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The February 2010 PDSS Newsletter presented some thoughts about the Toyota accelerator pedal quality problems, and now is a good time to revisit them. Last year at this time, Toyota was forced to recall several models of their automobiles because of numerous reports of sudden, uncontrollable acceleration. In some instances, this problem caused injury and death.
Since then, the Toyota accelerator system was tested by the National Highway Traffic Safety Administration (NHTSA) as well as the National Aeronautics and Space Administration (NASA). Recently, these agencies released their conclusions; the testing revealed no evidence to support the theory that the problems were caused by electronic failures in the systems.
Now that that outcome has been communicated across every major news outlet that could be tapped by Toyota's communications department and their media consultants, let's revisit the two items Toyota does admit were likely to be behind the acceleration problem; shifting floor mats and sticky pedal mechanisms. This is the kind of real-life product development challenge we can consider in terms of how the practices of Problem Prevention and Critical Parameter Management (CPM) could have helped.
A Critical Parameter Management Concept-Classifying Risk
In the practice of Critical Parameter Management (CPM) for product development, we identify areas of risk in the development of products and the processes that make them. Low Risk requirements and engineering parameters are those for which there is a great deal of data and experience. These requirements and engineering parameters are considered Easy, Common and/or Old also known as ECO.
High risk requirements and engineering parameters are those that are either completely New to us, or are Unique to us (even though our competitor is, or may be, already fulfilling these types of requirements), or are Difficult to fulfill. These are considered New, Unique and/or Difficult, also known as NUD.
 | | NUD vs. ECO Development Risk |
This risk accrual model above shows that as we move left to right along the x-axis from the static part characteristics to more complex functional dynamics, the risk of not meeting performance requirements increases. The solid red upper curve indicates that risk is higher for static and dynamic requirements and engineering parameters that are NUD (New, Unique or Difficult) than it is for those that are ECO (Easy, Common, or Old), indicated by the green dotted line.
The diagram above illustrates a rule-of-thumb used in CPM. Risk increases as one moves from:
- Static parts that do not move much on a micro-deflection basis or move very little relative to other parts in a macro-displacement context; no or extremely small mass-energy transfer or transformation such that we can say negligible "work" is measurable at this level of a product.
- Parts and materials that are integrated into functioning sub-assemblies that have some form of internal dynamics. These possess some form of mass-energy transfer or transformation we can classify and measure as "work".
- Integrated parts, sub-assemblies into functioning sub-systems that have additional and more complex forms of internal and across sub-assembly dynamics. These have additional and more complex forms of mass-energy transfer or transformation we can classify and measure as "work".
- Integrated parts, sub-assemblies and sub-systems that form a functioning system that has very high complexity within and across the functioning system. These have the most complex forms of mass-energy transfer or transformation we can classify and measure as "work".
How to Know if a NUD is also Critical-The "Big 7" Measurements
Each identified NUD requirement and engineering parameter should be measured on the following seven characteristics to determine which are Critical to the function of a sub-assembly, sub-system and thus to the integrated system. If the function being measured is associated with human health or safety, it is known as a Safety-Critical Requirement or Parameter and warrants special attention. The "Big 7" characteristics of a function or part/material that contribute to the function are:
- Measurability
- Stability
- Adjustability (also called Tunability)
- Independence or Interactivity and Statistical Significance
- Sensitivity
- Robustness
- Capability
Toyota's Two Possible Culprits...Plus One
- The shifting floor mat: This is a case of the lowest risk category mentioned above - a static part that is supposed to be a non-moving material. Since the floor mat was in proximity to the safety-critical function of the accelerator mechanism, anything that could co-opt the proper return of the accelerator should have been closely examined. To prevent accelerator return function problems, simple Poke-Yoke (Mistake-proofing) methods might have been applied to make it impossible for any designed-material near the accelerator function to impede its proper performance.
- The sticky accelerator mechanism: this is a problem of robustness, the sixth measurement of the "Big 7". If, over time, corrosion or contamination builds up and degrades the function of the accelerator mechanisms, then the function should have been made insensitive to those sources of unwanted variation. Robust Design focuses on this goal.
If Toyota was diligently practicing Problem Prevention at the Development Team Level and CPM as part of their Systems Engineering, they would have had a very high likelihood of preventing both of these problems. Problem Prevention requires the use of Mistake-proofing and CPM requires the use of Robust Design in all safety-critical functions. Of course, anyone can make a mistake, but these should have been prevented. It's also possible that the Development Team and Systems Engineering did their jobs, but were somehow unable to convince or communicate with management. In any case, it's likely that Toyota's Lessons Learned database has been expanded to include these issues. Both lives and money have been lost because of this problem. The cost of Problem Prevention and CPM is miniscule in comparison.
Plus One... Toyota also claims driver error could be a contributing factor to the accidents and deaths from the acceleration problem. I have experienced a problem with certain "wide-soled" shoes I own in conjunction with pressing both the brake and accelerator pedal at the same time...
 | | Accelerator & Brake too close together? |
Perhaps this ergonomic design situation bears some scrutiny by all car companies? I have experienced this problem in all my vehicles - none of which happen to be a Toyota product. What shoes you wear and the position of your heel may just matter....food for thought! I know what some of you are thinking.... the NHTSA will attempt to outlaw certain shoes! | 
