorial 1: Design an Indexing Motion


Motion Tutorial 1: Design an Indexing Motion


tog_minusStarting from the Default Motion


Default Motion:

Number of Segments = 4

Motion-Width = 360º

Motion Rise = 90º

Motion-Laws = All 4 are Flexible Polynomial

Motion 'type' = Rise-Dwell-Return-Dwell


The X-axis is the horizontal axis. You can see its units are 'degrees'.

The X-axis usually represents one machine shaft rotation rotation, 360º.

The Position, Velocity, Acceleration and Jerk graphs are exactly above each other, and are the same motion, but at different motion-derivatives. The X-axis is the same for all of the motion graphs.

Segment Names

Selected-Segment - is the RED segment

Previous-Segment - is to the left of the Selected-Segment.

Next-Segment - is to the right of the Selected-Segment.

tog_minusEdit the Y-axis units


If the Y-axis units are not 'degrees' then, to make them 'degrees':

1.Click the Edit toolbar > Active Motion-Settings


2.Click the Active Motion-Settings > Motion tab > Motion Units separator : Y-axis Position units
3.Select 'degree' in the drop-down box.

tog_minusDelete Two Segments


Indexing motions usually have 2 segments. Thus, we must delete two segments from the default motion.


Before we can delete a segment, we must make it the Selected-Segment

1.Click in the motion-graph area above or below the last segment

Step 1 is complete

The RED Selected-Segment is the last segment.



To delete the Red Selected-Segment

2.Click the Blend-Point & Segment Editor toolbar > Delete Segment

See image


Step 2 is complete

The last segment stretches.
The total Motion-Width does not change; it remains at 360º.
The first segment becomes the RED Selected-Segment.

We want to delete the last segment.

3.Do again Steps 1 and 2

That is: 'Click the last segment', and then 'Delete Segment', in the the Toolbar.


Step 3 is complete

The motion is to the left.

There are two segments.


Period [time period]: also applies to a number-of-degrees along the X-axis.
Index-Period: The number-of-degrees along the X-axis during which the Y-axis is in motion.
Dwell-Period: The number-of-degrees along the X-axis during which the Y-axis is not in motion.
Index-Angle: The range of the output motion - in Y-axis units.


tog_minusEdit the Motion-Law and Blend-Points


The motion we have:

The Segment-Width of the first segment is 90º. This is the Index-Period
The Segment-Width of the last segment is 270º - this is the Dwell-Period

The Motion-Width 90 + 270 = 360º

The Index-Angle is 90º.

The motion we want:

The Segment-Width of the first Segment to be 120º.
The Segment-Width of the last segment to be 240º.
The Index-Angle to be 120º.

The Motion-Width of the 'before and after' motions are the same:  is 90º + 270º = 120º + 240º = 360º.

We need to edit the Segment-Width of the two segments, but not change Motion-Width. We can:

Use the Blend-Point Editor to the 'slide' the Blend-Point to the right.
Use the Segment-Editor to make the first segment longer and then make the last segment shorter.

The best tool to edit the Segment-Width, when we do not want to edit the Motion-Width, is the Blend-Point Editor.

Blend-Point Editor button

Blend-Point Editor button

STEP 1.Open the Blend-Point Editor

Click: Blend-Point and Segment toolbar > Open Blend-Point Editor.



STEP 2.Make sure the active Blend-Point is #2
1.Use the Previous / Next buttons to move the active Blend-Point to #2

You can see the X-axis position is at 90º

Note: You can also make sure the second segment is the RED Selected-Segment before you open the Blend-Point Editor.

The Next / Previous buttons are important when you want to edit [or find] a 'very short' segment in the motion that you cannot click [or see] in the motion graph.






STEP 3.Edit the X-axis value

Note: The Blend-Point Editor can only edit the the motion-values at the left [the start] of the RED Selected-Segment.

We can use the spin-box tool to edit the X-axis value of the Blend-Point.

1.Click the bottom arrow-buttons in the spin-box control to increase the Spin-Increment from 1 to 10.
2.Click the top arrow-buttons in the spin-box control to increase X-axis motion-value from 90º to 120º.


1.Use your keyboard to enter a value in the X-axis box [enter 120]
2.Use the 'Enter' key on your keyboard to update to the new value [120]


General Reminder:

The Blend-Point Editor edits the active Blend-Point.

The active Blend-Point is ALWAYS to the left of the RED Selected-Segment.

As you edit the X-axis position of the active Blend-Point, the duration [period] of the Previous Segment and the RED Selected-Segment change at the same time.

The Motion-Width does not change.


STEP 4.Change the Motion-Law of the last segment to a Dwell

The default motion uses Flexible-Polynomial Motion-Laws for each segment.

1.Click the last segment so that it becomes the RED Selected-Segment
3.Select Dwell to change it from a Flexible-Polynomial to a Dwell motion-law


STEP 5.Change the Motion-Law of the first segment to a Modified-Sinusoid
1.Click the first segment so that it becomes the RED Selected-Segment
2.Click the Motion-Law Selector again
3.Select Modified-Sinusoid to change the motion-law from a Flexible-Polynomial to a Modified-Sinusoid motion-law.

VERY IMPORTANT: Click in the motion graph area above a different segment to remove the 'focus' from the Motion-Law selector. Otherwise, if you use your scroll mouse button, you will scroll through the motions! Not good!

STEP 6.Edit the Index-Angle [Y-axis-Values]

If necessary, open the Blend-Point Editor again - see STEP 1

Make sure the second segment is the RED Selected-Segment - see STEP 2

In the Y-axis values separator, edit the 'End-Position' of the Blend-Point. We want to increase it from 90 to 120º

1.Click the spin-box of the End-Position - see image to the left - to increase the Y-axis value of the Blend-Point from 90 to 120º

The graph auto-scales. Thus, look at the Y-axis scale of the Position graph [left of motion graph] to see the change to the motion.

Question: What if you do not change the 'Motion-Law' of the second segment to a Dwell?

Answer: The second Blend-Point moves up or down along the Y-axis, but the last Blend-Point does not move.

You will need to edit the End-Position of the last segment. To do this, you must first make Blend-Point #1 the active Blend-Point.

To edit the End-Position of the motion, you must click the FIRST SEGMENT.


Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd