Answer Geek: How Cruise Control Works

Q U E S T I O N: So you’ve explained how a speedometer works; how does cruise control work then?

— Adam D.

A N S W E R: Cruise control may not be rocket science — at least not yet, although as we’ll see later, it is getting closer all the time — but after last week’s foray into the quantum computer and the all-but-inexplicable world of quantum physics, this question comes as something of a relief. Besides, doesn’t that moment when you’re out on the open road and you flip on the old cruise control and sit back and watch the scenery roll by at a steady 80 — I mean, um, 65 miles per hour, rank as one of life’s little pleasures?

Speed Check

So how does that handy little device work? Pretty simple really. I live in the Pacific Northwest, so let’s just image that I’m driving from, say, Seattle to Portland. After snaking my way through traffic-clogged highways for 90 miles or so, I’ll eventually reach a less-densely populated stretch where traffic just might thin enough to finally allow me to switch into cruise control mode.

When I reach my ideal cruising speed — factoring in road and weather conditions, and that all-important legal speed limit — I press the button that kicks on the cruise control. When I do that, my current rate of speed is recorded in the memory of a control unit which contains a microprocessor. That control unit has three main responsibilities: it remembers the speed I want to maintain, monitors the vehicles actual speed, and controls the flow of fuel into the engine.

Information about vehicle speed is supplied to the control unit by a speed sensor, which typically consists of a magnet mounted on the drive shaft. This magnet spins past a sensing coil as the drive shaft turns, and the coils send a pulse to the cruise control unit each time the magnet flashes by. The control unit reads the pulse frequency to figure out how fast I am going, and if that speed is within a few miles-per-hour of the speed I was traveling when I pushed the cruise control button, all is well. If I begin to climb up a hill, and my rate of progress slows 5 miles an hour or so to 75 — I mean, 60 — the control unit signals a little servomotor connected to the accelerator linkage. That moves the engine throttle, sending more fuel into motor fuel, and allowing the car to accelerate. When I’m back at my ideal cruising velocity, the controller signals the servomotor to ease off, and the car settle back to a nice even cruising speed.

At least, that’s how the cruise control in your average vehicle works. But there is something new on the horizon, and believe me, it’s not the same thing at all. Currently available on just small handful of high end models from the likes of Jaguar, Mercedes-Benz, and Lexus, it may not be rocket science, exactly, but it does owe a lot to technology developed for military applications, most notably radar.

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