Takanori Mizobuchi

Basic performance of a desktop NC machine tool with compliant motion capability

In this paper, a new desktop NC machine tool with compliance controllability is presented for finishing metallic molds with small curved surface. The NC machine tool consists of three single-axis robots with high position resolution. A tool attached to the tip of the z-axis has a small ball-end shape. Also, the control system of the NC machine tool is composed of a force feedback loop, position feedback loop and position feedforward loop. The force feedback loop controls the polishing force consisting of tool contact force and kinetic friction force. The position feedback loop controls the position in pick feed direction. Further, the position feedforward loop leads the tool tip along cutter location data. It is expected that the NC machine tool delicately removes surface cusps with under about 0.3 mm height on a mold, and finishes the surface with high quality. In order to first confirm the application limit of a conventional industrial robot to a finishing task, we evaluate the backlash that causes the position inaccuracy at the tip of the abrasive tool, through a simple position/force measurement. Through a similar position/force measurement and a surface following control experiment along a lens mold, the basic position/force controllability of the proposed NC machine tool is demonstrated.

Desktop orthogonal-type robot with abilities of compliant motion and stick-slip motion for lapping of LED lens molds

In this paper, a new desktop orthogonal-type robot, which has abilities of compliant motion and stick-slip motion, is first presented for lapping small metallic molds with curved surface. The robot consists of three single-axis devices with a high position resolution of 1µ. A thin wood stick tool is attached to the tip of the z-axis. The tool tip has a small ball-end shape. The control system is composed of a force feedback loop, position feedback loop and position feedforward loop. The force feedback loop controls the polishing force consisting of tool contact force and kinetic friction forces. The position feedback loop controls the position in pick feed direction, e.g., z-direction. The position feedforward loop leads the tool tip along a desired trajectory called cutter location data (CL data). The CL data are generated from the main-processor of a CAM system. The proposed robot realizes a compliant motion required for the surface following control along a spiral path. In order to improve the lapping performance, a small stick-slip motion control strategy is further added to the control system. The small stick-slip motion is orthogonally generated to the direction of the tool moving direction. Generally, the stick-slip motion is an undesirable phenomenon and should be eliminated in precision machineries. However, the proposed robot employs a small stick-slip motion to improve the lapping quality. The effectiveness of the robot is examined through an actual lapping test of an LED lens mold with a diameter of 4 mm.

CAD/CAM-based force controller using a neural network-based effective stiffness estimator

In industries manufacturing metallic molds, various NC machine tools are used. We have already proposed a desktop NC machine tool with compliance control capability to automatically cope with the finishing process of LED lens molds. The NC machine tool has the ability to control the polishing force acting between an abrasive tool and a work piece. The force control method is called impedance model force control. The most effective gain is the desired damping of the impedance model. Ideally, the desired damping is calculated from the critical damping condition after considering the effective stiffness in the force control system. However, there is a problem in that the effective stiffness of the NC machine tool has undesirable nonlinearity. The nonlinearity has a bad influence on the force control stability. In this article, a fine tuning method of the desired damping is considered using neural networks. The neural networks acquire the nonlinearity of effective stiffness. The promise is evaluated through an experiment.

Skillful stick-slip motion control of a Cartesian-type robot

In this paper, a new desktop Cartesian-type (orthogonal-type) robot, which has abilities of compliant motion and stick-slip motion, is presented for lapping small metallic molds with curved surface. The size is 850 mm width, 645 mm depth and 700 mm height. The robot consists of three single-axis devices with a high position resolution of 1 μm. A thin wood stick tool is attached to the tip of the z-axis. The tool tip has a small ball-end shape. In order to improve the lapping performance, a small stick-slip motion control is considered in the control system. The small stick-slip motion is orthogonally generated to tools moving direction. The effectiveness and promise of the stick-slip motion control are examined through an actual lapping test of an LED lens mold with a diameter of 4 mm.

Desktop Cartesian-Type Robot with Abilities of Compliant Motion and Stick-Slip Motion

In manufacturing process of small lens molds, 3D CAD/CAM systems and high precision NC (numerically controlled) machining centers are used generally, and these advanced systems have drastically rationalized the design and manufacturing process. For example, recently, an ultra precision multi-axis control machining system has been developed for spherical micro-lens array molds. As a result, the mold with a spherical micro lens array has been effectively shaped with high accuracy (Oba et al., 2008). In case of an LED lens mold as shown in Fig. 1, however, the finishing process after the machining process has been hardly automated yet, because the LED lens mold has plural small concave areas to be finished, in which each diameter is 4 mm.

Position-based impedance control using inner servo system and its application to a desktop NC machine tool

A position-based impedance control using an inner servo system is introduced for industrial manipulators. Stiffness control, compliance control and impedance control can be easily realised. The impedance control does not have a force control mode but is the combination of position, velocity and force. The proposed impedance model force control is derived from the concept of the position-based impedance control. The force control method is applied to an NC machine tool to have a compliance. The machine tool consists of three single-axis robots with a high position resolution of 1 µm. The basic performance is demonstrated through a profiling control experiment.

Stick-slip motion control based on cutter location data for an orthogonal-type robot

In this article, a new desktop orthogonal-type robot, which has the capacity of stick-slip motion control based on cutter location data, is presented for lapping small metallic molds with a curved surface. The robot consists of three single-axis devices with a high position resolution of 1 μm. A thin wooden stick tool with a ball-end shape is attached to the tip of the z-axis. In order to improve the lapping performance, a novel stick-slip motion control method is developed in the control system. The small stick-slip motion is orthogonally generated in the direction of the tool’s movement. The effectiveness of stick-slip motion control is examined through an actual lapping test of an LED lens cavity.

Impedance model force control using neural networks for a desktop NC machine tool

In manufacturing industries of metallics molds, various NC machine tools are used. We have already proposed a desktop NC machine tool with compliance control capability to automatically cope with the finishing process of LED lens molds. The NC machine tool has an ability to control the polishing force acting between an abrasive tool and workpiece. The force control method is called impedance model force control. The most important gain is the desired damping of the impedance model. Ideally, the desired damping is calculated from the critical damping condition in consideration of the effective stiffness in force control system. However, one of the serious problems is that the effective stiffness of the NC machine tool has undesirable nonlinearity. The nonlinearity gives bad influences to the force control stability. In this paper, a fine tuning method of the desired damping is considered by using neural networks. The neural networks acquire the nonlinearity of effective stiffness. It has been observed that the desired damping generated from the learned neural networks allows the NC machine tool to achieve a stable finishing result.

Orthogonal-type robot with a CAD/CAM-based position/force controller

In this paper, a new desktop orthogonal-type robot with a CAD/CAM-based position/force controller is presented for finishing small metallic molds with curved surface. The robot consists of a three-axis robot whose single one has a high position accuracy of 1 µm, which means that it can perform higher resolutions of position and force, compared to general industrial robots. A thin wood stick tool with a ball-end tip is attached to the z-axis through a force sensor. The control system of the robot is comprised of a force feedback loop, position feedback loop and position feedforward loop. The force feedback loop controls the polishing force consisting of tool contact force and kinetic friction forces. The position feedback loop controls the position in pick feed direction. Further, the position feedforward loop leads the tool tip along a spiral path. We first evaluate the backlash that causes inaccuracy in position at the tip of the abrasive tool, by simply measuring the position and force. Next, a surface following control experiment along a lens mold is conducted, in which the mold has axis-symmetric concave areas. Finally, a LED lens mold is further finished by using the proposed system in order to demonstrate the performance and promise.