Control strategy for a mobile platform with an omni-directional drive

Authors:

G. Gruhler, Dr. Ing., Professor, Reutlingen University, Vice President (Research), Reutlingen, Federal Republic of Germany.

A. Bublikov, Cand. Sci. (Tech.), Associate Professor, State Higher Educational Institution “National Mining University”, Senior Lecturer of the Department of Automation and Computer Systems, Dnipropetrovsk, Ukrainе.

I. Gorlach, Dr. Sci. (Tech.), Professor, Nelson Mandela Metropolitan University (NMMU), Professor of the Department of Mechatronics, Port Elizabeth, the Republic of South Africa.

G. Cawood, Master of Science, Mercedes-Benz, Quality Incident Coordinator, East London, the Republic of South Africa

Abstract:

Purpose. Mobile robots are used in many areas of industry and commerce. This paper describes research on and development of a mobile platform, which is based on the concept of a ball-on-ball balancer, with two electrical drives at an angle of 90° providing a velocity vector in any direction in the horizontal plane. The purpose is to implement an originally novel principle for an omnidirectional mobile platform of very high agility, which is able at any given situation to move immediately in any direction without additional steering movements or steering mechanism.

Methodology. In advance of the design and implementation of the control strategy for the mobile device, the method of theoretical modelling of the vehicle’s properties and behaviour was applied. The developed theoretical and numerical dynamic models take into account all the control parameters which allow for the determination of the critical value of angular acceleration of the driving wheel. This is needed to prevent any slippage of the ball as this would result in the loss of accuracy of positioning. The equations of motion were implemented in the platform controller and tested.

The mobile platform consists of a ball of 0.2 m radius driven in the X-Y plane by two wheels that are attached to servo motors. The mobile platform is controlled by a CAN PLC controller interfacing with the motor drives, accelerometers and a laser sensor for feedback. Wireless communication provides the interface with the station controller via Wi-Fi and XBee Series 2 modules.

Findings. The experimentally obtained results show that the mobile platform can be reliably controlled using the ball-on-ball balancer principle with the developed control algorithm. Additional application of a sensor for guiding the mobile platform along obstacles or guiding lines improves the accuracy of the movement.

Originality. The originality of the control strategy for a mobile platform with an omnidirectional drive, proposed at the paper, is the avoiding slippage by limiting the platform acceleration to below the critical value by means of monitoring and limiting the lead values of the feedback control loop of the driving wheels.

Practical value. Development of control strategy for the mobile robot, which is based on the concept of a ball-on-ball balancer with two electrical drives at an angle of 90° providing a velocity vector in any direction in the horizontal plane.

References:

 

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9. Gruhler, G. and Bublikov, A.V. (2009), “New electrical drive system for mobile platforms with flexibility: Mathematical modelling and control strategy”, Proc. of the International Conference on “Problems of Automated Electrodrives Theory and Practice”, Alushta, Ukraine, September 14-19, 2009, Ekoinform, Lviv, pp. 262–265.

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