onsemi Inductive Sensors are Advancing Industrial Automation

In any application using motors for automation, there is a need to ensure accurate control of the motor. To make this possible, the position and speed of the rotor in the motor, which is usually attached to the shaft and moves in relation to the fixed stator, must be known. The most common technologies used for this are three varieties of rotary encoders: optical, magnetic and inductive. They each have advantages and disadvantages.

Optical encoders offer good accuracy and are insensitive to the magnetic fields found in many industrial applications. However, they are relatively expensive and are easily affected by dirt contamination in the rotating disc, light sources, and light detector or bearings runout.

Low-cost magnetic encoders are often used in high-volume applications where high accuracy is not required. While they are insensitive to dirt, they can be negatively impacted by magnetic fields.

Inductive encoders overcome these disadvantages because they can achieve higher position accuracy than magnetic encoders and are cheaper than optical encoders. Plus, they are ideal for rugged environments because they can handle high levels of contamination, vibration and external magnetic fields. Other benefits of inductive encoders include their insensitivity to changes in temperature, and the fact that they have a low component count that reduces size, cost and complexity. From an environmental point of view, another benefit is that they do not rely on the rare earth materials used in some magnets.

Sensor example

To illustrate the advantages of inductive encoder technology, let’s look at an example of an inductive sensor: the NCS32100 position sensor from onsemi. This sensor includes a controller and sensor interface for high-resolution, high-accuracy angular sensing when paired with a suitable contactless PCB (printed circuit board) sensor. It has flexible configuration capabilities, enabling it to be connected to a variety of inductive sensor patterns and it offers multiple digital output formats.

Figure 1: Full rotary encoder anatomy. Source: onsemi

Figure 1 shows the cross-section of a complete rotary inductor system. This includes a rotor and stator, which are both PCBs, with the rotor attached to the center shaft. This diagram also shows the NCS32100, which is mounted on the static stator PCB, along with connectors for power and data. The two PCBs are parallel to each other, separated by a typical air gap ranging from 0.1mm to 1mm.

The NCS32100 is an absolute encoder, meaning that it can deduce position without any rotation of the discs, which is especially helpful to acquire the position at start-up or if the system was altered during downtime. It provides position accuracy better than ± 50 arcsec, or 0.0138 degrees mechanical rotation—a level of precision that was previously only possible with optical encoders. This accuracy can be achieved at rotational speeds up to 6,000 revolutions per minute (RPM), and the NCS32100 can operate at speeds up to 45,000 RPM, albeit with slightly reduced accuracy.

Figure 2: NCS32100 reference design block diagram (simplified). Source: onsemi

To make integrating the NCS32100 as simple as possible, onsemi has developed tools and support, as well as a web-based design tool for PCB design. This includes a reference design for the NCS32100, which is shown in Figure 2, where items to the left of the blue dashed line are contained on the stator PCB board.

In a position sensing system, two sets of conductive traces or coils (fine and coarse) are printed on both disc surfaces (stator and rotor). A third conductive trace called the excitation coil is printed on the stator PCB (see Figure 3).

Figure 3: NCS32100 dual inductive positioning. Source: onsemi

The NCS32100 transmits a 4 MHz sine wave into the excitation coil, creating an electromagnetic field around it. Due to Faraday’s law of mutual induction, the rotor’s fine and coarse coils intersect with the electromagnetic field, coupling energy onto the rotor coils in the form of eddy currents. These rotor eddy currents couple voltages onto the stator’s fine and coarse coils (up to 100mV) connected to the NCS32100 eight receiver inputs.

The NCS32100 measures the rotor position by demodulating the eight receiver inputs, before converting them to the digital domain and processing them. This information is fed into an Arm Cortex M0+ microcontroller (MCU) to give the sensor high levels of configurability. The MCU provides absolute position and speed data, which are sent by the reference board over an RS-485 interface.

Self-calibration of the sensor can be completed in less than two seconds via a single command if the rotor is moving at between 100 and 1,000 RPM. The resulting coefficients are stored in embedded flash. Despite its performance, the dual inductive approach is simple to include. For comparison, an optical encoder system requires many parts, including an optical disk, a stator PCB and an LED driver—totaling 100 components. By contrast, an NCS32100 system only requires a pair of PCBs, with fewer than half the number of components and delivers the same accuracy as the optical alternative.

Bob Card is marketing manager in the Analog Mixed Signal group at onsemi.