stevieturbo wrote:No idea why or what radar is supposed to mean.
RADAR = RAdio Detection And Ranging - a term often used to describe something that is able to detect the presence of a metallic object (like a car) by bouncing radio waves off it. The returned radio wave will be frequency shifted by virtue of the fact that it bounced off a moving object. The difference in frequency can be detected by a radio frequency mixer - the two waves constructively and destructively interfere to generate a beat frequency which is equal to the difference in the two radio wave frequencies. By measuring the beat frequency one can determine the speed of the item that the wave bounced off. The most familiar of these devices are probably the infamous Gatsometer that police forces around the world have misused as a means of revenue generation [how else do you explain them being fitted to brand new dual carriageways ? Is someone seriously going to suggest we are now deliberately designing roads with built in accident blackspots ?? Nope ?? Didn't think so]
So what has this got to do with a Syvecs ? Well.... imagine for a moment that your car is 4 wheel drive, with fairly well locked diffs, and you do an emergency start on ice. The most likely outcome is that the speedometer will register some forward speed which is not a true reflection of the vehicle's actual motion - mostly because all four wheels are spinning. How, then, do you get traction control to work under those circumstances ?
The obvious answer is to grab that Stihl saw out of the garage, fit a nice fresh carborundum blade to it, wait until about 3am and then go out and liberate an unsuspecting Gatsometer and reassign it to more honest use - strap it onto the underside of your car and it will give you a speed reading that actually has some passing correlation to the real vehicle speed. It doesn't matter if the radar unit is stationary and measuring something that is moving, or if the radar unit is moving and measuring something that is stationary - you'll still get the relative speed.
On an installation that has 4 driven wheels, you would use four wheel speed sensors, plus one additional radar speed sensor for the true vehicle speed. Since there is no "rear axle" or "front axle" speed input, in the case where you only have one speed input from the gearbox we tend to use radar since that is more obviously NOT any individual wheel, but rather the output of the gearbox.
Of course the radar label is just a historic thing for the most common mechanism - you could also use LIDAR, GPS or some sort of IMU.
Hope this makes sense,
Pat.