The joints responsible for shoulder, elbow, and forearm movements are based on igus Robolink solution. Each joint is actuated by two tendons; one for clockwise, the other for counterclockwise rotation. The tendons are fed through bowden cables to protect them from breaking due to friction. The joints are operated using Maxon DCX35L motors. The tendons are attached to the motors using dedicated drive wheels from igus which are mounted directly onto the shaft of the motor. The construction of the drive wheels allows the tendons to be tensioned with ease with the help of clamping tools. Both the drive wheel and the clamping tools are shown in Fig. 1. Arm motors are mounted directly onto the robot's torso, as shown in Fig. 2.
Fig. 1 igus Robolink drive wheel and clamping tool.
Fig. 2 FLASH arm drive motors.
The whole arm setup consists of three different robolink joints: RL-50-TL1 (1 DoF, shoulder movement), RL-50-001 (2 DoF, shoulder/elbow movement, ±90° pivoting angle), and RL-50-002 (2 DoF, elbow/forearm movement, +130°/-50° pivoting angle). All joints have mechanical stop dogs to prevent exceeding the angle limits.
Wrist joint is based on a custom fabricated ball joint. The ball itself is machined from a solid block of teflon to decrease friction. The joint is also tendon driven and enables a two axis, ±30° movement (with relation to axis going through the forearm). In order to drive this joint, the large and heavy Maxon motors were replaced with Dynamixel MX-64 servomotors mounted next to the robot's shoulders. A special mounting bracket was designed to enable the robolink drive wheels to be used with MX-64 servo horns. The tendons for the wrist also rely on the same Bowden cable principle as the arm tendons. The motor section of the wrist drive can be seen in Fig. 3 while the wrist itself is shown alongside hand drive system in Fig. 4.
Fig. 3 FLASH wrist motors (left arm).
Hand joints are yet another example of a tendon driven system utilizing Bowden cable principle. The robot's fingers are driven by HS-5070MG servos equipped with custom, 3D printed horns to provide the full range of motion for each joint. The tendons are guided inside of the forearm support tube so they are normally not visible. For the index, middle and ring finger the joint setup is exactly the same. The fingers are made up of four segments. Each two segments are joined by a pin with a spring enabling finger retraction. The tendons can actuate the tip of the finger as well as the third segment from the tip to provide finger protraction. The fourth segment can be rotated around the axis perpendicual to the hand plane. The thumb is also actuated by three servomotors. Instead of having two actively protracted finger segments, the thumb has only one. However it is possible to protract the whole finger in a manner making the thumb opposable to the other fingers. The drive system for the hand joints is presented in Fig. 4.
Fig. 4 Hand and wrist drive.
FLASH is capable of keeping balance on mildly rough surfaces and slopes thanks to a differential balancing platform. Each of the two robot's identical balancing drives is based on a Maxon RE40 motor. The wheel hub is machined out of steel and mounted directly onto the motor's shaft. In order to reduce loads exerted on the gearhead, the hub is supported by a double bearing system. The bearings are installed inside a solid steel wheel mount, which is also connected to the frame of the robot. A heavy duty Rolko wheel mounted onto the hub completes the drive system. Complete overview of a balancing drive can be seen in Fig. 5.
Fig. 5 Balancing platform drive overview.
|Balancing platform drive|