ABS sensors live in a tough world.The underside of a vehicle is ex-posed to no end of road haz-ards—rain, salt, snow and iceamong them. There are the me-chanical shocks of potholes and

possible collisions and maybe even the slidingforces of a spinout to be concerned about. Then,too, there’s the heat from the braking and exhaustsystems nearby. In fact, it’s hard to think of anyplace on a vehicle that would be a worse locationfor a delicate sensor.

The three basic types of ABS sensors all de-pend on magnetic fields. Magnetic fields, in turn,have a set of rules that describe how they’remade, how they’re used and what can go wrongwith them. It turns out that sometimes, ABS sen-sors can find problems with magnetic fields thatmay indicate a problem elsewhere. Bad wheelbearings are one such case.

While we’re going to discuss ABS sensorshere, it’s important to point out that similar sen-

sor technologies are used elsewhere on the vehi-cle. Cam and crank sensors, for example, usemuch the same types of sensors, and what we sayhere will apply there as well.

We should first talk a little about magnets andmagnetic fields. ABS sensors generally use flat,rectangular magnetic materials made of rareearth materials. The major types of magnetic ma-terial (ferrite, alnico, rare earths) are all hard,brittle materials that chip or crack rather easily.

The location and intensity of the magnetismproduced by these magnets is referred to as themagnetic field. When you show this in a diagram,it’s represented by lines of flux that leave one ofthe magnetic poles, curve out and around andreenter at the other pole. Actually, there’s nosuch a thing as a line of flux, but it’s a way to de-scribe what the magnetic field might look like.

If the magnet cracks or breaks, one result willbe that the magnetic field will be distorted fromwhat was intended. (This can actually make onemagnet into two, with two sets of north and southpoles.) This can result in a distorted waveformoutput from the sensor. Damage to pole shoes orother parts of the assembly that conduct themagnetic fields also can distort the magneticfield, giving bad information to the sensor. This inturn can cause the ABS computer to set codes in-dicating a bad sensor when the problem really ismechanical damage.

The shape of the magnetic field determineswhat will be influenced by the power of the mag-net. Metal pole pieces are often placed on eachside of the magnet to concentrate and shape themagnetic field in a way to make it more useful.This works great in sensors because it focuses themagnetic field to give a sharp output change asgear teeth, tone wheel teeth or other metal ob-jects pass nearby.

A good example of how this works is the sim-plest of the ABS sensors—the variable reluctance(VR) type. It has a magnet, a pole shoe and a coilof wire. A permanent magnet creates a magneticfield that surrounds the coil of wire. So long asnothing is moving, that magnetic field is static

MikeDale

The ABS sensor’s hostile work environment and the demand for

greater sensor accuracy have led to several design innovations.

Magnets continue to play an important part in these designs.

mdale@motor.com

Eye On Electronics

This Jaguar wheel bearing announced its failure by setting an ABS code. Thecracked bearing affected the encoder ring’s magnetic field and caused a distortedoutput from the ABS sensor. The damaged area is between the two white marks.

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and no voltages are created. When atooth of a tone wheel (also called thepulse wheel or trigger wheel) comesaround, it offers a new, easier, less reluc-tant path for the magnetic field. Thisthen causes the magnetic field to move.

One of the rules of magnetism is thatwhen a moving magnetic field passesthrough a coil of wire, a voltage is gen-erated in that wire. This same ideaworks in alternators and ignition coils.The voltage generated in the coil can beused to indicate the passage and posi-tion of the passing tone wheel teeth.

For the VR sensor, the voltage outputhas the shape of a sine wave. The heightof the wave depends on wheel speedand the air gap between the sensor andthe teeth. It’s important to set the airgap correctly. Typically, this will be be-tween .02 and .05 in.

The frequency of the sine wave is di-rectly related to wheel speed. Once in-side the ABS controller, the VR signal ischanged into a square wave. The ABScomputer then counts the pulses ofeach wheel and compares them to

make sure one wheel is not rotating at adifferent speed than the others. A dif-ferent wheel speed could indicate alocked wheel or skid situation.

The vulnerability of VR sensors isthat damage to either the magnet or thepole shoe can distort the intendedshape of the magnetic field. This is alsotrue for the condition of the teeth of thetone wheel. There are limits as to howmuch variation from the original designcan be tolerated, and that’s why somesensors and even some tone wheelsmight need replacing.

What does not affect VR sensor out-put is rust or metal particles on the tonewheel. Even though the tone wheelteeth might be crusty and contaminat-ed, it does not make very much differ-ence to the output signal. Scrubbing,polishing, wire brushing, etc., mightmake it look better, but don’t really helpthe signal very much. One thing youcan do is to look at the sensor output onan oscilloscope. You want to see a sinewave that increases in frequency as thewheel spins faster. The height or ampli-

tude of the sine wave will also increasewith wheel speed. Note: If there’s an ex-cessive amount of metal filings on thesensor, it might very well be a sign thatthe wheel bearings are failing. This isanother example of how the sensor can“find” other trouble.

There are a couple of other things toadd about VR sensors. One is thatthey’re easy to identify. Because they’repassive devices (they generate their ownvoltage), they have no need for a con-nection wire to the battery or vehiclepower system. VR sensors have just twowires coming from them.

As for down sides to VR sensors, oneis that the magnet and wire of the sen-sor are more bulky than in other ABSsensor types. Another is that by theirnature, they’re not zero-speed sensors.In a skid, with the wheel locked up,there’s no output from a VR sensor.

Active sensors (the second type of ABSsensor) are powered by a vehicle’s elec-trical system and use a different sensingtechnology. These sensors are smallerthan VR sensors and indicate wheel ro-

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tation even down to almost zero. Theyalso cost more to make and install andare more fragile than VR sensors.

There are a couple of ways to makeactive ABS sensors. One is to use aHall-effect device. Hall cells are madeof silicon, much like transistors. The sili-con is situated in a tiny electrical pack-age that includes a bias voltage. When amagnetic field intercepts the silicon, thedevice will turn on, or switch, between ahigh and low voltage.

By the time you see a Hall-effect typeABS sensor output signal, it has alreadyhad a lot of help to improve the qualityof the signal from the Hall cell. Usually,electronic circuits are built right into thesilicon to improve, amplify and shapethe output signal. The characteristic out-put of the sensor is a square wave whosefrequency is directly related to wheelspeed. The amplitude of the signal isconstant and not related to wheel speed.

There are a couple of ways to controlthe magnetic fields that turn Hall-effectsensors on and off. One is to bias thecell with a permanent magnet. Whentone wheel teeth pass by, it changes thedirection of the magnetic field, causingthe sensor to switch states. Hall-effectcells are very sensitive to the directionof a magnetic field. If there’s damage tothe magnet or the magnet has beenmoved out of alignment, the cell maynot work properly.

A second possible way to activate aHall cell is to let the magnet itself do themoving. It’s possible to locate the mag-net on the moving part of the assemblyand let it trigger the cell when it’s in theright place. One way this is done is witha magnetic encoder ring, which is typi-cally a ring of bonded magnetic material.Mostly the ring is plastic, but embeddedin it are particles of neodymium ironboron magnetic material. Then the ringis magnetized in a special charger thatcan make many separate magnets allaround the circle of the encoder ring.Even though it looks like a ring, it func-tions like 70 or more magnets. Each ofthe different poles can represent an ex-act angle position on the ring.

Of these magnet types, it’s the en-coder ring that’s often most easily dam-aged. Typically, the bonding plastic thatholds the magnetic particles is a materi-

al like polyphenylene sulfide (PPS).This material can be somewhat brittle atcold temperatures and may be subjectto impacts from road debris. The shapeof the magnetic field can also be affect-ed by the surrounding metal. Crackedor damaged bearings located near themagnet pole can affect the shape of themagnetic field and thus change the out-put of the sensor.

The third major type of ABS sensor isa magnetoresistive sensor. This is a low-cost, zero-speed-capable sensor that us-es a different technology than the otherABS sensors. Here, a current is set upto flow through a magnetoresistive ma-terial. When the magnetic field inter-cepts the material, the resistance of thematerial changes, causing a change inthe voltage drop across it.

The sensor’s resistance will be largestif the current flow and the internal mag-netization vector are parallel. The resist-ance in ferromagnetic material is small-est if the angle between the currentflow and the internal magnetization vec-tor is 90°.

For practical ABS sensors, engineersput four of these magnetoresistive ele-ments into a something called a Wheat-stone bridge. In the presence of a mag-netic field, the values of the resistorschange. This in turn causes a bridge im-balance, which results in a generatedoutput voltage proportional to thestrength of the magnetic field.

There are a variety of advantages tousing the Wheatstone bridge configura-tion. The bridge reduces temperaturedrift. It also doubles the signal output tomaximize sensitivity. Another advantageof the magnetoresistive sensor is that it’ssensitive to relatively weak magneticfields. This means it’s not likely to be af-fected by small variations in sensor airgap. This type of sensor is also mechani-cally strong. Unlike VR sensors, magne-toresistive sensors are not subject tooutput variations due to vibration.

The take-away for you is to under-stand that ABS sensors—and for thatmatter, cam and crank sensors—de-pend on magnets and their magneticfields to function correctly. If there’sdamage to the magnet, the pole shoesor the tone wheel teeth, it can set codesindicating a bad sensor.

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