如何使用光電轉(zhuǎn)速表

要使用光電轉(zhuǎn)速表，首先要在要測量的旋轉(zhuǎn)表面上創(chuàng)建一個反射標記。您還必須確保旋轉(zhuǎn)曲面的其余部分相對而言不反光。一個簡單的方法是在黑色或暗淡的金屬軸上用油漆標記創(chuàng)建一個白點。然后，將光電轉(zhuǎn)速表發(fā)出的光束對準旋轉(zhuǎn)軸上的標記。每次反射點通過光電轉(zhuǎn)速表的光束時，一些光都會被反射回光電轉(zhuǎn)速表，并在那里被光傳感器接收。通過計算光傳感器在給定時間內(nèi)觸發(fā)的次數(shù)，可以確定被測機械的轉(zhuǎn)速。

例如，假設(shè)我們有一個電機以每分鐘1200轉(zhuǎn)的速度旋轉(zhuǎn)。我們將光電轉(zhuǎn)速表的光束對準我們在電機軸上標記的白點。讓我們假設(shè)光電轉(zhuǎn)速表計算它在一秒鐘的單位時間內(nèi)看到的脈沖數(shù)量來進行測量。在一秒鐘內(nèi)，白點將通過光束20次，在光束經(jīng)過時觸發(fā)我們的光傳感器。然后，光電轉(zhuǎn)速表中的微控制器進行一些簡單的數(shù)學(xué)運算——一秒鐘內(nèi)20個脈沖，乘以60秒——我們在顯示屏上得到每分鐘1200轉(zhuǎn)的轉(zhuǎn)速。

很容易想象，如果我們的軸旋轉(zhuǎn)得慢得多，大約每分鐘10轉(zhuǎn)，我們的光電轉(zhuǎn)速表每六秒鐘只會看到一個脈沖。如果我們把轉(zhuǎn)速提高到12轉(zhuǎn)/分，它仍然是每五秒只有一個脈沖。我們的轉(zhuǎn)速表在測量如此低的轉(zhuǎn)速時會受到影響，它需要很長時間才能注意到任何變化。在這種情況下，我們能做些什么嗎？

為什么是的，有！我們可以在旋轉(zhuǎn)軸上放置更多的反射點。讓我們在旋轉(zhuǎn)電機軸上畫十個點。現(xiàn)在，在10轉(zhuǎn)/分的轉(zhuǎn)速下，我們每0.6秒就會得到一個光脈沖，在12轉(zhuǎn)/分，每0.5秒就會得到。我們的轉(zhuǎn)速表現(xiàn)在能夠更快地響應(yīng)低速范圍內(nèi)的速度變化——我們只需要記住將顯示速度除以10，就可以考慮到我們的額外標記。

反光性更強的標記效果更好。

還有其他技巧可以用來提高性能。許多光電轉(zhuǎn)速表或激光轉(zhuǎn)速表的盒子里都有反光膠帶。這是一種特殊的膠帶，反光膠帶上的微?？梢宰屇z帶以任何角度反射光線。在旋轉(zhuǎn)機器上使用這種反光膠帶代替白色油漆，可以讓我們用光電轉(zhuǎn)速表在與標記垂直的角度之外進行測量。我自己用這個來測量我汽車的發(fā)動機轉(zhuǎn)速。不可能將轉(zhuǎn)速表直接指向曲軸，但有了反光帶，我可以從上方以一個奇怪的角度將激光指向曲軸皮帶輪，仍然可以獲得良好的讀數(shù)。

HOW TO USE A PHOTO TACHOMETER

To use the photo tachometer, you start by creating a reflective mark on the rotating surface you wish to measure. You must also ensure the rest of the rotating surface is comparatively non-reflective. An easy way to do this would be to create a white spot with a paint marker on an otherwise black or dull-metal shaft. Then, aim the beam of light from the photo-tachometer at the mark on the spinning shaft. Each time the reflective spot passes the beam of the photo-tachometer, some light is reflected back towards the device, where it is picked up by a light sensor. By counting the number of times the light sensor is triggered in a given time, it’s possible to determine the rotational speed of the machinery under test.

For example, let’s say we have a motor spinning at 1200 revolutions per minute. We aim the photo-tachometer’s beam at the white spot we’ve marked on the motor’s shaft. Let’s assume the photo-tachometer counts the number of pulses it sees in a unit time of one second to make its measurements. In one second, the white spot will pass the beam twenty times, triggering our light sensor as it goes by. The microcontroller in the photo-tachometer then does some simple maths – twenty pulses in one second, multiplied by 60 seconds – and we get a rotational speed of 1200 revolutions per minute on the display.

It is easy to imagine that if our shaft is rotating much more slowly, on the order of 10 RPM, our photo-tachometer will only see one pulse every six seconds. If we up this to 12 RPM, it’s still only one pulse every five seconds. Our tach is going to suffer trying to measure such low rotational speeds and it’s going to take a long time for it to notice any changes. Is there anything we can do to help in this situation?

Why yes, there is! We can place additional reflective spots on our rotating shaft. Let’s put ten spots on our rotating motor shaft. Now at 10 RPM, we’re getting a pulse of light every 0.6 seconds, and at 12 RPM, every 0.5 seconds. Our tach is now able to much more quickly respond to changes in speed at the low range – and we just need to remember to divide the display speed by ten to account for our additional markers.

A more reflective marker works better.

There are other tricks you can use to improve performance, too. Many photo-tachs come with a supply of retro-reflective tape in the box. This is a special tape filled with lots of microscopic glass spheres that allow the tape to reflect light at any angle. Using this instead of white paint on a rotating machine allows us to measure with the photo-tachometer at an angle other than perpendicular to the marking. I used this myself to make a measurement of my car’s engine speed. It was impossible to point the tachometer straight at the crankshaft – but with the retro-reflective tape, I was able to point the laser at the crank pulley from above at an odd angle and still get a good reading.