Gait Switching Lizard Inspired Robot
While extensive research allows most animal behavior patterns to be understood, one phenomenon still leaves researchers puzzled: the tendency of lizards to switch from a quadrupedal stance to a bipedal stance while running. The purpose of this project is to investigate any variations in energy consumption that some lizards may experience during motion when they actively enter a bipedal gait before natural rearing can occur. Prior research suggests that this active transition to a bipedal gait may occur in certain lizards, but offers no insight as to what advantages this behavior may offer over rearing naturally. The team hypothesized that lizards could achieve lower energy consumption by initiating active rearing during motion. Through experimentation on a lizard-inspired robot capable of modeling both passive and active rearing, the team hoped to determine the amount of time spent passively rearing after which our system would gain an energetic advantage by actively rearing.
Designed a Quadrupedal to Bipedal switching robot as a part of Bioinspired Robot Design and Experimentation course
Co-designed a robot that could switch from quadrupedal to bipedal gait during the run using its tail and trunk, the robot design drew inspiration from lizards and was used to test multiple research hypotheses.
Initial design calculations and experimentation: I did the initial calculation required to base the design on firm mathematical foundations, the calculations included calculating acceleration requirements for initiating rearing, minimum rpm of the legs required to stabilize the robot and motor selection calculations. Co-designed experiments and built simple test setups to conduct the experiments to quantify the behavior of Rhex legs and abs motor behavior.
I designed a PD controller to control the tail and stabilize the robot body during rearing and bipedal running, I codesigned the overall control flow of the robot and also implemented it on an arduino Mega and integrated it with IMU and Dynamixel servo motors.
Control Architecture
Just like how the lizards use their tail as counter-mass and swing it to balance and stabilize their body, our robot is designed to be able to actuate the tail using a servo motor controlled by an Arduino Mega board based on the body orientation readings from an IMU sensor. Our control architecture has separate designs for two different modes of operation. In the passive mode the robot would rear solely based on the reaction torque and the inertia without any actuation. In this mode the tail is actuated only to stabilize the robot’s posture. In the active transition phase firstly the frontal servo motor referred to as the ‘Abs’ motor is actuated to pull up the front wheels, followed by the actuation of the solenoid to impart the impulse force to trigger the rearing. Finally the crux of the rearing occurs due to the actuation of the tail in one swift downward motion, the reaction force of which pulls the front body into the reared position.
In both active and passive rearing modes the tail is used to stabilize the robot from toppling. This is achieved using a Proportional-Derivative controller, current IMU readings in the robot front body is constantly compared against a preset target rearing angle and the tail is actuated by the controller based on the calculated error. Implementing a Proportional-Derivative control was a clear first choice based on some of the previous work done on tail control and also due to its simplicity and ease of implementation within the limited time available. The range of motion of the tail is also capped to 130° from the tail being perfectly horizontal to a position where the tail mass is just above the robot body, this is done so that the tail doesn’t hit the ground and doesnt touch any circuitry on the robot body, but still provides enough range of motion to stabilize the robot.