...
After doing further research on ornithopter turning mechanisms, we found a hypocycloid gear mechanism for stroke control. It consists of a ring gear with a rotating planet gear inside with a pin that connects to a scotch yoke. By rotating the ring gear one can adjust the stroke that the scotch yoke moves through. This mechanism is nearly ideal, however, it requires a complex arrangement of gears that would be difficult for us to manufacturer.
The next one is a variable piston stroke mechanism used in automobile engines and uses a linear actuator to move a pivot point along a circular path to modify the stroke of a piston.
To change the stroke of the piston, the location of the pivot point on the speed control radius bar is changed along a circular slider. This mechanism requires a difficult to make circular slider and a linear actuator, so it was not choosen.
After some more research we found a simple variable stroke mechanism well suited to our task that is used in a ratcheting continuously variable transmission(CVT). In this mechanism, the oscillation amplitude of a link connected to a ratchet is adjusted to control speed. By decreasing oscillation amplitude, the link moves over a smaller amount of distance each cycle, thus the output connected to the ratchet rotates a smaller amount per cycle, thus speed is decreased. By using a several of these mechanisms in parallel, one can attain nearly continuous rotation of the output shaft. To change the stroke of the link, one changes the location of the pivot point of the speed control link. This works much the same way as the variable piston stroke mechanism shown above, except that a rotatable link is used instead of a slider. This mechanism is well suited for use in our project because
Sources:
- A Variable Stroke Mechanism for Ornithopters
- Fixed Frequency, Variable Amplitude (FiFVA) Actuation Systems for Micro Air Vehicles
- Mechanisms and Mechanical Devices Sourcebook Third Edition
http://www.zero-max.com/adjustable-speed-drives-c-21-l-en.html