Look at the laundry list of available automotive technologies these days and it reads like something Q would prepare for 007 before an important mission: radar cruise control, night vision, electronic stability control, and a whole host of acronyms that sound like they're meant to counteract SPECTRE's bag of tricks. However, these technologies are part and parcel of the electronic revolution that is changing the automobile from a purely mechanical device into an electro-mechanical one. According to Dr. Alois Seewald, global director, Systems Integration, Steering & Suspension R&D, at TRW Automotive, "Electronic control has fundamentally changed the design and development process in the auto industry, because it has shifted the boundaries between what is possible and what is not in ways that were once thought to be impossible." A couple of close-at-hand examples follow.
It sounds much more exotic than it is: an electro-hydraulic actuator located on one of the two anti-roll bar drop links–the links that connect it to the suspension–used to connect or disconnect the anti-roll bar, as well as adjust its stiffness. Dr. Dirk Nissing, part of TRW's Steering & Suspension R&D team, says ADC, as this technology is referred to, is ideal for light trucks, especially vans, "where ride and handling behavior is dependent on the load. Adding an algorithm to the software allows us to detect the load, estimate its distribution, and adjust the system accordingly."
TRW sees potential for the system in SUVs where the addition of ADC lets OEMs tune the suspension for a soft ride without adversely affecting the vehicle's ability to cope with emergency maneuvers, while also increasing the handling envelope of the vehicle before the electronic stability control (ESC) kicks in. "Because we disconnect the drop link in straight-ahead driving," says Nissing, "you can use a much stiffer anti-roll bar without affecting the ride quality. Head-toss is reduced, and there is an improvement in single-wheel impact performance." In fact, TRW claims there is a 20% reduction in vertical acceleration in straight-ahead driving, a roll reduction of up to 86%, a 50% reduction in understeer, and a 14% increase in the vehicle's handling limit. The latter was proven by what Dr. Nissing termed, "a very famous sports car maker," who discovered that adding ADC to both anti-roll bars was worth four seconds per lap and did not degrade the around-town ride when fitted to one of its cars.
Though more expensive than the cost of anti-roll bars alone, the system is relatively simple. A linear hydraulic actuator is connected to an electro-hydraulic control unit (EHCU) via hydraulic lines, and the EHCU varies the pressure–and, therefore, the force–dynamically. Hydraulic power is drawn from either an engine-driven or a separate electro-hydraulic power pack. With the system capable of pressures up to 180 Bar, but operating at or below 130 Bar in all but the most severe conditions, it's possible to use the same hoses and clamps found in hydraulic power steering systems in order to keep costs low. "If the vehicle already has ESC," says Nissing, "the same sensors are used to control the ADC, and often there is need for only one EHCU for both systems." OEMs can choose from a dual channel/dual axle, single channel/dual axle, or single channel/single axle system.
Nearly 20 years ago, TRW showed an electric power steering system (EPS) that used a concentric electric motor wrapped around the steering shaft. Though extremely elegant, the unit's 140-mm diameter took up valuable underhood real estate, and made its adaptation to world cars–those available in both right- and left-hand drive–problematic at best. "We were able to get the diameter down to 90 mm," says Aly Badawy, v.p., Steering Engineering, "but that increased the length of the shaft motor. To reduce this diameter further without increasing the motor length, we'd need much stronger, much more expensive magnets." So the company went in another direction.
TRW's electric power steering systems now come in two varieties: column drive and belt drive. The former places the power unit on the steering column just behind the instrument panel where the assist is transmitted through a worm gear to the column, while the latter places it to the side and drives the column through a ball nut mechanism via a rubber belt. A column-drive system can be provided as a single module (column and drive unit, steering wheel, airbag, switches) while the belt-driven unit can be packaged remotely and tuned for lower inertia, less friction, and enhanced steering response with a more direct feel. "There's a 90% energy savings with an EPS compared to a typical hydraulic power steering system," says Badawy, "and a 50 kg mass reduction." That weight difference brings an increase of about 1 mpg, and the "weight savings just about pays for the EPS," he says.
While EPS units are best suited to lighter B- and C-class vehicles, TRW has two EPHS–Electrically Powered Hydraulic Steering–systems that can be delivered as pre-tested modules line side for larger vehicles. "The EPHS eliminates the belts, pulleys, and other connections of traditional hydraulic systems," says Badawy, "and is capable of being tuned to provide multiple levels of feel and response." The current TRW system comes with either a two-speed pump whose lower speed is engaged when no steering input is required, or a variable-speed system that matches hydraulic flow to steering input. Its second-generation unit–scheduled to appear in two major European applications in 2007–delivers an 80% fuel economy improvement compared to standard hydraulic power steering systems, greater tuning possibilities, and better packaging.
When offered on a vehicle equipped with ESC, these steering units can be upgraded to adapt steering output to the driving conditions. For example, TRW's Steering Angle Overlay function calculates the required steering angle based on information from the ESC unit's yaw rate and lateral acceleration sensors. "This EPHS-based unit is similar to a steer-by-wire system in its capabilities, but without severing the mechanical connection to the wheels," says Badawy. Not only can it compensate during braking on split-coefficient surfaces, it also provides a speed-dependent steering ratio and reduces oversteer.
Smaller vehicles fitted with Steering Torque Control and ESC can do many of the same things with an EPS system. TRW claims it can reduce braking on a split-coefficient surface 8.2% at 80 km/h with improved stability, compensate for crosswinds and oversteer, and improve trailer stability by calculating the correct steering input and using the torque at the steering wheel to get the driver to steer in the right direction. "This is a software option on our EPS system, or it can be added to an EPHS system," says Badawy. It's also a stepping stone to TRW's Park Assist System that uses EPS or EPHS and ultrasonic sensors to calculate the steering trajectory necessary for successful parallel parking.
"Many of the systems created through the increase in electronic content have begun in Europe where driving conditions are much more congested than they are in North America," says Dr. Seewald. "However, as fuel prices stabilize at higher levels and U.S. automakers look to improve the safety, capabilities, and differentiation of the models in their lineup, their interest increases." To which Badawy adds: "You can't base your future on 'back of the envelope' calculations anymore. Technology prevails in the end."