Quadruped Trotting with Passive Knees

It is clear that to trot, the robot needs knees. The simulation below shows the Scout II trotting. To keep the robotic system simple, energy efficient, and light weight, I decided to investigate trotting with unactuated knees - that is, the knees have no motor or actuator. They rely on the dynamic coupling between the upper and the lower leg for angular control. The knees do have a locking mechanism which enables it to lock every 10 degrees using a magnetic latching solenoid. The problem with unactuated systems is that both the passive and the active joints cannot be controlled at the same time. Therefore, the system equations of motion were modeled in Matlab and solved while iterating through initial conditions and other leg trajectory parameters to determine the best sets for achieving toe clearance and a consistent stride. The system was simulated in the Working Model simulation environment, and the robot was made to trot experimentally.


Scout II with passive knees	Scout II with passive knees as rendered in ProEngineer

Simulation Results

click image to view simulation (avi 400 KB)

Trotting simulation - controlled velocity trot algorithm following a ramp up,
hold at 0.4 m/s, then ramp down

Videos

Note : click on image for high resolution video (>2.0 Mb),
or on the link below the image for lower resolution video (<2.0 Mb)

lower resolution video (771 kb)	This video shows Scout II on a stand with one leg moving through a trot motion. It can be seen that the knee unlocks and the knee angle is controlled by the motion of the hip. Concurrently, the entire leg swings forward and at the time of knee lock, it is in a position that would be appropriate for touchdown. The final motion is the support phase of a trot cycle where the knee is locked. Three different views of the motion are presented.
lower resolution video (235 kb)	This video shows all four legs operating together in a trot cycle while the robot is on a stand.
lower resolution video (828 kb)	This video shows the robot performing a step-trot, where it comes to rest after every step. The first sequence shows the robot moving from right to left without the treadmill moving. The second sequence shows a front view of the same motion. The final sequence shows the step-trot for a long period with the treadmill moving.
lower resolution video (1.06 Mb)	This video shows the robot walking with a controlled velocity trot. The treadmill is turned on, and the robot follows a ramping up and down profile similar to (but slower than) the simulation video above.