Development of three-chamber micro pneumatic wobble motor
Pneumatic wobble motor is a new stepping motor, which realizes high torque and high resolution. The motor has advantages of existing pneumatic actuators, which are simple structure and lightweight.
In this study, pneumatic wobble motor is miniaturized to φ6 mm by applying fine processing technology. Besides the number of pneumatic chambers are decreased from 6 to 3.
Driving principle and structure
Figure 1 shows the driving principle of wobble motor. Cylinder B is rolling on the inside of Cylinder A. Then the rotation center of Cylinder B is fixed on central axis. The motor, which has speed reduction function, drives on this principle. In the motor, Cylinder A is wobble ring, Cylinder B is rotor.
Fig.1 Driving principle of wobble motor
Figure 2 shows the structure of wobble motor. The motor consists of the following.
(1) Wobble generator made of silicone rubber
(2) Wobble ring with internal gear
(3) Rotor with external gear
Fig.2 Structure of wobble motor
Fig.3 Wobble generator Fig.4 Wobble ring and rotor
Rotor has an external gear, wobble ring has an internal gear (Fig.4). Wobble generator has three pneumatic chambers (Fig.3). Because the inner surface of wobble generator attaches on the outer surface of wobble ring, wobble ring does not rotate on its axis. When high pneumatic pressure is applied to the chambers of wobble generator sequentially, the orbital motion of wobble ring generates the rotation of rotor with gears engaged. Because the teeth number of rotor is one fewer than that of wobble ring, rotor rotates by tooth while wobble ring orbits a time.
Design of mechanism
The three-chamber micro pneumatic wobble motor is 6 mm in diameter, 5.6 mm in length, 0.46 g in weight. The output shaft is 1 mm in diameter. The teeth number of wobble ring is 36, and rotor 35. The module is 0.067, and the pressure angle is 60 deg. The tooth profile is designed based on involute.
Fig.5 Wobble motor Fig.6 Wobble motor mechanism removing the cover Fig.7 All parts of wobble motor
This experiment is performed under the condition that output shaft is given rotational load directly. Applied 0.4MPa, the motor realizes maximum speed of 57rpm, ratcheting torque of 0.6 mNm, reduction ratio of 1/35, and resolution of 210step/rev. The calculation torque is less than 1.15 mNm. Figure 8 shows the relation between pneumatic pressure and torque of experiment and calculation at 10rpm.
Fig.8 Relation between pneumatic pressure and torque at 10rpm
The φ6 three-chamber micro pneumatic wobble motor is developed by applying fine processing technology and fine modeling of rubber. The characters of this motor are listed below.
(1) Compact size, lightweight, and simple structure
(2) High torque
(3) Stepping rotation of high resolution
(4) Overload protection by ratcheting
Table 1 shows the comparison of a pneumatic wobble motor with an electromagnetic motor of same diameter. The table shows that the wobble motor can configure shorter assembly than existing electromagnetic motors.
Table 1 Comparison of a pneumatic wobble motor
with an electromagnetic motor
 Koichi SUZUMORI･Kohei HORI, A Study of Pneumatic Wobble Motors,
Transactions of Japan Society of Mechanical Engineers, 64-662-C, pp236-241, 1998-6
 Kiyohide OKAMOTO･Koichi SUZUMORI･Takefumi KANDA･Yoshiaki YAMADA,
Development of three-chamber micro pneumatic wobble motor,
The 6th. Machine Design and Tribology Division Meeting in JSME, No.06-6, pp295-297, 2006-5