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The purpose of the Support Arm is to support the weight of the camera plus sled while isolating the sled from the motions of the operator. The arm consists of two segments called bones connected together with a pivoting hinge, two joint hinges, a vest connector, and a gimbal connector. It acts as an additional arm of the cinematographer / operator.
Each arm segment is a sort of parallelogram with a spring-shock-absorbtion-system enclosed inside. Each arm is made up of two metal hollow cylinderical bars, fastened to two metal end-blocks. The lower bar in each arm segment is actually a hollow cylinder, with a large coiled spring inside. Just as with any parallelogram, the metal bars will remain parallel with each other (or nearly parallel) no matter how the arm is positioned. Since the end blocks are secured to the ends of the parallel bars, they will remain in the same position as the arm swings up and down.
In practice, an unloaded arm has its bones in the highest position. It then deflects to a more or less level position (parallel to the ground) as the camera and sled combination is attached to the end of the arm. This position is known as the neutral position and is where the sled naturally wants to float. Additional force must be applied to boom the arm either up or down and to keep it from the neutral position. Our arm requires the same amount of force to boom the arm along its entire range of motion and the arm will stay at any vertical position that you desire without any additional force.
You can see that the spring keeps the bone in its current shape. Without the spring, the bone would just flop down under its own weight. If a mass was attached to the end of the bone, the bone would deflect down and as it does the length of the diagonal increases. Since the spring is placed along this diagonal, it stretches and exerts a resisting force. The weight of the mass exactly equals the force of the spring because the bone has stopped deflecting.
The articulated arm essentially acts as a shock absorber for the camera sled. When the cameraman moves, the base of the arm moves as well. But the spring system in the rest of the arm responds to the weight of the sled. Instead of a sharp jolt, the camera shifts its position smoothly. The arm also frees up the cameraman's hands -- it hangs directly on the vest, so the cameraman doesn't have to do anything to hold the camera sled up. He or she can concentrate on positioning the camera to get the best shot. When the body of the cameraman jerks, camera glides. |