the case of the interfering systems

The Problem

A driver (engineer, and a “gentleman farmer”) was heading home with a load of straw for his barn, when he became inattentive for a few seconds, and found himself rapidly leaving the roadway.  He simultaneously hit the brakes and also attempted a rapid steering correction, and found that the effort to make the steering correction was very much higher than normal.  Fortunately, he did successfully maneuver back onto the roadway without being in, or causing an accident.

When looking into this phenomenon further (as engineers are wont to do), he found this particular model of vehicle had a user record of identical incidences.  He also found that the manufacturer offered no real feedback, nor did dealer service find a reason for this phenomenon.  He also learned of a class action suit by the dissatisfied vehicle owners.

The Investigation

We were asked if such an occurrence was in fact possible in this vehicle, and if we could establish the root cause.

We were supplied with the offending vehicle and all pertinent information. A review of the two systems in question showed that in this model of vehicle, the two systems were in fact connected hydraulically, utilizing the same power supply. Further analysis showed that the brake system was set up to have priority over the steering system, such that should the brake system temporarily require most (or all) of the available hydraulic power, the steering system would revert to its non-power-assisted manual mode of operation. Unfortunately, manual mode in this size and weight of vehicle meant that the steering wheel loads would be quite high, and drastically different in driver perception from the normal, boosted, load level. We also knew by historic experience that when steering wheel loads elevate drastically and without warning due to a loss of power assist, drivers tend to react as though they have lost all steering capability, meaning that often they do not try to steer with a greater level of input. This reaction is also related to the physical size and strength of the driver. 

We concluded that the problem was principally due to insufficient or improper systems engineering. If it had been properly done and validated, it would have pointed out the potential problem, and could have been addressed by changing some of the overall (combined) system operating parameters.

The Solution

Our next step was to verify that our suppositions were correct. To do this, the vehicle and the brake/steering system was fully instrumented, allowing data to be taken during evasive maneuver situations. A test track was rented and a professional test driver ran the vehicle on a series of predetermined tests while the data was recorded. Back home, the data was then reviewed to look for instances during the driving tests that conclusively showed the steering system loads required high input levels. A sufficient number of instances were indeed identified, which then led to a final report detailing the validation of our initial supposition. Our client reported that the suit did reach a satisfactory result for the various vehicle owners.

This article was written by Tom Weisgerber, Vice President of Engineering for Kevin Kennedy Associates, who has over 25 years of experience in systems engineering. Tom can be contacted at 317-536-7009, or via email at TomW@KevinKennedyAssociates.com.