Robot Arm Mechanical Design
The arm project is the most innovative mechanical project on the robot. It represents the first powered iso-elastic arm. Iso-elastic means that the motion at one end is isolated from the motion at the other end using elastically stored energy to counteract the force of gravity-the arm will hold it's position without the aid of the electric motors. In an ideal iso-electric design the servos should only lift the weight of the payload carried by the hand, allowing a considerable increase in the length of arm and size of payload that can be handled by a specific power of motor.
The idea was built from a simple design requirement, to maximize the limited resources available for the project. Our team was inspired by successful iso-elastic designs such an adjustable table lamp or a steady-cam. We knew is possible to devise a spring configuration that would perfectly counteract gravity, however it was a difficult task to prove how it could be done. The first step was realizing that horizontal and vertical motion should be separated. Horizontal motion will only be limited in acceleration by the vertical position of the arm, vertical motion will always be limited by the weight in the hand and the angle of the arm and not the horizontal angle. Any solution for finding a way to support the weight of the arm through a vertical arc could be used in any horizontal position. This arrangement is not typical for robot arms since it restricts the range of the arm, however we decided this was a worthwhile sacrifice. The initial attempt at proving the arm could be supported by a spring throughout it's vertical range was done using angles, and after a very complicated proof using difficult trig identities it was shown that specific configuration would work, however we were unsure of what other configurations would work as the math got too messy.
The general solution proved to be simple and elegant. In any position of the arm, the equilibrium force diagram forms a similar triangle with the geometry of the arm. Simple rearranging shows the spring constant necessary for the correct functioning of the arm. (fig 3) With this information, our design challenge became much simpler.
The arm was designed to be made almost entirely with a water jet cutter and very limited machining consisting of countersinks and a few taps only. The upper arm was designed to be sturdy, and weight was not much of a factor as it was attached directly to the chassis and any weight would be supported by the central spring in the arm. (fig 2) A main consideration was eliminating dangerous pinch points that could cause injury, and much of the arm's shape was inspired by making it finger safe. It requires purposefully jamming your fingers into the arm to sustain injury, it is almost impossible by casually resting your hand on the machine. The upper arm has an adjustment screw to change the height of the spring attachment point which can be used to calibrate it for the weight of the lower arm, the gripper, and the payload.
The lower arm was designed to be much lighter, with the calibration method to be adding or removing many small elastic springs. To protect the servo motor a slip clutch is implemented for the elevation of the arm. Because of the difficulties we had installing the main spring in the upper arm, the lower arm was designed to be very accessible. The structure was designed to be open and easy to access, with separate side plates to cover the pinch points once assembly or maintenance is complete. (fig 1) Of course it followed the principles of manufacturability and modularity that maximize our design mobility allowing us to improve the arm quickly and economically.