How can an incremental linear encoder achieve high-precision displacement measurement through photoelectric conversion?
Release Time : 2026-02-10
An incremental linear encoder, a core sensor that converts physical displacement into quantifiable digital signals, achieves high-precision displacement measurement through photoelectric conversion. Essentially, it utilizes the synergistic effect of a grating ruler and a photoelectric sensor to convert mechanical motion into periodic electrical signals, followed by precise displacement quantification through signal processing. This process involves a deep integration of optics, electronics, and mechanical engineering, and its core principle can be broken down into four key stages: grating structure, photoelectric conversion, signal processing, and displacement calculation.
The grating ruler is the core component of the incremental linear encoder. Its surface is engraved with equally spaced transparent and opaque stripes, forming a periodically arranged grating pattern. The spacing of these stripes determines the encoder's resolution—the denser the stripes, the more frequent the signal changes per unit displacement, and the higher the measurement accuracy. When the encoder moves along the guide rail with the object being measured, the relative motion between the grating ruler and the fixedly mounted photoelectric sensor causes the transparent stripes to periodically block or expose the light source, forming alternating bright and dark light signals. This process is similar to an "optical ruler," discretizing continuous mechanical displacement into detectable changes in light intensity.
The photoelectric sensor's role is to convert the light signal into an electrical signal. A typical sensor consists of a light-emitting diode (LED) and a phototransistor, located on opposite sides of a grating ruler. When the grating ruler moves, the transparent stripes allow light emitted from the LED to pass through and illuminate the phototransistor, generating current pulses; the opaque stripes block the light, causing the phototransistor to cut off. Thus, the mechanical motion of the grating ruler is converted into a periodic current switching signal. To improve signal quality, a Schmitt trigger is typically integrated inside the sensor to shape the raw current signal, eliminate noise interference, and output a regular square wave pulse.
An incremental linear encoder uses two sets of orthogonal grating stripes (phase A and phase B) to achieve direction determination and improve accuracy. The grating stripes of phase A and phase B are spatially offset by 1/4 period (90° electrical angle). When the encoder moves forward, the phase A pulse leads the phase B pulse by 90°; when it moves backward, the phase B pulse leads the phase A pulse by 90°. By detecting the phase relationship between the two phase signals, the control system can accurately determine the displacement direction. Furthermore, subdivision technology can further improve resolution—for example, by detecting the rising and falling edges of phases A and B, each original pulse cycle can be subdivided into four sub-cycles, increasing resolution by four times. This combination of "orthogonal encoding" and "subdivision processing" is key to achieving high precision in incremental encoders.
Displacement calculation relies on counting pulse signals. For every grating pitch moved by the encoder, phases A and B each generate a complete pulse. The control system records the number of pulses using a counter, and then calculates the total displacement by combining this count with the grating pitch (a known parameter). For example, if the grating pitch is 0.01 mm and the counter records 1000 pulses, the displacement is 10 mm. To eliminate accumulated errors, the encoder is typically equipped with a Z-phase zero-position signal—every fixed cycle (e.g., every meter) the Z-phase outputs a reference pulse to calibrate the counter, ensuring long-term measurement accuracy.
In high-speed or long-distance measurement scenarios, incremental linear encoders need to address the stability of signal transmission. During long-distance transmission, wire resistance and electromagnetic interference can cause signal attenuation or distortion. To address this, some encoders employ differential signal transmission technology, simultaneously outputting two inverted signals, A/ and B/. The receiving end extracts effective information by comparing the difference between the two signals, significantly improving anti-interference capabilities. Furthermore, the encoder housing is typically made of metal, creating a Faraday cage effect to further shield against external electromagnetic interference.
The accuracy of incremental linear encoders is affected by multiple factors. The manufacturing precision of the grating ruler directly determines the resolution. High-precision encoders require laser interferometers to calibrate the grating spacing, ensuring errors are less than micrometers. The response speed of photoelectric sensors affects dynamic performance; high-speed encoders need to use photosensitive elements with higher switching frequencies to adapt to linear velocities of several kilometers per minute. Environmental factors such as temperature, humidity, and vibration must also be considered—temperature changes can cause thermal expansion and contraction of the grating ruler, introducing measurement errors. Therefore, some encoders integrate temperature compensation modules to correct displacement data in real time.
From wafer positioning in semiconductor manufacturing to bed movement in medical CT scanners, from sorting precision in automated logistics systems to minute angle adjustments in robot joints, incremental linear encoders, as "hidden champions," provide fundamental support for industrial precision control. Its high-precision displacement measurement achieved through photoelectric conversion is not only one of the core technologies of modern manufacturing, but also a key force driving the industry towards intelligence and automation.