JPH04324391A - Compensating method for temperature of metal detector in excavator and metal detection circuit equipped with temperature compensating mechanism - Google Patents

Abstract

PURPOSE:To correct a change in the output voltage of a bridge circuit due to the temp. change around a coil by adding a correction DC component to an AC component prior to the comparison with reference voltage without using a temp. sensor. CONSTITUTION:The bias AC voltage from a bridge circuit 2 gradually falls in peak voltage with the temp. rise around coils 1a, 1b to reach reference voltage. Therefore, prior to the comparison with the reference voltage in a comparison circuit 10, a separation circuit 7 separates bias AC voltage into an AC component (Vac) and a DC component (Vdc). A correction circuit 8 generates DC correction voltage (Vdc') and an adder circuit 9 adds the DC correction voltage (Vdc') to the AC component (Vac) separated by the circuit 7. The added voltage is compared with the reference voltage by the circuit 10. By this constitution, peak voltage can be kept almost constant regardless of a temp. change and the erroneous detection due to temp. change is prevented.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a temperature compensation method for a metal detection device for preventing damage to buried metal objects such as buried pipes by detecting them when excavating underground with an excavator. The present invention also relates to a metal detection circuit equipped with a temperature compensation mechanism.

0002

2. Description of the Related Art Conventionally, in a relatively small-diameter excavator for use in gas pipeline work, for example, a pair of coils 1a forming a bridge type metal detection circuit are attached to a bit 11 at the tip of an excavation rod as shown in FIG. , 1b is built in, and the magnetic flux formed by this coil is leaked to the outside of the bit, and the change in coil inductance caused by the change in magnetic flux when the bit approaches the buried metal object 13 is suppressed by the bridge circuit. Some devices detect buried metal objects by detecting balance.

[0003] Such a metal detection circuit, as shown in FIG. 6, for example, includes a bridge circuit 2 which generally includes the pair of coils described above, an oscillator 3 that supplies a balancing alternating current signal to the bridge circuit, and an oscillator 3 that supplies a balancing alternating current signal to the bridge circuit. a DC bias circuit 4 that superimposes and supplies a bias DC signal to the bridge circuit; an amplifier circuit 14 that amplifies the output of the bridge circuit; and a comparison circuit 15 that compares the peak value of the bias AC voltage output from the bridge circuit with a reference voltage. The device detects the proximity of metal by detecting a change in the peak value of the bias AC voltage caused by the unbalance of the bridge circuit by comparing it with a reference voltage. In the figure, reference numeral 5 is a DC power supply, and 6 is an adder.

[0004] This metal detection circuit operates as shown below. First, when the bit is not close to a buried metal object, the bridge circuit is AC balanced, so the amplifier circuit outputs only DC voltage, or completely Small AC voltages appear due to the imbalance, and since these voltages are higher than the reference voltage in the comparator circuit, the comparator circuit does not generate an output corresponding to the metal proximity detection signal.

However, when the bit comes close to a buried metal object,
When the magnetic flux passing through the coil changes and the bridge circuit becomes unbalanced, as shown in part b of FIG. 4, the DC voltage does not change, and an AC voltage appears superimposed thereon. Therefore, the peak value Vp on one side of the peak value of the bias AC voltage, that is, the low voltage side in the figure, is compared with the reference voltage, and if this peak value Vp falls below the reference voltage, the metal buried object is detected. generates an output corresponding to the proximity detection signal.

[0006]

[Problem to be solved by the invention] The magnetic flux generated from the coil causes eddy current to flow through the bit made of metal such as aluminum alloy, and this eddy current reduces the leakage magnetic flux to the outside that contributes to metal detection. . For this reason, the signal level handled by the metal detection circuit is extremely low, so it is easily affected by impedance changes due to temperature changes around the coil.
Stable detection is relatively difficult.

The change in the ambient temperature causes a change in the DC resistance and inductance of the coil, and this change in DC resistance appears as a change in the DC component of the bias AC voltage, and the change in inductance causes a change in the DC component of the bias AC voltage. It appears as a change in the amplitude of the alternating current component of the voltage. All of these change the peak value Vp of the bias AC voltage compared with the reference voltage in the above-mentioned comparison circuit, resulting in a detection error. As a practical example, as shown in the figure, when the ambient temperature rises, the voltage value of the DC component increases and the amplitude of the AC component increases, and the increase in the voltage value of the former DC component is at the peak value Vp. The increase in the amplitude of the latter AC component contributes to the decrease in the peak value Vp.

That is, the increase in the voltage value of the DC component is expressed as ΔVd
c. If the increase in the amplitude of the AC component is ΔVac, the change in the peak value ΔVp is ΔVp=ΔVdc−ΔVac
It can be expressed as In this way, the changes in the voltage values of the DC and AC components tend to act in opposite directions and cancel out the changes in the peak values to be compared, but the rates of change of these changes are respectively Since they are different, the above-mentioned detection error becomes the difference between them, that is, ΔVp. For example, in the specific example shown in the figure, since ΔVac > ΔVdc, the peak voltage compared with the reference voltage gradually decreases from Vp to Vp' as the temperature rises.

[0009] Therefore, the above error is measured in advance as a correspondence relationship with temperature and stored in the circuit, and the output of the bridge circuit is corrected based on the temperature around the coil measured by a temperature sensor such as a thermistor. However, with this method, a new temperature sensor must be installed, which complicates the circuit configuration, increases the price, makes it difficult to downsize, and increases power consumption. This poses problems, such as making it difficult to perform long-term detection operations using batteries.

[0010] The present invention aims to solve such problems, namely, to correct changes in the output voltage of the bridge circuit caused by changes in the temperature around the coil without using a temperature sensor such as a thermistor. The purpose is to

[0011]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention first provides a balance AC signal and a bias DC signal to a bridge circuit whose components include a pair of coils built into the tip of an excavator. In a metal detection device that detects the proximity of metal by applying a superimposed voltage and detecting a change of more than a predetermined value in the peak value of the bias AC voltage caused by the unbalance of the bridge circuit by comparing it with a reference voltage, The bias AC voltage is separated into an AC component and a DC component prior to comparison with the reference voltage, and from the separated DC component, a change in the peak value due to the change,
A corrected direct current component that cancels the difference with the change in the peak value caused by the change in the AC component is derived and output, and the corrected direct current component and the separated AC component are added to obtain a corrected bias AC voltage. The present invention provides a temperature compensation method for a metal detection device in an excavator, which performs a comparison with the reference voltage.

The above method includes a bridge circuit whose components include a pair of coils built into the tip of the excavator, an oscillator that supplies a balancing alternating current signal to the bridge circuit, and a biasing direct current signal to the alternating current signal. A DC bias circuit that supplies superimposed signals; a separation circuit that separates AC and DC components from the bias AC voltage output from the bridge circuit; a correction circuit that multiplies the separated DC component by a coefficient and outputs the result; and the correction circuit. This can be achieved by using a metal detection circuit comprising an adder circuit that adds the output of the above and the separated AC component, and a comparison circuit that compares the output signal of the adder circuit with a reference voltage.

[0013]

[Function] As mentioned above, changes in the temperature around the coil
This causes a change in the DC resistance and a change in inductance of the coil, and this change in DC resistance appears as a change in the DC component of the bias AC voltage, and the change in inductance appears as a change in the amplitude of the AC component of the bias AC voltage. Therefore, the changes in the DC component and the amplitude of the AC component are each a function of temperature.

On the other hand, when the temperature is constant, even if the bridge circuit becomes unbalanced due to the proximity of a buried metal object and an AC voltage is generated at the output, the DC component of the bias AC voltage does not change. In other words, the DC component of the output of the bridge circuit is not affected by the change in inductance mentioned above, but changes in response to a linear change in the DC resistance of the coil that corresponds to a change in ambient temperature. is a function corresponding to the temperature around the coil. Therefore, a change in the amplitude of the AC component corresponding to a change in the temperature around the coil can be regarded as a function of the DC component, that is, it can be derived as a correspondence relationship with the DC component.

As described above, the variation ΔVp=ΔVdc−ΔVac of the peak value of the bias AC voltage compared with the reference voltage, which corresponds to the change in the temperature around the coil, is derived as a correspondence relationship with the DC component. Therefore, if the DC component of the bias AC voltage is shifted by this amount of change and then compared with the reference voltage, the change in the peak value due to temperature change can be compensated for as shown in FIGS. 2 and 3. be able to.

To shift the DC component, first, the bias AC voltage output from the bridge circuit is separated into an AC component and a DC component before being compared with the reference voltage, and the above change ΔVp is calculated from the separated DC component. This can be done by subtracting the corrected direct current component to derive a corrected direct current component, and then adding this corrected direct current component to the above-mentioned alternating current component again.

[0017] To derive the corrected direct current component from the separated direct current component, any suitable method can be applied as long as the above-mentioned change is substantially subtracted. is stored in the storage means of the correction circuit in the form of an approximation formula, data table, etc., and direct derivation can be performed by multiplying the separated DC component by a coefficient based on the correspondence relationship.

That is, in this method, the corrected direct current component Vdc
' is the DC voltage separated from the bias AC voltage, Vdc
Then, it can be derived as Vdc'=f(Vdc)·Vdc. Note that this f(Vdc) represents a coefficient based on the above-mentioned correspondence relationship, which may be obtained by approximating the correspondence relationship obtained by prior measurement using a constant or a formula, or by storing it as a data table.

[0019]

Embodiments Next, embodiments of the present invention will be explained with reference to the drawings. FIG. 1 shows an embodiment of a detection circuit to which the present invention is applied. Reference numerals 1a and 1b indicate a pair of coils built into a bit (not shown) at the tip of an excavator, and 2 indicates this pair of coils. This is a bridge circuit whose constituent elements are coils 1a and 1b. 3 is an oscillator that supplies a balance AC signal to this bridge circuit 2; 4 is a DC bias circuit that superimposes on the AC signal from the oscillator 3 and supplies a bias DC signal to the bridge circuit 2; and 5 is a DC bias circuit. The power supply 6 is an adder circuit.

Reference numeral 7 denotes a separation circuit connected to the output side of the bridge circuit 2 to separate the output voltage of the bridge circuit 2 into alternating current and direct current components. Note that an appropriate amplifier circuit is connected to the input side of the separation circuit 7, but is not shown. Reference numeral 8 denotes a correction circuit connected to the DC output side of the separation circuit 7. This correction circuit 8 stores a coefficient f (Vdc) based on the above-mentioned correspondence relationship, and adjusts the DC voltage Vdc input from the separation circuit 7. On the other hand, Vd
The configuration is such that a corrected direct current voltage Vdc' that satisfies the relationship c'=f(Vdc)·Vdc is generated. As described above, the coefficients are approximated by constants or formulas or stored as a data table based on the correspondence relationship obtained through previous measurements.

The AC component Vac separated in the separation circuit 7 is then input to the addition circuit 9 where it is added to the corrected direct current voltage Vdc', and then compared with the reference voltage in the comparison circuit 10. This reference voltage can be set to a ground voltage or any other appropriate voltage.

In the above configuration, for example, as shown in the figure, although a bias AC voltage due to the unbalance is output to the output side of the bridge circuit 2 at the temperature t1, its peak value is the reference value. If the temperature around the coils 1a and 1b rises in a state where the comparator circuit 10 is not outputting a signal corresponding to metal detection, the DC resistance of the coils 1a and 1b will change and the inductance will increase. The peak voltage gradually decreases as the temperature increases. Therefore, when the bias AC voltage as it is is compared with the reference voltage in the comparator circuit 10,
The peak voltage drops to the reference voltage at temperature t2, and the comparator circuit 10 outputs a signal corresponding to metal detection.

However, in the configuration of the present invention described above, the bias AC voltage is separated by the separation circuit 7 and corrected by the correction circuit 8 in accordance with the ambient temperature before comparison with the reference voltage. Since a direct current voltage is generated and this corrected direct current voltage is added to the separated alternating current component, the peak voltage can be maintained approximately constant regardless of temperature changes, as shown in FIGS. 2 and 3. Erroneous detection due to temperature changes can be prevented.

[0024]

Effects of the Invention Since the present invention is as described above, changes in the output voltage of the bridge circuit caused by changes in the temperature around the coil can be detected by changes in the direct current resistance of the coil itself without using a temperature sensor such as a thermistor. It can be corrected using
Therefore, the circuit configuration is simpler than when using a temperature sensor such as a thermistor as described above, and a metal detection circuit with reliable detection operation can be constructed at low cost and in a small size, and power consumption does not increase. This has the effect of enabling long-term detection operation using batteries.

[Brief explanation of the drawing]

FIG. 1 is a system explanatory diagram showing the configuration of a metal detection circuit to which the present invention is applied.

FIG. 2 is an explanatory diagram showing the operation of the method of the present invention in terms of the relationship between temperature and voltage values.

FIG. 3 is an explanatory diagram showing the operation of the method of the present invention in the state of bias AC voltage.

FIG. 4 is an explanatory diagram showing the operation of the conventional method in the state of bias AC voltage.

FIG. 5 is an explanatory diagram showing the concept of metal detection in an excavator.

FIG. 6 is a system explanatory diagram showing the configuration of a conventional metal detection circuit.

[Explanation of symbols]

1a Coil 1b Coil 2 Bridge circuit 3
Oscillator 4 DC bias circuit 5
DC power supply 6 Addition circuit 7 Separation circuit 8 Correction circuit 9 Addition circuit 10 Comparison circuit 11 Excavation rod 12
Bit 13 Metal buried object 14
Amplifier circuit 15 Comparison circuit

Claims (2)

[Claims] Claim 1: A balance alternating current signal and a bias direct current signal are applied in a superimposed manner to a bridge circuit consisting of a pair of coils built into the tip of the excavator, and the bias alternating current generated by the unbalance of this bridge circuit is applied. In a metal detection device that detects the proximity of metal by detecting a change in the peak value of the voltage by comparing it with a reference voltage, the bias AC voltage is detected as an AC component before being compared with the reference voltage. The DC component is separated from the DC component, and from this separated DC component,
A corrected normal current component that cancels the difference between the change in the peak value caused by the change and the change in the peak value caused by the change in the AC component is derived and output, and this corrected normal current component and the above A temperature compensation method for a metal detection device in an excavator, characterized in that the separated AC components are added together to obtain a corrected bias AC voltage, and then compared with the reference voltage. 2. A bridge circuit comprising a pair of coils built into the tip of the excavator, an oscillator supplying a balancing alternating current signal to the bridge circuit, and a biasing direct current signal superimposed on the alternating current signal. a DC bias circuit to supply, a separation circuit that separates an AC component and a DC component from the bias AC voltage output from the bridge circuit, a correction circuit that multiplies the separated DC component by a coefficient and outputs the result, and an output of the correction circuit. A metal detection circuit equipped with a temperature compensation mechanism, comprising an addition circuit that adds the separated AC components, and a comparison circuit that compares the output signal of the addition circuit with a reference voltage.