Wheeled Mobile Robots

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Marshall Jackson
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1 Wheeled Mobile Robots Most popular locomotion mechanism Highly efficient on hard and flat ground. Simple mechanical implementation Balancing is not usually a problem. Three wheels are sufficient to guarantee stability. If the number of wheels are more than three, a suspension system is needed to allow all wheels to maintain ground contact on uneven terrain. The focus of research in wheeled robotics is on traction and stability in rough terrain, maneuverability and control.

2 The Four Basic Wheels types a) Standard Wheel: Two degrees of freedom; rotation around the motorized wheel axis and the contact point. b) Castor Wheel: two degrees of freedom, rotation around the wheel axle and the offset steering joint.

3 The Four Basic Wheels Types c) Swedish (Omni) Wheel: Three degrees of freedom; rotation around the motorized wheel axis, around the rollers, and around the contact point. 45 degrees or 90 degrees types. d) Ball or Spherical wheel: Like the mouse ball. 3 DOF. Suspension is technically not solved.

4 Standard and Castor Wheels The main advantages : the easy implementation the high load capacity the high tolerance to ground irregularities. The main disadvantage : to make a vehicle using these wheels steerable, must be steered first along a vertical axis and then moved around a horizontal axis. So especially in case of heavy vehicles and when it is not moving during steering this steering method cause s high friction during steering as the wheel is actively twisted around its vertical axis, this increases the power consumption and reduces the positioning accuracy of the vehicle.

5 Swedish and Ball Wheel The Swedish wheel functions as a normal wheel, but it has little passive rollers around the circumference. These rollers provide low resistance in another direction. The wheels primary axis serves as the only actively powered joint. The spherical wheel is a real omnidirectional wheel. There are several implementations of spherical wheels.

6 How Ball Wheel is Actuated In the ball wheel design power from a motor is transmitted through gears to an active roller ring and then to the ball via friction between the rollers and the ball. Due to the rollers, fixed at the roller ring and the chassis, the ball is able to roll passively in any direction. A robot needs at least three spherical wheels to become mobile.

Stability A robot with a two wheeled differential drive can achieve stability if the center of mass is below the wheel axle or if there is a third point of contact striking the floor.

7 Stability A robot with a two wheeled differential drive can achieve stability if the center of mass is below the wheel axle or if there is a third point of contact striking the floor. Under normal circumstances a wheeled robot needs at least three wheels with ground contact to achieve static stability. The center of gravity has to be completely within the support polygon, formed by the three wheels with ground contact.

8 Maneuverability Maneuverability is a very important issue for a wheeled robot to solve its tasks. When a robot is able to move in any direction of the ground plane (x,y) it is omnidirectional. This level of movement requires usually actively powered wheels that can move in more than one direction like Swedish or spherical wheels. In contrast the Ackermann steering configuration, which is used by cars, is not omnidirectional. Vehicles using this configuration have usually turning radius which are larger than the vehicle itself It is not able to move sideways (that means in axis direction), such a movement requires several parking maneuvers consisting of repeated changes in wheel direction and forward and backward movement.

9 Controllability Driving a robot which uses four powered Swedish wheels straight forward, all wheels must be driven with exactly the same speed, to move in a perfectly straight line. Even little errors in the speed of the wheels will cause mistakes in the desired travel path of the robot. At this point the benefit of Ackermann steering appears, because controlling such vehicles is much easier. Driving straight forward means just locking the steerable wheels and driving the motorized wheels. These are connected by an axis, so the speed of the drive wheels is always the same by actuating just one motor. In general, controllability is inversely correlated with maneuverability.

10 Holonomic vs. non holonomic A robot is holonomic if the controllable degrees of freedom are equal to the total degrees of freedom. If the robot is able to move in an arbitrary direction out of any position at any time it is called holonomic. Consider a two-dimensional space; the degrees of freedom are the x axis, y axis, and rotation about the origin. In this space, a mobile base with three omnidirectional wheels in a triangular configuration would be considered holonomic. Consider a one-dimensional space; there is only one degree of freedom: the x axis. An example of this system is Rail transport, and in such a system the trains would be considered holonomic.

11 Odometry Odometry is the use of data from motion sensors to estimate change in position over time. Odometry is used by mobile robots, to estimate (not determine) their position relative to a starting location. In case of incremental encoder; sensitive to errors due to the integration of velocity measurements over time to give position estimates. Rapid and accurate data collection, equipment calibration, and processing are required in most cases for odometry to be used effectively. Robot can measure how far the wheels have rotated, and if it knows the circumference of its wheels, compute the distance.

12 Visual Odometry In robotics and computer vision, visual odometry is the process of determining the position and orientation of a robot by analyzing the associated camera images. Problems with rotary encoders: Cannot be used with legged robot. Precision problems, since wheels tend to slip and slide on the floor creating a non-uniform distance traveled as compared to the wheel rotations. Odometry readings become increasingly unreliable over time as these errors accumulate and compound over time. Visual odometry is the process of determining equivalent odometry information using sequential camera images to estimate the distance traveled. Visual odometry allows for enhanced navigational accuracy in robots or vehicles using any type of locomotion on any surface.

13 Wheeled Mobile Robots Stability of a vehicle is be guaranteed with 3 wheel center of gravity is within the triangle with is formed by the ground contact point of the wheels. Stability is improved by 4 and more wheels Bigger wheels allow to overcome higher obstacles but they require higher torque or reductions in the gear box. Most arrangements are non-holonomic require high control effort Combining actuation and steering on one wheel makes the design complex and adds additional errors for odometry.

14 Wheels Configuration Synchro Drive This mechanism consists of three steerable wheels arranged in a triangle. All wheels are driven and connected by a single belt which is actuated by one motor. Second belt, which is actuated by an additional motor and is connected to the wheels too, is used to spin each wheel around its individual vertical axis. In this way the robot can be driven and steered relatively simple by controlling just two motors.

15 Synchro Drive - Drawbacks The Robot is not omnidirectional; all wheels are steered with respect to the robots chassis together, so there is no way to re-orientate the chassis directly. Dead reckoning, the wheel which is closest to the motor begins spinning before the furthest wheel, this causes little changes in the orientation of the chassis, which accumulates to a large error in orientation when there are several changes in motor speed.

16 Tribolo Each ball wheel is driven by one To rotate the robot around its vertical axis all motors are driven with the same speed. To drive the robot straight forward one motor has to be turned off and two motors have to be driven, one with velocity v and the other with velocity v. The advantages of this design are the simple design and excellent maneuverability, but it is limited to flat surface and it is just capable to carry small loads.

17 Kovan robot Uses three actively powered Swedish 90 wheels. The robot is able to rotate in place by driving all wheels with the same velocity. To make a linear movement in direction v1 the first motor must move with velocity v, the third motor with velocity v and the second motor must be stopped, so that the second wheel will roll freely on the little rollers perpendicular to its powered axis of motion.

Carnegie Mellon Uranus robot This robot uses four actively powered 45 Swedish wheels. To move the robot straight forward or backward all wheels must spin with the same velocity in the same direction.

18 Carnegie Mellon Uranus robot This robot uses four actively powered 45 Swedish wheels. To move the robot straight forward or backward all wheels must spin with the same velocity in the same direction. The robot is also able to do a lateral movement. To do this the diagonal pair of wheels must spin with the same velocity in the same direction (v) and the other diagonal pair of wheels must spin with same velocity in the opposite direction(-v). Furthermore the robot is able to rotate in place. To rotate clockwise the wheels on the left side must spin with velocity v and the wheels on the right side with velocity v

19 Tracked Robot A tracked vehicle is steered by moving the tracks with different speed in the same direction or in opposite direction. The use of tracks offers a much larger area of ground contact, so the vehicles traction on loose surface is enhanced. Due to the large contact patches, tracked vehicles usually change their direction by skidding, where a large part of the vehicle slides against the ground, so: the vehicle needs a lot of space to change the orientation of the chassis. When the surface is hard, more friction during steering and with thus more power consumption of the vehicle.

20 Walking Wheels A hybrid solution which combines the advantages of legged and wheeled locomotion. Shrimp has six motorized wheels and is capable to climb barriers that are two times larger than its wheel diameter. it is able to overcome obstacles passively, that means that the robot has no sensors to detect an obstacle, the robot s mechanical structure is able to adapt the profile of the terrain.

Introduction to Robotics

Introduction to Robotics Ph.D. Antonio Marin-Hernandez Artificial Intelligence Research Center Universidad Veracruzana Sebastian Camacho # 5 Xalapa, Veracruz Robotics Action and Perception LAAS-CNRS 7,