Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2001-09-10
2003-04-08
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207240, C338S03200R
Reexamination Certificate
active
06545463
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of International application number PCT/DE00/00788, filed Mar. 10, 2000, which in turn claims priority to German patent application number 199 10 636.3, filed Mar. 10, 1999.
FIELD OF INVENTION
The invention relates to a length measurement system including one or more magnetic measuring rods and one or more magnetic field sensors.
The invention relates in particular to a magnetic length measurement system, such as is used for example for automatically determining positions, lengths and distances more particularly under rough surrounding conditions in machine engineering or in the automobile industry.
BACKGROUND OF THE INVENTION
Known magnetic length measurement systems include a magnet arrangement and one or more magnetic field sensors which indicate a component of the magnetic field strength or the magnetic field direction. In the simplest case the magnet arrangement is formed by a simple rod magnet and a magnetic field sensor is formed by a magnetoresistive sensor whose output voltage is determined by the field direction. This is described in the article “The magnetoresistive sensor” by A. Petersen in Electronic Components and Application 8 (1988) NO 4, 222-239. The sensor is guided at a certain distance from the magnet parallel to its north-south extension which is also the direction of measurement. The sensor plane extends in the direction of measurement and radially relative to the axis of the magnet.
A rod magnet with magnetization set in the longitudinal direction and whose length is not significantly greater than twice the width and thickness produces a magnetic field whose angle to the parallel of the longitudinal direction increases roughly linearly in the direction of measurement with increasing distance from the magnet centre. The magnetization in the magnetoresistive sensor lies in the plane of stratification and is set with a sufficiently high,field strength in the direction of the field. The output signal of the magnetoresistive sensor then changes proportional to the sine of the double angle. Since the sin(x) for small angles deviates only slightly from x a length range is produced in which the output voltage of the sensor is proportional to the position. In terms of accuracy this simplest arrangement has several drawbacks which are partly a result of the magnetic field distribution of the rod magnet and partly as a result of the properties of the magnetoresistive sensor.
The damaging marked temperature dependence of the output signal on magnetoresistive sensors and the restriction to small angular deviations from the parallel to the north-south direction of the magnet and thus to measuring lengths which are substantially smaller than the magnetic length has already been overcome by the arrangement indicated in patent DE 195 21 617. Here two magnetoresistive sensor bridges integrated onto one chip are used which supply both an output signal proportional to the sine and an output signal proportional to the co-sine of the double angle of the field direction. Through the formation of quotients the temperature-dependent amplitude no longer applies, and from the arcuate rod thus obtained it is possible to determine the angle without approximation over the entire length of the magnet. Thus measurement errors result here still mainly from the fact that there are deviations from the linear connection between angle and position. These deviations are however quite considerable for magnets where the lengths are long compared with the width and thickness. Furthermore the field strength over the middle part of a long magnet is only very slight so that the alignment of the magnetization in the resistance strip of the magnetoresistive sensors is no longer provided.
Therefore, for measuring large stretches measuring rods are used which consist of regions of uniform length magnetized alternately in the positive and negative longitudinal direction, as also indicated in DE 195 21 617. Determining the length is then carried out by counting the number of magnetized regions already passed from a starting position and adding the proportion of one region which results from the angular determination. However, information on the absolute position is no longer possible. After a breakdown of the measurement system it is necessary to return to the starting position in order to repeat the counting process.
In order to determine the absolute position it is also possible to undertake coding of the measuring rod wherein, however, uniformly magnetized regions of different length have to be used. Use of these regions results in the drawback already mentioned above of the small field strength close to the middle part of the region when several regions with the same code value lie next to each other. This problem can however be overcome by the arrangement described in EP 0 482 341 where twin tracks are used each with opposite magnetization whose direction always stands transversely to the direction of measurement. Unfortunately however in the said patent there is no arrangement given for producing the required twin track measuring rods, and the known magnetizing processes do not provide satisfactory results for the twin track.
Magnet arrangements comprised of several parts for determining position are proposed in the published specification DE 31 06 613. They are provided in order to achieve over quite specific short path lengths high local resolutions in position calculation. The drawback with all the different arrangements given is the extremely high dependence of the output signals on the distance between the magnet and sensor, which makes it necessary to provide very expensive guides for high-resolution position measurement and incurs high costs for adjustment and calibration.
DE 197 29 312 A1 describes an absolute magnetic length measurement system which contains a code track on a measuring rod and several sensors for scanning the magnetic fields which are present over the code track. One binary state of a code element in the code track is represented through a uniform magnetization and the other binary state is represented through a change in the direction of magnetization. This code track which is defined by binary states of one code element is scanned through several magnetoresistive sensors from whose output signals it is possible to determine the positions of the sensors relative to the code track. However several sensors are always required for this
In the Journal “Elektrotechnik” 370, Volume 4 of Apr. 4, 1991 there is a review on the use of different materials for manufacturing permanent magnets of different magnetization.
SUMMARY OF THE INVENTION
The invention is concerned with the problem of providing a length measurement system having a magnetic measuring rod which is characterised by a simplified structure.
According to one aspect of the invention, the length measurement system includes at least one magnetic measuring rod in which a significant component of magnetization or the magnetization as a whole lies (substantially) in a plane which extends perpendicular to the direction of measurement, and of one or more magnetic field sensors wherein the angle of the component of the magnetization or of the magnetization as a whole changes relative to a freely selectable preferred direction (reference direction) in this plane along the direction of measurement; i.e. the direction of the component of magnetization is rotated along the direction of measurement, namely about an axis which runs along the direction of measurement. To each longitudinal position of the magnetic field sensor is thereby assigned a direction of the component of the magnetization within the measuring rod at this longitudinal position from which the longitudinal position of the magnetic field sensor can be determined.
In one embodiment, the entire cross-sectional surface of the measuring rod or of the plurality of measuring rods is uniformly magnetized. It points for example at the start of the measurement path perpendicularly upwards. As one adva
Bischoff Heike
Dettmann Fritz
Dohles Hilmar
Loreit Uwe
Brose Fahrzeugteile GmbH & Co. KG Coburg
Christie Parker & Hale LLP
Lefkowitz Edward
Zaveri Subhash
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