The Medium Is the Measurement!September 1, 2005 By: Ed Ramsden Sensors
I used to have a car whose speedometer made an interesting statement about the designer's performance expectations: the highest speed on the dial was 85 mph. Although this limit didn't bother me much, the acceleration left something to be desired. I used to joke that this car had great pickup-it could do 0-60 in <5 s if I drove it off a cliff. I was thinking about this the other day while returning from a customer visit over the Portland (OR) West Hills. My current car (which has a much bigger engine and a speedometer that goes up to 120 mph) was having to work pretty hard to get up to highway speed while going uphill after creeping through some congestion. It took a lot longer to accelerate uphill than it would have on a level surface. Presumably, if I were going the other way (downhill), it would accelerate much more quiekcly. What is a good way to measure vehicle acceleration?
A vehicle's acceleration capability is sometimes given as the number of seconds it takes to go from 0 to 60 mph. This figure of merit is easy to measure-all you need is a clear track, a stopwatch, and a lead foot. It also is a relevant and simple metric for the consumer, providing an idea of how quickly the car can get up to highway speed. But this metric is dependent on many factors: road condition, weather (which way the wind is blowing), and, of course, whether the road is level. Because the 0-60 figure is so important to certain market segments, you can be sure that the automakers' spin doctors are doing everything they can to tweak it in their favor.
Another way to characterize acceleration is in m/s2, or maybe g's. Both have their attractions. The scientific-sounding m/s2 yields larger numbers (after all, bigger is better); g's, although yielding smaller equivalent numbers, connote fighter planes and Moon launches.
One obvious way to measure acceleration is as the time derivative of speed. A car has built-in speed sensors, so this measurement should be relatively straightforward. You tap into a speed sensor signal, record it with a data acquisition system, load the time-series data into your favorite data analysis program, take the derivative, and apply the appropriate conversion factor. The big problem with this technique, however, is the same as with the 0-60 metric-it still doesn't account for the grade of the road. On a steep enough downhill you could get a good acceleration figure for a car without even starting the engine. Just put it in neutral and release the brake. This is the kind of performance characterization test that warms the hearts of marketers everywhere because they can get about any figure of merit they want just by varying the test conditions.
Another way to measure acceleration is to use an accelerometer (gasp!). While this would require putting some additional instrumentation into the vehicle, it could be housed just about anywhere, even inside the cabin. Again, you would record measurements with a data acquisition system and analyze the results later.
Because an accelerometer measures true inertial acceleration, it also accounts for any gravitational help or hindrance the car experiences. On the way up hill, for instance, the engine output partly counters gravitational acceleration toward the rear. Driving upward at a constant speed, you are applying forward acceleration to cancel out the backward acceleration exerted by gravity. Measuring the actual attainable peak acceleration might provide a better picture of how a car really performs.
Needless to say, there are going to be problems with this measurement technique as well. It doesn't account for road conditions or weather, and, furthermore, a vehicle's peak acceleration will be a function of both its speed and the gear you're in. This would mean that a full characterization could be quite complex. Maybe the 0-60 figure of merit isn't so bad after all ...
The point of this discussion isn't so much a way to revolutionize automotive marketing as it is a demonstration of measurement techniques. When you measure a fundamental property, the way you measure it can make a huge difference in results. When you wind up with two different measurements of the same property, there is always the question of which one is right.
With a nod to Marshall McLuhan, The Medium Is the Measurement!
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