Advanced Braking

Advanced Braking

Safety Statement

Information and Administration : The course will start each day at 9.00am and finish at 5:00pm each day. Lunch and refreshments will be provided at the times advised by the training instructor. Workshop Safety • It will be expected that all necessary workshop health and safety procedures are followed e.g. the wearing of suitable work wear, safety footwear, eye and ear protection when in the practical workshops is mandatory. We recommend that barrier cream and workshop gloves are used at all times. • Ensure that you are fully aware of the emergency stop procedure for any of the rotating equipment used during the course. While working on rotary test equipment or engines, please ensure that all loose clothing is secure and can not get caught in the equipment or engine parts.

Contents

Aims and Objectives

Traction Control

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Braking Fundamentals

Hill descent/ascent Control

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Brake Inspections

Electronic Stability control

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Brake Measuring

• Electronic Parking Brake Systems

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Anti lock Brake Systems

• TPMS (Tyre Pressure Monitoring System)

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Aims and Objectives

• Describe the operating principles of ABS and Electronic Parking braking systems

• Identify their components

• Describe the features of the systems

• Explain the routine maintenance requirements

• Identify common faults

Braking Fundamentals

Before we jump to the advanced topics, let’s go over our basics .

Q. What is the function of a braking system ?

A. To slow down and stop a vehicle effectively and efficiently.

Q. How does the system achieve this ?

A. Friction between the moving mass and the stationary mass of the vehicle

Keeping in mind that friction is the key component of the braking system, we will not overlook it’s importance no matter how basic or advanced. Without it, no amount of technology can replace the laws of physics.

Braking Inspection

Let’s familiarize ourselves with the fundamentals of brake system inspections.

Brake Pads

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Brake Discs

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Brake Drums & Shoes

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Hydraulic lines and Hoses

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Master Cylinders

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Brake Calipers

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Braking Inspection (Brake Pads

A visual inspection of brake pad thickness is a good indicator or their condition but should be measured in accordance with manufacturers specifications.

Brake pad wear Measured with a depth gauge.

Uneven wear Suspected seizure of brake pad to carrier.

Friction material break up Suspected seizure of caliper or carrier, creating excessive friction heat.

Braking Inspection (Brake Discs)

Brake disc thickness and Run-out should be measured in accordance with manufacturers specifications. Brake disc wear Measured with a Micrometer. Visual signs of wear usually indicated by an excessive lip on the outer edge of disc. Min thickness should be taken into all accounts regardless of a discs visual condition. This value is typically indicated on the disc itself or in automotive data systems.

Braking Inspection: (Brake Discs)

Brake disc run-out A shudder in the brake pedal and in some cases the steering wheel at speed, under braking is a typical symptom of excessive run out.

There can be several reasons of this fault including: Seized brake caliper / carrier, excessive heat-build up.

To measure this, we us a DTI gauge, fixed to a stationary part of the vehicle, then applying a set gauge to the working face of the disc. By rotating the disc, we can measure any variance in the discs plane.

Manufacturer's specifications apply to this measurement.

Anti-Lock Brake System (ABS)

Anti-Lock Brake System (ABS)

How do brakes work?

Part of the first rule of thermodynamics or the conservation of energy, energy cannot be destroyed only transformed into something else. It takes fossil fuel to drive our vehicles forward when we apply the brakes pressure aids friction heat is the result.

Anti-Lock Brake System (ABS)

What is ABS?

Braking is achieved through friction being generated at two points:

1. Friction between the brake linings and the brake drums/discs. 2. Friction that exists between the tyre and the road.

Braking can be controlled in a stable manner as long as the friction created between the tyre and the road surface is greater than that between the brake linings and brake drums/discs. If the opposite is true, then the wheels will lock up. When lock up occurs, the friction between the road and the contact patch will change in nature – it becomes dynamic friction (moving) rather than static friction. i.e. the contact patch is being dragged across the road surface rather than laid onto it.

Anti-Lock Brake System (ABS)

Think about this,

If a vehicle is doing 60mph (brakes not applied), how fast are the contact patches going? 0 mph – they are stationary in relation to the road and the road is not moving at all: static friction.

If the same vehicle now locks its brakes, how fast are the contact patches going? The same speed as the vehicle: dynamic friction. Dynamic friction generates much less grip than static friction so stopping distances increase significantly and directional control is lost if the steered wheels lock up.

Anti-Lock Brake System (ABS)

Slip Ratio

Slip ratio is a means of calculating and expressing the locking status of a wheel and is vital to the effectiveness of any anti-lock braking system. When a vehicle is being driven along a road in a straight line its wheels rotate at virtually identical speeds. The vehicle’s body also travels along the road at this same speed. When the driver applies the brakes in order to slow the vehicle, the speed of the wheels becomes slightly slower than the speed of the body, which is traveling along under its own inertia. This difference in speed is expressed as a percentage and is called ‘slip ratio’.

The ideal slip ratio for maximum deceleration is 10 to 30%.

Slip ratio is calculated as follows:-

Anti-Lock Brake System (ABS)

Slip Ratio % = Vehicle Speed – Wheel Speed x 100 Vehicle Speed

Slip Ratio %= 60mph- 0 (wheel Locked) x100 60 mph

100% slip ratio

ABS Components

The ECU controls the entire system. It monitors wheel speed and determines wheel lock up. It uses control signals to influence the hydraulic actuator to reduce, hold or increase the brake fluid pressure. It carries out a self-check of- the system at start up and informs the driver of any abnormalities via the dashboard ABS warning light. Wheel speed sensors, These enable the ECU to detect individual wheel speed and also calculate vehicle speed. Sensor rotor, Attached to the hub or drive shaft, it has teeth that when passed in front of the ABS wheel speed sensors cause a signal to be generated.

ABS Components

ABS actuator/modulator, This controls the hydraulic brake fluid pressure to the individual brakes dependent upon control signals generated by the ABS ECU. Control relays, Usually two relays are required to facilitate electrical control of the ABS. One relay is the actuator pump relay and the other is for the actuator solenoids. They can be located on the actuator itself or an adjacent fuse/ relay block.

ABS Components

Limitations of ABS,

It should be noted that when a vehicle is driven on slippery or snowy roads, it might actually have a longer stopping distance than one that is not equipped with ABS.

This is due to the fact that a vehicle without ABS locks its wheels and therefore creates a ‘snow plough effect’ i.e.

Snow builds up in front of the locked tyre slowing it down, which cannot happen on an ABS equipped vehicle.

It should also be noted that no matter how advanced such systems become, the laws of physics still apply! If there is no grip available, the ABS cannot create it….

Operating Principals of ABS

The stop light switch provides a signal that the ECU can use to determine that the brakes are being applied.

The ECU, through the monitoring of the wheel speed sensor signals, calculates any sudden reduction in wheel speed. The ECU will now control the hydraulic brake actuator to provide optimum brake fluid pressure to each brake to achieve maximum deceleration conditions. The hydraulic brake actuators operates on control signals from the ECU to ‘reduce’, ‘hold’ or ‘increase’ brake fluid pressure as necessary in order to achieve and maintain an ideal slip ratio of 10% to 30% and avoid wheel lock up. These changes of braking state can be effected at a frequency of up to 60 times per second.

Operating Principals of ABS Electrical control variations In addition, the electrical control of the ABS varies from vehicle to vehicle:

individual control of the front wheels whilst controlling the rears together. This is known as ‘three channel control’. Individual control for all four wheels is known as ‘four channel control’

Operating Principals of ABS

Most ABS sensors detect wheel speed using an electrical process Known as “inductance”. These inductive sensors are effectively small electrical generators, operating in a similar fashion to an alternator.

A voltage is generated in a wire when a magnetic field is moved or changed in strength whilst in close proximity to the wire. The sensor consists of a sealed casing containing a permanent magnet and a coil of wire, the ends of which (output signal) are connected to the ABS ECU.

Advantages / Disadvantages

The advantage of passive induction speed sensors is the simple design of the components.

The disadvantage is their reliance on a precise gap dimension between the sender rotor and sensor. In addition, the passive induction speed sensors are heavier and require more installation space.

Reference system

Permanent magnet

( sender wheel

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Good Waveform

Faulty Waveform

The most likely cause of this is a broken tooth on the ABS reluctor wheel. Here's what was found:

Magneto-Resistive Sensors

The newer active sensor uses a digital signal created by the ABS controller. This type of sensor uses a hall effect or a variable reluctance signal with a square wave pattern. The sensor consists of two wires; one is used as the positive (DC) voltage from the controller and the other lead as the return (Ground) to it. The advantages of the active speed sensor are their ability to read more accurately at slow speeds than the passive speed sensors. It doesn’t have to self-generate the needed voltage by the spinning action of the wheel. Also, since these sensors use a DC voltage they can detect not only the speed of the wheel but the direction of travel. This allows the controller the ability to calculate not only wheel speed but can also be used for the hill holding and hill descent control features found on some vehicles.

What is the Correct Output?

The sensor produces a square wave output when the wheel is rotating. However, it is not switched to ground or to battery voltage as you would expect.

What you will see using a scope or multimeter is the voltage drop across a single resistor or a pair of resistors in parallel, depending on the position of the reluctor/tone ring, just by moving the wheel a small amount you should be able to detect this change. Try that with a passive sensor. The output should be around 1.2V and 0.6V or, if you are using a micro amps clamp, in the region of 14mA and 7mA. Only the frequency should change as the wheel accelerates and decelerates – unlike analogue AC sensors where the amplitude and frequency of the signal changes proportional to wheel speed. It is also possible to detect physical faults with the Reluctor/tone wheel using an oscilloscope.

What is the Correct Output

The output should be a square wave form the difference between the two voltages (Delta Voltage) should be approximately 0.6V or if you are using a micro amps clamp in the region of 14mA and 7mA. Only the frequency should change as the wheel accelerates and decelerates. Unlike analogue AC sensors where the amplitude and frequency of the signal changes proportional to wheel speed.

What is the Correct Output?

The active speed sensor or “Magneto Resistive Sensor” consists of two parallel resistors and a magnetic material located at a precise distance from a permanent magnet.

The resistors are about 1.4k Ohms each, however if you were to measure the resistance at the wire ends you would probably see 5 to 6 Mega Ohms. (That’s because you are reading the resistance values of the not only the resistors but the magnetic material inside the sensor.) These parallel resistors work together to create the voltage changes you’ll see on your scope as the tone wheel passes by the internal magnet. The tone wheel (if flattened out) looks like the square wave pattern it creates.

As the tone wheels high part of its tooth is near the sensor a higher voltage is created while the opposite is done when the lower part of the tone wheel is near the sensor.

Hall-Effect Sensor

Wheel Speed Sensor

Active wheel speed sensors have been in use since 1999 on models using the Teves Mark 20e system.

The Teves Mark 25, Teves Mark 25e, TRW EBC-340, some EBC-125 and the late model Bosch systems use this style sensor as well. This style sensor helps increase performance, durability and low speed accuracy. In other words, these sensors do not seem to have the false cycle problem like the passive sensors do.

Most vehicles with active sensors still use a toothed tone wheel which acts as the trigger mechanism for the sensor.

These have north/south pole magnets imbedded into the ring. The ring is then pressed on the axle shaft just like a tone wheel. In either case the result is a digital square wave signal.

Wheel Speed Sensor

It is important to note two things

1) If there is no battery voltage to this sensor, then the sensor will not work at all.

2) The signal return circuit is where the sensor can be tested. The normal voltage level, as the wheel is rotated, switches between approximately 0.8 volts and 1.6 volts. If using a scope for testing, this will appear as a square wave signal.

Wheel Speed Sensor

Teves Mark 25e.

This system uses the active wheel speed sensors, but testing them is different; it is tested on the power supply wire. To make things more complicated, the voltage only changes 0.2 volts as the wheel is rotated! A normal voltage switch on this system is approximately 12 volts to 12.2 volts. Testing this system can also be accomplished using a scope. Instead of putting the ground lead on ground, put it on battery voltage. Now, using the channel one lead of the scope, watch the signal on the sensor power supply wire. Set the scope to a low voltage range such as 0.5 volts per division.

As the wheel is slowly rotated, a square-wave signal can be seen. The voltage level is still only changing 0.2 volts, but with the scope set up like this, the signal is easier to see.

Wheel Speed Sensor

Even though it is close to battery voltage at the sensor (10.6v is what I’ve found) you won’t be able to use a test light to read it.

A test light consists of a ground lead, a bulb (resistance), and the positive end, which is basically the same thing the sensor has internally minus the magnets.

If you try stabbing a test light on the positive lead to the sensor the computer sees that as a possible short to ground. Meaning, it thinks your test light is the sensor, because the return voltage on the ground side of the sensor has drastically changed. In fact, in most of these ABS systems if you do “short” the lead the processor shuts that sensor off (0.0v) until the next key cycle. This way it avoids any harm coming to the internal circuits of the processor. But if you continued to check the circuit without turning off the key, you’d end up back at the processor and more than likely conclude the processor is bad. It’s not… just cycle the key and recheck it.

Wheel Speed Sensor

Wheel Speed Sensor

at low speed

Advantage/disadvantage

Active speed sensors supply a constantly accurate measurement result throughout the entire measurement range, because the signal strength is not dependent on the speed but is specified via defined currents.

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Low frequency

at high speed

The disadvantage is the difficulty of checking/ testing?

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High frequency

Two-Position Actuator / Modulator

The two-position solenoid systems are less sensitive to current levels. They also provide some benefits when used for traction control and other functions. The actuator assembly consists mainly of a series of electrically operated solenoid valves to control the hydraulic pressure within the brake lines. Most of these systems have 4 channels, one for each wheel with a pair of valves for each circuit. Two pumps are used, each one influencing a pair of hydraulic circuits (note that the pumps are integrated with the actuator assembly).

1. Reservoir 2. Brake servo 3. Brake pedal sensor system 4. Brake pressure sender 5. ABS/ESP control unit 6. Return flow pump 7. Pressure accumulator 8. Damping chamber 13. Front right ABS inlet valve 14. Front right ABS outlet valve 15. Rear left ABS inlet Valve 16. Rear left ABS outlet valve 17. Front left wheel brake cylinder 18. Front left speed sensor 19. Front right wheel brake cylinder 20 - Front right speed sensor

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Pressure Increase Phase

These systems have 4 channels, one for each wheel with a pair of valves for each circuit. Two pumps are used, each one influencing a pair of hydraulic circuits (note that as before the pumps are integrated with the actuator assembly).

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Each hydraulic circuit has an inlet (A) and an outlet valve (B) to control fluid flow.

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The valves are switched by the ECU based on signals from the wheel speed sensor for that hydraulic circuit.

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During normal braking, valve A is open allowing fluid to flow to the brake under pressure. Outlet valve B is closed.

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Pressure Hold Phase

If the wheel slows down too much, to the point where it might skid, the ECU supplies current to the inlet solenoid valve that closes the port from the master cylinder. As the outlet valve is still closed, pressure is trapped in the hydraulic line to the brake cylinder and the brake remains on but the driver’s input has no more influence.

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The aim is to prevent the wheel slowing any more.

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Pressure Release Phase

If the wheel continues to slow down more than it should for any particular road speed, it will eventually reach the stage where it skids. As before, changes in road surface, brake efficiency due to temperature rise or warped discs etc. can all cause the wheel to skid in an instant. Further intervention will be necessary to prevent skidding and loss of steering control. If the wheel gets close to skidding, the outlet valve will be opened by a signal from the ECU.

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Fluid will return from the hydraulic brake line to the reservoir and the pressure at the brake will drop, allowing the wheel to speed up again.

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As before, when the wheel reaches a certain speed and if the brake is still being applied, the system will repeat the pressure increase, hold and release phases until either the brake is released, or the vehicle comes to a halt. During this process, the pump will direct fluid back to the master cylinder, causing pulsing at the pedal.

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When the vehicle brakes are applied, the Centre of gravity of the vehicle shifts in the direction of travel. With less weight over the rear wheels, the available grip is reduced and the risk of locking these wheels is increased, reducing the braking effort applied between the tyres and road surface. The front tyres will have some extra grip due to the weight transfer and as long as the driver maintains or increases braking effort, the vehicle’s deceleration will be quite acceptable in most situations. The most dangerous effect of locked rear wheels is the loss of directional stability and a dangerous spin is very likely. For many years, motor vehicle manufacturers have employed various types of hydraulic valve to control brake balance between the front and rear pairs of wheels in order to prevent rear wheel lock up under heavy braking conditions. These valves are generally referred to as proportioning valves. Now that manufacturers are routinely equipping their models with ABS, the proportioning valve is effectively obsolete as this system can do the same job of controlling rear brake line pressure to prevent lock up during weight transfer. Electronic Brake Force Distribution (EBD)

However, for many years the proportioning valve was retained on ABS equipped vehicles as it proved difficult to control the rear brakes in an acceptably smooth way (due to low ABS ECU processing power and manufacturing limitations,

ABS intervention during non-emergency braking resulted in brake pedal feedback and system noise).

These issues have now been overcome and this mode of ABS operation is known as Electronic Brake Force Distribution (EBD).

EBD utilises the functions and components already used in ABS, although the software controlling the valves is modified to control the pressure increase and hold phases only, at lower braking efforts and less slip than those used in emergency braking. When braking in a straight line, the ECU compares the individual speed of each wheel and the average speed of each pair. The front to rear average speeds are compared and if the rears are showing signs of locking, the pressure hold (inlet) valves for both these wheels will close to prevent more pressure being applied. More pressure can still be applied to the front wheels if necessary. If the vehicle is fully laden, more pressure can be applied to the rear pair before intervention occurs.

Note that this load-sensing ability is automatic as the wheel speed sensors simply report on the slipping condition of the wheels, which is in proportion to the weight acting on them. Before the advent of EBD, specially designed load-sensing proportioning valves were needed for vehicles that experience a wide range of load conditions.

EBD Operation

If one or both rear wheels still show signs of skidding, the ABS will take over, releasing the pressure until the wheel speeds up.

When cornering and braking at the same time, the outside rear wheel has a big effect on directional stability so it is vital that this wheel doesn’t lock. The inside wheel will tend to lock early, and the ECU will notice a large difference between left and right wheel speeds. The ECU will regulate the pressure applied to both wheels based on the speed of the slower inside wheel to prevent too much braking causing the important outer wheel to slip.

Traction Control

Traction control has become increasingly important for various reasons, not least because cars have become more powerful.

In front-wheel-drive vehicles the weight transfer during start-off and sudden acceleration can promote a severe loss of grip (traction) between the front tyres and the road. In rear wheel-Drive cars the risk of yaw leading to a spin as the vehicle accelerates is very real, Particularly if the vehicle is being steered at the same time, at junctions for example. In the latter example limited-slip-differentials can be a great help but these devices can cause sudden unwanted steering responses in some front-wheel-drive vehicles and tyre wear can be an issue.

For safety reasons, traction control systems, whilst criticised by some “expert” drivers, have become a very necessary addition to a vehicle’s specification for most of us.

Traction Control

The TRC system uses exactly the same components as ABS and EBD, with the addition of extra valves for each hydraulic circuit.

The key difference between ABS and TRC is down to the pedal operated brake switch.

If the ECU detects significant differences in wheel speeds it checks to see if the brake switch is being operated at the same time.

If it is, then ABS is required – if not then traction control intervention is necessary and the ECU acts appropriately.

Some systems double-check by assessing the throttle position as well.

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Reservoir

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Brake servo

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Brake pedal sensors Brake pressure sender ABS/ESP control unit

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Return flow pump

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Pressure accumulator Damping chamber Front left ABS inlet valve Front left ABS outlet valve Rear right ABS inlet valve Rear right ABS outlet valve Front right ABS inlet valve

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Front right ABS outlet valve Rear left ABS inlet valve Rear left ABS outlet valve Front left wheel brake cylinder

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Front left speed sensor

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Front right wheel brake cylinder

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Front right speed sensor

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Rear left wheel brake cylinder

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Rear left speed sensor

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Rear right wheel brake cylinder

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Rear right speed sensor

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Switch valve

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High-pressure valve

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CAN data bus

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Control via the throttle valve

Speed sensor, front left

Nominal engine torque

Regulation via the injectors

Speed sensor, front right

Actual engine torque

Regulation via the ignition system

Speed sensor, rear left

Engine control unit

ABS/TCS control unit

Gearbox management

Traction Control System Operation

If one of the driven wheels starts to spin during acceleration the ABS/TRC ECU can initiate several actions:

• Retard ignition timing (to reduce power) • Close throttle valve (to reduce power) • Cut injectors (to reduce power) • Brake the spinning wheel

In early systems, braking the spinning wheel was the only option but unfortunately this system did nothing to prevent the driver applying even more power in an attempt to increase acceleration and subsequently brake pad wear was very high.

Traction Control System Operation

The ABS/TRC ECU is in constant communication with the engine management ECU, often via a CAN system on later vehicles. When wheel spin is detected, the intake airflow can be reduced to reduce power. Some vehicles use an additional, electrically controlled throttle in the throttle housing, although where “Drive-by-wire” systems are fitted the throttle motor can be controlled by the engine management ECU, overriding the driver’s input. In both cases the throttle can be progressively closed, reducing power. Some systems control the ignition timing or cut individual injectors for more rapid response and variable valve timing systems can also be affected.

TRC braking is still employed but the proportion of braking vs. power reduction varies according to vehicle speed and acceleration.

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When a wheel spins under acceleration the ABS/TRC ECU decides if brake intervention is required. If it is, the pump in the ABS actuator will run to supply pressurised fluid to the brake of the spinning wheel via the inlet solenoid valve (remember the driver is not applying the brake so brake pressure has to be generated by other means). fluid being pumped back to the brake master cylinder at this time, solenoid valve A is closed (see diagram). Solenoid B is opened to supply hydraulic fluid to the pump as necessary. The braked wheel should now slow down and more power will be supplied to the opposite wheel (via the differential) as long as power is still applied by the driver/ECU. As the wheel slows, the outlet valve allows fluid to be bled back to the reservoir/pump. The cycle is repeated until either power is reduced enough to prevent spin or the wheels start to turn at very similar speeds (equal grip).

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Electronic Stability Control (ESC)

ABS/ESP control unit

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Hydraulic unit

Brake light switch

Steering angle sender

Return flow pump

Speed sensor

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Sensor cluster

Data bus connection

Actuators

Sensors

Electronic Stability Control (ESC)

Nowadays, ESP means "electronic stabilisation programme". When the system was introduced, ESP stood for "electronic stability programme". With the aid of its sensors, the electronic stabilisation programme ESP recognises at an early stage that a critical driving situation is emerging. By specifically braking individual wheels and the possibility of intervention in the engine and gearbox management system, ESP then independently acts to counter this situation in such a way that vehicle stability and steer ability are maintained.

Control via the throttle valve

Speed sensor, front left

Nominal engine torque

Regulation via the injectors

Speed sensor, front right

Actual engine torque

Regulation via the ignition system

Speed sensor , rear left

Engine control unit

ABS/TCS control unit

Gearbox management

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Reservoir

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Brake servo

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Brake pedal sensors Brake pressure sender ABS/ESP control unit

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Return flow pump

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Pressure accumulator Damping chamber Front left ABS inlet valve Front left ABS outlet valve Rear right ABS inlet valve Rear right ABS outlet valve Front right ABS inlet valve Front right ABS outlet valve Rear left ABS inlet valve Rear left ABS outlet valve Front left wheel brake cylinder

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Electronic Stability Control (ESC)

Electronic Stability Program (ESP) - Audi, VW, Saab, Daimler, Chrysler Vehicle Stability Control (VSC) - Toyota, Lexus Vehicle Stability Assist (VSA) - Honda, Acura Advance Trac - Ford, Lincoln StabiliTrak - General Motors, Saturn

Vehicle Dynamic(s) Control (VDC) - Subaru, Nissan Dynamic Stability Control (DSC) - Mazda, BMW, Rover Dynamic Stability and Traction Control (DSTC) - Volvo Components of an ESC system

Anti-lock Braking System (ABS) Traction Control System (TCS or ASR) Yaw-rate control Roll-prevention System

How Electronic Stability Control Works

Let’s first mention what ESC doesn’t do. It doesn’t suspend the laws of physics.

It doesn’t help the driver make turns at higher speeds; better suspension, steering and of course wider/stickier tires all contribute to that goal.

ESC doesn’t help a vehicle stop in a shorter distance any more than ABS does.

What ESC does do is allow a minor amount of poor judgment while cornering to be turned into a minor amount of forgiveness by reducing engine power and applying the correct wheel(s) brake(s) when a skid is detected.

It all boils down to vehicle speed and the coefficient of friction between the road surface and the tyres.

Steering angle sender Engine control unit Brake light switch and brake pedal switch Brake pressure sender

Driver command Direction of travel Desired speed Desired deceleration

Nominal vehicle behaviour

ESP intervention when the vehicle does not behave as commanded by the driver

Vehicle behaviour

Brake pressure sender Speed sensor Longitudinal / transverse acceleration sender Yaw rate sender

Acceleration Deceleration

Yaw rate (yawing moment) Longitudinal and transverse acceleration

Actual vehicle behaviour

Gearbox control unit

How Electronic Stability Control Works

ESP has various options for stabilising the vehicle:

Via specific brake intervention

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• Via intervention in the engine management system and additionally

• Via intervention in the gearbox management system (in vehicles with automatic gearboxes) and into the four-wheel drive control system

By evaluating the input signals and comparing the vehicle's actual/nominal behaviour, the ABS/ESP control unit recognises an unstable driving situation. In certain situations, it is necessary for ESP to intervene in the engine management system. If, for example, a driver wishes to accelerate in an unstable driving situation, this is prevented via ESP's intervention in the engine management system.

What Triggers and ESC Intervention?

A criterion for ESP intervention exists when the yaw rate sensor senses an oversteering or understeering tendency of at least 4 ° /s (threshold depends on speed). If the plausibility analysis shows the same situation, action is taken to stabilize the driving condition. As the single-track vehicle model used for the calculations is only valid for a vehicle moving forward, ESP intervention never takes place during backup. In vehicles equipped with an automatic transmission, backup is recognised via the CAN message of the transmission control unit. In vehicles equipped with a manual transmission, the electronic control unit is required to recognize backup by means of calculations based on the sensor signals

What Triggers and ESC Intervention?

Once the beginning of a loss of stability is detected, ESC systems respond with reduced engine torque to lower vehicle speed in the hope of regaining the coefficient of friction with the tyres so the vehicle can make the turn safely without further steering control issues. Torque management has been around for years with ABS/TCS systems and works by either retarding ignition timing, upshifting the transmission, cutting several fuel injectors or physically reducing throttle angle. The latter has become an easy task these days with current throttle-by-wire systems. It literally becomes the process of an engineer writing some lines of code in a software calibration.

Often the driver will notice engine power reduction when applying too much steering angle while driving too fast for road conditions.

Essentially, what ESC systems do to combat steering and/or stability control problems is first detect an oversteer or understeer condition via the system’s lateral acceleration and yaw sensors, which measure sideways movement or spinning on an axis. Also factored in is the driver’s input to the steering wheel via a steering angle sensor. If a vehicle starts to oversteer in a turn and the rear end starts to come around (spin out), WSS variations between the left and right front wheels increase beyond the norm. If the vehicle understeers (loses front traction and moves wide in a turn), the difference in speed between the left and right front wheels decreases from the norm. The ABS/TCS electronic control module monitors actual yaw and compares it to a number in software called desired yaw. The difference is called yaw error. Lateral acceleration is factored into the ABS/TCS module with yaw error and combined with the delta (rate of change) of the steering wheel position sensor

In The Event Of Oversteering

Braking intervention takes place at the wheels on the outside of the bend. Most of the braking force is introduced via the front wheel, which is caused to slip up to 50% so that the centrifugal force contributes to stabilizing the vehicle. In this case, the ABS logic is blotted out by ESP for the wheels with ESP intervention.

In The Event Of Understeering

Braking intervention takes place at the wheels on the inside of the bend. In this case, the greater force is introduced via the rear wheel so that the lateral force is selectively reduced in exact doses to stabilize the vehicle. The ABS logic is again blotted out by ESP for the wheels with ESP intervention

TCS Overriding ESC

(at the driven axle only) Here, ESP intervention is overridden by the TCS logic, which means that the lesser of two requested brake pressures is adjusted at the wheel. However, unlike pure TCS intervention this type of intervention uses the ESP pressure modulator.

The reason for this is that the hydraulic system is ‘detuned’ by the precharging pump, which is always active during an ESP intervention.

ESP Overriding ABS

If an ABS intervention appears to be necessary while an EPS control cycle is in progress, the ABS logic is overridden by ESP. As the ESP system causes a wheel to slip up to 50% in order to stabilize the vehicle, the resulting wheel sensor signals would confuse the ABS logic (outside the ABS working range).

Engine torque under the influence of both ESP and TCS

If both ESP and TCS attempt to reduce the engine torque, the higher reduction takes precedence.

Engine torque under the influence of both ESP and EDC

If an EDC (Engine Drag Control) intervention becomes necessary while an ESP control cycle is in progress – releasing the gas pedal causes the slip at the driven wheels to be increased by engine drag – the EDC logic intervenes and increases the engine speed

Yaw rate sensor

Application Detect the yawing motion of the vehicle, triggering an ESP control intervention if the yaw velocity reaches roundabout 4 ° /s (= full circle in 90 s) Design and function The yaw rate sensor relies on the action of microscopic tuning forks. The plane in which these forks vibrate shifts when the car turns about its vertical axis. This shift is evaluated electronically.

Failsafe A faulty yaw rate sensor produces an output signal of 0 V.

Yaw Rate Sensor

The first step was to clear the code and go for a test drive with the scanner. It is one thing look at a scan tool for engine data when cruising or accelerating. It is very difficult and even dangerous to drive the vehicle and look at the readings from an accelerometer. Chances are you will see only minor movement in the sensor data. Lateral + longitudinal accelerometers and yaw sensors measure braking and acceleration force in G-Force. they are mounted in the centre of the vehicle at the centre of gravity. This is typically in the centre console

Yaw Rate Sensor

To test accelerometers you have the best “known value” on the planet, it is the planet’s constant 1-g of gravity that is keeping you from floating into space. To measure the performance of an accelerometer is to rotate it 90 degrees. When a lateral accelerometer is rotated 90º, it should read 1-G. When a longitudinal sensor is rotated on its end, it will read 1-G. This is because gravity is equal to 1-G. Since it is impossible to rotate a car 90 degrees on a lift (Don’t even try), pull the console and unbolt the sensor.

Most accelerometers are wired with three wires. These include a 5-volt power, ground and signal wires. The signal wire will vary the voltage from 0-5-volts depending on G-force

Steering Angle Sensor

Pressure Sensors

Application 1. Sense the driver’s braking intentions (braking while an ESP intervention is in progress) 2. Control the precharging pressure Design The sensor consists of two ceramic disks, one of which is stationary and the other movable. The distance between these disks changes when pressure is applied. Function • The pressure sensors operate on the principle of changing capacitance. • The distance (s) between the disks and, thus, the capacitance changes when pressure is applied to the movable disk by a braking intervention. • The characteristic of the sensor is linearized. • The fluid displacement of the sensor is negligible. • Max. measurable pressure: 170 bar

Electronic ESP control unit (integrated in ESP unit) The electronic control unit performs the following functions: 1. controlling the ESP, ABS, TCS, EBD and EDC functions 2. continuous monitoring of all electrical components 3. diagnostic help during servicing in the workshop

Applications of the ESP control unit The signals produced by the sensors are evaluated in the electronic control unit. From the information received, the control unit must first compute the following variables:

1. yaw rate 2. longitudinal acceleration 3. lateral acceleration 4. pressure in hydraulic system 5. wheel speed 6. reference speed 7. deceleration 8. slip

"active" booster is a non "conventional" booster where a solenoid is used to open the booster air valve to automatically push the master cylinder forward to perform some forms of dynamic stability control

Hill Descent Assist

S374_240

The hill descent assist system, also called Hill Descent Control HDC, supports the driver on hilly roads.

On descending a hill, the gradient which, in accordance with the force parallelogram, results from the weight pressure, also acts on a mass on an inclined plane.

Hill Descent Assist

If the mass has a separate drive power, which acts downslope, the gradient is added to this drive power.

The acceleration of this mass, which results from the sum of both forces, therefore increases constantly.

The result is that the longer a vehicle drives downslope, the faster it becomes in this situation.

Gradient greater than 20% A vehicle with hill descent assist system relieves the driver of this manual intervention and ensures that the desired speed is also maintained whilst descending the hill. How it works The hill descent assist system intervenes when the following conditions are met:

Speed less than 20 km/h

• •

Engine running

• Accelerator and brake pedal not actuated

If the trigger conditions are met and the hill descent assist system ascertains, based on the signals from the accelerator, the engine speed and the speed sensors, that the vehicle speed is increasing, the assist system assumes that the vehicle is driving downhill and that brake intervention is necessary. The system operates at a speed which is slightly higher than walking speed. The vehicle speed, which is to be maintained via brake intervention by the hill descent assist system at all four wheels, depends on the speed at which the gradient is approached and the engaged gear.

Hill Start Assist

If a vehicle stops on a hill, the vehicle's weight pressure does not act on a horizontal surface, but on an inclined plane. In accordance with the force parallelogram, the weight pressure results in a gradient, which allows the vehicle to roll downhill when the brake is released. If the vehicle starts off again uphill, the gradient first has to be overcome. If the driver accelerates too little or releases the brake pedal or the hand brake too early, the drive power is not sufficient to overcome the grade resistance. The vehicle rolls backwards on starting off. The hill start assist system, also referred to as Hill Hold Control HHC, is available to relieve the driver in this situation. The hill start assist system is based on the ESP system. The ESP sensor unit G419 is supplemented by a longitudinal acceleration sensor, which informs the system of the vehicle's position.

The hill start assist system is activated under the following conditions: • The engine is running. (Information from engine control unit) • Actuation of the foot-operated parking brake

The hill start assist system facilitates starting off on a hill without the need to use the hand brake for assistance. To do this, the function delays brake pressure build-up at the wheel brake cylinders on starting off. This prevents the vehicle from rolling backwards before sufficient drive power is available for starting off on the hill.

Phase 2

Phase 3

Phase 4

Phase 1

the slope.

The hill start assist system's function can be described in four phases.

1. Build Pressure 2. Maintain Pressure 3. Relieve Pressure 4. Reduce Pressure

The switch valve is opened gradually. Due to the open inlet valve, pressure can be reduced at the wheel brake.

The driver actuates the system by pressing the brake pedal

The vehicle is stationary. The driver removes his foot from the brake in order to actuate the accelerator

Auto-Hold

Auto-Hold

AUTO HOLD is also a pure extension of the ESP regulation system's software, and requires the vehicle to be equipped with ESP and an electromechanical parking brake. The following prerequisites must be met for the AUTO HOLD function to be activated:

● The driver door must be closed.

● The driver's seat belt buckle must be fastened.

The engine must be started.

●

● AUTO HOLD must be activated by pressing the AUTO HOLD button. Activation is indicated by the warning lamp in the button lighting up. If one of the prerequisites changes, AUTO HOLD shuts off. Each time the ignition is started again, it must be re-activated with the AUTO HOLD button.

Auto-Hold

How it works AUTO HOLD is switched on.

Based on the wheel speed signals and the signal from the brake light switch, AUTO HOLD recognises that the vehicle is stationary and that the driver is actuating the brake pedal. This brake pressure is then "frozen" by blocking the valves in the hydraulic unit, and the driver no longer has to actuate the brake pedal.

When the AUTO HOLD function is activated, the vehicle must therefore always be initially held via the four hydraulic wheel brakes when stationary.

If the driver does not actuate the brake pedal and the vehicle begins rolling again after standstill has been detected, the ESP system is activated. Hydraulic operation is carried out. This means that the brake pressure at the wheel brake cylinders is actively built up until the vehicle is stationary again.

Auto-Hold

The pressure required for this is calculated and set by the ABS/ESP control unit depending on the gradient. To achieve this, the function actuates the return flow pump and opens the high-pressure valves and the ABS inlet valves; the ABS outlet valves and switch valves are or remain closed.

If the driver actuates the brake pedal in order to start off again, the ABS outlet valves are opened and the return flow pump reduces the brake pressure, via the open switch valves, in the direction of the reservoir. The vehicle's inclination uphill or downhill is taken into consideration in this case in order to prevent it from rolling backwards.

Vehicle using the ESP function Vehicles using the electromechanical brakes

J104

J104

E540

E540

3minutes

J54 0

J54 0

V28 2

V28 2

V28 3

V28 3

After holding the vehicle for three minutes, a switch from the ESP hydraulic system to the electromechanical parking brake takes place.

The calculated holding torque is transferred from the ABS control unit to the electromechanical parking brake control unit.

The two parking brake motors on the rear wheel brakes are actuated by the electromechanical parking brake control unit

The brake is held electromechanically. The hydraulic brake pressure is automatically withdrawn. To do this, the ABS outlet valves are opened again and the return flow pump reduces the brake pressure, via the open switch valves, in the direction of the reservoir.

The hydraulic unit's valves are therefore protected against overheating.

Electronic Parking Brake

Electronic Parking Brake

Electric motor on brake caliper (EPB-M)

Electric motor with hand brake cable (EPB-C)

Conventional caliper

Brake caliper with actuator

1

1

2

3

3

1. Electronic Control Unit

1. Electronic Control Unit 2 . Electric motor (Actuator)2x 3. Brake caliper

2

2. Electric motor 1x 3. Brake caliper