JPH0460227B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resistive bridge transmitter used for remote measurement of physical quantities.

BACKGROUND ART Methods for measuring various physical quantities (pressure, load, acceleration, temperature, torque, etc.) using resistance value changes such as strain gauges have been widely used. Among these, for instrumentation in plants and dams where the detector and indicator (or recorder, data logger, etc.) are often installed far apart, there is a transmitter that has a resistive bridge circuit as a detector. A current transmission method is used to transmit signals from the side (resistance bridge transmitter) to the receiving side where an indicator or the like is installed.

FIG. 1 shows the configuration of a conventional resistance bridge transmitter using a current transmission method. In FIG. 1, the bridge circuit 1 constitutes a Wheatstone bridge, and its output voltage e is expressed as e={(R ₁ R ₃ −R ₂ R ₄ )/(R ₁ +R ₄ ) (R ₂ +R ₃ )}・E (1). R ₁ to _{R 4} are resistance values on each side of the bridge circuit 1. Note that the excitation voltage E applied to the power supply end of the bridge circuit 1 is based on the operational amplifier
A voltage (for example, 5 to 10 V) stabilized by a constant voltage circuit consisting of A ₁ , transistor Q ₁ , constant voltage diode V ₁ , and constant current diode I ₁ is used.

Now, the resistance value of each side of bridge circuit 1 when the strain gauge is not receiving strain is R ₁ = R ₂ = ₃ =
If R ₄ =R, the bridge circuit 1 will be in a balanced state, and its output voltage e will be 0 [V]. on the other hand,
If the strain gauge senses strain and the resistance value on each side changes as, for example, R ₂ = R ₄ = R + ΔR, R ₁ = R ₃ = R - ΔR, the output voltage e will be e = (ΔR /R)・E (2). Substituting the relational expression between the amount of strain and the change in resistance; ΔR/R=Ks・ε (3) (Ks is the gauge factor of the strain gauge, and ε is the amount of strain), e=Ks・ε・E ( 4) becomes. Here, Ks is generally around 2.0 and E is several V
Since ε is on the order of 10 ^-4 , the output voltage e
is on the order of 10 ^-4 to 10 ^-3 V, which is too small to drive an indicator or recorder on the receiving side.

Therefore, FIG. 1 shows the output voltage e of the bridge circuit 1.
is amplified by operational amplifier A ₂ , and then amplified by operational amplifier A ₃
The receiving _side load resistance R L (specifically, the input resistance of the indicator or recorder) is connected to the load resistance R _L (specifically, the input resistance of the indicator or recorder) through a voltage-current conversion circuit consisting of the transistor Q 3 and the current detection resistor Rs.
The configuration is such that the output power is supplied to the

Operational amplifier _A2 constitutes a feedback amplifier circuit with a resistor Rf, and its output voltage ea is ea={R ₃ (R ₂ +Rf) + R ₂ Rf/R ₁ (R ₄ +Rf) + R ₄ Rf・R _Four
It is expressed as Rf/R ₂ R ₃ −Rf/R ₂ }E (5). Equation (5) further states that if the conditions under which the strain gauge is subjected to strain are as described above, ea=2Rf ΔR/R ² −ΔR ² E ≒2Rf/R・ΔR/R・E =2Rf/R・Ks・ε・E (6) (∵R≫ΔR) This becomes the input voltage at the non-inverting input terminal of the operational amplifier _A3 in the voltage-current conversion circuit. If the gain of this operational amplifier _A3 is large enough, ea=
Since es=io・Rs (es is the input voltage at the inverting input terminal of operational amplifier _A3 ), from the relationship with equation (6), io=2/Rs・Rf/Rs・Ks・ε・E ( 7) becomes. Since all of the right-hand side of equation (7) except ε are constants, io changes linearly with the amount of strain ε, as shown by the solid line in FIG.

On the other hand, the current id flowing through the bridge circuit 1 is E=
When 8V and R = 350Ω, it is approximately 23mA. The current consumption is of the entire transmitter is (excitation current id of the bridge circuit 1 + output current io + current consumption in the constant current source _I1 ), so if io is assumed to be 4 to 20 mA, is≈50 mAmax.

Here, the power supply Vs on the receiving side is generally 24Vdc. In this case, the power consumption of the series control transistor Q ₁ of the constant voltage circuit is (Vs - E) is = (24V -
8V) x 50mA = 800mW. This is completely wasted power that is converted into heat by transistor _Q1 and dissipated to the surroundings, and is a factor that reduces the power efficiency of the transmitter. Therefore, not only heat dissipation measures are important, but also
Since the _Q1 itself required a certain size, there was a problem in that the overall shape of the transmitter became large.

SUMMARY OF THE INVENTION An object of the present invention is to provide a resistive bridge transmitter that consumes little power and is easily miniaturized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of the present invention will be described below based on embodiments with reference to the drawings.

FIG. 2 is a circuit diagram showing the configuration of an embodiment of the present invention.

In the same figure, operational amplifier A ₁ and transistor
Q ₁ , constant voltage diode V ₁ , constant current diode I ₁ ,
A constant current circuit 2 is configured by a current detection resistor Rd and a starting bypass resistor Rb.
This constant current circuit 2 is connected between the current input terminal 11 from the receiving side and the first power supply terminal _a1 of the bridge circuit 1, and provides a constant excitation current to the bridge circuit 1.
It supplies the id.

The output voltage e obtained between the first and second output terminals b ₁ and b ₂ of the bridge circuit 1 is the output voltage e obtained from the operational amplifiers A ₂ and 2.
The signal is amplified by an amplifier circuit 3 consisting of two feedback resistors Rf. The output voltage of this amplifier circuit 3 is converted into a current by a voltage-current conversion circuit 4 consisting of an operational amplifier A ₃ , a transistor Q ₃ , and a current detection resistor Rs, and is sent to a current output terminal 12 to the receiving side. be done. Here, a current corresponding to the output voltage of the amplifier circuit ₃ flows through the transistor _Q3 , but unlike the case shown in _FIG. It is connected between the current output terminal 12 to the side.
That is, a part of the excitation current id of the bridge circuit 1 is used as the output power io to the receiving side.

On the other hand, the operational amplifier A ₄ , the transistor Q ₄ , the constant voltage diode V ₄ and the constant current diode I ₄ constitute a parallel control type constant voltage circuit 5 . 2 feeding points
The voltage of _a2 , in other words, the collector potential of transistor _Q3 , which is the output element of voltage-current amplification circuit 4, is kept constant.

Next, the operation of the embodiment having the above configuration will be explained.

The resistance values on each side of bridge circuit 1 are the same as above.
When R ₁ = R ₂ = R ₃ = R ₄ = R = 350Ω, the excitation voltage id supplied from constant current circuit 2 to bridge circuit 1
If is set to approximately 23 mA, the applied voltage E of the bridge circuit 1 will be 8 V, which is consistent with the conventional case described above. In this case, the potentials applied to both the inverting and non-inverting input terminals of the operational amplifier _A2 are different from those shown in FIG. 1, but since these potentials are in-phase, they can be ignored if the common-mode separation ratio of _A2 is sufficiently large. Output current to receiver
Since the configurations of the amplifier circuit 3 and voltage-current conversion circuit 4 are the same as in the case of FIG. 1, io can be expressed as in equation (7). Furthermore, there is a constant relationship between the excitation current id of the bridge circuit 1, the output current io to the receiving side, and the emitter current _i4 of the parallel control transistor _Q4 in the constant voltage circuit 5: id=(io+ _i4 ). Since this holds, i ₄ is complementary to i ₀ (see Figure 3).

As described above, according to the configuration shown in Fig. 2, it is possible to send out the output current io corresponding to the output voltage (in other words, the amount of distortion) of the bridge circuit 1 to the receiving side as in the conventional case, and at the same time reduce the consumption as shown below. Power consumption can be significantly reduced compared to conventional methods.

That is, considering the power loss _P1 of the transistor _Q1 , which has conventionally been a problem, in the case of the configuration shown in FIG. 2, it is given by _P1 =(Vs- _V4 -E)id.
Note that V ₄ is the voltage drop across the constant voltage diode V ₄ . Here, as mentioned above, when R = 350Ω and E = 8V, id≒23mA, so assuming V ₄ = 12V, P ₁ = (24V - 12V - 8V) x 23mA = 92mW, which is the same as the conventional It can be seen that this decreases to about 1/9 of the configuration.

In addition, the power loss P ₃ in the transistor Q ₃ in the voltage-current conversion circuit 4, which was not considered a problem earlier,
In Fig. 1, P ₃ = Vs・io, but
In FIG. 2, P ₃ =V ₄ ·io, which also decreases due to the relationship of V ₄ <Vs. According to the above example, Vs=24V,
Since V ₄ =12V, the power loss in transistor Q ₃ in the case of FIG. 2 is reduced to about 1/2. As described above, the configuration shown in FIG. 2 greatly reduces power consumption and is extremely effective in reducing the size of the transmitter.

In the above explanation, we have described the case where the resistance values on all four sides of the bridge circuit 1 change according to the strain, but in reality, the resistance values on the two sides change as shown in Figure 4.
Strain gauge bridges configured such that only R ₂ and R ₄ change are also often used.

In Figure 4, R ₂ = R ₄ = R + ΔR, R ₁ = R ₃
= R, then from equations (1) and (3), e = {ΔR/(2R+ΔR)}・E = {Ks・ε/(2+Ks・ε)}・E (8), and the amount of strain ε and A problem arises in that the output voltage e is not proportional.

On the other hand, according to the present invention, as explained in the embodiment shown in FIG. is e=(id/2)・ΔR=(id/2)・Ks・ε(9), and the output voltage e can be made proportional to the amount of strain ε.

Note that the present invention is not limited to the embodiments described above, and can be implemented with various modifications without departing from the gist thereof.

For example, although a parallel control type is used as the constant voltage circuit 5 in FIG. 2, a simple constant voltage diode may be used instead if some deterioration in characteristics is tolerated.

Also, in Figure 2, the transmission line between the transmitter and the receiving side is a 4-wire system, but as shown in Figure 5, one end of the power supply Vs on the receiving side and one end of the load resistor _RL are common. , a three-wire system may be used, in which case the transmitter may be configured as shown in FIG. In FIG. 6, the output element of the voltage-current conversion circuit ₄ is replaced with the NPN transistor in FIG.
By inserting it between Q3' and _Q3 ', the load current feedback line from the receiving side is unnecessary.

As described in detail above, according to the present invention, power consumption is low, and the transistors used can be small in size, making it easy to reduce the overall size. It is possible to provide a current transmission type resistance bridge transmitter that can obtain an output current that changes linearly with respect to the physical quantity to be measured, regardless of the number of resistors whose resistance values change accordingly.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing the circuit configuration of a conventional resistance bridge transmitter, Fig. 2 is a circuit diagram showing the configuration of an embodiment of the present invention, and Fig. 3 is an output current and constant voltage circuit for strain amount. FIG. 4 is an explanatory diagram of a bridge circuit whose resistance value changes depending on the physical quantity to be measured; FIG. 5 is a schematic diagram for explaining another embodiment of the present invention; FIG. 6 is a circuit configuration diagram showing FIG. 5 in detail. 1... Bridge circuit, 2... Constant current circuit, 3...
...Amplification circuit, 4...Voltage-current conversion circuit, 5...
Constant voltage circuit.

Claims (1)

[Scope of Claims] 1. A bridge circuit that has a resistor whose resistance value changes depending on the physical quantity to be measured on at least one side, and a bridge circuit between the first power feeding end of this bridge circuit and the current input terminal from the receiving side. a constant current circuit that is connected to the bridge circuit and supplies a constant excitation current to the bridge circuit; an amplifier circuit that amplifies the output voltage of the bridge circuit; and a second circuit that supplies a current corresponding to the output voltage of the bridge circuit. a voltage-current conversion circuit configured to cause the current to flow through an output element connected between a power supply end and a current output terminal to the receiving side; and a constant voltage circuit that constantizes the voltage at the second power supply end of the bridge circuit. A resistive bridge transmitter featuring low power consumption. 2. The resistor bridge transmitter according to claim 1, wherein the constant voltage circuit is a parallel control type constant voltage circuit.