A Novel Design of a Prosthetic Hand
Wei-chen Lee, Chih-Wei Wu Department of Mechanical Engineering National Taiwan University of Science and Technology
Taipei, Taiwan, Republic of China [email protected]
Abstract—An innovative prosthetic hand prototype is presented in this paper. To increase the grasping stability, an underactuated finger is embedded in a traditional gripper. There are a total of three motors employed in the hand system: the gripper, the embedded finger, and the wrist. The preliminary performance shows that this prosthetic hand is capable of performing various daily jobs such as turning a key, pinching a card, grasping tubes of various sizes, etc. The maximum grasping force for the gripper is about 2 kg and that for the embedded finger is about 0.3 kg. The closing time for both the gripper and the embedded finger is about 0.2 sec, which is relative fast among similar prosthetic hands. The performance of this innovative prosthetic hand prototype shows the idea of the embedded finger is promising and further improvements will continue.
Keywords—prosthetic hand, gripper, embedded finger I. INTRODUCTION
With the advent of advanced personal safety equipment used in the war, the fatality rate reduces but the number of amputees increases in recent wars in Iraq and Afghanistan[1].
Various diseases and accidents also result in many people losing their hands. For people without hands, the inconvenience in their daily life is obvious; to let them have a life of quality, prosthetic hands are indispensable.
One of the most popular prosthetic hands is Touch Bionics’
iLimb[2], which was one of the TIME’s best inventions of 2008. This hand uses five actuators to control its five fingers, and the control signals make use of users’ EMG signals. The grasping force of this hand is about 4.5 kg. Its underactuated finger-type design can have better stability of grasping round tubes or irregular-shaped objects. iLimb can also pinch a credit card as well as grasp a mug, so the dexterity of iLimb is obvious. Another similar prosthetic hand is DARPA’s Revolutionizing Prosthesis 2009 (RP 2009)[3]. From the limited literature open to public, it can be observed that the mechanical design looks similar to iLimb. Both of these hands use underactuated finger design. However, the control signals of RP 2009 use nerves, requiring an invasive operation to reroute the nerves. Noise interference may be reduced, and more complicated motion may be achieved.
For the commercial prosthetic hand, Otto Bock has both the gripper-type and finger-type prosthetic hands. These hands
have one or two degrees of freedom, and can be easily learned to do uncomplicated jobs. They are also controlled by EMG signals. The speed for the hand to close can be as high as 300 mm/sec and the maximum grasping force is about 10 kg.
There are many other prosthetic hands that are developed by various research institutes. Iowa Hand[4] used springs to be its fingers. Cables are used to buckle the springs to control their motions. The structure is very simple, but the grasping force cannot be too large because it is related to the rigidity of the springs and the springs will not buckle if the rigidity is too large. Zhao et al.[5] developed a five-finger prosthetic hand by using three DC motors only. Each of the thumb and index finger are controlled by a motor, respectively, and the rest of the fingers are controlled by the third motor. The control signals are also from amputees’ EMG signals. The mechanism of the hand is complicated and the pinch force is only 0.5 kg.
In addition to the traditional prosthetic hand actuated by motors, Kargov et al.[6] applied a small hydraulic pump to a prosthetic hand. The total weight of this hand is 353 g, and the maximum grasp force is 11 kg, which is quite large due to the hydraulic force. The time for the fingers to move from full open state to full close state is less than 1 sec. From its specifications, the performance of this hand is quite impressive.
However, no experimental data was reported at that time.
Based on the previous discussion and our other survey, Fig.
1 shows the ways to actuate the fingers, which include DC motors with cables (or with cables and torsion springs), DC motors with belts, DC motors with linkages, and DC motors with spur gears. Other actuations include hydraulic motors and shape memory alloy. From a practical point of view, the use of DC motors with cables, belts and linkages are most popular.
Usually, people design underactuated fingers by using DC motors with cables or belts and design the gripper by using DC motors with linkages. The underactuated finger can adjust according to the shape of the object to be grasped. However, due to the limitation of the rigidity of the underactuated finger, it usually cannot provide large grasping force. The traditional gripper usually has a large grasping force but lacks flexibility to deform which result in less grasping stability. Therefore, the objective of this research was to redesign a prosthetic hand so that it can have both the benefits of the underactuated finger and the traditional gripper. In the following sections, the mechanical design, the control scheme and the preliminary evaluation results of the prototype of this innovative prosthetic hand will be presented.
This work was supported in part by the National Science Council, Republic of China, under Grant NSC98-2221-E-011-117.
,(((
Figure 1. The various typs of finger actuation
II. MECHANICAL DESIGN OF THE PROSTHETIC HAND The mechanical design of the prototype of this innovative prosthetic hand is shown in Fig. 2. An underactuated finger is embedded in the gripper. This embedded finger is composed of three phalanges as shown in Fig. 3(b). The first phalanx is fixed to the longer finger of the gripper. The second and the third phalanges can flex by pulling cables attached to their pulleys and can restore to the original positions with the help of the extension springs as shown in Figs. 3(a) and (c). Because the finger has two degrees of freedom (DOF) but one motor, it becomes an underactuated system. The gripper consists of two fingers. A motor is connected to the longer finger of the gripper, and then the longer finger and the shorter finger of the gripper can open and close together by using a coupler as shown in Fig.
4(a) and (b). The wrist is directly connected to a motor to allow it to be able to rotate. From the previous discussion, it is understood that the prosthetic hand has four degrees of freedom in total.
As described previously, the gripper and the embedded finger are driven independently by their own motors. If the object to be held is small or irregularly-shaped, the embedded finger can stick out of the longer finger of the gripper as shown in Fig. 4(c) to increase the grasping stability.
gripper
Figure 2. The structure of the prosthetic hand with an embedded finger
extension springs
Figure 3. The design of the embedded finger: (a) top view; (b) front view; (c) bottom view
Figure 4. (a) the gripper is at its full open position; (b) the gripper is at its full closed position; (c) the embedded finger is closed
III. KINEMATICS OF THE GRIPPER
The longer finger, the shorter finger, and the coupler of the gripper comprise a four-bar linkage as shown in Fig. 5(a). We use a, b, c, d to denote the linkage lengths, and θ1, θ2, θ3, θ4 to denote the angles between the linkages and the horizontal axis as shown in Fig. 5(b). Here θ1 is a fixed value because it is the angle of the fixed link. The input angle is θ2and the output angle is θ4. From Fig. 5(b), we can derive
(a) (b)
Figure 5. (a) top view of the gripper; (b) the corresponding mechanism of the gripper.
Summing up the square of Eqs. (1) and (2) gives
1cos 4 2sin 4 3 0, By using the tangent half angle formula and Eq. (3), we can obtain the following equation:
(
3 1)
tan2 4 2 tan2 4 1 3 0.2 2
k −k ⎛θ ⎞+ k ⎛θ ⎞+ + =k k
⎜ ⎟ ⎜ ⎟
⎝ ⎠ ⎝ ⎠ (7)
Thus we can solve Eq. (7) to obtain the relationship between the output angle θ4 and the input angle θ2 as Eq. (8).
When the gripper is in its fully open position and fully closed position, the input angles are 140° and 183°, respectively. By using Eq. (8), we can plot the output angles when the input angle varies from 140° to183° as shown in Fig. 6, and find out that the output angles corresponding to the fully open position and fully closed position are 26° and 1°, respectively. The input angle is the angle of rotation of the longer finger of the grip and the output angle is the angle of rotation of the shorter finger. It turns out that during the whole movement of the gripper, the longer finger rotates 43° in total and the shorter finger rotates 25° in total, which mimics human’s hand motion if we form a gripper by using index finger as the longer finger of the gripper and thumb as the shorter finger of the gripper.
( ) ( )( )
140 145 150 155 160 165 170 175 180 185
input angle (degree)
output angle (degree)
Figure 6. The relationship between the input angle and the output angle of the gripper.
IV. ACTUATOR
The gripper, the embedded finger, and the wrist use one motor, respectively. The motors that are employed for the gripper and the wrist are ROBOTIS’ Dynamixel RX-64 DC servo motors, and the one employed for the embedded finger is Dynamixel RX-28 DC servo motors. There are several reasons for us to use ROBOTIS’ Dynamixel DC motors. First, the controller, the driver and the encoder are integrated into the motors to make it compact and easy to use. Second, it can provide large torque. For example, RX-28 and RX-64 can provide 28 kg-cm and 64 kg-cm maximum holding torque respectively. Their weight is another consideration. RX-28 weights only 72 g and RX-64 weights only 116 g. Here we did not use very expensive mini or micro motors simply because the emphasis of this paper is to present the idea of the embedded finger rather than build a prosthetic hand that are ready to be used on amputees.
V. CONTROL SCHEME
The control scheme of the prosthetic hand was designed in the way as shown in Fig. 7. Since the motor controller is built in each motor, we can simplify the control scheme by using a DSP board to interpret the EMG signals and directly send the commands to the motors to move the mechanism to the desired position. The DSP board is still under construction, so we directly controlled the motors during the performance tests.
Pressure
Motor Gripper Encoder
Figure 7. The control scheme of the prosthetic hand
VI. EVALUATION RESULTS
The prosthetic hand prototype was tested for its performance and the results are listed in TABLE I. The total weight of the mechanism and the three motors is 721 g. The maximum grasping force for the gripper is 2 kg and the maximum pinch force for the embedded finger is 0.3 kg. Both the gripper and the embedded finger can finish their movement in approximately 0.20 sec. We also used this prosthetic hand to perform some daily jobs as shown in Fig. 8, such as turning a key, grasping rods of various sizes, pinching a card, etc. This prosthetic hand can complete these jobs without problems. The performance of the prosthestic hand
TABLE I. THE PERFORMANCE OF THE PROSTHESTIC HAND
Item Specification
Weight (mechanism and motors only) 721g
gripper
Maximum grasping force 2 kg
Maximum opening 140 mm
Closing time 0.18 sec
embedded finger Maximum pinch force 0.3 kg
Closing time 0.20 sec
wrist Maximum rotation angle 300°
Time for rotating 300° 0.8 sec
(a) (b)
(c) (d)
Figure 8. (a) turning a key; (b) grasping a pen; (c) grasping a thick tube; (d) pinching a card.
VII. CONCLUSIVE REMARKS
An innovative prosthetic hand prototype is presented in this paper. The major difference between this hand and the other prosthetic hand is that an underactuated finger is embedded in the traditional gripper. This embedded finger provides extra grasping stability when the hand is holding small objects.
There are four degrees of freedom for this prosthetic hand and three DC servo motors employed. The total weight of the mechanism and the motors is 721 g. Because this is only a prototype, the weight can be further reduced by optimizing the mechanical structure and choosing light-weight micro motors with high torque output. The closing time of both the gripper and the embedded finger is around 0.2 sec, which is relatively short compared to the other similar prosthetic hands. The hand prototype is capable of performing a lot of daily jobs according to our evaluation. Further improvement on the gripper and the embedded finger of this prosthetic hand will be performed to allow them to handle more objects of different sizes and shapes as well as increasing their grasping forces.
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