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Simple, Safe and Easy to Use – Inductive Motor Position Sensing For Industrial Applications

Figure 1 shows onsemi’s NCS32100 Rotary Position Sensor

By Alessandro Maggioni, Senior Regional Marketing Manager of EMEA, onsemi

In today’s world, expectations for machines are higher than ever.  Whether in factories, our homes or the vehicles we drive, we expect to control these machines with high levels of precision.

The drive towards machine automated tasks commonly requires physical movement, often facilitated at least one motor. Motors are found in robots, elevators, automobiles, and many more places. To ensure utility and safety, these motor applications require precise control and the ability to determine an absolute position, as a prerequisite.

Precision in motor control relies on accurate and reliable detection of a motor’s position which, in turn depends on the sensor technology used. Modern machines use inductive position sensors that offer high levels of positioning accuracy.

Encoder Technology

Historically, the preferred way of detecting motor position was to use magnetic or optical encoders. Both types of encoders rely on the relationship between motor position changes and its speed.

Many types of magnetic encoders are available and used. However, despite some differences (including size and weight), they all rely on variations of the same phenomenon: magnets are attached to a moving part of the motor and, as the magnets move in relation to a magnetic detector, the magnetic field changes in proportion to the movement.

Depending on the type of magnetic encoder used, the resolution available, which affects the precision, will vary and can be low – in the order of low hundreds of pulses per revolution (PPR). The Accuracy of magnetic encoders can be improved by precise manufacturing, but this increases cost.

Many industrial motor applications can be subject to high levels of electromagnetic interference (EMI) that can make magnetic sensors unreliable or even unusable. Also, temperature extremes will have an adverse effect on their accuracy.

As an alternative technology, optical encoders create and then detect pulses of light. Here, a motor is fitted with a disk-shaped grate that has a clear and opaque pattern. As the grate spins, light can shine through the clear sectors, creating a series of light pulses. These pulses are detected by a photodiode that facilitates the determination of the rotational speed and the position of the motor.

Since optical encoders are purely light-based, they are not affected by EMI or magnetic fields. However, they can be compromised by contaminants such as dirt, soot, or even moisture on the disc that can obscure the clear sections. They can also be adversely affected by temperature extremes.

Though costly, optical encoders can provide high resolution while delivering excellent accuracy, provided they are correctly installed. However, accuracy will tend to diminish when they are used at high speeds.

Current Based Inductive Sensors

Another type of motor position sensor is an inductive sensor. Inductive sensors also use magnets to operate, but do not rely upon changes in the magnetic field. Instead, they use induced current to determine motor operation.

With inductive sensors, a magnetic detection device senses the rotation of gear wheels. As the gear wheel rotates, teeth move past the sensor, thereby creating magnetic flux that, in turn, induces a voltage in the sensor. Monitoring the voltage allows the speed and direction of rotation to be precisely determined.

This technology is well established, having been used for decades. They tend to be quite simple and therefore robust and reliable. Also, they are unaffected by vibration, temperature variations and environmental contaminants.

Inductive sensors have been used extensively in automotive applications due to their high levels of reliability and relatively low cost. However, modern innovations are improving this technology.

Dual Inductive Rotary Sensor

A new type of motor position sensor, based upon traditional inductive sensors is the dual inductive rotary sensor. These new devices from onsemi are based upon a pair of printed circuit boards (PCBs). The first is a rotor with a pair of printed inductors while the second is a stator that includes printed inductors and a high-performance encoder IC. These new dual inductive position sensors meet the needs of modern applications, offering high-speed operation while remaining highly accurate.

onsemi’s NCS32100 has an output with a 20-bit single-turn resolution and a 24-bit multi-turn resolution. Based upon a 38mm sensor, accuracy exceeds +/-50 arcsec at speeds up to 6,000 RPM. Operation is possible at speeds up to 100,000 RPM, albeit with slightly reduced accuracy. As an absolute (instead of incremental) encoder, the innovative device can provide position data even when the rotor is stationary.

The NCS32100 module features an embedded, programmable M0+ – ARM® microcontroller (MCU) with firmware. With onboard signal processing, the NCS32100 delivers high-level position and velocity information. A high level of flexibility and configuration options allow for connection to different types of inductive sensor patterns.

Mechanically, the approach is not complex, requiring few components. It is also simple from an electrical perspective requiring just two external capacitors for bypass and tuning.

Similar to other inductive sensors, the NCS32100 is reliable, robust, and safe to use. Setup and use are also simple due to plug-and-play capabilities, ease of calibration, and advanced error correction and diagnostics.

onsemi’s NCS32100 is specifically intended and designed for use in demanding industrial applications. While greatly reducing overall cost when compared to similar optical encoders, the NCS32100 can be used to replace and upgrade medium- and high-end optical encoders in applications including industrial motor drives, factory automation (FA) systems, and a multitude of other industrial equipment.

Alongside the need for precision, modern applications demand long-term reliability. onsemi’s NCS32100 dual induction rotary position sensor offers simplicity, a reduced component count, and an extended operational lifetime.

 

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