增量式光学编码器介绍(中英文)
介绍 / Introduction
光学编码器是一种机电设备,使用光源、光探测器和光栅将旋转或线性位置转换为电信号。光学编码器可以是增量式的,也可以是绝对式的,这里主要介绍增量式光学编码器。主要有三种实现方式:透射式、反射式和干涉式。本文讨论了每种实现的技术和相对优点。
An optical encoder is an electromechanical device that uses a light source, light detectors, and an optical grating to convert rotary or linear position to an electrical signal. Optical encoders can be incremental or absolute but the focus here is incremental. There are three primary implementations: transmissive, reflective and interferential. This paper discusses the technology and relative merits of each implementation.
关键术语 / Key Terms
分辨率/ Resolution
定义可移动或测量的最小位置增量,通常以“计数”表示。高性能伺服系统需要高分辨率。定位系统在两个计数之间“抖动”,因此分辨率越高,抖动越小。分辨率对低速时的速度脉动也有显着影响。由于速度是从位置反馈得出的,如果分辨率较低,样本中可能没有足够的数据来准确地得出速度。在高速情况下,高分辨率设备可以生成超出控制器或伺服驱动器跟踪能力的数据速率。
Defines the smallest position increment that can be moved or measured and is typically expressed in “counts”. High resolution is required for high performance servo systems. A positioning system “dithers” between two counts so the higher the resolution, the smaller the dither. Resolution also has a significant impact on velocity ripple at low speed. Since velocity is derived from position feedback, if the resolution is low there may be insufficient data in a sample to accurately derive velocity. At high speeds, high resolution devices can generate data rates beyond the tracking capability of the controller or servo drive.
插值法 / Interpolation
光学编码器生成正弦和余弦信号。这些信号的周期由设备固有的“音调”定义。利用正弦/余弦信息,理论上可以通过计算信号比率来获得无限分辨率。这种技术称为插值。实际上,正弦/余弦信号的保真度和信噪比限制了可实现的分辨率。
Optical encoders generate sine and cosine signals. The period of these signals is defined by the inherent “pitch” of the device. With sin/cos information, it is theoretically possible to have infinite resolution by computing the ratio of the signals. This technique is known as interpolation. In practice, the fidelity of the sin/cos signals and signal to noise ratio limit the realizable resolution.
精度 / Accuracy
定义为每个测量位置与实际物理位置的接近程度。精度在很大程度上是一个系统问题,可能由偏心度、直线度和平面度等机械误差决定。传感器误差包括音调的非累积随机变化(线性)、累积音调误差(斜率)以及内部正弦/余弦信号保真度的变化。精密机器制造商通常通过偏移查找表来校准误差。
Defines how close each measured position is to the actual physical position. Accuracy is very much a system issue, and can be dominated by mechanical errors such as eccentricity, straightness and flatness. Sensor errors include non-accumulating random variations in pitch (linearity), accumulating pitch errors (slope), and variations in fidelity of internal sin/cos signals. Precision machine builders typically calibrate out errors via a lookup table of offsets.
重复性 / Repeatability
定义为系统多次返回同一物理位置时的测量位置范围。重复性可能比绝对精度更重要。为了有效校准系统误差,每个位置读数保持一致非常重要。传感器迟滞(不同的读数取决于测量位置的接近方向)是可重复性的重要因素。
Defines the range of measured positions when the system is returned to the same physical position multiple times. Repeatability can be more important than absolute accuracy. For system inaccuracies to be effectively calibrated, it is important for each position reading to be consistent. Sensor hysteresis (different readings depending on direction of approach to measure position) is an important factor in repeatability.
光学编码器 - 透射式 / Optical Encoder - Transmissive
透射式编码器使用由 LED 光源照明的精细光栅或“刻度尺”进行光学扫描。 旋转或线性刻度尺由透明和不透明“线”组成,按 5050 占空比排列。 光盘上透明区域的数量与定义编码器分辨率的刻度间距相对应。
传感器产生与入射光强度成正比的电压。 当传感器相对于标尺移动时,电压呈正弦变化。 添加了第二个光检测器,相位差 90°。 这涉及半个刻度线的位移。 传感器 A 的信号是否超前传感器 B 的信号(反之亦然)定义了相对运动的方向。 编码器输出可以是正弦/余弦信号,但信号更通常转换为方波:Aquad B(quad 与 90° 相移相关)。 控制器检测每个方波边缘的转变,这有效地将编码器分辨率提高了 4 倍。
与每条线的宽度相比,检测器往往较大。在较高的分辨率下,这可能会导致通道之间的溢出。添加与通道模式匹配的掩模有助于净化信号。这种设计的缺点是标尺和传感器之间的气隙必须非常小,对圆盘参数(例如平面度、偏心度和对准度)施加严格的规范,使设备更容易受到冲击和振动的影响。
相控阵增量编码器使用固态技术来提供更强大的解决方案。相控阵编码器不是每个通道都有一个离散检测器,而是具有一个检测器阵列,以便每个通道都被多个检测器覆盖。这种方法对光信号进行平均,最大限度地减少由制造误差(例如光盘偏心和未对准)引起的变化,并在放宽制造公差的同时提高性能。
编码器本质上是增量式的,通常有一个额外的标尺轨道,带有一条透明线和单独的传感器。传感器生成定义设备零位的索引信号。
传输式编码器通常封装在带有内部轴承和轴的外壳中,用于通过弹性联轴器连接到电机。这些外壳具有多种A猛片免费播放网等级,并且体积庞大。
The transmissive encoder uses optical scanning of a fine grating or “scale”, illuminated by an LED light source. The scale, rotary or linear, is made of transparent and opaque “lines” that are arranged in a 5050 duty cycle. The number of transparent regions on the disc corresponds to the scale pitch which defines the resolution of the encoder.
The sensor generates a voltage in proportion to the incident light intensity. As the sensor moves relative to the scale, the voltage varies sinusoidally. A second light detector is added 90° out of phase. This relates to a displacement of half a scale line. Whether the signal from sensor A leads sensor B, or vice versa, defines the direction of relative motion. The encoder output can be sin/cos signals, but the signals are more typically converted to square waves: A quad B (quad relates to 90° phase shift). A controller detects transitions on the edge of each square wave, which effectively increases the encoder resolution by a factor of 4.
The detectors tend to be large compared to the width of each line. At higher resolutions this can lead to spillover between channels. Adding a mask that matches the pattern of the channels helps clean the signal. The trade-off with this type of design is that the air gap between scale and sensor must be very small, imposing strict specifications on disc parameters such as flatness, eccentricity, and alignment, making the device more vulnerable to shock and vibration.
Phased-array incremental encoders use solid state technology to provide a more robust solution. Instead of a discrete detector for each channel, a phased-array encoder features an array of detectors so that each channel is covered by multiple detectors. This approach averages the optical signal, minimizing variations introduced by manufacturing errors, such as disc eccentricity and misalignment, and improves performance while relaxing fabrication tolerances.
Inherently incremental, the encoder typically has an additional scale track with a single transparent line and separate sensor. The sensor generates an index signal defining the null position of the device.
A transmissive encoder is typically packaged in an enclosure with internal bearings and a shaft for connection to a motor via a flexible coupling. The enclosures are available with a range of sealing ratings and are bulky.
光学编码器 - 反射式 / Optical Encoder - Reflective
反射式光电编码器的原理与透射式编码器非常相似。反射式编码器的工作原理是从与传感器相同的一侧(相对于码盘)发射光,并选择性地将部分光反射到传感器。减小物理尺寸是该解决方案的一个明显优势。无需透射式编码器通常所需的准直光学器件,并且 LED 光源与传感器位于同一侧,因此可以大幅减小编码器的总体积。然而,分辨率和精度可能不如透射式编码器。与传输编码器类似,附加索引轨道用于定义空位置。
反射式编码器通常是没有外壳或轴承的模块化设备,必须内置到机械系统中。它们更加紧凑,但可能需要更良好的环境。
The principle of the reflective optical encoder is very similar to the transmissive encoder. A reflective encoder works by emitting light from the same side as the sensor (relative to the code disc), and selectively reflecting portions of the light to the sensor. Reduced physical dimensions is a clear advantage of this solution. Without the collimation optics typically required in a transmissive encoder, and with the LED light source on the same side as the sensor, the total volume of the encoder can be reduced substantially. However, resolution and accuracy may not be as good as the transmissive encoder. Similar to the transmissive encoder, an additional index track is used to define the null position.
Reflective encoders are typically modular devices with no enclosure or bearings and must be built into the mechanical system. They are much more compact but may require a more benign environment.
光学编码器 - 干涉式 / Optical Encoder - Interferential
干涉式光学编码器通过相干激光光源产生发散光束来工作,该光束照亮印在标尺上的衍射光栅图案。光栅图案是使用玻璃刻度上的铬沉积或金属带刻度上的激光写入线创建的。20μm 节距光栅使光发生衍射,产生明暗高对比度的干涉图案,直接返回到探测器阵列上。本质上是增量的,通常使用第二个索引/标记轨道。
衍射光产生干涉图案的离散塔尔博特平面。在上图的示例中,使用了第三个 Talbot 平面。当标尺和检测器的相对位置发生变化时,衍射图案会在检测器阵列上平移,从而导致每个检测器单元中发生正弦变化。
干涉技术需要更少的光学元件,从而导致传感器尺寸较小。在没有插值的情况下,分辨率通常超过一个数量级,高于透射式或反射式光学编码器。由于正弦和余弦信号的保真度,可以进行高插值,从而产生高精度的纳米分辨率。考虑到设备的精度,对准公差要求并不过分。
此类编码器需要清洁的环境。采用相干性较低的 LED 光源,结合准直和过滤光学器件,可显着提高抗污染能力。编码器不可避免地更大,并且通常具有更严格的对准公差。
An interferential optical encoder operates by a coherent laser light source generating a diverging beam, which illuminates a diffraction grating pattern printed on the scale. The grating pattern is created using either chrome deposition on a glass scale, or laser written lines on a metal tape scale. The 20?m pitch grating diffracts the light to generate a high contrast interference pattern of bright and dark, directly back onto a detector array. Inherently incremental, a second index/marker track is typically used.
The diffracted light creates discrete Talbot planes of interference patterns. In the example from Figure 3 above, the 3rd Talbot plane is utilized. As the relative position of the scale and detector changes, the diffraction pattern translates across the detector array, resulting in a sinusoidal change in each detector cell.
Interferential technology requires minimal optical components, resulting in a sensor of small size. Resolution, without interpolation, is typically more than order of magnitude, higher than transmissive or reflective optical encoders. Because of the fidelity of the sine and cosines signals, high interpolation is possible, yielding nanometer resolution with high accuracy. Considering the precision of the device, alignment tolerances are not excessively demanding.
This type of encoder requires a clean environment. Employing a less coherent LED light source, combined with collimating and filtering optics, significantly improves contamination immunity. The encoder is inevitably larger, and typically has tighter alignment tolerances.
技术对比 / Technology Comparison
干涉式编码器在精度方面无疑具备更高地位。 干涉和反射技术使小尺寸和轻重量成为可能。 透射式编码器通常位于外壳中,并且根据外壳的额定值可以更加坚固。 干涉编码器的高精度需要更严格的对准公差,但考虑到设备的分辨率和精度,这一要求并不繁重。
The interferential encoder is the clear leader in terms of precision. Interferential and reflective technologies make small size and low weight possible. The transmissive encoder is typically in an enclosure and can be more rugged depending on the rating of the enclosure. The higher precision of the interferential encoder demands tighter alignment tolerances, but the requirements are not onerous considering the resolution and accuracy of the device.
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