Design Boundary Conditions

The wheelchair robotic arm system should be targeted for children, and operable by a user with limited hand and/or arm mobility. The system's payload should be 1 kg. The system's maximum reach should be at least 100 cm. The system's maximum and minimum speeds should be 0.2 m/s and 0.15 m/s respectively. The code for the controller should be written in C and executable on IBM PC. The cost of the system, excluding the computer, should be about $4,000.

Design Considerations

The system should also be robust, light-weight, modular, compact, easy-to-operate and safe. Aluminum was the material selected in virtue of its robust and light-weight properties, high strength to weight ratios and relatively low cost. In order for the robotic arm to serve as an extension of the user's arm, the proposed robotic arm must have a jointed-arm configuration. Regarding its articulation, the robotic arm should have 5 to 6 DOF. As to the physical size of the robotic arm, its length and cross-sectional area should be similar to that of a human arm. The robotic arm's workspace should be similar to the arm of an average man sitting on a wheelchair. The robotic arm's payload should be 1 kg. The robotic arm's speed, accuracy, repeatability and resolution are governed by its actuators and controllers.

Design I

Design I has a six degrees of freedom. The 1^st degree of freedom provides a lateral motion in a range varying from 0⁰ to 180⁰ , the 2^nd degree of freedom a vertical motion ranging from 0⁰ to 180⁰, the 3^rd degree of freedom another lateral motion ranging from 0⁰ to 180⁰, the 4^th degree of freedom another lateral motion ranging from 0⁰ to 180⁰, the 5^th degree of freedom provides a rolling motion about the wrist axis, and the 6^th degree of freedom handles the opening and closing of the gripping claw.

Design I has an extended length of 112 cm, and a 2:2:1 lengthwise member ratio. Figures 15-20 depict the dimensions of the link members and gripper of the robot system, and Figure 31 shows a prototypical illustration of the system. An assembly rendering of Design I is given in Appendix I.

Figure 15. Link member 1 of Design 1.

Figure 16.Link member 2 of Design I.

Figure 17.Link member 3 of Design I.

Figure 18.Link member 4 of Design I.

Figure 19. The gripper assembly of Design I.

Design II

Design II has a total of 5 degrees of freedom. The 1st degree of freedom provides lateral motion ranging from 00 to 1800, the 2nd degree of freedom a vertical motion ranging from 00 to 1800, the 3rd degree of freedom another lateral motion ranging from 00 to 1800, the 4th degree of freedom rolling motion about the wrist, the 5th degree of freedom handling the opening and closing of the gripper.

Design II, whose total length when fully extended is 112 cm, has a 1:1 lengthwise member ratio. Figures 21-25 depict the dimensions of the link members and gripper of the robot system, and Figure 32 shows a prototypical illustration of the system. An assembly rendering of Design II is given in Appendix I..

Members:

Figure 21. Member 2 of Design II

Figure 22.Member 3 of Design II

Gripper:

Same as Figure 19

Figure 24. Gripper Assembly of Design II

Same as Figure 20

Figure 25. Gripper Accessories of Design II

Design III

Design 3 consists of three link members. The first member rotates vertically; the second member and the third members rotates laterally. Drawing package can be found in Appendix-I.

When the total length of the arms is 1.0 meter, the first member is 0.40 meter long, and the second and third members are each 0.30 meter long. The grip is 5 cm long which will make the total length little longer than 1.0 meter. Thought it is specified that the maximum reach distance is 1 meter, in order to make the robotic arm more reliable and more efficient, the 5 centimeter is necessary to the design.

The general cross section shown in Figure 26 has three sides. Because the arm is only designed to pick up object with one kilogram weight and aluminum doesn't contribute too much weight and moment to the arm, it is not necessary to use solid aluminum, and it gives room to the installation of wires, belt and other accessary. The thickness of the aluminum plate is 0.5 centimeter.

Figure 26: Cross Section of middle part of arm member

Though the arm member structure is strong enough to perform the specified task, the arm needs to be strengthened at both ends. Any sudden strong force may cause the aluminum arm bent at the joint area, thus cause the joint to malfunction. The solution to prevent the bending is to add another side to the arm at both ends. The cross section at the end part of the arm member is shown in Figure 27. It decreases the bending moment at the two edge in Figure 26 dramatically.

Figure 27: Cross Section of end part of arm member

Figure 28: Back view of arm member 1

The bar is not only designed to strengthen the joints, it is also designed to limit the movement of each arm member. It is controlled by the distance between the edge of the arm member and the edge of the bar. In this project, there are two things needed to be considered which may limit the movement of the robotic arm. First is the effectiveness which is very important that the arm can reach most of the region around the user but also limit the arm to move too freely which may cause inconvenience to the user. Second the safety of the user is also a serious consideration that the arm should not reach further than the point where the robotic arm can barely touch the patient's face. Under these consideration, a through experiment needs to be done to finally determine the location of the bars to limit the arm movement. The angle limitation can range from 0 degree to 135 degree depend on the location. But bars only limit rotational movement on its side, on the other side same result can be achieved by cut that side of aluminum to create a gap allowing movements.

The initial design is that the joint between the wheelchair has 90 degrees of movement up and 120 degrees of movement down; the joint between the first and the second members has 135 degrees movement left and 135 degrees movement right; the joint between the second and the third members has 90 degrees left and right. After testing the design using simulation, more information will be collected to reach a more reliable design.

The arm member design is different between member 1 and member 2. Member 2 and member 3 have same design. Member one is longer and having connections with other members on different sides of the member, since at one end it rotates vertically and the other end it rotates horizontally. Thought the basic design is same in member two and three, member two have a small height so it will be able to fit in the ends of member one and three. The height of member two is 9 cm which is 1 cm shorter than member one and member three due to the thickness of the aluminum plate.

With a cross sectional area of 0.01 m² and aluminum bar thickness of 0.5 cm, it is a strong robotic arm for home use. Five degrees of freedom may not be enough to reach any point the users like to reach, but with the extra freedom that the wheelchair itself can provide, this design will cover all regions around the users. Another concern is that the direction of the grip may not be at the right direction that can perform the best results, but less the degrees of freedom the easier it becomes to the users to control. There has to be some compromise between the user and the design.

Kinematics

The Robix(tm) RC-6 Robotic Construction System was used to validate the kinematics and operation of each conceptual design. Robix RC-6 has six servo motors, and a Robix adapter - an electronics package with power attachable via to either LPT1 or 2 parallel printer port on a PC running DOS and can driving up to 6 servos simultaneously. This particular construction kit also comes with 16 punched and machined aluminum links, a parallel-jaw gripper with wrist subassembly, a weighted base and pivot post, and a manipulator breadboard.

It has also been determined that the maximum number of degrees of freedom of the design should be six. Although the additioanl degree of freedom provided by Design I may enhancing the robotic arm's reaching ability, it may hinder the user's ability to effectively control the mechanism. Figure 31-33 depict the Robix renditions of the three conceptual designs than five degrees of freedom would restrict the robotic arm's functionality and flexibility

Figure 31. A Robix rendition of Design II

Figure 32.A Robix rendition of Design II

Figure 33.A Robix rendition of Design III.

Rendered Image of Design I (1^st page)

Rendered Image of Design II (2^nd page)

Rendered Image of Design III (3^rd page)

Having validated all three designs with the Robix construction system, it was determined that design I and design II satisfy all the design requirements.

Design I has a solid cross-sectional area throughout its length. Its relatively small and compact cross-sectional area (approximately 0.0006 m2) enhances the device's functionality, aesthetics, robustness and the possibility of being easily stored when not in use. It is best suited for implementation using solely servomotors as actuators.

Design III is best suited for actuating systems made up of servomotors, chains and belts. The hollow cavities throughout its members provide the required space, shielding and support to the installation of such belt-driven actuating systems.