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Condition monitoring and explosion-proof design

Henry Tan's picture

This blog focuses on machinery condition monitoring, explosion-proof design, and related issues.


shengke zhi's picture

Hi Henry,

Thanks for letting me know the iMechanica. Since you start Condition Monitoring topic, I would like to give more details about Instantaneous Angular Speed technique in Condition Monitoring.

 Instantaneous Angular Speed (IAS) is investigated for last 10 years in Condition Monitoring. The content of IAS is coming from angular speed. If the time taken is reduced apporaching to zero, we can measure the angle travelled to extract IAS, or if the angle travelled is reduced to zero, we can measure the time elapsed to extract IAS.

Conventional measurements of IAS are mainly contact measurement, such as using a shaft encoder. However, the installation problems are still not been solved. It is difficult to find a place to mount an encoder into machinery. Non-contact measurement of IAS becomes popular in recent year. magnetic pick up, photo pick-up techniques have already been employed.

To reduce the cost of the hardware, and improve operation capability, another optical non-contact measurement of IAS is investigated. This method majorly employ a normal laser optical mouse to measure the shaft speed of machinery to achieve condition monitoring.

For more details, it will come soon.




Henry Tan's picture

Thanks for the information.
How is it technically realized to reduce the measuring time to zero?

shengke zhi's picture

The time reduction has two methods, including hardware technique based and software technique based.

For hardware based method, you can use an independent timer to trigger off the counter to measure the number of speed pulses in a suitable frequency, such as 1MHz. In that case, the time interval you obtained is so small that it apporaches to zero.

Another, for software based method, an internal timer is designed to trigger off the counter to measure the value of angle displaced. The interal timer can be programmed based on freuqnecy of a CPU within a computer which is used to acquire data, or be implemented based on the frequency of oscillater within the DAQ system.  

Meanwhile, an internal timer is always employed to measure the time elapsed between two successive speed pulses. In that case, the software timer starts when the speed pulses is going up at the rising edge and stops when the speed pulses is falling down at the falling edge. Because the number of speed pulses per revolution is decided by the resolution of an encoder sensor in contact measurement of IAS, the degree presented by the speed pulses thus is reduced. Therefore, combing an accurate time interval is measured and an precise angle displaced, the exact IAS can be extracted.

Henry Tan's picture

How is laser used as a photo picking-up technique? Can you elaborate it?

shengke zhi's picture

I am sorry to say it is not laser photo pick-up.

 It is Laser Speckle Pattern techniques, which means the reflected laser beam from the targeting area of the surface of a detecting shaft has less diffused scattering effect because of the coherent light, so that the reflected light is obtained by an 2D photo-array CMOS sensor placed in Fourier Plane. The light obtained generates plenty of light speckles whose size is bigger than the size of pixel in CMOS sensor. Assuming the position of the coherent light source is fixed, when the detecting shaft is rotating, the light speckle is moving along with the rotary shaft. In that case, the new speckle is generated when the previous speckle is moving out of the light beam.

The speckles has linear relationship witht the angle displacement of rotary shaft. Therefore, the IAS can extracted by measuring the angle displaced and the time elapsed.

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