ROTA (Remotely Operated Technic Arm) is a mobile robotic arm created with pieces from the MINDSTORM NXT 1.0 and the 8043 Excavator. ROTA uses 6 motors (2 NXT and 4 PF), 4 linear actuators, 2 PF receivers, 2 PF remotes, 2 NXT's, and 1 battery box. It is controlled via PF remotes and another NXT. It is around 20 in. high, and about 18 inches wide.
For the base I built a basic skid steer drive then strengthened it as I built the arm. The 2 NXT motors are geared down 1:667 and control the driving function. I reinforced the treads due to a slight amount of bending when the arm (weighing around 5 lbs) would attempt to lift something over 2 lbs. The base took the shortest amount of time to build only around 45 minutes with an additional 15–20 minutes fixing all the flaws.
I used 3 linear actuators for the arm, two for the whole arm and one for the bend in the middle. I slightly geared down the two actuators that lift the whole arm, but didn't use any for the bend in the middle. I originally was going to use two actuators too lift the middle bend but instead I used only one because I needed the other one for the hand. The arm took me about 2–3 hours too build and perfect.
At first I tried using a worm gear for the grabbing function of the hand, but the worm gear kept on popping off whenever it would start gripping something. So I switched and used an actuator instead, which worked great. It also gives it quite a bit more torque when gripping. The hand can spin around by an arrangement of gears that turn a worm gear which turns the turntable connected to the hand.