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Rotary sensors for electric vehicles (Part 2)

Posted: 25 Nov 2015     Print Version  Bookmark and Share

Keywords:electric vehicle  EV  rotary sensors  resolver  encoder 

Part 1 of this series on rotary sensors in electric vehicles tackles the use of resolvers and encoders.

In an electric vehicle, unless the manufacturer has a means to control vibrations where position sensing is required, especially for high shock applications, customers prefer a resolver sensor. Resolvers are able to survive harsh environments and are known for their environmental robustness. Resolvers are made of copper wires, iron laminations, steel housings, and high temperature coatings. Comparatively, encoders consist of optoelectronics, glass optics, plastic housings, and epoxy adhesives. Use of these materials makes encoders fragile and environmentally sensitive.

The environmentally demanding constraints of an electric vehicle generally eliminate encoders as a possible option. Sensitivity to shock and vibration can make an encoder's accuracy suffer much more than the accuracy of a resolver. The transparent optical disc used in encoders also makes them much more sensitive to contamination from dust particles, although, there are some ways to eliminate these issues. To improve robustness, encoder manufacturers try to use metal or plastic disks instead of glass disks and use higher thermally rated components to ensure thermal isolation. Moreover, in order to reduce the shock and vibration in an electric vehicle, mechanical isolation can be used, while certain enclosures can be used by the car manufacturers to reduce the contamination by oil and dust.

The resolver is available from manufacturers as a rotor and a stator as two distinct parts. Hence, it is easier for a car manufacturer to align the resolver components on a shaft and its base plate.

Who wins in measurement accuracy?
Let us first define the term "accuracy." Take one revolution of a shaft and divide it into 4096 parts (2^12 for a 12 bit resolution). Since there are 60 minutes in one degree and 360 degrees in one revolution, there are 21,600 arc minutes (60 x 360) in a shaft revolution. Thus, each division is spaced by 5.27 arc minutes (21,600 / 4096).

If everything is ideal, the error from the real value would be ± 0.00 but the accuracy would still be ± 5.27 arc minutes. Even though the system is presented with a 100% ideal exact value, it can't physically move from the point by 5.27 arc minutes before the next point kicks in. Every component in the system matters for the overall accuracy calculation. It could be attributed to the quality of construction of the sensor or its ability to be stable over temperature ranges and vibration levels; everything matters starting from the shaft to the conversion technology.

Resolver and encoder manufacturers specify the limits of their measurement in their datasheets. The circuit that converts the resolver signals to the digital data could also have inaccuracies because of influence of common mode noise in the system and tracking methods. The accuracy is generally specified in fractions of an angle. The accuracy is given as the limits of variation from the absolute position of the detected angle.

Typical accuracy specification for a brushless resolver is 10 arc minutes. The typical error for the entire resolver system, adding the sensor and the conversion error can be approximated to be ± 15.273 arc-minutes (10 arc minutes for the resolver sensor + 5.273 arc minutes for our above example). The improvements in design and process can increase the accuracy of resolvers to around 3 arc minutes. There are certain tricks that can be done to improve its accuracy.

The standard resolver has only one coil and only one north pole and one south pole per revolution. More coils and more poles can be added into the design to improve the accuracy. The angle that can be detected with the help of a multi-speed resolver is 360 degrees mechanical divided by the number of pole pairs. The number of poles reduces the angular error of a multi-pole resolver. Thus, with the help of such resolvers, the accuracy can be significantly improved.

Also, by paying careful attention to the copper winding and winding it evenly, resolver characteristics such as accuracy can also be significantly improved. Copper winding has to be tight and stable. Some resolver manufacturers [1] even use in-house unique nozzles that are based on the diameter of the copper and are used for passing the copper wire.

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