Estimation of an object's physical parameter by force sensors of a dual-arm robot

We are developing a Nursing-Care Robot in order to reduce the load in physical nursing care. The concept of this robot is to promote the cared persons by the robot to activate their own motion ability as long as possible. This may lead to the improvement of the cared person's movement volition and movement abilities. In order to realize safe and human friendly robot care tasks, full body manipulation is an important technology, for which it is necessary to estimate the subject's center of gravity from the contact positions and forces with the robot's two arms. In this paper, we estimate the center of gravity of object based on the contact point and the contact force estimated by force sensor on both robot arms. The position of gravity center is important to realize care tasks stably. We performed experiments and simulations for the single point contact and dual points contact cases using a cylindrical object. As a result, it is found that although some errors were recognized in the experiments compared with the simulations, the relations between the contact positions and such errors were observed. Such experimental error mainly comes from the difference of shape between the real robot and the model of the robot in simulation. As the future work, we have to improve the robot model so as to get better estimated information.


Introduction 
In the recent years, increase of age population becomes a serious problem in human societies. With the rapid development of the aging population, the elderly people's nursing care becomes more and more serious because of the lack of the care supporters. The research on nursing care robot is one necessary way to solve this problem.
Nursing care robots have been widely studied in recent years [1][2][3][4][5]. The appearance of its beneficial comes from both care supporters as well as cared persons. The nursing care robot can be roughly divided into three types, such as care-assisted type, self-assisted type and communication type． In order to reduce the burden of cared persons while realizing the action of carrying up care, it is important to clarify their physical conditions using the information from cameras and any other kinds of sensors.
When the robot is lifting up someone, it is necessary to concern two different purposes of assisting and sustaining to control the dual arm of the robot. Since the force of assisting arm depends on the force of sustaining arm, it is possible to encourage the cared persons to act by themselves instead of depending on the robots totally. In order to determine the force of assisting arm, physical parameters of cared person's body are required.
Since the robot is controlled basing on the measured parameters provided by sensors, how to get the parameters of the care receiver's body without noise is quite important. Nagase [6] proposed a method of estimating a contact point between a robot hand and an object by using a force sensor with measurement noise. He builds an objective function which represented the error and used the Lagrange multipliers to choose an optimal solution which makes error as small as possible.
In another hand, human body is a redundant system due to its lots of D.O.Fs. This makes the computation which serves the control of the care receiver's posture very difficu unnecessary model be sim control met Thus, when be lifted up geometrical task at the re In this stu the force an on the mea sensors, we arm and cont and the mod the center of Section 2 Section 3 object's p optimization perform the discuss the r   own. If there f the contact alance of the or e can be (5) sts an optimal r value e. We cted to two n problem.

Overvie
The second necessary condition of the Lagrange function can be described as It is possible to derive the analytical solution of the problem to satisfy these two conditions. By multiplying the F S H T on the left side of the Eq.
(8), we get 0 (13) Therefore, it is obvious that the optimal solution can be solved in terms of two situations λ 0 and

The Setti
The mode robot under radius of the shape is set between eac The gravity since there measured by add random

The Mod
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