3.1 OBJECTIVES

            At the initial meeting in October 2000, representatives from both Ohio State and Wright State Universities met to discuss the project and how the design of the 2000-2001 robotic arm would be achieved.  After reviewing the previous years’ work on the project, the group decided on some design objectives and constraints that would make sure that the 2000-2001 robotic arm would be an improvement upon the previous designs.

            The 2000-2001 design needed to be cheaper and lighter than that of Chris Fearon and the 1999-2000 team.  Other improvements that were to be implemented were to increase the lift capacity of the arm and to have a fully functional control system.  By the end of the meeting, the 2000-2001 team members from both universities decided on the following parameters for the new robotic manipulator:

·        6 Degrees of freedom

·        Maximum weight of entire assembly less than 30 lbs.

·        Fully-functional control system with working user interface

·        A maximum linkage movement speed of 0.5 m/s

·        Lift a 1.5 kg (~3.3 lb.) mass

·        Maximum cost of $4000 including the controls

 

3.2 DESIGN CONSTRAINTS

            Due to the physical layout of the wheelchair, many constraints had to be placed on the design to ensure that no clearance or interaction problems would occur.  The first constraint placed on the design was that no part of the assembly could extend more than 6 inches beyond the furthest edge of the wheelchair (see Figure 3.1).  This constraint was placed to make sure that with the arm mounted on the wheelchair, the wheelchair would still be able to fit through a standard doorway without too much difficulty.  Another constraint was that in the home position (see Figure 3.2), the arm was not taller than and does not interfere with the armrest of the wheelchair or with the rear tire.  The placement of the robotic arm was also limited by a cross member of the frame of the wheelchair.  This cross member had to be taken into consideration when selecting the placement and mounting widths of the mounting brackets.  The final constraint of the design was that no part of the assembly would interfere with the swivel and rotation of the front tire.  This constrained the mounting height of the arm and extension of any parts below the base of the arm.

 

3.3 CONFIGURATION

            Figures 3.1-3.4 show some different orientations of the completed 2000-2001 robotic arm.

 

3.4 TERMINOLOGY

            Early in the project, it was realized that describing the different motions, parts, and joints of the arm was difficult.  Therefore, members of both teams decided to create a standard terminology in order to make communication easier and less confusing.  The basis of the terminology was the human arm.  The main parts of the arm were chosen as the base, lower arm, forearm, and the gripper (see Figure 3.5).  The joints were defined as they would be on the human arm, shoulder, elbow, and wrist.  The shoulder is the joint between the base and the lower arm. The elbow is the joint between the lower arm and forearm. Finally, the wrist is between the forearm and the gripper.  Since these three joints account for 5 of the 6 degrees of freedom, the motion of each of the joints were added to the terminology.  The shoulder joint was broken into the shoulder twist and the shoulder bend motions (see Figure 3.6).  The shoulder twist is the motion of the arm rotating around a vertical axis as viewed from the side of the chair.  The shoulder bend is therefore the motion of the lower arm rotating around a horizontal axis.  Since the elbow only has one degree of freedom, its motion was simply defined as the elbow bend. (See Figure 3.7)  The wrist, like the shoulder, has two degrees of freedom. (See Figure 3.8)  The wrist bend motion refers to the rotation of the gripper around an axis that is located at the end of the forearm and runs along the width of the forearm tube.  The wrist twist motion is the rotation of the gripper about an axis that runs along the length of the forearm.  The sixth degree of freedom is the clamping action of the gripper and is referred to as the gripping motion.

 

 

3.5 PERFORMANCE HIGHLIGHTS

            The final design of the 2000-2001 robotic arm has the following performance and functional characteristics:

o       Freedom of Motion

o       Shoulder twist = 3600

o       Shoulder bend = 2100 max.

o       Elbow bend = 2550

o       Wrist twist = 3600

o       Wrist bend = 1400

o       Length

o       Shoulder to Elbow = 15.5 in.

o       Elbow to Wrist = 14.56 in.

o       Wrist to Finger Tips (open) = 11.05 in.

o       Full Extension = 41.11 in.

o       Width

o       Extension beyond the wheelchair width = 3.25 in.

o       Wheelchair width with the arm = 27.75 in.

o       Vertical Reach

o       Above the ground = 61.50 in. max.

o       Below the ground (plane wheelchair is sitting on) = 10.25 in. max.

o       Horizontal Reach

o       Extension from front wheel of the wheelchair = 38.11

o       Extension from side of the wheelchair frame = 45.11

o       Total Weight

o       22 lbs.

o       Lift Capacity

o       3+ lbs.

The above values are actual values measured from the final arm once it was completely assembled and mounted on the wheelchair.  The design criteria, as mentioned before, were not only met, but also exceeded in some areas.