A time-grating encoder is a position sensor that measures displacement in the time domain rather than by counting etched grating lines. The sensor generates a uniform traveling electromagnetic field and reads the phase shift of the induced signal against a high-frequency reference clock, so precision comes from time-domain electronics, not an ultra-fine physical scale. The method was developed by the team of Peng Donglin at Chongqing University of Technology, which reframed measurement as a space-to-time conversion: uniform motion turns distance into a time interval a clock resolves finely. Because the reference is a clock rather than a ruled scale, the encoder tolerates dust, oil, vibration, and temperature swing better than optical types, and the design extends to absolute output. Published time-grating angular prototypes have reported accuracy near ±0.0015°, about ±5 arc-seconds, after harmonic compensation. HansMotor produces time-grating angle and linear encoders matched to its own direct-drive motors.
Looking for available encoder hardware? Explore HansMotor time-grating angle encoders for rotary axes and time-grating linear encoders for linear motion.
How a Time-Grating Encoder Works
An optical or magnetic encoder subdivides space. Fine lines or magnetic poles are ruled onto a disk or scale at a known pitch, a readhead counts them, and the accuracy of the result is tied to how evenly and how finely that physical pattern was manufactured. Pushing accuracy higher means ruling a finer scale, which raises cost and makes the readout more sensitive to dust and gap.
Time-grating subdivides time instead. A traveling field, electric or magnetic, is driven around the sensor at a constant rate. As the moving element passes, the phase of the induced signal changes in step with position, and the electronics compare that phase against a stable high-frequency clock. Position becomes a time measurement, and a modern clock resolves time very finely at low cost. Peng Donglin’s group named this the space-to-time transformation, and its practical effect is to move the precision burden off mechanical scale-making and onto the timing electronics. The measurement is contactless, so there is no wear surface to degrade over life.
Electric-Field, Magnetic-Field, and Optical Time-Grating
The time-domain principle stays the same across three physical builds, and the field used sets the trade-offs.
The electric-field, or capacitive, type drives a traveling electric field between patterned electrodes. It runs at low power and packs into a thin, solid-state ring, which suits compact rotary axes. This is the build behind the electric-field time-grating patent assigned to Chongqing University of Technology.
The magnetic-field type generates an alternating magnetic field with a rotating magnetic ring and reads it with sensors set 90 degrees apart electrically. Magnetics tolerate contamination well, which is why the magnetic time-grating sensing mechanism is a common choice for rugged axes.
The light-field type modulates a moving light field rather than counting fixed rulings, and reported accuracy figures for this build come from the published time-grating prototypes out of Chongqing University of Technology. All three read position from the phase of a field against a clock; only the field differs.
Time-Grating vs Optical and Magnetic Encoders
No single sensing type wins on every axis. The honest comparison is by tier and environment, not by a headline resolution number.
| Property | Optical | Magnetic | Time-grating |
| Accuracy ceiling | Highest, sub-arc-second | Moderate | High, below the optical ceiling |
| Contamination tolerance | Low, needs a clean gap | High | High |
| Absolute output | Yes, coded track | Yes | Yes, absolute-capable |
| Large-diameter hollow shaft | Costly, sealed gap | Workable | Workable |
| Accuracy per unit cost, mid tier | Expensive | Good | Good |
| Best-fit axis | Metrology, wafer inspection | Rugged industrial, moderate accuracy | Large, harsh, or direct-drive precision axes |
At the metrology ceiling, an optical encoder still leads, and a wafer-grade inspection stage still calls for optical. Below that ceiling there is a wide tier where absolute position, a large hollow shaft, contamination tolerance, and cost matter as much as the very finest resolution. In that tier, time-grating is not an exotic outlier; it is often the better match. Optical and magnetic are not the only two options worth pricing.
Angular and Linear Time-Grating Encoders
Time-grating comes in both rotary and linear forms. HansMotor supplies time-grating angle encoders in the HTRA series for rotary axes, direct-drive rotary tables, torque motors, and robot joints, and time-grating linear encoders in the HTLIU series for linear stages and axes.
Each form is available as incremental or absolute. An incremental encoder sends counting pulses and needs a homing move after each power cycle. An absolute encoder assigns every position a unique code and reports true position the moment power returns, with no homing move, which is why absolute feedback is the default on direct-drive axes that cannot afford a homing sequence at startup.
Where Time-Grating Encoders Fit
Time-grating earns its place where the axis is large, harsh, or direct-drive, and where absolute output matters.
Direct-drive rotary tables and machine-tool fourth and fifth axes are a natural fit: the shaft is large and hollow, the environment carries coolant and swarf, and an optical scale would need a sealed, clean gap to survive. Contaminated production floors with oil mist, dust, or temperature swing are another, since a clock-referenced measurement does not depend on a clean optical window. Robot and cobot joints fit too, where a frameless torque motor has a hollow center and the feedback is a ring encoder built into the motor rather than coupled on afterward.
The boundary is worth stating plainly. A time-grating encoder is not the pick at the top metrology tier, where wafer-grade stages still run optical. For a structured walk-through of choosing a rotary encoder across optical, magnetic, capacitive, and time-grating types, the selection guide covers the full decision.
Time-Grating and Direct-Drive Motors from One Source
Most encoder write-ups skip time-grating or treat it as a curiosity. It is neither. It is a mature measurement method with a two-decade research record at Chongqing University of Technology and commercial hardware in the field. What is rare is a supplier that builds both the direct-drive motor and the time-grating encoder.
HansMotor is a direct-drive motor and encoder manufacturer in Shenzhen, with academic lineage in time-grating through the Peng Donglin team at Chongqing University of Technology. Because the motor and the encoder come from one source, the ring encoder is designed into the motor rather than bolted on and tuned out, and the mounting eccentricity that causes most field accuracy loss is handled at design time. The company offers absolute and incremental encoders matched to its frameless and framed torque motors, direct-drive turntables, and linear motors, backed by 230+ patents over 20 years of direct-drive work, ISO 9001 and ISO 14001 certification, and CE marking, as part of the Han’s Laser group (SZSE: 002008).
The positioning is deliberate. For the top metrology tier, an optical encoder from the leaders still leads, and that is the honest call. For a large, harsh, absolute, direct-drive axis, a time-grating encoder matched to its motor by one manufacturer is often the closer fit. HansMotor states the accuracy class per series spec, set against the axis error budget, with a typical prototype lead time of around 8 weeks for standard products.
Frequently Asked Questions
A time-grating encoder is a contactless position sensor that reads displacement from the phase of a traveling electric or magnetic field against a high-frequency clock, rather than by counting etched lines. It is absolute-capable and tolerates contamination well, which suits large or harsh precision axes.
An optical encoder measures position by reading a finely ruled physical scale, so accuracy is tied to how finely that scale is made and needs a clean gap. A time-grating encoder measures position in the time domain against a clock, so it holds accuracy in dust, oil, and vibration, at the cost of not reaching the optical sub-arc-second ceiling.
Both. Time-grating is available as an incremental type, which needs a homing move at power-on, and as an absolute type, which reports true position immediately with no homing move. Absolute is the usual choice on direct-drive axes.
Published time-grating angular prototypes have reported accuracy near ±0.0015°, about ±5 arc-seconds, after harmonic compensation across a full revolution. Production accuracy is given per series spec and set against the axis error budget. High-end optical encoders reach finer, sub-arc-second accuracy, so time-grating is chosen where hollow-shaft format, harsh conditions, absolute output, or cost weigh as much as the finest resolution.
An angular, or rotary, time-grating encoder measures shaft angle and suits rotary tables, torque motors, and robot joints; HansMotor lists these as the HTRA series. A linear time-grating encoder measures straight-line position for stages and linear axes; HansMotor lists these as the HTLIU series. Both use the same time-domain principle.
Use time-grating where the axis is large or hollow-shaft, where the environment is contaminated or vibrating, where the axis is direct-drive and needs absolute feedback, or where accuracy per cost matters more than the very finest resolution. For wafer-grade metrology at the top accuracy tier, an optical encoder is still the reference.
Talk to an engineer
Tell us your application and the specs you need, and our engineers will match a HansMotor direct-drive motor or encoder and share its datasheet and dimensions. Sample motors and encoders are available for validation before you commit to volume.
Written by Joe Chen, R&D Engineer, HansMotor.