1. Field of the Invention
The present invention relates generally to a method and apparatus for delivering a substantially constant reaction force in response to an applied displacement, regardless of the magnitude or change of the displacement.
2. Related Art
Many industrial applications can benefit by a device in which a substantially constant force is output in response to a varied, applied displacement input. Such devices apply a constant force in applications where the unit applying the force does not maintain a constant distance from the unit to which the force is applied. While simple to describe and understand, the concept of constant force application is not easily executed in practice. Most conventional materials and devices follow a typical force/displacement relationship: as the displacement applied to a particular body increases, the force increases correspondingly. This common relationship can, perhaps, best be understood by analyzing the traditional mechanics which describe the relationship between force and deflection of springs. The force (F) applied to a spring is proportional to the distance the spring is deflected (d) and the “spring constant,” k, illustrated by the well known equation F=kd. Although the spring constant may vary from one spring to the next, a conventional spring will typically output more force as the input displacement is increased. Conversely, as a displacement applied to a spring is decreased, the force output of the spring will decrease. Most naturally occurring materials exhibit the same response to an applied displacement: as the displacement increases, i.e., as the material is compressed, the force required to continue compressing the material increases proportionally. This relationship holds for most materials in an un-yielded state.
Despite these complexities, constant force devices have been developed. One field where constant force devices have been used is the field of materials testing. Manufacturing or developmental materials are frequently subjected to mechanical testing to determine the mechanical properties of the materials. Often materials must be qualified by undergoing a testing matrix before they can be used in production. Such testing often requires that the materials undergo constant stress testing. In order to perform such testing, machines were developed that sense the force applied to a material and adjust the displacement applied to the material in order to maintain a constant force. Similar machines have been developed to perform wear testing, a process by which a constant abrasion force is applied to a material over a period of time. Because the material abrades during the test, the abrasion force applicator must move in order to maintain contact with the material. Regardless of the required movement, the abrasive force applicator must maintain a constant force.
The machines developed for these tests are capable of precisely applying a uniform force, regardless of varying displacements, but are very sophisticated and require many components and relatively large spaces to operate. They usually include a force sensing and feedback control system in addition to the test hardware, making the constant force devices impractical for smaller applications and generally very expensive. The large expense associated with such devices is prohibitive in many fields where constant force devices are otherwise very desirable.
Because of these considerations, when a constant force device is required in smaller or simpler operations, the constant force device is often simulated using non-constant force devices and compensating for the variable force reactions. Such simulated devices often utilize conventional springs, which, as explained above, are not constant force devices. While constant force tension springs have been developed, it is believed that constant force compression springs have, to date, only been simulated with negligible success. Use of conventional compression springs as constant force simulators has led to many problems. For example, most motor brushes are equipped with springs that serve to maintain contact between the brushes and the rotor. Ideally, the brushes would exert a constant force on the rotor. However, as the brushes wear, the springs extend to compensate for the lost brush material. The springs are consequently extended beyond their initial displacement. As illustrated by the formula F=kd, the springs at this point are applying a force different than the originally applied force due to the difference in extension. Variations in spring forces can adversely affect the performance of the motors and can lead to uneven wear and premature failure of the brushes.
Another example where constant force devices are desirable is in the field of electrical contacts. The reliability of high-cycle electrical contacts is of great concern to designers. The factor that contributes most to the reliability of an electrical contact is the contact surface mating conditions. Two parameters most affect mating conditions, surface finish and contact normal force at mating. When contact normal force is maintained above a certain level, greater reliability is obtained. Contact normal forces must be small enough to minimize plating damage over the life of the contact, but must be large enough to overcome co-planarity differences and other geometric variations. Thus, a desirable electric contact would maintain a constant, optimal contact force regardless of variations in deflection due to assembly or use.
Conventional electrical contacts attempt to simulate this constant, optimal force by the use of conventional springs. Common examples include pogo type connectors and cantilever type connectors, both of which employ compression springs. While the type of spring used by these connectors differs, the objective is the same. The springs are selected in an effort to provide a constant reaction force at the electrical contact point. Due to the inherent limitations of conventional springs, however, the optimal force cannot be maintained through a range of displacements. To compensate for this, very tight assembly and use tolerances are established, as the designers of the connectors must ensure that contact is made with the spring only in a narrow range of the spring's travel. In this manner, a relatively constant force is maintained at the contact point, but considerable and costly restraints are imposed during assembly. Also, such simulated constant force contacts are not suitable in environments where vibration and movement are present. Applications such as airplanes, vehicles and heavy equipment require electric connectors that can maintain a constant contact force even in the presence of vibration and relative movement of parts.