Optical encoding

For an encoder utilizing this method, light (usually in the infrared frequency band) is emitted from an LED (or multiple LEDs) and passes through a disk into which notches have been placed. These notches will be at predetermined intervals to fit with the particular application requirements. As the disk rotates, light pulses are picked up by the photodiode on the opposite side to the LED, and an electrical signal is produced. Being of an optical nature, these encoders are totally dependent on having an unobstructed line of sight. The build-up of dirt, dust or grease over time can therefore have a major impact on their operational effectiveness, as the integrity of the notches can deteriorate. Though mechanisms can be put in place to combat the presence of dirt/dust (such as positioning an enclosure of some description around the apparatus), these will have cost implications. In addition, they may result in operational temperature and humidity levels being raised, which could in turn have a detrimental effect on system reliability. The working lifespan of the LED devices should also be taken into account. This will normally be in the region of 18 months to 2 years, and then replacements will needed – resulting in the expense of maintenance staff needing to be called out, and possibly unwanted downtime at the factory production line or processing plant while the work is carried out. The disk may also be prone to issues. Often these disks have a plastic construction (to keep costs down) and over time their shape can become deformed due to exposure to intense heat.

Magnetic encoding

Magnetic encoders have certain attributes that are quite different to optical encoders. Here, instead of relying on the emission of light, a magnetic field is employed. The disk featured in this setup is a magnetized one. This brings about alterations to the magnetic field as it rotates. These alterations are picked up by magnetic sensors, with a signal then being derived. Though this approach offers greater mechanical robustness than using optical encoders, and supports a longer operational lifespan, there are also major shortcomings that cannot be ignored. Magnetic encoders do not have the same levels of accuracy as their optical counterparts, which can be a problem in situations where a high degree of precision is required. Also, as you might expect, they are susceptible to electro-magnetic interference (EMI) – something that is very likely to be present in an industrial setting, where items of heavy machinery are located. Finally, like optical encoders, they aren’t able to support a particularly extensive temperature range. Their deficiencies when it comes to accuracy, EMI resilience and suchlike means magnetic encoders can only be employed in a fairly limited range of applications. 

Trying to implement a rotary encoder solution that will simultaneously deliver prolonged operation and maintain high levels of accuracy has not been realistically possible with the encoder mechanisms just discussed. This has given the industry an impetus to explore alternative options – leading to the technical staff at CUI Inc investigating the prospect of using capacitive technology instead, with the intention of combining the accuracy of optical encoding technology and the durability of magnetic encoding.



Figure 1: Schematic of capacitive encoder.

The company’s AMT series of robust, highly accurate modular encoders are based on capacitive technology, and are thereby able to overcome the operational disadvantages inherent in both optical and magnetic encoding arrangements. They comprise the following elements: a transmitter, receiver and a disk (attached to the motor’s rotating shaft), which separates them. The transmitter emits a signal that is subsequently modulated by the disk – working on the principle that as the disk’s orientation changes, alterations to the capacitance will occur. The receiver picks up the modulated signal and (using sophisticated electronics) this is converted into an output that can be utilized for controlling the motor. 

As these encoders are digital (rather than being of an analog nature, like their optical or magnetic rivals), they offer far greater scope for system design flexibility – with the programming of different resolutions, motor sizes and pole counts all able to be accommodated. AMT encoders are far easier to deploy than other solutions, too – for instance, in a BLDC context, they dispense with the need for a complex alignment process, where it is normally necessary to backdrive the motor and refer to the resulting back EMF output signal via an oscilloscope, then make mechanical adjustments to get the best match. Consequently, a significant time saving can be realized (with the procedure taking a few seconds, rather than 20 minutes). It ensures far more precise alignment (which will in turn improve efficiency), while requiring only minimal engineering effort (with obvious cost benefits). These encoders don’t suffer with the dirt/dust issues that can impinge on the optically based solutions. Furthermore, they support a wide temperature range – from -40°C to +125°C.