Measuring device

Abstract

To appropriately output a measurement amount while suppressing output saturation of a detection part.SOLUTION: A measuring apparatus for detecting a current flowing through a measuring object 9 includes a detection unit for detecting the magnitude of the current flowing through the measuring object 9, and the detection unit includes an operational amplifier 21 for amplifying a signal input from the measuring object 9, a feedback resistor 22 disposed in a feedback path Lf1 of the operational amplifier 21, and a capacitance circuit capable of connecting a capacitance Cp in parallel with the feedback resistor 22. The measuring device further includes a control part 43 for controlling the operation of the capacitance circuit so as to connect the electrostatic capacitance Cp in parallel to the feedback resistance element 22 when the output signal of the detection part exceeds a predetermined operation range, and a calculation part 42 for calculating the measurement quantity related to the measuring object 9 based on the output signal of the detection part.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a measuring device that detects a current flowing through an object to be measured. [Background technology]

[0002] Patent Document 1 discloses a current measuring device that is controlled to switch to one of a plurality of measurement ranges and converts the current flowing through the object to be measured into a voltage at a conversion rate according to the one measurement range. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-250831 Summary of the Invention [Problem to be solved by the invention]

[0004] In the detector that converts current to voltage as described above, AC noise may be superimposed on the current input from the object to be measured. In such a situation, it is possible to remove the AC noise superimposed on the input current using a low-pass filter or the like.

[0005] However, if the level of AC noise superimposed on the input current is extremely greater than the level of the input current itself, even if the AC noise is removed using a low-pass filter or the like, the output of the detection unit will become saturated and the measurement quantity will not be obtained correctly.

[0006] The present invention has been made in view of these problems, and has as its object to appropriately output a measurement quantity while suppressing output saturation of the detection unit. [Means for solving the problem]

[0007] According to one aspect of the present invention, a measurement device for detecting a current flowing through an object to be measured includes an operational amplifier for amplifying a signal input by the object to be measured, a feedback resistor element disposed in a feedback path of the operational amplifier, and a capacitance circuit for connecting a capacitance in parallel with the feedback resistor element, and further includes a detection unit for detecting the magnitude of the current flowing through the object to be measured. The measurement device also includes a control unit for controlling the operation of the capacitance circuit so as to connect the capacitance in parallel with the feedback resistor element when an output signal from the detection unit exceeds a predetermined operating range, and a calculation unit for calculating a measurand related to the object to be measured based on the output signal from the detection unit. [Effects of the Invention]

[0008] According to this aspect, the measurement device can estimate whether or not the output signal of the detection unit exceeds a predetermined operating range, thereby estimating whether or not the output of the operational amplifier is saturated.

[0009] If the measuring device estimates that output saturation has occurred in the operational amplifier, it connects a capacitance in parallel to the feedback resistor of the operational amplifier. This reduces the AC impedance of the detection unit itself with respect to the AC noise component of the signal input to the operational amplifier, thereby suppressing output saturation of the operational amplifier caused by AC noise.

[0010] Therefore, when the output signal of the detection unit falls within a predetermined operating range, the calculation unit calculates the measurement quantity based on the output signal of the detection unit in which output saturation does not occur in the operational amplifier, so that an accurate measurement quantity can be output.

[0011] In this way, according to this aspect, it is possible to appropriately output the measured amount while suppressing output saturation of the detection unit. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a circuit diagram showing the circuit configuration of an insulation resistance tester according to the first embodiment. [Figure 2]FIG. 2 is a flowchart showing a measurement method using an insulation resistance meter. [Figure 3] FIG. 3 is a diagram for explaining a change in the output signal of the current detection unit when a capacitance is connected in parallel to the feedback resistance element of the operational amplifier. [Figure 4] FIG. 4 is a circuit diagram showing the circuit configuration of an insulation resistance tester according to the second embodiment. [Figure 5] FIG. 5 is a circuit diagram showing the circuit configuration of an insulation resistance tester according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In this specification, the same or equivalent elements are designated by the same reference numerals throughout.

[0014] (First embodiment) FIG. 1 is a circuit diagram showing the circuit configuration of an insulation resistance tester 1 according to the first embodiment.

[0015] Insulation resistance meter 1 is a measuring device that detects the current flowing through a measurement object 9, and measures the insulation resistance of, for example, a household distribution board, power receiving equipment in buildings and factories, transformers, motors, etc. The insulation resistance values ​​that can be measured by insulation resistance meter 1 are, for example, several MΩ to several TΩ.

[0016] Insulation resistance meter 1 applies output voltage Vout, which is a DC voltage, from output terminal T1 to object under test 9, and in this state detects the magnitude of leakage current flowing from object under test 9 to detection terminal T2. Insulation resistance meter 1 then calculates the insulation resistance of object under test 9 by dividing the voltage value of output voltage Vout by the detected value of the leakage current.

[0017] A pair of cables with probes at the tips are connected to the output terminal T1 and the detection terminal T2, respectively, and the pair of probes are brought into contact with both ends of the object 9 to be measured by the measurer.

[0018] The insulation resistance meter 1 of the first embodiment performs constant current control to keep the leakage current flowing through the object to be measured 9 constant when the output voltage Vout is less than a predetermined voltage value, and performs constant voltage control to keep the output voltage Vout constant when the output voltage Vout reaches the predetermined voltage value.

[0019] The insulation resistance meter 1 may be configured to perform constant current control to keep the leakage current flowing through the object to be measured 9 constant when the insulation resistance of the object to be measured 9 is less than a predetermined resistance value, and to perform constant voltage control to keep the output voltage Vout constant when the insulation resistance of the object to be measured 9 is equal to or greater than the predetermined resistance value.

[0020] The insulation resistance meter 1 includes a voltage generating unit 10, a current detecting unit 20, an AD converting unit 30, a processing unit 40, and a display unit 50. The processing unit 40 includes a saturation detecting unit 41, a calculating unit 42, and a control unit 43.

[0021] The voltage generating unit 10 is a voltage generating circuit that applies an output voltage Vout to the object under test 9. The voltage generating unit 10 is realized by a known circuit configuration that includes, for example, a power supply, a transformer, a switching element that intermittently connects the power supply to the primary winding of the transformer, and a smoothing circuit that smooths the AC voltage of the secondary winding of the transformer.

[0022] The current detection unit 20 is a detection unit that detects the magnitude of the current flowing through the measurement object 9. The current detection unit 20 generates a detection signal that indicates the magnitude of the detected current, and outputs the generated detection signal.

[0023] In the first embodiment, current detection unit 20 detects the magnitude of leakage current flowing in from detection terminal T2 of insulation resistance meter 1. Current detection unit 20 then supplies a detection signal indicating the magnitude of the detected leakage current as an output signal of current detection unit 20 to AD conversion unit 30 and processing unit 40. Current detection unit 20 can detect current values ​​from several nA to several mA, for example.

[0024] The current detection unit 20 includes an operational amplifier 21, a feedback resistance element 22, feedback resistance circuits 23 to 25, and a parallel capacitance circuit 26. The current detection unit 20 configures a current-voltage conversion circuit by using the operational amplifier 21, the feedback resistance element 22, and the feedback resistance circuits 23 to 25, and outputs a voltage signal by converting the input current into a voltage drop across the feedback resistance element 22 and the feedback resistance circuits 23 to 25.

[0025] The operational amplifier 21 is an amplifier that amplifies a signal input by a current flowing through the object to be measured 9. In the first embodiment, the inverting input terminal (-) of the operational amplifier 21 is connected to the detection terminal T2 of the insulation resistance meter 1, and the non-inverting input terminal (+) of the operational amplifier 21 is connected to the ground potential G. The ground potential G is used as a reference potential that serves as a basis for the operation of the insulation resistance meter 1.

[0026] The output voltage range of the operational amplifier 21 is designed, for example, such that the upper limit voltage is from a positive several volts to a positive several tens of volts, and the lower limit voltage is from a negative several volts to a negative several tens of volts. In the first embodiment, the output voltage range of the operational amplifier 21 is designed, for example, to be from −15 volts to +15 volts.

[0027] The feedback resistor element 22 is a resistor element disposed in the feedback path Lf1 of the operational amplifier 21. The feedback resistor element 22 detects a current within a minute current range of several nA to several tens of nA, and the resistance value of the feedback resistor element 22 is designed to be several MΩ, for example.

[0028] The feedback resistance circuits 23 to 25 are series circuits in which switches 231 to 251 capable of forming feedback paths Lf2 to Lf4 of the operational amplifier 21 and feedback resistance elements 232 to 252 arranged in the feedback paths Lf2 to Lf4 are connected in series, respectively.

[0029] Specifically, the feedback resistance circuit 23 is configured with a switch 231 capable of forming a feedback path Lf2 for the operational amplifier 21 and a feedback resistance element 232 arranged in the feedback path Lf2. The feedback resistance circuit 24 is configured with a switch 241 capable of forming a feedback path Lf3 for the operational amplifier 21 and a feedback resistance element 242 arranged in the feedback path Lf3. The feedback resistance circuit 25 is configured with a switch 251 capable of forming a feedback path Lf4 for the operational amplifier 21 and a feedback resistance element 252 arranged in the feedback path Lf4.

[0030] The switches 231 to 251 respectively connect the feedback resistance elements 232 to 252 in parallel to the feedback resistance element 22 or cut off the parallel connection in accordance with a command from the control unit 43 in the processing unit 40 .

[0031] The feedback resistor elements 232 to 252 are used as resistor elements for detecting currents within a small current range of, for example, several hundred nA to several nA, a medium current range of from the upper limit of the small current range to several hundred μA, and a large current range of from the upper limit of the medium current range to several mA.

[0032] The resistance values ​​of the feedback resistance elements 232 to 252 are different from one another and are smaller than the resistance value of the feedback resistance element 22. In the first embodiment, the resistance values ​​of the feedback resistance elements 232 to 252 are designed so that the resistance values ​​decrease in the order of the feedback resistance element 232, the feedback resistance element 242, and the feedback resistance element 252.

[0033] The parallel capacitance circuit 26 is a capacitance circuit capable of connecting a capacitance Cp in parallel to the feedback resistance element 22. The parallel capacitance circuit 26 is a series circuit in which a switch 261 capable of forming a parallel connection path of the capacitance Cp to the feedback resistance element 22 and a capacitance element 262 having the capacitance Cp are connected in series with each other.

[0034] The switch 261 connects the capacitance element 262 in parallel with the feedback resistance element 22 or cuts off the parallel connection in accordance with a command from the control unit 43 .

[0035] Capacitor element 262 is an element for passing AC noise from the voltage input to current detection unit 20. Examples of AC noise include induced noise generated by electric wires present around insulation resistance meter 1 or measurement target 9. This induced noise enters from the input terminal of current detection unit 20 and can reach several hundred volts near high-voltage electrical lines, for example.

[0036] In the capacitive element 262 of the first embodiment, the capacitance Cp is designed so that inductive noise as AC noise passes through.

[0037] Specifically, the capacitance Cp is a value determined based on the frequency of the induced noise and the output voltage range of the operational amplifier 21. More specifically, the capacitance Cp is determined in advance so that the AC impedance obtained by the expected amplitude and frequency of the induced noise falls within the output voltage range of the operational amplifier 21.

[0038] In the first embodiment, the frequency of the induced noise is 50 Hz or 60 Hz, and the upper and lower limits of the output voltage range are +15 V and −15 V. In such a case, the capacitance Cp is designed to be, for example, several μF.

[0039] Next, the connections between the components that make up the current detection unit 20 will be described.

[0040] One end of each of feedback resistance elements 22, 232, 242, and 252 is connected to the inverting input terminal (-) of the operational amplifier 21. The other ends of the feedback resistance elements 232, 242, and 252 are connected to one ends of switches 231, 241, and 251, respectively, and one end of each of the switches 231, 241, and 251 and the other end of the feedback resistance element 22 are connected to the output terminal of the operational amplifier 21.

[0041] In addition, one end of a capacitance element 262 is connected to the inverting input terminal (−) of the operational amplifier 21, the other end of the capacitance element 262 is connected to one end of a switch 261, and the other end of the switch 261 is connected to the output terminal of the operational amplifier 21.

[0042] The AD conversion section 30 converts the output signal of the current detection section 20 from an analog signal to a digital signal, and outputs the converted digital signal to the calculation section 42 of the processing section 40.

[0043] The processing unit 40 is a computer that includes an arithmetic processing unit configured with one or more processors, a storage device configured with ROM and RAM, an input device that accepts input operations from the user, and a network connection device.

[0044] The saturation detection unit 41 detects output saturation of the operational amplifier 21 based on the output signal of the current detection unit 20. In the first embodiment, the saturation detection unit 41 determines whether the output signal of the operational amplifier 21 in the current detection unit 20 exceeds a predetermined operating range. This makes it possible to estimate whether excessive induced noise is mixed into the voltage input to the current detection unit 20, causing the operational amplifier 21 to be in a saturated state or a state where it may become saturated.

[0045] The predetermined operating range is determined based on the upper and lower limits of the output voltage range of the operational amplifier 21, and is set to a voltage range that is, for example, several tens of percent narrower than the output voltage range. In the first embodiment, the lower limit voltage of the operating range is set to −12 V, and the upper limit voltage is set to +12 V.

[0046] If the output signal of the operational amplifier 21 exceeds a predetermined operating range, the saturation detector 41 determines that the operational amplifier 21 is in a saturated state or is likely to be saturated, and generates and outputs a saturation detection signal.

[0047] On the other hand, if the output signal of the operational amplifier 21 does not exceed the predetermined operating range, the saturation detector 41 determines that the operational amplifier 21 is not in a saturated state or a state where it may become saturated, and generates and outputs a desaturation detection signal.

[0048] The saturation detector 41 is realized by, for example, a window comparator. The window comparator functioning as the saturation detector 41 compares the output signal of the operational amplifier 21 with a predetermined operating range.

[0049] The window comparator outputs a high-level signal as a saturation detection signal when the output signal of the operational amplifier 21 exceeds a predetermined operating range, and outputs a low-level signal as a desaturation detection signal when the output signal of the operational amplifier 21 does not exceed the predetermined operating range.

[0050] The calculation unit 42 calculates a measurement quantity related to the object 9 to be measured based on the output signal of the current detection unit 20.

[0051] In the first embodiment, the calculation unit 42 calculates, as a measurement quantity, the current value of the leakage current flowing through the object to be measured 9, using the output signal of the AD conversion unit 30. Furthermore, the calculation unit 42 divides the set value or detected value of the output voltage Vout applied to the object to be measured 9 by the voltage generation unit 10 by the output signal of the AD conversion unit 30, to calculate, as a measurement quantity, the insulation resistance value of the object to be measured 9.

[0052] The calculation unit 42 may also calculate the power consumption value of the object to be measured 9 as the measured quantity by multiplying the set value or detected value of the output voltage Vout by the output signal of the AD conversion unit 30. In this way, the calculation unit 42 performs calculation processing of the measured quantity. The calculation unit 42 outputs the calculated measured quantity to the control unit 43.

[0053] The control unit 43 selects an appropriate measurement range from among the minute current range, small current range, medium current range, and large current range based on the output signal of the current detection unit 20, and controls the operation of the feedback resistance circuits 23 to 25 according to the selected measurement range. The output signal of the current detection unit 20 is supplied via, for example, a saturation detection unit 41. The control unit 43 may also control the operation of the feedback resistance circuits 23 to 25 based on a user's input operation received by an input device (not shown).

[0054] Furthermore, the control unit 43 controls the operation of the parallel capacitance circuit 26 based on the output signal of the current detection unit 20 .

[0055] Specifically, when the output signal of the current detection unit 20 falls within a predetermined operating range, the control unit 43 controls the operation of the parallel capacitance circuit 26 to interrupt the parallel connection between the feedback resistance element 22 and the capacitance Cp. On the other hand, when the output signal of the current detection unit 20 exceeds the predetermined operating range, the control unit 43 controls the operation of the parallel capacitance circuit 26 to connect the capacitance Cp in parallel to the feedback resistance element 22.

[0056] In the first embodiment, when the control unit 43 receives a desaturation detection signal from the saturation detection unit 41, it sets the internal state of the switch 261 to a non-conductive state so as to interrupt the parallel connection between the capacitance Cp and the feedback resistance element 22. This prevents an increase in the time constant of the current detection unit 20, which is obtained by multiplying the resistance value of the feedback resistance element 22 by the capacitance Cp, and thus prevents an increase in measurement time due to a decrease in response speed.

[0057] Then, the control unit 43 instructs the calculation unit 42 to execute calculation processing. That is, the control unit 43 outputs the calculated measurement amount to the display unit 50 when the output signal of the current detection unit 20 falls within a predetermined operating range.

[0058] On the other hand, when the control unit 43 receives a saturation detection signal from the saturation detection unit 41, it switches the internal state of the switch 261 from a non-conductive state to a conductive state so that the capacitance Cp is connected in parallel to the feedback resistance element 22. In other words, when the output signal of the current detection unit 20 exceeds a predetermined operating range, the control unit 43 connects the capacitive element 262 of the capacitance Cp in parallel to the feedback resistance element 22.

[0059] As a result, excessively large inductive noise components in the input voltage of the current detection unit 20 pass through the capacitive element 262, and therefore, input of excessively large inductive noise to the inverting input terminal (−) of the operational amplifier 21 is suppressed.

[0060] After the capacitance Cp is connected in parallel to the feedback resistance element 22, when the control unit 43 receives a saturation detection signal from the saturation detection unit 41, the control unit 43 suppresses the execution of calculation processing in the calculation unit 42 or controls the operation of the display unit 50 to display the fact that measurement is impossible. In other words, after the control unit 43 connects the capacitance element 262 in parallel to the feedback resistance element 22, when the output signal of the current detection unit 20 exceeds a predetermined operating range, the control unit 43 does not display the measurement amount on the display unit 50.

[0061] On the other hand, after the capacitance Cp is connected in parallel to the feedback resistance element 22, if the control unit 43 receives a desaturation detection signal from the saturation detection unit 41, it instructs the calculation unit 42 to execute calculation processing. That is, after connecting the capacitance element 262 in parallel to the feedback resistance element 22, the control unit 43 outputs the calculated measurement amount to the display unit 50 if the output signal of the current detection unit 20 falls within a predetermined operating range.

[0062] The display unit 50 is a display device that displays the measured quantity calculated by the calculation unit 42. Furthermore, if the output signal of the current detection unit 20 exceeds a predetermined operating range, the display unit 50 displays error information in accordance with a command from the control unit 43. Examples of the error information include a message indicating that measurement is impossible or an error code.

[0063] As described above, in the first embodiment, insulation resistance meter 1 is configured as shown in Fig. 1. In the example shown in Fig. 1, the function of saturation detection unit 41 is provided in processing unit 40, but saturation detection unit 41 may be installed independently of processing unit 40. For example, saturation detection unit 41 installed independently of processing unit 40 may be connected to the output terminal of current detection unit 20, and may supply the result of detecting output saturation of operational amplifier 21 from the output signal of current detection unit 20 to control unit 43 of processing unit 40. Also, voltage generation unit 10 may be omitted from insulation resistance meter 1.

[0064] Next, the operation of the insulation resistance tester 1 will be described with reference to FIG.

[0065] Fig. 2 is a flowchart showing an example of the processing steps for outputting a measurement quantity using insulation resistance meter 1. The example shown in Fig. 2 assumes that the output of operational amplifier 21 is saturated due to excessive inductive noise when the user has set the measurement range to the minute current range.

[0066] In step S1, the processing unit 40 disconnects the parallel-connected capacitance element 262 from the feedback resistance element 22. Specifically, the control unit 43 of the processing unit 40 sets the internal state of the switch 261 to a non-conductive state.

[0067] In step S2, processing unit 40 determines whether the detection signal indicating the magnitude of the leakage current input to current detection unit 20 is within a predetermined operating range. That is, saturation detection unit 41 of processing unit 40 determines whether the output signal of current detection unit 20 exceeds the predetermined operating range.

[0068] If the output signal of the current detection unit 20 is within the predetermined operating range in step S2, the processing unit 40 proceeds to the process of step S3.

[0069] In step S3, the processing unit 40 calculates a measurement quantity related to the object of measurement 9 based on the output signal of the AD conversion unit 30. For example, the calculation unit 42 of the processing unit 40 calculates the current value of the leakage current flowing through the object of measurement 9 as the measurement quantity using the output signal of the AD conversion unit 30. Then, the control unit 43 of the processing unit 40 outputs the calculated measurement quantity to the display unit 50 as the calculation result.

[0070] On the other hand, if the output signal of the current detection unit 20 is outside the predetermined operating range in step S2, the processing unit 40 proceeds to the process of step S4.

[0071] In step S4, the processing unit 40 connects the capacitance element 262 in parallel to the feedback resistance element 22. Specifically, the control unit 43 of the processing unit 40 switches the internal state of the switch 261 from the non-conductive state to the conductive state.

[0072] In step S5, the processing unit 40 determines whether the output signal of the current detection unit 20 is outside the predetermined operating range. That is, the saturation detection unit 41 of the processing unit 40 determines whether the output signal of the current detection unit 20 exceeds the predetermined operating range.

[0073] If it is determined in step S5 that the output signal of the current detection unit 20 is within the predetermined operating range, the processing unit 40 proceeds to the process of step S3 and calculates the measurement quantity.

[0074] On the other hand, if the output signal of the current detection unit 20 is outside the predetermined operating range in step S5, the processing unit 40 proceeds to the process of step S6.

[0075] In step S6, the processing unit 40 generates error information indicating that measurement is impossible. For example, the processing unit 40 outputs an error code to the display unit 50 as the calculation result.

[0076] In step S7, the display unit 50 displays the measurement amount calculated in step S3 or the error information generated in step S6.

[0077] When the process of step S7 is completed, the series of processing steps for the measurement quantity output method shown in FIG. 2 is completed.

[0078] 2, capacitance Cp is connected in parallel to feedback resistor element 22 when the output signal of current detection unit 20 exceeds a predetermined operating range, regardless of the range of the measurement range, but this is not limited to this. For example, since the measurement range in which output saturation of current detection unit 20 is most likely to occur is the minute current range, capacitance Cp may be connected in parallel to feedback resistor element 22 when the output signal of current detection unit 20 exceeds the predetermined operating range only when the measurement range is set to the minute current range.

[0079] Next, a change in the output signal of the current detection section 20 that accompanies the execution of the process for suppressing output saturation of the current detection section 20 will be described with reference to FIG.

[0080] FIG. 3 is a diagram for explaining a change in the output signal of the current detection unit 20 when a capacitance Cp is connected in parallel to the feedback resistance element 22 of the operational amplifier 21. In FIG.

[0081] 3(a) shows the change over time of the output signal of the current detection unit 20 when output saturation occurs, and FIG. 3(b) shows the change over time of the output signal of the current detection unit 20 when capacitance Cp is connected in parallel to the feedback resistance element 22. The vertical axis of FIG. 3(a) and (b) represents the output voltage of the operational amplifier 21, and the horizontal axis represents time t.

[0082] Also, Figure 3(a) shows the lower limit voltage Vl and upper limit voltage Vu of the output voltage range R1 of the operational amplifier 21, and Figure 3(b) shows the lower limit threshold Tl and upper limit threshold Tu of the operating range R2 for detecting output saturation of the operational amplifier 21.

[0083] 3(a), inductive noise, which is AC noise, is superimposed on the detection voltage Vd, which has a voltage value proportional to the magnitude of the leakage current, which is a signal component, in the output signal of the operational amplifier 21. In this example, the amplitude of the induced noise is excessively large, and part of the output signal of the operational amplifier 21 drops to the lower limit voltage Vl and remains constant, causing output saturation of the operational amplifier 21.

[0084] In such a situation, the calculation unit 42 performs averaging or low-pass filtering on the output signal of the AD conversion unit 30, so the voltage value after processing deviates from the voltage value of the detection voltage Vd, making it impossible to obtain an accurate measurement quantity.

[0085] 3(a) exceeds the predetermined operating range R2, the saturation detection unit 41 outputs a saturation detection signal to the control unit 43. As a result, the control unit 43 connects the capacitance Cp in parallel with the feedback resistance element 22, which reduces the AC impedance of the current detection unit 20 and decreases the amplification factor of the current detection unit 20 with respect to the amplitude of the induced noise.

[0086] 3(b), the amplitude of the induced noise in the output signal of the operational amplifier 21 is reduced, and the output signal of the current detection unit 20 falls within the predetermined operating range R2. Therefore, the voltage value after processing by the calculation unit 42 becomes equivalent to the detected voltage Vd, making it possible to accurately measure the amount of leakage current of the object to be measured 9.

[0087] On the other hand, the response speed of the current detection unit 20 is slowed down because the capacitance Cp is connected in parallel to the feedback resistance element 22. As a result, the measurement time in the first embodiment is lengthened, for example, from about 1 [s] to about 30 [s].

[0088] In the example shown in FIG. 3, the operating range R2 of the output signal is set narrower than the output voltage range R1 of the operational amplifier 21, but it may be set to the same range as the output voltage range R1 of the operational amplifier 21.

[0089] Next, the effects of the first embodiment will be described.

[0090] In the first embodiment, insulation resistance meter 1 functions as a measurement device that detects a current flowing through an object to be measured 9. Insulation resistance meter 1 includes a current detection unit 20 that detects the magnitude of the current flowing through the object to be measured. Current detection unit 20 includes an operational amplifier 21 that amplifies a signal input by the object to be measured 9, a feedback resistance element 22 that is arranged in a feedback path Lf1 of operational amplifier 21, and a parallel capacitance circuit 26 that can connect a capacitance Cp in parallel to feedback resistance element 22.

[0091] Furthermore, insulation resistance meter 1 includes control unit 43 that controls the operation of parallel capacitance circuit 26 so that capacitance Cp is connected in parallel to feedback resistance element 22 when the output signal of current detection unit 20 exceeds predetermined operating range R2, and calculation unit 42 that calculates the measurement quantity related to object to be measured 9 based on the output signal of current detection unit 20.

[0092] With this configuration, insulation resistance meter 1 can estimate whether or not output saturation has occurred in operational amplifier 21 depending on whether or not the output signal from current detection section 20 exceeds predetermined operating range R2.

[0093] If insulation resistance meter 1 estimates that output saturation has occurred in operational amplifier 21, it connects capacitance Cp in parallel to feedback resistor element 22. This reduces the AC impedance in current detection unit 20 relative to the AC noise component of the signal input to operational amplifier 21, thereby preventing output saturation of the operational amplifier due to AC noise.

[0094] Therefore, when the output signal of the current detection unit 20 falls within the predetermined operating range R2, the calculation unit 42 calculates the measurement quantity based on the output signal of the current detection unit 20 in which output saturation does not occur in the operational amplifier 21, so that an accurate measurement quantity can be output.

[0095] As described above, according to the first embodiment, it is possible to appropriately output the measured amount while suppressing output saturation of the current detection unit 20.

[0096] In addition, in the first embodiment, when the output signal of the current detection unit 20 falls within a predetermined operating range R2, the control unit 43 controls the operation of the parallel capacitance circuit 26 to cut off the parallel connection between the feedback resistance element 22 and the capacitance Cp.

[0097] With this configuration, when the output signal of the current detection unit 20 falls within the predetermined operating range R2, the parallel connection between the feedback resistance element 22 and the capacitance Cp is interrupted, thereby reducing the time constant obtained by multiplying the resistance value of the feedback resistance element 22 by the capacitance Cp. This increases the response speed of the current detection unit 20, making it possible to shorten the time required for measurement.

[0098] Furthermore, in the first embodiment, after connecting the capacitance Cp in parallel to the feedback resistance element 22, if the output signal of the current detection unit 20 exceeds the predetermined operating range R2, the control unit 43 suppresses the calculation process of the measured quantity by the calculation unit 42. On the other hand, if the output signal of the current detection unit 20 falls within the predetermined operating range R2, the calculation unit 42 outputs the measured quantity calculated by the calculation unit 42.

[0099] With this configuration, when operational amplifier 21 is saturated or in a state where it may be saturated, calculation unit 42 cannot accurately determine the measurement quantity, and therefore output of the measurement quantity is suppressed, but in other cases the measurement quantity is output. This allows insulation resistance meter 1 to appropriately output an accurate measurement quantity.

[0100] In the first embodiment, insulation resistance meter 1 further includes display unit 50 that can display the calculated measurement quantity. After connecting capacitance Cp in parallel with feedback resistance element 22, control unit 43 controls the operation of display unit 50 to display a message that measurement is impossible if the output signal of current detection unit 20 exceeds predetermined operating range R2.

[0101] According to this configuration, when the output signal of the current detection unit 20 exceeds the predetermined operating range R2, the error in the measurement quantity may become large, and therefore a message is displayed indicating that the measurement is impossible, thereby making it possible to avoid displaying to the user a measurement quantity with low accuracy.

[0102] Furthermore, in the first embodiment, insulation resistance meter 1 functions as a measurement device that detects leakage current flowing through object to be measured 9 when a DC voltage is applied to object to be measured 9. As a result, even if excessive inductive noise is input to current detection unit 20, it is possible to appropriately output measured quantities such as insulation resistance while suppressing a decrease in measurement accuracy due to output saturation of operational amplifier 21.

[0103] Second Embodiment The circuit configuration of the insulation resistance meter 1 is not limited to the circuit configuration shown in Fig. 1, and the measurement method of the first embodiment can be applied to other circuit configurations as well. Therefore, a configuration having another circuit configuration will be referred to as the second embodiment and will be briefly described with reference to Fig. 4.

[0104] FIG. 4 is a circuit diagram showing the circuit configuration of an insulation resistance meter 2 according to the second embodiment.

[0105] In addition to the components of the insulation resistance meter 1 of the first embodiment, the insulation resistance meter 2 also includes an LPF 60 and a switch 61. Other components of the insulation resistance meter 2 are the same as or equivalent to the components of the insulation resistance meter 1 shown in Fig. 1, so the same reference numerals as those of the insulation resistance meter 1 are used and redundant explanations will be omitted.

[0106] The LPF 60 is a low-pass filter that performs low-pass filtering to remove noise components from the output signal of the current detection unit 20. Specifically, the LPF 60 removes induced noise components contained in the output signal of the operational amplifier 21. The cutoff frequency of the LPF 60 is designed to be lower than the frequency of the induced noise, for example, 50 Hz or 60 Hz.

[0107] The switch 61 is configured to be able to connect the LPF 60 between the current detection unit 20 and the calculation unit 42. In the first embodiment, a first input terminal of the switch 61 is connected to the output terminal of the LPF 60, and a second input terminal is connected to the output terminal of the operational amplifier 21. The output terminal of the switch 61 is connected to the input terminal of the AD conversion unit 30.

[0108] The switch 61 alternately switches the signal to be input to the AD conversion unit 30 between the output signal of the current detection unit 20 and the output signal of the LPF 60 in accordance with a command from the control unit 43. Specifically, the switch 61 internally connects its first input terminal and output terminal so as to supply the output signal of the current detection unit 20 to the input terminal of the calculation unit 42, and internally connects its second input terminal and output terminal so as to supply the output signal of the LPF 60 to the input terminal of the calculation unit 42.

[0109] The control unit 43 connects the LPF 60 between the current detection unit 20 and the calculation unit 42 based on an input operation by the user. For example, if the measured quantity displayed on the display unit 50 fluctuates, the user performs an input operation on the input device to change the noise removal function from disabled to enabled, and the control unit 43 accepts the input operation and switches the internal state between the first input terminal and the output terminal of the switch 61 from a non-conductive state to a conductive state.

[0110] As a result, the output signal of current detection unit 20 is input to LPF 60 via switch 61, where the induced noise component of the output signal of current detection unit 20 is removed, and the output signal from which the induced noise component has been removed is input to calculation unit 42 via AD conversion unit 30. This allows the user to recognize that the display unit 50 is displaying an accurate measured quantity from which induced noise has been removed.

[0111] However, if excessive inductive noise is mixed into the current detection unit 20, output saturation may occur in the operational amplifier 21 arranged in the stage preceding the LPF 60. In such a case, the output signal of the operational amplifier 21 contains not only the inductive noise but also signal distortion caused by the output saturation of the operational amplifier 21 itself, making it difficult for the LPF 60 to extract only the signal component indicating the magnitude of the leakage current from the output signal of the current detection unit 20.

[0112] To address this issue, similar to the first embodiment, the current detection unit 20 is provided with a parallel capacitance circuit 26 that can be connected in parallel to the feedback resistance element 22. In the second embodiment, the capacitance Cp of the parallel capacitance circuit 26 is determined based on the cutoff frequency of the LPF 60 that can remove induced noise and the output voltage range of the operational amplifier 21.

[0113] By disposing the parallel capacitance circuit 26 designed in this manner in the current detection unit 20, it is possible to suppress output saturation of the operational amplifier 21. In addition, if output saturation occurs in the operational amplifier 21 even when the electrostatic capacitance Cp is connected in parallel to the feedback resistance element 22, error information is displayed on the display unit 50 and the calculation process of the calculation unit 42 is stopped, thereby suppressing the output of the measured quantity calculated by the calculation unit 42.

[0114] Therefore, for a user who believes that an accurate measurement is being obtained by enabling the noise removal function of the insulation resistance meter 2, only the accurate measurement can be accurately output to the display unit 50.

[0115] In this embodiment, the LPF 60 and the switch 61 are arranged in the signal path between the current detection unit 20 and the AD conversion unit 30, but the functions of the LPF 60 and the switch 61 may also be provided in the signal path between the AD conversion unit 30 and the calculation unit 42 in the processing unit 40.

[0116] Next, the effects of the second embodiment will be described.

[0117] The insulation resistance tester 2 of the second embodiment has the same or similar configuration as that of the first embodiment, and therefore can achieve the same effects as those of the first embodiment described above.

[0118] Additionally, in the second embodiment, insulation resistance meter 2 further includes LPF 60 as a low-pass filter that removes AC noise components from the output signal of current detection unit 20. Control unit 43 then connects LPF 60 between current detection unit 20 and calculation unit 42, based on a user's input operation received by an input device (not shown).

[0119] According to this configuration, AC noise contained in the output signal of the current detection section 20 is removed, so that the measurand relating to the object to be measured 9 can be measured with high accuracy.

[0120] Furthermore, in a situation where the LPF 60 is connected between the current detection unit 20 and the calculation unit 42, it is possible that excessive AC noise may be mixed into the current detection unit 20, causing output saturation of the operational amplifier 21. Even in such a case, the control unit 43 can prevent output saturation of the operational amplifier 21 by connecting the electrostatic capacitance Cp in parallel with the feedback resistance element 22.

[0121] In addition, the control unit 43 controls the operation of the display unit 50 to display that measurement is impossible if the output signal of the current detection unit 20 exceeds a predetermined operating range R2 after connecting the electrostatic capacitance Cp in parallel to the feedback resistance element 22. Alternatively, the control unit 43 suppresses the calculation process of the measurement quantity by the calculation unit 42 if the output signal of the current detection unit 20 exceeds the predetermined operating range R2 after connecting the electrostatic capacitance Cp in parallel to the feedback resistance element 22.

[0122] These configurations make it possible to avoid a situation in which an inaccurate measurement quantity due to output saturation of the operational amplifier 21 is output to a user who believes that an accurate measurement quantity is being obtained by connecting the LPF 60 between the current detection unit 20 and the calculation unit 42.

[0123] In the second embodiment, the capacitance Cp of the parallel capacitance circuit 26 is a value determined based on the cutoff frequency of the LPF 60 capable of removing AC noise and the output voltage range of the operational amplifier 21.

[0124] According to this configuration, the AC impedance of the current detection unit 20 is reduced by the parallel capacitance circuit 26, and the amplitude of the output signal of the operational amplifier 21 is reduced, so that output saturation of the operational amplifier 21 can be suppressed.

[0125] (Third embodiment) In the above embodiment, the value of the capacitance Cp connected in parallel to the feedback resistance element 22 is fixed, but the value of the capacitance Cp may be changeable. Therefore, an embodiment in which the capacitance Cp is changeable will be described with reference to FIG.

[0126] FIG. 5 is a circuit diagram showing the circuit configuration of an insulation resistance tester 3 according to the third embodiment.

[0127] The insulation resistance meter 3 includes a current detection unit 20A instead of the current detection unit 20 of the insulation resistance meter 2 of the second embodiment, and the current detection unit 20A includes a parallel capacitance circuit 26A instead of the feedback resistance circuits 23 to 25 and the parallel capacitance circuit 26 of the current detection unit 20 shown in Figures 1 and 4.

[0128] Other components of the insulation resistance meter 3 are the same as or equivalent to those of the insulation resistance meter 2 shown in FIG. 4, and therefore are designated by the same reference numerals as those of the insulation resistance meter 1, and redundant explanations will be omitted.

[0129] The parallel capacitance circuit 26A is a capacitance circuit that can change the magnitude of the capacitance Cp connected in parallel to the feedback resistance element 22. The parallel capacitance circuit 26A includes a switch 261A and capacitance elements 262A to 262C.

[0130] The switch 261A is a selector that can connect one capacitance element selected from the three capacitance elements 262A to 262C to the feedback resistance element 22 in parallel.

[0131] The first to third input terminals of the switch 261A are connected to one end of the capacitance elements 262A to 262C, respectively, and the output terminal of the switch 261A is connected to the output terminal of the operational amplifier 21 and the other end of the feedback resistance element 22. The other ends of the capacitance elements 262A to 262C are connected to the inverting input terminal (-) of the operational amplifier 21.

[0132] The capacitance elements 262A to 262C have different capacitances, with the capacitance increasing in the order of capacitance element 262A, capacitance element 262B, and capacitance element 262C. The capacitance elements 262A to 262C are determined in advance based on the amplitude and frequency of the expected induced noise and the output voltage range of the operational amplifier 21.

[0133] When the control unit 43 receives a saturation detection signal from the saturation detection unit 41, it first controls the switch 261A to connect the capacitance element 262A in parallel to the feedback resistance element 22. In this state, when the control unit 43 receives a saturation detection signal from the saturation detection unit 41, it controls the switch 261A to connect the capacitance element 262B, which has a larger capacitance than the capacitance element 262A, in parallel to the feedback resistance element 22.

[0134] This reduces the AC impedance of the current detection unit 20 and reduces the amplitude of the output signal of the current detection unit 20 compared to when the capacitance element 262A is connected in parallel to the feedback resistance element 22. On the other hand, since the capacitance connected in parallel to the feedback resistance element 22 increases, the time constant of the current detection unit 20 increases and the measurement time becomes longer.

[0135] When the control unit 43 receives a saturation detection signal from the saturation detection unit 41 while the capacitance element 262B is connected in parallel to the feedback resistance element 22, it controls the switch 261A to connect the capacitance element 262C, which has a larger capacitance than the capacitance element 262B, in parallel to the feedback resistance element 22.

[0136] This reduces the AC impedance of the current detection unit 20 and reduces the amplitude of the output signal of the current detection unit 20 compared to when the capacitance element 262B is connected in parallel to the feedback resistance element 22. On the other hand, since the capacitance connected in parallel to the feedback resistance element 22 increases, the time constant of the current detection unit 20 increases and the measurement time becomes longer.

[0137] In this way, the control unit 43 controls the operation of the parallel capacitance circuit 26A so as to increase the capacitance Cp in stages until the output signal of the current detection unit 20 falls within the predetermined operating range R2.

[0138] Next, the effects of the third embodiment will be described.

[0139] The insulation resistance tester 3 of the third embodiment has the same or similar configuration as that of the second embodiment, and therefore can achieve the same effects as those of the second embodiment described above.

[0140] Moreover, in the third embodiment, the parallel capacitance circuit 26A is capable of changing the capacitance Cp, and the control unit 43 controls the operation of the parallel capacitance circuit 26A so as to increase the capacitance Cp in stages until the output signal of the current detection unit 20 falls within a predetermined operating range R2.

[0141] According to this configuration, by gradually increasing the capacitance Cp connected in parallel to the feedback resistor 22, it is possible to avoid unnecessarily increasing the time constant of the current detection unit 20A, thereby suppressing an increase in the time required for measurement. Therefore, compared to when the value of the capacitance Cp is fixed, it is possible to suppress the occurrence of output saturation of the current detection unit 20A while suppressing an increase in the measurement time.

[0142] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above embodiments.

[0143] In the above embodiments, the present invention is applied to insulation resistance meters 1 to 3 as measuring devices, but is not limited to this, and the above embodiments can also be applied to measuring devices such as clamp meters, testers, digital multimeters, etc. that measure AC current flowing through an electric wire as a measurement object. Examples of AC noise include induced noise from high-voltage power lines that exist around the measurement object, as well as harmonic noise generated from a supply circuit in a measuring device that has the supply circuit supplying an AC signal to the measurement object. [Explanation of symbols]

[0144] 1~3 Insulation resistance meter (measuring device) 2. Measurement Object 20 Current detection unit (detection unit) 21 Operational Amplifiers 22, 232 to 252 Feedback resistor 26, 26A parallel capacitor circuit (capacitor circuit) 262, 262A to 262C Capacitance elements 42 Arithmetic section 43 Control Unit 50 Display section 60 LPF (low pass filter) Cp capacitance Lf1 return path R1 Output voltage range R2 Operating range (predetermined operating range)

Claims

1. A measuring device for detecting a current flowing through a measurement object, a detection unit that includes an operational amplifier that amplifies a signal input by the object to be measured, a feedback resistor element that is arranged in a feedback path of the operational amplifier, and a capacitance circuit that can connect an electrostatic capacitance to the feedback resistor element in parallel, and that detects the magnitude of a current flowing through the object to be measured; a control unit that controls the operation of the capacitance circuit so as to connect the electrostatic capacitance in parallel with the feedback resistor element when the output signal of the detection unit exceeds a predetermined operating range; a calculation unit that calculates a measurement amount related to the measurement object based on an output signal from the detection unit; A measuring device comprising:

2. 2. The measuring device according to claim 1, the control unit controls the operation of the capacitance circuit to cut off the parallel connection between the feedback resistance element and the capacitance when the output signal of the detection unit falls within the predetermined operating range. Measuring equipment.

3. 2. The measuring device according to claim 1, further comprising a low-pass filter for removing noise components from the output signal of the detection unit; the control unit connects the low-pass filter between the detection unit and the calculation unit based on an input operation by a user. Measuring equipment.

4. 4. The measuring device according to claim 3, the capacitance is a value determined based on the cutoff frequency of the low-pass filter and the output voltage range of the operational amplifier; Measuring equipment.

5. 2. The measuring device according to claim 1, the control unit, after connecting the capacitance in parallel with the feedback resistance element, suppresses the calculation of the measured quantity by the calculation unit when the output signal of the detection unit exceeds the predetermined operating range; the calculation unit outputs the measured quantity when the output signal of the detection unit falls within the predetermined operating range. Measuring equipment.

6. 6. The measuring device according to claim 5, Further provided is a display unit capable of displaying the measured amount, the control unit controls the operation of the display unit to display a message that measurement is impossible when the output signal of the detection unit exceeds the predetermined operating range after connecting the capacitance in parallel with the feedback resistance element. Measuring equipment.

7. 2. The measuring device according to claim 1, the capacitance circuit is capable of changing the capacitance; the control unit controls the operation of the capacitance circuit to increase the capacitance stepwise until the output signal of the detection unit falls within the predetermined operating range. Measuring equipment.

8. 2. The measuring device according to claim 1, a measuring device including an insulation resistance meter that detects leakage current flowing through the object to be measured when a DC voltage is applied to the object to be measured;

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