Step 1: Wheelchair Mounting Bracket
Each powered wheelchair is highly customized, so mounting brackets will likely be case specific. In general, try to re-use as much standard mounting hardware as possible. For this project, a mounting bracket for a standard joystick was adapted to hold the arm by attaching an aluminum plate. We also made an effort to allow the client’s water bottle holder to remain in its usual position, requiring our arm to rotate around the water bottle.
Step 2: Arm Deployment Mechanism
To ensure the usability of an arm-type mechanism thought needs to be given to how a client will travel with the device mounted to the chair. For our arm, there is a stowed position and an operational position. In the stowed position, care was paid to minimizing the width added to the chair, and when deployed priority was given to placing the arm handle in a comfortable position for use. In order to easily deploy the arm, we designed a support plate that would rotate out at the press of a button. It was mounted to the end of the bracket plate via bushings, bearings, a shaft, and shaft collars. A gear was attached to the shaft which meshed with a gear attached to a servo mounted on the bracket plate. The support plate rotated outwards as the servo turned the gear mounted to the shaft.
Step 3: Build Arm and Pivot
For this project, the primary arm was a 1 inch OD fiberglass tube of approximately 20 inch in length. To allow for manual aiming by our client, a plastic tubing handle was epoxied through the main arm tube to allow for a secure place to grip. The arm allowed for two degrees of freedom: rotation about both the vertical and horizontal axes. These rotations allowed our client to adjust the horizontal position of the arm through approximately 150 degrees, and vertically through approximately 30 degrees, or 6 inches. Rotations were accomplished by mounting the arm to a custom bracket built using 1/4 inch aluminum plate cut on a water jet and assembled using the T-nut technique.
Step 4: Pusher Assembly
Button actuation requires a pushing force from the end of the arm. If your client lacks the arm and hand strength to accomplish this task mechanically, consider using a mechatronic solution. The pusher assembly was 3D printed to reduce machining time and incorporate necessary features to mount a servo and limit switches. Actuation was accomplished using a servo and a rack gear, and twin limit switches provided feedback. A commercial linear actuator would also work fine. In addition, we mounted springs on the opposite end of the arm in order to reduce the force needed to lift it.
Step 5: Incorporate Electronics
As the final step, the servos were connected to an Arduino nano programmed to control the arm. Three buttons were added: a master toggle switch, a toggle switch to deploy the arm, and a pushbutton mounted to the end of the handle to actuate the end effector. Limit switches were added to the deployment plate as safety measures. The entire system was powered off of the 24 volt wheelchair batteries and was stepped down to 12 volts using a DC to DC converter.
Author: MIT 6.811