JPS5843793B2 - Shiheitouno Jikikenshiyutsu Cairo - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic detection circuit for paper scraps, etc., and in particular, in a magnetic sensor for detecting ferromagnetic components contained in paper scraps, etc., various noise components are removed and S/ The present invention provides a novel circuit device for improving N.

A conventional magnetic sensor circuit, as shown in FIG. It is connected in series to the voltage power source 3, and detects and amplifies fluctuations in magnetic flux distribution due to paper damage containing a ferromagnetic component through a capacitor between the magnetoresistive elements 1 and 2, thereby generating a magnetic signal. .

The output e of the magnetic sensor circuit using such a system is expressed as follows, where the resistance values of the magnetoresistive elements are R, , R2.

The detection output is generated by magnetoresistive elements 1 and 2 due to variations in magnetic flux distribution.
Output as resistance valve fluctuation.

If the respective resistance values change to R□+△R1 and R2+△R2, the output fluctuation △e is expressed as follows.

Note that it is assumed that there is no variation in voltage E.

Basically, since the magnetoresistive elements 1 and 2 have the same characteristics, R1÷R2.

In addition, regarding various types of noise, such as thermal noise, hum noise, magnetic noise, piezo noise, etc., since both magnetoresistive elements are placed spatially very close to each other, the temperature, magnetic, and The variation due to mechanical causes can be considered to be equal, and △R1=△R2.

Therefore, regarding the noise component, the formula (2) is %.As for the signal component, since both magnetoresistive elements are arranged in parallel at right angles to the scanning direction, the magnetic flux fluctuations received by the two are different, and △R1\△R2 Therefore, equation (2) is the equation that indicates the output.

As is clear from the above explanation, regarding the noise component △e
=O has been considered an advantage of this sensor circuit.

However, this is conditional on the fact that the resistance values R1 and R2 of the magnetoresistive elements are equal.

If there is a difference between the resistance values Rt and R2, how will △R1f-
Even with ΔR2, noise is output as is clear from equation (2).

In reality, as shown in FIG. 2, the variation in the resistance value of the magnetoresistive element is wide, and the temperature coefficient of the resistance value is also large.

Therefore, it is very difficult to select and mount elements in which R1 and R2 are equal to each other on the sensor, and even if it were possible, it would be nearly impossible to guarantee R1 = R2 over a wide operating temperature range. In reality, R1-R2
Since this condition cannot be satisfied, noise from the side will be output even in the circuit shown in FIG.

The present invention is intended to improve the drawbacks of the conventional method described above and to obtain a detection output that is not affected by noise.

As a circuit for obtaining an output unaffected by noise even when there are variations in resistance values, the circuit shown in FIG. 3 using a constant current bias method can be considered.

In this circuit, the magnetoresistive elements 1 and 2 are resistors r1+
It is always biased with a constant current ■ by two constant current circuits consisting of r2 r r31 r4 (r3'''r4) and transistors Q1 and Q2.

In this circuit, the sensor output is e=IR1-IR2, so the output fluctuation is △e = I△R1-I△R2=I (△R1-△R2
), the resistance values R1 and R2 of the elements themselves are completely irrelevant, and the output Δe is always zero for the noise component where ΔRt=ΔR2.

However, in order for this circuit to not output noise, it is necessary that the bias currents of both magnetoresistive elements be completely equal.

That is, two constant current circuits with exactly the same characteristics are required.

In reality, there are variations in the elements used in each constant current circuit, so some adjustment is necessary to compensate for this, and this adjustment is effective over a wide temperature range. Must.

The present invention solves the drawbacks of the circuit shown in FIG. 3 described above, and FIG. 4 shows a circuit that is an embodiment of the present invention.

In this circuit, magnetoresistive elements 1 and 2 are connected in series and are always biased with a constant current (2) by a constant current circuit consisting of resistors rl+r2t'3 and transistor Q.

It should be noted that, as in the conventional example, the permanent magnets or DC electromagnets are arranged in parallel in a uniform bias magnetic field at right angles to the scanning direction of the banknote.

The combined output of the magnetoresistive element 1゜2 is sent to the differential amplifier 4 via a 1/2 voltage divider circuit consisting of a voltage follower 5 and a resistor 6J7, and the output of the magnetoresistive element 2 is sent to the differential amplifier 4 via a capacitor. Each is input.

Since the input impedance of the voltage follower 5 is extremely high, all of the current (2) from the constant current circuit flows through the magnetoresistive element 1.2.

Therefore, the input voltage to the voltage follower 5 is I (R1+R2) The output of the 1/2 voltage divider circuit with resistors 6 and 7 is Upper I (R1+R2) becomes.

On the other hand, since the output of the magnetoresistive element 2 is R2, the input to the differential amplifier 4 is 1 e = -: t (R1 + R2) - I R2 = T I (
Rt - R2) (3).

Since the detection output is this variation, 1 (4) △e : 2 I (△R1 - △R2). Since both magnetic resistance elements are connected in series,
The bias currents are always equal, and equation (4) always satisfies Δe=0 regarding the noise component where ΔR1=ΔR2.

FIG. 5 shows another configuration example of the present invention.

The magnetoresistive elements 1 and 2 are also connected in series, and are always bussed with a constant current (2) by a constant current circuit.

Since the junction type FET Q3 is used in the constant current circuit, the constant voltage source 3 unlike the circuit shown in FIG. 4 is not required, and a constant current can be easily obtained.

The combined output of the magnetoresistive elements 1 and 2 is input to the differential amplifier 4 via a capacitor, and the output of the magnetoresistive element 2 is input to the differential amplifier 4 via the amplifier 8 having a gain of 2.

If an operational amplifier is used for the amplifier 8, the input impedance can be extremely high, so the current due to the constant current circuit can be reduced.
All flows to the magnetoresistive elements 1 and 2.

Therefore, the combined output of magnetoresistive elements 1 and 2 is I (R1+R2) becomes.

Furthermore, since the output of the magnetoresistive element 2 is IR2, the output of the amplifier 8 with double the gain is IR2.

From now on, the input to the differential amplifier is e=I(Rt+R2
) 2IR2=I(RI R2) (5), and the detection output is this variation, so △e=I(△R1
-ΔR2), (6).

If an operational amplifier is used as the amplifier 8, it is easy to obtain an accurate amplifier with a gain of 2.

Since both magnetoresistive elements are connected in series, the bias currents are always equal, and as for the noise component where ΔR1=ΔR2, equation (6) always satisfies Δe=O.

As explained above, according to the circuit of FIG. 4 or 5 according to the present invention, only one constant current circuit is required, and the third
Unlike the circuit shown in the figure, there is no need to make adjustments to make the characteristics of the constant current circuits the same.

Furthermore, since the magnetoresistive elements 1 and 2 are connected in series, it is impossible for their respective bias currents to be different. Therefore, from equations (4) and (6), regarding the noise component, △e = 0, High S as a magnetic sensor circuit
/N can be obtained 0

[Brief explanation of the drawing]

Figure 1 is a conventional magnetic detection circuit using a magnetoresistive element, Figure 2 is a diagram of variation in resistance value and temperature characteristics of a magnetoresistive element, Figure 3 is a magnetic detection circuit using a constant current bias method, and Figure 4 and FIG. 5 illustrate an improved magnetic detection circuit according to the present invention. 1.2... Magnetoresistive element, 3... Constant voltage power supply, 4... Amplifier, 5... Voltage follower, 6°7...・Voltage dividing resistor, 8・
...Operation amplifier (gain 2).

Claims (1)

[Claims] 1. Two magnetoresistive elements connected in series that are magnetically biased and whose electrical resistance characteristics change due to magnetic flux fluctuations due to passage of a piece of paper or the like containing a magnetic material component; A constant current circuit for biasing an element with a constant current, 2 above.
A voltage follower circuit whose input is the composite output of two magnetoresistive elements, and the output of the voltage follower circuit is halved
a voltage divider circuit for intervening in the voltage divider circuit, the output of the voltage divider circuit and the
1. A magnetic detection circuit, such as a magnetic detection circuit, characterized by comprising an AC differential amplifier circuit whose input is the output of one of the magnetoresistive elements. 2. Two magnetically biased magnetoresistive elements connected in series whose electrical resistance characteristics change due to changes in magnetic flux due to the passage of a piece of paper or the like containing a magnetic material component; the magnetoresistive elements are biased with a constant current. Constant current circuit for
an operational amplifier circuit with a gain of 2 that receives as input the output of one of the two magnetoresistive elements, and an AC differential amplifier circuit that receives as input the output of the operational amplifier circuit and the combined output of the two magnetoresistive elements. 1. A magnetic detection circuit for paper-based products, etc., characterized by comprising: