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This bistable switch is an example of a compliant mechanism with a four-bar mechanism pseudo-rigid-body model. A contact force is created by causing the contacts to connect before the second stable equilibrium position is reached. Living hinges are used at the other joints. This mechanism could be used as a fully compliant electrical switch, or for other applications, such as on cabinet door hinges.
This can be 3D printed or milled. For ideal use, milling or cutting with polypropylene results in the best performance.
If 3D printing, depending on the filament material you use and its fatigue/flex properties, this will determine the number of cycles it can handle. See print settings
Depending on your 3D printer and settings, you will probably have to SCALE UP the model size to allow your 3D printer nozzle to reach the small members. You do not need to scale up if you are cutting it out with a mill.
You can print with PLA or ABS (however PLA will have fewer cycles). The model shown in the pictures was printed in PLA and was SCALED UP 160%.
Overview and Background
This lesson introduces the concept of bistable compliant mechanisms. It first offers an understanding of bistable systems in general, then goes into the compliant bistable mechanisms and the advantages thereof.
Lesson Plan and Activity
What is a Bistable Mechanism?
A Bistable Mechanism has two stable equilibrium positions in which it can rest. A common example of a bistable device is a light switch (Figure 2) which has two stable positions, resting in either the "on" or "off" position, but not in between. The mechanism remains in one of the stable positions until an external force is exerted on it.
A simple way to illustrate the stability of a bistable system is by using the "ball-on-the-hill" analogy (Figure 3). The ball rests in either of the two low points on the potential energy graph.
Figure 2: A light switch is a common example of a bistable mechanism
Figure 3: A graph of the potential energy of a bistable system; it has two local minima. A surface shaped like this with two "low points" can act as a bistable system; a ball resting on the surface can only be stable at those two positions. Balls marked "1" and "3" are in the two stable positions, while ball 2 is at the point of unstable equilibrium between them. [Wikipedia: Bistability]
Why use compliant mechanisms for bistable systems?
Just like any mechanism, a compliant mechanism also transfers or transforms motions, force, or energy. However, unlike rigid-linked mechanisms, it uses its own flexible members to gain mobility by storing strain energy internally, similar to potential energy stored in a deflected spring.
REDUCE PART COUNT AND COST
Using compliant mechanisms can be extremely advantageous because it reduces the number of parts required. This can make it easier, faster, and more affordable to manufacture and assemble. In the case of this compliant switch, you only need a single part to perform the task while a traditional switch may require multiple pieces such as springs, hinges, pins, etc. The compliant switch uses its own members to store energy to simulate the springs found in other switches.
Because of the reduced part count, there are also fewer movable joints which reduce wear and need for lubrication.
It can also increase precision because backlash is reduced or even eliminated since there is no "play" or "wobble" between separate parts.
By minimizing how many separate parts are needed in the design, the overall weight can be decreased significantly. This is very beneficial for aerospace and many other applications where weight is an issue. It can also help companies save money on shipping costs on consumer products.
Compliant mechanisms can easily be scaled down to miniature versions. It is impractical to create microscopic rigid body mechanisms that include pins and multiple assembly parts, because it is difficult to manufacture and assemble on such a small scale. it is more feasible to use compliant mechanisms in micro mechanisms because of the reduction of parts and joints as it reduces the need for assembly and separate part manufacturing.
Activity #1 - Build your own compliant bistable mechanism
Have the student (or yourself) attempt to build a bistable 4-bar compliant mechanism using our FlexLinks and LEGO links. Experiment with different lengths and pin locations to achieve different results.
Here is the link for the FlexLink member:
Simple Bistable mechanism build using LEGOs and FlexLinks. Notice how there are two stable positions
Activity #2 - Print the fully compliant bistable switch
*See "Print Settings" to see details on 3D printing and Water Jet cutting.
Two options to create:
- 3D Printer
- PLA or ABS filament (or other)
Mill, CNC Router, Water Jet Cutter
- compliant mechanisms
- living hinges
1/4" Polypropylene milled
This mechanism is patented. If you have interest in commercializing this product, contact Dr. Larry Howell at firstname.lastname@example.org
Bistable Compliant Mechanism
By Jensen, B.D. , Howell, L.L. , Roach, G.M.
U.S. Patent No. 6,215,081