A tracked vehicle (also called: track-type tractor, tractor crawler, or track-laying vehicle) is a vehicle that runs on continuous tracks instead of wheels. Tracked vehicles include construction vehicles, military armored vehicles and unmanned ground vehicles. The principal design advantages of tracked over wheeled vehicles are that they are in contact with a larger surface area than would generally be the case with a wheeled vehicle, and as a result exert a much lower force per unit area on the ground being traversed than a conventional wheeled vehicle of the same weight. This makes them suitable for use on soft, low friction and uneven ground such as mud, ice and snow. The principal disadvantage is that tracks are a more complex mechanism than a wheel, and relatively prone to failure modes such as snapped or derailed tracks.
Continuous tracks, also called tank treads or caterpillar tracks, are a system of vehicle propulsion in which a continuous band of treads is driven by two or more wheels. This band is typically made of modular steel plates in the case of military vehicles, or rubber reinforced with steel wires in the case of lighter agricultural or construction vehicles. The large surface area of the tracks distributes the weight of the vehicle better than steel or rubber tires on an equivalent vehicle, enabling a continuous tracked vehicle to traverse soft ground with less likelihood of becoming stuck due to sinking. The prominent treads of the metal plates are both hard-wearing and damage resistant, especially in comparison to rubber tires. The aggressive treads of the tracks provide good traction in soft surfaces but can damage paved surfaces. Special tracks that incorporate rubber pads can be installed for use on paved surfaces to prevent the damage that can be caused by all-metal tracks. Continuous tracks can be traced back as far as 1770 and today are commonly used on a variety of vehicles including
but can be found on any vehicle used in an application that can benefit from the added traction, low ground pressure and durability inherent in continuous track propulsion systems.
Control Algorithms in Tracked Robots
Constrained by track robots' kinematics and dynamics, stair-climbing is a complicated task and there is little research analysing robots' climbing behaviours. The autonomous tracked robot developed here is expected to explore stairs, move forwards facing the stairs, and change geometry configuration for climbing and landing safely. The steps of exploring and climbing stairs are as follows:
The basic track configuration for easily exploring stairs is to keep the front and rear track arms up. (Figure 1)
The front and rear arms rotate to the stair's angle θ1 when the robot is ready to climb the stairs. (Figure 2(a))
The track configuration changes to the straight type (Figure 2(c) and Figure 2(d)) when the robot is on the stairs. (Figure 2(b))
The robot changes the configuration of the front arm until the arm touches the floor at the moment when the robot reaches the top of the stairs. (Figure 2(e))
The front arm rotates to the stair's angle (Figure 3(b)) when the robot moves forwards and is then extending a little over the step surface. (Figure 3(a))
The track configuration of the robot changes to the straight type (Figure 3(d)) when the robot is on the stairs (Figure 3(c)).
The robot changes the configuration of the front arm until the arm touches the platform. (Figure 3(e))
Figure 1: Track configuration for exploring stairs (basic track configuration)
Figure 2: Variable track configurations for climbing upstairs
Figure 3: Variable track configurations for climbing downstairs
The steps of climbing stairs are depicted in Figure 1 & Figure 3, which show the capability of the tracked robot to adapt to stairs by changing its geometry configuration. In this paper, we expect the tracked robot to be able to autonomously explore stairs in an unknown environment and effectively complete the stair-climbing task using the developed intelligent control modes. For this purpose, five modes are developed, as shown in Figure 4. The exploration mode is expected to locate stairs only depending on the Kinect depth information, which is used to detect the features of the edge of the staircase. Then the robot autonomously moves toward the stairs while maintaining room to climb safely in control mode. Maintaining the robot's movement parallel with the stairs' centre line ls is an important issue when climbing. The tracked robot has to face the stairs. This will be done in the alignment mode. The purpose of the climbing mode is to supply the robot with enough friction force to climb stairs. Finally, the landing mode gives a function for landing.