Geometrical instruments – Distance measuring – Scale reading position sensor
Patent
1990-04-12
1991-11-05
Cuchlinski, Jr., William A.
Geometrical instruments
Distance measuring
Scale reading position sensor
33707, G01B 1102
Patent
active
050622145
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an absolute measurement scale system of the kind used in different measuring machines and measuring instruments for measuring the extent of displacement between a stationary and a movable object to a high degree of accuracy.
Such measuring systems often incorporate incremental scale systems, e.g. scale systems in which a linear scale is provided with a large number of graduation lines and with which a measuring head counts the number of graduation lines passed by the head, therewith providing information relating to the distance moved by the measuring head relative to the scale.
Incremental scale systems are encumbered with several drawbacks. One drawback is that all positional information is lost when the input measuring voltage to the system is switched off. The system must then be reset to a mechanical zero reference point and set to zero. Another drawback is that the arithmetical system is prone to interference from ambient electrical interference sources, therewith either losing pulses or gaining additional pulses. This erroneous state will then prevail until the system is reset to its mechanical zero reference. A further drawback is the likelihood of the mechanical zero reference being changed, e.g., as a result of wear, the presence of dust or dirt, and by thermal changes in the system.
It is desirable that measuring systems of the kind intended here are absolute measuring systems, i.e. with which measurements are made in conformity with a graduated straight-edge or measuring tape and with which the measurement (value) is obtained directly without counting intermediate graduations.
Such absolute measuring systems are previously known to the art from, e.g., Swedish Patent Specification No. 411 392. The measuring accuracy or resolution of measuring systems of this kind is often very high, i.e. in the order of micrometers, while such systems are capable of measuring long total linear dimensions, e.g. linear dimensions of several meters.
The Swedish Patent Specification 411 392 teaches a capacitive measuring system. Although the system affords a very high resolution, it can only effect absolute measuring operations over a limited measuring length or distance, namely a linear distance of about 10-20 mm.
Swedish Patent Specification 426 989 teaches a measuring system in which two different part-systems co-act with one another, namely a system in which one of said part-systems is constructed to measure solely within a minor interval and the other part-system is constructed for absolute measurements with regard to the number of such intervals entailed.
The measuring system taught by this latter patent specification, however, is encumbered with several drawbacks. For example, both part-systems operate electrically, i.e. the measuring signal is composed of signals that are transmitted electrically between a scale and a slide. As beforementioned, the systems are therefor prone to electrical interference from ambient sources, such as to obtain an erroneous measurement value in both one and the other of said systems. It is particularly disadvantageous when the absolute measuring part of the measuring system is subjected to ambient electrical disturbances such as to indicate a wrong value. According to Swedish Patent Specification 426 989, the absolute measuring system utilizes a capacitive measuring method, in accordance with which sensor plates are arranged on a measuring slide and electrodes on the scale. Experiences within this particular technical field, however, have shown that such sensor plates must have a particular size, in order to provide values of the coupling capacitance between scale and slide which can be used in practice. This necessitates a wide or broad scale, which consequently requires a commensurate amount of space. A further drawback with a capacitive, absolute measuring scale is that despite having a significant size, e.g. a width of 50 mm, the slide must be placed very close to the scale, e.g. at a distance of only a fraction of a millimeter therefrom. Thi
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Gustafsson Carl-Erik
Pettersson Bo
C. E. Johansson AB
Cuchlinski Jr. William A.
Fulton C. W.
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