1. Field of the Invention
The present invention relates to a sensing device for detecting the change in a magnetic field caused by the motion of a moving member of magnetic material, and more particularly, to a sensing device which is particularly suitable for detecting information about the rotation of for example an internal combustion engine.
2. Description of the Related Art
Magnetoresistance devices generally refer to those devices which change in resistance in response to the direction of a magnetic field applied to a thin ferromagnetic film with respect to the direction of a current flowing through the thin ferromagnetic film.
Magnetoresistance devices have minimum resistance when a magnetic field is applied in a direction at a right angle to the direction of current. On the other hand, when the angle between the direction of the current and the direction of the applied magnetic field is 0, that is when a magnetic field is applied in a direction the same as or opposite to the direction of current, the resistance has a maximum value. The change in the resistance is generally called the magnetoresistance effect, and the magnitude of the change in the resistance is referred to as the magnetoresistance variation ratio. A typical value of magnetoresistance variation ratio is 2 to 3% for Ni--Fe and 5 to 6% for Ni--Co.
FIGS. 32(a) and 32(b) illustrate the construction of a conventional sensing device, wherein its side view and perspective view are shown in FIG. 32(a) and FIG. 32(b), respectively.
The sensing device shown in FIGS. 32(a) and 32(b) includes: a rotating shaft 1; a rotary member of magnetic material 2 having at least one protruding or recessed portion wherein the rotary member of magnetic material 2 is adapted to rotate in synchronization with the rotation of the rotating shaft 1; a magnetoresistance device 3 disposed at a location a predetermined distance apart from the rotary member of magnetic material 2; and a magnet 4 for applying a magnetic field to the magnetoresistance device 3. In the above construction, the magnetoresistance device 3 includes a magnetoresistance pattern 3a and a thin film surface (magnetic field sensing plane) 3b.
If the rotary member of magnetic material 2 rotates, the magnetic field applied to the magnetic field sensing plane 3b of the magnetoresistance device 3 changes in response to the rotation of the rotary member of magnetic material 2, and, as a result, the resistance of the magnetoresistance pattern 3a changes correspondingly.
FIG. 33 is a block diagram illustrating the construction of the sensing device using the magnetoresistance devices described above.
The sensing device includes: a Wheatstone bridge circuit 11 including magnetoresistance devices disposed a predetermined distance apart from the rotary member of magnetic material 2 so that a magnetic field is applied from a magnet 4 to the magnetoresistance devices; a differential amplifier 12 for amplifying the output signal of the Wheatstone bridge circuit 11; a comparator 13 for comparing the output of the differential amplifier 12 with reference values V.sub.T1,V.sub.T2 and outputting a "0" signal or a "1" signal depending on the comparison result; a holding circuit 30 for holding the output of the comparator 13; a waveform shaping circuit 14 for shaping the waveform of the output of the holding circuit 30 and supplying a "0" or "1" signal having sharp rising and falling edges to the output terminal 15.
The operation will be described below with reference to FIGS. 34(a)-34(c).
If the rotary member of magnetic material 2 rotates, the magnetic field applied to each of the magnetoresistance devices changes in response to the passage of the protruding and recessed portions of the rotary member of magnetic material 2 as shown in FIG. 34(a). As a result, the above change in the magnetic field is detected by the magnetoresistance devices, and the midpoint voltages of the Wheatstone bridge circuit 11 also change in a similar fashion.
The difference between the mid-point voltages is amplified by the differential amplifier 12. Thus, as shown in FIG. 34b, the differential amplifier 12 outputs a signal corresponding to the passage of the protruding and recessed portions of the rotary member of magnetic material 2 shown in FIG. 34a.
The comparator 13 compares the output signal of the differential amplifier 12 with the reference values V.sub.T1,V.sub.T2 and outputs a "0" or "1" signal in response to the comparison result. The output signal of the comparator 13 is shaped by the waveform shaping circuit 14 so that a "0" or "1" output signal having sharp rising and falling edges is provided to the output terminal 15 as shown in FIG. 34(c).
However, the conventional sensing device having the above construction has the following problems.
That is, in the conventional sensing device, as shown in FIG. 35, no hysteresis exists in the characteristic of resistance versus applied magnetic field. As a result, in the operation of detecting the protruding and recessed portions of the rotating member of magnetic material, the output signal varies at each edge as shown in FIGS. 34(a)-34(c). However, as can be seen from FIGS. 34(a)-34(c), the signal level for protruding portions is equal to that for recessed portions. Therefore, the edges at the protruding and recessed portions must be detected and held by a holding circuit. Furthermore, since there is no difference in the output signal between the recessed and the protruding portions, it is impossible to obtain a signal precisely corresponding to the protruding and recessed portions of the rotating member of magnetic material for an instant after the electric power to the sensing device is turned on (the ability to start precise operation immediately after the power is turned on will be referred to as "instantaneous starting capability").
As described above, the problem of the conventional sensing device is that it is impossible to obtain a signal precisely corresponding to the protruding and recessed portions of the rotating member of magnetic material. Another problem is that it is impossible to start a correct operation immediately after the electric power is turned on.
It is a general object of the present invention to solve the above problems. More specifically, it is an object of the present invention to provide a sensing device capable of outputting a correct signal precisely corresponding to a particular position (angle) such as a protruding or recessed portion of a rotating member made of a magnetic material. Another object of the present invention to provide a sensing device capable of starting a correct operation immediately after the electric power is turned on.