Final Design
STRUCTURALLY SMART ANIMATRONIC FIGURE (SSAF)
The final design uses five total Dynamixel robotic servos in the torso, shoulder, and arm components. The torso serves as a sturdy base for the figure and houses an MX-106T motor, timing pulley power transmission, and bearing system that rotate and support the shoulder. The shoulder connects the arm to the torso and has a 3D-printed box containing steel plates to counterbalance the arm. The arm consists of three joints, two linkages, and a hand. The joints are made of two master/slave configured MX-106T motors at the shoulder, one MX-106T motor at the elbow, and one MX-28T motor at the wrist. The upper and lower arm linkages are based on t-slotted aluminum construction, and their modular design allows stiff and flexible interchangeable linkages. The hand is designed to hold a cup or similarly shaped payload weighing up to 0.45kg. The motors were chosen by deriving the torques at the joints (shoulder, elbow, wrist) in the worst-case scenario, which was defined as fully-extended arm moving at max acceleration.
Figure 3: Final Assembly
Linkage Design
Figure 4: a) Stiff Linkage; b) Flexible Linkage
The stiff linkage consists of a pair of solid aluminum plates that unify the sections of t-slotted aluminum in the upper and lower arm, constraining motion in all directions. The flexible linkage replaces the stiff plates and consists of brackets housing a shaft that supports two torsion springs mounted in opposing directions. This linkage constrains side-to-side (transverse) motion, but allows for flexibility in the same rotational plane occupied by the motors in the arm. These designs were chosen for their simplicity and use of commercial off-the-shelf hardware, allowing for more straightforward analysis and ease replication and modification. The linkage material selection was validated with Finite Element Analysis and hand calculations to ensure it would be strong enough to support the arm.
Pulley-Bearing System
Figure 5: Bearing Pulley System
The torso rotated the arm via a pulley-bearing system. The system consisted of a shaft attached perpendicularly to the shoulder and rotated via a pulley system connected to a motor. The belt of the pulley system could be tensioned by adjusting the motor’s position through a slit in the mounting plate. The design was sturdy, portable, and allowed access to the pulley bearing system.
Mechatronics
Figure 6: a) ArbotiX-M Robocontroller; b) UartSBee FTDI driver
SolidWorks motion analysis and hand calculations were used to find required motor torques at each joint. These calculations were based on the extreme case of movement at maximum acceleration with full arm extension. Taking factors of safety and operation criteria into account, motors were selected to support stall torques of 20 Nm at the shoulder, 10 Nm at the elbow and 3.1 Nm at the wrist.
The motion profile was hard coded using Arduino and uploaded to the Dynamixel motors via the ArbotiX-M Robocontroller and UartSBee FTDI driver. The code loops through a series of motor positions with defined velocity and acceleration values for each motor. A 12-volt 10-amp power supply was selected and confirmed to provide enough current with a factor of safety of 2.25.
Performance
The parts and components fit together nicely and the joints had the full desired range of motion. The stiff linkage was very solid and the pully-bearing system rotated easily. Although the motors were able to move simultaneously and smoothly before assembling SSAF, the fully-assembled figure was not able to repeat the motion profile due to overheating of the ArbotiX-M Robocontroller. The figure was then disassembled and the five motors were retested. When not attached to the linkages, the motors performed the motion profile as intended. A possible explanation for this incident was that motors drew more current than the board could handle to support the weight of the arm. The solution was that instead of powering the motors through the ArbotiX-M Robocontroller, the power was supplied through a 6 Port AX/MX Power Hub, and a 1-Amp fuse was utilized to limit the current flowing through the ArbotiX-M Robocontroller. As a result, motors were able to perform the motion profile smoothly.
Figure 7: a) 6 Port AX/MX Power Hub; b) New Hardware configuration with 1-Amp Fuse