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.
Another mechanism we considered was the
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.
Sources:
- A Variable Stroke Mechanism for Ornithopters
- Fixed Frequency, Variable Amplitude (FiFVA) Actuation Systems for Micro Air Vehicles