Anti-Lock Braking System (ABS)

General description
The main function of the ABS system is to control the speed of the wheels when the brake pedal is pressed by checking whether any of the wheels decreases its speed faster than the others. This means that there’s a possibility of a wheel “block”. The modern equivalent is now called “Stability Control” and is much more advanced than the standard ABS. New cars are fitted with anti skid system, which basically works opposite to the ABS. If there is some increase to the individual rate of any of the wheels when accelerating, this wheel receives breaking pressure to reduce the difference. During this time, the electronic control unit changes the torque. The ABS indicator light does not flash before, during and after the above action, i.e. control unit does not perceive the situation as an error, but as the normal operation of ABS.

Used types of sensors
ABS sensors are divided into two types:

Passive (analog)
Active (digital)

Principle of operation of the ABS

ABS consists of three main elements:
           • Wheel speed sensors
• Electronic control unit (ECU) (i.e. ABS controller).
• Actuator – hydro-pneumatic unit.

 

Each controlled wheel is fitted with pinion and inductive sensor comprising a permanent magnet and a coil. Rotation of the pinion induces AC voltage in the coil sensor, which frequency is proportional to the angular speed and the number of teeth of the wheel.
Hydro-pneumatic unit incorporates hydraulic accumulator, electro-hydraulic pump and valves. Individually adjustable valves are fitted in pair at each wheel: normally open input valve and normally closed output valve. By controlling these valves, the ECU increases, decreases or maintains constant pressure in the brake chamber. In the initial state of the hydro-pneumatic unit, the two electromagnetic valves and the hydraulic pump motor are empty. Brake chamber is connected to the master cylinder through the open output valve and the input valve is closed.

Also Read:

ENGINE COOLANT TEMPERATURE SENSOR (ECT)

At normal operating pressures (no wheel “block”) brake fluid passes from the master cylinder to the brake chamber without restriction as the pressure of the fluid in the cylinder and the chamber is equal and proportional to the amount of pressure applied on the brake pedal. In this case ABS does not affect the brake system. During an emergency brake (possibility of wheel “block”) the ECU controls the electromagnets of two valves simultaneously causing them to operate. The input valve releases the brake chamber from the master cylinder and the output valve connects it with the hydraulic accumulator, thus reducing the pressure. Simultaneously, the ECU turns on the hydraulic pump motor to return liquid from the hydraulic accumulator in the master cylinder. Brake chamber pressure continues to decrease. When the possibility of wheel lock disappears, the ECU closes the output valve. The brake chamber is disconnected from the master cylinder and from the hydraulic accumulator, and the pressure in the chamber remains constant and less than the master cylinder. When the wheel speed increases, the ECU turns off the input valve which opens and the brake chamber is connected to the master cylinder again. The pressure in the chamber increases and equalizes to the pressure in the master cylinder. This completes one cycle of operation of ABS.
If the wheel tries to block again, next cycle is being started. System frequency is 5Hz – 10Hz.
When ABS operates the average pressure in the brake chamber does not depend on the applied pressure to the brake pedal. It is determined by the ECU and depends on the condition of road surface.
In its full configuration ABS includes four sensors and four pairs of valves, which allows individual control of each wheel to achieve maximum effect and allows keeping the diagonal distribution of brake actuators. Such systems are used to be called four-channel systems.

Operation principle of passive wheel speed sensors

      During its operation the ABS ECU receives signal from four identical in design sensors, one for each wheel (Fig. 3).


Fig. 3

      The sensor consists of a coil with a core magnetization. Sensor output is AC, and generates a voltage pulse each time any of the teeth of the rotating cogwheel passes through the sensor’s magnetic field. The shape of the output signal depends on several factors:

  • internal magnetic field strength of the sensor;
  • number of windings of the sensor’s coil;
  • shape of the rotating cogwheel;
  • distance between the sensor and the rotating cogwheel;
  • Speed at which the cogwheel passes along the magnetic field of the sensor.

Each of these factors plays a significant role in forming the output signal from the sensor. When one of cogwheel’s teeth falls into the magnetic field of the sensor, a positive voltage peak on the output signal is present, and after leaving the magnetic field a negative voltage peak is present, equal to the positive one.
The sensor has two output terminals, one of which is connected to the ground and the other (signal) is connected to the ECU.
The output signal of the sensor depends on the speed of the car: lower speed means smaller output amplitude and the period of impulses is bigger, and opposite – at higher speed, the amplitude is greater and the period of pulses is less.
In case of a mechanical failure of any of the teeth of the ABS reluctor, the output signal looks like the signal in the diagram below (Fig. 4).


Fig. 4

If wear of the teeth is present, output signal will look like the signals in the following diagram (Fig. 5).


Fig. 5

Operation principle of active wheel speed sensors
(Digital ABS sensors)


Fig. 6                                        Fig. 7

      Active ABS sensors offer an advantage of being able to read very slow speed. Passive sensors, normally quit reading around three miles per hour. Newer active sensors can also determine the direction of rotation. They can be built much smaller than passive sensors. Often they are incorporated into the wheel bearing assembly. This saves assembly time for the manufacturer, but greatly increases the cost of repair. When an integral sensor fails, the entire hub bearing has to be replaced.
Active ABS sensors produce a square wave, digital output. The operation of the active sensor can be likened to the Hall type sensor found in distributors etc. The pick-up assembly has an inbuilt amplifier, producing a strong signal even at a very low speed and thus relies on a supply voltage, normally 5V but it can be 12V. The rotating element consists of a multi-pole (north-south, north-south) magnetic ring, which can be located onto a rotating assembly as with the passive sensor. The rotating, alternating, magnetic poles generate a magnetic flux within the sensor element, which then amplifies and regulates the signal for the ECU to use as wheel speed information. The output of an active sensor is capable of sending wheel speed information down to 0mph, whereas the passive sensor’s accuracy is usually dubious below, 25mph. Active sensors generally have three wires: power supply (reference input),  signal return and  earth/shield. But some active sensors have only two wires (reference voltage and signal return).
Also, some vehicle manufacturers now use active sensors that have small aerials built in, to transmit information to the ECU (vehicle’s computer) via radio waves.

It is important to note, there is no way to quickly tell if a Wheel Speed Sensor is passive or active.
Both sensors have two wires and are located in the same position on the vehicle. Often, the service information will not indicate what types of sensors are on the vehicle. Trouble codes will not indicate if the sensors are passive or active.

Procedure to verify the reliability of the passive ABS sensor

  • Perform an external visual inspection of gear and sensor for broken or damaged wires.
    Use voltmeter to check whether power supply is applied.
  • Check whether the air gap between the pinion and the sensor is within the required limits.
  • Disconnect the sensor connector.
  • Measure with ohmmeter the active resistance between the terminals of the sensor. Check the database what should be the value of the measured resistance for car you are dealing with. If the reading shows extremely high resistance, sensor is broken. Zero or close to zero indication means that the sensor coil is in a short circuit.
  • Repeat the following procedures to the other wheels to see the condition of the remaining sensors.

Oscilloscope measurements

  • Plug in the sensor connector.
  • Lift the vehicle with a gig or a jack so that the wheels can rotate freely.
  • Connect the active end of the oscilloscope probe to the signal terminal of the sensor and the other end to the chassis ground. Spin the wheel. You should observe the following waveform (Fig. 8)

Fig. 8

  • Remember that one period of the signal corresponds to one revolution of the wheel.
  • It is possible to observe the following waveforms – high signal from the sensor (Fig. 9) or low signal (Fig. 10)


Fig. 9

Fig. 10

  • Repeat the procedure to the other wheels to see the condition of the remaining sensors.

Procedure to verify the reliability of an active ABS sensor
Because of the added electronic circuitry inside the sensor, and active ABS sensor cannot be tested with an ohmmeter like the passive ABS sensor. The easiest way to identify whether the sensor is active or passive,is with a voltmeter.

  • Perform an external visual inspection for broken or damaged wires.
  • Turn the ignition on, and with the wheel setting still, back probe the connector.
    If you see a voltage reading (5 to 12V), you have an active sensor. If you see no voltage (0V), you have a passive sensor.
  • Turn ignition on. Connect a digital voltmeter to the power supply terminal (reference voltage) of the sensor.
  • Lift the vehicle with a gig or a jack so that the wheels can rotate freely.
  • Turn on of the wheels very slowly and look for a change in the voltage output from high (around 1.65V) to low (from 0V to 0.9V).
  • Repeat the following procedures to the other wheels to see the condition of the remaining sensors.

Oscilloscope measurements

  • Make sure the sensor’s connector is plugged in.
  • Lift the vehicle with a gig or a jack so that the wheels can rotate freely.
  • Connect the active end of the oscilloscope probe to the signal terminal of the sensor and the other end to the chassis ground. Spin the wheel. You should observe the following waveform as in Fig. 11.
  • Remember that one period of the signal corresponds to one revolution of the wheel.
  • Repeat the procedure to the other wheels to see the condition of the remaining sensors.


Fig. 11

Possible reasons for failure of the active (digital) ABS sensor:
The principle cause of failure is the same as most other automotive sensors, the environment in which it operates. Wheel speed sensors operate in harsh conditions; they have to cope with extreme vibration, water, temperature fluctuations and dirt. The connecting wires have to cope with the demands brought by suspension and steering movements. All in all it’s not good news.

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