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Near Constant Force Electrical Contact

ID: 2000-28 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.

ID: 2000-28
Principal Investigator: Chris Mattson

This invention provides a constant force apparatus having a cam with a non-planar surface; a compliant member with a free end, a fixed end, and an intermediate contact area there between, and includes:
1. A constant force method and apparatus of simple construction that produces a constant reaction force in response to an applied displacement.
2. An electrical contact that provides a substantially constant force.
3. A docking station for use with a lockable unit with a substantially constant contact force.

The free end of the compliant member slid­ably engages the non-planar surface of the cam and the compliant member continuously provides a substantially constant reaction force at the intermediate contact area regardless of magnitude or change of displacement of the intermediate contact area.

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.

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 have been 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 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.

For more information, contact Spencer Rogers (801-422-3676)

Links and Resources

  1. One Page Summary PDF
  2. Inventor Webpage - Larry Howell
  3. Inventor Webpage - Spencer Magleby
  4. Compliant Mechanisms Research Group Webpage