Electricity: measuring and testing – Magnetic – Displacement
Patent
1992-07-29
1994-07-05
Wieder, Kenneth A.
Electricity: measuring and testing
Magnetic
Displacement
32420712, 32420719, 307358, 307520, 307551, 328149, 328167, G01B 734, G01D 5245
Patent
active
053270789
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a position detecting apparatus which magnetically detects the position of, for example, a cylinder rod in a construction or other industrial machine, a piston of an automotive engine, a scaler of a magnetic scale, and so forth.
BACKGROUND ART
Hitherto, for the purpose of detecting the position of a cylinder rod of a construction or industrial machine, a position detecting apparatus has been often used in which a variation in the magnetic characteristics is created at a predetermined spacing along the cylinder rod or a detecting rod connected thereto, by attaching magnets to the rod or varying the magnetic permeability at the predetermined spacing, whereby the displacement or position of the rod is detected on the basis of the change in the magnetic characteristics. The change in the magnetic characteristics can be detected by various detecting means such as, for example, a detecting means using a Maxwell bridge circuit. The Maxwell bridge circuit has four sides, one of which is formed by a coil having a reference inductance L.sub.1, and a coil having an unknown value of inductance L.sub.3 is inserted in another side of the bridge. In operation, an adjustment is conducted to nullify the output voltage between the output terminals, thereby determining the inductance L.sub.3.
More specifically, a known position detecting apparatus incorporating a Maxwell bridge circuit has a detecting rod made of a magnetically permeable metal. The detecting rod has a plurality of magnetic permeability changing portions which are formed by locally increasing or decreasing magnetic permeability at a predetermined pitch along the rod. The detecting rod is received in the coil having the unknown inductance L.sub.3. When the detecting rod is axially moved to cause the magnetic permeability changing portion to pass through the coil, the self-inductance L.sub.3 of the coil is changed to cause a difference between the voltage across this coil and the voltage across the coil having the inductance L.sub.1. Consequently, an A.C. voltage produced by this phase difference is obtained from the output terminals, and the amount of displacement, i.e., the position, of the detecting rod is determined on the basis of the zero-cross point of the A.C. voltage.
The voltage between the output terminals of the Maxwell bridge circuit should be zero when there is no displacement of the detecting rod. Actually, however, a voltage appears between the output terminals even when there is no displacement of the detecting rod, due to variations or fluctuations of various factors such as the sensor sensitivity, the state of mounting of the detecting rod, the magnetic permeability of the detecting rod, and the characteristics of the circuit elements, as well as the temperature at which the detecting apparatus operates, with the result that the zero-cross point is deviated. Thus, the output voltage obtained from the Maxwell bridge circuit in response to an axial movement of the detecting rod is the sum of the A.C. voltage generated as a result of passage of the magnetic permeability changing portion through the coil and a D.C. voltage (offset voltage) which is attributable to the offset of the zero-cross point caused by fluctuations in the factors such as the sensor sensitivity and the characteristics of the circuit components. Therefore, the determination of the position or displacement of the detecting rod through the detection of the zero-cross point of the A.C. output voltage from a Maxwell bridge circuit essentially requires that the offset voltage component be removed from the output voltage of the Maxwell bridge circuit.
There are two methods for removing the offset voltage: a method called A.C. coupling method which utilizes a capacitor, and a method called voltage adjusting method which uses a trimmer resistor. The A.C. coupling method, which relies upon charging and discharging of a capacitor, suffers from various disadvantages such as the generation of a delay of a phase or delay of zero-cross of
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patent: 4749951 (1988-06-01), Tanaka
patent: 4992731 (1991-02-01), Lorenzen
patent: 5144321 (1992-09-01), Tenenbaum et al.
Kabushiki Kaisha Komatsu Speisakusho
Patidar Jay M.
Wieder Kenneth A.
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