One step closer to full brake-by-wire.
Wheel speed sensors detect the rotation speed of a wheel that's moving. Active wheel speed sensors detect rotation speed all the way down to zero, and the newest sensors can detect the direction of rotation, too. Four different sensor designs are in use today.
The original design is a variable reluctance magnetic pickup, which is nothing more than a coil of wire wrapped around a magnet. The sensor is mounted close to a target wheel called a tone ring, which looks like a gear with square teeth. The sensor's magnetic field is disturbed as each tooth moves through it, and that disturbance generates A/C voltage in the coil. As a tooth on the tone ring approaches the magnet, voltage increases. As it passes directly in front of the magnet, voltage drops to zero. As it moves away from the magnet, voltage continues decreasing (negative voltage) until it leaves the magnetic field and voltage returns to zero again.
This type of sensor generates a voltage sine wave that continuously changes from positive to negative and back again. It's an analog signal, but computers can use only digital signals. So when the voltage reaches zero, the control unit counts each zero-volt cross-over event and converts the cross-over frequency to a digital speed signal.
A magnetic pickup is a passive sensor: it generates a voltage only when the wheel is moving above a certain speed, typically about 5 mph.
A Hall-effect sensor is an active sensor. It detects the presence of a magnetic field and generates a digital yes/no signal. The Hall effect, discovered by Edwin Hall in 1879, is generated by the interaction of electric current and a magnetic field. When current flows through a thin flat semiconductor (left to right), a magnetic field with the lines of flux passing through the semiconductor (front to back) will induce a voltage at right angles to the current.
A Hall-effect sensor has a vane wheel between the semiconductor and magnet that blocks the magnetic field, "turning off" the induced voltage. The wheel has gaps or windows, so as it rotates, the magnetic field is alternately interrupted, turning the induced voltage on and off. The frequency of that on/off signal is proportional to rotation speed, and the window spacing can represent degrees of rotation.
A Hall-effect sensor has a three-wire circuit: power, ground and signal. It can detect slower wheel speeds than a magnetic pickup, and the windows in the vane wheel can be shaped to form the output signal for detecting a reference point or even the direction of rotation. This sensor has been used in ignition systems for decades, but it is sensitive to temperature changes and physical contamination. Though three-wire Hall-effect sensors have been used as wheel speed sensors, they're not the best option.
Two newer types of active wheel speed sensors are quickly becoming more common. Like the others, they detect changes in magnetic flux, but they are quite different from a Hall-effect sensor.
The Continental Teves Mark 20 brake system introduced in the 1990s is the first system that can operate brake calipers without the driver pressing the brake pedal and is the first to use active sensors. The sensing element is called a magneto-resistive sensor. It's constructed similar to a magnetic pickup, with a coil of wire wrapped around a magnet. But in this case, a voltage is applied to the wire. As a tone ring tooth passes the magnet, the changes in magnetic flux cause changes in the circuit's resistance, which changes the current flow through the circuit.
A regulator chip built into the sensor detects these up-and-down resistance changes and attempts to maintain constant current flow by regulating voltage on the circuit. This produces digital high-low voltage signals. The control unit counts the high-low switching frequency to calculate wheel speed, and because high or low voltage is always present, speed can be detected down to zero.
The newest type of sensor is completely different. Though it still senses changes in magnetic flux, this time the magnet is moving. Actually, there are many magnets arranged around a wheel in alternating poles, so as the wheel rotates, the sensor is exposed to alternating north-south magnetic fields.
The sensor assembly consists of two sensing elements mounted side-by-side with an amplifier chip built into the sensor assembly. The output from each sensing element rises and falls the same way as with a magnetic pickup, and the amplifier converts that to digital high-low voltage signals. Also, as before, the high-low switching frequency is proportional to wheel speed, and because the signal is always present, zero wheel speed can be detected. But because the sensing elements are next to each other, the two voltage signals are always slightly out of phase: one rises or falls just a few degrees after the other. That's how the sensor detects the direction of rotation. If the signal from element A lags behind the signal from element B, the wheel is turning clockwise. If B lags behind A, the wheel is turning counterclockwise. The exact same out-of-phase dual-sensor technique is used in radio knobs.
Today, active wheel speed sensors are commonly built into the wheel bearing. The sensor itself, if it can be removed, looks similar to other two-wire sensors you've seen before, but the tone ring might not be visible. Versions with dual sensing elements have a ring of tiny magnets that looks like a roller thrust bearing, but often it can't be seen because it's sealed inside the wheel bearing housing. On newer versions, the magnets are built into the wheel bearing seal.
Why Active Sensors?
Beginning with the 2012 model year, all light trucks will be required to have some type of Electronic Stability Control (ESC) system (see June 2007 Motor Age). The Continental Teves system mentioned earlier was developed for cars and introduced by Mercedes-Benz in 1995, but today's size, weight and price of the components are low enough that some manufacturers are installing ESC on all of their models. We're now entering the second generation of ESC technology, and it usually uses dual-element active wheel speed sensors.
Toyota's Stability Control System introduced on the 2008 Tundra (sometimes referred to in their service information as "Vehicle Skid Control") is typical of what we'll see in the next decade. Since it must be able to apply the brakes without the driver pressing the brake pedal, the Antilock Brake System (ABS) pump is more robust than previous models. A master cylinder pressure sensor built into the hydraulic assembly provides a feedback loop for the control unit. With this feedback and wheel speed sensors that can read zero wheel speed, plus a bit of programming, the brake system can hold the vehicle in place after the driver releases the brake pedal, providing a hill-hold feature.
The Tundra also has Brake Assist, so there's a brake pedal load-sensing switch and a pedal position sensor. Brake Assist automatically applies full brake boost if the control unit decides the driver is making a panic stop, but we won't cover that feature here.
Scan Tool Testing
A factory or enhanced scan tool can check the wheel speed sensors while the vehicle is being driven. On the Toyota system, wheel speed sensor faults will set chassis (C) codes C0200 through C0231. Some are circuit faults and some are plausibility codes. Though these sensors can detect zero wheel speed, plausibility decisions are made at 6 mph or greater in forward direction, or 1.8 mph or greater in reverse direction.
Vehicle speed is used to determine plausibility, so that sensor must be working properly, too. Codes will be set if there's a 5 mph difference between wheel speed and vehicle speed. Codes will also be set if one wheel speed is zero and vehicle speed is between 6 mph and 9 mph.
If the ABS activates the pressure release valve at any caliper for 28 seconds or more, that will set a plausibility code. A code will also be set if there's a difference in wheel speed sensor readings twice for 30 seconds or more (could driving around a big circle for 40 seconds set a wheel speed sensor code?). The control unit can also decide there's metal stuck to sensor rotor magnets.
Self Test Mode
The Tundra skid control system has a self-test mode that lets you test the system while driving either with or without a scan tool. The test should be performed any time a wheel speed sensor or wheel bearing is removed or replaced. Testing the system in self-test mode will generate live scan tool data and also pass/fail signals from the instrument panel lights and warning buzzers. It will also generate fault codes, but if the test is taken to completion and everything passes, they will automatically be erased.
Any real faults will be stored in permanent code memory, and all the codes will be stored if the engine is switched off before the test is completed. Don't enter the self-test mode unless you're prepared to complete the job.
There's not enough space to describe the self-test mode here. Go to www.techinfo.toyota.com and buy a subscription to the Technical Information System (a one-day subscription to TIS is $10; one month is $50). Select the make, model, year, Brakes, Brake Control/Dynamic Control System and the keywords "wheel speed sensor." Once you've figured out how to navigate TIS (it's pretty intuitive), spend some time learning how the system works. Though this brake system has appeared first on the Tundra, we expect it will become Toyota's standard brake system in the future. When you find the Test Mode Procedure, read the instructions carefully and make sure you understand the whole job before entering the self-test mode.
Codes shown in self-test mode include C1271 through C1278, which indicate circuit problems or metal stuck to the rotor magnets. Additional codes are C1330/35 or 1331/36, which indicate opens in the front sensor circuits, and 1332/37 and 1333/38, which indicate opens in rear circuits.
Bare Sensor Tests
On the Tundra, the wheel speed sensors are double magneto-resistive elements, and they read a ring of 48 magnets built into the inner wheel bearing race. Voltage is applied to the sensor on one wire, but the circuit must be complete and in good condition for accurate readings.
Any time the ignition switch is on, a reference voltage is applied to the active wheel speed sensors. That voltage comes from the brake system control unit, and on this particular model it can be anywhere from 5.5 volts up to 20 volts (yes, 20 volts, even with the engine not running). On other vehicles, it's usually battery voltage, but the important thing is that it's the same on all four sensors. Because it's a two-wire sensor and the control unit measures the return signal, it's not possible to test for reference voltage across the connector terminals. With the connector unplugged, check for voltage between one terminal and chassis ground.
There's one other Toyota brake system that we expect will become more common a few years from now. The new Lexus LS460 has what is essentially brake-by-wire laid over a hydraulic brake system similar to the one already described, but it has a hydraulic booster and pressure accumulator.
Using vehicle data already on the CAN bus, plus information from the Adaptive Cruise Control radar and several optical sensors, the "Driving Support Control Unit" can apply the brakes in an effort to avoid a collision or minimize its effect. Some European cars have a similar pre-collision or collision avoidance system, and it eventually might show up on domestic models, too. Although wheel speed sensor data is still gathered by the ABS controller, that data and all braking functions may be integrated into another more powerful control unit.
Whether it's a simple anti-lock brake system or full brake-by-wire, the system won't operate without plausible signals from all four-wheel speed sensors. Since replacing an active sensor may require replacing a wheel bearing assembly, tools and information for proper diagnosis are just as critical.