Yohichi Nakao

Diamond turning using position and AE dual feedback control system

A control scheme to improve machining accuracy by means of an acoustic emission root mean square (AErms) signal measured on-line during diamond turning is proposed. Preliminary experiments have verified a correlation between a machining error and measured AErms. Based on the correlation, a control algorithm to apply a compensation signal to a position feedback control loop, named the position and AE dual feedback control scheme is proposed. Machining tests using a diamond turning machine controlled with a PC-based open architecture controller showed that the proposed control scheme accomplishes 20% reduction of machining error compared to conventional position feedback control. Machining tests using new and worn diamond tools showed that an on-line tool-wear recognition is possible using the AE feedback control system.

Frequency response of a water hydraulic servovalve

A servovalve for use with clear water is designed and constructed. The spool of the valve is supported by hydrostatic bearings, and the bearing fluid is used in its nozzle flapper system. The frequency response of the valve is measured and verified that it has a bandwidth greater than 30 Hz. A linearized analysis is made to explain the dynamic characteristics of the valve.

Angular Position Control of Fluid-Driven Bi-Directional Motor

This paper describes the design of a rotational speed-control system and an angular position-control system for a fluid-driven bi-directional motor. The fluid-driven bi-directional motor has a driving principle similar to that of the fluid-driven spindle, which is designed for use in ultra-precision machine tools. The fluid-driven bi-directional motor was designed so that it is driven by low viscosity oil flow power. In this paper, the rotational speed controller for the motor is first discussed. In order to reduce the influence of external load torque on the rotational speed, a conventional disturbance observer is combined with the rotational speed-control system. The angular position-control system, which possesses the rotational speed feedback loop with the disturbance observer in the angular position feedback loop, is then discussed. The designed rotational speed and angular position-control systems are conventional I—P control and proportional control systems, respectively. The performance of the designed rotational speed-control system and the angular position-control system is studied via simulations and experiments. The performance of the designed control system is tested by the step response method as well as by the frequency response method, respectively. The simulation and experimental results show that the rotational speed and the angular position of the motor can be controlled by the rotational speed controller and angular position controller, respectively. In addition, the influence of the external load torque acting on the motor is successfully compensated for by means of the disturbance observer. The experimental result shows that the designed angular position-control system suppresses the steady-state positioning error to less than 0.02°, even if external constant load torque acts on the motor.

Design of Short-Pipe Restrictor of Hydrostatic Thrust Bearings

High stiffness hydrostatic bearings are needed in order to achieve precise motions of the machine components of ultra-precision machine tools or other precision machines. Design procedure of restrictors of hydrostatic thrust bearings, making bearing stiffness maximize under given conditions, is considered in the paper. In particular, the paper focuses on design of short pipe restrictors that are used in the hydrostatic thrust bearings in many industrial applications. Derived mathematical model predicting load capacity and stiffness of the hydrostatic bearings with short pipe restrictors are verified by compared with experimental results. Based on the derived mathematical model, an optimum condition of a ratio between the diameter and length of the short pipe restrictor is then derived. Designed short pipe restrictors are used in a water hydrostatic thrust bearing. Then the experimental results show that the bearing stiffness significantly increased. It is noted that the optimum condition of the restrictors is represented by defined non-dimensional parameters.Copyright

Model Verification and Design of Speed Control System of Water Driven Stage

The present paper describes a design of speed control system of the water driven stage that has been developed for a feed table of an ultra-precision machine tool. The stage has a piston-cylinder mechanism to drive a table of the stage. Since the piston-cylinder mechanism is used, the flow rate supplied to the piston-cylinder controls the speed of the table. For diamond turning applications, the constant feed motion of the stage is highly desirable in order for obtaining fine diamond-turned surfaces. In the present paper, mathematical models of the water driven stage and a flow control valve are introduced. Based on the derived models, a conventional P-I control system is then designed in order to achieve desired control performances, aiming no steady-state error and minimized extraneous disturbance effects on the response. Performances of the designed controller are studied through experiments and simulations.Copyright

Simulation of Energy Consumption of Machine Tool Motion for 3-Axis MachiningSimulation of Energy Consumption of Machine Tool Motion for 3-Axis Machining

In recent years, the shortage of the energy source is a serious problem in the world. Thus, the reduction of the energy consumption in manufacturing fields has been demanded. The energy consumption of NC machine tools has been also focused on. However, the energy consumption of the machine tool motion of each control axis during machining process has not been considered. In this study, we focus on the energy consumption during the machining process and we proposed the simulation model of the energy consumption of the feed drive systems of NC machine tool. Based on the proposed model, the energy consumption during the machining motion was simulated and evaluated. From these results, if the CAD/CAM systems can generate the tool paths considering about the energy consumption of NC machine tools, the energy consumption will be reduced without replacing or overhaul the machine tools.

Development and Modeling of Water Driven Stage

The water driven stage that is developed for ultra-precision machine tools is presented. The stage is designed for the diamond turning of small precise parts, such as various small lenses or mirrors. The moving table of the water driven stage is supported by water hydrostatic bearings. The stage is driven by the water hydraulic piston-cylinder mechanism that is designed inside the table. A feature of the stage is that the driving force by the piston-cylinder mechanism acts on the center of gravity of the moving table, minimizing undesirable pitching and yawing motions. In order to investigate the characteristics of the stage, a mathematical model of the water driven stage is derived. Performances of the water driven stage are examined through experiments and calculations. The present study verifies that the feed motion needed for the diamond turning operations can be obtained by supplying few flow rate of water, several tens milliliters per minutes. Stiffness of the water hydrostatic bearing is also experimentally investigated. The result shows that stiffness is approximately 330 N/μm if the supply pressure was 0.5 MPa.Copyright

Design of Rotary-Type Flow Control Valve for Control of Water-Driven Spindle

Water-driven spindle was developed for producing small and precise parts by the diamond turning processes. Rotational speed of the spindle can be controlled by the flowrate supplied to the spindle. The paper describes a newly developed rotary-type flow control valve that is designed for controlling rotational speed of the water-driven spindle. In particular, the paper focuses on the establishment of the mathematical model capable of representing the characteristics of the open loop control system composed of the pump, flow control valve and spindle. Mathematical models are then derived so that a feedback control system can be designed using the models. Performances of the flow control valve and the spindle are examined through simulation as well as experiments. It is then verified that the derived mathematical models are capable of representing the performance of the system. In addition, the required positioning accuracy of valve rotation for achieving desired control of the rotational speed of the spindle is considered based on the derived linearized mathematical model.Copyright

Angular Position Control of Fluid-Driven Spindle

A precise spindle is essential to achieve precision machining, such as diamond turning. A fluid driven spindle supported by hydrostatic bearings was thus designed and tested. A feature of the spindle is that several flow channels are designed in its rotor so that driving torque can be generated by supplying pressurized flow into the channels. Rotational speed of the spindle can be controlled by the flow rate. In addition, the rotational direction of the spindle can be controlled by switching supply ports. Thus angular position control of the spindle is achieved by designing appropriate feedback controller. In the present paper, mathematical model of the spindle was thus derived in order for designing an angular position control system. Then spindle characteristics calculated by the mathematical model were compared with experimental results. Furthermore, the angular position control system that has a disturbance observer in its feedback loop was designed based on the mathematical model. The performance of the designed control system was experimentally investigated through the step response. Experimental results verified that the designed controller minimizes the steady state error of angular position of the spindle. Consequently, the steady state error was comparable with the resolution of the rotary encoder, 0.018 degree. In particular, the experimental results indicated that the disturbance observer effectively reduced the influence of various load torque on the angular position of the spindle.Copyright

Design of Angular Position Control System for Fluid Drive Motor

This paper describes the angular position control system for a fluid drive motor. The fluid drive motor has a similar driving principle with the fluid drive spindle that is designed for the spindle of precision machine tools. The fluid drive motor is driven by the water or oil flow power. In the present paper, the rotational speed controller is first discussed. In order to reduce the influence of external load torque on the rotational speed, a conventional disturbance observer is combined with the rotational speed control system. The angular position control system, which possesses the rotational speed controller as a minor feedback loop, is then discussed. Performances of the designed angular position control system are studied via simulations and experiments. It is verified that the influence of the external load torque on the rotational speed is successfully compensated. The angular position control system is tested through simulations and experiments, as well. Experimental result shows the designed angular position control system suppresses steady state positioning error less than 0.02 degree, even if external constant load torque acts on the motor.© 2008 ASME