A New Type of Motor: Pneumatic Step Motor

Abstract

This paper presents a new type of pneumatic motor, a pneumatic step motor (PneuStep). Directional rotary motion of discrete displacement is achieved by sequentially pressurizing the three ports of the motor. Pulsed pressure waves are generated by a remote pneumatic distributor. The motor assembly includes a motor, gearhead, and incremental position encoder in a compact, central bore construction. A special electronic driver is used to control the new motor with electric stepper indexers and standard motion control cards. The motor accepts open-loop step operation as well as closed-loop control with position feedback from the enclosed sensor. A special control feature is implemented to adapt classic control algorithms to the new motor, and is experimentally validated. The speed performance of the motor degrades with the length of the pneumatic hoses between the distributor and motor. Experimental results are presented to reveal this behavior and set the expectation level. Nevertheless, the stepper achieves easily controllable precise motion unlike other pneumatic motors. The motor was designed to be compatible with magnetic resonance medical imaging equipment, for actuating an image-guided intervention robot, for medical applications. For this reason, the motors were entirely made of nonmagnetic and dielectric materials such as plastics, ceramics, and rubbers. Encoding was performed with fiber optics, so that the motors are electricity free, exclusively using pressure and light. PneuStep is readily applicable to other pneumatic or hydraulic precision-motion applications.

Figures

Fig. 1

Kinematic diagram of the PneuStep motor.

Fig. 2

Transmission ratio and step size.

Fig. 3

Isometric view and two central cross sections of the motor.

Fig. 4

Two sizes of motor prototypes.

Fig. 5

Rotary distributor.

Fig. 6

Optical sensor using hoop gear (left) or 3-mark code-wheel (right).

Fig. 7

PneuStep driver.

Fig. 8

Torque versus speed with 3-m hose and rotary pump.

Fig. 9

Motor stall speed versus (a) hose length with rotary pump (b) supply pressure with electronic pump and 7-m hose.

Fig. 10

Open-loop, PID, and modified PID tests.

Fig. 11

MRI-compatible robot with six PneuStep motors.