Holonomic omnidirectional mobile robots are useful because of their high level of mobility in narrow or crowded areas, and omnidirectional robots equipped with normal tires are desired for their ability to surmount difference in level as well as their vibration suppression and ride comfort. A caster-drive mechanism using normal tires has been developed to realize a holonomic omnidiredctional robot, but some problems has remain. Here we describe effective systems to control the caster-drive wheels of an omnidirectional mobile robot. We propose a Differential-Drive Steering System (DDSS) using differential gearing to improve the operation ratio of motors. The DDSS generates driving and steering torque effectively from two motors. Simulation and experimental results show that the proposed system is effective for holonomic omnidirectional mobile robots.

Push-to-talk over cellular (PoC) is considered as one of important applications in next generation networks (NGN). The main objective of this study is to investigate the performance of our PoC system evaluated over three operational 3G networks in the US. Measurements are conducted in two stages. First, several generic measurements such as round trip time (RTT), UDP, TCP, and dormant mode are made to understand the fundamental characteristics of 3G packet switched (PS) networks for the voice and data transmissions of the PoC service. Next, various parameters and features in our PoC software are tested to evaluate the performance of startup latency, voice latency, and voice quality. In conclusion, through empirical study we have shown that PoC applications can achieve high quality in current 3G PS networks. Furthermore from these results we study the network requirements to be solved in NGN for the voice and data communications.