Mechanical Design Mechanically, the quadruped is quite simple. The main body is small compared to the size of the legs, but large enough to house a high capacity Lithium Polymer (LiPo) battery, four ultrasonic distance sensors, the Arbotix controller and the interconnecting wires. This relatively small body to leg ratio means that the quadruped body can have a low weight. It also means that the body can be kept at a distance from any obstacle, leaving the legs to deal with bumps or knocks. The diagram to the right shows the basic layout of the main body. The components can be accessed easily via a removable panel on the bottom of the quadruped.

Project Information The exploration of hazardous areas is a task best suited to robots due to their smaller size and potential immunity to dangers like radiation, toxic fumes and fire. However the environment of such areas is often complex, fraught with obstacles and uneven terrain, making ground navigation of these areas a difficult task. For exploratory robots, the use of actuated legs rather than traditional wheels can improve manoeuvrability and improve the robot’s ability to overcome obstacles. This aim of this project is to develop and build a prototype quadruped (four-legged) robot capable of navigating an whilst avoiding or overcoming obstacles. Along with this, the quadruped will use sensors to create a 2D map of a room, and send this back to a computer for processing. The first part of this project involves the mechanical, and electrical design and assembly. Following this software will be developed to implement co-ordinated movement, autonomous navigation, wireless control and distance mapping.

Quadruped robot for remote exploration of hazardous areas

MEng Electrical and Electronic Engineering (WIE) School of Electrical and Electronic Engineering The University of Manchester

Project student: Mr Joshua Elijah joshua.elijah@gmail.com Project supervisor: Professor Barry Lennox barry.lennox@manchester.ac.uk

Quadruped Controller An Arbotix-M Robocontroller is used as the main controller for the quadruped. This is a modified version of the more renowned Arduino Mega, and is available as a ready-built component. This board will implement all the control algorithms and acquire data from sensors onboard the quadruped. It will then implement a control algorithm and send commands to the Motors.

Servo Motors Each quadruped leg is constructed using three Dynamixel AX-12A motors. These motors provide the high torque needed to support the chassis of the quadruped. They also have many other features such as position feedback and temperature sensing., allowing the robot to sense it’s own position. They are controlled via serial communication from the Arbotix controller, All twelve motors can move simultaneously for co-ordinated movement.

Automonous navigation and 2D mapping The quadruped has four distance sensors on each of it’s sides. It uses these sensors for both local obstacle avoidance and longer range distance mapping. In the bottom left is an example of simple short-range obstacle avoidance using the quadruped’s distance sensors. In this example an obstacle is detected, so the quadruped rotates until the obstacle is no longer visible. This means an immediate clear path has been found and the quadruped can continue moving The diagram in the bottom right shows an implementation of 2D mapping. The robot navigates to the centre of a room and takes an initial distance measurement. The robot then rotates by a small angle and takes another distance measurement. It continues this until it has rotated a full 90˚. An array of distances and angles will be generated from this, that can be sent to a computer to be converted into a 2D map.

Abstraction of movement Movement of the quadruped is the first task of the quadruped software . Achieving co-ordinated, omnidirectional movement of a legged robot is not a simple task. To achieve it, the task can be broken down into a series of abstraction layers. At the start is Angle Control. At this level, the angles of joints of the legs can be varied. Above this is the Cartesian Control layer which uses Inverse Kinematics to control each leg in terms of x,y and z co-ordinates. The Walking Gait Generation layer generates a series of cartesian co-ordinates to implement a particular style of walking gait. The Movement of the CoG (Centre of Gravity) layer calculates the intermediate positions of the legs between walking steps to ensure that the CoG of the quadruped moves smoothly. The end goal is to achieve omnidirectional movement by manipulating the quadruped’s CoG.

Electronic Design Shown below is the system diagram of the quadruped robot It’s split into two parts; on the left is the remote control and on the right, the actual quadruped. The Arbotix is at the heart of the quadruped, and interacts with the motors, sensors, switches, LEDs, and an XBEE wireless bluetooth module. The remote control uses an Arduino Nano to interface with various input components, and sends data to the quadruped via another XBEE wireless module

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[1] – Image available at http://www.trossenrobotics.com/p/arbotix-robot-controller.aspx [2] – Image available at http://www.trossenrobotics.com/dynamixel-ax-12-robot-actuator.aspx