METHOD OF CONTROLLING A/K PROSTHESIS FOR ASCENDING WITH DISABLED SIDE HIP JOINT TORQUE

Hiroshi FUJIMOTO Assistive Technology Lab. Human Environment System Department National Institute of Bioscience and Human-Technology Tsukuba, JAPAN

ABSTRACT

This paper deals with the controlling method of A/K prosthesis which enables lower limb amputees to ascend the staircase. As conventional A/K prostheses are designed for walking on the level surface, the disabled persons wearing them have been compelled to walk on stairs with unnatural posture. The construction of powered A/K prostheses requires not only the prosthesis mechanism but also its controlling method. Measuring the axial force and moment acting on the socket that is the human-machine-interface during walking with the six axis force-moment sensor, the torque of disabled side hip joint was calculated. We devised a controlling method using this torque for a feedback signal. The results of clinical walking experiment indicated that the subjects wearing the A/K prosthesis could walk with joint angle patterns similar to those in a normal subject. Furthermore, with the results of the inverse dynamics analysis, the amputee subject generated a torque and power pattern at his disabled side hip joint similar to that in the normal subject.

BACKGROUND

To disabled persons who lost their lower limbs at the thigh level, A/K prosthesis is an indispensable assistive system for their daily activities. However the function of the conventional A/K prostheses is limited, because they do not enable the amputee to walk on a step or staircase. To solve this problem, the author had developed a multi-functional A/K prostheses. It generated enough power at the knee joint for stair walking(1)(2). This type of powered A/K prosthesis has not been developed with some exceptions. Active artificial Leg was one of them. That had been developed by one of the National Research and Development Programs for Medical and Welfare Apparatus in Japan. The controller of this leg receives signals from a foot-switch equipped in the shoe, and starts extending, the knee joint following a preset angle pattern(3).

STATEMENT OF THE PROBLEM

In the development of power artificial limbs, not only the development of mechanisms but also the controlling method which drive it adequately are important subjects. Because, if it is not controlled appropriately, the powered A/K prosthesis may injure the amputee or people around him/her with its power. The aim of this study is to construct the new controlling method which has the following function. Amputees can control the movement of their A/K prosthesis by with moving the hip joint of the disabled side same as the normal side, without any special unnatural operation to drive the prosthesis.

RATIONALE

Amputees drive their A/K prostheses with their hip joint of the disabled side. In this study, the author considered a new controlling method using a hip joint torque of the disabled side as a controlling signal. With this signal, a voluntary and natural controlling method will be accomplished.

DESIGN AND DEVELOPMENT

MECHANISM OF THE A/K PROSTHESIS

The basic specification of the powered A/K Prosthesis is that it can generate enough power to the joint when it is needed. Figure 1 shows the basic mechanism which contains two systems; a closed hydraulic circuit system and an electronic power system. The former consisted of the piston-cylinder systems linked with movements of ankle and knee joints, and the latter consisted of battery, DC-motor and ball-screw to drive the knee joint. With these two systems, necessary power could be transmitted to the knee joint immediately during stairs ascending(1)(2). Results of an inverse dynamics analysis of gaits carried out by normal subjects showed the two phases of driving the knee joint from the external electric power supply. 1. when the knee joint extends to push up the upper body, during the stance phase. 2. when the knee joint flexes to pull up the toe of the prosthesis not to touch the edge of the staircase.

CONTROLLING METHOD

To control the prosthesis with the torque of the hip joint of the disabled side, it was necessary to calculate the torque in real-time. So the six axes force-moment sensor (NITTA, UFS-4520A) was equipped between the socket and the knee joint of the A/K prosthesis(4). Figure 2 shows the sensor mounted on top of the prosthesis. Figure 3 shows the two dimensional link model on sagittal plane containing the information gathered by the sensor. The torque acting on each joint was represented by eq.[1]-[3].

As inertial terms was smaller than other terms, the gravity term and acceleration term can be neglected. So eq.[1]-[3] are transformed into eq.[4]-[6], respectively. As a result, they can be calculated in real-time, and the torque of the disabled side hip joint, MH, can be utilized as a feedback signal.

CLINICAL WALKING EXPERIMENT FOR EVALUATION

Clinical experiments were carried out on two amputees who lost their left thigh. After several trials by them, the trigger torque, MH, that starts the extension of the knee joint was determined to 40[Nm]. This torque is almost equal to that in normal persons. Another trigger compression force, Fz, that starts the flexion of the knee joint was also determined to 100[N]. This force is almost equal o that in normal persons, too.

RESULTS AND DISCUSSION

Figure 4 shows the data on a normal person (left) and these on an amputee (male,aged 27)(right). The upper and lower parts of Fig.4 show the hip joint and knee joint, respectively. The abscissa represents normalized time[%], and 0 and 100[%] mean toe contact. Four domains divided by vertical lines indicate, from left to right, double supported phase, single supported phase, double supported phase and swing phase. The dotted curve and two solid curves show the joint angle, joint torque and joint power, respectively. As to the power curve, the positive value and negative value mean the driving power and breaking power respectively, because the power is a product of torque and angle velocity. These data revealed that the amputee wearing the A/K prosthesis could walk with a joint angle pattern similar to that in the normal subject. Furthermore the results of the inverse dynamics analysis demonstrated that the amputee generated a torque and power pattern at his disabled side hip joint similar to that in the normal subject. Sequential pictures are shown in Figure 5. Within one hour training, the amputees could master stair walking without the help of his upper limbs. A foot switch has often been used as a sensor to generate a controlling trigger signal. However, due to the instability of this signal, the movement of A/K prostheses tends to be unstable. The present study demonstrated that the stability of the A/K prosthesis movement was improved greatly by using the the axial force moment as a controlling signal. In fact, the amputee reported that he felt relieved because when he wanted to move the knee joint of the prosthesis it began moving automatically without any delay.

REFERENCES

1. H. FUJIMOTO, H. TAKITA, M. ARITA, K. KOGANEZAWA, and I. KATO. RO.MAN.SY7: 419-429, 1989

2. H. FUJIMOTO and I. KATO. ADVANCES IN ECHE 10: 413-424, 1990

3. Technology Research Association of Medical and Welfare Apparatus: Active Artificial Leg(in Japanese), 1985

4. H. FUJIMOTO, Y. SHIMURA, and I. KATO. Journal of Robotics and Mechatronics, 4-4: 268-272, 1992

ACKNOWLEDGEMENT

The author is indebted to the late Prof. Ichiro KATO of Waseda University for his help and suggestions.

Hiroshi FUJIMOTO 1-1 Higashi Tsukuba 305, Japan telephone; +81-298-54-6725 fax; +81-298-54-6727 e-mail; fujimoto@nibh.go.jp