Compare Piggyback Sliders and Blend-Curve

We will use the two ways to design a path that moves up and over an obstacle.

Piggyback Sliders

- and -

A Motion-Path FB

tog_minus        Piggyback Sliders

MD-16-Obstacle

STEP 1: Add an Obstacle

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.

MD-16-2-Piggyback2

STEP 2: Add Piggyback Sliders
1.Add a vertical Slider1s-red
2.Add a horizontal Slider to the vertical Slider.2s

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.

3.Add a Motion FB and then a Linear-Motion FB as the input to each.

16-4-XSlider

STEP 3: Design Motions for the X and Y Sliders.

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)).

16-4-YSlider

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.

 

MD-16-3-TracePoint

STEP 4: Add a Trace-Point.

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

MD-16-4-Point Vectors

STEP 5: Display the Velocity and Acceleration Vectors to find the Peak values.

[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.

MD-16-5-NewMotions

STEP 6: Edit the X and Y Motions to return the Slider more quickly in the reverse direction - so it is a quick-return.

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 :

1.Not collide with the Obstacle
2.Reduce the Peak Acceleration of the Point.

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.

MD-16-6-NewMotionInput

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

STEP 7: Add a new Motion FB
STEP 8: Connect it to the inputs of the X and the Y Motion FBs.

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.

tog_minus        A Motion-Path FB.

MD-16-Obstacle

STEP 1: Add an Obstacle

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.

 

 

MD-16-8-BC

STEP 2: Add a sketch in the Base-Part to give the XY-Path
a)Add a vertical Line1s-red to the right of the rectangle/obstacle.
Make sure you 'Drag upwards' from the start-Point to the end-Point.
b)Add a vertical Line2s to the left of the rectangle/obstacle
Make sure you 'Drag downwards' from the start-Point to the end-Point.
c)Add a Blend-Curve3s
Drag from the top of the Line in the right side, to the top of the Line on the Left side.
Make sure you Hover+Drag to make the start and end-Points merge.
d)Close the Part-Editor

Blend-Curve dialog box

STEP 3: Look in the Blend-Curve dialog-box

Open the Blend-Curve dialog

1.Double-click Blend-Curve sketch-element

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

Angle: Start +90, End-90),
Curvature: Start 0,End 0 and
Curvature Rate: Start 0,End 0)

The image shows the Start Point separator expanded.

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

MD-16-10-MPFB

STEP 4: Add a Motion-Path FB to the sketch of the XY-Path.
a)Click Function-Block menu (or Toolbar) > Motion-Path FB

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

b)Click the Line1s-red on the right-side of the XY-Path
c)Click 'OK' in the Command-Manager

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

The Motion-Point2s shows at the start-Point of the Line.

MD-16-11-FBs

STEP 5: Add a Motion FB and Linear-Motion FB
a)Edit the Motion FB
Change the Output Data Type to Linear
b)Connect the FBs
c)Cycle the Mechanism.
You will see that Motion-Point1s-red does not move all the way along the XY-Path.

MD-16-13-Motion

STEP 6: Edit the Motion that is linked to the Motion FB

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

a)Edit the Motion so it equals the projected length of the XY-Path, and returns to zero.
Keep the Output Data Type in the Motion FB as Linear.

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)

Motion-Path4

b)Edit the Motion so it changes from 0 to 360 [See Note] and returns to zero(0).
In the Motion FB, change the Output Data Type to Rotary.

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.

MD-16-15-MotionAssym

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

STEP 7: Edit the Motion so the first segment is longer than the second segment.

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

The segment types are the same.

In this case:

The Velocity of the Motion-Point when moving in the 'forward' direction has 1/2 of the Velocity in the 'return' direction.
The Acceleration of the Motion-Point when moving in the 'forward' direction has 1/4 of the Acceleration in the 'return' direction.

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