In the design of robot joints, servo motors, AGV wheel systems, and even humanoid robots, magnetic encoders (Robot Magnetic Encoder Sensors) are gradually replacing traditional optical encoders as the core components for position and velocity feedback. Their advantages—non-contact measurement, contamination resistance, vibration resistance, and compact structure—have led to widespread adoption in industrial automation and intelligent robotics.
When faced with the numerous parameters and output interfaces of magnetic encoder sensors on the market, engineers often find it confusing: Is higher resolution always better? What is the relationship between resolution and accuracy? How do you choose between SPI, SSI, and ABZ? This article provides a clear selection guide for robot developers around these three core issues.
1. Distinguishing Resolution from Accuracy: High Resolution ≠ High Accuracy
Resolution and accuracy are the two parameters most easily confused, but they have very different meanings.
Resolution refers to the smallest angular change that the encoder can read and output, reflecting the “fineness” of measurement. Absolute encoders typically use bits, e.g., 14 bits (16384 steps/rev), 17 bits (131072 steps/rev); incremental encoders use pulses per revolution (PPR), e.g., 1024 PPR. Simply put, resolution determines how finely you can divide a full 360° circle—the higher the bits, the finer the division.
Accuracy refers to the deviation between the encoder’s output signal and the actual physical angle, reflecting the “correctness” of measurement. Accuracy is usually expressed in degrees (°) or arcminutes (arcmin), and is affected by multiple factors: magnet quality, mounting eccentricity, temperature drift, magnetic noise, etc. Generally, the quality of the magnetic ring determines accuracy, while the readhead (chip) determines resolution and repeatability.
There is a common pitfall: high resolution does not necessarily bring high accuracy. A 14-bit magnetic encoder can divide one revolution into 16384 steps, but if the magnet’s magnetization accuracy is poor or there is mounting eccentricity, the actual measured accuracy might only be ±1.0°, with resolution far exceeding accuracy. In extreme cases, the error between resolution and accuracy can be more than 50 times. When selecting a sensor, priority should be given to the calibrated accuracy specification rather than simply pursuing high resolution.
How to reasonably match resolution? An empirical formula: Resolution ≥ 360° ÷ positioning accuracy requirement. For example, if the positioning accuracy requirement is ±0.1°, then the resolution must be at least 360 ÷ 0.1 = 3600 lines (about 11.8 bits). In practice, it is advisable to leave a margin and choose one level higher than the calculated value.
2. How to Choose Communication Protocols: ABZ, SPI, SSI Scene Matching
The communication protocol of a magnetic encoder sensor directly affects wiring complexity, noise immunity, and real-time performance. They can be roughly divided into incremental interfaces and absolute interfaces.
Incremental Interface (ABZ) : A/B quadrature pulse outputs, with a 90° phase difference to determine speed and direction, and a Z channel for one zero pulse per revolution. The biggest advantages of the ABZ interface are good compatibility and low cost; it is the standard input format for most servo drives and PLCs. However, incremental encoders do not retain position information after power-off and require a homing cycle on startup. Suitable for stepper motor drives, conveyor speed measurement, and other speed control or simple position detection applications.
SPI Interface : Synchronous serial interface, can directly read absolute angle values, and also supports on-chip register configuration and magnetic field diagnostics. SPI offers high real time performance and simple wiring, making it suitable for applications like FOC control that require fast angle reading.
SSI Interface : An industrial version of the synchronous serial interface, using clock+data differential transmission, with strong noise immunity and a transmission distance of up to 100 meters. SSI supports 12 25 bit resolution and is the mainstream absolute encoder interface in industrial environments. Suitable for absolute positioning over long distances in strong electromagnetic interference environments.
Quick Selection Guide :
· Short distance, low cost, speed control oriented → ABZ single-ended interface
· Long distance, high interference, absolute position required → Differential ABZ or SSI interface
· High precision, no homing needed, FOC control → SPI/SSI/I²C absolute interface
3. Selection Recommendations for Three Core Robot Applications
3.1 Robot Joints (Collaborative Robots, Humanoid Robots)
Robot joints demand the highest accuracy, resolution, and reliability from the encoder. Absolute encoders using TMR or AMR technology are typically chosen. Recommended resolution is 18 bits or higher, with accuracy no worse than ±0.05°. For communication, the SPI interface can communicate directly with the joint driver chip, suitable for high real-time FOC control. Also, due to the compact space in robot joints, small package products (e.g., QFN 3×3 mm) should be prioritized, used with radially magnetized NdFeB magnets.
3.2 AGV/AMV Wheel Systems
AGV wheel encoders are mainly used for speed closed-loop control and odometry. Resolution requirements are moderate (14-17 bits sufficient), but environmental adaptability and reliability are critical. Because AGVs often operate in dusty, humid environments, the contamination resistance of magnetic encoders is a clear advantage. The ABZ interface can be used to connect directly to the motor driver, or the SSI interface for longer transmission distances.
3.3 Servo Motors (Industrial Automation)
Servo motors require both high resolution to improve low-speed smoothness and dynamic stiffness, and sufficient accuracy to ensure positioning correctness. Recommended resolution starts at 15-17 bits, with accuracy better than ±0.1°. For communication, absolute interfaces have become the mainstream choice for high-end servos. SSI or BiSS interfaces ensure stable transmission in industrial environments with strong electromagnetic interference.
4. Pitfalls to Avoid After Selection
Even if the selection parameters are correct, practical applications may encounter the following issues:
· Mounting accuracy : The eccentricity between magnet and chip must be strictly controlled, typically ≤0.3 mm, with an axial gap of 0.5-1.5 mm. Exceeding these limits introduces additional nonlinear errors.
· Electromagnetic interference : Strong EMI from motors, inverters, etc., is a major cause of signal distortion. Differential output interfaces combined with twisted-pair shielded cables (shield grounded at one end) are recommended.
· Environmental adaptability : For applications with continuous water immersion or high-humidity condensation, choose products with an ingress protection rating of IP67 or higher. Industrial grade typically requires an operating temperature range of -40°C to +85°C.
5. SDM Robot Magnetic Encoder Sensors
In the process of domestic manufacturing of magnetic encoders, SDM has taken a differentiated technological path in manufacturing. The core advantages of their Robot Magnetic Encoder Sensors are reflected in the following three areas:
Injection-molded integrated process : SDM uses an injection-molding process to form magnetic materials and engineering plastic in one shot, replacing the traditional multi-part assembly process. Injection-molded integration offers significant benefits: short process flow, low energy consumption, few shape limitations, high production efficiency, and good dimensional accuracy. This process greatly improves the dimensional consistency and mechanical strength of the encoder magnetic ring, laying the foundation for subsequent magnetic performance consistency.
Magnetic printing magnetizing technology : In the magnetizing stage, SDM employs high-precision “magnetic printing” technology—writing pole patterns point by point. Compared to conventional bulk magnetization, this significantly improves pole position accuracy and magnetic field uniformity. High-pole-count, high-accuracy magnetization processes demand extremely precise equipment and tooling; they must be completed on dedicated multi-pole magnetizing fixtures with precise arrangement and high-intensity pulsed magnetic fields. SDM’s accumulated expertise in this area enables their magnetic encoder sensors to achieve a high level of pole-division accuracy.
Full waveform inspection : Unlike most domestic magnetic encoder manufacturers that rely on sampling inspection, SDM performs a full waveform inspection on every sensor before it leaves the factory. Each product undergoes signal waveform scanning under multiple operating conditions, covering all performance indicators: inter-pole angle error, magnetic field strength fluctuation, signal distortion, etc. Full inspection means that every sensor a customer receives has been individually verified through actual measurement, ensuring better product consistency and reliability—a critical advantage in applications like robot joints where sensor reliability is paramount.
From injection-molded integration to ensure the mechanical datum of the magnetic ring, to magnetic printing magnetizing to ensure the electrical accuracy of the magnetic poles, and finally to full waveform inspection to guarantee the outgoing quality of each product—SDM’s complete process closed loop ensures full-chain controllability of every magnetic encoder sensor from material to finished product, providing users with high consistency, high-reliability domestic magnetic encoder choices.
