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21Automated Remote Calibration of Electrical Measuring Tools on the Example of a Digital Multimeter NI PXI 4072
O.V. Stukach
National Research University Higher School of Economics, Moscow, Russia Novosibirsk State Technical University, Novosibirsk, Russia
Abstract: There has been a need for a more efficient approach to measurement calibration. Remote calibration will significantly reduce the material and time costs of the client for transportation and calibration of the measuring tools. A method to maximize the possible automation within the framework of modern technology is proposed to reduce human participation in the calibration procedure. The problems of introducing innovative technologies are described. Such a promising way as the creation and constant expansion of the set of measuring tools is discussed which remote calibration is possible for. The software for remote calibration of the NI PXI 4072 digital multimeter in the LabVIEW graphical programming environment has been created. In order to prove the effectiveness of the remote calibration, the demonstration experiments were performed. Calibration of the multimeter in various modes on simple client-server architecture is carried out, the measurement method is given. The experimental results are also shown. Using simulation of the proposed remote calibration setup it is demonstrated that the method ensures good calibration precision. This technology is relevant for use in large network organizations geographically remote from each other.
Keywords: virtual tool, calibration of measuring tools, remote calibration system, measuring tool, client-server architecture, Internet of things, electrical value.
INTRODUCTION
Currently, the results of measurements, tests and control are being used more and more intensively in every sphere of human activity. To date, more than one billion measuring tools are operated on Russian territory. More than 10 million specialists of various qualifications work in the country to ensure and directly perform measurement, testing and control operations. With such a volume of measurements, the issue of reliability of measurement information and increasing its accuracy is becoming more and more relevant. Consequently, the costs of enterprises for metrological support are constantly increasing. After all, only guaranteed high accuracy of measurement results can ensure the correctness and completeness of decisions at all management levels in the enterprise.
Calibration of measuring tools in remote mode is attractive to increase economic efficiency due to the intensive design of computer networks and communication technologies. This method allows to reduce both financial and time costs because it is no necessary to send a calibration specialist or transportation of measuring tools, which is important for modern enterprises with territorial remoteness of branches.
The paper gives an example of a software system design for remote calibration of the PXI module NI PXI 4072, which is a digital multimeter, using a reference calibrator tool Fluke 5520A within the LabVIEW graphical programming environment. If necessary, the system can be equipped only with modules for which remote calibration is possible [1].
1. Calibration of measuring tools
Measuring tools should be calibrated in order to control their metrological characteristics. The term "calibration of measuring tools" has several interpretations.
Calibration is a set of operations performed in order to determine and confirm the actual values of metrological characteristics and/or suitability for the use of measuring tools that are not subject to state metrological control. In one case, during the calibration process the laboratory determines the actual values of the metrological characteristics of the measuring tool at a certain time and confirms them through a calibration certificate. Since the defined characteristics may differ from those given in the certificate, the laboratory does not make a conclusion about the suitability of the measuring tool, because the customer is competent to determine whether this measuring tool is suitable for operating. in another case, the calibration laboratory makes a conclusion about the suitability of the measuring tool based on the compliance of the actual values of its metrological characteristics with the defined requirements in the documentation or directly by the customer [2]. Secondly, calibration is often understood as the process of adjusting the output value or the indication of the measuring device until the moment of agreement between the reference value at the input and the output.
External calibration is carried out directly with the means of an external standard of physical value or a reference measuring device that is not part of the tested equipment. If the measuring tool has a built-in standard of a unit of physical value, then internal calibration of the device becomes possible.
To date, information and telecommunication technologies are at a high level of development, which makes it possible to design automated complexes based on the interaction of measuring and computer equipment. It becomes possible to carry out the procedure of comparing the metrological characteristics of measuring tools in remote mode directly at the places of operation. Remote calibration allows reducing the cost of transportation and calibration procedures. It significantly decreases the time during which the device is not available for operation. In addition, this method allows minimizing the participation of personnel in the metrological support of measuring tools and automating the acquisition and processing of calibration results. However, it is possible if measuring tools have a standard computer interface, for example, IEEE 488, IEEE 1394, PCI/PXI, and USB.
2. Internet of measurement (IoM) and
AUTOMATION IN METROLOGICAL SUPPORT OF A MODERN ENTERPRISE
The fundamental influence of the Internet of Measurement on metrological support is connected with an increase in the level of automation, which makes it possible to increase the accuracy of measurements and significantly reduce the complexity of metrological tests [3]. The main tasks for information technologies of metrological support are:
• standardization of metrological characteristics;
• input of the received measurement information into the computer;
• control of reference signal source and measuring information processing tools by a software.
The need to measure a large amount of different physical values requires the design of measuring tools that allow to derive the necessary information without direct human involvement, that is performing measurements automatically.
Automation of measuring processes allows to provide:
• acquisition of measurement information in places inaccessible to men;
• long-term, multiple measurements;
• simultaneous measurement of a large number of values;
• measurement of parameters of fast-flowing processes;
• large scale measurements and complex algorithms for its processing.
Measurement automation does not decrease the role of a researcher, engineer or technician planning and using measurement information. Conversely, it increases their productivity, requires them to have a higher level of knowledge not only of measuring tools, but also of the tasks that are solved when receiving and processing measuring information, the ability to use an optimal measurement program and give the correct interpretation of measurement outcomes [4].
Automatic measuring tools in the process of design have had a lot of formation stages. At the first stage, only the means of measuring information acquisition and registering on analog indicating and recording devices were automated. The operator carries out measurements and elaborates of conclusion. In such control systems, measuring tools represent a set of separate measuring instruments. As a result, when measuring a large number of the object parameters, the operator was unable to recognize all the received information and hence make the optimal decision on object control. This led to an expansion of the maintenance staff, a decrease in the reliability, quality of control, and an increase in operating costs.
At the second stage, the increasing requirements for measuring tools due to the rise in measuring information flow, led to the creation of information-measuring systems. Unlike a measuring device, an information-measuring system provides measurement of a large number of object parameters and performs automatic processing of the received information using computing tools built into the system. The task of the control system operator now began to include only making decisions based on the measurement outcomes and obtaining control commands.
At the third stage, information and control systems and information and computing complexes were arisen. It is characterized by a complete closed cycle of information circulation from input to processing, making appropriate decisions and issuing control commands to the object without an operator. The main advantage of such systems is software-controlled algorithms. It is easily tunable when changing operating modes or facility conditions. The operator's work is reduced to the diagnosis of the control system state, development of measurement methods and functioning programs.
Automation of the calibration procedure allows to significantly reduce labor costs during calibration and verification of products, to carry out measurements by low-skilled personnel, which in turn reduces the cost of products.
In order to minimize the participation of personnel in the calibration and consequently eliminate the influence of subjective error on the calibration results, many tools have the automatically calibrated function (self-calibration), which is carried out without an operator and any external means. The measuring tool will automatically detect the need and perform the calibration procedure when the external environmental conditions change. Self-testing and self-calibration are used when the power is turned on.
Automatic calibration uses schemes specially built into the final equipment. These schemes are very diverse and have different functionality. For example, it can use digital communication channels between the end equipment and a remote host system or a factory test system. After the connection is
established, the end equipment uploads data to the host system. Then, using commands and received data, the host calibrates the parameters of the end equipment. In another mode, the calibration circuit can be fully integrated directly into the equipment. In this case, the circuit can measure the characteristics of an embedded precision component, for example a precision resistor or a reference voltage source, to ensure tuning and verify the accuracy of the components.
3. Overview of remote calibration
SYSTEMS FOR MEASURING TOOLS
A large number of automatic measuring tools require constant metrological maintenance, including calibration. Remote calibration is one of the most promising directions in the automation of metrological research. This method makes it possible to observe the metrological characteristics of measuring tools just in time and is becoming increasingly widespread due to decrease of metrological work, reducing its cost and problems with measuring tool transportation.
The possibility of remote calibration by Internet was first announced in 1997 [5], and in 2000 the US National Institute of Standards and Technology (NIST) carried out the first remote calibration of measuring tool using a multifunctional mobile working standard (RE) for the Sandia laboratory [6].
The introduction of a large number of automated measuring tools into production leads to the need to introduce systems for automatic control of the metrological characteristics of these measuring instruments. For example, the paper [7] presents a software-hardware complex for automated calibration of thermal imaging systems of brightness pyrometry used to study the structural macro-kinetics of fast-flowing processes of high-temperature material synthesis. The authors proposed a software-hardware complex based on Matlab to automate the calibration process of a thermal imaging system with "Videosprint" highspeed digital camera. A lamp with a known dependence of the brightness temperature on the current flowing through its tungsten strip is used as a temperature reference. As a current source, a special device is used, which can be controlled through a port using a set of commands. Calibration consists in comparing the video signal levels of the thermal imaging system with fixed parameters of the electron-optical path: magnification of the microscope, amplification and displacement of the video signal, exposure, with the reference current levels of the tungsten lamp strip. It is enough to open in Matlab a file with the port name for current source. Writing to the file it is necessary to transmit the required control command to PSH-2035, or by reading from the file to find out the current and voltage values in the output circuit of the device. The average duration of the calibration cycle is about 15 min.
In the paper [8], the author considers two approaches ensuring the reproduction of accurate astronomical time in embedded computing systems of low temporal accuracy that is calibration and time synchronization. The time calibration procedure is the determination of the clock frequency error relative to the reference time source and calculation of the calibration coefficient. In the proposed clock calibration device, a GPS receiver is used as a reference source, and the reference signal is 1 Hz. The scheme of the time calibration complex includes a calibrated module and a clock calibration device. All computing work is concentrated in the clock calibration device and does not require human participation. The algorithm of the clock calibration device consists of three stages: search for GPS satellites, measurement of the frequency of the test signal by reference and calculation of the calibration coefficient. The duration of the calibration procedure is 2 min., accuracy is 1 ppm (~2.6 s/month).
The advantages of this solution include simplicity of implementation and accessibility, and the short duration of the calibration procedure. A significant disadvantage is the limited signal propagation area. As a result, the uncertainty of the GPS satellite search time at the beginning of the calibration procedure makes the duration of the entire process uncertain.
Professional analytical and standard laboratory scales with self-calibration functions are widely used, for example, Shimadzu scales (AUW 120D, AW 220D, AUX 220), Ohaus (EP 64C, EX 324), Mettler Tolero (AB 135-S/Fact DR, AB 265-S/Fact DR) etc. Such models are used for complex and long-term work in pharmaceutical or jewelry laboratories.
The scales independently determine the need for a calibration procedure and carry it out without any operator involvement. In some models it is also possible to adjust the value of the internal built-in weight, which may change over time due to corrosion or other damage caused by the external working environment. This function adjusts the value of the internal weight according to the external mass. The corrected mass value is stored in the device's memory.
The next example is the self-calibrating angular displacement sensor [9]. A distinctive feature of the sensor is self-calibration during scanning by two spatially connected control points at the edges of the scanning range, which increases the measurement accuracy in a wide temperature range.
However, such an additional function as self-calibration in measuring tool leads to a significant increase of the device cost, directly related to the need for a built-in standard of a unit of physical value. With the existing number of measuring instruments, for example, in large-scale production, the costs of providing and updating the company's measuring equipment increased significantly. It may be the case therefore that this calibration method cannot be used increasingly.
Having appreciated all the advantages, a significant number of enterprises are engaged in the design and implementation of remote calibration systems for a number of measuring tools. Some of these systems are described in [10-12]. It main disadvantage is operating only at certain enterprises for calibrating specific devices, and adapting the software of such systems to other equipment is either impossible or difficult and expensive. From a technical point of view, remote calibration can be implemented using a client-server architecture [11], while the calibrated and reference measuring tools must be located in one place and connected to the client system. A client system consisting of at least two different applications (corresponding to the calibrated measuring tool and to the calibrator) receives commands from a server installed on the equipment of the calibration laboratory and forwards them to these devices via the selected interface. Then it records the readings of the device and transmits to the server through an active Internet connection. All operations required to prepare the calibration certificate are performed on the server side (Fig. 1).
The work [12] describes a calibration system with Internet support. The main purpose of creating such a system is to be able to control and supervise remote standards and measuring tools that are used in the calibration process under the international standard ISO/IEC 17025-2006. This system consists of a transported high-precision calibration device, which is controlled by a personal computer on the side of the service provider (server) and a server-side application that controls the full calibration process.
The transported standard or calibration device must have a communication interface in order to connect to the client computer for calibration control. The application on the Service Provider's side performs the calibration procedure without a person on the customer's side. In addition, the clientside equipment must independently recognize and automatically configure the available interfaces connecting measuring instruments and standards. The operator on the client side should only ensure correct communication between the equipment that is undergoing the calibration procedure and the reference equipment. Consequently, there is no need for specialized engineers or technicians in
laboratories at the client enterprise. The system was implemented to provide calibration of an external USB DAQ card.
This technology has not yet received sufficient spread [13]. Some researchers are interested in this topic. For example, in [14] the issues of remote calibration of measuring instruments via the Internet in accredited laboratories are considered, and the paper [15] is devoted to remote calibration of automatic circuit analyzers. The new features of the design system eliminate the need to periodically send analyzers to the metrological service for metrological tests. Instead, the metrological service sends high-precision calibration measures to the consumer and uses software to provide remote measurements via the Internet. Based on the received data, a conclusion is made about the suitability of automatic circuit analyzers. The system of remote calibration for analyzers can be metrologically considered in the form of three main components: the circuit analyzer itself, software, calibration measures. The operation of the circuit analyzer is provided through a public channel under program control. The software accompanies the consumer throughout the calibration process. An employee of the consumer of the measuring instrument accesses the electronic page of the system, downloads specialized software, and connects to the system server. The software for automatic circuit analyzers controls the measuring equipment, decrypts the data and corrects them using the calibration database.
However, there are a few of such works because the introduction of remote calibration system requires solving many problems related to ensuring compliance with the competence requirements of calibration laboratories.
One of the few systems successfully operating and widespread in Russia has become a system for transmitting reference signals of frequency and time. An accurate time system is a complex of technical means providing periodic transmission of digital information about the value of the current time from a reference source to all network elements in order to synchronize their internal clocks [16]. In Russia, the State Service of Time, Frequency and Determination of the Parameters of the Earth's Rotation has three main standards of atomic time, primary and two secondary.
When transmitting values from state primary and secondary standards to working standards, measuring instruments and time counting devices in various systems including communication networks, the additional errors arise due to the imperfection of channels and devices for transmitting accurate time signals. In particular, when transmitting accurate time signals by radio engineering means (via air and television channels), a relative error is about ±10-6 %. An absolute error is provided in the range from ±100 to ±10 ns (relative error ranging from ±10-7 to ±10-8) when receivers of accurate time signals transmitted by spacecraft of the global positioning system are used.
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Fig. 1. Front panel of the virtual instrument (server) for calibration of the Keithley 2000 multimeter using the Fluke 5500A calibrator
Thus, one of the most accessible and accurate ways to spread information about the exact time from the state standard of time and frequency to consumers is using global positioning system. To get a time scale from satellite systems, it is necessary to use special equipment containing receivers of global positioning system signals. Such specialized equipment is called the Time Server. When transmitting time signals from the server to remote network clients, special Internet protocols NTP (Network Time Protocol) and PTP (Precision Time Protocol - IEEE1588) are used. The exact time system is built on the principle of hierarchy based on network protocols. There are primary time servers with receivers of satellite signals that form the first layer. Interacting with them, time servers belong to the second layer, etc. Thus, an accurate time signal transmission network is formed. It is a connection between primary and secondary time servers, clients, and transmission lines.
Atomic frequency standards with a relative instability of 5 10-13 are installed on satellites board. It serves as the main source for creating an on-board time scale and ensuring synchronization of all the processes of functioning of the orbital grouping, measures the parameters of the orbits of artificial Earth satellites, transmitted the work program and special information to the satellites. Information to the ground control complex and navigation information is transmitted to users via different radio channels. In order for all processes in such a complex system was on the same time scale, a central synchronizer is included in the control center equipment. Thus, the onboard time scales are synchronized with the central synchronizer, and then with the state standard of time and frequency. The unified accurate time system is a clear example of the successful implementation of remote calibration of time measuring instruments.
4. NI PXI 4072 MULTIMETER REMOTE CALIBRATION SYSTEM
The software for the remote calibration system was designed in the graphical programming LabVIEW ecosystem of National Instruments (NI) http://www.ni.com/. The system is a complex of hardware and software interact with each other. The hardware includes a Fluke 5520A multifunction calibrator, a NI PXI 4072 digital multimeter, as well as two personal computers. The Fluke 5520A calibrator is an adjustable multivalued AC/DC measure. Performing the calibrator is a reference and provides high-precision reproduction of the values of DC and AC voltage. Software is based on clientserver architecture, and consists of two parts: the main module (client program) and the device control module (server program). The client program is hosted on a PC in the calibration laboratory, and the server program is hosted on a PC in the customer's laboratory. The interaction between the client program and the server program takes place through the TCP/IP data transfer protocol.
A feature of the client-server architecture is the ability to remove the client from the server at any distance without significantly reducing the speedrate characteristics of the system, even of complex requests and without any changes in the software. In our case, the client means a program that uses resources, and the server is a program that serves customer requests for certain types of resources. The client application operates at the user's workplace computer. The server program operates on a computer connected to a multimeter and calibrator, which is interacted with a GPIB interface.
The program includes two so-called virtual instruments (VI): Server_VI running on a personal computer directly connected to the measuring equipment, and Client_VI launched at the client's workplace. For correct operation of Server_VI the server computer requires installation of the LabVIEW software with drivers for specific equipment involved in the calibration process. Client_VI may be launched and run on any computer with Web access. The logic of interaction between the server and client parts is described as follows. Server_VI receives commands from Client_VI and transmits it to the calibration equipment via the selected interface. Then Server_VI reads the measuring information from the instruments and transmits it to the client's computer though the Web. Data transfer between the client and the server takes place via TCP/IP protocol. The calibrated measuring tool must have a standard interface. Based on the obtained measurement results, the measurement uncertainty is calculated, and a calibration protocol is formed.
The virtual instrument receives commands from a graphical block diagram. It is a visual representation of the solution. The block diagram also contains the source codes for the virtual instrument. The pictogram-connector of the virtual instrument is a graphical list of parameters that provides the possibility of exchanging data of the virtual instrument with other devices and virtual "sub-instruments".
A generalized block diagram of system for remote calibration is shown in Fig. 2.
IEEE 488 (GPIB), RS-232C, and USB interfaces are used for switching equipment. The IEEE-488 bus and the corresponding protocol are widely used in hardware and software complexes for connecting computers and workstations with measuring instruments, in particular, in data acquisition systems. Most manufacturers of automated measuring systems and tools integrate GPIB interfaces into their products as the main data transmission channel because the IEEE-488 bus is well standardized and tested.
Fig. 2. General diagram of the remote calibration system
Devices connected to the IEEE-488 bus can be in the "listener" mode (reads messages from the bus) or "talker" (sends messages to the bus), or be of the "controller" type. There is only one device in the "talker" mode at any given time, but there are a lot of devices in the "listener" mode. The controller determines which of the devices are currently in the "talker" and "listener" modes.
Systems with a large number of slots and multiple segments are built using PCI-PCI bridges. For a 32-bit PCI bus with a frequency of 33 MHz, the bandwidth of the PXI system is 132 MB/s, and for a 64-bit PCI version with a frequency of 66 MHz, the value of this parameter is 528 MB/s. The operation of PXI devices is also possible in Compact PCI systems, due to the universal design of the hardware based on the IEEE 1101.1 standard and their complete software compatibility.
The role of the calibrated measuring tool play the NI PXI 4072 digital multimeter, which converts input signals into digital form by a high-speed ADC. Structurally, the module occupies one slot in the PXI base unit. The front panel of the Client_VI contains controls for setting the programming parameters of
the Fluke 5520A calibrator. In the window "Access settings", the user needs to specify the IP addresses and port numbers for the local and remote computers. In the fields "File name" and "Path to the file", it is necessary to determine the name and location of the file where the calibration results will be recorded.
In the field "Fluke status", it is possible to control the selection of the calibrator state: "Waiting" (passive mode) and "Operating" (active mode). It is necessary to set the option "Working" to perform the calibration. In this mode, Fluke 5520A will generate the selected value in the field "Value". The unit of measurement in the field "Unit" is displayed automatically depending on the value type. The calibration process is started and stopped by the buttons "Calibration" and "Stop" accordingly.
After starting the process, calibration points are automatically formed in accordance with the requirements of the regulatory manual for the NI PXI 4072 multimeter. The set of calibration points is formed according to the following rules:
• six points on each decimal range for the main measurement range;
• five points on each decimal range for other measurement ranges.
Fig. 3 shows an example of the calibration point formation for the measurement mode "AC power". Arrays of data "AC power", "Iac Range", "Iac Permissible Error", "Iac Frequency" are formed according to the equipment manual. Initially, all data arrays are hidden from the user's eyes, and only when a certain value is specified, one or another array appears by selecting a property "Visible" in the Property Node of each value.
Fig. 3. Block diagram of automatic formation of PXI DMM 4072 calibration points in the DC power measurement mode
For example, when selecting in the field "The Set Value" an option "Idc power" on the front panel "Client_VI" an array of DC power amplitudes is displayed, for which the actual values will be determined later.
To synchronize parallel tasks and transfer data between such tasks, a virtual synchronization device is used with the functions "Obtain Queue", "Dequeue Element", "Enqueue Element", and "Release Queue". These functions are used to
accumulate data in a queue with subsequent data extraction in the form of individual elements or an array of all elements [17]. To implement the first task, the "Event Structure" is used, which is waiting for the event on the front panel (press the button "Calibration"). After that, the appropriate option is made in order to process this event. The program is in standby mode until the expected action on the front panel. The reaction comes faster than in the case of constant interrogation of all elements. After
pressing the button "Calibration", the values of the ith calibration point are processed for subsequent sending via TCP/IP connection to the server
computer, according to the block diagram shown in Fig. 4.
Fig. 4. Diagram of the module "Client_VF
Calibration will be performed only for one point. If it is necessary to calibrate M points, it is necessary to repeat the event detection process, so the Event Structure is placed in the While Loop.
In Fig. 4 a part of the program "Client_VI", that is a case structure with the Waits function marked by 1. It defines the time delay in milliseconds during which the function waits for an ongoing operation to be performed. Depending on the set value, the waiting time for the new calibration point is changed. The minimum value corresponds to the Fkuke parameter setting time, which is 5 s [18]. The
second part of the program, marked by 2 is responsible for the accuracy of the transmitted data. The number of decimal digits is 6-8 for all values except capacitance and inductance, and 14 characters for the specified values. The "Number to Fractional String" function has a precision of 6 digits by default. This choice is explained by the small dimension of these values.
The second task, obtaining measurement information over a TCP/IP connection and forming a calibration protocol is solved according to the algorithm shown in Fig. 5.
Fig. 5. The program "Client_VI"
Let's consider in detail the second part of the Client_VI diagram. The result of the whole process is the calibration protocol displayed on the Client_VI front panel (Fig. 6).
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Fig. 6. Example of calibration protocol of the NI PXI 4072 multimeter in the DC voltage measurement mode
The calibration protocol contains the following information:
• the actual value (reproduced by Fluke);
• frequency of the reproduced signal (Hz);
• the value measured by the PXI DMM 4072 multimeter;
• measurement limit of the PXI DMM 4072 multimeter;
• absolute measurement error;
• permissible measurement error (according to the documentation for PXI DMM 4072);
• relative error (%).
For convenience, the calibration protocol duplicates the value in which the multimeter provides measurements. In addition, the result of the i-th measurement is displayed in the field "Measurement result". The absolute error is calculated according to the formula:
AX= Xm —Xa,
where Xm is the measured value, Xa the actual value.
One of the most important issues is estimating the permissible error of the measuring tool. The
possibility of calculating the permissible error of various measuring tools was accounted as one of the main factors when choosing a programming environment. The LabVIEW library of functional blocks allows to create a part of the program that can be quickly and easy adapted to various ways of setting the permissible error given in the manual for a variety of devices. In this case, the error is calculated in accordance with the manual for the calibrated measuring tool based on the specified values of the measurement limit and voltage. So, the expression for the measurement error of capacitance with a NI PXI 4072 multimeter with an intercalibration interval of two years for the operating temperature range (23±10) °C has the form:
AX= ±(Xm-a % + Xr b %),
where Xm is the measured value, Xr is the measuring range, a, b are positive numbers independent of X.
Based on this specification, the measurement error may be calculated. For example, the error for an electrical capacitance of 1 nF measured at the limit of 10 nF, is as follows:
AX = ±(1,0000 10-9x0,0015 + 10,0000 10-9x0,001) = ±0,011510-9 F.
Not to fall outside the limit norm, the error value is compared with the error calculated based on the measurements with the permissible error value for a specific measuring tool regulated by a manual. If the absolute error value modulo exceeds the allowable one, the LED opposite this value lights up orange, as shown in Fig. 6.
The calibration protocol is also saved in a file on the client computer, and if necessary the measurement information can be extracted for a further processing. The software implementation of the algorithm for recording a file is shown in Fig. 7.
Fig. 7. Diagram of the program part "Client_VI", responsible for saving data to a file
In order to save data to a file, you need to input the name and path of the file in the fields "File name" and "Path" accordingly. First of all, using the block "Write to a text file", the header of the Table is saved to the file. Next, the time and date of file creation (Get Date/Time String function), and i-th line of the calibration protocol is saved one by one.
Another important issue was rounding the error and the measurement result according to metrological rules [19]. Here, standard rounding according to mathematical rules by LabVIEW cannot be applied. To solve the rounding problem, an additional "sub-device" was created that performs rounding according to the following.
1. The error in the measurement result is indicated by two significant digits, if the first of them is equal to one or two, and one if the first digit is three or more.
2. If the digit of the oldest of the discarded digits is less than five, then the remaining digits of the number do not change. Extra digits in integers are replaced with zeros, and in decimals are discarded.
3. If the digit of the oldest of the discarded digits is greater than or equal to five, but it is followed by non-zero digits, then the last digit left is increased by one.
The following blocks are used in this virtual device: Format Into String, Concatenate Strings, Fract/Exp String To Number, Replace Substring, String Subset, Match Pattern, as well as a number of
mathematical and logical functions. Before transforming a number, it is converted to a string format. Then significant digits are compared, and the result of the error rounding is displayed (see Fig. 8). According to the rules, the measurement result is rounded to the same decimal place that ends with the rounded value of the absolute error. If the decimal fraction in the numerical value of the measurement result ends in zeros, then zeros are discarded to the digit that corresponds to the digit of the numerical value of the error.
An additional "sub-instrument" was created for the measurement rounding (Fig. 9). In this virtual instrument, similar blocks and functions are used, as in a virtual instrument for error rounding.
Fig. 8. Diagram of a virtual device performing error rounding by metrological rules
Fig. 9. Subroutine "Rounding of result"
The front panel of the Server-VI for completely calibrating the NI PXI 4072 digital multimeter is shown in Fig. 10. The front panel of the program is simple and understandable even for a user who is not aware of LabVIEW (window "Visa resource name"). The front panel of the Client_VI contains controls for setting the programming parameters of the Fluke 5520A calibrator (field "Visa resource name"). In the window "Setting up access", the user needs to specify the IP addresses and port numbers for the local and remote computers in the appropriate fields by clicking on the tabs alternately "Local" and "Remote". So, connections with computers where Client VI is installed will be established.
Fig. 10. The front panel of the "Server_VI" to calibrate a digital multimeter
It is must select the name of the measuring tool to be calibrated (NI PXI 4072) in the field "Instrument Descriptor" to configure the communication. The front panel automatically displays the set value of the amplitude and frequency
of the signal supplied to the Fluke 5520A calibrator, the measured value of the amplitude by the NI PXI 4072 digital multimeter, and the value itself. In the field "Abs. Error", the calculated absolute error of the i-th measurement is displayed.
The program Server_VI performs two main tasks:
• receiving of parameters for the i-th calibration point via TCP/IP connection from the client computer, setting/receiving values on Fluke 5520A;
• receiving the i-th generated value with Fluke 5520A, measuring with a NI PXI 4072 multimeter, transmitting measurement information via a TCP/IP connection to a client computer.
Functions "Queue operations" of the virtual synchronization instrument (Acquire Queue, Dequeue Element, Enqueue Elements, Release Queue) are used similarly Client_VI to synchronize
parallel tasks and transfer data between ones. The LabVIEW uses streaming programming, in which the sequence of execution is determined by the data stream. It is necessary to explicitly specify the order of the program performing to operate with the Fluke 5520A calibrator. So, during programming the "Flat Sequence Structure" structure was utilized. The structure is used to control the order of execution of data nodes that do not depend on each other. It looks like a set of frames and provides sequential execution of program fragments placed in its frames. The application of this structure is shown in Fig. 11.
Thus, data from Fluke 5520A will not be received until the previous frames are carried out due to this structure.
The second task as obtaining the i-th generated value with Fluke 5520A, measuring with a NI PXI 4072 multimeter, transmitting measuring information to a client computer is solved according to the algorithm in Fig. 12.
I □ □ □ □ o □ □ □ □ n :□ ma □ □ □ □ □ □ □ □"□□□□□□ b □□ o □□□□ a aTTn □□□□□□□□□□□□ o n □ □ EHEM Dinanpananondi
1) 2) 3) 4) 5)
1) setting the parameters of the output value (amplitude, frequency); 2) setting the operating mode; 3) delay; 4) obtaining the parameters of the value; 5) comparing the received and initial values
Fig. 11. Part of the Server_VI diagram providing operation with the Fluke 5520A working standard
Fig. 12. Part of the "Server_VI" diagram providing work with a calibrated NI PXI 4072 multimeter
Let's consider in detail the second part of the "Server_VI" diagram. In order to identify a measuring tool, the virtual instrument "NI DMM Initialize" is used. The value of the descriptor in the corresponding field on the front panel is supplied on
the input. Then the automatic zero setting on the multimeter using the virtual instrument "NI DMM Auto Zero" took place. The operation of the virtual instrument "NI DMM Configure Measurement Digits" provides the setting of the multimeter
measurement limit and resolution (6.5 digits). Then the readings of the virtual device "NI DMM Read" and the necessary processing of measurement information for transmission over a TCP/IP connection to a client computer is performed. After shutting down, the virtual instrument "NI DMM Close" closes the established connection between the computer and the multimeter and stops data acquisition.
5. Remote calibration of the ni pxi 4072
MULTIMETER IN VARIOUS MEASUREMENT MODES
of the NI PXI 4072 out for the following
Remote calibration multimeter was carried measurement modes:
• DC voltage, V;
• AC voltage, V;
• DC resistance, Ohms;
• capacity, F.
Let's discuss in more detail the features of the software implementation and the calibration results. The parameters of the points for which calibration is required are set for each value separately in the manual for the measuring tool. The limits of permissible errors are taken according to the errors with an inter-calibration range of two years for all values. The parameters of the calibration points for the multimeter in the DC voltage measurement mode are given in Table 1.
Table 1. Parameters of calibration points for a multimeter in DC voltage measurement mode
The actual Measurement Permissible
value, V limit (NI PXI 4072), V error, V
0 1 ±6 10-6
0 10 ±60 10-6
0 100 ±600 10-6
0 300 ±6 10-3
0.1 0.1 ±6 10-6
-0.1 0.1 ±6 10-6
1 1 ±31 • 10-6
-1 1 ±31 • 10-6
10 10 ±6 10-5
-10 10 ±6 10-5
100 100 ±4,110-3
-100 100 ±4,110-3
300 300 ±0.0165
The delay time for acquisition parameters of new points was selected for 7 s in this mode (Fig. 6). This time is sufficient to set the required values at the output of the calibrator. An array of actual voltage values is displayed (column 1) after launching the program on the Client_VI front panel. The remaining parameters of the points are displayed directly in the calibration protocol.
According to the calibration protocol, it can be concluded that when measuring DC voltage U=-0,1 V the permissible measurement error of the NI PXI 4072 multimeter at the corresponding limit is less
than the calculated absolute measurement error. This calibration protocol is automatically saved into a text file.
It turned out that the Fluke 5520A calibrator cannot functionally reproduce combinations of parameters of some calibration points, for example, an AC voltage with amplitude of 50 V at frequencies of 30 Hz and 300 kHz. There are six such values in total. The delay time for issuing parameters for new points was 5 s to operate in this mode (Fig. 13). This is the minimum time required to set the required values at the output of the calibrator. After launching the program on the front panel of the virtual instrument Client_VI an array of actual voltage values (column 1) and an array of frequency values (column 2) are displayed. The remaining parameters of the points are displayed directly in the calibration protocol. This protocol is automatically saved in a text file.
Fig. 13. Calibration protocol of the NI PXI 4072 multimeter in the AC voltage measurement mode
The delay time for the parameters of new points was 20 s to work in the mode of electrical capacitance measurement (Fig. 15). This time is necessary to set the necessary values at the calibrator output. The permissible error of measuring the electrical capacitance was calculated by the formula:
AX= ±(Xm^% + Xdb %),
(1)
where a, b are positive numbers independent of X
(Tabl. 2).
Table 2. Parameters of calibration points for a multimeter in the mode of electrical capacitance measurement
Actual Measureme Permissible error
value, F nt limit (NI % ofthe % of the
PXI 4072), measured meas.
F value limit
270T0-12 300-10-12 ±0.15 ±0.1
110-9 10 ±0.15 ±0.1
110-9 100 ±0.15 ±0.1
Actual Measureme Permissible error
value, F nt limit (NI % ofthe % ofthe
PXI 4072), measured meas.
F value limit
10010-9 300 ±0.18 ±0.1
10010-9 0.1 ±0.18 ±0.1
1010-6 0.1 ±0.18 ±0.1
1010-6 0.1 ±0.18 ±0.1
110-6 0.1 ±0.18 ±0.1
110-6 0.1 ±0.18 ±0.1
Table 3. Parameters of calibration points for a multimeter in DC resistance measurement mode
After launching the program on the front panel of the Client_VI an array of actual electrical capacitance values is displayed (column 1). The remaining point parameters are displayed directly in the calibration protocol (Fig. 14).
(0.00000009972=0.00000000029) F Calibration Protocol capacity. F
Actual Value Fiequency Measuied Value Measui.Limit Abs.Enoi Ailiniss.Enoi Relat.Eiior |
F j Hz IF F F F %
2.7E-10 jo |2,963E-I0 |3E-10 2,636-11 |l,94445E-12 |9,74074074074 1
1E-9 |0 Jl,024E-9 JlE-9 2,«Ell |2,S36E-I2
IE-9 |0 |l,025IE-9 |lE-8 2.5IE-1I Jl,153765E-ll I»,
llE-7 |0 |l,0H2456E-7 L, 2.456E-I0 J2,503684E-10 ¡0,2456
|lE-7 jo |9,97t56C-6 11E-6 2.844E-10 Jl,1495734E-9 |0,2844
|lE-5 jO |l,44925652E-5 u 4.4925652E-6 13,17388478E-6 |44,925652
|lE-5 jO j9,9670797E-6 |0,000! 3,292Q3E-6 jl,14950619S5E-7 |0,329203
Fig. 14. Calibration protocol of the NI PXI 4072 multimeter in the electrical capacitance measurement mode
The software implementation of the error calculation is shown in Fig. 15.
Permissible Error
Fig. 15. Block diagram for calculating the permissible error of electrical capacitance measurement
The parameters of the calibration points for the multimeter in the DC resistance measurement mode are given in Table 3.
The permissible error of measuring DC resistance was calculated using (1). The delay time for setting the parameters of new points was 20 s for operation in this mode. This time is necessary to set values at the output of the calibrator. After launching the program on the front panel of the Client_VI virtual instrument an array of actual DC resistance values is displayed (column 1).
Actual value, Ohm Measurem ent limit (NI PXI 4072), Ohm Permissible error
ppm of the measured value ppm of the measure ment limit
0 10103 ±40 ±20
0 1103 ±400 ±200
0 100 ±4000 ±2000
100106 100106 ±6040 ±920
10106 10106 ±410 ±102
1106 1106 ±100 ±22
100103 100103 ±86 ±17
10103 10103 ±83 ±14
1103 1103 ±83 ±14
100 100 ±90 ±25
Conclusion
Based on the calibration results, it can be concluded that the metrological characteristics of the multimeter correspond to those stated in the operational manual for the measurement modes of AC voltage and electrical resistance. In the DC voltage measurement mode, the measurement error at the limit of 0.1 V exceeds the permissible limits. In the capacitance measurement mode the measurement error is within the limits of the permissible error only in the measurement modes of 10 nF, 1 uF.
Thus, software was created for remote calibration of the PXI NI 4072 digital multimeter in the LabVIEW graphical programming environment. The results of the study and the multimeter remote calibration system were tested in the measurement modes of AC, DC voltage, resistance, and electrical capacitance. Two programs have been designed that operate on remote computers and form the software of the remote calibration system, as well as two subroutines that implement rounding of the error and the measurement result. The programs provide calibration of the multimeter according to the operational manual. The user manual of the digital multimeter remote calibration system has been created, which can be recommended for inclusion in the set of documents when certifying the designed software.
The use of remote calibration systems in metrological practice will provide a significant decrease in financial costs while reducing the time spent on all calibration operations of measuring tools, and minimizing the share of personnel participation in the calibration process.
Acknowledgments
The study was carried out with the financial support of the Russian Scientific Foundation in the framework of scientific project № 22-29-00024.
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[18] Fluke 5520A Multi-product calibrator, Manual Set, 2003.
[19] MI 1317-2004. The state system of ensuring the uniformity of measurements. Results and characteristics of measurement error. Forms of representation. Methods of it use during testing of product samples and control of their parameters of product samples and control of their parameters (MI 1317-2004. Gosudarstvennaya sistema obespecheniya edinstva izmerenii. Rezultati i kharakteristiki pogreshnosti izmerenii. Phormi predstavleniya. Sposobi ispolzovaniya pri ispitaniyakh obraztsov produktsii i kontrole ikh parametrov).
Oleg V. Stukach is the
founder of the Tomsk IEEE Chapter, Dr. of Sci., Professor of Moscow Institute Electronics and Mathematics of National Research University Higher School of Economics and Novosibirsk State Technical University.
E-mail: tomsk@ieee.org The paper has been received on 20/03/2023.
Автоматизированная дистанционная калибровка средств измерений электрических величин на примере цифрового
мультиметра N1 РХ1 4072
О.В. Стукач
Национальный исследовательский университет «Высшая школа экономики», Москва,
Россия
Новосибирский государственный технический университет, Новосибирск, Россия
Аннотация: Дистанционная калибровка позволит существенно снизить материальные и временные затраты клиента на транспортировку и проведение калибровки средства измерений. Предложен способ максимального достижения возможной автоматизации в рамках современных технологии, то есть привести к минимуму участие человека в процедуре калибровки. Обсуждается такое перспективное направление, как создание и постоянное расширение базы средств измерений, для которых возможна калибровка в дистанционном режиме. Создано программное обеспечение для дистанционной калибровки цифрового мультиметра N1 РХ1 4072 в среде графического программирования ЬаЬУШШ. Проведена калибровка мультиметра в различных режимах, приводится методика измерения. Данная технология актуальна для использования в крупных организациях с сетью филиалов, географически удаленных друг от друга.
Ключевые слова: виртуальный прибор, калибровка средств измерений, система дистанционной калибровки, средство измерений, клиент-серверная архитектура, Интернет вещей, электрическая величина.
Литература
[1] I.A. Ershov, O.V. Stukach, "Internet of measurement development based on NI PXI remote calibration". Dynamics of Systems, Mechanisms and Machines (Dynamics), Omsk, Russia, 5-7 November 2019. Publisher: IEEE, pp. 1-5, doi: 10.1109/Dynamics47113.2019.8944613.
[2] Селиванов М.Н. О понятии "калибровка средств измерений" // Измерительная техника. - 1992. -№ 7. - С. 16 -17.
[3] Ершов И.А., Стукач О.В. Архитектура системы дистанционной калибровки как часть концепции Internet of Measurements (IoM) / Современные технологии поддержки принятия решений в экономике: сборник трудов III Всероссийской научно-практической конференции студентов, аспирантов и молодых ученых, 24-25 ноября 2016 г., г. Юрга. - Томск: Изд-во ТПУ, 2016. - С. 140142.
[4] Aimagmbetova R.Zh., Ershov I.A., Stukach O.V. Towards the problem of measurement traceability in the Internet of measurement concept / Dynamics of Systems, Mechanisms and Machines (Dynamics), Conference, Omsk, 14-16 Nov. 2017. ISBN: 978-15386-1820-2. Doi: 10.1109/Dynamics.2017.8239425.
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internet // Proceedings of 4th Optical Fibre Measurement Conference. - Teddington, UK, October 24-27, 1997. - P. 155-158.
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[18] Fluke 5520A Multi-product calibrator, Manual Set, 2003.
[19] МИ 1317-2004. Государственная система обеспечения единства измерений. Результаты и характеристики погрешности измерений. Формы представления. Способы использования при испытаниях образцов продукции и контроле их параметров.
Стукач Олег Владимирович -
основатель Томской группы Института IEEE, доктор технических наук, профессор Национального исследовательского университета «Высшая школа экономики» (г. Москва) и Новосибирского государственного технического университета (г. Новосибирск).
E-mail: tomsk@ieee.org Статья получена 2G.G3.2G23.
