Step 16.4: Compare XY Piggyback Sliders and Blend-Curve

Draw a simple shape in the Base-Part and add a Profile to it.

Use Tools menu (or Toolbar) > Show Model in Mechanism to see the Solid.

Step 1 is complete - I have added a rectangle.

1.	Add a vertical Slider

2.	Add a horizontal Slider to the vertical Slider.

Note, the Part in the image to the left, keeps the Parts vertical. There is a Line in both Parts that we use for the Slider moving in the Horizontal direction. We use Points in the Lines to add the Motion-Dimension FB.

Design an XY-Path for each Slider so that the XY Point does not hit the 'obstacle' as it moves along the Path.

If you want to understand the disadvantages of the Piggyback Slider technique, then do not look at the two motions to the left! Please use MotionDesigner to design your own.

Alternatively, this is the Motion for the X-Slider.

The numbers (Θ,Y) (for example (45,0) on the motion graphs are the Machine Angle (Θ) and Slider Position (Y)).

This is the Motion for the Y-Slider

These motions make the Point follow the same Path in the Pick and the Place directions.

To do this, the Motions are symmetrical at the 180.

To show the path of a Point in the Horizontal Slider.

You can see the Path of the Slider avoids (does not collide) the Blue Solid.

You know the motions in the X and Y directions from the MotionDesigner. But you do not know the absolute values.

They are Vel = √X2 + Y2

[Double-click the Trace-Point to open the Point Vectors and Position dialog-box. Select the Show Velocity and Show Acceleration check boxes].

This image shows the Sliders at the position of the Peak Acceleration of the Point.

To find the peak vertical and horizontal velocity and acceleration you can look directly at the X and Y Velocity and Acceleration graphs in MotionDesigner.

This is one advantage of Piggyback Sliders.

The disadvantage is to arrange the motions that do not collide with the obstacle.

When you edit the motions, you will notice that it is not very easy to make the 'Place' Path match the 'Pick' Path. The target is to :

The image shows the Peak Acceleration for the return path - with the Red Vector - 14.67m/s/s

It takes a while to create suitable motions with Piggyback Sliders. Small changes to the motion can give large changes to the Path.  There is a different approach.

Alternatively, you can keep the same motion path, but move along it with a different velocity.

Design the Motion for the Motion FB to give two segments: both constant velocity; one is faster than the other. The Mid-Position has a position of 180, the end position is 360. The first segment takes 230º degrees and the second takes 130º. [Compare to a Linear Motion FB, which increases steadily from 0 to 360].

The output from the Motion FB is the input to the X and Y motions. The first 180º (Y-axis) takes longer than the second 180º. With this technique, you can keep the same XY-path, but change the speed along it.

You must take care to design this motion so it 'marries' with the X and Y motions.

Draw a simple shape in the Base-Part and add a Profile to it.

Use Show Model in Mechanism to see the Solid if you like.

Step 1 is complete - I have added a rectangle.

Open the Blend-Curve dialog

In the Start Point separator and the End-Point separator, accept the values for the:

The image shows the Start Point separator expanded.

Note: Click the image to the left to 'Expand and then Collapse'.

The Command-Manager selection-box will accept a sketch-element

b)	Click the Line on the right of the XY-Path

The Motion-Path FB shows in the graphic-area.

The Motion-Point shows at the start-Point of the Line.

There are two ways to make sure the Motion-Point moves along all of the XY-Path, and returns.

The disadvantage to this approach is when you edit the XY-Path. The length of the XY-Path will change.

We do not recommend you do this.

Use Method b)

In this case, a motion from 0 to 360 ALWAYS moves the Motion-Point along the complete length of a XY-Path.

When you edit the XY-Path, the Motion-Point will always move from the start of the first sketch-element to the end of the last sketch-element.

Note: Make the motion value equal to 359.999 (not 360) to make sure the Motion-Point does not, for an instant, appear at the start of the first sketch-element when it should be at the end of the last sketch-element. This is because it is possible that the motion-value that should be 360 is actually 360.000001.

Notice that the velocity along the XY-Path is proportional to the Velocity of the Motion.

In this example, the first segment is 2x longer than the second segment.

The segment types are the same.

In this case: