如何使用光電轉速表

如何使用光電轉速表

要使用光電轉速表，首shou先xian要yao在zai要yao測ce量liang的de旋xuan轉zhuan表biao麵mian上shang創chuang建jian一yi個ge反fan射she標biao記ji。您nin還hai必bi須xu確que保bao旋xuan轉zhuan曲qu麵mian的de其qi餘yu部bu分fen相xiang對dui而er言yan不bu反fan光guang。一yi個ge簡jian單dan的de方fang法fa是shi在zai黑hei色se或huo暗an淡dan的de金jin屬shu軸zhou上shang用yong油you漆qi標biao記ji創chuang建jian一yi個ge白bai點dian。然ran後hou，將jiang光guang電dian轉zhuan速su表biao發fa出chu的de光guang束shu對dui準zhun旋xuan轉zhuan軸zhou上shang的de標biao記ji。每mei次ci反fan射she點dian通tong過guo光guang電dian轉zhuan速su表biao的de光guang束shu時shi，一yi些xie光guang都dou會hui被bei反fan射she回hui光guang電dian轉zhuan速su表biao
，並bing在zai那na裏li被bei光guang傳chuan感gan器qi接jie收shou。通tong過guo計ji算suan光guang傳chuan感gan器qi在zai給gei定ding時shi間jian內nei觸chu發fa的de次ci數shu
，可ke以yi確que定ding被bei測ce機ji械xie的de轉zhuan速su。

例如
，假設我們有一個電機以每分鍾1200轉(zhuan)的(de)速(su)度(du)旋(xuan)轉(zhuan)。我(wo)們(men)將(jiang)光(guang)電(dian)轉(zhuan)速(su)表(biao)的(de)光(guang)束(shu)對(dui)準(zhun)我(wo)們(men)在(zai)電(dian)機(ji)軸(zhou)上(shang)標(biao)記(ji)的(de)白(bai)點(dian)。讓(rang)我(wo)們(men)假(jia)設(she)光(guang)電(dian)轉(zhuan)速(su)表(biao)計(ji)算(suan)它(ta)在(zai)一(yi)秒(miao)鍾(zhong)的(de)單(dan)位(wei)時(shi)間(jian)內(nei)看(kan)到(dao)的(de)脈(mai)衝(chong)數(shu)量(liang)來(lai)進(jin)行(xing)測(ce)量(liang)。在(zai)一(yi)秒(miao)鍾(zhong)內(nei)，白(bai)點(dian)將(jiang)通(tong)過(guo)光(guang)束(shu)20次
，在光束經過時觸發我們的光傳感器。然後，光電轉速表中的微控製器進行一些簡單的數學運算——一秒鍾內20個脈衝，乘以60秒——我們在顯示屏上得到每分鍾1200轉的轉速。

很容易想象，如果我們的軸旋轉得慢得多，大約每分鍾10轉，我們的光電轉速表每六秒鍾隻會看到一個脈衝。如果我們把轉速提高到12轉/分(fen)，它(ta)仍(reng)然(ran)是(shi)每(mei)五(wu)秒(miao)隻(zhi)有(you)一(yi)個(ge)脈(mai)衝(chong)。我(wo)們(men)的(de)轉(zhuan)速(su)表(biao)在(zai)測(ce)量(liang)如(ru)此(ci)低(di)的(de)轉(zhuan)速(su)時(shi)會(hui)受(shou)到(dao)影(ying)響(xiang)，它(ta)需(xu)要(yao)很(hen)長(chang)時(shi)間(jian)才(cai)能(neng)注(zhu)意(yi)到(dao)任(ren)何(he)變(bian)化(hua)。在(zai)這(zhe)種(zhong)情(qing)況(kuang)下(xia)，我(wo)們(men)能(neng)做(zuo)些(xie)什(shen)麼(me)嗎(ma)
？

為什麼是的，有！我們可以在旋轉軸上放置更多的反射點。讓我們在旋轉電機軸上畫十個點。現在，在10轉/分的轉速下，我們每0.6秒就會得到一個光脈衝
，在12轉/分
，每0.5秒就會得到。我們的轉速表現在能夠更快地響應低速範圍內的速度變化——我們隻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。許多光電轉速表或激光轉速表的(de)盒(he)子(zi)裏(li)都(dou)有(you)反(fan)光(guang)膠(jiao)帶(dai)。這(zhe)是(shi)一(yi)種(zhong)特(te)殊(shu)的(de)膠(jiao)帶(dai)，反(fan)光(guang)膠(jiao)帶(dai)上(shang)的(de)微(wei)粒(li)可(ke)以(yi)讓(rang)膠(jiao)帶(dai)以(yi)任(ren)何(he)角(jiao)度(du)反(fan)射(she)光(guang)線(xian)。在(zai)旋(xuan)轉(zhuan)機(ji)器(qi)上(shang)使(shi)用(yong)這(zhe)種(zhong)反(fan)光(guang)膠(jiao)帶(dai)代(dai)替(ti)白(bai)色(se)油(you)漆(qi)，可(ke)以(yi)讓(rang)我(wo)們(men)用(yong)光(guang)電(dian)轉(zhuan)速(su)表(biao)在(zai)與(yu)標(biao)記(ji)垂(chui)直(zhi)的(de)角(jiao)度(du)之(zhi)外(wai)進(jin)行(xing)測(ce)量(liang)。我(wo)自(zi)己(ji)用(yong)這(zhe)個(ge)來(lai)測(ce)量(liang)我(wo)汽(qi)車(che)的(de)發(fa)動(dong)機(ji)轉(zhuan)速(su)。不(bu)可(ke)能(neng)將(jiang)轉(zhuan)速(su)表(biao)直(zhi)接(jie)指(zhi)向(xiang)曲(qu)軸(zhou)，但(dan)有(you)了(le)反(fan)光(guang)帶(dai)，我(wo)可(ke)以(yi)從(cong)上(shang)方(fang)以(yi)一(yi)個(ge)奇(qi)怪(guai)的(de)角(jiao)度(du)將(jiang)激(ji)光(guang)指(zhi)向(xiang)曲(qu)軸(zhou)皮(pi)帶(dai)輪(lun)，仍(reng)然(ran)可(ke)以(yi)獲(huo)得(de)良(liang)好(hao)的(de)讀(du)數(shu)。

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.