增量式光學編碼器介紹（中英文）
介紹 / Introduction
光學編碼器是一種機電設備，使用光源、光探測器和光柵將旋轉或線性位置轉換為電信號。光學編碼器可以是增量式的，也可以是絕對式的，這里主要介紹增量式光學編碼器。主要有三種實現(xiàn)方式：透射式、反射式和干涉式。本文討論了每種實現(xiàn)的技術和相對優(yōu)點。
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.
關鍵術語 / Key Terms
分辨率/ Resolution
定義可移動或測量的最小位置增量，通常以“計數(shù)”表示。高性能伺服系統(tǒng)需要高分辨率。定位系統(tǒng)在兩個計數(shù)之間“抖動”，因此分辨率越高，抖動越小。分辨率對低速時的速度脈動也有顯著影響。由于速度是從位置反饋得出的，如果分辨率較低，樣本中可能沒有足夠的數(shù)據(jù)來準確地得出速度。在高速情況下，高分辨率設備可以生成超出控制器或伺服驅動器跟蹤能力的數(shù)據(jù)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
光學編碼器生成正弦和余弦信號。這些信號的周期由設備固有的“音調(diào)”定義。利用正弦/余弦信息，理論上可以通過計算信號比率來獲得無限分辨率。這種技術稱為插值。實際上，正弦/余弦信號的保真度和信噪比限制了可實現(xiàn)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
定義為每個測量位置與實際物理位置的接近程度。精度在很大程度上是一個系統(tǒng)問題，可能由偏心度、直線度和平面度等機械誤差決定。傳感器誤差包括音調(diào)的非累積隨機變化（線性）、累積音調(diào)誤差（斜率）以及內(nèi)部正弦/余弦信號保真度的變化。精密機器制造商通常通過偏移查找表來校準誤差。
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.
重復性 / Repeatability
定義為系統(tǒng)多次返回同一物理位置時的測量位置范圍。重復性可能比絕對精度更重要。為了有效校準系統(tǒng)誤差，每個位置讀數(shù)保持一致非常重要。傳感器遲滯（不同的讀數(shù)取決于測量位置的接近方向）是可重復性的重要因素。
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.
光學編碼器 - 透射式 / Optical Encoder - Transmissive
透射式編碼器使用由 LED 光源照明的精細光柵或“刻度尺”進行光學掃描。 旋轉或線性刻度尺由透明和不透明“線”組成，按 5050 占空比排列。 光盤上透明區(qū)域的數(shù)量與定義編碼器分辨率的刻度間距相對應。
傳感器產(chǎn)生與入射光強度成正比的電壓。 當傳感器相對于標尺移動時，電壓呈正弦變化。 添加了第二個光檢測器，相位差 90°。 這涉及半個刻度線的位移。 傳感器 A 的信號是否超前傳感器 B 的信號（反之亦然）定義了相對運動的方向。 編碼器輸出可以是正弦/余弦信號，但信號更通常轉換為方波：Aquad B（quad 與 90° 相移相關）。 控制器檢測每個方波邊緣的轉變，這有效地將編碼器分辨率提高了 4 倍。
與每條線的寬度相比，檢測器往往較大。在較高的分辨率下，這可能會導致通道之間的溢出。添加與通道模式匹配的掩模有助于凈化信號。這種設計的缺點是標尺和傳感器之間的氣隙必須非常小，對圓盤參數(shù)（例如平面度、偏心度和對準度）施加嚴格的規(guī)范，使設備更容易受到?jīng)_擊和振動的影響。
相控陣增量編碼器使用固態(tài)技術來提供更強大的解決方案。相控陣編碼器不是每個通道都有一個離散檢測器，而是具有一個檢測器陣列，以便每個通道都被多個檢測器覆蓋。這種方法對光信號進行平均，最大限度地減少由制造誤差（例如光盤偏心和未對準）引起的變化，并在放寬制造公差的同時提高性能。
編碼器本質上是增量式的，通常有一個額外的標尺軌道，帶有一條透明線和單獨的傳感器。傳感器生成定義設備零位的索引信號。
傳輸式編碼器通常封裝在帶有內(nèi)部軸承和軸的外殼中，用于通過彈性聯(lián)軸器連接到電機。這些外殼具有多種密封等級，并且體積龐大。
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 “l(fā)ines” 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.
光學編碼器 - 反射式 / Optical Encoder - Reflective
反射式光電編碼器的原理與透射式編碼器非常相似。反射式編碼器的工作原理是從與傳感器相同的一側（相對于碼盤）發(fā)射光，并選擇性地將部分光反射到傳感器。減小物理尺寸是該解決方案的一個明顯優(yōu)勢。無需透射式編碼器通常所需的準直光學器件，并且 LED 光源與傳感器位于同一側，因此可以大幅減小編碼器的總體積。然而，分辨率和精度可能不如透射式編碼器。與傳輸編碼器類似，附加索引軌道用于定義空位置。
反射式編碼器通常是沒有外殼或軸承的模塊化設備，必須內(nèi)置到機械系統(tǒng)中。它們更加緊湊，但可能需要更良好的環(huán)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.
光學編碼器 - 干涉式 / Optical Encoder - Interferential
干涉式光學編碼器通過相干激光光源產(chǎn)生發(fā)散光束來工作，該光束照亮印在標尺上的衍射光柵圖案。光柵圖案是使用玻璃刻度上的鉻沉積或金屬帶刻度上的激光寫入線創(chuàng)建的。20μm 節(jié)距光柵使光發(fā)生衍射，產(chǎn)生明暗高對比度的干涉圖案，直接返回到探測器陣列上。本質上是增量的，通常使用第二個索引/標記軌道。
衍射光產(chǎn)生干涉圖案的離散塔爾博特平面。在上圖的示例中，使用了第三個 Talbot 平面。當標尺和檢測器的相對位置發(fā)生變化時，衍射圖案會在檢測器陣列上平移，從而導致每個檢測器單元中發(fā)生正弦變化。
干涉技術需要更少的光學元件，從而導致傳感器尺寸較小。在沒有插值的情況下，分辨率通常超過一個數(shù)量級，高于透射式或反射式光學編碼器。由于正弦和余弦信號的保真度，可以進行高插值，從而產(chǎn)生高精度的納米分辨率?？紤]到設備的精度，對準公差要求并不過分。
此類編碼器需要清潔的環(huán)境。采用相干性較低的 LED 光源，結合準直和過濾光學器件，可顯著提高抗污染能力。編碼器不可避免地更大，并且通常具有更嚴格的對準公差。
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 20μm 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.
技術對比 / Technology Comparison
干涉式編碼器在精度方面無疑具備更高地位。 干涉和反射技術使小尺寸和輕重量成為可能。 透射式編碼器通常位于外殼中，并且根據(jù)外殼的額定值可以更加堅固。 干涉編碼器的高精度需要更嚴格的對準公差，但考慮到設備的分辨率和精度，這一要求并不繁重。
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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