JPH0697170B2 - Sensor drive circuit - Google Patents

Description

The present invention relates to a drive circuit for a sensor used in a magnetic rotary encoder.

[Conventional technology]

Conventionally, in a magnetic rotary encoder as a drive circuit of a sensor, as shown in FIG. 4, four magnetoresistive effect elements 11 to 14 whose resistance values change according to the displacement of an object to be detected.
Is assembled in a bridge circuit and its output signal is amplified by an operational amplifier 15. Here, the four magnetoresistive elements 11 to 14 are composed of a large number of N poles and S poles alternately magnetized to the rotor of the motor when the magnetic rotary encoder is used to detect the rotational position and rotational speed of the rotor. The magnetic field due to this pattern is detected so as to be opposed to the pattern in the rotation direction thereof.

Further, in a magnetic rotary encoder, two magnetoresistive effect elements that are arranged so that the rotating magnetic fields given by the magnet rotors have an antiphase relationship and that are electrically connected in series, and the two magnetoresistive elements that are connected in series A constant current source for supplying a detection current to the resistance effect element, a first stage differential amplifier for detecting a voltage generated across each magnetoresistive element, and an output signal of the first stage differential amplifier. Japanese Patent Laid-Open No. 58-68615 discloses a sensor driving circuit including a second stage differential amplifier that outputs a detection signal of only a resistance change due to a magnetic field.

[Problems to be Solved by the Invention]

In the drive circuit of the sensor described above, since the magnetoresistive effect elements 11 to 14 are voltage driven in the former case, the power consumption of the magnetoresistive effect elements 11 to 14 itself is large. Further, if the resistance values of the magnetoresistive effect elements 11 to 14 are increased in order to reduce the power consumption, the pattern becomes large when the magnetoresistive effect elements 11 to 14 are integrated. In the latter case, since the magnetoresistive effect element is not used as a part of the amplifier and its output is small, it is necessary to make the differential amplifier of the next stage highly efficient.

It is an object of the present invention to eliminate the above-mentioned drawbacks and provide a sensor driving circuit which does not require the use of a very high-performance sensor output amplifier capable of reducing power consumption.

[Means for Solving the Problems]

The present invention is directed to a sensor drive circuit which alternately faces a rotor magnetized with a large number of N poles and S poles, and which drives a sensor element whose resistance value changes in accordance with the displacement of the rotor at a constant current. A constant current circuit having two constant current transistors is connected to a constant current source via a load transistor, and two magnetoresistive effect elements are provided as resistors for determining the current value flowing in the constant current circuit. Is connected to each of the two constant current transistors, and the sensor output corresponding to the difference in resistance value between the two magnetoresistive elements is taken out from the output terminal of the constant current circuit. It is characterized by.

[Work]

The resistance value of the sensor element composed of two magnetoresistive elements changes according to the displacement of the rotor, and the current value flowing in the constant current circuit is determined by the resistance value of the sensor element. A sensor output corresponding to the difference in resistance value of the magnetoresistive element is taken out.

〔Example〕

 FIG. 1 shows an embodiment of the present invention.

The constant current circuit 21 is composed of two constant current transistors 22 and 23 and magnetoresistive effect elements 24 and 25.
The emitters of 2, 23 are connected to the DC power source V _EE via the magnetoresistive effect elements 24, 25, respectively. The bases of the transistors 22 and 23 are connected to the collector of the transistor 22, and the output terminal 26 is led out from the collector of the transistor 23. The magnetoresistive elements 24 and 25 are connected as resistors that determine the value of the current flowing in the constant current circuit 21, and the two transistors 27 and 28 are load transistors for the constant current circuit 21. The collectors of the transistors 22 and 23 are connected to one end of a constant current source 29 via the collectors and emitters of the transistors 27 and 28, respectively, and the other end of the constant current source 29 is connected to the DC power supply Vcc. The DC power supplies 30 and 31 are connected between the bases of the transistors 27 and 28 and the ground point and are used for offset adjustment. This embodiment is used in a magnetic rotary encoder, and the magnetoresistive elements 24 and 25 are alternately magnetized to the rotor of the motor when the magnetic rotary encoder is used to detect the rotational position and rotational speed of the rotor of the motor. The magnetic field due to this pattern is detected by being arranged in the rotational direction so as to face the numerous N-pole and S-pole patterns. The magnetoresistive effect elements 24 and 25 are elements whose resistance value changes according to the displacement of the rotor. The transistors 22 and 23, the transistors 27 and 28, and the magnetoresistive effect elements 24 and 25 are kept paired, and are formed, for example, as one integrated circuit.

Next, the operation of this embodiment will be described. Note that transistor 2
Ignore the base currents of 2,23,27,28.

First, the magnetic field generated by the motor rotor is
When not applied to 4,25, since the resistance values R _S1 and R _S2 of the magnetoresistive effect elements 24 and 25 are equal, the emitter currents I _E3 and I _{E4 of} the transistors 22 and 23 are equal and the output voltage of the output terminal 26 is equal.
Vout becomes 0.

In addition, the magnetic field generated by the rotor of the motor is
When applied to 5, the magnetoresistive element 2
When a difference appears in the resistance values R _S1 and R _S2 of 4,25 and R _S1 > R _S2 , the balance of the transistors 22 and 23 is lost and the collector currents I _C3 and I _{C4 of} the transistors 22 and 23 become I _C3 <I. _It becomes _C4 , and the output voltage Vout changes. The relation between I _C3 and I _C4 is that if the base-emitter voltage of the transistor 23 is ΔV _BE4 and I _C3 · R _S1 > ΔV _BE4 is satisfied. Therefore, a current flows from the output terminal 26 in accordance with the difference between the resistance values R _S1 and R _S2 of the magnetoresistive effect elements 24 and 25. Here, since the output terminal 26 is the output terminal of the constant current circuit 21, the output resistance of the transistor 23 becomes large, and the output voltage Vout changes greatly even if the output current changes small. Therefore, even if the amount of resistance change of the magnetoresistive effect elements 24 and 25 is small, the output voltage Vout
The change in will be greater.

When the ambient temperature changes, the magnetoresistive element 24,2
Since 5 are formed on the same substrate and are arranged at intervals of μ, resistance values R _S1 and R _S2 are independent of changes in ambient temperature.
Is unlikely to change, and it is considered that the resistance values R _S1 and R _S2 change the same. That is, when the ambient temperature changes by ΔT with respect to the nominal temperature T, the magnetoresistive effect element
The resistance values R _S1 and R _S2 of 24 and 25 respectively change at the same time as R _S1 = R _S1 + ΔR R _S2 = R _S2 + ΔR. At this time, the resistances R _S1 and R _S2 of the magnetoresistive effect elements 24 and 25 of the transistor 22 and the transistor 23 change at the same time, so that the balance is maintained and the output voltage Vout does not change.

Therefore, the output voltage Vout does not change with changes in the ambient temperature.

In this embodiment, the magnetoresistive effect element is controlled by the constant current circuit 21.
Since the current flowing to 24 and 25 can be set, low power consumption can be achieved. Further, since the magnetoresistive effect elements 24 and 25 are used as the emitter resistance (of the constant current circuit 21) of the active load, the amplification degree of the first stage is increased and a large output can be obtained with a small current. Moreover, since a large output can be obtained even with a small output of the magnetoresistive effect elements 24, 25,
When forming a thin film sensor or the like, the area of the thin film can be reduced, and the sensor can be reduced to a practical size. The magnetoresistive effect elements 24 and 25 are used in a magnetic rotary encoder, and a large number of N poles and S magnets are alternately magnetized to the rotor of the motor.
When detecting the magnetic field due to the pattern of the poles, it must be made smaller according to the magnetization pattern.If a metal thin film sensor of 50 KΩ or more is used, the area of the thin film will increase and It's too big to be practical. Therefore, the magnetoresistive effect elements 24 and 25 are, for example, 10K
When the metal thin film sensor has a resistance of Ω or less, preferably 1 KΩ, the area of the thin film is smaller than the magnetized pattern of the rotor, which is practical. In addition, this embodiment is easy to form a one-chip integrated circuit by connecting to the differential amplifier circuit used in the integrated circuit.

FIG. 2 shows another embodiment of the present invention.

In this embodiment, the bases of the transistors 32 are connected to the collectors of the transistors 22 and 27 in the above embodiment,
The collector of the transistor 32 is connected to the DC power source Vcc, the emitter of the transistor 32 is connected to the DC power source V _EE via the resistor 33, and the connection point between the emitter of the transistor 32 and the resistor 33 is the base of the transistors 22 and 23. It is designed to be connected. Transistors 22 and 23 are transistor 2
A base current is supplied from the transistor 32 according to the collector voltage of 2, and equal currents are supplied to the magnetoresistive effect elements 24 and 25. In the above embodiment, the base current is supplied from the collector of the transistor 27 to the transistors 22 and 23, but the base current of the transistors 22, 23, 27 and 28 can be ignored. However, in order to keep I _C3 = I _C4 , it is better to use this embodiment.

FIG. 3 shows another embodiment of the present invention.

The above-described embodiment is an example of a current sink type in which current flows in from the output terminal 26, but this embodiment is an example of a current discharge type in which current flows out from the output terminal 26. This embodiment is the same as the embodiment shown in FIG.
Connect _EE in reverse and connect the constant current source 29 in reverse.
In addition, instead of the NPN type transistors 22 and 23, PNP
This is an example in which NPN type transistors are used instead of PNP type transistors as transistors 27 and 28.
It operates similarly to the illustrated embodiment.

[Effect]

As described above, according to the present invention, a plurality of N-poles and S-poles are alternately opposed to a magnetized rotor, and a sensor element whose resistance value changes according to the displacement of the rotor is driven by a constant current. In the drive circuit of the sensor described above, a constant current circuit having two constant current transistors is connected to a constant current source via a load transistor, and the constant current circuit is used as a resistor for determining a current value flowing in the constant current circuit. A sensor element including a plurality of magnetoresistive effect elements is connected to each of the two constant current transistors, and a sensor output corresponding to a difference in resistance value between the two magnetoresistive effect elements is output from an output terminal of the constant current circuit. Since it is configured to be taken out, it is possible to achieve low power consumption, and it is not necessary to use an extremely high-performance sensor output amplifier. Further, as a sensor element, a magnetoresistive effect element is used. Been able sensor element can be practical small by using, practically can be realized to detect the magnetic field by the rotor magnetization pattern of the motor used in a magnetic rotary encoder.

[Brief description of drawings]

1 to 3 are circuit diagrams showing respective embodiments of the present invention, and FIG. 4 is a circuit diagram showing a conventional sensor drive circuit. 21 …… Constant current circuit, 22,23 …… Constant current transistor, 2
4,25 …… Magnetoresistive element, 26 …… Output end, 27, 28 ……
Load transistor, 29 ... Constant current source.

Claims (1)

[Claims] 1. Driving a sensor in which a large number of N poles and S poles are alternately opposed to a magnetized rotor, and a sensor element whose resistance value changes according to the displacement of the rotor is driven with a constant current. In the circuit, a constant current circuit having two constant current transistors is connected to a constant current source via a load transistor, and two magnetic resistors are provided as resistors for determining the value of the current flowing in the constant current circuit. A sensor element composed of an effect element is connected to each of the two constant current transistors, and a sensor output corresponding to a difference in resistance value between the two magnetoresistive effect elements is taken out from an output terminal of the constant current circuit. A sensor drive circuit characterized by the above.