Abstract: This paper uses the HP34401A high-precision digital multimeter as the control object and designs a four-channel switching and recording system based on the principle of virtual instruments. This system enables a single digital multimeter to simultaneously measure signals from up to four channels. The entire system has a user-friendly interface, is easy to operate, has high practical value, and has already been successfully applied in practice.
Keywords: Virtual instrument; LabVIEW; Digital multimeter; Multichannel testing
A Four-channel Switching System of Digital Multimeter Based on Virtual Instrument XIE Jie, YANG Bo, HE Qiang
Abstract: This paper describes a four-channel switching system, which can perform four channels or less measurement at the same time by one digital multimeter. The four-channel switching system, used in HP34401A high-performance digital multimeter, is designed based on virtual instrument. The interface is friendly and it is easy to operate. The switching system can meet practice needs and perform well.
Key words: virtual instrument; LabVIEW; digital multimeter; multi-channel testing
1. Introduction
High-precision multimeters are commonly used testing tools, typically with only one test channel. However, modern testing often requires simultaneous measurement of multiple signals due to testing objectives. Using several testing instruments simultaneously increases costs and makes coordination difficult. Designing a system that allows a single digital multimeter to measure multiple signals in a time-sharing manner would greatly simplify the work of testing personnel and save costs. Based on this principle, the author uses the HP34401A high-precision digital multimeter as an example, designing a four-channel switching and recording system based on the principle of virtual instruments. This system enables a single HP34401A to simultaneously measure up to four signals. Furthermore, a soft panel VI (Visual Identity) for measurement and control was developed using LabVIEW. This VI not only controls parameters but also records data and displays graphically, enriching the testing content and making the results more intuitive. The soft panel design also enhances the system's flexibility, facilitating modifications and upgrades.
2. Overall Plan
The system consists of two parts: software and hardware.
The hardware components are shown in Figure 1. It consists of a computer with a serial port, a four-channel switching circuit, a high-precision digital multimeter, and a power supply. The computer communicates with the four-channel switching circuit via USB or RS-232; the HP34401A communicates with the computer via RS-232.
Figure 1 Hardware Composition
The entire testing process is as follows: the four analog signals to be measured first enter the four-channel switching circuit; under the control of the computer, the four-channel analog circuit switches the four signals in a time-division manner according to the software settings, so that only one analog signal can be transmitted to the HP34401A at any given time; the HP34401A measures the received signal according to the software settings, and then transmits the digital information after AD conversion to the computer through the RS-232 interface.
The software component, as the core of the system, is written using the graphical language LabVIEW. LabVIEW comes with a rich set of extended library functions, including data acquisition, communication interface bus and serial port instrument control, data display, analysis, and storage. It supports platform operation, has a good visual and user-friendly interface, and can effectively perform functions such as control of circuits and digital multimeters, signal acquisition, data recording, and control.
3. HP 34401A
To enable computer control of a digital multimeter, the multimeter must have a communication port and be programmable. A program needs to be written based on the multimeter's performance and requirements to set various parameters on the soft panel, enabling remote control of the digital multimeter. High-precision digital multimeters generally meet these requirements. Here, we introduce the performance of the HP 34401A.
The HP 34401A is a high-precision 6.5-digit digital multimeter with the following system features that enable it to meet a variety of testing needs:
• Features HP-IB (IEEE-488) and RS-232 remote interfaces;
• The maximum read rate can reach 1000 reads per second;
• Compatible with three standard programmable languages: SCPI, HP 3478A, and Fluke 8840;
• It has a storage capacity of 512 read data entries;
As can be seen from the above performance, the HP34401's flexible system features enable it to have remote ports and programmable functions, allowing for remote control, and it also has a high read speed, which can meet the testing requirements.
Table 1 describes the correspondence between programming languages and remote interfaces, which allows you to select the appropriate programming language and interface type.
Table 1. Language and Interface Relationship Table
Because the circuit uses an RS-232 interface, and neither the HP 3478A nor the Fluke 8840A/8842A programming languages support RS-232, SCPI was chosen as the programming language for the digital multimeter.
4. Four-channel switching circuit
The four-channel switching circuit is used for switching analog signals. It is the only part of the hardware that needs to be designed and manufactured, and its structure is shown in Figure 2.
Figure 2 Composition of the four-channel switching circuit
After the test signal passes through a voltage follower, it is time-division switched by a multiplexer under SCM control, so that only one signal can be transmitted to the digital multimeter through the interface at any given time.
The control section of the circuit is a microcontroller, which connects to the computer via a communication port. Since some computers only have one serial port, both RS-232 and USB communication ports were designed. The microcontroller controls the multiplexer based on signals from the computer, enabling channel selection and controlling the switching time. The Cygnal C8051F310 microcontroller is used here. The multiplexer used is the Maxim DG408, an 8-channel CMOS analog multiplexer characterized by short switching time and low signal loss. Only four channels are used here; if needed, the number of multiplexed channels can be increased to eight with appropriate hardware and software modifications. Because the output impedance of the test signal is uncertain, and the multiplexer has a certain impedance, the input signal to be tested must pass through a voltage follower before being input to the multiplexer to prevent signal attenuation.
5. Software
The software component is the core of this system, responsible for controlling the four-channel switching circuit and the HP 34401A, as well as data acquisition and storage. It is programmed using the visual language LabVIEW, leveraging its powerful software capabilities and structured programming style to make the software concise and clear.
The software includes two modules: parameter setting and data acquisition.
5.1 Parameter Settings
The parameter settings include three parts: initializing HP34401A, test settings, and system settings.
5.1.1 Initialize HP34401A
It only takes 7 steps to program the HP34401A for data testing via a remote interface.
① Set HP34401A to reset state;
② Configure HP34401A as needed;
③ Set the trigger state;
④ Initialize HP34401A;
⑤ Trigger HP34401A;
⑥ Read data from buffer or internal memory;
⑦ Read the test data into the computer.
Steps ①, ②, and ③ involve manually configuring the multimeter before starting the software. Since the communication port used here is RS-232, the HP34401A must be manually set to RS-232 remote operation mode before using the multimeter, and the baud rate and offset must be set to match those in the program.
Steps ④, ⑤, ⑥, and ⑦ are performed by software. This section mainly focuses on step ④.
First, initialize the serial port. After initialization, send the "*CLS" command to clear the multimeter's registers. Then, send the "SYST:REM" command to put the multimeter into RS-232 remote control mode. Finally, set the measurement parameters for the HP34401A.
The HP34401A's measurement parameter settings include test function settings, test range settings, and test accuracy settings. In the function settings, the test signal can be set to DC voltage, AC voltage, DC current, or AC current. In the test range settings, depending on the test accuracy settings, different ranges such as 0.1, 1, and 10 can be selected for the test value. In the test accuracy settings, different accuracies such as 4 1/2, 5 1/2, or 6 1/2 can be set for the test result. The accuracy is set to 1/2 because the result can only be represented by "0" or "1".
5.1.2 Test Setup
In the test configuration, you can select whether the sampling method is fixed-point or timed, and select the channel. If timed sampling is set, the set sampling time and the sampled time will be displayed during the sampling process; if fixed-point sampling is set, the set number of sampling points and the number of sampled points will be displayed during the sampling process.
The flowchart for initiating HP34401A and selecting a channel is shown in Figure 3.
5.1.3 System Settings
The system settings include sampling mode selection and program configuration. It's used to set the port for communication between the computer and the switching circuit and multimeter, as well as the default filename for saving data.
Since the communication port between the computer and the HP 34401A is RS-232, the HP 34401A is programmed using SCPI language embedded in the program. The instruction manual for high-precision digital multimeters typically provides commands for programming the multimeter.
Figure 3. Initialization of HP34401A and Channel Selection Flow Code
5.2 Data Acquisition
Data acquisition consists of three parts: data collection, data display, and data storage.
Data acquisition is used to collect data from the multimeter and convert it into various data formats as required, then output it to the graphical display or data recording. During sampling, the data from the four channels can display the waveform of the acquired signal in real time, and the graph-specific function in LabVIEW can be used to zoom in on specific areas of the graph. Data storage stores the acquired data in .txt file format in a file with a name set before sampling or a default filename, and the file header indicates the sampling channel and sampling start time. There is also a numeric box on the front panel for real-time display of the acquired data.
6. Test Examples
This testing system has been successfully applied in testing. Figure 4 shows the front panel of the VI and the test results of simultaneously testing the zero-point stability of four sensors of a certain model. The controls in the middle of the figure are used to switch between test settings and system settings.
Figure 4. VI front panel and test results
This test demonstrates that this system enables a high-precision digital multimeter to perform multi-channel signal measurements, meeting testing requirements. Furthermore, the system's hardware is simple and low-cost; appropriate modifications to the hardware and software allow for expansion to include more channels, offering great flexibility and making it ideal for extending high-precision multimeters for multi-channel measurements. The LabVIEW-based software boasts a beautiful and concise interface, while also providing more flexible data acquisition methods and making the data acquisition results more intuitive. This greatly facilitates testing work. It provides testing personnel with an effective measurement tool and has high practical value.
References:
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