Tutorial 11: Two Degrees-of-Freedom Planar Kinematic-Chains

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Tutorial 11: Two Degrees-of-Freedom Planar Kinematic-Chains

Two Degree of Freedom Planar Kinematic-Chains

Background: Properties of a Point that moves along an XY-Path

If a Point has a general motion on the Mechanism-Plane, we can think of its:

A.Path on the Plane, or, 'Where it moves on the Plane'.

B.Motion along the Path, or, 'How it moves along the Path'.

In MechDesigner, to define the motion for a Point on the Mechanism-Plane, you can use:

1.Piggyback Sliders : Slider(A), to which we add Slider(B) that moves at right-angles to Slider(A). Design two motions: a motion for Slider(A) and Slider(B).

2.Motion-Path FB : sketch the Path for the Point. Design one motion for a Point along the Path.

Some designs are easier to design with Piggyback Sliders and others with a Motion-Path FB.

This tutorial uses Piggyback Sliders.

See also: Tutorial 16 - Motion-Path FB and Blend-Curves

An example of a Piggyback slider application:

A tool (say a vacuum cup) must take a carton from a stationary magazine and insert it into a pocket on a moving conveyor.

To place the carton, you must move the tool along with the moving conveyor (say, the X-direction) and also move it down into the pocket (say, the Y-direction) at the same time. To make sure the carton does not touch the sides of the pocket, we must synchronize the motion of the tool in both the X and Y directions. This design problem is easier to solve with Piggyback Sliders.

GST-Icon-AddMechanism Two Degree-of-Freedom Kinematic-Chains

Step 11.1: Introduce the Piggyback Sliders

Step 11.2: Add Dyads to the XY Piggyback Sliders

Step 11.3: Remove the XY Piggyback Sliders

Examples: Two Degree-of-Freedom

Robots: XY-Path to give the letters 'MD'

2D Point Motion

Machines that move a point in the X- and Y-axis directions, include:

Pen Plotters

Water-Jet Cutters

Laser Markers or Cutters


Each machine is a two degree-of-freedom (2 DOF) system.

Many kinematic-chains have two degrees-of-freedoms.

tog_minusGantry Robot

tog_minusSCARA Robot

tog_minusParallel '5-bar' Robot

2D Planar, Curvilinear Motion

A Plane that is coplanar with a fixed-plane has 3 degrees-of-freedom.

However, if each Point on the Plane has the same general XY-motion - that is, the Plane moves, but does not rotate relative to the fixed-plane - then it has Curvilinear Motion.

Planar Guidance Click to Play Video

In this model:

we have fixed three 'motors', shown as black cylinders, to 'Ground'.

we have connected a Part (a Dark Green color) to each motor.

we keep the tool platform at the same angle relative to the frame while the other two axes write the letters 'MD'.

This can be used with DNA sequencing where multiple pipettes need to take sample or add an 'ingredient'.