by Jim Kerr
Power steering, once an option, is now standard equipment on almost all vehicles. Power steering has evolved over the years from a simple control valve and external hydraulic ram on the regular steering mechanism to integrated rack and pinion systems with variable assist under different driving conditions. Now electric power steering is making inroads into the marketplace and it offers several advantages.
One of the most appealing advantages of electric power steering to auto manufacturers has to be simplicity. There is a lot of design work and technology in the electric steering systems themselves, but the use of electric systems simplifies vehicle assembly and frees up space in the engine compartment. Hydraulic steering requires a pump mounted on the engine, a belt to drive the pump, hoses to connect the pump to the steering gear and fluid to transfer the energy from the pump to the steering gear. All these components can now be eliminated. This may not sound like much, but in design terms, it is significant.
Look at hydraulic pumps. Vibrations from the pump can cause unwanted interior noise so mounting brackets have to be strong and stiff. Pump placement is also a problem. Different models from the same manufacturer may need different mounting locations so they don’t interfere with body panels and underhood accessories. The manufacturer must build and stock several different pump housings and mounts to meet repair and manufacturing requirements.
Cars already have drive belts for the generator, water pump and air conditioning, so it seems adding a power steering pump isn’t a problem. However, every time another drive pulley is added, the belt must be longer, extra idler pulleys are required and vibrations from the belt
Hoses and fluid are simple parts but hoses cause most power steering problems. Leaks are annoying and messy. Of course, electric systems don’t need fluid or hoses. Vehicle assembly and repair is also simplified because hoses don’t need to be connected and air evacuated out of hydraulic systems.
For drivers, the big benefit of electric power steering has to be fuel economy. Engineers talk of up to 5% gain in fuel economy because the systems only use power when the wheels are turned compared to hydraulic systems that pump fluid all the time the engine is running.
So what vehicles have electric power steering? While TRW did make some systems in the early 80’s, such as the Fiero that used an electric motor to drive the hydraulic power steering pump, the first real application had to be the Acura NSX. Compact, light and responsive, the steering system matched the characteristics of this aluminum bodied sports coupe. Honda again introduced a system on the S2000 sports car. Steering response and feel are excellent. Smaller, lighter electric units are also used on Honda’s Hybid Insight and Civic sedan.
Saturn is using electric power steering on the Vue SUV and the Ion sedan. QuadraSteer, GM’s rear wheel steering system optional on some full size trucks is a true electric steering system with no mechanical connection to the steering wheel. When GM introduces the 2004 Malibu, it will use Delphi’s new E*STEER unit. Other OEM vendors of electric power steering systems are Visteon with EPAS and ZF Freidrichshafen AG with ZF Servolectric.
So how do they work? Two sensors on the steering rack are used. One sensor monitors driver input from the steering wheel. As soon as the wheel is turned, the steering module provides power to an electric motor that moves the steering gear. The second sensor monitors the steering gear position and provides feedback on correct operation to the steering module. Other inputs such as vehicle speed or vehicle lateral acceleration can be used to modify steering assist for better steering feel.
The electric steering unit is still connected to the steering wheel by a steel shaft, but this is only used to provide input to the sensor during normal operation. If the steering module detects a fault, it shuts down the electric motor and the steel shaft allows the vehicle to still be steered.
European cars, because of their smaller sizes and lighter weight have utilized electric steering sooner. Industry observers expect electric power steering units will replace traditional hydraulic power steering units within the next five to seven model years. In 2003, approximately 20 million new cars are expected to be built with electric power steering. Maybe your next vehicle will have it.