Development of Crown Motor.

Abstract

Conventional fluid motors require high-pressure fluid (10 MPa∼) and the rotation speeds of these motors are extremely high (10, 000 rpm∼). Therefore, it is difficult to apply these motors to mobile robots. This paper describes a new fluid motor composed of new mechanisms. The motor consists of two crown gears and 3 or more cylinders. It is named 'Crown Motor' after the two crown gears, the precession gear and the output gear. The precession gear is driven by the cylinders meshing the output gear. In each cycle, the output gear rotates an angle equal to the difference of the numbers of teeth between precession and output gears divided by the number of teeth of the output gear. With this mechanism, compact size and high gear ratio can be obtained. Thus, a high-torque fluid motor with slow rotation speed is accomplished. The rotation speed of the motor can be continuously controlled from CW (max) to CCW (max) by changing the timing of valve opening, similar to the sequence of electric DC brushless motors. ln addition, the Crown Motor can be driven with low- pressure fluids, such as tap water (0.2 MPa∼). Two control systems of Crown Motors are proposed in this paper. One is with conventional electric valve system (stepping drive system) and the other is with new mechanical valve system without any electric device (self-excited drive system). With stepping drive system, Crown Motors can be used as a stepping motors with one-by-one cylinder operation. On the other hand, self-excited drive system is proper and reliable if water is used as driving fluid in practical applications. In general, high-torque motors with free or lock states available require large clutches or brakes. However, the prototype of Crown Motor achieves both states without any additional components. With these properties, Crown Motors can be used in not only mobile robotic applications but also other various applicatlons. The prototype accomplished a high torque of 16 Nm at 15 rpm under 0.5 MPa and with efficiency of 35%.