RESEARCH ANALYSIS: Review of steering systems
For many years, power steering technology was focused on hydraulic systems but demand is increasingly favouring electric mainly due to fuel consumption and logistical benefits. Matthew Beecham reports.
A turn for the better
According to research by just-auto, two-thirds of passenger cars and light trucks built in Europe in 2008 were equipped with electrically-assisted steering. Although the electrically-assisted steering market is relatively small in North America, manufacturers say this market will grow. Demand on both sides of the Atlantic is being fuelled by a range of factors, not least that electric systems are more economical to run, lighter, recyclable and easier to package and install compared to conventional hydraulic systems.
Electrically-assisted steering systems effectively eliminate the traditional hydraulic system's power steering pump, hoses, hydraulic fluid, and drive belt and pulley on the engine. Such systems eliminate the need for the engine to provide mechanical power for the steering system, providing cost and performance benefits. The technologies accommodate the needs of manufacturers as they migrate vehicles from hydraulics, through semi-hydraulics to fully electric systems.
More specifically, electrically-assisted steering offers significant fuel efficiency advantages and reduced CO2 and greenhouse gas emissions as steering assist is only supplied upon driver demand through an electric drive system that is independent of the engine. Because electric steering systems operate under computer control, steering feel can be tuned through software and enables integration with other electronically controlled technologies such as electronic stability control braking systems.
Most types of electrically-assisted steering in Europe are either EPS (electric power steering) or EPHS (electrically powered hydraulic steering). An EPS system can effectively change the assist force for steering operation depending on information such as steering angle, steering angle velocity, speed and acceleration of the vehicle and each wheel, and yaw rate. It can therefore not only reduce the steering load but also optimise the assist force depending on the driving situation.
The main factors that will help accelerate the adoption of EPS are better reliability, improved control, down-sized and more powerful electric motors and enhanced battery performance. Going forward, the adoption of EPS by mid- and large-size vehicles depends, in part, on fine-tuning the existing technology to ensure drivers of those vehicles detect no change in 'steering feel' compared to their old model.
Steering towards an electric future
In North America, most of the EPS market is accounted for by Japanese and European imported cars. However, manufacturers report that the North American market could gradually increase its use of EPS over the next few years. Last April, Ford announced a commitment to fit 80% - 90% of its Ford, Lincoln and Mercury models with EPS systems by 2012, in an effort to improve the overall fuel economy, performance and reliability of the company's line-up. The 2008 Ford Escape, Mercury Mariner, and their respective hybrid spin-offs already have EPS. For 2009, the Ford Fusion and Mercury Milan will also get EPS, with other new and refreshed vehicles, including the Ford Mustang, Ford Expedition and Lincoln Navigator, to follow over the next three years. Ford said the EPS system in the Escape and Mariner features active returnability and active damping that help correct for road irregularities and improve overall handling and steering feel. EPS also improves fuel economy, because unlike the common hydraulic-powered systems that continuously draw power from the engine, EPS draws power from an independent electric motor only as needed. According to some estimates, this on-demand capability can equal up to a one-mile per gallon improvement in fuel efficiency. For the Escape, EPS - and a number of other new features and technologies - contribute to an 8% increase in fuel economy versus the previous model. "We have a very aggressive plan that by the time we finish with our migration in 2012, we will have a strong position as one of the leaders in this technology," says Brad Hochrein, Ford technical specialist, electrical steering. "Fuel economy is now such a large issue in our country and is one of the main drivers of more efficient vehicle systems like EPS," added Hochrein.
In the early days of EPS, customers and reviewers criticised the poor steering feel, especially vagueness around the centre position. To a large extent, these issues have now been cleared-up.
Charlie Cregeur, director, global steering product planning for TRW Automotive's chassis division, points out that there have been great strides made in improving the performance of EPS systems to match the overall handling and feel compared to hydraulic systems. He said: "One of the big advantages of EPS is that the actual tuning of the steering system can be done when the car is on the test track through tunable software parameters versus having to take the car back to the garage, remove the steering gear and physically make changes to the valve shape which can sometimes take several days or weeks to achieve."
To date, the electrical power required for EPS has limited the system's usage mainly to smaller vehicles, i.e. part of the issue with a 12-volt system is getting enough instantaneous power to have a full electric power steering for a large car. Forsaking the emergence of 42 volts, there are some advances being made. Cregeur believes that there are many parameters that define the 'signature' of a steering system, but in terms of system type selection the "C" factor is one of the most critical. The C-factor, says Cregeur, is the ratio of how far the ground wheels move in relation to one full revolution of the hand-wheel. "The higher the C-factor the more energy it takes to create the necessary force to move the ground wheels rapidly. As the front axle weight of the vehicle increases, so does the amount of energy to maintain a given C-factor. When an electric motor is trying to drive a large load with a typical 12-volt power system, the current draw from the battery and alternator can reach well over 100-amps. If not properly managed this can lead to voltage drops on the vehicle causing dimming of lights or other noticeable impacts. By increasing the supply voltage to the EPS, the current draw can be reduced accordingly and thereby reduce or eliminate these impacts. Some vehicles have been put in the market where the steering system creates or is supplied with an artificially higher voltage like 24- or 42-volts to provide the extra power. The downside to this is the cost of the voltage converter itself."
Among those steering manufacturers pushing back the technical boundaries is tedrive, a specialist in developing and producing driveshafts, differentials and steering systems. In predicting the prospects for hydraulic steering, Salvatore Oliveri, chief technology officer for tedrive Group, believes that the hydraulic steering market will decline over the next few years in Europe but with a very slow rate after 2012. He adds that the reasons for these developments are:
- gross vehicle weight greater than two tons results in steering load demands greater than 16 kN.
- improvements in HPS (hydraulic power steering) to gain EPS features
- improvements in low CO2 adaptive hydraulic pumps
- a premium steering feel
- careful evaluation of cost versus benefit
Despite early predictions of electro-hydraulic steering being a transitional technology, it looks as if it is here to stay, albeit for the foreseeable future. Oliveri adds: "The main reasons for [electro-hydraulic steering] maintaining a presence in the market are due to certain packaging benefits, niche application solutions, cost benefits and the fact that EPS requires a high investment."
In outlining the next generation EPS, Cregeur concluded: "For the next decade, the market will continue to increase fitment of EPS and will strive to have large segment vehicles matching the performance of the lower end B and C segments. OEMs will add more functionality to steering systems thru software algorithms that can, for example, help offset the effects of side-wind forces on the vehicle, suppress some hand-wheel vibrations due to chassis components and enable higher fitment rates of marketable features like park assist."