如何提高角度测量的精度一直是二维测量仪器的难题。 市面上流行的二维测量仪器上，角度测量基本上有两种方法，一种是切线法，一种是点计算法。
切线法是指手动旋转屏幕上或镜头中的刻线，分别对准工件的两个边缘，通过编码器或圆光栅计数来测量角度的方法。 该方法又分为两种：投影切线法，如投影仪、工具显微镜等；图像切线法，如带视频功能的成像仪、工具显微镜，依靠软件自带的米线旋转测量 .
切线法操作方便简单，但测量精度低，适用于快速批量检测。 如果被测件的角度精度读数较高，另一种方法，即点计算法更为合适。
所有的几何元素都是由点组成的，包括直线、曲线和圆弧的基本元素。 二维平面角由两条直线组成，基本几何元素，一条直线由无数个点组成。 因此，角度测量是否准确，点是最关键的。
测角技巧1：直线采集尽可能长。
影像测量仪，由于屏幕显示有限，放大倍率大（一般为0.7到4.5，28X到180X），屏幕上显示的工件尺寸实际上只有几毫米，很多测量员习惯只在屏幕上进行测试 在显示部分收集点和线元素。 如果采集点有偏差，线段越短，测量角度值偏差越大，线段越长，测量角度值偏差越小。 如图1所示，理论角度为30度，点偏差为0.25mm，可以清楚地看到线段长度对实测值的影响。
所以，我们在测量角度的时候，尽量把角度两边的线尽量收长。 如果屏幕显示范围太小，可以移动工作台，在角度所在线的起点附近选取一点，然后在终点处选取一点。 角度测量误差将大大降低。
测角技巧二：回归线偏差小。
很多检测人员反应，测量角度时重复性很差。 同一个人用同样的方法，两次测量的重复性高达0.5度。
在很多图像测量软件中，包括三坐标测量软件，默认的线采集是两点。 对于一些比较规则、线性度好的零件，不会造成太大的误差，但是对于线性度差、毛刺较多的零件，直线两点采集的方法会带来很大的误差。 而且重复性很差，多次测量的重复性对于这样一条直线形成的角度肯定不好。
如果我们用多点求回归线的方法来确定角度的两边，直线会更接近被测工件的实际边缘，直线偏差会减小，测量误差也会 大大降低，测量值的重复性将大大提高。
角度测量技术3：放大倍数尽可能大。
对于很多机械零件来说，被测角的边缘很短，只有2mm到3mm，例如轴类零件的倒角。 如果我们也用0.7或1的最小镜头齿轮进行点测，工件的像只有48mm到120mm，点偏差会对测量值有很大的影响。 如果我们切换到3倍或4倍的放大倍率，工件成像可以达到240mm到480mm，图像边缘的真实情况更容易观察，采样点偏差也会最小化。 不过这种方法也带来了很多不便，图形太大，显示窗口只能显示一小部分，但对于技术熟练的检验人员和追求高精度的品控来说，这些应该不是问题。
影像测量仪是二维测量仪器中功能最多、应用范围最广的仪器。 它的基本功能是有限的：点、线、圆和弧的集合。 但是，它的组合计算的功能是相当广泛的。 只要动动脑筋，用好，很多测量问题都能很好的解决。
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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