Encoders are commonly used to measure data such as rotation angle and speed in servo motor systems. Currently, the main types used in the market are optical encoders and magnetic encoders. Compared to optical encoders, magnetic encoders have gained increasing attention in recent years because they are unaffected by dirt, dust, oil, condensation, and other contaminants. Advanced magnetic encoder technology can provide accuracy (resolution) comparable to optical encoders and significantly reduce production costs while improving system efficiency.
1. Introduction
In the field of industrial automation, speed and position sensors, represented by encoders, are playing an increasingly important role. Among them, optical encoders are favored by many users due to their fast response time and high precision. However, these devices, which use photoelectric signals to output pulses, are highly susceptible to interference from environmental factors such as vibration, dust, humidity, and temperature differences, thus affecting signal output and their own lifespan. Compared with optical encoders, magnetic encoders, which use magnetic fields and Hall effect sensors to generate signals, are more adaptable to the environment, and the feedback from the integrated chip is more accurate. These significant advantages have made them stand out in applications related to high-precision measurement and control.
2. Coaxial encoders with numerous limitations
Magnetic encoders typically employ two different designs. Currently, coaxial magnetic encoders remain the mainstream product in the market.
A coaxial magnetic encoder mounts a single pair of rare-earth magnets on the end of a rotating shaft and uses a ring array of 4-8 Hall elements for sensing. Two opposing Hall sensors are connected by a differential amplifier, generating a sine and cosine signal for each rotation, which is converted (often by a DSP chip) into quadrature or serial position input signals.
Coaxial magnetic encoders offer lower detection costs, but they place high demands on equipment installation. The midpoints of the north and south magnetic poles of the magnet, the center of the chip, and the center of the motor shaft end must be aligned, as any installation error will reduce the encoder's accuracy. However, in practical applications, not only is radial jitter of the motor shaft unavoidable, but deviations caused by the relative displacement of the chip and magnet also affect the accuracy of the actual output signal. Furthermore, coaxial designs require high-rate interpolation from one cycle to the final resolution; once the rotational speed exceeds a certain range, it becomes impossible to accurately read point-by-point position information, leading to reduced accuracy in the actual output. For these reasons, coaxial magnetic encoders have long been concentrated in relatively low-end applications.
Figure 1: Coaxial (left) and Off-axis (right) magnetic encoders
3. Pioneering groundbreaking off-axis design
With continuous technological innovation, magnetic encoders now largely rival the performance of optical encoders. The groundbreaking off-axis design, in particular, has opened up opportunities for magnetic encoders to excel in a wider range of applications. As a leader in this field, Timken's Timken® off-axis magnetic encoders solve practical problems such as calibration.
Timken® off-axis magnetic encoders employ a multi-pole code disk with the chip mounted off-center from the rotating shaft. During operation, each pole pair generates a one-cycle sine and cosine signal, which is converted into a quadrature or serial position signal via a hardware interpolator. As the motor shaft rotates, the magnetic field strength sensed by the Hall element inside the chip remains constant within the radial range. Radial jitter of the motor shaft (as long as it remains within the radial range of the Hall sensing magnetic field) does not affect the accuracy of the Hall sensing output signal. Therefore, by ensuring the chip position and radial mounting distance (related to the pole pair spacing) during actual installation, the accuracy of the output signal can be guaranteed.
Figure 2: Interpolator with 4% error
The improved accuracy of off-axis magnetic encoders is also related to changes in their code disk diameter and the number of magnetic pole pairs. Under mechanical gain effects, the smaller the diameter of a coaxial encoder, the greater the magnetic pole error, while the larger the diameter of an off-axis encoder, the smaller the magnetic pole error. As shown in Figure 2, regarding the interpolator accuracy gain, the coaxial design directly displays a 1:1 ratio, while the off-axis error is reduced by a multiple of the magnetic pole pairs. If the off-axis design contains 32 pole pairs, only a low-magnification interpolation with a resolution of 1/32 is needed from each pole pair; in contrast, the coaxial design requires high-magnification interpolation from the entire cycle to the final resolution.
Furthermore, coaxial encoders typically use DSP chips to interpolate sine and cosine signals, which, while providing higher magnification, often requires longer response times. Off-axis encoders, on the other hand, only require low-magnification hardware interpolators and can operate normally even at spindle speeds exceeding 10,000 rpm, exhibiting exceptional stability.
Figure 3 shows the comprehensive test results of coaxial and off-axis encoders within radial and tolerance ranges. The experiment compared three coaxial encoders from different manufacturers, and the results showed that the Timken® off-axis encoder had a significantly smaller error than these three coaxial encoders. Even when the chip was not correctly positioned during installation, the off-axis encoder still had a smaller error than the coaxial encoder.
Figure 3: Error measurement results
Timken® off-axis magnetic encoders can achieve resolutions of 0-5000 pulses/revolution or even higher on mainstream optical incremental encoders. Leveraging its patented magnetized magnetic field detection and processing technology, Timken can flexibly design and manufacture code disks with different magnetic pitches ranging from 0.8mm to 4mm, tailored to customer requirements, and combined with chip sensing to meet various practical application needs.
4. Easily handles harsh working conditions
Optical encoder code disks are generally made of materials such as metal, glass, and plastic. In actual installation, the distance between the code disk and the photoresistor is extremely small, and any axial movement can damage the encoder. For example, in the textile industry, the vibration of rapier looms causes the motor shaft to be constantly wobbling, resulting in a significantly shortened lifespan of optical encoders. Frequent replacements not only do not improve system efficiency but also bring enormous cost pressures. At the same time, due to the limitations of photoelectric sensing technology, dust and fabric debris in the production process can easily contaminate the light source emission, making optical encoders less adaptable to harsh environments. Magnetic encoders, on the other hand, do not rely on a light source, and the magnetic field generated by the magnetic poles is unaffected by contaminants. The code disk is extremely robust, and therefore their lifespan is often several times that of optical encoders. In addition, due to their vibration resistance, corrosion resistance, pollution resistance, interference resistance, and wide temperature range, magnetic encoders can be used in many fields where traditional optical encoders cannot be applied.
Currently, in overseas markets, Timken® off-axis magnetic encoders have been successfully applied in closed-loop stepper systems and automotive EPS steering systems. In China, magnetic encoders are used in industrial sectors with stringent environmental requirements, such as injection molding machines, woodworking engraving machines, textile machinery, and waterproof motors. Off-axis magnetic encoders are also well-suited for general applications requiring position control, including highway ETC automatic barrier gates, motion theaters, rotating stages, mobile fountains, and electric forklifts, as well as in environments with significant temperature differences.
5. Considerable cost-effectiveness
In harsh operating conditions, optical encoders must be encapsulated to avoid contamination, while off-axis magnetic encoders, which do not contain bearings, circuit boards, brackets, or other accessories, can reduce encapsulation costs for users. In addition, ordinary optical encoders require at least 30-40mm of installation space, while off-axis magnetic encoders occupy only about 12-14mm of space after installation, further converting into considerable cost-effectiveness through a more compact structure.
In recent years, the global magnetic encoder market has been gradually expanding. According to the "12th Five-Year Plan for China's Instrument Industry," by 2020, the industry scale of incremental encoders alone will reach 50 billion yuan. This ever-expanding market capacity also indicates the enormous development potential of magnetic encoders, especially off-axis magnetic encoders. With reliable magnetic coding technology, strong environmental adaptability, and significant cost advantages, off-axis magnetic encoders will enter more and more application fields, serving as a complementary technology to optical encoders and providing customers with richer and more clearly defined product choices.
