1. Field
Embodiments of the invention relate to the field of counterbalance mechanisms; and more specifically, to spring force counterbalance mechanisms for rotating loads.
2. Background
In load positioning systems the effects of gravity on the load may be compensated for with a counterbalance or counterweight, a constant-force spring, or a pneumatic equilibrator. A counterweight is often undesirable due to its large size or to the weight and inertia that it adds to the system. In many situations a constant force spring is not capable of generating a sufficient compensating force. These springs have innately high stresses which induce metal fatigue. Consequently, the springs have a relatively short dynamic life span. The disadvantage of pneumatic equilibrators is that they are long and bulky and generally have to be made on a custom basis.
In a load positioning system with a servomechanism the servo amplifier may be biased in an attempt to compensate for the force of gravity. This procedure is not optimum inasmuch as it only reduces the steady-state error. When the bias is added, the maximum acceleration and maximum velocity remain asymmetrical—that is, the acceleration and the velocity in the direction opposite the gravity vector are less than the acceleration and the velocity in the direction of the gravity vector. Motor selection must therefore be made according to the performance demands of the worst case. This results in greater apparatus cost, volume and weight. Additionally, the extra load of the bias force going through the load positioning system's transmission will increase the friction in the joint.
Static balancing systems incorporating springs may be advantageous because they do not add an undue amount of weight and inertia. A spring balance system may offer the advantage of not requiring external power. However it can be difficult to provide a spring balance system for a load that rotates freely on a shaft. For example, U.S. Patent Application 2007/0156122 shows a spring balance system that can balance a pivoted arm where the arm moves through less than one-half of a revolution. U.S. Patent Application 2004/0035243 shows a spring balance system that can balance a pivoted arm where the arm can rotate freely but the spring balance system must be connected to an unobstructed end of the rotating shaft that supports the load.
It would be desirable to provide static balancing systems incorporating springs that can be coupled in the middle of the shaft that supports the load and permit the shaft to rotate freely.