From the bounding gait simulation, it was determined that the amplitude the legs should move through should be 25° for maximum speed and stability. It was also found that in order to obtain a suitably small turning radius(~ 3 body lengths), the amplitude of the legshad to increase by 5° on the outer radius of the turn and decrease by 5° on the inner radius of the turn. Thus a mechanism is needed to vary the stroke of the legs. This sort of mechanism is known as a variable stroke mechanism and have myriad number of applications. In order to not reinvent the wheel, research was carried out on mechanisms capable of producing variable stroke. Research revealed several candidate mechanisms that could be used to produce a variable stroke. Variations of this mechanism exist using more complicated paths for variable valve stroke mechanisms in engines. This was not found to particularly useful for our purposes as it produces a translational and not rotary oscillation. It also requires a circular path that is difficult to manufacturer.
The first mechanisms considered were locomotive valve gears. A valve gear is a device on a locomotive that enables a steam engine to be throttled and reversed. An example of one of the most well known valve gears, the Stephenson Valve Gear is shown below. By adjusting the a lever on the locomotive, one is able to vary the stroke of the valve rod which determines the flow rate of steam through the valve. As can be seen in the diagram below, valve gears tend to be very complicated mechanisms and output a linear oscillating motion instead of the rotary oscillation need, making them ill-suited for use in this project.
However, valve gear mechanisms have been modified to be used as steering mechanisms for ornithopters, where the mechanism is used to modulate the wing stroke on both sides of the ornithopter much in the same way we intend to steer. A mechanism designed by Valentine is shown below.[1] While this mechanism has the advantage that it directly outputs an oscillating rotary motion of a link with a decent mechanical advantage, it is very complicated and requires linear actuators for stroke adjustment.
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