Robot Magnetic Encoder Sensor Demagnetization at High Temperature – Solutions for Continuous Operation

I. The Problem: The Hidden Risk of Robot “Joint” Failure

In industrial robots, collaborative robots, and precision servo systems, magnetic encoders play a critical role as “joint sensors.” They detect changes in the magnetic field of the encoder disk (magnetic ring) and provide real-time feedback on motor speed and position. However, one easily overlooked issue is that when robots operate continuously under high-temperature conditions, the magnetic encoder disk may suffer high-temperature demagnetization, leading to position feedback drift, increased current, and even step-loss.

Engineers have encountered this in real-world debugging: when the motor runs above 80 °C, the position feedback gradually becomes inaccurate, and after thorough checks, the root cause is traced to high-temperature demagnetization of the magnetic encoder. For industrial robots that need to run continuously for long periods, this problem is especially critical -it not only affects machining accuracy but can also trigger equipment failures or even safety incidents.

Currently, the two most widely used materials for magnetic encoder disks are injection-molded ferrite and rubber magnet (also called flexible ferrite magnet). Why do these two materials demagnetize at high temperatures, and how can this be solved? This article provides an in-depth analysis from the perspectives of material characteristics, demagnetization mechanisms, and solutions.

II. Two Common Materials and Their Temperature Characteristics

1. Injection-Molded Ferrite

Injection-molded ferrite is made by mixing nylon (or other thermoplastic resins) with iron oxide powder to form pellets, which are then processed by injection molding. Its advantages include high shape freedom, excellent dimensional accuracy, and suitability for mass production. In the field of magnetic encoder disks, injection-molded ferrite rings can be precision-magnetized with multiple poles and are widely used in motor control and robot joints.

However, injection-molded ferrite has a significant shortcoming – high sensitivity to temperature. Under elevated temperatures, its magnetic properties degrade, and demagnetization may occur. Typically, the operating temperature range of injection-molded ferrite is between -40 °C and 120 °C; beyond this range, magnetic performance begins to decline. More concerning is that injection-molded ferrite has a relatively low coercivity, making it susceptible not only to external magnetic fields but also to irreversible magnetic loss at high temperatures.

2. Rubber Magnet (Flexible Ferrite Magnet)

Rubber magnets are made by blending bonded ferrite powder with synthetic rubbers (such as nitrile rubber or chloroprene rubber) and then processing via calendering, extrusion, or injection molding. They are flexible, elastic, and cuttable, and are also widely used in magnetic encoders.

Conventional rubber magnets typically have an upper operating temperature limit of about 80 °C. Ordinary rubber magnets show a magnetic attenuation of less than 3% after 72 hours at 80 °C; however, if the temperature rises further, the risk of demagnetization increases significantly. It should be noted that the temperature resistance of rubber magnets is mainly determined by the rubber matrix -by using high-temperature-resistant rubbers such as silicone rubber, the upper limit can be raised to 150-200 °C.

III. Physical Mechanism of High-Temperature Demagnetization: Magnetic Domains “Lose Direction” in Thermal Motion

To understand high-temperature demagnetization, one must first understand why magnetic materials are magnetic.

The magnetism of a material originates from the ordered alignment of tiny magnetic regions inside the material – magnetic domains. At room temperature, these domains are largely aligned in the same direction, producing macroscopic magnetism. When the temperature rises, the thermal motion of atoms within the domains intensifies, disturbing the originally ordered domain structure and causing the magnetic moments to deviate. As a result, both the remanence (Br) and coercivity (Hc) of the magnet decrease to varying degrees, manifesting as a weakening of the magnetic field strength.

For injection-molded ferrite and rubber magnets, the situation is more complex: both materials are bonded magnets -composed of magnetic powder (ferrite powder) compounded with a polymer binder. High temperature not only affects the magnetic domain arrangement of the powder itself but also alters the binder properties (softening, aging, etc.), which in turn affects the dispersion of the powder and the overall magnetic performance.

It is important to distinguish between two types of high-temperature demagnetization:

· Reversible demagnetization: magnetic performance decreases with rising temperature but recovers when the temperature returns to normal.

· Irreversible demagnetization: when the temperature exceeds a certain threshold, the domain structure changes permanently, and the original magnetic properties cannot be restored even after cooling.

For continuous robot operation, the most critical concern is irreversible demagnetization -once it occurs, the magnetic signal strength of the encoder disk will be permanently reduced, directly compromising the measurement accuracy of the encoder.

IV. Systematic Solutions

To address high-temperature demagnetization of injection-molded ferrite and rubber magnet encoder disks, solutions can be pursued from two dimensions: the material level and the system design level.

Solution 1: Material-Level Optimization and Upgrading

1. Select High-Temperature-Resistant Grades of Injection-Molded Ferrite

The operating temperature of injection-molded ferrite is not fixed. By optimizing the magnetic powder formulation and the binder system, its temperature resistance can be significantly enhanced. For example, ferrite materials using nylon 6 or PPS (polyphenylene sulfide) as the binder have shown satisfactory test results at around 180 °C. Research indicates that adding an appropriate amount of TAF lubricant can improve pellet flowability and magnetic properties; surface treatment of the magnetic powder with coupling agents can also effectively enhance the performance of injection-molded magnets.

2. Select High-Temperature-Resistant Rubber Magnets

Conventional rubber magnets have an upper temperature limit of 80 °C, but by adopting high-temperature rubber systems (e.g., replacing ordinary rubber with silicone rubber), the upper limit can be raised to 150-200 °C. Patented technologies have shown that with proper formulation and process optimization of high-temperature rubber systems with ferrite powder, the maximum service temperature of rubber magnets can reach 120-170 °C. Blended rubber systems such as SR/NBR also exhibit good thermal stability above 120 °C.

In practical selection, it is recommended to match the material’s temperature rating with the equipment’s operating temperature range.

Solution 2: System Design and Thermal Management

1. Optimize Heat Dissipation Design

Since high-temperature demagnetization is ultimately caused by excessive temperature, effective thermal management is the most direct approach. Specific measures include:

· Adding heat sinks or forced air cooling around the encoder to reduce the operating temperature of the magnetic ring.

· Placing thermal insulation structures between the motor windings and the encoder to block heat transfer.

· Adopting water-cooled jacket designs to achieve continuous cooling under high-temperature conditions.

2. Reserve Margin in Magnetic Circuit Design

During the magnetic circuit design phase, an appropriate flux margin can be reserved to compensate for partial magnetic loss at high temperatures, ensuring stable performance throughout the product’s life cycle.

3. Control Injection Molding Process Temperature

In the manufacturing of the magnetic encoder disk, attention should be paid to controlling the processing temperature during injection molding, avoiding excessive temperatures that could damage magnetic properties.

V. Summary

The demagnetization of injection-molded ferrite and rubber magnet encoder disks at high temperatures is essentially a physical process in which thermal motion disrupts the ordered magnetic domain structure, reducing magnetic order. This issue is particularly prominent in continuous robot operation.

Solutions can be approached from two levels: at the material level, by selecting high-temperature-resistant grades of injection-molded ferrite or high-temperature-resistant rubber magnets; at the system level, by optimizing heat dissipation, implementing proper thermal management, and reserving flux margins in the magnetic circuit design.

In real engineering applications, it is advisable to comprehensively evaluate and choose the most suitable solution based on the specific temperature range, cost budget, and accuracy requirements. After all, for a robot that needs to operate stably over long periods, the reliability of every “joint” is critical to the overall success.