Koyo Yu

A nonlinear stability analysis for the robust position control problem of robot manipulators via disturbance observer

In this paper, a new nonlinear stability analysis is proposed for the robust position control problem of robot manipulators via Disturbance Observer (DOb). Although a DOb has long been used in the robust motion control systems, it suffers from the insufficient and impractical analysis and design methods. Nonlinearities of motion control systems are generally ignored to simplify analyses; however, they may significantly influence the stability and performance of motion control systems. The paper shows that a DOb based two degrees of freedom robust position controller equals to a passivity based controller. The error of a DOb based two degrees of freedom robust position control system is uniformly ultimately bounded when it is applied to a trajectory tracking control problem of robot manipulators. The error bound is directly determined by the bandwidth of DOb and nominal inertia matrix. As they are increased, the error bound shrinks. However, the bandwidth of DOb and nominal inertia matrix are limited by the practical constraints such as noise and sampling period; therefore, the error cannot be freely decreased. Asymptotic stability is achieved if the robust position control system is applied into a regulator, i.e., point to point, position control problem of robot manipulators. It is shown that not only the robustness but also the stability of the robust position control system is improved by increasing the bandwidth of DOb. Besides, decreasing nominal inertia may degrade the stability of the robust position control system, drastically. New practical design methods are given by using the proposed analysis method. Simulation results are given to show the validity of the proposals.

Modeling and experimentation of drilling vibration for implant cutting force presenting system

Since uncontrolled force can cause drill-bit break-age, excessive heat generation, and bone damage, prediction of the drilling force is important parts to increase success rate of implant operations. This paper presents a vibration model for prediction of the drilling vibration. The force from the drill is modeled by considering variation in force caused by CT value and feed velocity change. This method can be applied for a cutting force presentation method using haptic drilling system. This system allows dental students to learn procedures such as cutting the jaw bone. It can also be used for prior confirmation before implant surgery because doctor can experience the force response. The advantages of this system are the high force output quality and fine position sensing quality. The validity of the proposed method was confirmed through experiments using pine wood mentioned in Mischs bone density classification.

Motion control of tendon-driven rotary actuator using Twist Drive system

This paper proposes a method to achieve high-precision control for tendon driven system by using the Twist Drive. Twist Drive is one of transmission system which is consisted a pair of strings and rotary motor. This transmission system converts torque into a pulling force by using the pair of strings twisted on each other. However, it cannot realize pushing motion. Therefore the Twist Drive needs to be combined with an other actuation system for the realization of pushing motion. The problem is solved by tendon drive using Twist Drive system in this paper. High-precision control is achieved by modal-space disturbance observer. Modal-space disturbance observer is designed to realize the acceleration based control. It also compensates the modal interference. Furthermore work space observer is able to estimate the force in the work space without any force sensors. The proposed method was applied to rotational angle control and torque control of the tendon driven rotary actuator. The effect of the proposal is verified through experiments.

Force-sensorless bilateral control using piezoelectric cantilever with Nonlinearity Compensation

In the research about force sensorless bilateral control using piezoelectric cantilever, nonlinearities of piezoelectric constant and creep phenomenon has not been considered in reaction force observer (RFOB). Therefore, there are room for improvement of the performance of force estimation. In this paper, piezoelectric constant and creep phenomenon are modeled based on experimental results and built into RFOB to improve the performance of reaction force estimation. The validity of proposed method is confirmed through experiments of micro-macro bilateral control between linear motor and piezoelectric cantilever.

Real-time CT value estimation method for robotic drilling system based on thrust force and torque

An estimation of bone density is important for dental implant. CT value has been used in clinical and diagnostic as a quantitative evaluation method of bone density. In this paper, a method for estimating the CT value is proposed by modeling the relation between the cutting force and the CT value. The cutting force is calculated by the thrust force of the linear motor and the torque of the rotary motor. The estimated CT value can be obtained in real time by comparing between the force from reaction force observer and the formula of CT value with the pre-estimated parameters. The validity of the proposal was confirmed through experiments using wood mentioned in Mischs bone density classification. The experimental results indicated that the error of CT value estimation was ±91 HU and the estimation accuracy was 84 %.

Weight estimation system using surface EMG armband

Knowing the force exerted by a human operator while he/she is performing a specific task is important in many different field. For instance, limiting or optimizing the effort in sport activities allows for the development of specific training patterns for athletes, while knowing the effort made by a worker when he/she lifts a weight is important from the point of view of the safety. The effective effort is not only related to the net force/torque generated, but also to the force generated by each muscle. This aspect is the most crucial to be evaluated, as a non properly designed/handled exercise/task can lead to an excessive muscle strain and, in turn, to injuries. In this paper we report some preliminary results obtained by using low-cost wearable sensors in the estimation of the weight lifted by a human operator, through the simultaneous measurement of motion (via inertial sensors) and the muscles activations (via surface ElectroMyoGraphy — sEMG). The armband has 8 sEMG sensors and a 9-DoF inertial sensor, it has electrically safe setup with low voltage battery and Bluetooth protocol. The relationship between the EMG and inertial signals and the exerted force was made using a biarticular model of the arm. The model was used in order to have a theoretical value of the shoulder and elbow torques performing a weightlifting standardized tests. The value was then compared with the one estimated by the identification of a model and by means of a neural network. In both cases, the results show the relationship between signals and torque, but, in both cases, the results are affected by error. Nevertheless, even if it doesnt accurately estimate the weight lifted, both the presented techniques highlight the possibility of developing a classification of the exerted force that, calibrating the system for each person, can identify whether the weight lifted is light or heavy.

Robotic finger rehabilitation system for stroke patient using surface EMG armband

Several technological solutions have ben recently proposed for fingers rehabilitation in patients who were affected by a stroke, in particular for those who cannot generate the finger force but with preserved sEMG signals. In fact, a very promising approach uses a robot rehabilitation system controlled through sEMG signals. However, this system has two main problems: 1) the placement of electrodes is made manually and this could bring to a non-optimal detection of signals; 2) the cost of treatments is very high, mainly due to the usage of single-use, dispodsable electrodes. Our proposal is to use a robot rehabilitation system with a low-cost armband instead of standard electrodes. The armband has 8 sEMG sensors and a 9-DoF inertial sensor, it has electrically safe setup with low voltage battery and Bluetooth protocol. Moreover it is a very low cost and reusable equipment. The validity of the proposal was confirmed through experiments.

Construction of motion reproduction system using haptic forceps robots for needle insertion

In medical field, teleoperation robots for surgery have been studied and developed to support surgeons. Surgical tasks such as needle insertion and suturing are realized by multi degree of freedom (DoF) forceps robots which are a component of a surgical robot. Besides, surgical robots have some advantages such as automation of tasks to reduce the burden to surgeons. Motion reproduction system is a method to automate tasks. The haptic data is used as the data of human motion for motion reproduction system. However, realization of tasks by using multi DoF robots as a motion reproduction system is still difficult. In this paper, in order to realize support for surgeons by automatic motion, needle insertion is realized by the haptic surgical robot composed of multi DoF haptic forceps robots. Experimental results show that the motion of operator was reproduced by using the haptic data, and automatic needle insertion based on motion reproduction system was achieved.

A quantitative stiffness assessment method in liver biopsy teleoperations

This paper reviews the study progress of existing liver fibrosis assessment methods. After reviewing, this paper proposes a method to assess stiffness which intends the fibrosis, in the liver biopsy teleoperations. In this method, the liver biopsy operation is modeled. Also, teleoperated liver biopsy operations are conducted by a doctor. During the teleoperation, the force data is recorded by constructing reaction force observer (RFOB) without utilizing force sensors. The recorded force data is then treated in the force modeling equation of needle insertion movements. Finally, the quantitative stiffness assessment method is presented. The proposed method is applied to the evaluation of stiffness of three different environments which all imitate the liver. The experimentally assessed stiffness values are compared with the stiffness measured by the durometer. The comparisons validate that the proposal is effective to quantitatively assess the stiffness of needle inserting environments.

Consideration on function mode design for motion construction

With the development of robot technology, the activities of robots are expected to enter the human environment. Robots in human environment need the large-scale system with multi-degrees-of freedom for environmental adaptation. Therefore, it is necessary to design a large-scale system efficiently and simply. Tsuji et al. proposed function based control method. This method decouples a large control system into small independent components called “function” extracted by the “function mode”. However, design method of function mode has not been clarified. Therefore, this paper considers the design method of function modes by fragmentation of the function mode. By using this method, independent control of complex tasks can be achieved. This method can be a guide for designing complex functions and improve the adaptability to a designers intention and unknown environment. Proposed method is implemented in hybrid control system using three robot arms and each robot arm has two degrees of freedom. The validity of proposed method is verified by the experimental results.