Shaft encoder

Details Shaft encoder Shaft encoder

1998-11-09

2001-03-13

Phan, Trong (Department: 2818)

Incremental printing of symbolic information

Electric marking apparatus or processes

Electrostatic

C347S152000

Reexamination Certificate

active

06201555

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of shaft encoders and more specifically to an improved shaft encoder which provides a pulsed output signal upon rotation of a shaft, the improvement relating to the low cost, reliability and simplicity of the encoder of the invention.
2. Prior Art
Shaft encoders provide electrical signals indicative of a shaft's angular position, speed of rotation and direction. Modern shaft encoders typically provide an output in the form of a series of electrical pulses the rate of which is proportional to the rotational rate of the shaft. The angular resolution of the shaft encoder is dependent upon the number of pulses generated per revolution of the shaft. The larger the number of pulses per revolution, the more complex and sophisticated is the encoder and therefore the more costly. Moreover, the complexity of the encoder leads to a reduction in reliability because there are more components that could become faulty or inoperative. However, there are applications for shaft encoders where less precise resolution is needed such as within 5 or 10 degrees as opposed to tenths of a degree. Unfortunately there are no prior art shaft encoders which fully exploit a reduction in resolution requirements. As a result, even low resolution shaft encoders are relatively expensive and unreliable and are still sufficiently complex to make size reduction a still formidable challenge.
There is therefore an ongoing need for a low cost, highly reliable shaft encoder capable of modest angular resolution and which can be made in relatively small package sizes.
SUMMARY OF THE INVENTION
The present invention comprises a shaft encoder providing a pulsed output signal with rotation of a shaft, the number of pulses being dependent upon a shaft-mounted gear and its number of teeth. In a preferred embodiment, a piezo film element is mounted in intimate contact with a spring element, the latter being positioned to engage the teeth of the gear. When the shaft rotates, the gear rotates as well. The gear teeth cause the spring element to be displaced and then return to a nominal position for each tooth. Accordingly, given a sufficient number of regularly spaced teeth around the gear, a relatively high resolution pulsed signal is generated as the shaft rotates. In the illustrated embodiment, there are 60 teeth on the gear. Thus, the pulses occur at 6 degree intervals as the shaft rotates. The piezo film element generates electrical pulses due to the mechanical displacement of the spring element to which the film is in contact. Thus, no electrical power source is required to generate the pulses. Moreover, because of the simplicity of the invention, size scaling is readily available. Thus, the shaft encoder of the invention may be readily fabricated in extremely small configurations depending upon the desired application and the availability of machining or other techniques for producing small, accurately placed teeth. Furthermore, in an alternative embodiment, one tooth of the gear may be made slightly different from the others (i.e., higher) to provide a different pulse characteristic (i.e, greater amplitude) to indicate a reference position for the shaft. Moreover, the polarity of the generated pulse will indicate the direction of shaft rotation; i.e., positive for clockwise and negative for counterclockwise. The resulting shaft encoder is inexpensive, reliable and provides good angular resolution for its cost.
OBJECTS OF THE INVENTION
It is therefore a principal object of the present invention to provide a shaft encoder wherein a series of electrical pulses are generated in response to rotation of a shaft.
It is another object of the invention to provide a mechanical rotation sensor that by generating electrical pulses, indicates the position, speed and even direction of a shaft.
It is yet another object of the invention to provide a shaft encoder which needs no source of electrical power to generate electrical pulses in response to rotation of a shaft.
It is still another object of the invention to provide a shaft encoder which may be made smaller and more reliable than conventional shaft encoders while still providing acceptable angular resolution.
It is still another object of the present invention to provide a shaft encoder wherein a gear having a plurality of regularly spaced teeth engages a spring member to which a piezo film element is in contact whereby rotation of the shaft causes periodic displacement and recovery of the spring member and the attached film element.


REFERENCES:
patent: 4962393 (1990-10-01), Fulton et al.

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