Position encoder using saturable reactor interacting with magnet

Details Position encoder using saturable reactor interacting with magnet Position encoder using saturable reactor interacting with magnet

1998-05-06

2000-09-12

Strecker, Gerard

Electricity: measuring and testing

Magnetic

Displacement

32420724, 32420725, 34087033, 341 15, G01D 520, G01B 730

Patent

active

061182710

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to position encoders generally. The invention has particular but not exclusive relevance to non-contact rotary and linear position encoders. Some embodiments of the invention are suitable for use in relatively small systems which operate at relatively high temperatures and in which there may be magnetic and electromagnetic interference. Other embodiments are suitable for use in systems having a large measurement range which require a sensor head which is relatively insensitive to vibrations, mechanical misalignments and dirt, grease and the like.


DISCUSSION OF THE PRIOR ART

Position encoders are well known in the art and typically comprise a movable member, whose position is related to the machine about which position or motion information is desired, and a stationary member which is coupled to the moving member either optically, capacitively or magnetically. The stationary member includes a number of detectors which provide electrical output signals which can be processed to provide an indication of the position, direction, speed and/or acceleration of the movable member and hence those for the related machine.
One example of this kind of prior art encoder is one which uses a number of Hall effect detectors to detect magnetic features on the movable member, from which positional information can be derived.
As those skilled in the art know, a Hall effect detector detects the magnetic field at a point and consequently, in order to determine position unambiguously between each magnetic feature, at least two Hall effect detectors must be provided. Further, each Hall effect detector is likely to have a different non-zero DC offset (i.e. the output from the detector when there is no external magnetic field) resulting in a positional error in the output signals. Additionally, since several (typically 2 or more) Hall effect detectors are used to determine the positional information, this type of encoder is relatively expensive to manufacture.
Another disadvantage of the Hall effect encoder is that when it is used in a system where the spacing between the magnetic features on the rotating member is small (i.e. about 2 mm), the Hall effect detector s sensitivity to the magnetic features on the movable member is severely reduced due to its packaging. Typically, the packaging is plastic or ceramic and is used to avoid piezoelectric effects, to prevent contamination and to facilitate the provision of output leads. The inventors have established, by experimentation, that the magnetic field strength at the active surface of the Hall effect detector is attenuated by a factor of about 500 when the packaging has a thickness of 1 mm and when the magnetic features comprise a continuous series of alternating north and south magnetic poles having a pitch of 1 mm. The practical limit for the packaging thickness is approximately 0.5 mm, and even at this thickness the magnetic field strength is attenuated by a factor of about 25.
Yet another problem with Hall effect encoders is that they are sensitive to external electromagnetic interference. In particular, the Hall effect detector is sensitive to magnetic fields from nearby magnets and to the earth's magnetic field.
Another type of magnetic detector currently being used is the magnetoresistive detector, which has an active element whose resistance changes in dependence upon the external magnetic field. However, this type of detector suffers from poor temperature performance (practical magnetoresistive technology being limited to operation at or below about 80.degree. C.) and either need to be precisely installed as part of the encoder assembly or require magnetic biassing and bridge circuitry (for detecting the change in resistance of the element) which requires accurate sensor matching.
A third type of magnetic detector which is currently being used in position encoders is the reluctance detector. This detector has a number of sense windings which are arranged to detect the changing magnetic field which occurs in their vi

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