Measurement and real-time display of critical spee

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

in the operation of rotating machinery, when the speed of rotating shaft reaches a certain value, the operation of rotating shaft will become unstable and vibration will occur, and the shape of rotating shaft will also have obvious bending deformation; When the rotating speed of the shaft continues to rise, the above phenomenon gradually disappears; However, when the rotating speed of the shaft continues to rise to another new fixed value, the above phenomenon will reappear. The speed of the shaft when the above phenomena occur is called the critical speed. Since the rotating shaft will vibrate when it is at (or close to) the critical speed, and even damage the whole machine in serious cases, the working speed of the rotating shaft should be designed away from the critical speed. If the rotating speed of the rotating shaft cannot be changed at will, the critical speed value can be changed by changing the size of the rotating shaft, so as to ensure the normal operation of the shaft, shafting and the whole machine. Although the measurement of the critical speed of the rotating shaft is very important, most of the methods to determine the critical speed are still in the theoretical calculation stage, and the theoretical calculation value is often inconsistent with the measured value. Therefore, we have developed a measuring 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, directly reflect the change process of speed and displacement, and the measured critical speed value can be in good agreement with the actual speed

2 measurement principle

the measurement principle of critical speed is to determine the speed value (i.e. critical speed value) at the maximum displacement by tracking the change of shaft vibration displacement. Since the measurement of speed and displacement needs to be one-to-one corresponding, the speed pulse can be used as the starting measurement point to measure speed and displacement at the same time on a time scale. The measurement method is as follows: when the speed pulse arrives, turn on the timer T1 to measure the time, turn on the timer T0 to measure the speed, and conduct 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, the speed value and the time value at this time are stored in the corresponding storage area

when the speed is measured by the timing method (i.e. the principle of perimeter measurement), the clock pulse is interpolated between two adjacent speed pulses as the counting pulse. If 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 by single chip microcomputer clock or after frequency division. The measurement error of rotating speed measured by timing method is ± 1 clock pulse. In order to improve the measurement accuracy, the frequency of interpolation clock signal can be increased. This measuring device adopts a 16 bit counter of single chip microcomputer, and the crystal oscillator frequency is 12Mhz, so the frequency of interpolating clock signal between adjacent speed pulses is 1MHz, and the maximum error of each cycle is 1 s, which can meet the measurement requirements even in high-speed measurement occasions

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 byte flash memory, 256 byte on-chip RAM, 32 i/o ports, and 2 16 bit timers/counters. In order to measure the speed, a keyway with a depth of several millimeters is pre machined on the rotating shaft, and an electromagnetic switch is used as the speed sensor. When the rotating shaft rotates, the electromagnetic switch outputs a pulse signal (one pulse per revolution), and the generated pulse meets the interrupt trigger 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 measured object and have a wide frequency range (DC ~ 10kHz), which is particularly suitable for measuring the vibration of the rotor. The eddy current sensor converts the displacement change with the measured object into voltage change, and then sends the voltage value to the a/d conversion chip for a/d conversion. 2. Because the gapless drive a/d conversion chip adopts 12 bit parallel ADC chip MAX197, its power supply voltage is +5v, the conversion time is 6 s, the sampling rate is 100ksps, and there are 8 analog input channels (all or part can be selected by programming). Through serial communication, the data of single chip microcomputer can be transmitted to the upper computer. The level conversion is realized by MAX232

led display can realize the real-time display of speed adapting to the new normal measurement, such as oil industry, gasification industry, nuclear power industry and so on. MAX7219 is selected as the display chip, which is called conditional yield limit or conditional yield strength. 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 packet, which is sent to the din end. The rising edge of each CLK is moved into the internal 16 bit shift register, and then the data is latched on the rising edge of load. The display mode is on-chip dynamic scanning mode, and the brightness can be controlled by programming. In order to prevent the LED display from getting out of control, a 47 f tantalum capacitor is connected in parallel near the MAX7219 power end. 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. Timer T0 is set as mode 1, the pre installed value is 0, and the crystal oscillator frequency used is 12Mhz, so the timing time is 65536 s; Timer T1 is set to mode 1, and the pre installed value is 0. When the falling edge of the speed pulse arrives, it will enter the interruption process, open timers t0 and T1 to start timing, and open MAX197 to start a/d conversion at the same time. After the speed measurement and displacement measurement, 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 and the timing is stopped, and the speed value can be calculated. Set the pulse period as t, the overflow times of the timer as N1, and the last timing value in the timer as N2, then t is set =65536n1+n2 (s). Since there are 8 pulses in the timer, the speed calculation formula is

v=60/t=60 (/t set/8) = (60 × eight × 106)/(65536n1+n2) (r/min)

compile the lower computer program. The initialization procedure of single chip microcomputer is as follows:

clr EA

MOV SP,# 60H;

MOV TMOD,# 01H; The preset value of timer T0 is 0

mov tl0, # 00h

mov Th0, # 00h

mov tmod, # 10h; The preset value of timer T1 is 0

mov TL1, # 00h

mov Th1, # 00h

mov scon, # 00h; The serial port is initialized to mode 0

setb p3.2; Interrupt port set 1

setb EA; Open total interrupt

setb ET0; Timer T0 interrupt overflow position 1

setb ET1; Timer T1 interrupt overflow position 1

setb ex0; Open INT0 interrupt, and enter the speed measurement module

setb px0 when the interrupt comes; Make INT0 high priority

setb it0; Set INT0 as the edge to trigger

setb EX1; Turn on INT1 interrupt, and enter AD conversion measurement module

setb it1 when the interrupt comes; Set INT1 as the edge to trigger

here:ajmp here; Block diagram of interrupt program when waiting for the arrival of interrupt

speed pulse. Ad interrupt program block diagram

this measuring device realizes remote data acquisition through serial communication. The PC sends data acquisition commands to the lower computer through serial communication. After receiving the commands, the lower computer carries out 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 reliability of data transmission. The upper computer communication program is written in visual c++6.0. It can provide an active control MSComm, which can be used to realize the reading and writing management of the serial port of AT89C51 single chip microcomputer. Through the main interface, you can enter the speed change interface, displacement change interface and speed displacement change interface respectively to monitor the speed and displacement change curve in real time

5 conclusion

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

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