JP2993532B2 - Excitation circuit of Wheatstone bridge type load cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an excitation circuit for exciting a load cell formed by connecting a load conversion element such as a strain gauge to a Wheatstone bridge.

[0002]

2. Description of the Related Art In an electronic balance, a load converter for converting a load into an analog electric signal is configured as shown in FIG. The load cell is constituted by connecting a strain gauge Ra, Rb, Rc, and Rd attached to a strain body to a Wheatstone bridge.
The voltage is applied during -IN to be excited. Bridge circuit 1
Output voltage of the balanced input 3
Through the A / D converter.

[0003]

However, this amplifier 3
Requires three good quality operational amplifiers Aa, Ab, Ac, resistors R ₁ and R ₂ , R ₃ and R ₄ , and R ₅ and R ₆
Has a problem that the gain and the common mode voltage suppression ratio must be adjusted by the semi-fixed resistors Rv ₁ and Rv ₂ .

[0004] In addition, there are the following problems. -Since the gain is adjusted by the resistors in the differential section so that the common-mode voltage does not enter, it is not possible to adjust the gain by putting the entire system into the feedback loop.

The common mode suppression ratio is good at DC and low frequencies, but worsens with increasing frequency. The same applies to the case where the power supply 2 outputs a rectangular wave voltage.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an excitation circuit capable of performing a single-ended amplification process on an output voltage of a bridge circuit and sufficiently removing a common-mode voltage.

[0007]

SUMMARY OF THE INVENTION An excitation circuit according to the present invention is an excitation circuit for differentially exciting a load cell formed by connecting a load converting element to a Wheatstone bridge with a rectangular wave voltage. One end of the output signal source is connected to one input terminal of the bridge circuit of the load cell via the first amplifier, and the other end of the signal source outputting the rectangular wave signal is connected to the other end of the load cell via the second amplifier. Connect one output terminal of the load cell bridge circuit to the third input terminal.
Negative feedback to the input sides of the first and second amplifiers via the amplifier.

[0008]

According to this configuration, in order to set one of the output terminals of the bridge to the ground potential in phase, the fact that the negative input terminal of the inverting amplifier subjected to a large amount of negative feedback becomes a virtual ground is utilized. That is, the excitation voltage is applied to the differential first and second amplifiers while only the differential voltage is stored by the floating capacitor. The common-mode voltages of the first and second amplifiers are controlled by applying a large negative feedback through the third amplifier so that the ground terminal of the output of the bridge circuit becomes the ground potential. The output of the bridge circuit is output from the other terminal as a single-ended output, and is applied to the first-stage signal amplifier.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. It is to be noted that components having the same functions as those of the conventional example are denoted by the same reference numerals and described.

FIG. 1 shows an excitation circuit according to the present invention, in which V _EXT
Is a signal source for outputting a square wave signal, which is a floating power source that is not conductively connected in phase. In order to fix the common mode potential, series high resistances R ₁ and R ₂ having the same resistance value are connected in parallel. The voltage Va is the first
Is applied to the input terminal + IN of the bridge circuit 1 via the buffer amplifier A ₁ as an amplifier, the voltage Vb is applied to the input terminal -IN of the bridge circuit 1 via a buffer amplifier A ₂ of the second amplifier I have.

Here, the input terminal + IN of the bridge circuit 1
Is applied to the input terminal -I of the bridge circuit 1.
The voltage Vb is applied to N, and the bridge circuit 1 is excited by Va−Vb = _VEXT .

The output terminal -OUT of the bridge circuit 1 is connected to the third terminal.
Inverting input terminal of the operational amplifier A ₃ as the amplifier (-)
It is connected to the non-inverting input terminal of the operational amplifier A ₃ of the high degree of amplification is connected to the ground. The output of the operational amplifier A ₃ is connected in series with high resistance R _1, the R ₂ midpoint P.

[0013] The output terminal + OUT of the bridge circuit 1 is output as a single-ended output, it is added to the first stage of the signal amplifier A _4. Here, if the excitation voltage is ignored and the feedback loop including the buffer amplifiers A ₁ and A ₂ and the differential amplifier A ₃ is omitted, the circuit shown in FIG. 1 can be expressed as shown in FIG. Moreover, if you look at a differential amplifier A _3, it is composed of a signal path composed of a resistor R ₁ and the buffer amplifier A ₁ and the strain gauge Rc, a resistor R ₂ and buffer amplifier A ₂ and the strain gauges Rd 3 can be regarded as circuits having the same transfer characteristics, and can be omitted and represented as shown in FIG.

In FIG. 3, A ₁₂ is a buffer amplifier A
₁ and A ₂ , C _S1 is a stray capacitance generated by a V _EXT generating circuit and wiring, and C _S2 is a stray capacitance generated by a bridge, a cord, and the like, and these are high loop gains formed by the differential amplifier A _3. Oscillation and unstable development sometimes occur in the feedback circuit. Therefore, in an actual circuit, it is necessary to insert a phase compensation circuit Nw as shown in FIG.

In FIG. 4, a differential amplifier A_Three Inverting input terminal
The potential of the child (-) is A_Three Of high amplification
Therefore, it is very close to the + input terminal, that is, the ground potential. Differential amplification
Vessel A _Three Is the output terminal of the bridge circuit 1.
Output terminal + OUT is a bridge circuit
Output voltage V_SIG And is completely single-ended
Signal amplifier A as a type_Four Is added to

FIG. 5 shows a specific circuit of the excitation circuit shown in FIG. In this specific example, the signal source V _{EXT includes} capacitors C ₁ and C ₂ and switches S _{1 to} S ₄ .
Switch S ₁ and the switch S ₂ is switched in conjunction with each other. Switch S ₃ and the switch S ₄ also switched in conjunction with each other.

The capacitor C ₁ is connected to the contact b and is charged by the DC power supply voltage V _C , and is connected to the contact a and charges the capacitor C ₂ . The terminal voltage of the capacitor C _2, which is charged to the voltage V _C 'are first, second timing contact c, the switch repeatedly switched to the side of d S
₃ , and are applied to buffer amplifiers A ₁ and A ₂ via S ₄ . The polarity of the excitation voltage applied between the input terminals + IN and -IN of the bridge circuit each time the switches S ₃ and S ₄ are switched from the contact c to the contact d and from the contact d to the contact c. Switches. Switching of the switches S ₁ and S ₂ is repeatedly performed in synchronization with the switching of the switches S ₃ and S ₄ , and the capacitor C ₂ is always charged to the voltage V _C ′.

Furthermore, the excitation method a DC supply voltage V _C switches S ₃ even polarity is either positive or negative as shown in FIG. 5,
Together we are obtained exactly the same effect by reversing the time switching of S _4, one end of the DC power supply voltage V _C may be grounded. This is effective when used as a reference voltage of an AD converter. For example, a double integral type A-
The D converter needs to input a negative reference voltage for a positive signal input, and accurate AD conversion is usually performed by normalizing the signal voltage by the reference voltage. A negative ground reference voltage is a very significant advantage. Also, A-D reference voltage inputted to the converter is sometimes a voltage proportional to the DC supply voltage V _C, but the DC power supply voltage V _C is readily DC power supply voltage V _C for a ground voltage
Can be obtained.

[0019]

As described above, according to the present invention, there is provided an excitation circuit that differentially excites a load cell formed by connecting a load converting element to a Wheatstone bridge with a rectangular wave voltage, and outputs a rectangular wave signal. One end of the signal source is connected to one input terminal of a bridge circuit of the load cell via a first amplifier, and the other end of the signal source for outputting a rectangular wave signal is connected to the other input terminal of the load cell via a second amplifier. , And one output terminal of the bridge circuit of the load cell is negatively fed back to the input side of the first and second amplifiers via the third amplifier, so that the output of the bridge circuit is not affected without affecting the excitation voltage. Only the voltage becomes a purely common mode voltage, and the following effects are achieved.

(1) The bridge output voltage can be subjected to single-ended amplification processing. (2) Be independent of common-mode noise voltage. (3) The whole system can be processed in a single feedback loop.

(4) Countermeasures against temperature changes and aging are easy. (5) It can be realized at low cost and is highly economical. (6) It is easy to make the relationship between the reference voltage and the excitation voltage exactly proportional.

(7) Since rectangular wave excitation can be easily performed, offset and zero point movement of an amplifier and the like can be extremely reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of an excitation circuit according to the present invention.

FIG. 2 is a circuit diagram obtained by simplifying the explanatory diagram focusing on only common-mode voltage control.

FIG. 3 is a simplified circuit diagram of the circuit diagram with a single feedback circuit.

FIG. 4 is a configuration diagram in which a phase compensation circuit is inserted to stabilize the excitation circuit.

FIG. 5 is a specific configuration diagram of an excitation circuit according to the present invention.

FIG. 6 is a configuration diagram of an input stage of a conventional electronic balance.

[Explanation of symbols]

Reference Signs List 1 bridge circuit A ₁ buffer amplifier [first amplifier] A ₂ buffer amplifier [second amplifier] A ₃ operational amplifier [third amplifier] V _EXT Signal source for outputting a rectangular wave signal R ₁ , R ₂ high resistance + IN, -IN Input terminal of bridge circuit + OUT, -OUT Output terminal of bridge circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01L 1/22 G01B 7/18

Claims (1)

(57) [Claims] An excitation circuit for differentially exciting a load cell formed by connecting a load conversion element with a Wheatstone bridge with a rectangular wave voltage, wherein one end of a signal source for outputting a rectangular wave signal is connected to a first amplifier. Is connected to one input terminal of a bridge circuit of the load cell, and the other end of a signal source that outputs a square wave signal is connected to the other input terminal of the load cell via a second amplifier. An excitation circuit for a Wheatstone bridge-type load cell, wherein the output terminal of the load cell is negatively fed back to the input sides of the first and second amplifiers via a third amplifier.