Magnetic resonance sensor

Details Magnetic resonance sensor Magnetic resonance sensor

1999-06-28

2001-10-16

Snow, Walter E. (Department: 2862)

Electricity: measuring and testing

Magnetic

Displacement

C324S207130

Reexamination Certificate

active

06304075

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a magnetic field sensitive sensor whose output signal with speed-independent amplitude yields information about the distance, rotational velocity (speed) and direction of rotation of a magnetic field simultaneously.
b) Description of the Related Art
Magnetic sensors with speed-independent output voltage in use heretofore rely on the large Barkhausen effect (English-language abbreviation: LBE). The LBE is a pulse-like complete magnetic reversal of special magnetic materials which have a privileged or preferred orientation of magnetic domains due to their composition and method of production. Since two stable states exist for the preferred orientation of the magnetic domains, structural component parts made from LBE materials are also known as bistable magnetic elements (hereinafter abbreviated as BME).
The magnetic reversal takes place within a time frame of roughly 50 &mgr;s resulting in a limit frequency for the LBE of f
Gr
≈20 kHz. Above f
Gr
, sensors in use heretofore do not generate a usable signal.
Since the magnetic reversal in LBE materials always takes place in a pulse-like manner when an external magnetic field has a frequency f less than f
Gr
, this effect is suitable for use in magnetic sensors. Previously used sensors with LBE materials can be divided into two classes: sensors without a magnetic exciting field and sensors with a magnetic exciting field.
Sensors without a magnetic exciting field are also known as pulse wire sensors (DE 3729949, DE 4107847, DE 3824075, DE 3406871). Devices for measuring velocity (DE 9014753, DE 3112709) can be realized by coupling pulse wire sensors with evaluating electronics. Patents have been applied for in a large number of sensors and devices in which LBE is combined with other physical operating principles (DE 3817704, DE 3008581, DE 3008582, DE 3046804, DE 3008526, DE 3008527, DE 3008560, DE 3008561, DE 3008562, DE 3008581, DE 3008582, DE 3008583, DE 3225499, DE 3225500, DE 342419, DE 3427582, DE 3637320, DE 3538514). However, all of the patented solutions have in common that they rely on the complete course of the LBE, i.e., complete magnetic reversal, and are capable of use only up to the limit frequency f
Gr
of the LBE.
In sensors with a magnetic exciting field, a constant magnetic reversal of the BME is carried out by the magnetic field of an exciting coil with exciting frequency f
Err
, wherein the condition for the occurrence of the LBE f
Err
<f
Gr
must be met again for the exciting frequency. Sensors with a magnetic exciting field contain a sensor coil. The voltage U
s
induced in this sensor coil has voltage peaks due to the magnetic reversal of the BME in every half-wave. Depending on the orientation of the external magnetic field, the voltage peaks in every half-wave of the sensor signal can either be strengthened or weakened by an external magnetic field. The working point of the sensors can be adjusted by means of superposing a constant magnetic field on a working point coil (DE 3241018, DE 3718857, DE 4037052, DE 421358).
The magnetic resonance sensor differs in its function from other magnetic field sensitive sensors which likewise employ an oscillatory circuit (DE 8227446, DE 8316996, DE 8517733, DE 9010779, DE 9412765) through the use of a BME as a core of the sensor coil and the possibility of measuring speed, rotating direction and distance of the magnetic field from sensor simultaneously.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to develop a sensor whose working frequency distinctly exceeds the limit frequency f
Gr
established in the prior art for magnetic sensors with a BME core and whose output signal simultaneously contains information about the distance, speed and rotating direction of the magnetic field. Since the limit frequency f
Gr
of the LBE is determined by objective physical processes, the sensor function must use a different physical principle.
This object is met, according to the invention, by the magnetic resonance sensor and a process for detecting the position and change in position of objects interacting with magnetic fields (FIG.
1
).
The resonator system (sensor) includes the following: a bistable magnetic core (
1
); at least one exciting coil (
3
); means serving to generate a magnetic field comprising either a permanent magnet and/or a coil; a high-frequency oscillatory circuit constructed from at least one sensor coil (
5
) and at least one capacitor (
6
).
Further, the following are required for operation of the sensor: a high-frequency generator (
2
); a DC voltage source (
8
); evaluating electronics (
7
).
The high-frequency generator (
2
) feeds an advantageously sinusoidal AC voltage of constant amplitude U
Err
and constant resonant frequency f
Res
of the high-frequency oscillatory circuit into the exciting coil (
3
). As long as an external magnetic field at the location of the sensor has not reached a threshold field strength H
Schw
characteristic for the sensor, the exciting coil induces a periodic voltage of constant amplitude and identical frequency by means of the bistable magnet core (
1
) in the sensor coil. A periodic voltage of the resonant frequency f
Res
with constant amplitude U
s
is taken off as sensor output voltage by way of the high-frequency oscillatory circuit. The amplitude of the sensor output voltage is determined by the amplitude U
Err
of the high-frequency generator (
2
) and the working point of the sensor. This working point can be established by at least one of the following:
a) permanent magnet,
b) working point coil (
4
) with applied DC voltage U
DC
of the DC voltage source (
8
),
c) exciting coil (
3
) with applied DC voltage U
DC
of the DC voltage source (
8
).
When an external magnetic field reaches H
Schw
, the working point of the sensor shifts to the steeper area of the induction-field strength characteristic line (B=f(H) characteristic line) of the bistable magnetic core without causing magnetic saturation or magnetic reversal of the core. A voltage is induced in the sensor coil (
5
) which leads to a change in the amplitude of the sensor output voltage U
s
. If the magnetic field falls below the value H
Schw
again, the sensor returns to the established working point. The sensor accordingly emits a pulsed signal for the period during which the value exceeds or falls below H
Schw
, this pulsed signal being evaluated by the evaluating electronics (
7
). The peak amplitude of the pulse U
p
is not dependent on the change over time in the magnetic field and depends only on the maximum magnetic field strength H
max
at the location of the sensor; the pulse width is proportional to the time period during which the value exceeds or falls below H
Schw
.
The sensor function is based on the following physical processes: The magnetic domains of the LBE materials are also exposed to the force of the exciting magnetic field in the event of high-frequency external magnetic fields where f
Err
>f
Gr
. However, since the period of the high-frequency exciting field T
Err
<(1/f
Gr
), the domains cannot completely change their orientation, but rather begin to oscillate at frequency f
Err
because of their preferred orientation. This collective effect is also still observable when f
Err
>1 MHz. In magnetic materials without an ordered position of the domains, the oscillation behavior is appreciably poorer because the domains conflict. The collective oscillation of the magnetic domains in LBE materials leads to an oscillation of the magnetic flux density B at frequency f
Err
. Based on the law of inductance, a periodic voltage is induced in a sensor coil at frequencies n*f
Err
. If an additional external magnetic field is superposed on the exciting magnetic field at the location of the sensor, the sensor output voltage U
s
can be increased or reduced, depending on the orientation of the additional magnetic field, due to the resulting displacement of the working point of the sensor in the B=f(H) charact

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