Design of measuring product roundness using LVDT (Linear Variable Differential Transformer) using AD698

Mr.Xiang

Design of measuring product roundness using LVDT (Linear Variable Differential Transformer) using AD698 [Copy link]

 

background

In the industrial production process, there are usually requirements for form and position tolerances for various parts. The most common one is the roundness requirement for parts with circular surfaces. In actual projects, I have encountered customers who want us to provide a measurement system for measuring the roundness of shaft parts. Figure 1 shows a shaft part with shape error. By arranging linear displacement sensors on the two degrees of freedom of the x and y axes, the roundness of the circular section can be converted. Roundness = (Dmax-Dmin)/2, that is, the difference between the maximum and minimum diameters divided by 2. In order to achieve accurate measurement of linear displacement, we adopted the LVDT (Linear Variable Displacement Transducer) solution and selected Analog Devices' AD698 as the signal conditioner.

Figure 1 Schematic diagram of roundness measurement

AD698 Chip Introduction

AD698 is a general-purpose LVDT signal conditioner chip, which is used with LVDT. The working principle of LVDT is simply a core movable transformer. The size of the AC voltage output by the secondary side depends on the size of the core displacement. Figure 2 is the functional block diagram of AD698. The AD698 chip outputs an excitation signal through the internal oscillator OSCILLATOR to energize the primary coil of the LVDT through the power amplifier AMP; the chip collects the voltage of the primary and secondary sides, and after decoding, obtains the ratio of the output signal voltage to the input drive voltage (A/B), and outputs a DC voltage after passing through the filter FILTER and the output amplifier AMP. The output DC voltage is proportional to the position of the transformer core.

Figure 2 AD698 functional block diagram

Evaluation Board System Solution

First, let's take a look at the example solution provided by Analog Device: CN0301. Some schematic diagrams of the CN0301 solution are shown in Figures 3 and 4. As can be seen from Figure 3, the AD698 chip requires a ±15V power supply. The peripheral resistors and capacitors framed in red in Figure 3 determine the amplitude, frequency, and bandwidth of the excitation signal. The displacement information of the LVDT core is transmitted in analog form through the LVDT_OUT signal. Figure 4 shows the subsequent processing of the analog signal LVDT_OUT. R19 and C34 form an RC low-pass filter to remove the high-frequency noise of the signal, and the operational amplifier U3 is used as a voltage follower to ensure that the voltage at the output of the operational amplifier U3 always follows LVDT_OUT, and there is no voltage loss due to the different impedances on both sides. The AD7992 in Figure 4 converts the analog signal, converts it into a digital signal, and transmits it using the IIC bus.

Figure 3 CN0301 partial circuit schematic 1

Figure 4 CN0301 partial circuit schematic 2

Figure 5 is a physical picture of the evaluation board CN0301. I have marked the functions of the main circuit modules on the picture. On the right is the LVDT sensor. The CN0301 evaluation board supports two LVDT signal inputs. The lower part of the evaluation board is the power supply circuit, which generates ±15V and 5V voltages respectively.

Figure 5. Example solution for AD698—CN0301

System solution with host computer

In practical applications, we need to use PC to monitor the linear displacement information sampled by LVDT, so we designed the LVDT measurement system solution shown in Figure 6. The LVDT acquisition board transmits the original analog signal or IIC digital signal to the acquisition signal conversion board. The main function of the signal conversion board is to communicate with the PC. Of course, the LVDT acquisition board and the acquisition signal conversion board can also be designed on one board. Finally, the acquisition signal conversion board uses the UART protocol to send the message containing the LVDT linear displacement information to the PC, and the dedicated host computer interface on the PC side displays the linear displacement information or the roundness of the part.

Figure 6 LVDT measurement system with host computer

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