r's web
About Spider
Year: 2003-2004
MCU: 2x Fujitsu MB90560 (16bit)
Clock: 16MHz
Memory: 2kB on chip RAM (data + stack)
64kB on chip flash (program + data)
Battery: 6x 1.2V NiMH AA cels
Sensors: 4x infra-red optosensor (line detection)
3x infra-red optosensor SFH5110 (obstracle detection)
2x bumper (obstracle detection)
Actuators: 12x standard hobby servo HS-322
Programming language: "C"
Body: Duraluminium 190x250x140mm
Description: Inspired by nature, Spider is the robot with six independent legs, each with two degrees of freedom, driven by standard hobby RC servos. The purpose of the robot is to demostrate different types of gains and its advantages or disadvantages, respectively. Simple tasks as line following and obstracle avoidance are used as background for this research.

Configuration of the robot is composed of rectangular body with six legs mounted around the body. The oveall motion of the robot is achieved using a two identical "brain" boards, responsible for sending commands to each of the individual leg motors on one side of robot. It is fully autonomous, it had all of its power and computing on board.

Four infra-red sensors at the bottom of robot are used to detect black line on white background from working distance (about 50mm). Three infra-red sensors and two bumpers are used to detect obstracles in front of and on the sides of robot. With this configuration is robot able to follow line and avoid obstracles which lies on it.
The most common types of gaints for six legged robots are the tri-pod gate and the ripple gait (or wave gait).

The ripple gait keeps the maximum number of feet in contact with the ground to provide the maximum stability of robot. In the ripple gait, the rear pair of legs is moved, then the middle pair and then the front pair. The body is then translated forward, with all feet remaining on the ground. The sequence is then repeated. Unfortunately, because the robot only advances agter all six legs have moved, the walking motion is very slow - three beats are needed to complete only one cycle.

To improve the speed of the robot, a tri-pod gait can be used. In the tri-pod gait, three legs move at a time: the midle on one side and the front and rear legs on the other side. Provided the three legs form tri-pod around the centre of mass of the robot this gait will maintain stability. The tri-pod is much faster than the ripple gait because the body is translated forward each cycle by three legs, while the other three legs get into the position. Two beats are needed to complete one walk cycle.

To improve stability but not speed while walking, wave gait can be used. Body of robot is continuously slowly translated forward. When first leg gets in its rear position, it is moved fast to the front starting position. In this moment second leg gets to its rear position and is translated to the front. This repeats foreach leg. In wave gait is very important to keep all legs synchronized and the displacement of each leg is 1/6 of phase. Six beats to complete one step.