Mechanism Synthesis Techniques and Mechanism Design and Analysis

We usually use MechDesigner to design and analyze cams, motions and mechanisms.

However, it is also possible to use the Constraint-Based Sketch-Editor in MechDesigner for Mechanism Synthesis

Thus, we have prepared these topics to give a few 'classic' mechanism synthesis techniques.


Notes:

Mechanism Synthesis is also called Kinematic Synthesis, or Linkage Synthesis.

We use the term Mechanism in this section. Each mechanism is a kinematic-chain, which is the term we have used in the Tutorials and Reference.


To complete these tutorials you should be able to:

Use the Part-Editor to add and delete sketch-elements, dimensions and constraints to sketch-elements
Assemble basic kinematic-chains
Use the Measurement FB

tog_minus        Machine Design

When we design a packaging machine, we often start by specifying the position and motions of the packaging and then the tools.

In some respects, how we specify the positions and motions for the machine parts also indicates how we will design the machine.

Cams and Servomotors:

When you must specify the motion for the tools at every instant, and also specify their motion-derivatives, then we would normally use Cams or Servomotors.

Each mechanism must have the necessary degrees-of-freedom, and the same number of drive inputs to the mechanism, to fully control the tool's motion at every machine instant.

- versus (compared with) -

Mechanism Synthesis:

When we can specify the motion for a tool at a reduced number of instances, and the motion derivatives are not so important, we may be able to synthesise a mechanism that does not have cams or servos.

The simpler mechanism may guide the tool sufficiently well with only one degree-of-freedom.

Advantages and Disadvantages of Mechanism Synthesis

Advantages: Simpler mechanisms are cheaper, easier to manufacture, can often run faster, do not wear out as quickly as cams.

Disadvantages: Less easy to design, with a reduced set of possibilities. You must be careful to avoid the possible 'defects'.

tog_minus        The Mechanism Editor compared to the Part Editor

In MechDesigner, the Mechanism-Editor is different to the Part-Editor.

Mechanism-Editor

The Mechanism-Editor uses closed-loop equations to give exact kinematics results.

The Mechanism-Editor does not find a soution with numerical techniques. We use exact equations for Position, Velocity and Acceleration. This means you can specify all possible machine angles and get the exact position of a Part, with velocity and acceleration. The Mechanism-Editor does not need to 'interpolate'. The solutions are also very fast, perhaps 1000s of times faster than numerical techniques.

Part-Editor

The Part-Editor uses 'Constraint-Based-Programming' techniques.

The Part-Editor has a 'solver' to minimize errors for the constraints you add to a sketch.

For example, when you add the constraint 'make that line equal to the length of this line', MechDesigner might use Newton Raphson (or other numerical technique when necessary) to minimise the error between the two lengths. When one of the lengths changes, we must find the solution again. The more constraints that you give to a sketch, and the more sketch-elements you add, the more maths is being completed. Numerical solving techniques are much slower, perhaps 1000s times slower, and less exact than closed form techniques.

However, Constraint-Based-Programming offers opportunities for Mechanism Synthesis.

tog_minus        Mechanism Synthesis in MechDesigner

Constraint Based Sketch-Editor

In these tutorials, we use the power of the constraint based sketch-editor to synthesise a mechanism. At this stage, the mechanism is only a sketch.

We transfer the 'sketch-based' mechanism from the Part-Editor to the Mechanism-Editor.

Then, we assemble the mechanism with Parts and Joints. Finally, we can add a Motion.

tog_minus        Possible 'Defects' with Mechanism Synthesis

When synthesising a mechanism, it is possible that you get a solution that, in practice, you can not use. There are three reasons for this.

1.Circuit Defects: You may need to disassemble the linkage then reassembly it to get to the next position. For example, the Dyad Closure may need to change.
2.Branch Defects: A part may need to go in to a toggle position between the positions and it would not be able to pass through this position in a real mechanism.
3.Order Defects: The Crank gives the positions you want, but in the wrong order.

AddMechanism1 Mechanism Synthesis.

Translating Plane - Six-bar - coming soon

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