Tech bit: Precaution on the EPS and drive-by-wire
As EPS has exploded into the market, a number of troubling issues have plagued the system. The National Highway Traffic Safety Administration has opened investigations into the Chevy Cobalt and Toyota Corolla, which share the column-mounted JTEKT EPS system. Cobalt, which moved to an EPS system for the 2005 model year, has been haunted by […]
As EPS has exploded into the market, a number of troubling issues have plagued the system. The National Highway Traffic Safety Administration has opened investigations into the Chevy Cobalt and Toyota Corolla, which share the column-mounted JTEKT EPS system. Cobalt, which moved to an EPS system for the 2005 model year, has been haunted by an accelerating number of failures since the switch, while the Corolla investigation centers on Corollas built since the 2009 model-year switch to EPS.
In both of these vehicles, pinning down exact steering defects is proving to be difficult. Many of the Corolla complaints are related to sudden veering, particularly at speeds above 40 mph. Again, the inability to stay in a set traffic lane is being targeted as the most dangerous symptom of the possible defect.
These symptoms fit conveniently into a category that an early report (by Amit Rohidas Bendale of the Vishwakarma Institute of Technology, publication date unknown) on EPAS technology posted at college-seminars.com calls “auto steer,” a term the paper asserts “has crept into the lexicon as an adjunct to the development of EPAS system.” Bendale attempts to explain this phenomenon in his paper’s section on the disadvantages of EPS thusly:
“To-date, technical and product liability concerns have precluded the introduction of such systems in the market though it is expected that niche application may be expected in the near-to-mid term mix of future vehicles. Such system design have yet to prove themselves sufficiently reliable and safe to prevent dangerous “auto steer” events. “Auto steer” has crept into the lexicon as an adjunct to the development of EPAS system. As the name implies, “auto steer” denotes an uncontrolled steering event neither commanded nor stoppable by the vehicle’s driver due to catastrophic failure in the electron hardware or software. In truth, these systems are control servo systems, similar in function to aircraft control servo systems, and must have multiple redundancies. Although these new EPAS systems are said to have multiple redundancy, their design and broad application within the automotive industry have been, and will continue to be, subject to economic pressure, more extreme than found in the aircraft industry. For instance, one obvious safety related item has been universally deleted from such system specifications: a clutch for physically disengaging the reduction gear box and drive motor assist assembly from the host steering system in the event of system failure. This means that a driver encountering an EPAS system failure will have to exert additional force to “back drive” (i.e. manually over ride) the systems reduction gear box and drive motor assist assembly while attempting to maintain control of the vehicle in the absence of normal power steering assist.”
Reviews in the automotive press often comment that certain steering systems with electric assist do not have a satisfactory amount of “road feel”. Road feel refers to the relationship between the force needed to steer the vehicle and the force that the driver exerts on the steering wheel.
With the drive-by wire, apart from the extra cost to manufacturing, failures in the control system could theoretically cause a runaway vehicle, although this is no different from the throttle return spring snapping on a traditional mechanical throttle vehicle. The vehicle could still be stopped by turning the ignition off if this occurred. Another disadvantage is that manufacturers often reduce throttle sensitivity in the low-mid throttle range to make the car easier or safer to control – or to protect the drive train (gearbox, clutch, etc.) from driver abuse. Even though economic pressures drive auto makers to adapt these innovations some still don’t have full confidence with it seen in the BMW set up which still retains the mechanical system.