For being something that is seemingly boring and rather mundane, the wheel speed sensor is ingenious. This little device has made possible the innovation of many of the modern vehicle control systems that most of us take for granted, and that many of us would never want to do without. On every single new car sold today, every wheel has a speed sensor that monitors the speed of just the one wheel to which it is assigned. Under normal vehicle operation, things are better when every wheel is spinning at the same speed. The sensors operate in a couple of different ways, but usually create a voltage signal with a variable frequency. The faster the wheel is turning, the higher the frequency of the oscillating voltage.
During those rare moments when wheels are not spinning at the same speed, we might have a tendency to lose control of the vehicle. It’s during these moments that the computer monitoring the voltage signals can see a difference in frequency and from this infer that the car might be headed into a ditch, or maybe into the back of a bus. Is there ever a time when it’s okay to have two wheels doing one thing, and the other two doing something else? Think about that for a minute and it’s pretty obvious that this is what happens just before certain doom.
The first feature made possible by the wheel speed sensor is anti-lock braking systems. These systems have been common for the past 20 years. The idea behind ABS is the fact that a spinning wheel allows for maintaining vehicle control and directional stability during heavy braking. This means you can steer while romping on the brake pedal. Also, a wheel that is skidding maintains less friction between the tire and the road than a wheel that is barely turning. With ABS a computer watches wheel speed during braking and makes sure that all wheels decelerate at the same rate. If one wheel stops before the others it is considered locked up and the computer will back off the brake pressure to that wheel so that it will rotate at the same speed as the others. The ability to steer anytime a vehicle is in motion is never over-rated, and often taken for granted. Why on earth would you want to lose the ability to steer while hard braking?
Traction control systems use the wheel speed sensors in a manner similar to ABS, except it functions as the vehicle is accelerating and not while stopping. During acceleration on a slick surface, one of the drive wheels might slip causing it to accelerate quicker than the other drive wheel. The wheel speed sensors will see this and the computer they report to will take action to correct the problem. If the brake is applied only to the slipping wheel, the power will be kicked over to the other wheel that isn’t slipping through the normal action of the differential gears between the two drive wheels.
Related to ABS and traction control we have anti-skid control, or vehicle stability assist, or dynamic stability assist, or the nanny for bad drivers, or whatever you want to call it. This system takes ABS and traction control to another level. In addition to monitoring wheel speed while braking, VSA also monitors the direction the vehicle is traveling compared to the direction the vehicle is facing. If the vehicle is traveling in the 12 o’clock position but the front of the vehicle is pointed to the 1 o’clock position the computer can see the car is in a skid, and it can take control from the driver to correct the situation. Individual wheel speed is important for the computer to know in this situation. Many other calculations are made by the computer to determine what must be done to get the vehicle pointed in the direct of travel. This will almost always happen through the use of the same modulator that is used during ABS action. If the vehicle is traveling at 12 o’clock but pointing towards 1 o’clock, a quick tap of the brake on the left front wheel will get it going in the right direction again.
Many advanced all-wheel drive systems use the wheel speed sensors to know which wheels should receive the most torque from the drivetrain. Naturally this should be the wheels that are spinning more slowly because this always means they have more traction (as long as the vehicle is not in the middle of a turn, but that’s a story for another time). This keeps all tires firmly planted and driving the vehicle forward. Sounds like traction control but it is more complicated than that. On modern AWD systems the computer will just manipulate torque application between the four wheels without the need to try and stop any of those that don’t have traction, such as occurs with common traction control systems. Electronic actuators in the axles and transmissions can essentially turn individual axles off and on.
This is similar to the last example of where we find a need for wheel speed sensors, torque vectoring systems. Many vehicles that are built for good handling can change the amount of torque supplied to the left or right drive wheel as the vehicle is going around corners. Shifting torque is this manner will prevent things like oversteer and understeer. The vehicle will rotate through corners in a way that’s much more planted and certain.
Considering that the vehicle will go the way of the wheels, having wheel speed sensors in place to allow the computer to watch the wheels, means the computers on modern vehicle can do so much more to control the vehicle. The input of the driver at the steering wheel is not the only way to control the direction of the car. This is a good thing since people so commonly steer their vehicles into trouble.