You dont have javascript enabled! Please enable it!

ABS

Subjects:

  • History
  • Purpose
  • Operation
  • Speed ​​sensors
  • Hydrogen unit
  • Hydraulic circuit
  • ABS control cycle
  • Control principles to avoid µ-split
  • Measurements of a vehicle with and without ABS

History:
ABS (an abbreviation for Anti-Blocking System) As early as 1961, tire manufacturer Dunlop successfully experimented with ABS on the Ferguson P99 Formula 1 racing car. That's about fourteen years before something like that was introduced on 'normal' cars. Nowadays all new cars are fitted with ABS.

Target:
The purpose of ABS is to use the maximum bond between tire and road surface while driving. In addition, the ABS also ensures that driving stability is maintained. These include:

  • Steering stability: when the ABS kicks in, the vehicle remains steerable. In the event of a slipping wheel, the vehicle will slide in one direction and the steering movements will not be able to be transferred to the road surface.
  • The course stability: with a locking wheel, the vehicle can take a different course. For example, a locking rear wheel can cause the vehicle to rotate around its axis, causing the vehicle to end up backwards on the road.

Operation:
The braking system is responsible for braking the wheels. Under no circumstances should the wheel lock, because then it will lose its grip with the road surface. The wheel then slides over the asphalt, so that no more steering movements can be transferred. In that case, the vehicle is uncontrollable. The ABS system prevents the wheel from locking.
The moment the wheel threatens to lock up, the ABS system ensures that the brake pressure (the brake fluid pressure on the wheel brake cylinders) on the wheel in question is reduced. It does not matter at that moment how hard the foot is pressed on the brake pedal. The ABS system regulates the brake pressure so that the wheel does not slip. At a certain point, the ABS system will gradually build up the pressure again, because the wheel must of course be braked as much as possible. This continues until the slip limit is reached again; then the pressure is released again. This process takes a few milliseconds. A vibration is then felt in the brake pedal. The ABS pump is often audible.

The figure below shows an overview of the ABS system components.

Two red pipes are shown in the image above. These run from the master cylinder to the hydraulic unit. Hydrogen aggregate is another word for the ABS pump. The two red lines have to do with the separate braking system; left front with right rear and right front with left rear. If, for example, there is a leakage at the left front wheel, which means that all the brake fluid has leaked away, it is still possible to brake with the other brake circuit. Orange pipes run from the hydraulic unit to all wheels. The braking force can be adjusted for each wheel in the hydraulic unit.

A speed sensor is mounted on each wheel. This makes it possible to continuously monitor the speed of all four wheels. The blue lines are signal wires connected to the speed sensor. A signal wire runs from each wheel to the control unit. The signals from the brake pedal and from the hydraulic unit are also sent to the control unit. In the car shown, this is located under the seat, in the interior of the car. Nowadays you see more and more that the control unit is attached to the hydraulic unit. It is then one whole. If there is a fault in the system, for example due to a faulty or contaminated sensor, faulty cable or a fault in the hydraulic unit, a fault light will illuminate in the instrument panel. The fault can then be read out with diagnostic equipment.

Speed ​​sensors:
The figure below shows the inductive speed sensor mounted. This is a photo of a McPherson strut from the front suspension. The gear ring can also be seen here, where the sensor measures the speed.

An ABS sensor can be designed as an inductive sensor (see image above), or as a magneto-resistive sensor (MRE sensor), ie a Hall sensor (see image on the right). The operation of this sensor is described on the page Hall sensor described. The latter sensor is used with the ABS magnet ring that is used in the wheel bearing processed.

The signals from the inductive and Hall sensors can be oscillscope are measured. Examples of these measurements are shown and described below.

Inductive speed sensor:
The inductive speed sensor consists of a permanent magnet with a coil around it. The magnetic field strength changes when a tooth of the ring of teeth (attached to the drive shaft) moves through the magnetic field of the permanent magnet. The change in the magnetic field causes a voltage to be generated in the coil. Each period in the speed signal corresponds to the passage of a tooth past the sensor. The amount of teeth present on the ring and the rotational speed of the drive shaft determine the frequency and amplitude of the signal.

Hall sensor:
Also with the magneto-resistive sensor (MRE sensor), or the Hall sensor, a metal ring with magnets moves along the sensor. The magnetic ring is located on the drive shaft or in it wheel bearing. The frequency of the square-wave voltage depends on the speed of rotation and the number of teeth of the metal ring. The amplitude (the height of the signal) remains the same. 

MRE sensors require a power supply to operate. Yet these sensors often only have two wires (and therefore two connections). The sensor sends the signal via the negative cable to the ABS control unit. The signal is formed because the electrical resistance of the semiconductor wafers changes when they are exposed to an alternating magnetic field.

The signals from the speed sensors are passed on to the ABS control unit. The signals from four wheels are compared with each other. The moment the vehicle drives through a bend, the speed of the wheels in the inner bend will be lower than that of the wheels in the outer bend. This is measured, but of course falls well within the margins.
If the speeds differ too much during braking, the ABS control unit will ensure that the hydraulic unit will reduce the braking pressure on the relevant wheel (braking too hard). With too much speed difference during acceleration, the engine power will be abruptly reduced by the engine management system.

In the event of malfunctions in the ABS system, the signals can be measured with the oscilloscope. These can be measured at the wheel, but also at the control unit. By measuring at the wheel you can check whether the ABS sensors are working properly. When measuring at the control unit, it can be ruled out whether faulty wiring is the cause of the malfunction.
During the measurement it can be checked whether the frequency and amplitude at the inductive sensor are correct. With the Hall sensor it is possible to check whether the frequency of the signal is correct while the wheel is turning. To do this, turn the wheel full rotations so that any defects on the teeth can be quickly identified. With damaged teeth, a deviation in the purity of the sensor signals will be visible (think of a frequency that is wider than intended with each rotation).

Hydrogen unit:
The image below left shows a hydraulic power pack with a built-in control unit. This can be seen from the large number of pins in the plug connection.
The connections of the pipes from the master cylinder and to the wheels are also visible here. The separate brake circuits (front left with rear right and front right with rear left) are incorporated in this pump unit.

When we take the hydraulic unit apart, the valve block can be seen. The image at the bottom right shows the inside of the hydraulic unit.

Hydraulic circuit:
The components in and around the hydraulic unit are visible in the hydraulic diagram below. To understand the operation, parts and symbols, the page basic principles of hydraulics be consulted.
The diagram below is drawn for one wheel. The numbers 5, 6 and 9 are internal. Another wheel uses the same components, except for the 2/2 valves (6), just with different connections. In other words, if the schematic of the complete car is drawn, there are still six 2/2 valves next to it, all with their own pipes. For the sake of clarity, only the schematic for one brake circuit is now shown.

Situation 1: With no braking and with stable braking:
The diagram on the right shows the situation with no and stable braking. The brake pedal (2) is depressed causing fluid pressure to be applied to the left 4/2 valve (2) by the master cylinder (6). This 2/2 valve has an open connection to the caliper (7). As the fluid pressure to the caliper increases, the brake pads will be pressed against the brake disc. There will then be braking. The speed sensor (8) registers the number of revolutions the wheel makes.

Situation 2: ABS active, hold brake pressure:
This diagram shows the situation when braking is hard and the wheel deceleration is too great. The ABS sensor at the brake has transmitted the speed signal to terminal 5 of the control unit, which is lower than from the other wheels. The control unit reacts to this and shuts down the system towards the brake caliper.
This is done as follows: a certain current is applied to pin 3 of the control unit, which energises the solenoid valve on the left 2/2 valve. The valve is pushed to the left against the spring force. As a result, the access of new brake fluid to the caliper is blocked. The right 2/2 valve remains in the same position, so no brake fluid can go to the brake or return. This keeps the pressure constant. The control unit again checks whether the speed difference between the wheel in question and the other wheels differs too much. If the mutual speed difference is minimal, or if there is no longer any speed difference because the brake pressure has been kept constant, the control unit takes the power off pin 3 again. The 2/2 valve springs back into its original position, so situation 1 applies again. If the speed difference does not change, or even increases, the braking pressure of the wheel concerned must be reduced. This happens in situation 3.

Situation 3: ABS active, reduce brake pressure:
To reduce the brake pressure, brake fluid in the line between the 2/2 valve and the caliper must be pumped out. This is done in the diagram above.
Now pin 4 is also supplied with power, so that the right 2/2 valve is energized. This too is now slid into the left-hand position, so that the passage between the brake caliper and the hydraulic pump has become clear. At this point, the pump motor will spin and pump the brake fluid from the caliper to the master cylinder. The fluid is now pumped back to the reservoir against the force of the master cylinder. The pressure is reduced and the wheel will rotate again.

Summarized:
Situation 1 applies when driving and braking lightly. During braking where the wheel threatens to lock, situation 2 and where the pressure has to be reduced due to the blocking wheel situation 3. During braking, the situation will always change. If situation 3 applies, where brake fluid is pumped away from the brake, then the wheel must then be braked again. Otherwise, the vehicle would not be able to brake strongly enough. It then switches back to situation 1, then situation 2 again and then situation 3. This happens again until the driver stops braking, or until the driver is driving on a different surface, which is, for example, more difficult (a higher coefficient of friction).

ABS control cycle:
The graph below shows the ABS control cycle. Various factors have been added to this, such as the vehicle speed (A) with the wheel speed, the wheel circumference acceleration (B), the activity of the system (C) and the brake pressure (D).
The graph is also divided into 9 time periods. In every period a change is visible because the system is adjusted. The time period is about 20 milliseconds in total and is divided into 9 unequal pieces. Below the graph is the explanation of the lines.

A: The black line is the vehicle speed, the green line is the wheel speed and the red line is the reference speed. The vehicle speed decreases (period 1), but the wheel speed decreases much faster. In doing so, the red reference line is cut. The moment the green line falls below the red line (from period 2), wheel slip can occur. The ABS will therefore intervene.

B: The line indicates the circumference acceleration. An example: by turning the wheel and slowing down, the line at B stays close to the zero line. By now turning the wheel at the same speed and braking more forcefully, the line will swing further downwards. That also happens with getting up to speed; by turning the wheel very quickly from 0 to 10km/h, the line will shoot up further if it takes you 5 seconds to turn the wheel from 0 to 10km/h. Basically, this is the wheel circumference gear.

C: This line indicates where the pressure in the system is stabilized; the ABS is then activated. Where the line at C is low (on the zero line), the ABS system is not operating. In period 7, the ABS is pulsed, so that the wheel speed does not decrease too quickly.

D: This line indicates the brake pressure. The brake pressure increases until the green line (A) of the wheel speed intersects the red reference line. The ABS kicks in (C) and ensures that the wheel circumference acceleration does not become too low. The wheel circumference acceleration is on the zero line in period 4; exactly the moment when the wheel speed in (A) goes from negative to positive. The pressure is kept constant at that time. In period 7, the pulsating control is clearly visible. The brake pressure is now carefully increased, so that the wheel will not brake too quickly.

Control principles to avoid µ-split:
The ABS can be set individually for each wheel with this information. The wheel speed sensors register the speed of each wheel. This is necessary because in all situations the maximum achievable coefficient of friction must be weighed against the steerability of the vehicle. When the vehicle is driving with the left wheels on dry asphalt and with the right wheels in the soft shoulder and braking with full braking force, the vehicle will become uncontrollable and spin on its axis. The difference in braking power between the wheels on asphalt and on ice causes a yaw moment that causes a course deviation. This situation is called the µ-split situation. The µ is pronounced as “mu”. To avoid this scenario, a number of control principles are applied:

  • The individual control (IR): here the brake pressure is set to the maximum coefficient of friction of each wheel. This allows high yaw moments to occur, but the maximum braking forces are achieved.
  • The select-low control (SL): the wheel with the lowest coefficient of friction determines the braking pressure for the other wheel. The maximum achievable braking force is not used here, but the yaw moment is low.
  • The select-high control (SH): the wheel with the highest coefficient of friction determines the braking pressure for the other wheel. The select-high scheme is only applied for ASR schemes.
  • The select-smart or modifying control: during braking, control is taken from select-low to the individual control. This allows a compromise between yaw moments and maximum braking forces. This arrangement is often applied to company cars.

Usually the braking system of a passenger car is separated diagonally (cross-left). An example of this is shown in the image below. This shows the red brake system of the left front and right rear and the blue brake system of the right front and left rear.

The brakes of the front wheels are controlled with the individual control (IR). The braking pressure of one front wheel is set to the maximum coefficient of friction of the other front wheel. During an emergency stop, the front wheels will individually search for the maximum achievable braking force.
The brakes of the rear wheels are controlled according to the select low (SL) principle. The adjusted braking pressure of the rear wheel with the least coefficient of friction determines the braking pressure of the other rear wheel. The braking torque of both rear wheels will remain the same.

Measurements of a vehicle with and without ABS:
To get a good idea of ​​the influence of the ABS system on a vehicle, this section shows two graphs of measurements that show the difference between a braking vehicle without and with ABS.

Vehicle speed relative to wheel speed without ABS:
The graph on the right shows the vehicle speed in relation to the wheel speed.
From t = 0 second, the vehicle speed is 15 meters per second. At that moment, the brake pedal is pressed to the maximum. The vehicle speed decreases linearly to 0 m/s at between
t = 2,75 and 3,00 seconds. The wheel speed drops completely to 0,5 m/s between t = 1,0 and 0 seconds. This means that the wheel already has a speed of 0 m/s, i.e. is stationary, while the vehicle is still moving. At that moment, a wheel is blocked. The wheel slips on the road surface while the vehicle is not yet stationary. In this situation, the ABS is not in operation.

Vehicle speed versus wheel speed with ABS:
In the graph on the right, the blue line is the same; at a vehicle speed of 15 m/s, the maximum braking is done to 0 m/s. This again happens in a time frame of 3 seconds. Now that the ABS is in operation, the red line at t = 0,3 seconds does not fall to 0 m/s, but the speed of the wheel increases again. This can be seen from the red line that first runs downwards and rises again just before t = 0,5 second. The braking pressure is reduced by the ABS at a speed of 7,5 m/s. The speed of the other wheels is equal to the vehicle speed and thus to the blue line. The ABS sensor of the left front wheel registers the deceleration. The ABS computer detects the difference in speed and intervenes. The brake pressure is reduced with the hydraulic unit until the blue and red lines are equal again. At that moment, the brake pressure is again kept constant. Until the vehicle comes to a stop, the ABS continues to control the speed of the spinning wheel.

The pressure in the master cylinder relative to the wheel brake cylinder without ABS:
The force exerted on the brake pedal is converted into brake pressure in the master cylinder by means of fluid displacement. This brake pressure is shown by the blue line in the graph below.
Regardless of whether the wheel slips or not, the brake pressure in the wheel brake cylinder (the red line) remains the same as the pressure in the master cylinder. So this is the situation without ABS.

The pressure in the master cylinder relative to the wheel brake cylinder with ABS:
In the situation where the ABS kicks in, the pressures in the master brake cylinder and in the wheel brake cylinder are no longer equal. The pressure in the master cylinder remains high as the driver keeps the brake pedal depressed. In the graph, the red line drops at t = 0,3 seconds; here the ABS lowers the brake pressure. The reduction in brake pressure causes the wheel to roll again. From t = 0,4 seconds, the brake pressure is increased again in steps until the speed of the wheel is equal to that of the other wheels. That is the case at t = 2,35 seconds.