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.
† 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|
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) -
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'.
In MechDesigner, the Mechanism-Editor is different to the Part-Editor.
The Mechanism-Editor uses closed-loop equations to give exact kinematics results.
The Mechanism-Editor does not find a solution 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.
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.
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.
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.