Q & A for chassis control systmems.pdf

Automotive Electrical and Electronic Systems [email protected]

Nandakumar M. B. Dept of Automobile Engineering, Dayananda Sagar College of Engineering, Bangalore

Questions and Answers for Chassis control system.

Below are the questions from the previous examination and also important from examination

point of view.

Note: some answers are elaborate for better understanding. Use your discretion depending on

the marks and the question.

1. what are the essential components of ABS. What are their function. Locate them with

the help of neat sketch. Or Explain the working of ABS with its functional components

with a neat sketch.

Ans: There are a few variations between manufacturers involving a number of different

components. For the majority of systems, however, there are three main components.

Wheel speed sensors.

Electronic control unit.

Hydraulic modulator

Wheel speed sensors Most of these devices are simple inductance sensors and work in conjunction with a toothed

wheel. They consist of a permanent magnet and a soft iron rod around which is wound a coil

of wire. As the toothed wheel rotates, the changes in inductance of the magnetic circuit

generate a signal; the frequency and voltage of which are proportional to wheel speed. The

frequency is the signal used by the electronic control unit. The coil resistance is of the order

of 1k_. Coaxial cable is used to prevent interference affecting the signal. Some systems now use Hall effect sensors.



Electronic control unit The function of the ECU is to take in information from the wheel sensors and calculate the

best course of action for the hydraulic modulator. The heart of a modern ECU consists of two

microprocessors, which run the same program independently of each other. This ensures

greater security against any fault, which could adversely affect braking performance because

the operation of each processor should be identical. If a fault is detected, the ABS disconnects

itself and operates a warning light. Both processors have non-volatile memory into which

fault codes can be written for later service and diagnostic access. The ECU also has suitable

input signal processing stages and output or driver stages for actuator control.

The ECU performs a self-test after the ignition is switched on. A failure will result in

disconnection of the system. The following list forms the self-test procedure.

Current supply.

Exterior and interior interfaces.

Transmission of data.

Communication between the two microprocessors.

Operation of valves and relays.

Operation of fault memory control.

Reading and writing functions of the internal memory. All this takes about 300 ms.

Hydraulic modulator The hydraulic modulator has three operating positions.

Pressure build-up (brake line open to the master cylinder.)

Pressure reducing (brake line open to the accumulator)

Pressure holding (brake line closed.)

The valves are controlled by electrical solenoids, which have a low inductance so they react

very quickly. The motor only runs when ABS is activated

2. Explain the active suspension system with the help of a block diagram showing all

their functional components. Or

what are the importance of suspension system? How active suspension system is better

version of conventional suspension system. Explain with a neat sketch. Or



Explain with a block diagram the general layout of an active suspension system. 10

marks, Dec 2013

Ans: Conventional suspension systems are always a compromise between soft springs for comfort and

harder springing for better cornering ability. A suspension system has to fulfil four main

functions.

Absorb bumps.

Manage nose dive( pitching)when braking.

Prevent roll when cornering.

Control body movement. This means that some functions have to be compromised in order to fulfil others to a greater

extent

operation:Active suspension allows the best of both worlds. This is achieved by replacing

the conventional

springs with double-acting hydraulic units. These are controlled by an ECU, which receives

signals from various sensors. Oil pressure in excess of 150 bar is supplied to the hydraulic

units from a pump. A servo valve controls the oil, which is arguably the most critical

component.

The main benefits of active suspension are as follows.

Improvements in ride comfort, handling and safety.

Predictable control of the vehicle under different conditions.

No change in handling between laden and unladen.

. A representation of an electronically controlled suspension system or Active suspension

system is shown in Figure.



The layout of the suspension system also shows a simplified view of the hydraulic unit. This

is, in effect, a hydraulic ram and can have oil under very high pressure fed to the upper or

lower chamber. The actual operation of the whole system is as follows. As a wheel meets a

bump in the road there is increased upward acceleration and vertical load. This information is

fed to the ECU, which calculates the ideal wheel displacement. A control signal is now sent

to the servo valve(s), which control the position of the main hydraulic units. As this process

can occur hundreds of times per second, the wheel can follow the contour of the road surface.

This cushions the vehicle body from unwanted forces. By considering information from other

sensors, such as the lateral acceleration sensor, which gives data relating to cornering, and the

longitudinal sensor, which gives data relating to braking or acceleration forwards, the

actuators can be moved to provide maximum stability at all times. Active suspension looks

set to have an easy ride in the future. The benefits are considerable and, as component prices

reduce, the system will become available on more vehicles. It is expected that even offroad

vehicles may be fitted with active suspension in the near future

Sensors, actuators and system operation To control the hydraulic units to the best advantage, the ECU needs to know certain information. This is determined from sensor readings from various parts of the vehicle. A

number of sensors are used to provide information to the suspension ECU.

Load sensor A load cell used to determine whether actual load is positioned on each hydraulic ram.

Displacement and vertical acceleration This sensor can take a number of forms, as simple as a variable resistor or a more accurate

and sensitive linear sensor such as the LVDT Lateral and longitudinal acceleration Acceleration can be determined from a pendulumtype sensor using strain gauges linked to a

mass, or devices similar to an engine knock sensor.

Yaw transducer Yaw can be determined from lateral acceleration if the sensor is mounted at the front or rear

of the vehicle.

Steering position As well as steering position, rate of change of position is determined from a rotary position

sensor. This device can be a light beam and detector type or similar. If the rate of change of

steering position is beyond a threshold the system will switch to a harder suspension setting.

Vehicle speed The speed of the vehicle is taken from a standardtype sensor as used for operating the

speedometer.

Throttle position Similar to the existing throttle potentiometers. This gives data on the drivers intention to accelerate or decelerate allowing the suspension to switch to a harder setting when

appropriate.

Driver mode selection A switch is provided allowing the driver to choose soft or hard settings. Even if the soft

setting is selected, the system will switch to hard, under certain operational conditions

3. What is traction control system? Explain the function of essential components

Ans: The steerability of a vehicle is not only lost when the wheels lock up on braking; the same effect arises if the wheels spin when driving off under severe acceleration. Electronic

traction control has been developed as a supplement to ABS. This control system prevents the

wheels from spinning when moving off or when accelerating sharply while on the move. In

this way, an individual wheel, which is spinning is braked in a controlled manner. If both or

all of the wheels are spinning, the drive torque is reduced by means of an engine control

function. Traction control has become known as ASR or TCR.

Traction control will intervene to achieve the following:

Maintain stability.



Reduction of yawing moment reactions. _ Provide optimum propulsion at all speeds.

Reduce driver workload. The following list of advantages can be claimed for a good traction control system.

Improved tractive force.

Better safety and stability on poor surfaces.

Less driver stress.

Longer tyre life.

No wheel spin on turning and cornering. An automatic control system can intervene in many cases more quickly and precisely than the

driver of the vehicle. This allows stability to be maintained at time when the driver might not

have been able to cope with the situation.

Traction control is not normally available as an independent system, but in combination with

ABS. This is because many of the components required are the same as for the ABS. Traction

control only requires a change in logic control in the ECU and a few extra control elements

such as control of the throttle. Figure shows a block diagram of a traction control system.

Note the links with ABS and the engine control system.

Contol functions:

The following are controlled by the ECU to control traction

Throttle

Ignition

Brakes

Figure 15.8 shows an ABS and traction control modulator, complete with an ECU.



3. Explain the functioning of electronically controlled automatic transmission with

suitable diagrams OR

Explain briefly about electronic control system for automatic transmission 8 marks Jan

2007

The main aim of electronically controlled automatic transmission (ECAT) is to improve

conventional automatic transmission in the following ways.

Gear changes should be smoother and quieter.

Improved performance.

Reduced fuel consumption.

Reduction of characteristic changes over system

life.

Increased reliability. Gear changes and lock-up of the torque converter are controlled by hydraulic pressure. In an

ECAT system, electrically controlled solenoid valves can influence this hydraulic pressure.

block diagram of an ECAT system is shown below.



Most ECAT systems now have a transmission ECU that is in communication with the engine

control ECU (by a CAN controller area network databus in many cases). The system as a whole consists of a number of sensors providing data to the ECU, which in turn is able to

control a number of actuators or output devices.

4. With the help of a motor reverse circuit, explain the ECU controlled seat adjustment

system. 05 marks

Electric seat adjustment Adjustment of the seat is achieved by using a number of motors to allow positioning of

different parts of the seat. Movement is possible in the following ways.

Front to rear.

Cushion height rear.

Cushion height front.

Backrest tilt.

Headrest height.

Lumber support. Figure 16.2 shows a typical electrically controlled seat.

This system uses four positioning motors and one smaller motor to operate a pump, which

controls the lumber support bag. Each motor can be considered to operate by a simple rocker-

type switch that controls two relays as described above. Nine relays are required for this, two

for each motor and one to control the main supply. When the seat position is set, some

vehicles have set position memories to allow automatic re-positioning if the seat has been

moved. This is often combined with electric mirror adjustment. Figure shows how the circuit

is constructed to allow position memory. As the seat is moved a variable resistor, mechanically

linked to the motor, is also moved. The resistance value provides feedback to an electronic

control unit. This can be remembered in a number of ways; the best technique is to supply the resistor with a fixed voltage such that the output relative to the seat position is

proportional to position. This voltage can then be analogue-to-digital converted, which produces a simple number to store in a digital memory. When the driver presses a memory recall switch, the motor relays are activated by the ECU until the number in memory and

the number fed back from the seat are equal. This facility is often isolated when the engine is

running o prevent the seat moving into a dangerous position as the car is being driven. The

position of the seats can still be adjusted by operating the switches as normal.



5. Explain briefly the ventilation and heating system in a car. Show the path of flow with a

neat sketch.

Ans: Ventilation: To allow fresh air from outside the vehicle to be circulated inside the cabin, a pressure difference must be created. This is achieved by using a plenum chamber. A

plenum chamber by definition holds a gas (in this case air), at a pressure higher than the

ambient pressure. The plenum chamber on a vehicle is usually situated just below the

windscreen, behind the bonnet hood. When the vehicle is moving the air flow over the

vehicle will cause a higher pressure in this area. Figure 14.2 shows an illustration of the

plenum chamber effect. Suitable flaps and drains are utilized to prevent water entering the car

through this opening.

By means of distribution trunking, control flaps and suitable nozzles, the air can be directed as required. This system is enhanced with the addition of a variable speed blower motor.

Figure 14.3 shows a typical ventilation and heating system layout. When extra air is forced

into a vehicle cabin the interior pressure would increase if no outlet was available. Most

passenger cars have the outlet grills on each side of the vehicle above or near the rear quarter

panels or doors.

Heating system in a water-cooled engine Heat from the engine is utilized to increase the temperature of the car interior. This is

achieved by use of a heat exchanger, called the heater matrix. Due to the action of the

thermostat in the engine cooling system the water temperature remains broadly constant. This

allows for the air being passed over the heater matrix to be heated by a set amount depending

on the outside air temperature and the rate of air flow. A source of hot air is therefore

available for heating the vehicle interior.



However, some form of control is required over how much heat (if any), is required. The

method used on most modern vehicles is the blending technique. This is simply a control

flap, which determines how much of the air being passed into the vehicle is directed over the

heater matrix. The main drawback of this system is the change in air flow with vehicle speed.

Some systems use a valve to control the hot coolant flowing to the heater matrix.

By a suitable arrangement of flaps it is possible to direct air of the chosen temperature to

selected areas of the vehicle interior. In general, basic systems allow the warm air to be

adjusted between the inside of the windscreen and the driver and passenger foot wells.

Most vehicles also have small vents directing warm air at the drivers and front passengers

side windows. Fresh cool air outlets with directional nozzles are also fitted. One final facility,

which is available on many vehicles, is the choice between fresh or recirculated air. The main

reason for this is to decrease the time it takes to demist or defrost the vehicle windows, and

simply to heat the car interior more quickly to a higher temperature. The other reason is that,

for example, in heavy congested traffic, the outside air may not be very clean

8. List out the functional requirements of heating and ventilation system.

or

List out the requirements of typical conventional heating and ventilation system. 06

marks july 2013

Any heating and ventilation system has a simple set of requirements, which are met to

varying standards.

These can be summarized as follows.

Adjustable temperature in the vehicle cabin.

Heat must be available as soon as possible.

Distribute heat to various parts of the vehicle.

Ventilate with fresh air with minimum noise.

Facilitate the demisting of all windows.

Ease of control operation.

The above list, whilst by no means definitive, gives an indication of what is required from a

heating and ventilation system. As usual, the more complex the system the more the

requirements are fulfilled. This is directly related to cost.

6. Explain with a neat sketch the working of a car air conditioning system.

Or Question: explain typical air conditioning unit

Or question: Describe the air-conditioning used in Automobile. 07 marks Dec 2013

Or question: Explain the basic air-conditioning or refrigeration cycle with a neat

diagram. Explain the function of each component of the system. 14 marks Jan 2014

The operation of the system is a continuous cycle. The compressor pumps low pressure but

heat laden vapour from the evaporator, compresses it and pumps it as a super-heated vapour

under high pressure to the condenser. The temperature of the refrigerant at this stage is much

higher than the outside air temperature, hence it gives up its heat via the fins on the condenser

as it changes state back to a liquid.



This high-pressure liquid is then passed to the receiver-drier where any vapour which has not

yet turned back to a liquid is stored, and a desiccant bag removes any moisture (water) that is

contaminating the refrigerant. The high-pressure liquid is now passed through the

thermostatic expansion valve and is converted back to a low-pressure liquid as it passes

through a restriction in the valve into the evaporator. This valve is the element of the system

that controls the refrigerant flow and hence the amount of cooling provided.

As the liquid changes state to a gas in the evaporator, it takes up heat from its surroundings,

thus cooling or unheating the air that is forced over the fins. The low pressure vapour leaves the evaporator returning to the pump, thus completing the cycle. The cycle is represented in

Figure. If the temperature of the refrigerant increases beyond certain limits, condenser

cooling fans can be switched in to supplement the ram air effect.

A safety switch is fitted in the high-pressure side of most systems. It is often known as a

highlow pressure switch, as it will switch off the compressor if the pressure is too high due to a component fault, or if the pressure is too low due to a leakage, thus protecting the

compressor.



7 what are the requirements of seat heaters? Discuss in view of passenger comfort. 06

marks Dec 2013

The concept of seat heating is very simple. A heating element is placed in the seat, together

with an onoff switch and a control to regulate the heat. However, the design of these heaters is more complex than first appears.

The heater must meet the following criteria.

The heater must only supply the heat loss experienced by the persons body.

Heat to be supplied only at the major contact points.

Leather and fabric seats require different systems due to their different thermal properties.

Heating elements must fit the design of the seat. The elements must pass the same rigorous tests as the seat, such as squirm, jounce and bump

tests

9. Describe the circuit diagram of screen heater that includes timer relay. 05 marks july 2013

Ans: Screen heating



Most vehicles contain a heated rear window. The heating is performed by an element that is

almost invisible when viewed from the drivers seat. Close inspection reveals that there are

many small resistance wires embedded within the glass and crossing from one side to the

other. When the rear demister switch is turned on, electrical current flows through the wire

causing it to heat. This gradually clears a fogged glass allowing the driver to see through it.

Figure below shown the heating elements sandwiched with in the rear wind screen.

Front wind screen can be heated with hot air from the blower directly.

Heating of the rear screen involves a very simple circuit as shown in Figure.

The heating elements consist of a thin metallic strip bonded to the glass. When a current is

passed through the elements, heat is generated and the window will defrost or demist. This

circuit can draw high current, 1015 A being typical. Because of this, the circuit often contains a timer relay to prevent the heater being left on too long. The timer will switch off

after 1015 minutes. The elements are usually positioned to defrost the main area of the screen and the rest position of the rear wiper blade if fitted. Front windscreen heating is being

introduced on some vehicles. This of course presents more problems than the rear screen, as

vision must not be obscured. The technology, drawn from the aircraft industry, involves very

thin wires cast into the glass. As with the heated rear window, this device can consume a

large current and is operated by a timer relay.

10. Write a short note on a. Air bag b. seat belt tensioner c. Central locking.



Air Bag: A seat-belt, seat-belt tensioner and an airbag are, at present, the most effective restraint system in the event of a serious accident. At speeds in excess of 40 km/h the seat-

belt alone is no longer adequate. Research after a number of accidents has determined

that in 68% of cases an airbag provides a significant improvement.

Operation of the system The sequence of events in the case of a frontal impact at about 35 km/h, as shown in Figure

is as follows.

Note to students:Sketch no 4 is sufficient to illustrate the use of Airbag.

1. The driver is in the normal seating position prior to impact. About 15 ms after the impact

the vehicle is strongly decelerated and the threshold for triggering the airbag is reached. The

igniter ignites the fuel tablets in the inflater.

2. After about 30 ms the airbag unfolds and the driver will have moved forwards as the

vehicles crumple zones collapse. The seat-belt will have locked or been tensioned depending on the system.

3. At 40 ms after impact the airbag will be fully inflated and the drivers momentum will be absorbed by the airbag.

4. About 120 ms after impact the driver will be moved back into the seat and the airbag will

have almost deflated through the side vents, allowing driver visibility.

Passenger airbag events are similar to the above description. A number of arrangements are

used with the mounting of all components in the steering wheel centre becoming the most

popular. Nonetheless, the basic principle of operation is the same. Consideration is being

given to the use of a seat switch on the passenger side to prevent deployment when not

occupied. This may be more appropriate to side-impact airbags mentioned in the next section.

Seat-belt tensioners Taking the slack out of a seat-belt in the event of an impact is a good contribution to vehicle passenger safety. The decision to take this action is the same as for the airbag inflation. The

two main types of tensioners are:

Spring tension.

Pyrotechnic. The mechanism used by one type of seat-belt tensioner is shown in Figure



When the explosive charge is fired, the cable pulls a lever on the seat-belt reel, which in turn

tightens the belt. The unit must be replaced once deployed. This feature is sometimes

described as anti-submarining.

11. Explain the working of Electronically controlled vehicle heating system.

Ans: Air conditioning can be manually controlled or, as is now often the case, combined with

some form of electronic control. Figure 14.6 shows a block diagram representing an

electronically controlled vehicle heating system.

This system requires control of the blower motor, blend flap, direction flaps and the fresh or

recirculated air flap. The technique involves one or a number of temperature sensors suitably

positioned in the vehicle interior, to provide information for the ECU. The ECU responds to

information received from these sensors and sets the controls to their optimum positions. The

whole arrangement is, in fact, a simple closed loop feedback system with the air temperature

closing the loop. The ECU has to compare the position of the temperature control switch with

the information that is supplied by the sensors and either cool or heat the car interior as

required.

12. What is the use of seat heaters? Explain in brief. 05 marks



Figure shows a seat containing heating elements. In order for the passengers (including the

driver) to be comfortable, rigorous tests have been carried out to find the optimum heat

settings and the best position for the heating elements. Many tests are carried out on new

designs, using a manikin with sensors attached, to measure the temperature and heat flow.

The cable used for most heating elements is known as a Sine Cable and consists of multi-

strand alloyed copper. This cable may be coated with tin or insulated as the application

demands. The heating element is laminated and bonded between layers of polyurethane foam.

The traditional method of control is a simple thermostat switch. Recent developments,

however, tend to favour electronic control combined with a thermistor. A major supplier of

seat heaters, Scandmec Ltd, supplies an electronic system that includes push button switches,

potentiometers, timer function, short and open circuit detection. This is in addition to accurate

control of the chosen temperature setting. These seat heaters will heat up to provide an initial

sensation in 1 minute and to full regulated temperature in 3 minutes.

The following is not recommended for study.

Some cars use wiper blades with heating elements



More recent vehicle designs use a similar wire embedded within fixed glass as a radio antenna which may look like scree heaters. The wire is usually separate to the resistance wire and can be located in any fixed window, including the windscreen. Some manufacturers fit the embedded antenna in more than one window.

Q & A for chassis control systmems.pdf

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