increases. In addition, it requires a lot of control units, so it moves at low speed. Therefore, there is a demand for lightweight robots with excellent operability.  
For the purpose of movement to iron structures, the authors invented a lightweight actuator with excellent operability and a very simple structure [15; 16]. When this actuator is built into a robot, direct drive is possible without reduction gear mechanisms because this actuator has a high propulsion force. In addition, consideration about a reciprocating movement using two vibration components was accomplished [17]. However, these actuators [15; 16; 17] could not move even a slight step in a horizontal plane. The movement characteristics were significantly reduced due to vibration interference between the components. Furthermore, the author proposed a new actuator system that combines magnetic wheels and vibration components [18; 19]. These wheel-type actuators can move on small steps on a horizontal surface, but cannot move on steps on a vertical surface. Further, the operation of movement on the step is complicated, and these cannot automatically move on the step. Thus, these actuators cannot move across horizontal surfaces that have large steps or vertical surfaces that have very small steps. Thus, despite the development of the above-mentioned robots and actuators, a technology for inspecting structures has not yet been established.  
In the present study, a novel vibration actuator system capable of movement on a complex iron structure with large steps in the horizontal and vertical planes was proposed. The cooperative operation of the two vibration component, this actuator system can automatically move the large step in a vertical plane. First, the direction of movement for the actuator was examined, and a vibration actuator capable of reciprocating movement that has high efficiency was manufactured. In consideration of mobility, a scale size of the vibration actuator is designed to be smaller than 50 mm and a total mass is lighter than 20 g. In addition, a vibration actuator system in which two vibration actuators and a wheel having a vibration force were connected by a flexible silicon rubber material was prototyped. By extending the operating principle proposed by the authors, a new type of the vibration actuator system that can be directly driven without using an electromagnetic motor or device has been proposed. The actuator system can move on vertical surfaces of complex iron structures with a step height of 25 mm. Furthermore, as a result that the vibration actuator can be driven directly, the actuator system can be driven by simple devices such as signal generators, amplifiers and DC power supplies and does not require any special operation. Therefore, it is possible to move a step with a height of 20 mm on a vertical surface at a speed exceeding 20 mm/s. The total length of this actuator system is designed to be about 300 mm and the total mass is 68 g. For this reason, the actuator system is superior in operability and mobility, and has the potential to be applied to on-site inspection. In addition, this actuator system can switch the movement characteristics by changing the attractive force of the actuator according to the inspection distance. When increasing the attractive force, the experimental results obtained by the actual mechanical test showed the possibility of visual inspection of the main tower of a large bridge over 100 m.  
Finally, a movement test was performed with a frame consisting of horizontal and vertical surfaces by varying the attractive force of the permanent magnets attached to the actuator system. By changing a part of silicone rubber frame in the actuator system, the possibility of inspection in the bridge girder was shown. It was shown that this actuator system could be applied to inspection of iron bridges and tanks by installing sensors such as cameras.