Design of the hottest digital tachometer

Design of digital tachometer

introduction

in industrial control, speed measurement is often carried out. Generally, contact tachometer is used. This kind of tachometer must be placed at the center of the rotating shaft to measure. It is inconvenient to use, has great limitations, and its safety is not very good. Therefore, we designed a non-contact tachometer with photoelectric sensor. The measurement range is from 1.0 to 9999 rpm. The four digit digital tube shows that when the measured speed is less than 1000 rpm, the accuracy is 0.1 rpm

1 measurement principle

stick an aluminum foil on the measured rotating shaft as a reflector. When the reflector turns to the front of the photoelectric sensor, the infrared beam sent by the photoelectric sensor is reflected back, and at the same time, it is received by the infrared receiving tube on the photoelectric sensor to generate a pulse signal. We use the edge of this signal to trigger the high-precision timer inside the single chip microcomputer for timing, and the accuracy can reach 1 μ s. When the reflector turns to the front of the photoelectric sensor again, the edge of the light reflection signal is used to stop the timing of the single chip microcomputer. In this way, the rotation period T of the rotating shaft is accurately measured, and then the single chip microcomputer converts the period of ordinary plastic into the rotation speed with the smaller tonnage of the experimental machine and displays it through the LED digital tube

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circuit structure is shown in Figure 1. Because it is required to be small and have high display brightness, the scanning display mode is adopted. However, it is not possible to complete the measurement and display with one piece of CPU at the same time, and the scanning function will seriously affect the measurement of CPU. Therefore, two AT89C2051 single-chip computers produced by ATMEL company are used. AT89C2051 has built-in 2KB EEPROM program memory and 128 bytes of ram, and each pin can draw 20mA current. CPU1 is used to measure the rotation cycle of the rotating shaft and convert it into speed, and then send the display data to CPU2. CPU2 usually just scans the display data continuously. When CPU1 sends data, CPU2 interrupts, receives the data immediately, and then updates the display data, so that the data on the four digit led nixie tube is updated immediately. The advantage of using two CPUs is that the circuit structure is clear, the programming is simple, and the modular design is realized. Serial interface mode 2 is adopted between CPUs to communicate in the form of interruption. The program list of the communication part of CPU2 is as follows:

3 conclusion

after practical use, the tachometer we designed has achieved the purpose envisaged in advance, and it is convenient and reliable to use. Moreover, because of this structural design, the input/output port of CPU1 occupies very little, so its function can be expanded. For example, we installed a switch after the user purchased the product, and added the function of photoelectric counter (the probe is shared). In addition, we can also use CPU2 and four LED common positive nixie tubes to form a general display module. In addition to displaying numbers such as 0 ~ 9, we can also display characters such as A-J, l, O, P, Q, R, u, y, etc. In this way, if we need to use LED nixie tube display when designing other circuits, we don't need to program plate making, just use this general display module, which is simple and convenient. If this display module does not expand the input and output lines, it can display five digit seven segment nixie tubes at most