JPS6082936A - Device for temperature correction of load cell - Google Patents

Abstract

PURPOSE:To improve measuring accuracy of a load cell by using the same circuits in the load cell as those of the prior art and adding a correction circuit to the outside so that dirft is adequately compensated even for an abrupt change in temp. CONSTITUTION:A temp. detecting terminal 9 which takes out the forward voltage Ep of an internal diode as a temp. detecting signal is provided in a load cell 1. On the other hand, the extent that the temp. detection voltage Ep changes in certain time is calculated to calculate dtheta/dt and the drift amt. DELTAV that arises according to dtheta/dt is measured by the preliminary experiment. The relation DELTAV=f(dtheta/dt) is preliminarily made and the above-mentioned voltage is converted to the drift amt. DELTAV by a function generating circuit 11 which makes DELTAV=f(dtheta/ dt) with respect to the output dtheta/dt of a differentiating circuit 10. Said value is added as the correction rate for the output 6 to the output V of the load cell 1 by an amplifier 12 by which the said output is corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a load cell temperature correction device that corrects temperature drift of a load cell.

[Technical background of the invention and its problems]

Load cells are often used to measure rolling pressure in rolling mills, and in this case they are installed right next to the hot mass being rolled, so in addition to normal temperature changes, temperature changes due to the influence of the hot mass are added. For this reason, it is necessary to consider that the load cell used in rolling mt will have a stable output within a temperature range of -10'Q to +75°C.

FIG. 1 shows a block diagram of the principle of a capacitive load cell.

This involves making a circular hole in an iron block, forming a parallel plate capacitor 2 in it, deforming the circular hole under load, and detecting the capacitance that changes accordingly with a capacitance detection circuit 3. The width is amplified and output.

In this case, parallel plate capacitor 2, capacitance detection circuit 3
, amplifier 4 are each designed to have a minimum change with respect to temperature change, and the output signal V is suppressed to have a small change with respect to temperature change.

However, since there are cases where the temperature does not fall below a certain threshold value due to variations in elements, etc., a temperature correction circuit 5 is provided to perform correction. Note that 6 is an output terminal.

FIG. 2 shows details of the temperature correction circuit 5.

The circuit shown in FIG. 2 corrects the drift of the output V in the positive direction when the temperature rises, and utilizes the fact that the forward voltage of the diode 7 decreases by about 2 mV/''Q depending on the temperature.

In other words, when the output V drifts in the positive direction due to temperature, if the forward voltage of the diode 7 is appropriately divided by the resistor 8 and applied to the non-inverting input terminal of the amplifier 4, the temperature drift △V of the output V is caused by this non-inverting input. Canceled.

As shown in Fig. 1, the parallel plate capacitor 2, capacitance detection circuit 3, amplifier 4, and temperature correction circuit 5 are contained within the same load cell, so their temperatures change at almost the same rate as the ambient temperature changes. Follow.

However, in an extremely rare phenomenon, when a sudden temperature change is applied to a load cell, for example, a temperature rise of 40°C/20 minutes, the parallel plate capacitor 2, capacitance detection circuit 3, amplifier 4, and temperature compensation circuit 5 inside the load cell Since the temperature time constants are different, there will be a difference in temperature, and each temperature drift will be different, causing a transient drift in the output.

For example, the parallel plate capacitor 2 is in direct contact with the iron block of the load cell 1, so it has good heat conduction and follows the ambient temperature quickly, but the capacitance detection circuit 3 and the like have internal air intervening, so it is difficult to follow the temperature. There will be a delay.

That is, there is a problem in that the parallel plate capacitor 2, the capacitance detection circuit 3, etc. generate a temperature difference in response to changes in ambient temperature, which causes a drift in the output.

[Purpose of the invention]

In the present invention, the circuit inside the load cell is the same as the conventional -1,
It is an object of the present invention to provide a temperature correction device for a load cell that can appropriately compensate for drift even in sudden temperature changes by adding an external correction circuit.

[Summary of the invention]

The present invention includes a differentiating circuit that differentiates an internal temperature signal of a load cell, and a function that has a relationship between a temperature change rate and a required drift correction voltage determined by experiment in advance, and inputs the output of the differentiating circuit and outputs a drift correction voltage. It is equipped with a generator and an adder circuit that adds the output voltage of the load cell and the drift correction voltage output from the function generator, and the output of the adder circuit is used as the final detection output. This is a temperature correction device for a load cell that performs appropriate drift compensation even for a load cell.

[Embodiments of the invention]

An embodiment of the present invention is shown in FIG.

In the present invention, a temperature detection terminal 9 is added to the load cell 1 as shown in FIG. i.e. diode 7
The forward voltage ED of is taken out to the outside as a temperature detection signal.

As shown in Figure 5, this forward voltage ED is a function of temperature θ and has a slope of approximately -2 mV/''G, so by measuring forward voltage ED, the temperature θ inside the load cell can be determined. .

In FIG. 4, the temperature detection output FiD is calculated by calculating how much it changes over a certain period of time, for example, 10 seconds, and then dθ
/d t! put out

That is, the amount of drift ΔV that occurs depending on dθ/di is measured in advance through experiments, and as shown in FIG. 6, dθ R: do/dt , ! :△V (7) Relational expression △V=
1 (-aT) e-creation si' The amplifier 12 adds the amount of correction to the output V of the load cell to perform correction.

It is convenient to use a microcomputer for the above correction calculations. An example of the block diagram is shown in FIG. 7, and FIG. 8 shows its basic flowchart.

dθ The function △V=fCi> shown in Figure 6 differs depending on the base temperature, so for example, if you create data with the ambient temperature as a parameter every 5℃ from O”G, then △V=/(7ρ Use.

If each block shown in Figure 1 follows the same temperature change without a sudden temperature change, which is a normal phenomenon, then dO
/dt = O, and the temperature correction circuit 5 performs the drift correction described in FIG.

According to the above method, the internal structure of the load cell can be modified from the conventional one, and can be realized by simply adding the forward voltage output terminal 9 of the diode 7.

It is also possible to detect the temperature inside the load cell using a diode, a thermal resistor, a temperature sensor IC (analog device AD590...analog type, AD537...frequency conversion type), etc. instead of a diode. In this case, as shown in the flowchart of FIG. 9, d0 is shown in FIG.
The correction of the amount of drift with respect to the absolute temperature when /dt=Q may be added, and the role of the temperature correction circuit 5 in FIG. 2 may be performed by microcomputer processing.

〔Effect of the invention〕

As explained above, the present invention provides a rational load cell that improves the measurement accuracy of the load cell by appropriately correcting the transient temperature drift caused by sudden temperature changes of the load cell, while leaving the load cell itself unchanged. A temperature compensation device is obtained.

[Brief explanation of drawings]

Figure 1 is an internal circuit diagram showing the general configuration of a conventional load cell, Figure 2 is a detailed diagram of the temperature correction circuit in Figure 1,
Fig. 3 is an internal circuit diagram of a load cell used in this invention, Fig. 4 is a system diagram showing an embodiment of the invention, and Fig. 5 is a characteristic diagram showing the relationship between diode forward voltage and temperature. ,
Fig. 6 is a characteristic diagram showing the relationship between the temperature change rate and the drift correction value, Fig. 7 is a system diagram showing an embodiment of the present invention using a microcomputer, and Figs. It is a flowchart showing the operation of the microcomputer. ■ Load cell 2 Parallel plate capacitor 3 Capacitance detection circuit 4.12 Amplifier 5 Temperature correction circuit 7 Diode 10 Differentiation circuit 11 Function generation circuit 13 + 13B A/D conversion circuit 14 Microcomputer 15 D/A conversion circuit Figure 1 2 causes Figure 3 Figure 4 Figure 5 Figure ρ Figure 6 t

Claims (1)

[Claims] a differentiating circuit that differentiates an internal temperature signal of the load cell; a function generator that has a relationship between a temperature change rate and a required drift correction voltage determined in advance through experiments; and that inputs the output of the differentiator circuit and outputs a drift correction voltage; An 8-inverter correction device for a load cell, comprising an adder circuit that adds the output voltage of the load cell and the drift correction voltage output from the function generator, and uses the output of the adder circuit as a temperature-corrected detection signal.