| Better fuel efficiency, and a new toy for the performance driver
The time of electric power steering has finally come. Not that it hasn’t been here before. Electric steering has been available in production cars since the launch of the Honda NSX in 1990. Honda followed this up with a similar system in the Honda S2000 in 1999. BMW introduced its first electric system on the Z4 in 2002, and added an electric “active steering” system to its 5-series in 2003.
Earlier electric systems on vehicles such as on the Buick Roadmaster were either expensive to produce or provided the kind of steering “feel” that plagued other attempts to improve power-steering, such as the system on the famous Citroen CX: the car would go where it was pointed, but provided almost no feedback to the driver. As computer systems have become both cheaper and more effective, most of these technical obstacles have now been overcome.
Probably the best testament to this is the following snippets from reviews for the upcoming BMW 5-series, which features EPS.
The combination of BMW’s new four-wheel-steer tech with electric-assist steering felt superb throughout all of the hundreds of quick left-right-left manoeuvres we negotiated at speed. We preferred the Sport mode in the DDC, especially the ability to make customized modifications within that mode through the fourth-generation iDrive interface. (Portuguese roads and drivers are not of the highest quality, so a little electronic oversight is best.) [Edmunds.com]
[The previous 2010 generation BMW 5-series] generally went where it was pointed, but I never felt like I could take full credit for getting it there. Early Active Steering was a mess, although it was improved markedly during the production run of the E60/E61 5er.
...
At a recent BMW technical day, for example, some of the engineers confided they now prefer working with electric power steering (now standard on the 5), because they know exactly how to program the desired steering feel they want into EPS, while tuning the feel of hydraulic steering assist requires more work. The result: The new 5 ... at least when fitted with EPS and optional Active Steering, and practically every other chassis trick in the book as our test car was, is far more rewarding to drive on a twisty coastal road than its predecessor. [Motor Trend]
For the first time on the 5-series, power steering is a purely electric setup. As in the 1- and 3-series, steering feedback and weight have been executed almost flawlessly. [Automobile magazine]
The electric advantage
One of the main advantages of electric steering is its potential to save fuel through greater efficiencies. Mechanical hydraulic systems were first patented in 1932, and became available in production cars during 1951 with the debut of the Chrysler Imperial. In mechanical hydraulic systems a belt drive from the engine itself is used to power the rotary hydraulic pump to pressurize the hydraulic system and provide a power boost to the steering system. This means that there is a constant drain on engine power, as the hydraulic pump is constantly driven.
Electro-hydraulic systems (sometimes called hybrid systems) solved some of this problem by using an electric motor to power the hydraulic pump. This also enabled these systems to easily vary the level of power steering assist, so that maximum boost was provided at low speeds, and minimal boost at high speeds.
However, the hydraulic system itself, though optimized, continued to be inefficient. With an ever-increasing need to make vehicles more fuel-efficient, and advances in computer technology making on-board vehicle computers both more powerful and cheaper, electric power steering (EPS) looks set to become the standard for the future.
|
Helping to drive this adoption is the way in which these systems can interact with other onboard driver aids. A central computer can calculate the various forces on the car through feedback from inertial and braking systems, and actually make steering corrections to prevent unwanted skids and slides. Yet another possibility, exploited on the BMW 5-series and upcoming systems, is that drivers can effectively tune these systems to suit their own preferences, with the driver programming in more or less assistance at set speed ranges.
Differences in EPS systems
Early EPS systems, such as that on the Honda NSX, used a brush DC motor installed concentrically around the steering rack. This had the advantage of simplicity – for example the amount of power assist could be easily controlled by the amount of current. This design remains popular, and has been used on cars such as the 2005 Toyota Prius. However, this design does not work well on larger vehicles, and can suffer from problems such as brush arcing.
One solution that seemed promising was the switched reluctance electric motor. Though robust, powerful and highly reliable, these motors tended to produce unacceptable vibration through the steering wheel due to high torque ripple.
The next solution to come along was the brushless permanent magnet (BPM) electric motor. These provide a high level of efficiency and power density, making them ideal for EPS systems of larger vehicles. Manufacturers such as Delphi have been selling these systems since 1999.
Bigger vehicle systems
These advances have made it possible for companies such as Nexteer in the USA to develop EPS systems for use on larger pickup trucks, such as the Chevrolet Silverado. According to a review of these systems on pickuptrucks.com:
“After driving both trucks, it’s clear that the changes that drivers notice when EPS finally makes it to their driveways won't be near as dramatic as the technology switch itself. In fact, drivers probably won't notice much difference at all. And, in some ways, it's better than hydraulic steering could ever hope to be.”
But, still...
And yet, there are bound to always be some people who will notice a difference, or are simply not happy with the new systems. If you haven’t yet had the pleasure of reading Dan O’Neil’s articles on cars in the Los Angeles Times, you really owe yourself that treat. Dan is a Pulitzer-Prize winning automotive writer, and he takes a unique, passionate approach to reviewing cars. Here is part of his take on the BMW 2011 5-series:
If owners should ever take the 5-series on the track, they'll find them benign, easy, capable, progressive and slightly boring. And numb as a well digger's . . . let's just say numb, OK? What with the electric power steering, the active this and that, the four-wheel thingy, the car has been sapped of anything approaching vividness and tactility.
If you want to fiddle at the margins with the car's ride and handling, you can, via the Driving Dynamics Control, a system that progressively dials up the adaptive suspension, transmission sharpness, rev limit, steering response and the thresholds of the stability control system.
The settings include Comfort, Normal, Sport and Sport+, the last of which finally puts some whalebone in the car's corset. It feels tighter and firmer, corners flatter -- of course, with an active anti-roll bar in the rear, it ought to -- and bites harder. But still, the steering feel is uncommunicative and artificial and just plain over-assisted. [LA Times]
|
