High-speed data acquisition and arbitrary sensor functions combined with the application of Doppler effect

High-speed data acquisition, arbitrary sensors, these new and very practical functions are of epoch-making significance for multimeters, making project test development work more handy. In order to experience the charm of the first domestic high-performance six-and-a-half-digit digital multimeter, the author applied DM3064 to the development of the Doppler velocity measurement experimental system.

The experimental system is designed as follows:

the ultrasonic signal reflected by the moving object is multiplied by the original signal through an analog multiplier, and the difference frequency information is obtained by filtering and then sampled for signal processing, thereby obtaining reliable and usable information, and finally calculating the speed of the moving object.

The Doppler effect is used in the system to measure the speed of the moving object. The transmitter and the receiver are all stationary, and the moving object reflects the sound wave. The Doppler effect formula is derived and simplified to obtain:

where is the speed of the moving object to be measured in the experiment, is the Doppler frequency shift. is the speed of sound, and is the frequency of the sound wave.

In order to quickly develop this system, the RIGOL DM3064 digital multimeter is used for feasibility verification.

Expected project information to be measured:

1. Ambient temperature. Because the speed of sound is closely related to temperature, we need to measure accurate temperature information to calculate the speed of sound, that is, the size of .

2. The difference frequency information after the multiplier. That is, the size of the Doppler frequency shift value.

3. Photoelectric gate trigger detection. In order to compare the Doppler speed measurement value with the classic photoelectric gate speed measurement value, it is necessary to start the multimeter while the object passes through the photoelectric gate speed measurement to capture the speed measurement information at this moment. That is, the photoelectric gate is used as the external trigger source for measurement.

System construction and measurement

The system framework is shown in the figure:

Figure 1. Doppler velocity measurement system

Figure 2. Doppler velocity measurement experimental system with DM3064

1. Sound velocity measurement

Under laboratory conditions, a platinum resistance temperature probe is used as a temperature sensor. Use the DM3064's arbitrary sensor function and configure the temperature sensor function in combination with the platinum resistance temperature probe's resistance value temperature comparison table:

Enter the arbitrary sensor menu, name it, and select the type as two-wire resistance (2WR), with the unit as. Then add the reference value.

Figure 3. Arbitrary sensor setup

After adding, perform a zero reset operation. Start measuring.

This measurement method needs to calculate the sound velocity after obtaining the temperature information, so it is improved to a sensor with resistance->sound velocity mapping. It only needs to simply update the reference value on the DM3064, which is convenient and fast.

2. Difference frequency information measurement

The key to speed measurement in this system is to obtain accurate difference frequency information, and in order to draw a continuous speed curve, the difference frequency information needs to be continuously and quickly collected.

RIGOL DM3064 digital multimeter inspection software Ultralogger provides 2 measurement function options, namely multi-channel multi-measurement task inspection (Scan) and high-speed single-channel large-scale data acquisition (Data Log). For current monitoring needs, the latter is used.

First, create a task project. When the project is established, the system will automatically create a database corresponding to the project to store user-customized task items and measurement data.

Figure 4. DataLog settings [page]

Figure 5. DataLog difference frequency waveform

Connect the RIGOL DM3064 digital multimeter to the PC via USB, click the Download button, download the task list created in the above steps to the RIGOL DM3064 digital multimeter and save it. Click the Start button to start the task. (When connected for the first time, if the multimeter is in the idle state, the PC will automatically synchronize the RTC time in the multimeter to prevent errors caused by different times.)

The task is saved in the RIGOL DM3064 digital multimeter. If the current PC is not needed to perform operations or display collected data, you can disconnect it, which will not affect the operation of the multimeter.

The RIGOL DM3064 digital multimeter collects measurement values ​​according to the set tasks. The PC can read the measurement values ​​through USB or LAN and save them to the PC. At the same time, the FLASH in the multimeter can also save the measurement values, but its storage space is naturally much smaller than that of the PC.

After the PC reads the data, it is saved in the database. The database is automatically created when the project is established, so there is no need to develop a database separately. For the read data, the software provides two display modes: table and image. If the software is in monitoring state, the table and image will be automatically refreshed. If an alarm occurs, there will be a display and sound alarm prompt.

According to the above method, a continuous high-precision difference frequency waveform can be measured, and the entire process data is recorded in a data file for easy analysis and processing.

In this way, the results obtained by applying high-speed data acquisition can provide raw data for various processing algorithms, such as direct Fourier transform, fast Fourier transform, and equivalent filtering method, etc. However, it cannot be used to directly draw the difference frequency quantity curve. At this time, if you want to get an intuitive speed curve, you can continue to apply the powerful arbitrary sensor function of DM3064 to establish a frequency-speed type sensor, and then you can get the speed curve.

Figure 6. Frequency-speed sensor curve

3. Photoelectric gate trigger detection

In order to compare the speed, the Doppler frequency shift when passing through the photoelectric gate is measured here. Connect the DM3064 test lead to the difference frequency output terminal of the Doppler system, and set the frequency->speed correspondence according to the setting method of any sensor. Select the trigger source as external trigger, and lead the trigger cable to the shaping output terminal of the photoelectric gate.

After configuration, every time it passes through the photoelectric gate, the DM3064 displays the speed value at that moment, which can be compared with the photoelectric gate speed measurement value.

Problem analysis in the system

The system built according to the above scheme meets the expected requirements and can well complete the accurate Doppler speed measurement function. However, it was found that the starting section of the speed curve was distorted during the measurement. With the help of the powerful high-speed acquisition function of DM3064, the initial section waveform was analyzed. It was found that spike interference appeared. Analysis shows that it is the interference caused by the moment of starting the brushed motor. After adjusting the capacitance between the two wires of the brush and the ground, use the Data Log function to check again, and the interference is effectively solved.

Summary

The introduction of RIGOL DM3064 has greatly accelerated the development process of the system and facilitated the testing and fault analysis of the functional modules of each part of the system. The arbitrary sensor and high-speed data acquisition function make the desktop digital multimeter integrated with the system and take on the functions of the system, which is very necessary in feasibility verification and system debugging.

As a sharp tool for engineers, the DM3064 of Beijing Puyuan Jingdian Technology Co., Ltd. has broad application prospects, and its powerful and complete functions can handle more innovative tasks!