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Contactless encoders

Contactless encoders Could this be the death of the rotary encoder as we know it? We look at a new design with no delicate disk and no moving parts.

It's not often that we see a genuine revolution in the world of industrial sensors. The introduction of the inductive proximity switch as an alternative to the mechanical limit switch was certainly one such moment; eliminating the mechanical parts dramatically increased reliability, whilst maintaining the form and function required of the limit switch. However the mechanical limit switch lives on, because in some applications its inherent weaknesses are never seen.

Factor 1 Inductive sensors, with their ability to sense all metals at the same distance, were another important moment in the history of industrial sensors. Turck had the revolutionary idea of replacing the wound coil and ferrite core at the heart of every inductive sensor with multiple coils printed on circuit boards. Although this once again eliminated a number of shortcomings of a traditional sensor, the outdated inductive proximity switch lives on.

Turck has now unveiled its latest innovation in sensing technology. The company has thrown out almost every part of a traditional rotary encoder and replaced it with state-of-the-art, fully encapsulated electronics. Most importantly there are no moving parts inside the encoder.

There is no contact between the passive positioning element fixed to the rotating machine shaft and the encoder. No bearings to wear. No possibility of dust or moisture penetrating the electronics. No problems with vibration or magnetic fields. In fact, according to the company, the mean time to failure is calculated at 138 years. Managing director Peter Gardner explains: "With traditional encoders, there is always the challenge of misalignment. This stress on the bearings accelerates wear. You can mitigate misalignment through some form of coupling, but that adds cost to the system, and the coupling itself has a finite life.

"Then there is the transmission of vibration," he continues. "The delicate optical disks in traditional encoders are not at all tolerant to vibration, or indeed to other environmental factors such as dirt and thermal stress. These are problems inherent to all encoders."

In eliminating the moving parts within the encoder, Turck has also eliminated one of the major problems for the encoder stockist. A traditional encoder has a glass or plastic disk printed with a pattern of transparent and opaque areas. Every variant of encoder resolution and output type requires a different disk. Manufacturers have warehouses full of these different disks, yet relatively few fully assembled and finished products, simply because the economics don't allow it. The majority of orders have to be built on an as-needed basis, thereby extending delivery times.

By contrast, Turck's new encoder has no disk, and the majority of parameters are all programmable, resulting in very low requirements for stock or spares, and a fast turnaround on orders. A single model can replace thousands of different encoder types and standard mounting arrangements accommodate retrofitting with ease. There is now no need to compromise between resolution and rugged design: the Turck RI-QR24 utilises a 32-bit microprocessor, is sealed to IP69K and has over 18-bit resolution.

In a conversation with Kelvin Lewis, managing director of Industrial Encoders Direct in Wrexham, he confirmed that the biggest problem the company encounters with encoders is unreliability caused by vibration, shock and fluid ingress. "We get encoders sent to us for repair after as little as two years, although some can last 30 years. It depends upon the environment they are in and how much work they have to do." A typical repair can cost anywhere from £60 to £300 depending on the encoder. Looking at Turck's new development, Lewis commented that he particularly liked the depth of the housing, which is considerably shorter than other encoders, and confirmed that the price was competitive with traditional encoders.

The Turck RI-QR24 is currently available for use with shaft diameters of 6 to 20mm as standard. Output types for IO-Link, Modbus RTU, 4...20mA, 0...10V, SSi, RS485, and Gray coded. Single and multiturn versions are available, and the encoders are e1 compliant for use on mobile machines.

So what's the secret behind the encoder? Turck has developed an inductive resonance coupling circuit utilising emitter and receiver coils manufactured as printed circuit boards with exceptional precision. The coils and the positioning element form an inductively coupled circuit capable of precisely sensing the rotational position of the positioning element whilst ignoring lateral movement or a distance change. The positioning element is fixed to the rotating shaft and the sensor is mounted next to it with a small gap using standard encoding mounting points. The 32-bit microprocessor evaluates the received signals and generates the output in whatever format has been configured.

Physically, the new encoder is much shorter than the conventional designs that it replaces, and the elimination of the need for a coupling further reduces overall package size. That means it's much easier to integrate into applications where space is an important factor, and it means potential for accidental damage to the encoder in challenging industrial environments is significantly reduced.

The patented technology was developed by Turck in Germany, and the products are being manufactured there as well, with Turck focusing very strongly on the OEM market where the company has ambitious goals for the market share it can capture. Early products have reportedly been enthusiastically welcomed in trial applications with several customers, and products will be generally available by the end of July. Turck also says that explosion-proof versions will be available towards the end of the year.

So could this be the death of the rotary encoder as we know it? The market will decide.
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