Introduction
Robotic arms are used for positioning a payload within a 3D space. While simple robotic arms with 3 degrees of freedom (DOF) are sufficient for lateral translation, at least 6 DOF are required for rotation along the pitch, yaw and roll of a payload (Figure 1).
Problem Statement
Typically, a robotic arm is anchored at the base and stretches itself out to get the payload in position. However, stretching out stresses the axes significantly due to large torque generated by the weight of the robotic arm and payload. If the motors and drives are unable to generate sufficient torque, the robotic arm will stall, collapses or fail to move payloads to precise positions. Therefore, it is crucial to reduce the weight of robotic arm where possible.
The wrist is a major contributor of torque on the axes because it is located the furthest from the base (Figure 1). Reducing the weight of the wrist and/or shifting its center of weight closer to the anchor would alleviate stresses on the axes. While weight reduction is important, the function of the wrist should not be compromise. It is critical that the wrist remain stiff so it can still provide an accurate yaw and pitch rotation.
Solution
Differential bevel gear mechanism is one of the many good solutions to tackle the weight issues of the wrist. It involves three bevel gears arranged perpendicular to each other. Depending on the direction and magnitude of rotation of the bevel gear on both sides, the bevel gear at the center will rotate to provide yaw rotation and/or the whole mechanism will swivel too to provide pitch rotation (Figure 2).
Such mechanism allowed two stepper motors to power either the yaw or the pitch rotation motion of the payload simultaneously and none of the motor would be idling at any point in time. This is beneficial because any idling motor is a dead weight and minimize the performance of a robotic arm. Secondly, heavy motors powering the wrist can be placed nearer to the base and help reduces moment on the joints. Lastly, such mechanism is compact and well suited for tight spaces at the end of a robotic arm.
Design
A variety of components ranging from ball bearing and specially designed brackets were used to position the bevel gears while allowing the gears to rotate freely. It is crucial that these bevel gears were positioned accurately to minimise backlash or excessive wear. Some of the specialized brackets such as “Side gear mount and lock” (Figure 3) are responsible for locking onto bevel gears and transmit large torque onto them. Other specialized bracket such as “Top gear mount and lock” extracts large torque from the mechanism and transmits it outside to enable to wrist to lift heavy payloads. Other specialized brackets such as “Top bracket” and “Side bracket,” assist in holding large ball bearings in place and serve as an anchor point for screws during assembly.
One unique aspect of this wrist design is the incorporation of a planetary gearbox within the wrist structure (Figure 5). Planetary gearbox amplifies torques and help the wrist to lift heavier payload (Figure 4). Most planetary gearbox were sold separately and integrated into robotics. However, integration requires interfacing surfaces and mounts which add weight and volume to the design. For this design, the incorporation of planetary gearbox within the wrist enables its output to couple directly to bevel gears, which results in a more lightweight and compact design.
The planetary gearbox inside the wrist has a sun gear with elongated shaft (red). The shaft extends through a large opening in the bevel gear and towards the center of the wrist. The end of the shaft is equipped with rough teeth that allow a timing belt to mesh against and transfer torque down the shaft and onto the sun gear. The sun and planetary gears (blue) then rotate around the ring gear to amplify torque which is then transmitted directly to the bevel gears (Figure 6). This design features allows for a more compact wrist by receiving torque through its center where there are plenty of spaces.
The wrist is constructed from three sub-assemblies: two sub-assemblies at the side with built-in planetary gearboxes and one at the top that houses the bevel gear responsible for lifting heavy loads. These sub-assemblies are secured together with M3x16 bolts followed by two timing-belt loops, and a 5mm diameter rod running through the center (Figure 7a). A cover protecting the mechanism is screwed on to complete the assembly (Figure 7b).
To summarize, the design of the wrist is important given that it is located at the furthest end of a robotic arm. The design must prioritize compactness and lightness to minimize strain on subsequent joints to avoid the use of heavier motors and gearboxes. Our approach is to integrate a planetary gearbox into the wrist structure and directly couple its output to the bevel gears. This reduces interfacing surfaces and hence keeping the wrist compact and lightweight.
