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Break differential-drive limits for an omnidirectional robot

15 Feb 2013  | Eileen Su, Yeong Che Fai, Tey Wei Kang

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Most mobile robots use differential-drive concept, where they are equipped with two actuators that permit only single-direction rotation at a time. This concept limits the robot's navigation because its orientation must always change according to direction of movement. We present here the development of an omnidirectional mobile robot that uses three actuators, aligned at 120° separation and each attached to an omniwheel. By manipulating actuator speed, the robot can navigate to any direction without changing its orientation.

The common form of autonomous mobile robot uses differential-driven method, where two separate motorized wheels propel the robot into motion. When both wheels rotate at the same speed, the robot moves in a linear motion, either forward or backward. The robots change their direction by rotating each wheel at different speed. This type of steering has several disadvantages. Firstly, the wheels can rotate in a single direction at any one time limiting its navigational capabilities. Secondly, the robot must change its orientation if turning motion is required. This generally increases the complexity in path planning during navigation.

Conversely, omnidirectional mobile robot uses omniwheels that can rotate in two directions simultaneously, hence allowing lateral movement. This gives omnidirectional mobile robot an added advantage of being able to navigate to any direction, without the need to alter its orientation. A right side-parking comparison is shown in figure 1. A differential drive robot has to make minimum four separate movements: forward–backward right–backward left–reverse.

An omnidirectional robot only needs to make one direct lateral motion to achieve the same result. In addition, the differential drive robot requires more space for side parking compared to the omnidirectional robot. This is only one example to show how omnidirectional robot allows more economical movement, simplifies trajectories and reduces time to arrive at target location. It will not be surprising if someday the same concept is adopted by vehicle manufacturers looking for a competitive edge.

Omni-directional navigation

Figure 1: Comparing the differential mobile robot concept with that of an omnidirectional version.

System overview, theory
The design of a three wheeled omnidirectional mobile robot is implemented in this project. To navigate to a desired location, the robot needs two feedbacks from the environment. First is the orientation feedback. An APM Ardupilot gyroscope is used to provide fine orientation reading and then send the data to the main controller, the NIsbRIO9632xt, via serial communication.

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