The Benefits of Contactless Torque and Position Sensing for EPS

In many new vehicle designs, hydraulic power-assisted steering is gradually being replaced by electric power steering (EPS). Improvement in fuel economy, albeit a small improvement, has largely motivated the use of electric motors to drive the steering rack in the passenger-car market.

Additionally, EPS contributes to greener vehicle end-of-life by requiring no disposal of hydraulic fluid. It also continues to be the unstoppable trend towards computer-controlled drive-by-wire systems, which began with anti-lock braking and traction control, and is now taking the market towards fully autonomous vehicles.

Early EPS designs lacked experience and had criticism about their lack of 'feel'. Generally, it is accepted that specific improvements in electric power steering systems, with refinement in sensor and control technology, have overcome such concerns. Nowadays, most car drivers experience performance from EPS that is every bit as good as hydraulic assist, if the drivers are even aware of or can tell the difference.

The Challenge with Larger Vehicles

The aforementioned descriptions are successful for smaller road vehicles, however, it is a different matter for larger commercial vehicles, such as buses and trucks. The challenges can be further demanding for industrial off-road vehicles (ORVs), like vehicles used in the mining and quarrying industries, in agriculture and in factories and warehouses.

These larger vehicles require a much higher torque to provide steering assistance, as well as the need to ensure the reliability of vehicles that are subject to much higher mileages, heavier loads and more constant use than passenger cars, and so issues arise.

Furthermore, some of these commercial or industrial vehicle applications are likely to adopt autonomous or semi-autonomous driving technology sooner than regular road vehicles, making EPS a prerequisite. This has already been implemented in factories and warehouses, where forklift trucks are quickly being replaced by driverless vehicles, operating completely under computer control. Like this, long-haul freight delivery is an early candidate.

Durability and Reliability of EPS

Reliability over the life of a commercial vehicle is a necessity for the adoption of EPS, which must be more durable to minimize breakdowns or any safety-related occurrences. Over its entire lifetime, an ordinary passenger car might travel 160,000 km. In comparison, some commercial vehicles are expected to travel over 400,000 km in just three years.

In addition to this, the operating environment in a commercial vehicle is more susceptible to noise and vibration, placing a higher demand on the delivery of performance and reliability of an EPS system.

Heavier steering loads require a higher power assist in commercial and off-road industrial vehicles. Steering column mounted EPS units usually produce a force of approximately 5kN. Repositioning of the EPS closer to the wheels to drive either the pinion shaft or steering rack can show a rise in this to between 5kN and 12kN. However, this will certainly subject the EPS to higher temperatures and vibration.

It is probable that larger vehicles will require an even greater force, at least 15kN, and possibly much more for the most demanding off-road applications. EPS systems for such vehicles will need to operate from higher voltages, e.g. 42V, and be capable of delivering output powers that are over 3kW.

A Commercial Vehicle's Demands

Sensors are integral to the design of an EPS system to be able to detect the angular position of the steering wheel and the resulting torque being applied to the steering column, pinion, or rack. Since less force is required at higher speeds to achieve the same change in direction, the vehicle's speed is another factor, that the algorithm implemented by the EPS' electronic control unit (ECU), needs to take account of in driving the EPS motor.

Rotary torque and position sensors employ sliding electrical contacts and problems can sometimes arise due to vibration and, as already noted, this will be worse for commercial vehicles. As part of a column-mounted EPS system, they can also contribute to in-cabin noise. For a passenger car, this is required to be below 40dB.

A solution to this problem is to use non-contacting position and torque sensors that are based on magnetic sensing principles, and then mounted on the steering rack. This has the advantage that the system is less affected by vibration, and in turn reduces noise and increases reliability.

On top of this, another highly feasible option is the combined Magnetorque Plus steering sensor from TT Electronics. It is precisely designed for rack-based EPS systems and can be customized to fit larger shafts like those found in trucks, buses, and ORVs. Rigorously tested to meet the vibration challenges posed by commercial vehicles, this sensor has been shown to survive acceleration forces of 8.5G in the x- and z-axes and 5G in the y-axis over periods of eight hours for each axis.

The innovative sensor offers a torque resolution ±5°, allowing the ECU to achieve a smooth output. For a car, 2.5 turns is typically enough, but for a commercial vehicle this might need to be as many as four turns. Importantly, rotational angle can also be customized.

Typical schematic diagram of an Electric Power Steering (EPS) system

Figure 1. Typical schematic diagram of an Electric Power Steering (EPS) system

Can Hall-Effect Technology Provide the Answer?

Using Hall-effect technology, non-contact position and torque sensing can both be implemented which detects the influence of a magnetic field on a current-carrying conductor and generates a voltage difference across that conductor, but transverse to the current flow. These sensors are applied as integrated circuits (ICs) where the effect of the magnetic force on the charge carriers in the semiconductor can be measured as an output voltage of the chip.

An appropriate arrangement of Hall sensors and magnets can be used to measure the angular position and rotation of a shaft. This can be done by using an assembly that comprises of both rotating and stationary elements, for exmaple, a rotor and stator. For an EPS application, this assembly could be mounted on the steering column, pinion shaft or steering rack.

Inside the Magnetorque Plus Sensor

Figure 2. Inside the Magnetorque Plus Sensor

For position sensing the Hall sensors sit on a circuit board in the stator assembly above the two pinion wheels. The flux from magnets on the pinion wheels is sensed and from these signals a microprocessor calculates three outputs; two provide high-resolution angular position signals, while the third is a turn-counting signal that delivers absolute multi-turn position information over ±900°.

For torque sensing, the crown rings can capture the magnetic flux from the torque rotor's ring magnet. With the benefit of the concentrators, they direct the flux to the Hall sensors that are located on the other stator circuit board. The torque rotor is located on the steering shaft and has no mechanical contact with any of the other components in the sensor assembly.

The elements of the rotor assembly

Figure 3. The elements of the rotor assembly

The concentrators are immobile, thus the flow of magnetic flux is from the magnet to the first crown ring and concentrator, and then through the Hall sensor to the second concentrator and crown ring and to return to the magnet.

In order to increase the magnetic flux across the concentrators, the crown rings are arranged so the first is located over the north pole of a magnet and the second over its south pole. Next, if the magnet is rotated so both concentrator fingers are centered over the line between the north and south poles, there will be zero flux flowing through the Hall chips.

The flux flowing through the Hall sensor will be proportional to the relative angle of rotation between the torque rotor assembly and the position rotor assembly at any intermediate position. This provides the necessary measure of the torque applied to the steering wheel.

Conclusion

The adoption of electric power steering in passenger vehicles is proceeding rapidly, but its implementation in heavier commercial and industrial vehicles has been slow. Such applications, especially the industrial off-road class of vehicle, give way to certain challenges for EPS system. The system needs to achieve reliable, long-life operation in harsh environments and yet deliver precise and responsive control.

Luckily, TT's Magnetorque Plus combined position and torque sensor achieves all of this. Its non-contact design makes best use of its mechanical durability ensuring a minimum lifetime of one million shaft rotations without degradation of the output signal. Its torque sensor resolution range is ±5° and its position sensor provides multi-turn position information.

An increase in its system accuracy is possible with customer programmable torque offsets, and custom torque output slopes are accessible along with other custom design options. The integrated style is smaller, lighter and more cost-effective compared to alternatives and uses two separate sensors. Meanwhile, the fully calibrated solution simplifies the route to EPS system design for the harsher and tougher necessities of larger vehicles.

This information has been sourced, reviewed and adapted from materials provided by TT Electronics plc.

For more information on this source, please visit TT Electronics plc.

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