Measuring and testing – Testing of apparatus
Reexamination Certificate
2000-01-27
2001-10-23
Noland, Thomas P. (Department: 2856)
Measuring and testing
Testing of apparatus
C250S231180, C324S262000, C356S615000
Reexamination Certificate
active
06305234
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an absolute-type encoder for generating position information by being mounted on a drive shaft of a rotationally driven element such as a servomotor or the like, or on a linearly driven element such as a linear actuator or the like.
Encoders serve as devices for detecting rotational and linear positions of machines such as servomotors, linear actuators, tachometers, and the like, to allow accurate positioning of such machines, and determination of such quantities as velocity and acceleration. Many different kinds of encoders are available for such purposes.
Incremental-type encoders are typically used for positioning servomechanisms due to their simplicity and low cost. However, incremental-type encoders require initializing whenever power is removed from the device. Initializing the encoder requires sensing a “home” position and then using the home position as a reference for subsequent position measurements. If the initializing process includes an error, then all subsequent measurements will include the error.
Absolute encoders also require initializing to a home or reference position upon being powered up for the first time. However, in contrast to incremental encoders, absolute encoders do not need to be re-initialized every time the power is turned on. Thus, after a one-time adjustment, an absolute encoder will provide position information immediately upon start up, without the need for re-initialization.
Typical absolute encoders are of the so-called discrete or digital type wherein position information is encoded on a rotary disk or a linear transducer element as binary ones and zeroes, or as on and off states. The binary information is typically encoded as a series of concentric rings or bands on a rotary disk, or as a series of adjacent bands or strips on a linear encoder. Typically, one ring or strip, corresponding to the most significant binary digit, is divided into two equal parts, with one part representing a one, and the other part representing a zero. Successive adjacent rings or strips are divided into twice as many equal alternating ones and zeroes as the previous rings or strips. The final ring or strip, having the greatest number of equal-sized ones and zeroes, corresponds to the least significant binary digit. Each distinct binary number can be associated with a unique rotational or linear position.
The ones and zeroes may be represented as alternately light and dark optically reflective domains, or as magnetic fields of alternating polarity, or as discrete variations in height or width wherein one height or width measurement corresponds to a one, and a second different height or width measurement corresponds to a zero.
Because of the two-valued nature of binary or digital encoders, intermediate values are not permitted as they give rise to ambiguous or indeterminate values, and hence, loss of precision in rendering position measurements. Thus, typical digital or binary encoders require sharp transitions between alternating regions, and relatively constant values within regions.
Digital or binary encoders are inherently limited in their fundamental accuracy by the number of rings or strips they possess, as well as by the size of the smallest regions in the least significant ring or strip. Within one of such regions, a rotary disk or linear actuator may be positioned anywhere, yet yield the same digital value, so that the potential measurement error is approximately equal to the size of the smallest binary region within the least significant ring or strip. Also, in order to achieve ever finer resolution, additional rings or strips must be added, thereby consuming more space, requiring additional sensors for each ring or strip, and adding to encoder complexity.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide an improved non-contact absolute encoder which is capable of reliable, accurate, and continuous position detection of a rotationally driven element or a linearly driven element.
Another object of the invention is to provide an improved non-contact absolute encoder which is simple in design and inexpensive to manufacture.
Yet another object of the invention is to provide an improved non-contact absolute encoder which provides high resolution position measurement information as a continuous function of workpiece position.
In accordance with a broad aspect of the invention, a non-contact absolute-type encoder for determining the position of a movable workpiece comprises: a movable transducer element adapted for coupling to said workpiece, said transducer element having a predetermined characteristic profile that varies in a substantially continuous way as a function of said transducer element position; a fixed sensor positioned proximate said transducer element to sense said characteristic profile to provide a substantially continuous output signal representative of the position of said transducer element; and means for positioning said sensor relative to said characteristic profile.
In a first embodiment of the invention, the workpiece includes a rotationally driven element; the transducer element is adapted to be coupled to the rotationally driven workpiece element with the transducer element further defining an axis of rotation; the characteristic profile of the transducer element varies substantially continuously as a function of the rotational position of the transducer element; and the sensor produces a substantially continuous output signal representative of the rotational position.
In a second embodiment of the invention, the workpiece includes a linearly driven element; the transducer element is adapted to be coupled to the linearly driven workpiece element with the transducer element further defining an axis of translation; the characteristic profile varies substantially continuously as a function of the linear position of the transducer element; and the sensor produces a substantially continuous output signal representative of the linear position.
The characteristic profile may be magnetic field strength, optical reflectivity, magnetic reluctance, or some other characteristic that is measurable by non-contact means and may be representative of transducer element position.
The invention will be further described and illustrated in the following description, accompanying drawings, and claims.
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Dowis William F.
Thies Edward L.
Biebel & French
Noland Thomas P.
Thies Edward L.
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