Comparing optical and inductive sensors
Why is this? Optical encoders shine a light source, usually an LED, through a coded disc that is physically connected to the object whose position is being measured. The disc has a pattern – often a set of equally spaced transparent and opaque lines – and so the light passing through it is alternately blocked or revealed to a photodetector as the disc moves. The resultant pulse-train provides the raw data needed to extract the position or motion of the disc and the shaft on which it is mounted. In incremental optical encoders, this pulsed signal indicates motion and can be counted to determine relative position. In absolute optical encoders, the disc’s pattern is usually coded to provide unique signals for specific positions. For increased resolution, an optical encoder often has two output channels, 90 degrees out of phase, which allows the direction of motion to be determined.
The advantages of optical encoders include their ability to provide high resolution and precision, their ability to provide absolute positioning data and the fact that they are a non-contact form of measurement, which reduces wear and tear.
Inductive encoders use induction, instead of light, to sense positional changes. They usually have a coded disc, made from a conductive material and patterned to interact with an applied EM field, and a sensor head. The sensor head generates an EM field using an excitation coil, so that when the coded disc moves relative to the sensor head, it changes the field induced in the disc’s patterns. This change is then picked up by coils in the sensor head.
Depending on the encoder design, its output may provide absolute position information, or just incremental changes in position. The precision of inductive encoders can be quite high, as it depends on the geometric accuracy of the patterns on the coding wheel and the sensitivity of the sensing coils. The resolution, or the smallest detectable movement, is determined by the ability of the sensing electronics to discern minute changes in the induced voltage or current in the sensing head.
Comparing optical and inductive rotary encoders
Optical and inductive encoders have different advantages and disadvantages. For example, inductive encoders are durable and can work in harsh environments, while optical encoders can be more sensitive to dust, dirt, and other contaminants which obstruct the light path and degrade the signal quality. Optical encoders may even need to be mounted in sealed enclosures to work properly in harsh environments.
Optional encoders are widely thought of as being able to deliver high resolution, because of their ability to detect very fine divisions on the coded disc. The resolution of inductive encoders is highly dependent on the quality of the processing applied to the changing EM field picked up by the sensor head, so high resolutions are possible but may take sophisticated signal analysis to achieve.
It’s a similar story for measurement accuracy, with inductive encoders offering the moderate to high accuracy needed for many industrial applications, while optical encoders can provide very high accuracy, especially in controlled environments.
In terms of operating costs, inductive encoders may need less careful handling than optical encoders, whose sensitivity to dust and dirt can lead to higher maintenance costs and more regular replacement.
Inductive encoders can also be more affordable than optical encoders due to being built with less complex manufacturing processes with lower precision requirements. And they can be smaller than optical encoders because they are not constrained by the need to integrate a light source and a photodetector.
Adapting encoders for precise positioning
Encoders play a pivotal role in motion control systems, especially in controlling high-speed motors, linear axes, and lever angles, by providing vital information related to position, speed, and direction. They are important in providing real-time feedback, for position monitoring, speed, and direction control, especially in closed-loop systems. Rotary encoders are also important for enabling systems to be versatile, provide dynamic control, and helping avoid accidental damage by providing immediate feedback on unexpected changes or errors in the system.
The accuracy of encoders is important to enable precise positioning in applications such as CNC machining, robotics, and automation. It also enables such systems to offer consistency, so that movements can be repeated with very low variability, and can help systems correct their own errors, such as overshoot or undershoot, by providing continuous, accurate feedback.
Custom PCB design for encoder stators and rotors
A PCB stator/rotor pair for an inductive rotary encoder, made on a pair of PCBs
One way to ensure that rotary encoders provide the precision, accuracy and repeatability needed for an application is to have them designed to your specifications. With inductive rotary encoders, this means designing custom PCBs for the stator and rotor to create a pattern of changing EM fields which, upon rotation, can be sensed accurately enough by the sensing coil to be accurately interpreted by the controlling electronics.
For example, in the diagram below, an entire sensing system, including a control IC, transmitter loop and receiver, is built on a PCB stator. It then interacts with a rotor, built on a single-layer PCB, which has both a five-period coarse-positioning loop (the inner, hexagonal trace on the board) and a 64-period fine-positioning loop (the fine pattern around the rotor’s circumference) to provide two levels of positioning data.
The example above shows that it is not hard to imagine producing custom rotor designs to achieve the performance necessary for your design, considering design factors such as how the sensing coil is wound, the layout of the rotor coil, the PCBs’ substrate materials, and the connectivity needs for the encoder’s output.
Introducing onsemi’s NCS32100 dual-inductive rotary position sensor
To support wider use of inductive rotary encoders, onsemi offers the NCS32100 encoder chip, a PCB reference design to build a 38mm rotary sensor using it, and an evaluation board. The part includes a sensor frontend, a DSP, and an MCU with firmware. It outputs digital position and velocity data, instead of raw analogue signals. It can be used to build a contactless absolute encoder that can provide position data even when the rotor is not moving, by applying a battery signal to one pin on the device to retain its positional information.
The NCS32100 is designed to work with two PCBs: a rotor with two printed inductors and a stator with printed inductors and the encoder IC. It has a 20bit-resolution output for a single turn, and 24bit across multiple turns. The encoder’s accuracy is better than +/-50 arc seconds for a 38mm sensor, operating at up to 6,000 RPM. The part will continue to work at up to 100,000 RPM, but with less accuracy.
The encoder IC can be configured to work with different-sized PCB sensor designs. It has flexible mechanical specifications (+/- 0.25mm alignment) and self-calibration facilities that deal with asymmetries in the rotor PCBs and the effect of other mechanical errors.
It’s built in a process that has been proven for making inductive parts for the automotive industry, and is less sensitive to temperature, vibration, or contaminants than many optical encoders of similar specifications. The results is that it can be used to build smaller, less costly rotary encoders that have fewer components than alternative solutions, reducing manufacturing cost, supply chain risks, and minimizing the number of points of failure.
NCS321000 block diagram
An evaluation board provides all the facilities necessary to experiment with the NCS32100 and to understand how it can support your application, including the use of customised rotors with differing diameters from the 38mm example provided.
NCS32100 evaluation board
NCS32100 evaluation board block diagram
Conclusion
The choice between inductive and optical encoders depends on the application’s needs. It will involve considering factors such as the likely operating conditions, required resolution, available space, and budget for the part. Inductive encoders, with their flexibility, robustness, and ability to deliver high resolution with careful design, are an increasingly practical, lower-cost alternative to optical encoders in many applications.
