We usually use MechDesigner to design and analyze cams, motions, mechanisms, and multi-axis machines.
However, it is also possible to use the Constraint-Based Sketch-Editor in MechDesigner for Mechanism Synthesis†
Thus, we have prepared these topics to explain the classic mechanism synthesis techniques.
† Mechanism Synthesis is also called Kinematic Synthesis, or Linkage Synthesis.
In this section, we use the term Mechanism to replace the term kinematic-chain.
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 an equal number of drive inputs, 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 synthesize 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 - when the mechanism is kinematically-defined.
We use exact equations for Position, Velocity and Acceleration. This means you can specify any machine angle and MechDesigner evaluate the exact position of a Part, with its velocity and acceleration. 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 Equal Constraint, or a rule to make the length of that line equal to the length of this line, MechDesigner might use Newton-Raphson (and other numerical techniques) to minimize 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 math is being completed. Numerical solving techniques are much slower, perhaps 1000s times slower, and less exact than the closed form techniques that we use in the Mechanism-Editor.
However, Constraint-Based-Programming that we use in the Part-Editor offers some design opportunities for Mechanism Synthesis.
Constraint Based Sketch-Editor
In these tutorials, we use the power of the constraint based sketch-editor to synthesize a mechanism. At this stage, the mechanism is only a sketch.
We transfer the sketch from the Part-Editor to the Mechanism-Editor, as a Mechanism.
Then, we assemble the mechanism with Parts and Joints. Finally, we can add a Motion.
When a generic sketch is complete, with its 'rules' embedded in its constraints, we can use the Measurement FB, Math FB, Parameter-Control FB, and Design-Set FB to explore the dimensions of the synthesis in a dynamic way.
When synthesizing 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.
➢Translating Plane - Six-bar - coming soon