JPH0212012A - Sensor driving circuit - Google Patents

Abstract

PURPOSE:To reduce the power consumption of the title circuit by connecting a constant-current circuit provided with two transistors for constant current to a constant-current source through a transistor for load. CONSTITUTION:A constant-current circuit 21 is constituted of two transistors 22 and 23 for constant current and sensor elements 24 and 25 and the emitters of the transistors 22 and 23 are connected with a DC power source VEE respectively through the elements 24 and 25. The collectors of the transistors 22 and 23 are connected with one end of a constant-current source 29 through between the collectors and emitters of transistors 27 and 28 and the other end of the power source 29 is connected with a DC power source VCC. The resistance values of the elements 24 and 25 change following the displacement of an object to be detected. The value of the current flowing to the circuit 21 changes depending upon the resistance values of the elements 24 and 25 and a sensor output is fetched from the output terminal of the circuit 21. Thus the power consumption of this sensor driving circuit can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a drive circuit for a sensor used in a magnetic rotary encoder or the like.

[Conventional technology]

Conventionally, as a drive circuit for a sensor, in a magnetic rotary encoder, etc., four magnetoresistive elements 1 whose resistance value changes according to the displacement of the detected object are used, as shown in Fig. 4.
There is one in which the circuits 1 to 14 are assembled into a bridge circuit and the output signal thereof is amplified by an operational amplifier 15. For example, when a magnetic rotary encoder is used to detect the rotational position and rotational speed of a rotor, the four magnetoresistive effect elements 11 to 14 are used to form a large number of N poles that are alternately magnetized on the rotor of a motor, etc. It is placed opposite to the S-pole pattern in its rotational direction and detects the magnetic field due to this pattern.

In addition, in a magnetic rotary encoder, two magnetoresistive elements are arranged and electrically connected in series so that the rotating magnetic field given by the magnet rotor has an opposite phase relationship, and two magnetoresistive elements electrically connected in series are used. A constant current source that supplies a detection current to the resistance effect element, and a first stage differential amplifier that detects the voltage generated across each magnetoresistive element.

A sensor drive circuit equipped with a second-stage differential amplifier that outputs a detection signal corresponding to only the resistance change due to the magnetic field from the output signal of the first-stage differential amplifier is known from Japanese Patent Laid-Open No. 58-68615. It is being

[Problem to be solved by the invention]

In the sensor drive circuit described above, in the former case, the magnetoresistive elements 11 to 14 are driven by voltage, so the power consumption of the magnetoresistive elements 11 to 14 themselves is large. In addition, in an attempt to reduce power consumption, the magnetoresistive effect elements 11 to 14
If the resistance value is increased, the pattern becomes larger when the magnetoresistive elements 11 to 14 are integrated into an integrated circuit. In the latter case, since the magnetoresistive element is not used as part of the amplifier and its output is small, the differential amplifier in the next stage must be of very high performance.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a sensor drive circuit that eliminates the above-mentioned drawbacks, can reduce power consumption, and does not require the use of a very high-performance amplifier for the sensor output.

[Means to solve the problem]

The present invention relates to a sensor drive circuit configured to drive a sensor element whose resistance value changes according to the displacement of a detected object with a constant current.
A current circuit is connected to a constant current source via a load transistor, and a sensor element is connected to the constant current transistor as a resistor that determines the value of current flowing in the constant current circuit, and the sensor outputs from the output terminal of the constant current circuit. It is configured to take out.

[For production]

The resistance value of the sensor element changes in accordance with the displacement of the object to be detected, and the resistance value of the sensor element determines the value of the current flowing through the constant current circuit, and the sensor output is taken out from the output end of the constant current circuit.

〔Example〕

FIG. 1 shows an embodiment of the invention.

The constant current circuit 21 includes two constant current transistors 22゜2.
3 and magnetoresistive elements 24 and 25,
The emitters of the transistors 22 and 23 are connected to the DC type *V, through magnetoresistive elements 24 and 25, respectively.The bases of the transistors 22 and 23 are connected to the collector of the transistor 22, and the output is output from the collector of the transistor 23. Terminal 26 is led out. Magnetoresistive element 2
4 and 25 are connected as resistors that determine the value of current flowing through the constant current circuit 21, and two transistors 27 and 28 are load transistors for the constant current circuit 21. The collector of transistor 22.23 is transistor 27.2.
Constant current source 2 is connected between the collector and emitter of 8.
9, and the other end of the constant current source 29 is a DC type gV
Connected to cc. DC power sources 30 and 31 are connected between the bases of transistors 27 and 28 and the ground point, and are used for offset adjustment.

This embodiment is used in a magnetic rotary encoder, etc., and the magnetoresistive elements 24 and 25 are used, for example, to determine the rotational position of the rotor of the magnetic rotary encoder.

When used to detect rotational speed, a magnet is placed in the direction of rotation facing a large number of N-pole and S-pole patterns that are alternately magnetized on the rotor of the motor, and detects the magnetic field caused by this pattern. The second magnetoresistive elements 24, 25 may be any element whose resistance value changes according to the displacement of the rotor, etc., and the transistors 22 and 23. The transistors 27 and 28 and the magnetoresistive elements 24 and 25 are maintained as a pair, and are formed as, for example, one integrated circuit.

Next, the operation of this embodiment will be explained. Note that transistor 2
2.23.27.28 Base currents are ignored.

First, when the magnetic field from the motor rotor or the like is not applied to the magnetoresistive elements 24 and 25, the resistance values R8 and R5 of the magnetoresistive elements 24 and 25 are equal, so that the emitters [lu, , r , become equal, and the output voltage Vout of the output terminal 26 becomes O.

Furthermore, when a magnetic field from a motor rotor or the like is applied to the magnetoresistive elements 24 and 25, a difference appears in the resistance values R, □1R12 of the magnetoresistive elements 24 and 25 due to the magnetic field, and when R□〉Ro, 1 ~The balance between transistors 22 and 23 is lost, and the collector currents of transistors 22 and 23 (C3+IC4) are broken. 3, the output voltage V out changes. ■. , and IC4, the base-emitter voltage of the transistor 23 is ΔV f
i cff-R as i e 4, i) ΔV 8, . 4
The resistance value of the magnetoresistive element 24.25 is R5□.

A current flows from the output terminal 26 according to the difference in Rs□.

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 even if the output current changes small, the output voltage Vout changes greatly. Therefore, even if the amount of change in resistance of the magnetoresistive elements 24, 25 is small, the change in the output voltage Vout becomes large.

In addition, when the ambient temperature changes, the magnetoresistive element 24
．． 25 are made on the same substrate and arranged at intervals of μ, the resistance value R can be adjusted independently against changes in ambient temperature.
It is unlikely that s□+Rcz will change, and the resistance value R8□
, Rs2 are considered to change in the same way. That is, when the ambient temperature changes by ΔT with respect to the nominal temperature T, the resistance values Rs□, R5□ of the magnetoresistive element 24.25 change.
change simultaneously with Rs□=R, □+ΔR R, □=R, 2+ΔR, respectively. At this time, since the resistance values Rs□IRs□ of the magnetoresistive elements 24 and 25 of the transistors 22 and 23 change simultaneously in the same manner, balance is maintained and the output voltage V out does not change.

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

In this embodiment, since the current flowing through the magnetoresistive elements 24 and 25 can be set by the constant current circuit 21, power consumption can be reduced. Furthermore, since the magnetoresistive elements 24 and 25 are used as the emitter resistance of the active load (of the constant current circuit 21), the amplification degree of the first stage is increased, and a large output can be obtained with a small current. Furthermore, since a large output can be obtained even with a minute output from the magnetoresistive elements 24 and 25, the sensor can be made smaller when the magnetoresistive elements 24 and 25 are formed into a thin film sensor or the like. Furthermore, this embodiment can easily be made into a one-chip integrated circuit by connecting it to a differential amplifier circuit used in an integrated circuit.

FIG. 2 shows another embodiment of the invention.7 This embodiment is similar to the transistor 22.2 in the above embodiment.
The base of the transistor 32 is connected to the collector of the transistor 7, and the collector of the transistor 32 is connected to the DC power supply Vc.
e, and the emitter of the transistor 32 is connected to the DC power supply v, e via the resistor 33, and the connection point between the emitter of the transistor 32 and the resistor 33 is connected to the transistor 22.23.
It is designed to be connected to the base of The transistors 22 and 23 are supplied with base currents from the transistor 32 in accordance with the collector voltage of the transistor 22, and the magnetoresistive elements 24 and 25 are supplied with equal currents.

In the above embodiment, the base current is supplied from the collector of the transistor 27 to the transistors 22, 23, but it is assumed that the base currents of the transistors 22, 23, 27, and 28 can be ignored. However, in order to maintain ICJ"IC4, it is better to do as in this embodiment.

FIG. 3 shows another embodiment of the invention.

The above embodiment is an example of a current sink type in which current flows into the output terminal 26, but this embodiment is an example of a current discharge type in which the current flows out from the output terminal 26. In this embodiment, the DC power supply Vcc and the DC power supply V are connected in the opposite direction in the embodiment shown in FIG. Transistor 27
．． This is an example in which an NPN type is used instead of a PNP type as 28, and it operates in the same manner as the embodiment shown in FIG.

〔effect〕

As described above, according to the present invention, in a sensor drive circuit configured to drive a sensor element whose resistance value changes according to the displacement of a detected object with a constant current, the sensor drive circuit includes two constant current transistors. A constant current circuit is connected to a constant current source via a load transistor, a sensor element is connected to the constant current transistor as a resistor that determines the value of current flowing through the constant current circuit, and the output terminal of the constant current circuit is connected to the constant current source. Since the sensor output is configured to extract more sensor output, it is possible to reduce power consumption, and there is no need to use a very high performance amplifier for the sensor output.

[Brief explanation of the drawing]

1 to 3 are circuit diagrams showing each embodiment 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, 24, 25... Sensor element, 26...
Output end, 27.28...Load transistor, 29.
...Constant current source. EE

Claims (1)

[Claims] In a sensor drive circuit configured to drive a sensor element whose resistance value changes according to the displacement of an object with a constant current, a constant current transistor including two constant current transistors is provided. A current circuit is connected to a constant current source via a load transistor, a sensor element is connected to the constant current transistor as a resistor that determines the value of current flowing through the constant current circuit, and a sensor output is generated from the output terminal of the constant current circuit. A sensor drive circuit characterized in that it is configured to take out.