Getting Started Tutorials - MechDesigner > Tutorial 16: Motion-Path FB and Blend-Curves > Step 16.4: Compare XY Piggyback Sliders and Blend-Curve

Compare Piggyback Sliders and Blend-Curve

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

•Piggyback Sliders in the X-axis and Y-axis direction

•Sketch-Path and Motion-Path FB

Piggyback Sliders

STEP 1: Add an Obstacle to the Base-Part

To see the obstacle, we will add a rectangle.

1.Part-Editor: Geometry toolbar > Add 4 × Lines as a sketch-loop

2.Part-Editor: Constraints toolbar > Add 3 × Perpendicular

3.Part-Editor: Geometry toolbar > Add 1 × Horizontal to a Line

4.Part-Editor: Geometry toolbar > Add Dimension (70mm wide, 50mm tall )

5.Exit Part-Editor

STEP 2: Add Piggyback Sliders

1.Edit Base-Part: Geometry toolbar > Add Line, Vertical Constraint, Dimension

2.Exit Part-Editor

3.Mechanism-Editor: Kinematic elements toolbar, Add Part, Add Slide-Joint

4.Mechanism-Editor: Kinematic FB toolbar, Add Motion-Dimension FB (Y-axis)

5.Edit the Slider:

6.Part-Editor: Add vertical Line in the Slider Part (which is Horizontal in the Mechanism-Plane)

7.Exit Part-Editor

8.Mechanism-Editor: Kinematic elements toolbar, Add Part, Add Slide-Joint

9.Mechanism-Editor: Kinematic FB toolbar, Add Motion-Dimension FB (X-axis)

X,Y plot values for each Blend-Point

STEP 3: Design Motions for the X-axis Slider.

1.Motion-Designer: Right-Click the motion graph. Rename to “X”

2.Motion-Designer: Blend-Point Editor. All Blend-Point Position=0,

3.Motion-Designer: Close Blend-Point Editor

4.Motion-Designer: Insert Blend-Point: Click the Motion graph

5.Motion-Designer: Blend-Point Editor. Edit X-axis Position values of all Blend-Points.

6.Motion-Designer: Blend-Point Editor. Edit Y-axis Position values of all Blend-Points.

X,Y plot values for each Blend-Point

STEP 4: Design Motions for the Y-axis Slider.

1.Motion-Designer: File toolbar > Add new Motion

2.Motion-Designer: Right-click the motion graph Rename to “Y”

3.Motion-Designer: Blend-Point Editor. Edit X-axis values of all Blend-Points.

4.Motion-Designer: Blend-Point Editor. Edit Y-axis values of all Blend-Points.

STEP 5: Add Function-Blocks

1.Mechanism-Editor: Click Kinematic FB toolbar > Linear-Motion FB, and click the graphics-area.

2.Mechanism-Editor: Click the Kinematic FB toolbar > Motion FB, and click the graphics-area

3.Mechanism-Editor: Click the Kinematic FB toolbar, Motion FB AGAIN, and click the graphics-area.

4.Mechanism-Editor: Edit a Motion FB - select “Y” in the Select-Motion drop-down list box.

5.Mechanism-Editor: Edit a Motion FB - select “X” in the Select-Motion drop-down list box.

6.Mechanism-Editor: Drag wires to connect FBs to the Motion-Dimension FBs

STEP 6: Add a Trace-Point to see the Path

1.Mechanism-Editor: Kinematic elements toolbar > Add Trace-Point

2.Graphics-Area: Click a Point on the X-axis Slider.

3.Command-Manager: Click to close

<<< click to play video

Point Properties dialog

STEP 7: Open the Point Properties dialog

1.Mechanism-Editor: Filters toolbar > Filter for Points

2.Mechanism-Editor: Double-Click the Point with the Trace-Point

The Point Properties dialog is now open.

STEP 8: Select the Vectors to show

1.Enable Show Velocity Vector

2.Enable Show Acceleration Vector

3.Enable Velocity Vector Options: Mag. (Magnitude)

4.Click to close the Point Properties dialog

Magnitude = √(X2 + Y2)

STEP 9: Cycle the model

1.Click Run toolbar > Cycle

1.Press ALT+C on your keyboard

The Maximum Vertical and Horizontal velocity and acceleration vectors are equal to those of the X and Y Velocity and Acceleration graphs in MotionDesigner.

You can find the maximum acceleration with these methods:

•Move the MMA until the Acceleration Vector is a maximum value - see left

•Use a Point-Data FB with a Graph FB to measure and plot the motion-values

•Use a Stats FB to review the data values - e.g. Min, Max, RMS.

We will imagine that this motion is to guide a Pick and Place Robot.

We may want the Robot to move more rapidly in the Pick direction (say from right-to-left) as compared with the Place direction (say from the left-to-right direction)

STEP 10: Edit the Motions to Return more quickly.

1.Try to edit the X and Y motions to move the Sliders quickly in the Pick direction as compared to the Place direction

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

You will notice that it is not easy to make the 'Place' and 'Pick' paths the same.

There is a different approach - or an Alternative Strategy.

Alternative Strategy for “Fast Pick” and “Slow Place” motion.

Motion to modify the MMA

When you want a “fast Pick” and “slow Place”, it is easier design and edit the input motion function to the X-Y-axis Slider motions.

In the standard motion strategy - as above -

•The output from the Linear-Motion FB increases at a constant rate from 0 to 360.

•It is the input-function to the X-Y-axis motions. Its output mirrors the MMA.

In the image to the left, this “Pink Motion” is the output from the Linear-Motion FB motion.

The alternative motion increases slowly (X-axis: 0-270) until its output, the Y-axis, is 180, and then increases rapidly (X-axis from 270 to 360) until its output, the Y-axis increases from 180 to 360.

In the image to the left, the alternative motion has the Blue then Orange segments

The output of this motion is connected to the X-axis and Y-axis Motion FBs for the Piggyback Sliders in exactly the same way as the output from a Linear-Motion FB in the original example, above.

With this technique, XY-path does not change, but you change the speed along it.

Thus, if you do not want to change the path, but only change the velocity along, it, and you have Piggyback Sliders, it is often the best strategy.

Output of Motion FB is input to Motion FB

Motion-Path FB.

STEP 1: Add an Obstacle

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

Use Visibility toolbar > Show Solids in Mechanism to see the Extrusion if you like.

STEP 2: Add a sketch in the Base-Part to give the XY-Path

1.Add a vertical Line to the right of the rectangle/obstacle.
Make sure you 'Drag upwards' from the start-Point to the end-Point.

2.Add a vertical Line to the left of the rectangle/obstacle
Make sure you 'Drag downwards' from the start-Point to the end-Point.

3.Add a Blend-Curve
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.

4.Close the Part-Editor

STEP 3: Look in the Blend-Curve dialog

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 is the Start-Point separator, only

STEP 4: Add a Motion-Path FB to the sketch of the XY-Path.

1.Click Function-Block menu (or Toolbar) > Motion-Path FB

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

2.Click the Line on the right of the XY-Path

3.Click in the Command-Manager

The Motion-Path FB is in the graphics-area.

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

STEP 5: Add a Motion FB and Linear-Motion FB

1.Edit the Motion FB
Change the Output Data Type to Linear

2.Connect the FBs

3.Cycle the Mechanism.

The Motion-Point does not move all the way along the XY-Path.

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.

Method A

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

You can find the length of the Path in the Motion-Path dialog.

Disadvantage: If you edit the XY-Path, the length of the XY-Path will change. You will also need to edit the Motion in MotionDesigner.

We do not usually recommend Method A

Method B

b)Edit the Motion so it the X and Y-axes change from 0 to 360, (See Note below) and also returns to zero(0).
In the Motion FB, change the Output Data Type to Rotary.

Advantage: If you edit the XY-Path to change the length of the Path, he Motion-Point ALWAYS moves along the total length of a XY-Path.

Note:

Make the maximum Y-axis motion value is 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 should be 360 , but it is actually 360.000001.

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

In this example, the Segment-Width of Segment 1 is 2× Segment-Width of Segment 2

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.