Steering Committee: Electric Power Steering Basics

[Counterman]

Electric power steering systems have gained widespread popularity in the U.S. since their introduction in 1990, primarily due to the increasing number of hybrid and electric vehicles in today’s market. Like any new(er) technology, each manufacturer has a slightly different method of achieving the same goal, in this case effortless power steering assist, and some are better suited than others for certain applications.

The first (but never fully-realized in production) was an electro-hydraulic system intended for the 1989 Pontiac Fiero. When GM decided that 1988 would be the last year for the Fiero, the system was shelved for later use in its short-lived EV-1 battery electric vehicle. Electro-hydraulic power steering (EHPS) is itself a sort of hybrid, with an electric motor-driven hydraulic pump replacing the belt-driven unit common to “traditional” power steering systems, but retaining the familiar hydraulic rack and pinion assembly, the associated hoses and hard lines, and often a system-specific hydraulic fluid. Found across a wide variety of marques, EHPS remains relevant today as we find ourselves transitioning between ICE, hybrid and BEV technologies.

Fully-electric power steering systems use DC motors rather than hydraulic pressure to provide the assistive force required to turn the wheels. Electric motors are long-wearing and quiet, eliminating the squeals and groans common to hydraulic systems, and the power losses associated with belt-driven accessories. These features make them an ideal choice for luxury cars as well as those quiet-running BEVs and hybrids. When compared to hydraulic systems, EPS also represents a weight reduction, adding to vehicle efficiency. Current EPS designs fall into three general categories, based upon the location of the assist motor(s). 

C-EPS, or “column assist” systems are commonly found in compact vehicles. The motor, sensors and other electronics are integrated into the upper steering column assembly. This location maximizes underhood space, with the bulk of the assembly hidden below the dashboard, and still allows for integration with ADAS features like self-parking, lane assist, handsfree and self-driving technologies. This system is the only one of the three EPS designs that does not attach to or integrate with the rack and pinion. With no plumbing or wiring, the C-EPS rack unit is effectively a manual steering gear. 

R-EPS, also known as “rack assist” systems feature assist motors integrated into or attached in parallel to the rack body. A recirculating ball gear and toothed rubber belt convert the assist motor’s rotation into a linear (side-to side) motion. Capable of high applied force, this “parallel axis” design is used primarily in light trucks, SUVs and other vehicles where extra steering effort is required. The rubber belt is a common failure point for this type of rack, but repair kits are widely available for many domestic applications, and offer substantial savings when compared to the cost of a complete steering gear.

The last category is the “pinion-assist” or P-EPS system. Single-pinion designs locate a relatively large assist motor at the lower end of the steering column, and force is applied directly to the pinion gear at the input shaft. Due to space and safety considerations, many manufacturers have eliminated this system in favor of a dual-pinion setup. The input pinion gear connects to the column, but the assist motor drives a second pinion gear at the opposite end of the rack, isolating the motor from the column, and resulting in improved steering feel. Limited mostly to mid-size cars, P-EPS is not powerful enough for use in heavy vehicles and most light trucks. 

Vehicle electrification will continue to drive future EPS technologies, but existing ICE vehicles have already proven the advantages of these systems across multiple platforms. The progression from manual to hydraulic to electric power steering systems leaves us on the verge of the next technology, known as “steer by wire.” Just as “throttle by wire” has largely replaced the accelerator cable with a pedal position sensor, engineers are removing the physical linkage between the steering wheel and the steering gear. Steering angle sensors, torque sensors and vehicle speed sensors contribute information to the steering module, which determines the amount of assist required under different driving conditions. This data is sent to actuators in the rack unit that perform the commanded steering functions. Once the realm of science fiction, SBW can now be found in the Infiniti Q60, the Lexus RZ and the Tesla Cybertruck. 

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