With today’s control algorithms we no longer deal with systems, rather we work with systems of systems. (For a comparison the refrigerator I purchased two months ago has a more advanced ECU then my first car)(1). Because of this, we need to consider fail-safe systems to protect against emergent properties.
Emergent properties can be defined as “a phenomenon whereby larger
entities arise through interactions among smaller or simpler entities such
that the larger entities exhibit properties, the smaller/simpler entities do not
Until a solution to the halting problem is found the best we can do is the creation of fail-safe systems. Fail-safe systems are characterized as
Inherently responding in a way that will cause no or minimal
harm to other equipment, the environment or to people.
My safe failures…
Starting off in the automotive industry there was a saying “a critical failure is too much energy; everything else is a fault.” Think of a vehicle oscillating out due to bad shocks or an engine overheating due to a lack of coolant; examples of kinetic and thermal energy overabundance.
If the problem is too much energy that the solution, obviously(2), is to remove energy. However, let’s take the two cases outlined above.
First in the case of the bad shocks. The “obvious” method for removing energy is to slow the car down by applying the brakes to slow the vehicle down. In this case, hard braking can add energy into the system; therefore a slow controlled braking event is required.
Something know is wrong, what should we do?
First off how do you know something is wrong? The answer should be that you have a fault detection system in operation. The fault detection sits outside of your system of systems and monitors the system. To monitor the system, you need the following information
- What are the “standard” operational parameters of the system? E.g. maximum engine temperature, biggest “bounce” of your spring?
- How long can you be outside of the “normal”?
- Are there multiple ways you can be abnormal?
- How do you pull the energy out?
The Northstar engine
During my first job, I worked at the GM development center in Flint Michigan. The work focused on simulations of the Northstar engine and ABS braking systems. At the time a unique feature of the Northstar engine was the ability to selectively shut down cylinders during steady state cruise; this was shown to provide a 5 ~ 6% fuel economy savings.
At some point during the diagnostic phase of development a bright engineer, not me, realized that the ability to selectively shut down cylinders also provided a way to cool down an overheating engine.
As our system of systems become more complicated, the protections for those systems have the worrisome trend of becoming more complex. Ensuring that the fault monitoring systems are independent of each other and do not interject faults into the system will be a topic for a future post.
(1) A sad but true story, the car caught on fire before I was able to drive it more than 100 miles.
(2) Like most obvious solutions it is true in some cases but tragically wrong in others.
(3) Growing up in Michigan during the 80’s Flint and the metro Detroit area was well past their heydays; still was and is a living community.