Electric power-assisted steering (EPAS) has tempted automotive engineers since the 1950s. Even so, the promise of smaller, lighter, and more efficient systems never quite matched the low cost and performance of hydraulic power-assisted-steering (HYPAS).
Advances in microelectronics, however, have rekindled interest in EPAS. Motor drive stages, electronic control units, and torque sensors can now be manufactured relatively cheaply. These components, coupled with complex control algorithms implemented in software, can rival or better the performance and functionality of conventional hydraulic steering systems. In addition, software can be fine-tuned to deliver the desired mix of stability, robustness, and steering "feel."
So says Tony Burton, control engineer with automotive component supplier LucasVarity plc (Solihull, England). The company is presently working on an EPAS system that offers a lower component count than HYPAS, is three to five kg lighter than a comparable HYPAS system, and consumes 4 to 5% less fuel. Attached to the steering column, the self-contained unit is said to be easy to install and particularly cost-effective on smaller vehicles.
"There are often considerable difficulties fitting HYPAS to a small car," Burton explains. The LucasVarity system, he estimates, could be installed in four minutes on a production line.
How it works. Operation of the LucasVarity EPAS is straightforward. An optoelectronic sensor measures driver torque applied to the steering wheel. The electronic control unit takes this measurement and, through the control software, drives the motor.
The brushless dc motor generates an additional torque which acts through a reversible gearbox to the steering mechanism, assisting the driver. This assistance torque varies from about 15 Nm in a small "city" car, to 75 Nm in a larger car such as a Ford Taurus, and diminishes with road speed.
A frequent complaint made of HYPAS systems is their tendency to "over assist" the driver at higher speeds. In poorly designed systems, this can result in the vehicle oscillating or "yawing" around the center line as the driver attempts to correct the oversteer.
In the new EPAS system, software allows precise control over steering behavior. Algorithms programmed into the system define speed sensitivity, yaw damping, and steering self-centering. Added algorithms can give steering a "sports" feel or offer light load settings.
A safety relay incorporated into the design improves fault tolerance, while the electronic control unit includes diagnostic functions for fault detection and management. If the system fails, its "fail stop" design cuts all torque assistance and returns the driver to manual steering.
For a torque sensor, LucasVarity incorporates a dual-channel optical device. Its non-contacting design and mechanical simplicity provide system reliability, while the use of optics offers immunity to EM interference.
To operate, two patterned disks mount on either end of the torsion bar separating the steering wheel and steering column. Torque applied to the steering wheel creates a relative movement between the two discs. Light intensity reaching the photodetectors varies in proportion to torque.
Because either detector can be used to measure light intensity and thus torque, the system is redundant. Offset patterns on the two discs, furthermore, allow the software to calculate the steering wheel's relative position and velocity by comparing the two sensor signals.
Motor design. Achieving a smooth, progressive feel at the steering wheel requires a motor with low levels of ripple and cogging torque. LucasVarity, therefore, uses a three-phase inverter to control motor phase currents, and hence torque. An array of power MOSFETS make up the circuitry; pulse width modulation (PWM) regulates switching time and sequence for the MOSFET stages.
Since the power-switching stage encompasses the most complex dynamics of the whole EPAS system, optimization requires computer simulation. Lucas uses the Saber simulator program from U.S. software house Analogy Inc. to analyze alternative PWM strategies, and to model the complex patterns of secondary currents induced when the MOSFET stages are switched.
The aim, Burton explains, is to ensure smooth control of the switching stages and also reduce the ripple currents fed into the battery harness. "These currents have to be filtered out to protect the electronics in the EPAS control unit. By minimizing the ripple current, we are able to use filter components with lower ripple specifications and, therefore, lower cost."
LucasVarity's Electric Power Assisted Steering system is targeted primarily at the European market where smaller cars dominate and the advantages of EPAS offer the greatest potential. "But there is no reason why the system cannot be scaled up to larger cars," Burton adds. The company claims to have negotiated a development contract with a leading European car maker which could see EPAS installed on production models in the year 2000.
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