JPS6275326A - Thermocouple input unit - Google Patents

Abstract

PURPOSE:To achieve a simpler and lower-cost construction while enabling highly accurate detection of a disconnection, by integrating a converter and an analog input card of an input unit. CONSTITUTION:Micro voltages generated from a plurality of thermocouples 1, 2 and 5 are drawn while switching them with a first multiplexer 9; after being amplified with an amplifier 10, the result is converted to a digital signal with an A/D converter 11 and then subjected to a linearity and span correction digitally with a corrector 13. On the other hand, a second multiplexer 16 runs a microcurrent to the respective thermocouples 1, 2 and 5 while sequentially switching them. This can reduce the period during which the current flows to the thermocouples 1, 2 and 5 so much that the difference in the DC impedance can be ignored between the thermocouples 1, 2 and 5, thereby enabling highly accurate detection of a disconnection in the thermocouples 1, 2 and 5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thermocouple input device capable of detecting disconnection of a thermocouple.

[Conventional technology]

FIG. 2 is a block connection diagram showing a conventional thermocouple input device. In the figure, (1), (2), and (5) are thermocouples,
υ, (c), (e) are thermocouples (1), (2), (5)
A voltage converter that converts the starting voltage of 1 to 5 standard voltages, (to) is 1 to 5 V from each voltage converter -, @, (to)
is an analog input card that converts standard voltages into digital values. Also, Fig. 3 shows a specific circuit of the voltage converter CIJ among the voltage converters Qυ, ■, and (c), in which (6) is the reference junction compensation circuit, 6z is the preamplifier, and Yu is the linearizer. ) is an isolator, αη is a pull-up circuit for detecting disconnection, and this connects the voltage VB input terminal US and high resistance θ9 to thermocouple +11 and preamplifier CJ5.
It consists of a line connected to the

Next (we will explain this operation. First, for example, a thermocouple (
The minute voltage generated in step 1) according to the temperature is applied to the preamplifier G.
A standard voltage of 1 to 5 V is obtained by amplifying the voltage by 3. At this time, the linearizer (to) corrects and linearizes the nonlinear relationship between the voltage generated by the thermocouple (1) and the temperature. Also,
The reference junction compensation circuit (6) operates to compensate for the temperature difference between the reference junction of the thermocouple (here the terminal of the converter) and the measurement point. In the pull-up circuit (lη, for example, a thermocouple (
1) is in a normal state with no disconnection, a small amount of current flows through the thermocouple (11) due to the voltage VB, or the thermocouple (1)
Since the impedance of the thermocouple (1) is small, the voltage drop across the thermocouple (1) due to this current becomes close to 0, and the potential difference between both ends of the resistor 11 becomes large.

On the other hand, when the thermocouple (11) is disconnected, the voltage VB is directly applied to the preamplifier C13, and the potential difference between both ends of the resistor 01 is almost eliminated. It becomes possible.

Furthermore, the linearizer (g) is adjusted to obtain linearity of the input and output of the preamplifier C33 according to the temperature fljil constant range and the drawing order of thermocouples +1+, (2), and (5).

[Is this an invention? - Problem to be resolved] Since the conventional thermocouple manual device is configured as described above, when the number of measurement points increases, thermocouples (1), (2), (5) ) and converter C! It is necessary to install a large number of υ, @, (c) accordingly, which causes variations in the output levels of each converter Qυ, ■, (c), increases the cost of the system, and causes variations in the DC impedance of thermocouples. Therefore, there were problems such as the inability to detect disconnections with high accuracy.

This invention was made to solve the above-mentioned problems, and by integrating the conventional converter and analog human power card, it is possible to simplify the configuration and reduce costs, as well as to detect disconnections with high accuracy. The purpose is to obtain a thermocouple input device that can be used.

[Means for solving problems]

The disconnection detection circuit for a thermocouple input device according to the present invention takes in minute voltages generated by a plurality of thermocouples while switching them with a first multiplexer, amplifies the minute voltages with an amplifier, converts them from analog to digital, and converts them into a corrector. In addition to performing digital linearity and span corrections, the second multiplexer allows a minute current to flow through each thermocouple, making it possible to detect thermocouple disconnection.

[Effect]

The second multiplexer for detecting disconnection in this invention sequentially switches a minute current through each thermocouple, so the period during which current flows through each thermocouple is short, and therefore the difference in DC impedance of each thermocouple is ignored. This function allows thermocouple disconnection detection to be performed with high accuracy.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, (1), (2), (5) are thermocouples, (6)
is the reference junction compensation circuit, (7) and (8) are the reference voltage generation circuit for correction, (9) is the minute voltage generated by the thermocouple, the output voltage of the reference junction compensation circuit (6), and the reference voltage generation circuit for correction. The first multiplexer α [
is a differential amplifier that differentially receives and amplifies minute voltages, αυ is an analog-to-digital converter that converts the analog output voltage of differential amplifier QG into digital, the opposite is a digital corrector, αa is an interface with the outside, and Q2 is thermocouple 1
The storage unit for the measurement range and thermoelectric range for each point, αη is a disconnection detection circuit, and the input terminal α frame for DC voltage VB input and the resistor 1f
jRB and resistance αj.

Further, ue is a second multiplexer that switches the disconnection detection circuit αη for each thermocouple point, and α9 is a control unit that sends control signals to each part of the circuit.

Next, the operation will be explained. Thermocouples (1), (2), (
5) generates a minute voltage depending on the temperature. The small voltage is selected by a multiplexer (9), amplified by a differential amplifier, and converted to a digital value by an analog-to-digital converter t1υ. The span is corrected by performing digital calculations based on the obtained data, and the output is outputted to the outside via the interface 04).

Next, a method of correcting offset drift by the reference voltage generation circuits (7) and (8) will be explained. thermocouple (1),
The true generated voltage in (2) and (5) is vl, vl, v5, the reference voltage generation circuit (power), the reference 7L pressure in (8) is V7, ■8
These are selected by the multiplexer (9) and taken in sequentially, these are amplified by the amplifier, and the digital values converted by the analog-to-digital converter 0υ are converted to d.
If l, dl, d5, dl, and d8, the following relationship holds true.

v1 = 1 d1 + a1, V2 = 2 d2 + 32,
V 5 = k5 d 5 +a 5V7=to 7 d7
+a7, VB = 8dB+8, amplifier [+1. A
/D converter Uυ is L distribution voltage v1, v2, v5, v7
, v8, so 1, k2, k5, k7, k8 and al, a2, a5 are added to each of these circuit constants.
, a4, a5 are 11 = 2; 5 = 7
:8, a1=a2=a5=a7=a8 holds true, and from these, the following relationship holds true.

ko c"c', v7, VaLt known tear f knee, dl
Even if the values of , d2, and d5 fluctuate due to drift, etc., d
By measuring l and d8 simultaneously, the thermocouple (
1), (2), (5) true generated voltages v1, v2, v5
can be calculated. Also, thermocouples (1), (2)
, (5) generates a voltage according to the temperature difference between the reference junction (terminal here) and the measurement point, so the reference junction compensation circuit (6) generates a voltage according to the ambient temperature of this reference junction. The output is also selected by the multiplexer (9) and taken in.
By using this output, the offset drift described in if is corrected, and later the corrected data is added as the temperature of the reference junction to the temperature obtained from the voltage generated by the thermocouple to obtain the true measured temperature.

Next, the operation of the corrector αJ will be explained. Thermocouple (11,
There is generally a nonlinear relationship between (2), (5) and the surrounding temperature, and the level and nonlinearity of the generated voltage differ depending on the type of thermocouple +1+, (2), (5).
For each temperature measurement range, set the above ambient temperature, thermocouple (1),
It is necessary to perform span correction depending on the types of (2) and (5). The computation performed by this corrector αJ is as follows. First, the voltage V of the thermocouple and reference junction compensation circuit is as follows, and here it is a digital value after dit analog-to-digital conversion.

Also, if the ambient temperature of the thermocouple reference junction is T, then T=f
(Vl......This is a correction for nonlinearity.In addition, the lower limit values of the temperatures at which you want to measure TA and TB,
Upper limit value (stored in storage unit 0), 0 people, DB to T
Assuming a digital 1st shift corresponding to A and TB, the following is obtained. The calculations for each voltage (2), ambient temperature (T1) and span (D) are efficiently performed using a microprocessor.

Next, the disconnection detection circuit CL71 will be explained. Third
When measuring the voltage generated by a thermocouple, as shown in the disconnection detection circuit shown in the figure,
If the voltage VB was constantly being supplied, the thermocouple would be
When extending over 0m and acquiring voltage by switching several thermocouples, each thermocouple (1), (2), (5)
The difference in DC impedance between the two cannot be ignored, resulting in an offset error. Therefore, the second multiplexer t1
e to sequentially switch and supply the voltage Va to the thermocouple measured by the switch αe via the high resistance α9, and after first detecting a disconnection, immediately cut off the supply of the voltage VB, and use the circuit described above to convert the voltage generated by the thermocouple. If measured, offset errors caused by the disconnection detection circuit can be removed.

〔Effect of the invention〕

As described above, according to the present invention, the minute voltages generated by a large number of thermocouples are taken in by the first multiplexer,
Each minute voltage is amplified by an amplifier and then converted from analog to digital, and the linearity span is also corrected by a corrector.
Therefore, compared to the conventional system in which a converter equipped with a disconnection detection circuit is installed for each thermocouple, it is possible to simplify the configuration and significantly reduce costs.
By switching and supplying a minute current from the DC power source to each thermocouple using the multiplexer, errors in the DC impedance of each thermocouple can be ignored, and thermocouple disconnection can be detected with high accuracy.

[Brief explanation of drawings]

Fig. 1 is a block connection diagram of a thermocouple input device according to an embodiment of the present invention, Fig. 2 is a block connection diagram schematically showing a conventional thermocouple input device, and Fig. 3 is a detailed block connection diagram of a converter. It is a diagram. (1), (2), (5) are thermocouples, (6) is a reference junction compensation circuit, (7), (8) is a reference voltage generation circuit, (9) is a first multiplexer, Ql is an amplifier, αυ is an analog-to-digital converter, t13 is a corrector, αe is a second multiplexer, αD is a disconnection detection circuit, and u9 is a resistor. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] A first multiplexer that switches and takes in minute voltages corresponding to the temperatures measured by multiple thermocouples, an amplifier that amplifies the minute voltages, and an analog-to-digital converter that converts the voltage amplified by this amplifier into a digital value. , a corrector that corrects the digital value signal according to the ambient temperature and type of each of the thermocouples, and a small direct current flowing through each of the thermocouples while switching sequentially through a high resistance for detecting disconnection. and a second multiplexer.