Design of Digital Tachometer Based on Single Chip Microcomputer

Introduction

In industrial control, speed measurement is often required. Generally, a contact tachometer is used. This tachometer must be placed against the center of the shaft to measure. It is inconvenient to use, has great limitations, and is not very safe. Therefore, we designed a non-contact tachometer using a photoelectric sensor. The measurement range is from 1.0 to 9999 rpm, and the four-digit digital tube display is accurate to 0.1 rpm when the measured speed is less than 1000 rpm.

1 Measurement principle

A piece of aluminum foil is attached to the rotating shaft to be measured as a reflector. When the reflector moves to the front of the photoelectric sensor, the infrared beam emitted by the photoelectric sensor is reflected back and received by the infrared receiving tube on the photoelectric sensor, generating a pulse signal. We use the edge of this signal to trigger the high-precision timer inside the microcontroller for timing, with an accuracy of up to 1μs. When the reflector moves to the front of the photoelectric sensor again, the edge of the light reflection signal is used to stop the microcontroller timing. In this way, the rotation period t of the shaft is accurately measured, and then the microcontroller converts the period into the rotation speed and displays it through the LED digital tube.

2 Circuit structure and software design

The circuit structure is shown in Figure 1. Because of the requirement of small size and high display brightness, the scanning display mode is adopted. However, it is not possible to use one CPU to complete measurement and display at the same time. The scanning function will seriously affect the measurement of the CPU. Therefore, two AT89C2051 microcontrollers produced by ATMEL are used to perform the functions respectively. AT89C2051 has built-in 2KB EEPROM program memory and 128 bytes of RAM, and each pin can absorb 20mA of current and other good characteristics. CPU1 is used to measure the rotation period 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 generates an interrupt, receives the data immediately, and then updates the display data, so that the data on the four-digit LED digital tube is immediately updated. The advantage of using two CPUs is that the circuit structure is clear, the programming is simple, and the modular design is realized. The serial interface mode 2 is used between the CPUs to communicate in the form of interrupts. The communication part of the program list of CPU2 is as follows:

3 Conclusion

After actual use, the tachometer we designed has achieved the pre-conceived purpose. It is easy to use and reliable. Moreover, because of this structural design, the input/output port of CPU1 is rarely occupied, so its function can be expanded. For example, we installed a switch to add the function of photoelectric counter (the probe is shared). In addition, we can also use CPU2 and four-digit LED common anode digital tube to form a universal display module. In addition to displaying numbers such as 0 to 9, it can also display characters such as A-J, L, O, P, q, r, U, Y, etc. In this way, if we want to use LED digital tube display when designing other circuits, we don’t need to program and make plates. Just use this universal display module, which is simple and convenient. If this display module does not expand the input and output lines, it can display up to five digits of seven-segment digital tubes.