JP3309380B2 - Digital measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital measuring instrument having a self-diagnosis function.

[0002]

2. Description of the Related Art Various measuring instruments for measuring a voltage, a current, a resistance or the like and digitally displaying the measured values have been provided.
Such a measuring instrument is usually provided with a plurality of input ranges so that measurement can be performed in an optimum state.
On the other hand, a digital measuring instrument having a built-in self-diagnosis function for automatically checking whether or not a recent range is operating normally has also been provided.

FIG. 4 is a configuration diagram of an example of a conventional measuring instrument having a self-diagnosis function. 4, reference numeral 1 denotes a terminal to which an input Vin to be measured is applied, 2 denotes a reference voltage source for generating a reference voltage Vref, and 3 denotes a changeover switch. Reference numeral 4 denotes a range switching circuit having three stages of switching.
, The resistors 43 to 45 and the switches 46 to 4 connected to the feedback circuit of the amplifier 41
It consists of eight. 5 is a programmable gain amplifier,
Reference numeral 6 denotes an A / D converter, 7 denotes a CPU (microprocessor), and 8 denotes a display unit.

[0004] In a measuring instrument comprising such components,
When the input is measured, by connecting the changeover switch 3 to the NC side, the input Vi to be measured applied to the terminal 1 is measured.
The current corresponding to n flows into feedback resistors 43 to 45 via an input resistor 42 of an amplifier 41 constituting the range switching circuit 4 and is converted into a voltage. By switching the switches 46 to 48 connected to the feedback resistors 43 to 45, respectively, the input range is switched in three stages, for example, at × 1, × 10, and × 100.

As described above, the current flowing through the input resistor 42 is converted into a voltage at the specified magnification of the range switching circuit 4 and is taken into the CPU 7 via the programmable gain amplifier 5 and the A / D converter 6. At the time of input measurement, the gain of the programmable gain amplifier 5 is set to 1. The CPU 7 reads the output of the A / D converter 6 and calculates a measured value together with switching of each switch of the input changeover switch 3 and the range changeover circuit 4, and displays the result on the display means 8.

Next, the operation at the time of self-diagnosis for automatically checking whether or not the range is operating normally will be described. In this case, by connecting the input changeover switch 3 to the NO side, the reference voltage Vref is supplied to the range changeover circuit 4, whereby the range is changed. As described above, the current flowing through the input resistor 42 is converted into a voltage at the designated magnification of the range switching circuit 4 and then amplified by the programmable gain amplifier 5 with a preset gain, and the amplified output is converted to the A / D converter 6. A / D converted by the CPU 7 and taken into the CPU 7.

Here, for example, when the input resistance 42 constituting the range switching circuit is 1 KΩ and the feedback resistances 43 to 45 are 10Ω, 1 KΩ, and 100 KΩ, respectively, the reference voltage source 2
The output of the range switching circuit 4 is "0.01", "1", and "100" times the output Vref. Therefore, the gain of the programmable gain amplifier 5 is set to “1”.
00 "," 1 ", and" 0.01 "times. Even if the range is different, the values are almost in the same range. Then, those values are measured by the CPU 7, and the range operates normally with this CPU. Is automatically checked.

A measuring instrument having such a self-diagnosis function is conventionally known. However, in order to perform a self-diagnosis, a programmable gain amplifier 5 having an accuracy higher than the diagnosis accuracy is required. The programmable gain amplifier 5 is not used for the measurement of the input Vin to be measured, which is the original function of the measuring instrument (the gain is 1).
Set to double), redundant. There is a problem.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and an object of the present invention is to use an inexpensive general-purpose product using a circuit used for ordinary measurement and to operate the range normally. An object of the present invention is to realize a digital measuring instrument having a built-in self-diagnosis function capable of automatically checking whether or not the measurement is performed.

[0010]

SUMMARY OF THE INVENTION According to the present invention, a measured input is
An input terminal applied a reference voltage generating circuit, these the
Switching the measurement input and the output of the reference voltage generation circuit
A first changeover switch applied to the inverting input terminal, and two
A resistor is connected in series and one end is connected to the output terminal of the amplifier
A resistor series circuit whose other end is connected to a common potential point,
One fixed contact is connected to the output terminal of the amplifier and the other is
Fixed contact is connected to the series connection point of the two resistors in the resistor series circuit.
Connected to the non-inverting input terminal of the amplifier.
The second changeover switch and one fixed contact
The other fixed contact is connected to the output terminal of
A / D is connected to the series connection point of the two resistors
A third switch connected to the input terminal of the converter
Range switching circuit, which is used during self-diagnosis and during normal measurement.
Depending on the operation and setting range, the amplifier
And the output of the reference voltage generation circuit at the set magnification.
Movable contacts of each of the first to third changeover switches to amplify
While switching the points to the predetermined fixed contact side,
At the time of self-diagnosis, pass / fail judgment is made based on the result of the A / D converter.
During the measurement of the input under measurement, the measurement result and the reference
Circuit components by comparing the output of the
The measurement value while canceling the effects of fluctuations in
And a display for displaying a pass / fail judgment result at the time of self-diagnosis and displaying a measured value of the input to be measured during the normal measurement.

[0011]

According to the present invention, at the time of self-diagnosis, a pass / fail judgment is made based on the result of the A / D converter. During the measurement of the input under measurement, the measured value is canceled while canceling the influence of the fluctuation of the circuit component. Is calculated.

[0012]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the following embodiments, the same parts as those in FIG. 4 are denoted by the same reference numerals as those in FIG. 4, and the description thereof will not be repeated. In FIG.
Reference numeral 9 denotes a range switching circuit which includes an amplifier 91, changeover switches 92 and 93, and range resistors 94 and 95. The (-) input terminal of the amplifier 91 is connected to the input changeover switch 3, and the output terminal is connected to a common potential point via a series circuit of resistors 94 and 95. The changeover switch 92 is connected to the (+) input terminal of the amplifier 91, and the changeover switch 93 is connected to the input terminal of the A / D converter 6. The changeover switches 92 and 93 have contacts NC and NO, respectively.
A contact NC of both switches is connected to a connection point between the amplifier 91 and the resistor 94, and a contact NO is connected to a connection point between the resistors 94 and 95.

In such a configuration, (1) operation at the time of self-diagnosis and (2) operation of a normal measuring instrument will be described. In this embodiment, the measurement range has three steps (full scale: 0.1 V, 1 V, 10 V) and the output Vre of the reference voltage 2
f is 1 V, the value of the range resistor 94 is 18 KΩ, the value of the resistor 95 is 2 KΩ, and the full scale of the A / D converter 6 is 1.2 V.
The case will be described. (1) Operation at the time of self-diagnosis First, the switches 92 and 9 in the range switching circuit 9
3 is connected to the NC side to constitute an amplifier 91 having a gain of 1. The switch 3 is connected to the NO side to connect the reference voltage Vr of 1V.
ef is added to the range switching circuit 9. The output of the range switching circuit 9 is converted into a digital signal by the A / D converter 6 and taken into the CPU 7, and a pass / fail judgment is made based on the result. Next, the switch 93 is switched to the NO side to constitute the 0.1-fold range switching circuit 9, and the pass / fail is determined by comparing the A / D converted value obtained in the CPU 7 with the 1-fold time.
At this time, the resolution of the A / D-converted value is one digit lower than that of the A / D-converted value.
If a high-precision 95 is used, there is no problem in the accuracy of diagnosis. Further, the switch 92 is also switched to the NO side to compare with the A / D conversion value at the time of 0.1 obtained by switching to the NO side. In these processes, the switches 92,
93 is operating independently and correctly, and the range resistor 9
It is checked whether the resistance values of 4,95 are normal.

(2) Operation as a normal measuring device When measuring in the 0.1 V range, first, switch 3 is set to N
On the O side, the output of the reference voltage source 2 is applied to the range switching circuit 9 and the switch 9 in the range switching circuit 9
2 and 93 are connected to the NC side, the gain of the amplifier 91 is increased by one, and the reference voltage Vref is measured by the A / D converter 6.
As a result, the output of the reference voltage source 2 is set to Vref, and A /
When the D conversion value is Vr, it is expressed by the following equation. Vref = Vr (1) Next, the switch 3 is connected to the NC side, the input Vin to be measured is applied to the range switching circuit 9 from the terminal 1, and the switch 92 is set to the NO side to make the gain of the input amplifier 91 ten times. And measure similarly. The result is that the gain is G1
Assuming that 0, the A / D conversion value is Vm, it is expressed by the following equation. Vin · G10 = Vm (2) From the equations (1) and (2), the value of the measured input Vin can be obtained by the following equation. Vin = (Vm / Vr) · (Vref / G10) (3) The calculation of the expression (3) is performed by the CPU 7, and the calculation result is displayed on the display 8. When measuring in the 1 V range, the reference voltage Vref is measured in the same manner as described above, and then the switches 3, 92, and 93 are connected to the NC side, and the input Vin to be measured is measured by the input amplifier 91 of 1 time. From the result, Vin is obtained by the following equation. Vin = (Vm / Vr) · Vref (4) When measuring in the 10 V range, similarly, Vref
Then, the switches 3 and 92 are set to the NC side and the switch 93 is set to the NO side, and the input Vin to be measured is measured by the input amplifier 91 of 0.1 times. From the result, the value of Vin can be obtained by the following equation. Vin = (Vm / Vr) · (Vref / G0.1) (5) The calculations of the equations (3) to (5) are also performed by the CPU 7, and the calculation results are displayed on the display 8. The first term of these equations (3) to (5) is the ratio of the A / D converted values. Therefore, the A / D converter 6 only needs to be stable for a short period of time between the measurement of the input Vin to be measured and the measurement of the reference voltage Vref. Inexpensive general-purpose products can be used as the components constituting the A / D converter 6.

FIG. 2 shows a configuration in which a range switching circuit 9a having the same configuration as that of the range switching circuit 9 is provided at a stage preceding the range switching circuit 9. In the apparatus shown in FIG. 2, the full-scale ranges of 0.1 V, 1 V, and 10 V correspond to the range switching circuit 9.
a switch 92a is connected to the NC side and the input amplifier 91
By configuring the gain of “a” to be one, it can be realized in the same manner as the device of FIG. 1, but in FIG. 2, a measurement range of 0.01 V can be further obtained.

In this case, first, the switch 3 is connected to the NO side to apply the reference voltage Vref to the range switching circuit 9a.
The switch 92a is set to the NO side to configure the gain of the input amplifier 91a to be 10 times. Further, the switch 92 of the range switching circuit 9 is set to the NC side, and the switch 93 is set to the NO side, and the gain of the amplifier 91 is set to 0.1 times.
Measure with Assuming that G10 'is the gain of the amplifier 91a, G10' is expressed by the following equation. Vref · G10 ′ · G0.1 = Vr (6)

Next, the gain of the range switching circuit 9a is set to 1
The switch 3 is set to the NC side with 0 times, and the measured input Vi
n, and set the switch 92 of the range switching circuit 9 to NO.
Switch 93 on the NC side and the range switching circuit 9
The gain is set to 10 times, and the measurement is performed similarly. The result is expressed by the following equation. Vin · G10 ′ · G10 = Vm (7) From the equations (6) and (7), the value of the measured input Vin can be obtained by the following equation. Vin = (Vm / Vr) · (G0.1 / G10) · Vref (8) If G10 ′ is stable in the short term only during the measurement of the measured input Vin and the reference voltage Vref, (6), (7) )), G10 'no longer exists in equation (8). Therefore, inexpensive general-purpose products can be used as the components constituting the range switching circuit 9a. In conventional devices of this type, extending the range required the addition of a relatively expensive reference resistor, but this is relatively easy in the present invention.

FIG. 3 shows an embodiment in which a measurement range of 100 V full scale is similarly obtained. In FIG. 3, reference numeral 9b denotes a voltage dividing circuit provided at a stage preceding the range switching circuit 9. The voltage dividing circuit comprises a changeover switch 93b and a resistor 94b.
(9MΩ) and 95c (1MΩ), and has the same configuration as that of FIGS. 1 and 2 except for the voltage dividing circuit 9b. In such a configuration, first, the switch 3 is set to the NO side and the reference voltage source 2 is connected, and the switch 93b is set to the NO side to configure the voltage dividing circuit 9b by 0.1 times. Then, the contact of the switch 92 in the range switching circuit 9 is set to the NO side, and the switch 93 is set to the NC side, the gain of the amplifier 91 is configured to be 10 times, and the A / D converter 6 measures. As a result, the following equation is established when G0.1 ′ is a voltage dividing ratio of the voltage dividing circuit 9b. Vref · G0.1 ′ · G10 = Vr (9)

Next, the switch 3 is connected to the NC side while the gain of the voltage dividing circuit 9b remains 0.1 times, and the input Vin to be measured is
And switch 92 of range switching circuit 9 is set to NC.
The switch 93 is set to the NO side and the amplifier 91 is configured to be 0.1 times, and the measurement is performed in the same manner. The result is expressed by the following equation. Vin · G0.1 ′ · G0.1 = Vm (10) From the equations (9) and (10), the value of the measured input Vin can be obtained by the following equation. Vin = (Vm / Vr) · (G10 / G0.1) · Vref (11) The gain G0.1 ′ of the voltage dividing circuit 9b is canceled in the same manner as the gain G10 ′ of the range switching circuit 9a described in FIG. Therefore, the components constituting the voltage dividing circuit 9b may be general-purpose products.

According to the present invention, a digital measuring instrument having a self-diagnosis function is realized by utilizing a circuit used for measuring a normal input to be measured, such as an amplifier constituted by a reference voltage or a reference resistance. be able to. Further, the measured value is calculated by comparing the measured object with a built-in reference voltage using an input amplifier constituted by a reference resistor, whereby A
This has the effect of canceling out the aging of the elements constituting the / D converter or the influence of the temperature coefficient and replacing them with inexpensive general-purpose products.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a digital measuring instrument according to the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the digital measuring instrument according to the present invention.

FIG. 3 is a circuit diagram showing another embodiment of the digital measuring instrument according to the present invention.

FIG. 4 is a circuit diagram showing an example of a conventional digital measuring instrument of this type.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Input terminal to be measured 2 Reference voltage generating source 3 Changeover switch 6 A / D converter 7 CPU 8 Display 9 Range switching circuit 91 Input amplifier 92, 93 Switch 94, 95 Reference resistance

Claims (1)

(57) [Claims] An input terminal to which an input to be measured is applied, a reference voltage generating circuit, and switching between the input to be measured and the output of the reference voltage generating circuit.
First changeover switch applied to the inverting input terminal of the amplifier
And two resistors are connected in series and one end is the output terminal of the amplifier.
Connected in series and the other end is connected to a common potential point.
And one fixed contact is connected to the output terminal of the amplifier.
The other fixed contact is a series connection of two resistors in a resistor series circuit
And the movable contact is connected to the non-inverting input terminal of the amplifier.
The connected second changeover switch and one fixed contact
The other fixed contact connected to the output terminal of the amplifier
The movable contact connected to the series connection point of the two resistors in the column circuit
A third switching switch connected to the input terminal of the A / D converter
Range changeover circuit consisting of switches , depending on the operation and set range during self-diagnosis and during normal measurement.
The amplifier measures the input to be measured and the output of the reference voltage generation circuit.
First to third to amplify at the set magnification respectively
The movable contact of each changeover switch is fixed
Side, and at the time of self-diagnosis, the A / D conversion
The pass / fail judgment is made based on the results of the measuring instrument.
Compare the measurement result with the output of the reference voltage generation circuit.
Cancels the effects of fluctuations in circuit components.
A digital measuring instrument comprising a microprocessor for calculating a measured value while performing a measurement, and a display for displaying a pass / fail judgment result during self-diagnosis and displaying a measured value of a measured input during normal measurement.