These devices indicate the angular position of a rotating shaft. They are described most often as incremental or absolute.
Incremental encoders: This version needs an input voltage, often between 5 and 30 Vdc. The design is similar to a rotary switch in that it is on or off, so the output is 0 or the input (supply) voltage. When supplying an encoder with 5 Vdc, that will be the high output voltage. Because the output is either 0 or the input voltage, the output is always a square wave.
These are further identified in terms of pulses per revolution (ppr). A common incremental resolution is 1,024 ppr. So for every complete rotation, the encoder produces 1,024 pulses. Another common resolution is 2,048 ppr, but incremental encoders come in many different resolutions, from 1 to 10,000 ppr. Most applications in a wind turbine will be 1,024. On the generator, however, expect 3,072 ppr. Each pulse is exactly the same. A controller would count the number of outputs to know a shaft’s position.
Absolute encoders: These are a bit different. Their output data is relative to their position. They output a unique digital word for each individual position. They also output in several different field-bus-communication protocols. Instead of using pulses-per-revolution to describe their resolution, they use bits. Rather than say an absolute encoder has a resolution of 1,024 ppr, it is described as 10 bits.
Here are a few windpower applications. A common one is inside the turbine hub on pitch control units. Encoders are either sandwiched between pitch motors and brakes or on the back ends of motors. Shafted encoders are picked for these applications as well as hollow-shaft devices. An important element to keep in mind when looking at encoders for these applications is the temperature spec. it’s not surprising that encoders sandwiched between motors and brakes get hot, so make sure the selected device can tolerate the heat. Specs up to 130°C are not unusual.
Not all encoders are built to tolerate heat, so when they get hot, several things can happen. For one, glued components can come apart and cause the device to fail. Also, signal quality can be affected by extreme heat causing the drive system to misunderstand the position reported by the encoder. Both of these failures can be avoided by understanding the encoder’s capabilities.
Another common wind-turbine application is on the back end of a generator. Encoders there are used to regulate speed and, in some cases, determine position. Like the feedback used on pitch systems, generator feedback applications are considered mission critical. if either of these feedback applications fails, the turbine cannot produce power. Also expect encoder problems on those with undersized bearing on a generator. Failed bearings are the number one cause of generator-encoder failures. Electrical and thermal isolation further enhances feedback reliability.
One OEM uses a pulse encoder on each rotor blade and one to track where the turbine is pointing. The main control system uses that position information to optimize generator speed relative to wind speed and direction. It is essential to use reliable components because wind turbines are sited in areas with large temperature fluctuations. In addition, many wind turbines are offshore, so their components must be reliable and durable, otherwise servicing and maintenance costs can spiral out of control.
Magnetic hollow-shaft encoders: These provide a third type. They are hermetically-sealed magnetic encoders, heavy-duty devices said to detect wear and maintenance issues. one line of these encoders can be mounted onto generator shafts up to 740-mm diameter, yet have only a 27-mm profile. These encoders provide 17-bit single- turn resolution, which provides accurate speed, rotation, and position data even at low turning speeds. With FpgA signal processing, the encoders are precise, generating maximum incremental square pulses of 524,288 periods/ rev and a maximum sine/cosine interpolation of 32,768 periods/rev. The units come with an ip68 stainless steel encoder wheel and an ip67-rated sensing head. The encoders have no moving parts, light source, or breakable components. The manufacturer also makes hermetically sealed magnetic encoders also ip69K-rated. Such encoders are protected from salt, dust, temperature extremes, moisture, and other contaminants. The
most recent encoders of this sort feature a monitoring system that notifies users in advance regarding wear and maintenance issues. A checking system in the encoder monitors all functions over the full speed range and sends an alarm when needed.
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