Currently available instrument couplings run the risk of unwanted rotational errors being introduced by shaft misalignment. When extreme precision is needed in connecting rotary encoders, imaging drums and lead screws to motors, these slight rotational errors can result in serious performance problems. In the connection between the nut and carriage on a lead screw driven carriage arrangement, any angular misalignment between the lead screw and carriage rails can produce cyclic errors in carriage position, even if the lead screw is perfectly machined.
It would appear that the aforementioned difficulties could be alleviated by utilizing couplings of appropriate configuration. Constant velocity joints are routinely used in front wheel drive automobiles to minimize the transmission of speed pulsations when the wheels are turned, but to the inventors' knowledge, similar joints have not been employed in electromechanical instrumentation or as non-rotational couplings between mechanical components.
Exemplary of constant velocity joints of particular interest with respect to drives such as automobile drives are U.S. Pat. Nos. 4,033,144; 1,522,351; 1,524,161; 1,665,280; and 1,916,442. While the constant velocity joints shown in these patents are useful for relatively large devices such as motor vehicles, they are not disclosed as being of particular interest with respect to instrumentation where only very slight rotational errors may be introduced due to shaft misalignment.
Patents such as U.S. Pat. Nos. 4,623,322; 4,572,682; and 4,547,177 are of interest in that they utilize an intervening central element to effect coupling in either a constant velocity joint or universal joint. Patents such as U.S. Pat. Nos. 4,609,302; 4,033,145; 3,903,711; 3,392,548; and 2,552,682 disclose effecting coupling with radially oriented pins. There are advantages to both of these approaches. However, neither of these groups of patents combine the advantages of both approaches in a single device to provide a constant velocity joint which is especially suitable for instrumentation applications and the like.
The prior art also includes U.S. Pat. Nos. 4,620 514; 4,392,837; 2,380,602; and 459,325 wherein radially off-set protrusions are connected to one another by structural elements in tension or compression. The devices disclosed in these patents are not identified as being of specific interest with respect to constant velocity couplings useful in instrumentation and tend to have somewhat complex configurations not necessarily suitable for relatively inexpensive, small scale applications.
Couplings between members which are not designed to rotate with respect to one another, such as couplings between the carriage and nut in a lead screw driven arrangement can also introduce slight errors due to misalignment and flexing. This misalignment may result from exceeding tolerances during manufacture and assembly or may be introduced by environmental factors such as thermal expansion, vibrations or humidity. Accordingly, there is also a need for a coupling having the characteristics of a constant velocity joint for such rigid connections.