As automation levels increase across industries, the importance of motion control is becoming increasingly prominent. Control inputs describing speed and position are essential for effectively driving motors. However, there are various technologies for achieving this sensing, each with different characteristics and application scenarios.
This article will compare different rotation sensing technologies and discuss the reasons for their selection. Then, we will look at some of the latest devices on the market.
Position sensing applications
To improve accuracy, increase yield, and reduce operating costs, many processes that previously required manual operation have been automated, leading to the rapid growth of position sensing applications. In fact, whenever there is some form of motion, sensors are needed to provide position information to the controller.
Industry 4.0 is driving significant advancements in automation across the industrial market. Robotics is becoming increasingly prevalent, enabling 24/7 "unmanned" operation without fatigue or errors—requiring a sensor for every axis of motion. The same applies to "collaborative robots" that work alongside humans in traditional factories.
Today, many parts are manufactured by machines—some using CNC machine tools, some using laser cutting machines, and others using 3D printers. These machines have moving parts and require precise position control to meet quality targets. After the parts are manufactured, they are usually transported via automated material handling or conveyor belts, which also requires position sensing capabilities.
Outside of factories, location control is needed in many places, such as large medical equipment that can move patients or scanners. Additionally, robots are now capable of performing surgery, which also requires highly precise control. In the transportation sector, every application involves motion. Whether it's traditional transportation such as trains, agricultural machinery, and construction machinery, or emerging applications like autonomous mobile robots (AMRs) in warehouses and thousands of drones, all require position sensing.
As passenger vehicles of all drive types (internal combustion engine (ICE), pure electric drive (EV), and hybrid) move towards electrification, mechanical control schemes are being replaced by systems such as "drive-by-wire" and "steer-by-wire." For these systems to function properly, the position information of the accelerator pedal must be transmitted to the electronic control unit (ECU), or the position information of the steering wheel must be transmitted to the steering control system.
As electronic control extends to almost every aspect of vehicle operation, position sensing technology is also widely used in suspension components (for leveling/driving control), powertrains, and power windows, sunroofs, door locks, and other applications.
Comparison of position sensing technologies
Rotational position sensing mainly uses three technologies—optical, magnetic, and inductive technologies, each with its own different working modes, advantages, disadvantages, and application scenarios.
Optical encoders are generally considered the most accurate (although not in all cases). They work by passing light through a perforated disk and using light pulses to detect motion as the disk rotates.
These devices are typically used in applications requiring extremely high precision, such as precision robots and machine tools like CNC lathes or laser cutting machines. While they are highly accurate and insensitive to magnetic fields, they are susceptible to vibrations and contamination on the disk, factors that can cause them to fail.
Magnetic encoders tend to have lower accuracy and are primarily used in cost-sensitive applications. They perform well in the presence of vibration and contamination, but external magnetic fields can significantly affect them, limiting their applicability.
Inductive encoders offer superior accuracy compared to magnetic encoders, can withstand higher levels of vibration and contamination, and are insensitive to magnetic fields. Other advantages include good repeatability, temperature insensitivity, fewer components, smaller size, and no need for rare-earth materials (i.e., magnets).
NCS32100 Dual Inductive Position Sensor
Onsemi's NCS32100 dual-inductive position sensor achieves excellent non-contact position accuracy, better than +50 arcseconds or 0.0138 degrees of mechanical rotation, through two simple yet innovative PCB disks. One PCB is fixed to the motor stator (stationary part), while the other single-layer PCB is fixed to the rotor or shaft. The two PCBs are placed parallel to each other, separated by an air gap of 0.1mm to 2.5mm. The NCS32100 is located on the stator PCB.
The thick and thin (double) conductive traces or coils are printed on two disks. The third conductive trace, called the excitation coil, is printed on the stator PCB. The NCS32100 sends a 4MHz sine wave to the excitation coil, generating an electromagnetic field around the stator excitation coil. According to Faraday's law of mutual inductance, the thick and thin trace coils of the rotor intersect with the electromagnetic field, coupling energy into the rotor coils and forming eddy currents.
Simultaneously, the stator's coarse and fine coils are connected to up to eight NCS32100 receiver inputs. As the rotor rotates, eddy currents in the rotor interfere with the stator receiver coils. The NCS32100 processes these interferences using proprietary algorithms from its internal DSP (Digital Signal Processor) to measure the rotor position.
Using a 40mm PCB sensor, the NCS32100 achieves a position accuracy of ±50 arcseconds at 6,000 RPM, and at speeds up to 45,000 RPM, at a slight sacrifice in accuracy. Even higher accuracy, within +/- 10 arcseconds, can be achieved by using a larger PCB sensor or by precisely aligning the rotor and stator.
This simple solution requires only a small number of electronic components, thus ensuring a compact size and low cost. Furthermore, it is completely insensitive to temperature fluctuations, contamination, and external magnetic fields.
Dual Inductor Technology Integration Solution
ON Semiconductor's NCS32100 supports high-precision rotary position sensors designed for industrial applications and environments. It is an absolute position device that determines position without requiring motion. The NCS32100 can also calculate rotational speeds up to 45,000 RPM.
At speeds up to 6,000 RPM, the NCS32100 delivers full accuracy of ±50 arcseconds, comparable to the performance of many optical encoders. The device also integrates an Arm® Cortex® M0+ MCU, providing high configurability and an internal temperature sensor.
The NCS32100's built-in calibration routines allow the sensor to self-calibrate with a single command, a process that takes only two seconds. It requires no reference encoder and the program can run at any time as long as the rotor speed is between 100 and 1000 RPM. All calibration coefficients are stored in non-volatile memory (NVM).
A typical optical solution requires a total of three PCBs—an optical disk, a stator PCB, and an LED driver PCB—and approximately 100 components are needed to achieve full functionality.
In contrast, the NC32100-based solution requires only two PCBs: the rotor is a single-layer PCB without any components, while the stator PCB contains only 12 components.
In automotive applications, while cost and reliability are important, safety is paramount, especially in applications such as steering and braking. ON Semiconductor's automotive-grade absolute position sensor, the NCV77320, conforms to the ISO 26262 standard and is specifically designed for these critical applications. The NCV77320 offers a position accuracy of 194.3 arcseconds or 0.0539 degrees of mechanical rotation (depending on PCB geometry), primarily because it has only 3 receiver inputs, compared to 8 for the NCS32100, and the NCV77320 does not support coarse or fine coil PCB configurations. Both the NCV77320 and NCS32100 can operate as rotary encoders or linear encoders.
Applications of the NCV77320 include brake pedal sensors, accelerator pedal sensors, motor position sensors, braking system sensors, vehicle level sensors, transmission gear sensors, throttle position sensors, and exhaust gas recirculation valve sensors.
Like the NCS32100, the NCV77320 is insensitive to pollution, temperature changes and magnetic field interference, and can be used in automotive environments with ambient temperatures ranging from -40ºC to +150ºC.
The NCV77320 can operate at speeds up to 10,800 RPM and communicate with the accompanying MCU via SENT, SPI, or analog interfaces.
Summarize
With the increasing prevalence of automation, the demand for position sensing of rotating motors is growing. Currently, several technologies can achieve this function, including optical, magnetic, and inductive technologies. Optical technologies offer high precision but are expensive and susceptible to contamination. Magnetic technologies are low-cost but easily affected by magnetic fields.
Inductive technology is gaining increasing popularity, and with the advent of dual-inductive sensors, it is now possible to create sensors that offer both optical-grade precision and greater cost-effectiveness.
