Takaaki Oiwa

Three-dimensional touch probe using three fibre optic displacement sensors

This paper presents a touch probe system with improved sensitivity and repeatability. In this probe, a tip ball is attached to the probe stylus through elastic bodies that enable the ball to be displaced in three directions. When the tip ball touches the workpiece, three fibre optic lever displacement sensors measure directly the balls displacement and direction. Because the ball is located in sensitive directions of all of the sensors, this probe complies with Abbes principle, which is a basis of precision measurement. Various performance tests have been performed using one-dimensional stages. The measurement resolution of the tip ball displacement was less than 12 nm in these experiments. Thus, the probe triggering level was set safely to 50 nm, taking into account the associated noise levels. The variation of the pretravel was less than 25 nm in all directions. The measurement force was approximately 0.6 mN in the horizontal direction and 4.5 mN in the vertical direction.

A design consideration of a novel bearingless disk motor for artificial hearts

A novel bearingless disk motor for a possible application of implantable centrifugal artificial hearts is proposed in this paper. To reduce a motor height, C-shaped stator cores with radially expanded coil windings are employed. End windings are not located in the axial direction. The motor height and an outer diameter are designed to be 10mm and 70mm, respectively. Magnetic suspension characteristics of the bearingless disk motor are analyzed by using finite element method. The design consideration of a suspension winding configuration has been discussed especially on radial suspension force variations. A prototype machine is fabricated and tested. Experimental results have provided the feasibility of the design of the bearingless disk motor.

Error Compensation System for Joints, Links and Machine Frame of Parallel Kinematics Machines

This paper describes error compensation methods for parallel kinematics machines (PKMs). First, joint errors and link deformations caused by external forces and heat are discussed. Because a linear scale unit is connected to each spherical joint located at both ends of an extensible limb through two rods, the scale unit can measure not only displacement of the prismatic joint but also the limb’s deformation. Moreover, the scale unit also measures the joint’s runout and its deformation in the limb’s direction because the rod end is in contact with the sphere of the spherical joint. Consequently, the unit accurately measures any change in the distance between the spherical joints independent of the external force. Further, temperature fluctuation has little effect on measured distance because the rods are made of a low expansion alloy. This device was built using experimental limbs and tested for use with coordinate measuring machines. Results showed that, without any additional sensor, the device acquired the same effect; it improved the stiffness and the thermal coefficient of expansion of the whole limb. Secondly, another compensation method for elastic and thermal deformations of the machine frame supporting the mechanism is proposed. Nine displacement sensors with Super-Invar rods measure the distance between the surface place and three joint supports or the base platform of the PKM. Then, the forward kinematics of the hexapod mechanism calculates displacement and orientation variations of the base platform during operation from these measured displacements. Consequently, coordinates of the PKM’s end-effector can be compensated by the dimensions, the displacement, and the orientation of the base platform during operation. This device was installed in the experimental coordinate measuring machine. Coordinate measurements using a master ball mounted on the surface plate showed that this device decreased the influences of temperature fluctuation and external force on machine accuracy.

Miniaturized three-dimensional touch trigger probe using optical fibre bundle

This paper presents a miniaturized touch trigger probe system using optical fibres. In this probe, a 1.2 mm diameter tip ball is attached to the end surface of a 0.8 mm diameter stylus stem by an elastic body that enables the ball to be displaced in three directions. One illuminating fibre and three bundles of receiving fibres are incorporated into the stem. Light reflected on the tip ball surface is detected by three photodetectors, one for each fibre bundle. Consequently, the probe system measures directly the balls displacement and direction when the tip ball touches the workpiece. Because the offset between the fibre bundle and the measuring point or the ball is very small, this probe complies with the Abbe principle, which is a basis of precision measurement. Various performance tests have been performed using a one-dimensional stage. In those experiments, the measurement resolution of the tip ball displacement was less than 0.5 µm. Thus, the probe triggering level was set safely to 0.6 µm, taking into account the associated noise levels. The pretravel variation was less than 0.65 µm in all measurement directions. The measurement force was less than 0.9 mN in all directions.

Friction control using ultrasonic oscillation for rolling-element linear-motion guide

This article reports a friction-control method for rolling-element linear-motion guides used for precision positioning. In general, static friction greater than dynamic friction generates stick-slip motion and diminishes the positioning accuracy. Two ultrasonic actuators excite both the rail and the carriage of the guide to give relative displacements to bearing surfaces. In order to effectively propagate the vibration over the entire rail without damping, the actuator drives at that frequency with a half wavelength corresponding to the distances between the rail mounting bolts. This also minimizes undesirable vibration of the machine structure. Moreover, the bearing surfaces of the carriage are resonated by a second ultrasonic actuator. The experiments using a force sensor showed that the static and dynamic friction forces were reduced by approximately 25% at any place on the 600-mm-long rail. Moreover, excitation only at very low velocity decreased the static friction peak.

Optimal Suspension Winding Configuration in a Homo-Polar Bearingless Motor

The authors have proposed a six-pole homo-polar type bearingless motor intended for a use as a small power and ultra high-speed motor. In a six-pole homo-polar rotor, crucial suspension force interference is caused. Thus, the suspension winding configuration should be carefully designed. In this paper, theoretical calculation is carried out with several structures to find the optimal winding configuration for reduction of the force interference. To verify the theory, finite-element-method (FEM) analysis is carried out. A test machine is also fabricated. FEM analysis and experimental result have demonstrated that the proposed optimal winding configuration can successfully reduce the force interference.

Linear rectangular air bearing based on squeeze film generated by ultrasonic oscillation

This article reports an actively controllable linear motion air bearing that utilizes air films generated by ultrasonic oscillation. The films on eight bearing surfaces of a horn, which serves as the stator, lifted and guided a carriage consisting of four flat plates. Moreover, the oscillation that modulated the amplitude via the positioning signal varied with the thickness of the air film. Thus, this bearing regulated the position of the carriage in a vertical direction using a proportional and integral (PI) controller with a displacement sensor so that its motion error became zero. Furthermore, no pneumatic plant was needed in this bearing system. The motion straightness errors were 0.41μm peak to peak (p-p) in the vertical direction and 1.52μm p-p in the horizontal direction without compensation. Nonrepetitive motion errors were 0.34μm 2σ (two standard deviations) in the vertical direction and 0.39μm 2σ in the horizontal direction. The PI feedback control, in which a fiber optic displacement sensor mea...

Squeeze air bearing based on ultrasonic oscillation: Motion error compensation using amplitude modulation

This article reports an active air bearing that utilizes air films generated by ultrasonic oscillation. The films on the bearing surfaces of a horn, which is a stator, lift and restrain the rotor. Moreover, the oscillation that modulates the amplitude via the positioning signal varies the thickness of the air film. Thus, this bearing regulates the position of the rotor using a PI controller with a displacement sensor so that its runouts become zero. Furthermore, no pneumatic plant is needed in this bearing. The repetitive runouts were 0.23 μm p-p in an axial direction and 1.0 μm p-p in a radial without compensation. Nonrepetitive runouts were 0.061 μm 3σ in an axial direction and 0.065 μm 3σ in a radial without compensation. PI feedback control, in which a fiber optic displacement sensor measures the movement of a master ball mounted on the rotor, decreased repetitive axial runout from 0.232 μm p-p to 0.118 μm p-p. The settling time for 0.2 μm step positioning in an axial direction was less than 0.2 s. Th...

Motion error compensation of a mechanical contact spindle using a multi-degree-of-freedom micropositioning stage

A three-degree-of-freedom micropositioning stage has been developed for controlling spindle runout. The stage consists of three wedge-shaped plates and PZT-stacked actuators. This stage can expand to 6 degrees of freedom and moves precisely without a stick-slip effect because there is no friction. Moreover, this stage has high loading capacity and stiffness because most of the load is placed on actuators having high compressive strength. In a trial, a PI controller actuated the stage to decrease the runout of a ball-bearing spindle weighing 400N. As a result, radial and axial motion errors of the spindle mounted on the stage were decreased from 0.2μmto0.04μm.

Hand-operated translation stage with subnanometer resolution

In this article, a simple micrometer-driven stage with subnanometer resolution has been developed. The stage consists of a displacement reduction mechanism using three toggle links, a fine motion stage guided by a notched linear spring, and a micrometer head rotated by a hand wheel. Displacement of the manually rotated micrometer head drove the stage with subnanometer resolution through the reduction mechanism. The travel range of the stage was 3.4 μm and the reduction ratio reached 6300. Further, 0.5-nm step positioning was successfully performed using a fiber optic displacement sensor with a 1-Hz lowpass filter. Moreover, a one-hour measurement of the displacement and the temperatures shows that this device has an extremely high thermal stability.