Comparison of Single-Track And Dual-Track Magnetic Encoder Disks

In industrial automation, robot joint control, and new energy vehicle motor systems, magnetic encoders are playing an increasingly critical role. Through non-contact magnetic field sensing, they convert rotary or linear motion into precise electrical signals, providing the system with position, speed, and direction information. The "heart" of a magnetic encoder is precisely that seemingly simple magnetic disk (magnetic ring).

When engineers begin selecting components, they often face several unavoidable questions: What exactly is the difference between single-track and dual-track? What scenarios are rubber magnets and injection-molded ferrites respectively suited for?

1. Single-Track Disks: Sufficient, but Not "Smart" Enough

A single-track magnetic disk has only one magnetized track and works with a single sensor. It calculates the rotation angle by outputting pulse signals; in essence, it is an incremental encoder—it can only tell you "how much it has rotated," but cannot directly determine "which direction it is rotating" or "what absolute position it is currently in."

For this reason, the reference zero point of a single-track system must be re-established every time it is powered on. If the device is powered off and restarted, all previous position information is lost.

However, single-track disks also have irreplaceable value: simple structure, extremely low cost, and a short signal processing chain. They are very suitable for applications that only need speed measurement and do not require direction or absolute position—such as fan speed detection, unidirectional motion systems, and cost-sensitive ordinary motor control.

2. Dual-Track Disks: Measuring Absolute Position with "Two Rulers"

The core idea of a dual-track disk is to integrate two magnetized tracks on the same magnet—a main track and a secondary track (usually called a vernier track)—with their numbers of magnetic pole pairs differing by one, for example, 64 pole pairs paired with 63 pole pairs, or 32 pole pairs paired with 31 pole pairs.

The ingenuity of this design lies in the fact that, because the two tracks have different numbers of pole pairs, at any angular position the combination of magnetic fields generated by the two tracks is unique. The encoder chip reads the phase difference between the two signal channels and, through the vernier principle (similar to the reading logic of a vernier caliper), can calculate the current absolute angular position within 360°—even after a power-off restart, the exact position can be known the moment power is restored, without the need to "return to zero."

The performance improvements brought by dual tracks are multidimensional:

• Resolution leap: With quadrature decoding, the dual-channel signals can increase the effective resolution by a factor of 4. For a magnetic ring with 50 pole pairs, a single-track system can only obtain 50 counts per revolution, while a dual-track system with quadrature decoding can achieve 200 counts.

• Direction detection capability: The phase sequence of the two signal channels directly reflects the rotation direction, without the need for additional logical inference.

• Interference-resistant redundancy: The dual-channel signals can verify each other, making it easier to filter out erroneous pulses in vibration or electromagnetic interference environments, significantly improving system stability.

• Space efficiency: Compared with using two single-track magnetic rings side by side, an integrated dual-track structure can reduce axial space occupation by about 30%–40%, which is crucial for volume-sensitive applications such as robot joints.

Of course, a dual-track solution requires two sensors and more complex signal processing circuitry, so the hardware cost is relatively higher. However, in scenarios with strict positioning accuracy requirements, such as servo systems, robot joints, and precision control of brushless motors, this investment is worthwhile.

3. The Material Debate: Rubber Magnets vs. Injection-Molded Ferrite

Whether single-track or dual-track, the choice of substrate material for the disk is equally critical. At present, the two most commonly used materials for encoder magnetic rings are rubber magnets and injection-molded ferrite, each with a distinct character.

Rubber Magnets: The Flexible "All-Rounder"

Rubber magnets are made by mixing ferrite magnetic powder with elastomers such as nitrile rubber, followed by compounding, calendering, and vulcanization molding. Their greatest advantage lies in flexibility and ease of processing—they can be made into thin sheets or rolls and adapted to curved or specially shaped installation requirements, making them especially suitable for space-constrained, lightweight scenarios.

The magnetic performance of modern rubber magnet encoders is also continuously improving. By optimizing the rubber formulation, the filling ratio of ferrite powder can be further increased, and manufacturers such as NTN have achieved a 25% increase in magnetic force compared with traditional rubber encoders. The magnetic rings in Hutchinson's Elasto-Ferrite series can achieve a remanence of up to 3500 G, a pole width as small as 2 mm, and a period tolerance controlled within 0.05°.

However, rubber magnets also have obvious shortcomings: limited temperature resistance, usually between 60°C and 80°C, and relatively weak magnetic performance. They are more suitable for small and medium-sized motors, automotive ABS sensors, and other occasions with less demanding temperature requirements.

Injection-Molded Ferrite: The Rigid "Precision Faction"

Injection-molded ferrite is made by mixing strontium or barium ferrite magnetic powder with engineering plastics such as nylon (PA66) or PPS and then forming it by injection molding. Its core advantages lie in dimensional consistency and molding freedom—complex geometric shapes can be injection-molded in one step, and dimensions are highly stable from batch to batch, which is crucial for the precise alignment of the two tracks in a dual-track disk.

Injection-molded ferrite has significantly better temperature resistance than rubber magnets. The heat resistance of nylon-based injection-molded magnets can reach 120–150°C, far higher than the 60–80°C of rubber magnets. In terms of mechanical durability, injection-molded ferrite has higher surface hardness, stronger wear resistance and impact resistance, and is less likely to develop cracks or damage during long-term use. In addition, as an oxide material, ferrite itself has excellent corrosion resistance and requires no additional coating protection.

Comparison of Key Parameters of the Two Materials

The selection logic is actually quite clear: if the application pursues high precision, high durability, and a wide temperature range, injection-molded ferrite is more suitable; if there are higher requirements for flexibility, lightweighting, and cost control, rubber magnets are the more pragmatic choice.

4. Combination Strategy: Matching Track Count with Material

Once the track structure and material characteristics are understood, selection becomes a matrix-style decision:

• Single-track + rubber magnet: The most economical solution, suitable for simple speed measurement, fan speed detection, and similar scenarios.

• Single-track + injection-molded ferrite: Improves durability and temperature adaptability while keeping costs controllable; suitable for incremental applications in relatively harsh environments, such as automotive wheel speed sensors.

• Dual-track + rubber magnet: Balances absolute positioning capability and lightweighting. NTN's high-precision angle sensor uses a dual-track structure made of rubber magnet material, achieving an angular accuracy of ±0.1° and a 27% weight reduction.

• Dual-track + injection-molded ferrite: Aimed at the most demanding industrial scenarios—servo motors, robot joints, and CNC machine tool rotary tables—requiring high-precision absolute positioning, wide-temperature stability, and long-term reliability.

It is worth noting that the accuracy of a dual-track disk depends not only on the material but also, more importantly, on magnetization accuracy. The spacing between the two tracks is extremely small, and traditional batch magnetization methods are prone to magnetic crosstalk, causing deformation of the magnetic poles in adjacent tracks. The industry-leading approach is "single-pole data-writing" magnetization—like writing data to a hard drive, micro magnetic heads magnetize pole by pole in sequence, thereby improving the phase alignment accuracy between tracks to the sub-micron level.

In the final analysis, selecting a magnetic encoder disk is a process of "requirement matching." Single-track disks solve the question of "how much it has rotated" in the simplest way, while dual-track disks use two "rulers" with different scales to answer the more fundamental question of "where it is now." Rubber magnets are known for flexibility and economy, while injection-molded ferrite wins with rigidity and precision. When track structure and material characteristics are properly matched, magnetic encoders can deliver maximum performance in industrial automation and intelligent equipment.