JP2536822B2 - Temperature compensation circuit for weighing device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a temperature compensation circuit for a weighing device, which converts a strain amount of a flexure element that converts weight into mechanical strain into an electric signal and outputs the electric signal. .

(Prior Art) As shown in FIG. 4, a circuit for converting the strain amount of a flexure element such as an electronic scale into an electric signal is attached to the flexure element on each of two opposite sides with a signal output terminal interposed therebetween. Strain gauges A, A ', B,
A bridge circuit D connected to B ', a signal output terminal of the bridge circuit D is connected to a non-inverting input terminal, and an inverting input terminal is connected to a temperature-sensitive resistance element E closely attached to the strain body. Composed of operational amplifiers F and G and a differential amplifier circuit H that receives the outputs of these operational amplifiers F and G. The temperature characteristics of the Young's modulus of the metal, usually aluminum alloy, which constitutes the strain-generating body, and the strain gauge The fluctuation of the weight signal due to the temperature characteristic of the resistance is corrected by the change of the amplification factor of the amplifier circuit including the temperature sensitive resistance element E.

(Problem to be Solved) According to such a circuit, although the weight signal can be corrected with high accuracy for a relatively narrow temperature between which the compensation temperature set point is sandwiched, the temperature characteristic of the output from the bridge circuit, Since the temperature characteristic of the output of the amplifier circuit including the temperature-sensitive resistance element E has a positive second-order characteristic, the error ΔL rapidly increases with distance from the compensation temperature set point T ₀ , as shown in FIG. I still have problems.

In order to solve such a problem, it is conceivable to connect a temperature-sensitive resistance element in series with the bridge circuit to improve the temperature characteristic of the bridge output, but such temperature compensation lacks general versatility, which leads to cost reduction. Not only does this cause a rise in temperature, but it also introduces a new problem in that it is necessary to prepare a large number of temperature-sensitive resistors that match the characteristics of the flexure element and strain gauge.

The present invention has been made in view of the above problems, and an object thereof is to prevent an increase in cost,
By providing a new temperature compensating circuit for weighing devices that can correct the temperature characteristics of the bridge circuit including the strain gauge and the amplifier circuit connected to it with high accuracy over a wide temperature range. is there.

(Means for Solving the Problems) In order to solve such a problem, in the present invention,
The load circuit includes a bridge circuit configured by a strain gauge provided in the strain generating element and first and second operational amplifiers, and non-inverting input terminals of the first and second operational amplifiers are connected to the bridge circuit. At the same time, the inverting input terminals of the first and second operational amplifiers are made to match the secondary temperature characteristics with a metal having no positive secondary temperature coefficient and a metal having a positive secondary temperature coefficient. The connection was made by a temperature-sensitive means whose primary temperature coefficient was adjusted by a precision resistor.

(Embodiment) Therefore, the details of the present invention will be described below based on an illustrated embodiment.

FIG. 1 shows an embodiment of the present invention, in which reference numeral 1 denotes strain gauges 3, 3 ', 4 attached to a flexure element.
This is a bridge circuit having 4'on four sides. Reference numerals 6 and 7 denote first and second operational amplifiers, respectively, whose non-inverting input terminals are connected to the signal output terminals of the bridge circuit 1, and whose inverting input terminals are connected to the output terminals via precision resistors 8 and 9. And the temperature-sensitive resistance element described below between the inverting input terminals.
It is connected by 10 and precision resistor 12.

Reference numeral 10 is the above-mentioned temperature-sensitive resistance element, which is a first temperature-sensitive resistance element 10a such as copper having no quadratic coefficient component or platinum having a negative quadratic coefficient, and nickel having a positive quadratic coefficient. The second temperature-sensitive resistance element 10b is connected in series or in parallel. Reference numeral 12 is a precision resistor 12 for matching the primary temperature coefficient, which is connected in series to the temperature-sensitive resistance element 10 in this embodiment. In addition, reference numeral 14 in the figure
A differential amplifier circuit that receives outputs from the operational amplifiers 6 and 7 and cooperates with them to form a high input impedance differential amplifier circuit is shown.

By the way, the temperature of the first temperature-sensitive resistance element 10a made of copper −
The resistance characteristic shows a positive primary temperature characteristic with respect to temperature as shown in FIG. 2 (I), and the temperature-resistance characteristic of the second temperature-sensitive resistance element 10b made of nickel is shown in FIG. ), The positive first-order and positive second-order temperature-resistance characteristics are shown.

In this embodiment, the temperature-output characteristic of the bridge output is investigated by measuring the change of the output signal with respect to the temperature change of the bridge circuit 1 including the strain element in advance. This temperature-output characteristic is approximated by an expression of 1 + α _SP25 ΔT (where α _SP25 is a temperature coefficient and ΔT is a temperature difference from the compensation temperature set point) to obtain a coefficient α _SP25 .

In this way, when the temperature coefficient α _SP25 of the bridge output is determined, the temperature coefficient of the output of the high input differential amplifier circuit including the operational amplifiers 6 and 7 and the differential amplifier circuit 14 is obtained, and the output from this is almost the same. The resistance values of the temperature-sensitive resistance element 10 and the precision resistance 12 in which copper, nickel, etc. are combined so as to be flat are adjusted.

Assuming that the reference temperature is 25 ° C and the rated output of the bridge circuit at (25 + T) ° C is f (T), f _SP (T) −A _SP25 (1 + α _SP25 T) where α _SP25 : Bridge output temperature at 25 ° C Coefficient T: Temperature difference from 25 ℃ α _1SP : Temperature coefficient constant term of bridge output at 25 ℃ α _2SP : Temperature dependence term of temperature coefficient of bridge output when temperature changes α _SP25 × α _1SP + α _2SP T You can

On the other hand, if the amplification factor of the high input differential amplifier circuit at (25 + T) ° C is f _a (T), Where R _SO25 : Temperature-sensitive resistance value of input resistance at 25 ° C R _S1 : Precision resistance value of input resistance R _f : Feedback resistance value α _S25 : Temperature coefficient of amplification factor at 25 ° C α ₁ : Temperature of amplification factor at 25 ° C Coefficient constant term α ₂ : It can be expressed as a temperature-dependent term α ₂₅ = α ₁ + α ₂ T of the amplification factor temperature coefficient when the temperature changes.

Letting f _t (T) be the entire output when the output of the bridge and the high input differential amplifier circuit are combined, Here, in order to suppress the temperature change of the output f _t (T), if the terms of the third or higher order are ignored, the first and second terms should be zero. Becomes To satisfy this condition, each resistance value R _f , R _S25 ,
You can set R _S1 .

However, α _iSP and α _2SP are uniquely determined by the temperature characteristics of the bridge output obtained by the experiment, and α ₁ and α ₂ depend on the temperature coefficient of the temperature-sensitive resistance element to be used, and the amplification factor (1 + 2R _f / The ratio of the resistance values R _f , R _SO25 , and R _S1 is determined by how much (R _SO25 + R _S1 ) is set. The resistance value of is determined.

As a result, the positive secondary temperature change of the signal output from the bridge circuit 1 is canceled by the negative secondary temperature change of the high input differential amplifier circuit itself, so that the temperature compensation set point T ₀ is centered. A signal proportional to only the weight is output regardless of the temperature change over a wide temperature range.

In this embodiment, the high input impedance type amplifier in combination with the differential amplifier 14 is applied,
It is apparent that the same operational effect can be obtained even when applied to the high input impedance inverting circuit as shown in FIG.

(Effects of the Invention) A bridge circuit including a strain gauge provided in the strain generating element of the load cell and first and second operational amplifiers are provided, and the non-inverting input terminals of the first and second operational amplifiers are The inverting input terminals of the first and second operational amplifiers are connected to the bridge circuit by a temperature-sensitive resistance element made of a metal having no positive second-order temperature coefficient and a metal having a positive second-order temperature coefficient. The temperature characteristics of the bridge circuit including the strain gauge and the amplification circuit of the subsequent stage are combined in a wide temperature range because the temperature characteristics of the secondary temperature characteristics are adjusted and the temperature coefficient of the primary temperature coefficient is adjusted by a precision resistor. It can be easily fitted so that it becomes flat.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an apparatus showing an embodiment of the present invention, FIG. 2 is a diagram showing an example of a temperature resistance element used in the same apparatus,
FIG. 3 is a block diagram showing another embodiment of the present invention, and FIG.
FIG. 5 is a block diagram showing an example of a conventional temperature compensation circuit and a diagram showing its temperature compensation characteristic. 1 ... Bridge circuit 3, 3 ', 4, 4' ... Strain gauge 6, 7 ... Operational amplifier 10 ... Temperature-sensitive resistance element 10a ... Copper resistance element 10b ... Nickel resistance element 12 ... Precision resistance element

Claims (1)

(57) [Claims] 1. A bridge circuit composed of strain gauges provided in a strain element of a load cell, and first and second operational amplifiers, wherein non-inverting input terminals of the first and second operational amplifiers are provided. A temperature-sensitive resistance element that is connected to the bridge circuit and has inverting input terminals of the first and second operational amplifiers made of a metal having no positive second-order temperature coefficient and a metal having a positive second-order temperature coefficient. By combining the secondary temperature characteristics with
Further, a temperature compensating circuit for a measuring device, which is connected by a temperature-sensing means whose primary temperature coefficient is adjusted by a precision resistor.