8. Other system and brake theories

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Hugh Boone
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1 8. Other system and brake theories Objective To understand the limiting valve, proportioning valve, load sensing proportioning valve and brake theories, which were used immediately before the development of ABS system. Main contents 1. Load sensing proportioning valve 2. Matters about the vehicle stopping process. 3. Fade 4. Vapor lock 8.1. Load Sensing Proportioning Valve (1) Overall In this system, starting point of hydraulic control is decided by rear wheel, namely by the weight. It is installed between master cylinder and rear wheel cylinder. Brake hydraulic pressure of rear wheel cylinder is controlled after sensing vehicle weight when brake is applied. Then, braking force distribution between front and rear wheels can be obtained. (2) Operation Valve piston pushes up the piston with the hydraulic pressure applied on master cylinder A plus spring tension. Then, reaction force of hydraulic pressure generates on wheel cylinder B. When the brake pedal is pressed, spring tension lifts the ball to open the path so that hydraulic pressure is transferred to wheel cylinder. Then, hydraulic pressure of master cylinder increases to a specified pressure and force on B exceeds the force on A. Now, piston moves down and shut the path to reduce the pressure. Spring To wheel cylinder From master cylinder If master cylinder pressure increases further when the path is closed, increased hydraulic pressure is applied on A. Then, piston moves up and ball opens the path so that hydraulic pressure moves to wheel cylinder. Since master cylinder pressure applies on the wheel cylinder and B together when the path is open, piston moves down and closes the path to repeat the pressure reduction. Rod [Figure 8-9. Brake operation] 51

2 To wheel cylinder From master cylinder [Figure Operation of load sensing proportioning valve] 8.2 Matters about vehicle stopping process (1) Reaction time It refers to the critical time for the driver to take in applying brake from sensing the dangerous situation or signal from eye or ear until taking an action against the danger. In general, they say reaction time is about 0.4~0.5 seconds. (2) Foot changing time It refers to the time for the driver to take in changing the foot from accel pedal to brake pedal. In general, foot-changing time is about 0.2~0.3 seconds though it is subject to pedal position. (3) Pedal pressing time It refers the time for the driver to take until hydraulic pressure in the brake circuit start increasing after foot moved to the brake pedal. In general, pedal-pressing time is about 0.1~0.2 seconds though it is subject to pedal clearance and the gap between brake shoe and drum. (4) Transitional brake and main brake Once hydraulic pressure in the brake circuit increases and braking force generates, deceleration is accompanied by. It takes a certain time for the hydraulic pressure reaches to maximum value and this transitional time is called transitional brake. Main brake is the interval from the maximum braking force until the complete vehicle stop. Refer to the following figure. 52

3 Driving direction Danger A Finding the danger A1 Start moving the right foot (release the accel pedal) A2 Placing the right foot on the brake pedal B Braking starts A Finding the danger TIME Actual braking time Deceleration time Preparation time t1 :Reactiontime t2 : Foot changing time t3 : Pedal pressing time TIME Actual braking distance Stopping distance Preparation distance Relative frequency Hand operation Male Female Foot operation Reaction time [Figure 8-11, Driver s operation, relationship between time and deceleration] 8.3 Fade It refers to the phenomena of gradual disappearing of light, sound or power, which means the same terminology used in cinema or TV to change the screen. If the brake has been applied repeatedly on the long descending road, the temperature increases at the frictional plate of brake shoe and braking force reduces due to the friction reduction. 8.4 Vapor lock It refers to loss of function when some part of liquid related system is locked due to the vaporization of liquid by the heat. In fuel system, fuel supply fails because of vapor concentration in a fuel pipe and engine stops eventually. When the brake fluid evaporates in wheel cylinder or brake pipe of hydraulic brake system, brake does not work and soft as if the sponge is pressed when the brake pedal is pressed. 53

9. ABS system Objective To understand the advantages of ABS system and its construction and operation. Main contents 9.1 Advantages of ABS 9.2 ABS type 9.3 Physical principles 9.4 ABS construction 9.

4 9. ABS system Objective To understand the advantages of ABS system and its construction and operation. Main contents 9.1 Advantages of ABS 9.2 ABS type 9.3 Physical principles 9.4 ABS construction 9.1 Advantages of ABS Anti-lock Brake Systems are designed to prevent wheel lockup under heavy braking conditions on any type of road condition. The result is that, during heavy braking, the driver : retains directional stability(vehicle Stability) stops faster (Shortened Stopping distance, except gravel, fresh snow..) retains maximum control of vehicle (Steerability) If the front wheels lock it is no longer possible to steer the car If the rear wheels lock the car can become unstable and can start to skid sidewaysif a car on the different conditions of surface brakes, the wheels on the slippery surface easily lock up and the vehicle begins to spin. But ABS provides vehicle stability until it stops. 9.2 ABS type 1) 4-Sensor 4-Channel type ( Independent control type ) This type has four wheel sensors and 4 hydraulic control channels and controls each wheel independently. Steering safety and stopping distance maintains optimum condition on the homogeneous road surface. However, on the split- road surface, uneven braking force between left wheels and right wheels generates a Yawing Moment of the vehicle body resulting in vehicle instability. 54

Therefore,most of vehicles with a 4 channel ABS incorporates a select low logic on rear wheels to maintain the vehicle stability at any road conditions.

concentrated on front wheels and the center of the mass of vehicle also moves forward while braking allowing almost 70% of braking force to be controlled by front wheels.

However, rear wheels which performs relatively less braking force are very important to guarantees vehicle safety while braking.

5 Therefore,most of vehicles with a 4 channel ABS incorporates a select low logic on rear wheels to maintain the vehicle stability at any road conditions. <FF car, X-line brake system> 2) 4-Sensor 3-Channel type (Front wheels: independent control, Rear wheels: Select low control ) In case of FF(Front engine Front driving) car, most vehicle weight concentrated on front wheels and the center of the mass of vehicle also moves forward while braking allowing almost 70% of braking force to be controlled by front wheels. This means that most braking power is generated by front wheels and to get a maximum braking efficiency while ABS operation, independent control of front wheels is necessarily required. However, rear wheels which performs relatively less braking force are very important to guarantees vehicle safety while braking. That is, while ABS operation of rear wheels on the split road surface, independent control of rear wheel generates uneven braking force resulting in vehicle yawing moment. To prevent this yawing and to maintain vehicle safety with ABS operation on any kinds of road surface, rear wheel braking pressure is managed according to the wheel which shows more lock-up tendency. This control concept is called Select-low control. 3) 4-Sensor 3-Channel type (Front wheels;indendent control,rear wheels ; Select contnrol ) Vehicle with H-bake line system has this ABS control system. 2 channels are for front wheels and the other one is for rear wheel control. Rear wheels are controlled together by a select low control logic. In case of X-brake line system, 2 channels (2 brake ports in the ABS unit) are required to control rear wheel pressure because each rear wheel belongs to different brake line. 4) 1-Sensor 1-Channel type ( Rear wheels: Select low control ) Vehicle with H-bake line system. Only controls rear wheel pressure. One wheel speed sensor is installed on a rear differential detecting rear wheel speed. Front wheels are locked up while heavy braking, vehicle loses its steering stability and stopping distance on a low- road surface also increases. This system helps vehicle have a straight stop. <FF car, H-line brake system> <FF car, H-line brake system> 55

<FR car, H-line brake system> 9.3 Physical principles 1) Tire force Forces which act on a moving vehicle are gravity, air force(air resistance) and tire force (rolling resistance).

The tire force consists of the following components: - driving force F D caused by the drive, - lateral force F S caused by the steering, and - normal force F N as a result of the vehicle weight.

6 <FR car, H-line brake system> 9.3 Physical principles 1) Tire force Forces which act on a moving vehicle are gravity, air force(air resistance) and tire force (rolling resistance). A desired movement or change in movement can be achieved only via the tire force. The tire force consists of the following components: - driving force F D caused by the drive, - lateral force F S caused by the steering, and - normal force F N as a result of the vehicle weight. The lateral force FS transfers the steering movement to the road and makes the vehicle turn. The normal force FN is F determined by the vehicle weight and its load, that is, it is the N weight component acting perpendicularly on the road. The degree to which the forces can actually come into effect F D depends on the condition of the road and tires and on the weather condition, that is, on the friction force between the F S tires and road surface. 2) Relationship among forces The relationship between frictional force, side force, braking force and driving force can be expressed using a friction circle. The friction circle assumes frictional force between the tire and road surface to be identical in all directions. It can be used to visualize the relationship between side forces, braking force, and driving force. While cornering at a fixed speed, for example, all of the tire s frictional force is the side force that is turning the vehicle. When brake are applied during cornering, however, part of the frictional force of the tire is used for braking force, which reduces the size of the side force. Conversely, turning the steering wheel while applying the brakes reduces braking force, because part of the tire frictional force normally used for braking becomes cornering force. Portion of frictional force acting as braking force Braking force Fractional force generated at tire patch Side force Side force Driving 56force

7 <Friction circle> 3) Friction force The friction FR is proportional to the normal force FN: FR = B x FN The factor B is the braking force coefficient (or Frictional coefficient). The factor can be influenced by the characteristics of the different tire/road material pairings. The braking force coefficient is thus a measure of the transferable braking force. For vehicle tires, the braking force coefficient reaches its maximum values on a dry and clean road surface and its lowest on ice. <Example> Road condition Braking force coefficient( B) Dry concrete 0.8 ~ 1 Wet asphalt 0.2 ~ 0.65 Ice 0.05 ~ 0.1 The braking force coefficient depends greatly on the vehicle speed. When braking at high speeds, and under certain road conditions, the wheels may lock if the braking force coefficient is so low that the grip of the wheels to the road surface can no longer be available 4) Slip While vehicle driving or braking, complex physical forces occurs in the tire s contact area with the road. The tire s rubber elements become distorted and are exposed to partial sliding movements, even if the wheel has not yet locked. The measure of the sliding components of the rolling movement is the slip : =(V V - V W )/ V V Slip Ratio Slip Ratio = (V V - V W )/ V V 100, V V : Vehicle Speed, V W : Wheel Speed Maximum braking force Approximately 10~30% Slip This means that some tire rotation is necessary to achieve maximum braking. The optimum slip value decreases as tire-road friction decreases. Where V v is the vehicle speed and V W is the circumferential speed of the wheel. The formula shows that brake slip occurs as soon as the wheel starts to rotate more slowly than the wheel speed which corresponds to the driving speed. Braking forces can be generated only in this condition. 0% When a tire is rolling freely 100% When a tire locks up completely 5) Lateral force (side force) In addition to the braking force and driving force acting on the contact area in the direction that the tire is rotating, there is also a Lateral force that acts laterally on the tire. Side force 57

8 is the basic force that occurs when the vehicle turns. The basic force during cornering by a vehicle is the force of the part of the tire in contact with the road surface wanting to return its normal shape from its currently deformed state. This force pushes the tire sideways against the road surfaces, and is therefore called Side force. And the moment generated at the deformed tire is called Over turning moment Tire shape when vehicle is traveling straight Tire shape when vehicle is cornering Tire overturning moment Side force Normal force 6) Understeering and oversteering Keeping the steering wheel turned at a fixed angle and traveling at a fixed speed causes the vehicle to move in a circle with a fixed radius. Increasing the vehicle s speed at this point causes the vehicle to move either outside the original circle due to Understeering, or inside the original circle due to Oversteering. The actual steering characteristic (Understeering or Oversteering) produced by a particular vehicle depends on the interrelationship between the weight distribution between its front and rear wheels, tire specifications, suspension characteristics, and drive system 9.4 ABS construction HCU Proportioning valve (Without EBD) ABSCM G-Sensor (with 4WD) 58

1) ABSCM ABS consists of wheel speed sensors which detects a wheel lock-up tendency, on the basis of wheel speed sensor signal an ABSCM(Control Module) which outputs control signal and HCU(Hydraulic

wheel SPEED SENSOR 1 2 I 3 4 5 6 1 Electronic Cable 4 Winding 2 Permanent Magnet 5PolePin 3 Housing 6 Tone Wheel [Sectio1] 1 Electronic Cable 5PolePin 2 Permanent Magnet 6 Winding 3 Housing 7 Air gap

9 1) ABSCM ABS consists of wheel speed sensors which detects a wheel lock-up tendency, on the basis of wheel speed sensor signal an ABSCM(Control Module) which outputs control signal and HCU(Hydraulic Control Unit) which supplies brake pressure to each wheel according to the ABSCM output signals.wheel SPEED SENSOR 1 2 I Electronic Cable 4 Winding 2 Permanent Magnet 5PolePin 3 Housing 6 Tone Wheel [Sectio1] 1 Electronic Cable 5PolePin 2 Permanent Magnet 6 Winding 3 Housing 7 Air gap 4 Housing Block 8 Tone wheel [Section2] 1 Magnet 2 Winding 3 Tone Wheel 4 Rotates 5 High Speed 6 Low Speed 7AirGap When the Tone Wheel rotates, the magnetic field changes and induces a voltage in the winding. - Permanent magnetic produce a voltage - Higher speeds produce a higher frequency - Lower speeds produce a lower frequency 3) G-sensor ABS control for 4WD uses the signal of G-sensor to solve the problems that is early all 59

10 wheel-lock on Lm and that late response in case of m change of road surface. G-sensor signal is got every 7ms, and filtered. ABSCM sets m-flags (High, Medium, Low) to calculate detailed gradient of reference velocity and control threshold compared with 2WD.When driving in 4WD, all four wheels are mechanically locked, so all wheel speed decrease with almost same rate in many case. This phenomenon is more notable when driving on low (friction) road, so ABS control become unstable. To prevent this happening, G sensor is installed. With this signal, ABSCM recognize that the vehicle is now stopping on a low road or high road, thereby modifying the ABS operating cycle(algorism). That is, Small(or Great) G braking G value Low (or High) Low (or High) road detected ABSCM advances(or delays) to decrease hydraulic pressure Wheel lock is delayed(or advanced) Stopping distance increases(or Decreases). 60

HECU Clock frequency 32 MHz 50 MHz Memory 128 KB 512 KB Switch Orifice Orifice. Operating temperature - 40 C to 150 C - 40 C to 150 C

489000 113 1. SPECIFICATION Unit Description Specification ABS ESP HECU Clock frequency 32 MHz 50 MHz Memory 128 KB 512 KB Switch Orifice Orifice Wheel speed sensor ABS / ESP CBS Operating temperature