Specialist report TWK ELEKTRONIK: Electronic switching cam encoders with SIL 2 certificate in The Windfair Newsletter

Safety is now playing an increasingly important role in mechanical and plant engineering. Demand for corresponding components has been increasing continuously in recent years. The reason: man and the environment must not be placed in danger as a result of machine damage or malfunctions. New specifications and guidelines specifying the design of new constructions exist to meet this requirement. This topic is also of concern to sensor manufacturers.

The EN 62061 and EN ISO 13849-1 standards, for example, specify how machines and system such as e.g. lifting platforms or paper production machines are to be designed, whilst Germanischer Lloyd is determining the approach and issuing corresponding guidelines in the case of wind power plants. However, certain specifications, which can be summarised under the term "safety technology", have to be met in each case.

As an established and successful supplier of angle and displacement measurement components – this year marks our 50th anniversary – TWK ELEKTRONIK reacted to this trend at an early stage. We now offer a range of sensors which meet these safety requirements. A few months ago, our long-serving rotary encoders with SIL 2 certificate according to IEC 61508 were joined by the NBN65 series inclinometers, also certified to SIL 2.

These will in turn be joined by our NOC series electronic switching cam encoders. They contain a full-size rotary encoder and additional, galvanically separated relay switching contacts. They are used primarily, but not exclusively, in wind power plants, as these devices combine a compact design with high robustness and a high protection class at the same time. Based on TWK's successful T model series, the versatility of these switching cam encoders enables us to offer diverse flange and shaft designs as well as housing materials (aluminium / stainless steel) with little effort. The CANopen variant can be programmed very easily by the user. This applies to the rotary encoder signal and the adjustable cams. Or the customer can order pre-programmed devices which then only have to be adjusted using the preset function on installation. Analogue interfaces and the digital interface SSI are also available. Both are equipped with the preset function.

One major advantage in comparison with the familiar mechanical switching cam encoders with integrated rotary encoder is that the rotary encoder signal and the switching contacts operate without play compensation. An important point for increasing the efficiency of a wind power plant. This advantage is supported by the play compensating toothed gear ZRS developed by TWK, which can be connected to the ring gear together with the NOC.

As safety requirements are also constantly being tightened up in the wind industry, TWK is additionally having this device certified by TÜV Nord, ensuring that a SIL 2 version will be available soon.

This objective is achieved through a redundant design, through safety firmware and software and through secure data transfer to the control system via the CANopen safety interface. The switching contacts which undertake the switching cam encoder function are implemented using relays. These are designed in dual form per switching contact to guarantee safe switching. Contact sticking and the risk of the contact's failing to switch are therefore out of the question. The actuating microcontroller electronics are in any case designed to minimise the probability of failure (SIL 2 = PLd). If a fault nevertheless occurs, it is detected and forwarded to the control system, causing the system to assume a safe status.

The electronic switching cam encoders are used primarily in pitch and yaw regulation. The rotary encoder in the NOC constantly outputs the precise position to the control system for regulation purposes. The switching contacts function as limit switches. This enables frequency inverters or other peripheral devices to be initialised e.g. to stop the motor.

These devices are also suitable for a variety of other safety-relevant applications.

As is customary, TWK ELEKTRONIK is meeting market requirements at an early stage by further developing the established NOC switching cam encoder in the field of safety technology, and is therefore extending its range of certified safety sensors for mechanical and plant engineering.