A new device for measuring the magnitude and direction of rotor rotation speed

The more advanced rotor speed measuring devices in the existing technology mainly use the rotation of the rotor to drive the turntable in the measuring device to rotate, and then perform photoelectric or electromagnetic measurement on the turntable, convert the speed signal of the turntable into an electrical signal, and then process it through the subsequent processing circuit (or secondary instrument) to output the magnitude and direction of the rotor speed. Since the turntable of the speed measuring device is in direct contact with the rotor when measuring the speed, it will undoubtedly affect the rotation of the rotor itself, and the installation structure is also relatively complicated when this method is used for detection. In recent years, with the development of optoelectronic technology, some rotor speed measuring devices that are non-contact with the rotor to be measured have also appeared, mainly for detection in the radial direction of the rotor. This type of speed measurement method usually uses a speed measurement sensor to measure the speed of the rotor, but cannot determine the direction of rotation of the rotor. In actual industrial measurement, especially in some special occasions such as aviation and aerospace, it is necessary to detect both the speed and direction of the rotor. Therefore, a rotor rotation speed and direction measurement device with a compact structure, simple and convenient installation, and high stability, reliability, and measurement accuracy has been developed. It will be widely used in industrial measurement and scientific experiments and solve some key problems. This study proposes a photoelectric measurement method for detecting the magnitude and direction of rotor speed using a four-quadrant photoelectric detector, and develops a measuring device. The device is installed in the axial position of the rotor, and only one speed sensor can be used to realize online detection of the magnitude and direction of the rotor rotation speed. 1 Principle of rotor speed magnitude and direction detection Figure 1 is a schematic diagram of the measurement principle of the magnitude and direction of rotor speed. A high-reflectivity film is pasted on one side of the center point of the top surface of the rotor. Its reflectivity to light is significantly higher than that of the rest of the rotor end surface. After passing through the optical system, the image on the photosensitive surface of the four-quadrant photoelectric detector is shown in Figure 1.

When light shines on the photosensitive surface of the four-quadrant detector, the voltages generated by the four silicon phototubes are: V1, V2, V3 and V4. The output signals of the four-quadrant detector are processed by sum and difference to obtain the A and B signals VA and VB:

Assuming that the light spot is a circular spot with uniform light intensity distribution, the rotor rotates counterclockwise, starting from the direction coincident with the positive direction of the X-axis, the output signal waveforms of A and B can be calculated and simulated as shown in Figure 2. Both A and B signals are AC signals crossing the zero point, and the phase of A is 90° ahead of the phase of B. Using a similar method, the output waveforms of A and B when the rotor rotates clockwise can be obtained as shown in Figure 3, at which time the phase of A lags behind the phase of B by 90°.

Therefore, when using a four-quadrant photoelectric detector to measure the speed of the rotor, two signals with a phase difference of 90° can be obtained. After the two signals are processed by quadrupling the frequency and distinguishing the direction, a quadrupled frequency pulse signal for speed measurement and a signal for judging the forward and reverse rotation are obtained. Then they are sent to the single-chip microcomputer for signal processing and calculation, and the speed of the rotor can be calculated and the direction of rotation of the rotor can be output at the same time.

2 Structure of the speed sensor and composition of the measurement system

Figure 4 is a schematic diagram of the photoelectric imaging system structure of the speed sensor. The light emitted by the high-brightness light-emitting diode passes through a semi-transparent and semi-reflective prism and is focused by a lens onto the end face of the rotor. The light reflected from the end face is then imaged onto the photosensitive surface of the four-quadrant photoelectric detector through a lens and a semi-transparent and semi-reflective prism. Only the light focused on the high-reflectivity film can be reflected back to the photoelectric detector. When the rotor rotates, the position of the light spot imaged on the photoelectric detector will change accordingly.

Figure 5 is a schematic diagram of the overall system. A high-reflectivity film is pasted on one side of the center point of the top surface of the rotor. The measuring device consists of a speed sensor and a signal processing system. The speed sensor includes a photoelectric imaging system and a preamplifier circuit. The signal processing part consists of an addition and subtraction processing circuit, a quadruple frequency and direction identification circuit, a single-chip microcomputer and a digital display system. In the single-chip microcomputer, the rotation frequency of the rotor, i.e., the speed, is calculated by combining the measurement of the pulse period and the number of pulses, and then the rotation speed and direction of the rotor are output through a digital display.

3 Signal Processing

Figure 6 is a block diagram of the preamplifier circuit and sum-difference processing circuit of the four-quadrant detector. After the four-quadrant photodetector converts the received optical signal into an electrical signal, the current signal is converted into a voltage signal through the preamplifier circuit. The op amp device uses a high-precision SMD fast op amp. The entire preamplifier circuit board is very small and is packaged in the speed sensor together with the four-quadrant photodetector to avoid the influence of external electromagnetic interference on the measurement signal. The above-mentioned output voltage signal is output as two signals A and B after sum-difference processing. The signals A and B are similar to sine signals. Due to the use of differential signal processing, the influence of system noise and external interference signals can be effectively overcome.

The A-channel and B-channel signals are respectively sent to the zero-crossing detection circuit composed of a high-speed voltage comparator to obtain two square wave signals, which are then sent to the quadruple frequency direction determination processing circuit for processing to obtain the quadruple frequency signal of the rotor rotation frequency and the rotation direction signal, as shown in Figure 7. Due to the use of the zero-crossing detection signal processing method, the intensity change of the light spot will not affect the measurement of the rotation speed.

4 Single-chip microcomputer speed measurement system

The speed is measured by combining the measurement pulse period and the measurement pulse number. Figure 8 is a signal processing structure block diagram with the single-chip microcomputer as the core. The quadruple frequency signal and the speed identification signal are connected to the T0 interface and P3.0 port of the single-chip microcomputer respectively. The 89C51 single-chip microcomputer is used, and the crystal oscillator with a frequency of 12MHz is used. The calculated data can be displayed digitally or output as serial data through the photoelectrically isolated RS485 interface. In order to ensure the reliable operation of the single-chip microcomputer, the X5045 chip is selected. The chip integrates E2 PROM, power supply monitoring and watchdog circuits to save calibration parameters and system parameters and monitor the operation of the single-chip microcomputer. When designing the circuit, the characteristics of the SPI serial interface are used to make the measuring instrument small in size, low in power consumption, and stable and reliable in operation. According to experimental tests, the speed measurement range of the measuring device is 1-200,000 r/min, and the relative error of measurement is ±0.001%.

Figure 8 MCU system block diagram

5 Conclusion

The measuring device has outstanding features: it uses a four-quadrant silicon photodetector as a detection element to directly measure the rotor's rotation speed without contact, and can measure both the rotor's rotation speed and the rotor's rotation direction; the measuring device is small and simple; it can effectively overcome the influence of noise and external interference signals; it can also overcome the influence of changes in light spot intensity on the measurement results; it has high measurement stability and test accuracy. A measurement method that can detect the size and direction of the rotor's rotation speed online using only one speed sensor is realized.