Geometrical instruments – Distance measuring – Scale reading position sensor
Reexamination Certificate
2000-06-09
2001-12-11
Fulton, Christopher W. (Department: 2859)
Geometrical instruments
Distance measuring
Scale reading position sensor
C033S710000
Reexamination Certificate
active
06327791
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to an apparatus for measuring the position of a moving member relative to a stationary member and, in particular, a scale on a rod of a linear motion cylinder (having a rod, housing, gland and driving element such as a piston or rotor) and a detector on the cylinder housing for detecting the scale to indicate the position of the rod relative to the housing, thus detecting the length of the cylinder.
2. Description of the Prior Art
Computerized motion control of actuated cylinders requires rod position and velocity measurements. These measurements must be accurate, repeatable and have sufficient resolution for the particular application. Many applications also require the measurement to be cost effective, easily redundant, immune to mechanical noise and insensitive to debris. The problems with measuring the position of a rod of a linear motion cylinder are particularly acute. Current hydraulic cylinder measurement practice uses either string encoders with multi-turn potentiometers, linear potentiometers or magnetostrictive devices. All of these approaches can be problematic for commodity applications. Magneto-strictive devices are expensive and require either hollow rods or extensive external mountings. On the other hand, string encoders are less expensive but are frail, are subject to animal damage, and are degraded by mechanical noise. Linear potentiometers are length limited and subject to mechanical noise.
SUMMARY OF THE INVENTION
The apparatus of the invention has several advantages over the prior art including instant position detection, increased resolution, inherent redundancy, low maintenance, and vibration tolerance. The apparatus of the invention addresses many of the hydraulic cylinder position measurement problems-to provide an excellent solution in that area as well as in any area where position detection is required. Furthermore, according to the invention, the general approach can be applied to other position measuring problems where one element moves along another.
The invention measures the position of the cylinder rod by observing a scale having sequences of coded marks on the cylinder rod. The cylinder rod presents a rigid base for scales and is thus resistant to mechanical vibration. The scales are inexpensively applied to the cylinder rod through such techniques as photo engraving or laser marking. Preferably, the detector for detecting the sequences of coded marks comprises an array of detectors such as an electronic camera. The detector preferably takes multiple readings of the sequence so that the resultant information is resistant to sporadic errors. From a manufacturing standpoint, the detectors are structured to be replicated on a consistent basis to enhance system reliability.
Preferably the invention employs a stationary detector on the cylinder housing and a scale (preferably illuminated) having a code marked on the cylinder rod which code does not repeat itself within a desired range. For example, the scale may use a chain code to provide an absolute position while viewing a minimum length of the scale. The detector uses the two-dimensional array of detector elements to detect the image of the scale. A lens or other focusing device may be used to transfer the image to the detector. The multiple detector elements provide multiple signals which, when evaluated, provide redundancy, error tolerance and improved resolution.
In one form, the invention comprises a device for sensing the position and velocity of a movable member which moves along an axis relative to a stationary member. A scale encompassing a nonrepeating code is on the movable member aligned substantially along the axis of movement. A detector is fixed to the stationary member and positioned to sense a portion of the scale. The detector generates a plurality of signals, each signal indicative of the portion of the scale sensed by the detector. A controller determines the position of the movable member relative to the stationary member as a function of the signals.
Other objects and features will be in part apparent and in part pointed out hereinafter.
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PCT/US00/16370, International Search Report Sep. 19, 2000.
Bunch W. Robert
Engvall David P.
Norcross Richard J.
Fulton Christopher W.
Senniger Powers Leavitt & Roedel
The Government of the United States as represented by the Secret
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