Here is the GeoGebra model of our double articulated arm. The Shoulder and Elbow Motor sliders represent spinning the drive motor. Both joints are controlled by the motors at the base of the main 2 inch support bar, blue colored points, the lower blue point is the drive shaft of the shoulder joints motor. The points spinning around are to show a visualization of how fast they spin, so the sprockets they are chained to will have spinning points as well, but their rate of spin will vary depending on sprocket teeth and subsequent reduction.
http://coffman.dublin.k12.oh.us/teac...ble/Arm_2.html