Hydraulic vs. EPS
Hydraulic power-assisted steering is steadily being replaced by electric power steering (EPS) in many new vehicle designs. In the passenger-car market, the use of electric motors to drive the steering rack is largely motivated by the resulting, albeit small, improvement in fuel economy.
EPS also contributes to greener vehicle end-of-life by having no hydraulic fluid to dispose of. It also continues the inexorable trend towards computer controlled drive-by-wire systems, which started with anti-lock braking and traction control and is now taking us towards fully autonomous vehicles.
Despite the poor experience of early EPS designs, with criticism of their lack of 'feel', it is generally accepted that refinements in electric power steering systems, with improvements in sensor and control technology, have overcome such concerns. Now, for most car drivers, EPS delivers performance every bit as good as hydraulic assist, if indeed the drivers are even aware of or can tell the difference.
Big Vehicle Challenge
This is all fine as far as smaller road vehicles are concerned but it is a different matter for larger commercial vehicles, such as buses and trucks. The challenges can be even more demanding for industrial off-road vehicles (ORVs); for example, those vehicles used in the mining and quarrying industries, in agriculture and in factories and warehouses. The problem arises with the much higher torque needed to provide steering assistance, combined with the need to ensure the reliability of vehicles that are subject to much higher mileages, heavier loads or more constant use than normal passenger cars.
Additionally, 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. Long-haul freight delivery is one such early candidate; however an area where it is already happening is in factories and warehouses, where forklift trucks are rapidly being replaced by driverless vehicles operating entirely under computer control.
EPS Durability And Reliability
Reliability over the life of a commercial vehicle is vital for the adoption of EPS, which must be more durable to avoid breakdowns or any safety-related incidents. An ordinary passenger car might travel 160,000 km over its entire lifetime whereas some commercial vehicles are expected to cover more than 400,000 km in just three years. Moreover, the operating environment in a commercial vehicle is more prone to noise and vibration, which places an even greater demand on the performance and reliability an EPS system must deliver.
The heavier steering loads presented by commercial and off-road industrial vehicles require a higher power assist. Steering column mounted EPS units typically produce a force of about 5 kN. Moving the EPS closer to the wheels to drive either the pinion shaft or steering rack can increase this to between 5 kN and 12 kN but will subject the EPS to higher temperatures and vibration.
Larger vehicles are likely to require even greater force, at least 15 kN and potentially 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 greater than 3 kW.
Commercial Vehicle EPS Demands Better Sensors
Integral to the design of an EPS system are sensors to detect the angular position of the steering wheel and the resulting torque being applied to the steering column, pinion, or rack. 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, since less force is required at higher speeds to achieve the same change in direction. This is illustrated in figure 1.
Fig. 1:. Typical schematic diagram of an Electric Power Steering (EPS) system
Rotary torque and position sensors that employ sliding electrical contacts are subject to problems 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, which for a passenger car is required to be below 40 dB.
A better solution is to use non-contacting position and torque sensors, based on either magnetic sensing principles, and mount them on the steering rack. This has the benefit that the system is less affected by vibration, which reduces noise and increases reliability.
Another highly viable option is the combined Magnetorque Plus steering sensor from TT Electronics that is specifically designed for rack-based EPS systems and can be customized to fit larger shafts such as those found in trucks, buses, and ORVs. This sensor has been rigorously tested to meet the vibration challenges posed by commercial vehicles and 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 sensor provides a torque resolution ±5°, allowing the ECU to achieve an extremely smooth output. Rotational angle can also be customized. For a car 2.5 turns is typically enough but for a commercial vehicle this might need to be as many as four turns.
Hall Effect Technology Provides The Answer
Non-contact position and torque sensing can both be implemented using Hall-effect technology, 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. Hall-effect sensors are implemented 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.
So a suitable arrangement of Hall sensors and magnets can be used to measure the angular position and rotation of a shaft, using an assembly that comprises both rotating and stationary elements, i.e., a rotor and stator. For an EPS application this assembly could be mounted on the steering column, pinion shaft or steering rack.
The Magnetorque Plus sensor is designed for rack-mount EPS and combines non-contact torque and multi-turn position sensing into a single robust unit that is ideal for handling the harsher and tougher requirements of larger vehicles. Figure 2 shows the overall construction of this sensor while figure 3 provides a more detailed illustration of the rotor elements. The crown rings rotate together in the position rotor assembly, while the magnet and yoke, which is designed to increase available magnetic flux, rotate together in the torque rotor assembly, but separately from the position rotor.
Fig. 2: Inside the Magnetorque Plus Sensor
Fig. 3. The elements of the rotor assembly
The Hall sensors for position sensing sit on a circuit board in the stator assembly above the two pinion wheels that can be seen in figure 2. They sense the flux from magnets on the pinion wheels and from these signals a microprocessor calculates three outputs – two of these provide a high-resolution angular position signals while the third is a turn-counting signal that provides absolute multi- turn position information over ±900°.
For torque sensing, the crown rings shown in figure 3 capture the magnetic flux from the torque rotor's ring magnet and, with the benefit of the concentrators, direct the flux to the Hall sensors that are located on the other stator circuit board (shown green in figure 2). 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 concentrators are stationary so 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 back to the magnet.
To increase the magnetic flux across the concentrators, the crown rings are arranged so the first is positioned over the north pole of a magnet and the second over its south pole. Then, 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. At any intermediate position 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. This provides the required measure of the torque applied to the steering wheel.
The adoption of electric power steering in passenger vehicles is proceeding apace but its adoption in heavier commercial and industrial vehicles has been slow. Such applications and especially the industrial off-road class of vehicle present particular challenges for EPS systems, which need to achieve reliable, long-life operation in harsh environments and yet deliver precise and responsive control.
TT's Magnetorque Plus combined position and torque sensor achieves all this. Its non-contact design maximizes mechanical durability ensuring a minimum lifetime of one million shaft rotations without degradation of the output signal. Its position sensor provides multi-turn position information while the torque sensor resolution range is ±5°.
Increased system accuracy is possible with customer programmable torque offsets and custom torque output slopes are available along with other custom design options. The integrated approach is smaller, lighter and more cost effective than alternatives using two separate sensors, while the fully calibrated solution simplifies the route to EPS system design for the harsher and tougher requirements of larger vehicles.
About the Author
Teng Hoong Swee is a Global Product Line Director with TT Electronics. In this role, Swee manages TT Electronics - BI Technologies products including electric power steering torque and position sensors. He has more than 10 years of experience in magnetic and optical sensing industries. TH Swee holds a B.Sc.E. with Honors in Electrical Engineering from The University of Iowa, Iowa City, USA.