Project Proposal - Team 1

Introduction: 

Conventional walking robots usually employ one to two motors per leg, offering precise dynamic control. However, in cases characterized by terrain predictability and the specific requirements of forward, backward, and lateral motion, a simplified configuration utilizing just two actuators becomes a feasible choice. Our primary objective is to engineer a multi-leg walking mechanism that utilizes a single motor for locomotion and a secondary actuator for steering.  Minimizing actuator count reduces cost, complexity, and weight of the dynamic robot.

Complexities: 

One of the primary complexities we face is determining the appropriate link lengths to ensure the continuous and seamless motion of the walking mechanism. Discontinuities in the gait may occur if the link lengths are not appropriate. There are can be complexities such as the foot of the mechanism moving too quickly in half of the gait and slowing down in other half, we need to consider the sensitivity of the change in gait when we need to turn such that the gait doesn’t change drastically for small changes in input. We also need to be aware of binding and inversions. We want to achieve a gait path that has a flat bottom to ensure the main body remains level and reduce physical uncertainties.

Description of Proposed Mechanism: 

The proposed mechanism includes six identical leg subassemblies, each containing a linkage mechanism that moves a "foot" through a walking gait relative to a grounded "hip" joint. Rotation at the hip joint serves as the sole input for the mechanism. A consistent rotational speed at the hip joint will result in a walking gait pattern at the foot. Additionally, we plan to introduce some vertical compliance in each foot to ensure that the feet maintain contact with the ground during the forward step phase, even if the foot's movement is not perfectly parallel to the ground.

The six leg subassemblies will be arranged with three legs on each side of the robot. Within each side, the center leg will be phased 180 degrees apart from the outer two legs. The two sides will also be 180 degrees out of phase from each other. This configuration keeps three legs on the ground during the forward step phase (two on one side, one on the other), while the other three legs are raised and moving forward. This ensures that the center of gravity of the robot will always be between the 3 contact points so that the robot is passively stable and there is no risk of tip over during any point in the gait. All six legs will be mechanically linked using a timing belt or a similar power transfer solution to ensure that all six hip joints rotate at the same speed, maintaining constant phase separation. This allows all six legs to be driven by a single motor. The motor will most likely be a brushed DC motor with an attached worm drive gearbox, delivering low-speed, high-torque performance and relatively simple electronic control.

Steering of the walking robot will be accomplished using a single secondary actuator. This actuator will modify the length of one link in each leg linkage, thus altering the foot's path of motion during the walking gait. The linkage from the steering actuator will be configured such that a single turn can simultaneously lengthen the gait for three legs on one side while shortening the gait for the three legs on the other side. Such an asymmetric gait adjustment will result in the robot steering to one side. The steering actuator may operate a set of rocker arms, tension a set of cables, drive a pair of lead screws, or utilize other methods to adjust the effective length of a link within the leg linkage mechanisms. The actuator itself may be a servo, stepper, or DC motor.

Proposed Scope of Work:

1. Develop mechanism designs for leg mechanisms with dynamic gait adjustment capabilities.
2. Turn the mechanism designs into mechanical designs, considering physical attributes such as leg strength, joint integrity, motor power, and transmission design.
3. Create rapid prototypes and CAD models to validate leg motion before final production.
4. Construct an initial functional prototype using 3D printing and laser cutting.
5. Later iterations will address any issues and potentially incorporate machined parts to enhance reliability and functionality.

Preliminary Design Ideas:

The linkage mechanism depicted above on the left generates a gait that can be modified by adjusting the distance between the prismatic joint and the grounded rotary joint. The top row displays three legs with a short step, while the bottom row shows three legs with a long step. In both cases, the center leg is phased 180 degrees apart from the outer legs. A similar mechanism is expected to meet the performance objectives of the robot. The image on the right proposes an alternative design to achieve a flat bottom of the gait path. This mechanism has a large dependence on prismatic joints, which is not ideal. This is just to show that we are considering more mechanism designs. 

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