Measurement and real-time display of critical speed of rotating shaft

In the operation of rotating machinery, when the speed of the shaft reaches a certain value, the operation of the shaft will become unstable and vibration will occur, and the shape of the shaft will also be significantly bent and deformed; when the speed of the shaft continues to increase, the above phenomenon will gradually disappear; but when the speed of the shaft continues to increase to another new fixed value, the above phenomenon will reappear. The speed of the shaft when the above phenomenon occurs is called the critical speed. Since the shaft will vibrate when it is at the critical speed (or close to it), it may even damage the entire machine in serious cases. Therefore, the working speed of the shaft should be designed to be far away from the critical speed. If the speed of the shaft cannot be changed at will, the critical speed value can be changed by changing the size of the shaft to ensure the normal operation of the shaft, shaft system and the entire machine. Although the measurement of the critical speed of the shaft is very important, most of the current methods for determining the critical speed are still in the theoretical calculation stage, and the theoretical calculation value is often inconsistent with the measured value. To this end, we have developed a measurement device that can measure the critical speed of rotating machinery in real time. The device can display the measurement curve in real time on the computer, intuitively reflecting the change process of speed and displacement, and the measured critical speed value can be well consistent with the actual speed.

2 Measurement principle

The measurement principle of critical speed is to determine the speed value at the maximum displacement (i.e., critical speed value) by tracking the change of the vibration displacement of the rotating shaft. Since the measurement of speed and displacement needs to correspond one to one, the speed pulse can be used as the starting measurement point, and the speed and displacement can be measured simultaneously with time as the scale. The measurement method is as follows: when the speed pulse arrives, turn on timer T1 to measure the time, turn on timer T0 to measure the speed, and perform A/D conversion at the same time; after the A/D conversion is completed, enter the interrupt, and store the conversion result and the time value at this time in the corresponding storage area; after the speed measurement is completed, store the speed value and the time value at this time in the corresponding storage area.
When the timing method (i.e., the cycle measurement principle) is used to measure the speed, the clock pulse is interpolated between two adjacent speed pulses as the counting pulse. The design value is N, the speed pulse period is Tx, and the clock pulse period is Tc, then Tx=NTc=N/fc, and the speed v=60/Tx=60fc/N. The clock pulse can be obtained through the microcontroller clock or after frequency division. The measurement error of the speed measured by the timing method is ±1 clock pulse. To improve the measurement accuracy, the frequency of the interpolated clock signal can be increased. This measuring device uses a 16-bit counter of a single-chip microcomputer and a crystal oscillator frequency of 12MHz. The frequency of the interpolation clock signal between adjacent speed pulses is 1MHz, and the maximum error of each cycle is 1µs, which can meet the measurement requirements even in high-speed measurement situations.

3 Hardware Design

Hardware circuit design of this measuring device. Atmel's high-performance microcontroller (MCU) AT89C51 is used as the lower computer. The standard configuration of AT89C51 is: 4K bytes of flash memory, 256 bytes of on-chip RAM, 32 I/O ports, and 2 16-bit timers/counters. In order to measure the rotation speed, a key slot with a depth of several millimeters is pre-processed on the shaft, and an electromagnetic switch is used as a speed sensor. When the shaft rotates, the electromagnetic switch outputs a pulse signal (one pulse is generated for each rotation), and the generated pulse meets the interrupt triggering requirements of the single-chip microcomputer. Two mutually perpendicular eddy current sensors are installed in the direction perpendicular to the rotor plane and the axis. They have no direct contact with the object to be measured and have a wide operating frequency range (DC~10kHz), which is particularly suitable for measuring the vibration of the rotor. The eddy current sensor converts the displacement change between the object to be measured into a voltage change, and then sends the voltage value to the A/D conversion chip for A/D conversion. The A/D conversion chip uses a 12-bit parallel ADC chip MAX197, with a power supply voltage of +5V, a conversion time of 6µs, a sampling rate of 100ksps, and 8 analog input channels (all or part of them can be selected by programming). The microcontroller data can be transmitted to the host computer through serial communication. The level conversion is achieved by MAX232.

The LED display can realize the real-time display of the speed measurement value. The display uses MAX7219 as the display chip. MAX7219 is a new type of serial input and output common cathode LED display driver. Its 3-wire serial interface can be easily connected to various general-purpose microcontrollers. The serial data is a 16-bit data packet sent to the DIN terminal, shifted into the internal 16-bit shift register at each rising edge of CLK, and then the data is latched at the rising edge of LOAD. The display mode is an on-chip dynamic scanning mode, and the brightness can be controlled by programming. To prevent the LED display from getting out of control, a 47µF tantalum capacitor is connected in parallel near the Max7219 power supply terminal. The display circuit has the characteristics of simple structure, low power consumption and good flexibility.

4 Software Design

Connect the speed pulse signal to P3.2 of AT89C51. Set timer T0 to mode 1, preload value to 0, and the crystal frequency used is 12MHz, so the timing time is 65536µs; set timer T1 to mode 1, preload value to 0. When the falling edge of the speed pulse arrives, the interrupt process is entered, and timers T0 and T1 are turned on to start timing, and MAX197 is turned on to start A/D conversion. After the speed measurement and displacement measurement are completed, the measurement results and the time value at this time are stored in the corresponding storage area. When measuring the speed, a total of 8 pulses are calculated. When the falling edge of the 8th pulse arrives, TR0 is cleared, the timing is stopped, and the speed value can be calculated. Let the pulse period be T, the number of timer overflows be N1, and the last timing value in the timer be N2, then T is set to = 65536N1+N2(µs). Since the timer has 8 pulses, the speed calculation formula is
v=60/T=60(/T constant/8)=(60×8×106)/(65536N1+N2)(r/min)

Prepare the lower computer program. The microcontroller initialization program is as follows:
CLR EA;
MOV SP, # 60H;
MOV TMOD, # 01H; Timer T0 preload value is 0
MOV TL0, # 00H
MOV TH0, # 00H
MOV TMOD, # 10H; Timer T1 preload value is 0
MOV TL1, # 00H
MOV TH1, # 00H
MOV SCON, # 00H; Serial port is initialized to mode 0
SETB P3.2; Interrupt port is set to 1
SETB EA; Turn on the general interrupt
SETB ET0; Timer T0 interrupt overflow position 1
SETB ET1; Timer T1 interrupt overflow position 1
SETB EX0; Turn on INT0 interrupt, and enter the speed measurement module SETB PX0 when the interrupt comes
; Set INT0 to high priority
SETB IT0; Set INT0 to edge trigger SETB EX1; Turn on INT1 interrupt, and enter the AD conversion measurement module SETB IT1
when the interrupt comes ; Set INT1
Edge trigger
HERE: AJMP HERE; Wait for interrupt
Interrupt program flowchart when the speed pulse arrives. AD interrupt program flowchart.
This measuring device realizes remote data acquisition through serial communication. The PC sends a data acquisition command to the lower computer through serial communication. After receiving the command, the lower computer performs on-site data acquisition and sends the data to the PC through serial communication. Compared with parallel communication, serial communication has the characteristics of long transmission distance, simple connection, and high data transmission reliability. The upper computer communication program is written in Visual C++6.0, which can provide an Active control MSComm, which can be used to realize the read and write management of the serial port of the AT89C51 microcontroller. Through the main interface, you can enter the speed change interface, displacement change interface and speed displacement change interface respectively, and monitor the speed and displacement change curves in real time.

5 Conclusion

The circuit design of this measuring device is simple and reliable. After actual testing, the host computer can draw the working curve of the shaft in real time and monitor the speed change. The measured critical speed value is consistent with the actual processing state, achieving satisfactory results.