Plug and play speed sensors are insensitive to electromagnetic fields

Paul Boughton

New speed sensor technology for industrial electric vehicles has been developed by to cope with the harsh environmental demands created by the switch to more powerful, compact ac motor designs.

The growing trend towards replacing dc motors with ac designs is resulting in new challenges for designers of mechatronics solutions in control and sensing applications.

While there are space-savings and material cost reductions in adopting more compact and powerful ac drive motors, the greater compactness of motor design means that all the related components such as the electronics, bearings etc. are closer together and have to exist and perform in an even more demanding environment. For certain components such as the sensor electronics, the increasingly harsh electromagnetic and physical environments can be a real issue.

The move from dc to ac designs for drive motors is having the effect of pushing up the power density in a smaller unit. While the increased compactness has the advantage of offering better space utilisation for the vehicle design and reduced material component costs, it does have the effect of exposing electronic sensing systems to difficult conditions, particularly due to the inherent ac motor design.

The main issue until now, has been that the existing sensor bearing technology employing Hall Effect devices, used mainly to detect the speed, direction and position of the drive shaft cannot operate optimally or as expected because of interference by the strong magnetic fields generated by the higher motor currents (between 600-800 A) of these ac motors. In the future, currents up to 1000 A could easily be the norm for the next generation of electric motors for industrial vehicles.

New solutions

SKF has developed a new generation of speed sensors for electric ac motors that are insensitive to the effects of strong magnetic environments.

As important, the sensor can be integrated into the bearing design, continuing the general trend towards increased component integration and 'plug and play' solutions for electric vehicle systems.

SKF has a long history of incorporating sensing technology into its bearing designs with applications that span cars, industrial applications, locomotives to wind turbines. Consequently, its designers understand the need to produce robust sensing elements with low maintenance and long operational life.

The experience that SKF has gained both in mechatronics and sensing technology has supported the development of sensor bearing technology that can meet the current and future needs of industrial vehicle drive systems and without any magnetic compatibility concerns.

For this particular application, the designers have sought to meet a number of challenges. One of the central concepts for the motor encoder unit has been its design as a 'plug and play' unit offering high reliability and easy integration. At the same time, the ambition has been to reduce maintenance costs. The device that has been developed is not only capable of use with new drive designs but also able to be retrofitted to current industrial electric vehicles in production.

Of course one of the strongest development drivers has been the desire to make it robust in both mechanical and electromagnetic environments. The new motor encoder unit, BMH series designation, combines the compact range of SKF Explorer deep groove ball bearings, which are characterised by small size, high performance and long life, with novel sensing technology. The first model to be launched is based on the most popular compact bearing size for industrial vehicle applications, the 6206 deep groove ball bearing. The additional sensing element adds only 6 mm to the overall design width. This new motor encoder unit is available as a standard off-the-shelf design or can be provided as a customised solution complete with tailored cables and connectors. It provides two open collector outputs that can be incorporated into the overall drive control by suitable resistive loading.

Inductive system

The system itself uses an inductive microcoil technology that is applied to sensor bearing applications for the first time. Here, two arrays incorporating inductive microcoils sense electromagnetic (EM) pulses that are bounced off a moving metallic target wheel that rotates with the bearing inner ring.

The microcoils transform the EM signal into current, which is then converted into a voltage by a Schmitt Trigger that produces the pulses which are directly related to the speed of rotation. The signal created is clearly detectable and separable from any background noise and provides a versatile signal output. One of the attributes of this sensing system is that it is valid for extremely low speeds and can provide accurate measurements over the range from zero up to about 20,000 rpm. Many other speed-sensing techniques have difficulty in accurate measurements at very low speeds. Another feature of this high performance motor encoder unit is its ability to work at ambient temperatures up to 150°C. Such thermal performance is important in environments that are experienced in vehicle applications that produce thermal shocks, heavy vibration and extreme electromagnetic conditions.

This technology is insensitive to magnetic disturbance and this avoids the need to include any magnetic shielding for the components, which increases the overall system design flexibility. They can be mounted inside the motor coils or magnetic brake, for example. This opens up the possibility for electric drive designers to develop new weight and space saving design concepts that incorporate encoding, power and control electronics plus electric brakes.

Also, the fact that the unit is a true 'plug and play' device means that the electric drive development team does not need to consider the particular requirements of the sensor in terms of environment. This also eliminates additional testing and qualifications for the sensor bearing system. For manufacturers, as well as being a system that is built with future developments in electric drives in mind, it offers less maintenance and repair during warranty periods. Because the new motor encoder unit is an off-the-shelf product, availability is not an issue and replacement parts generally have short lead times.

As industrial vehicle design moves increasingly towards ac motors and away from dc designs, SKF has seen an increased interest in its sensor bearing solutions with a significant growth in sales. This new complete bearing sensing solution offers benefits in providing a more controllable, efficient and compact design in a standard solution that is easy to fit into new and existing electric vehicle drive designs.

SKF offers many other mechatronic solutions for industrial vehicles including steering units that are essential components for Steer-by-Wire systems, which combine reliable encoder technology with the simplicity of 'plug-and-play'. The compactness of these units enabling the steering wheel to be located where it fits best, means operator efficiency can be increased due to improved ergonomics and better overall working conditions.

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