Blend-Curve sketch-element and Motion-Path FB

When we want to guide a Motion-Point along an XY-Path, we often use a Blend-Point sketch-element. This is because it can give curvature continuity with other sketch-elements. Curvature Continuity is important to prevent needless vibration of the mechanism elements that are made to follow the Motion-Point FB.

The two basic steps are:

1.Edit a Part to add a sketch-path, which includes a Blend-Curve sketch-element [in this example], to define the shape of the XY-Path on the Mechanism-Plane.
2.Add a Motion-Path FB to the Blend-Curve

Add a Blend-Curve

In this tutorial, we will add a Blend-Curve between the end-points of a Line and an Arc.


Preparation - in the PART-EDITOR

In the image, I have edited the Base-Part to add an Arc and a Line.

The two Points at the end of the Arc are horizontal with the top Point of the vertical Line.

Add Blend-Curve

Add a Blend-Curve with the same hover+drag technique to merge its end-points with other sketch-elements.


Add the Blend-Curve between the Line and the Arc.

In the Part-Editor:

STEP 1: Click Add Blend-Curve in the Geometry toolbar to the left of the graphic-area.
STEP 2: Hover + Drag from the end of the Line1s-red
STEP 3: ...the Arc2s - see image

When you release your mouse, MechDesigner draws the Blend-Curve.

Step 3 is Complete


Zoom in to Examine the start and end of the Blend-Curve

Start-Point 1s-red

Angle of the Blend-Curve is 90º, vertically upwards. The Blend-Curve aligns with the end of the vertical Line3s.
Curvature of the Blend-Curve is '0'. It is a Straight-Line4s. Its Radius-of-Curvature is infinite.
Curvature Rate of the Blend-Curve is '0'.  That is to say, the Blend-Curve is a straight-line for a 'short distance' after the Blend-Curve [Mathematically, that is nonsense, of course].

Note: 'Angle' is Counter-Clockwise from the the X-axis of the Part to which you add the Blend-Curve.


End-Point 2s

Angle of the Blend-Curve is 90º vertically downwards. The Blend-Curve aligns with start of the Arc5s.
Curvature of the Blend-Curve is equal to the curvature of the Arc6s.
Curvature Rate of the Blend-Curve is '0'.  The Curvature of the Blend-Curve is the same for a short distance before the end of the Blend-Curve.


Open the Blend-Curve dialog-box

STEP 4: If necessary, de-select Add Blend-Point command
STEP 5: Double-click the Blend-Curve to open the Blend-Curve dialog-box.

Step 5 is Complete

There are two separators in this dialog

Start-Point parameters
End-Point parameters

If you expand the Start-Point separator you will find the:

Parameters are equal to the observations we have made for the start and end-points of the Blend-Curve.
Blend check-box is selected for each parameter. Thus the Parameters are 'Read-Only]

These are the default settings.

The Blend-Curve is continuous:

Curvature [Tangent] and
Curvature Rate [Same Radius at the 'Start-Point


Edit the Blend-Curve

STEP 6: Click the 'Blend' check boxes, to de-select it, to the right the Curvature Rate in the Start Point and End-Point separators.
STEP 7: Edit the Curvature Rates

Step 7 is complete

Here, I have changed the Curvature Rate values

Start-Point = 4300
End-Point = -4300

The units of Curvature Rate are 1/m2.

Rather than calculate values for Curvature Rate, it is best to observe the shape of the Blend-Curve as you use the Spin-Box tool.

What is the best value of the Blend-Curve?

It is not possible to know unless you know the application of the Blend-Curve.


This is the new 'shape' for the Blend-Curve.

STEP 8: Close the Part-Editor

Add a Motion Path FB


The Motion-Path FB is in the in the Kinematic-FB toolbar [R13.2: Kinematics-GB toolbar]

GA-Motion-Path FB

STEP 1: Click Add Motion-Path FB in the Kinematic-FB toolbar
STEP 2: Click one of the three sketch-elements
STEP 3: Click OK in the Command-Manager

Step 3 is Complete. You will see a:

1s-red Motion-Path FB

2s Motion-Point, at the start-Point of the sketch-element you select

3s Dimension between the Motion-Point and the start-Point of the sketch-path - 0 [deg] because the Motion-Point is at the start-point.

The Dimension is not shown in the default case. Open the Motion-Path FB dialog-box > Point Params. tab > Point List... separator . You can show it as 'Displacement' or 'Phase'.

'Phase' normalizes the sketch-path between 0 and 360 degrees, with 0 at the start and 360 at the end of the sketch-path.
'Dimension' gives the literal distance from the start-Point to the Motion-Point.

When sketch-element Points are 'merged', the Motion-Point will be at the start-Point of the sketch-Path and at the start-Point of the first sketch-element that you added.

When sketch-elements Points are 'coincident', the Motion-Point will be at the start-Point of the sketch-element you select.

STEP 4: Add a Motion FB and a Linear-Motion FB
STEP 5: Edit the Motion FB to change the Output Data Type to Linear - in the Motion FB dialog-box
STEP 6: Connect the FBs in the usual way

Step 6 is Complete


STEP 7: Cycle the 'Machine'

You will see the Motion-Point move up the vertical line and then along the Blend-Curve.

It will stop, somewhere along the sketch-elements, then return to the start position.

What is the Motion?

The Motion that is in MotionDesigner and linked to the Motion FB.

Why only so far along the sketch-elements?

The Motion-Point moves the distance along the sketch-elements by the maximum Y-axis value in the Motion. In this case, 90mm.


Cycle the Motion-Point between 1s-red - 2s : the ends of the sketch-elements

STEP 1: Double-click the Motion FB to open the Motion FB dialog-box
STEP 2: In the Output Data Type, click the Rotary radio button option
STEP 3: Click OK


STEP 4: Edit the Motion in MotionDesigner, to have a Rise of 360 units.

Why 360?

I find it easiest to think that the Motion-Point is a 'Crank'.

One complete rotation of the Crank = 360º.
One complete movement along the sketch-elements for the Motion-Point = 360 Rotary Units.

The Motion-Point will move the complete length of the sketch-elements that are merged, when the:

Motion in MotionDesigner has a 'Rise' of 360 [units].
Output Data Type of the Motion FB is set to Rotary

See the Video, to the left, cycle the Motion-Point with the Motion in MotionDesigner.

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