如何提高角度測量的精度一直是二維測量儀器的難題。 市面上流行的二維測量儀器上，角度測量基本上有兩種方法，一種是切線法，一種是點計算法。
切線法是指手動旋轉屏幕上或鏡頭中的刻線，分別對準工件的兩個邊緣，通過編碼器或圓光柵計數來測量角度的方法。 該方法又分為兩種：投影切線法，如投影儀、工具顯微鏡等；圖像切線法，如帶視頻功能的成像儀、工具顯微鏡，依靠軟件自帶的米線旋轉測量 .
切線法操作方便簡單，但測量精度低，適用于快速批量檢測。 如果被測件的角度精度讀數較高，另一種方法，即點計算法更為合適。
所有的幾何元素都是由點組成的，包括直線、曲線和圓弧的基本元素。 二維平面角由兩條直線組成，基本幾何元素，一條直線由無數個點組成。 因此，角度測量是否準確，點是最關鍵的。
測角技巧1：直線采集盡可能長。
影像測量儀，由于屏幕顯示有限，放大倍率大（一般為0.7到4.5，28X到180X），屏幕上顯示的工件尺寸實際上只有幾毫米，很多測量員習慣只在屏幕上進行測試 在顯示部分收集點和線元素。 如果采集點有偏差，線段越短，測量角度值偏差越大，線段越長，測量角度值偏差越小。 如圖1所示，理論角度為30度，點偏差為0.25mm，可以清楚地看到線段長度對實測值的影響。
所以，我們在測量角度的時候，盡量把角度兩邊的線盡量收長。 如果屏幕顯示范圍太小，可以移動工作臺，在角度所在線的起點附近選取一點，然后在終點處選取一點。 角度測量誤差將大大降低。
測角技巧二：回歸線偏差小。
很多檢測人員反應，測量角度時重復性很差。 同一個人用同樣的方法，兩次測量的重復性高達0.5度。
在很多圖像測量軟件中，包括三坐標測量軟件，默認的線采集是兩點。 對于一些比較規則、線性度好的零件，不會造成太大的誤差，但是對于線性度差、毛刺較多的零件，直線兩點采集的方法會帶來很大的誤差。 而且重復性很差，多次測量的重復性對于這樣一條直線形成的角度肯定不好。
如果我們用多點求回歸線的方法來確定角度的兩邊，直線會更接近被測工件的實際邊緣，直線偏差會減小，測量誤差也會 大大降低，測量值的重復性將大大提高。
角度測量技術3：放大倍數盡可能大。
對于很多機械零件來說，被測角的邊緣很短，只有2mm到3mm，例如軸類零件的倒角。 如果我們也用0.7或1的最小鏡頭齒輪進行點測，工件的像只有48mm到120mm，點偏差會對測量值有很大的影響。 如果我們切換到3倍或4倍的放大倍率，工件成像可以達到240mm到480mm，圖像邊緣的真實情況更容易觀察，采樣點偏差也會最小化。 不過這種方法也帶來了很多不便，圖形太大，顯示窗口只能顯示一小部分，但對于技術熟練的檢驗人員和追求高精度的品控來說，這些應該不是問題。
影像測量儀是二維測量儀器中功能最多、應用范圍最廣的儀器。 它的基本功能是有限的：點、線、圓和弧的集合。 但是，它的組合計算的功能是相當廣泛的。 只要動動腦筋，用好，很多測量問題都能很好的解決。
How to improve the accuracy of Angle measurement has always been a difficult problem for two-dimensional measuring instruments. There are two basic methods for Angle measurement of popular two-dimensional measuring instruments in the market, one is the tangent method, the other is the point calculation method.
Tangent method refers to the method of manually rotating the lines carved on the screen or in the lens, respectively aligned with the two edges of the workpiece, and measuring Angle by encoder or circular grating counting. This method is divided into two kinds, projection tangent method, such as projector, tool microscope, etc., and image tangent method, such as image instrument, tool microscope with video function, rely on the software with metre line rotation measurement.
Tangent method is convenient and simple to operate, but the measurement of intensive reading is low, suitable for rapid batch detection, if the Angle of the measured parts intensive reading requirements are high, using another method, the point calculation method is more suitable.
All geometric elements are made up of elements, including the basic elements straight lines, curves and arcs. A two-dimensional plane Angle consists of two straight lines of the basic geometric elements, consisting of an infinite number of points. So the Angle measurement accuracy, the point is the most critical.
Angle measurement technique 1: straight line collection as long as possible.
Image measuring instrument, due to the screen display is limited, coupled with large magnification (generally in 0.7 ~ 4.5 files 28X ~ 180X), the screen display part of the workpiece size is actually only a few millimeters, many surveyors in the detection of the habit of only on the screen display part of the collection of points, line elements. If the collected point is biased, the shorter the line segment is, the greater the measured Angle value deviation will be, and the longer the line segment, the smaller the measured Angle value deviation will be. As shown in Figure 1, the theoretical Angle is 30 degrees and the sampling point deviation is 0.25mm. We can clearly see the influence of line length on the measured value.
Figure 1
So when we measure the Angle, try to collect the line on both sides of the Angle longer, if the screen display range is too small, you can move the table, at the starting point of the Angle of the line, and then at the end point, so that the Angle error will be greatly reduced.
Angle measurement technique 2: small deviation of regression line.
There are a lot of detection personnel reaction, in the measurement of Angle, the repetition accuracy is very poor, the same person with the same method, two measurement repetition error reached as much as 0.5 degree.
Many image measurement software, including CMM software, line acquisition is two points by default. For some of the more regular, linear parts, will not cause too much error, but for the linearity is not good, more burr parts, two point collection of linear method will bring a lot of error, and the repetition accuracy is very poor, such a straight line Angle, repeatability of multiple measurements will not be good.
If we use the method of multi-point search regression line to determine the Angle of both sides, the line will be more close to the actual line of the measured workpiece, the line deviation will be reduced, at the same time, the measurement error will be reduced a lot, the repeatability of the measurement is greatly improved. See Figure 2.
Figure 2
Angle measurement technique three: magnification as large as possible.
Many mechanical parts, measured Angle edge line is very short, only 2mm ~ 3mm, for example, shaft parts chamfering. If we also use the lens minimum 0.7, or 1 for point measurement, the workpiece imaging is only 48mm ~ 120mm, point deviation will have a great impact on the measured value. If we change to magnification 3 or 4, the workpiece image can reach 240mm ~ 480mm, the real situation of the image edge is easier to observe, and the point deviation will be minimized. However, this method also brings a lot of inconvenience, the graph is too large, the display window can only display a small part, but for skilled inspection personnel and the pursuit of high precision quality control, these should not be a problem.
Image measuring instrument is a two-dimensional measuring instrument with the most functions and the widest range of application. Its basic function is limited: point, line, circle, arc collection. But its combination calculation function is quite extensive, as long as open the mind, good at using, a lot of measurement problems can be very good solution.
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