A new solution of disc permanent magnet synchronous motor (PMSM) directly driven wheel is proposed as a design customized for low floor tramcar. And the motors are overhung on the bogie frame to make the weight as the sprung mass. Meanwhile, the universal coupling is installed between the driven wheel and motor shaft. A disc PMSM is designed according to the demand of traction power. The motors are not only traction and steering actuators but are also regarded as sensors to obtain the rotational speed of motor directly driven wheel. Through the obtained data, an active sensorless steering control method is applied using the relative rotational speed between wheel pair. Finally, models combined with motor control and steering control are set up to check the control strategies. The simulation results indicate that sliding mode observer has the functionality of estimating the rotating speed with high accuracy for active steering control. The tramcar exhibits self-steering and better negotiation under active steering control. The tramcar is under a better condition of running along the central line of track with small attack angle and low power consumption while passing the shape curve track. 1. Introduction For the convenience of passengers getting aboard, independently rotating wheel (IRW) is introduced into low floor tramcar. However, the improvement is required to overcome the shortage of poor steering performance in IRW. Coupled wheelsets are one of the most widely used techniques. This technology can be separated as mechanical coupled and electrical coupled methods. Mechanical coupled methods in low floor tramcar are represented by the GTxN manufactured by ADtranz and the Citadis series from Alstom. To be specific, two wheels on the same axle are coupled by a lateral mechanical axle in these tramcars. Meanwhile, the Combino, Combino Plus, and Avenio of Siemens, the Sirio from Ansaldo Breda, the Jtram built by Kinki Sharyo, and 100% low floor tramcar from Changchun Railway Vehicles Co. Ltd (Changchun, China) [1] also use the mechanical coupled means as well, in which the front wheel and rear wheel on the same side of those tramcars would run in the same speed under longitudinal mechanical coupled technology. Although it is easy to develop early production of low floor tramcar under mechanical coupled means, the fact that electrical coupled ones replace the mechanical ones is an inevitable fact, especially with the development of power electronic devices and modern control. The bogies with four-motor independently driving wheels have been successfully
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