Experiment 7

Aim: Study of vehicles control systems.

Examples of Automotive Closed-loop Control Systems

Control Indirectly Directly Manipulated Sensor Actuator

System controlled controlled variable
variable variable
Fuel Air-fuel ratio Exhaust Quality of Zirconia or Fuel
injection oxygen injection fuel Titania based injector
system content electro-
chemical
Knock Knock Knock Ignition timing Piezo-electric Ignition
control sensor accelerometer coil
output switch.
Transistor
Anti-lock Wheelslip Wheelspe Brake time Magnetic ABS
braking limit ed pressure reluctance solenoid
system valve

ECU (based on Micro Computers)

Engine Management Sensors

Measured Direct/indirect Sensor technology/ Sensor mounting location
variable measurement reference
Intake manifold Indirect measurement Wheatstone bridge Within intake manifold
absolute pressure of engine load or arrangement of thick
mass air-flow intake film resistors bonded
onto a thin alumina
diaphragm
Mass airflow Direct and indirect Various forms Within air intake
measurement of fuel including ‘flap’ type,
injector basic pulse ‘hot-wire’, Karman
width vortex and thick-film
diaphragm
Temperature Direct measurement Thermistor or Intake air, outside air,
at various locations thermocouple catalytic converter, engine
depending on coolant, hydraulic oil
temperature range
Engine speed Direct measurement Magnetic reluctance or Flywheel on end of engine
and crankshft Hall effect device crankshaft
reference
position

Engine Management Sensors

Measured Direct/indirect Sensor technology/ Sensor mounting
variable measurement reference location
Battery voltage Direct measurement Resistive attenuator
Throttle position Direct measurement Potentiometer Accelerator pedal
Knock (engine Direct measurement Piezoelectric Cylinder block or head
cylinder pressure accelerometer type.
oscillations during
ignition)
Oxygen Direct measurement Zirconia or Titania based Exhaust manifold (normal
concentration in exhaust gas oxygen operation above 3000 C)
exhaust gas sensors
(Lambda sensor)
Chassis Control Sensors

Measured variable Direct/indirect Sensor technology/ Sensor mounting

and application measurement reference location
Wheelspeed and Direct measurement Magnetic reluctance or Brake assembly and
engine speed, (ABS, Hall effect device crankshaft flywheel
TCS and electronic respectively
damping)
Steering wheel Direct measurement Potentiometer or Steering shaft
angle, (Electronic optical encoder
damping)
Throttle position Indirect measurement Potentiometer Accelerator pedal
of vehicle accel.
Chassis and wheel Direct Piezo-electric Engine compart-ment
acceleration, accelerometer and wheel assembly
(electronic damping)
Brake system Indirect measurement Flexing plate sensor Brake master cylinder
pressure (electronic of vehicle decelerat- with strain gauges
damping) ion mounted on plate
Steering shaft torque Direct measurement Optical device relying Steering shaft
(Electric power on steering shaft
assisted steering) distortion under
driver’s twisting action
Safety and Onboard navigation

Measured Direct/indirect Sensor technology/ Sensor mounting location

variable measurement reference
Vehicle Direct ‘G’ sensor (Piezo- Single-point electronic
deceleration (air- measurement electric sensing, location in
bag systems) accelerometer) dashboard or steering
wheel
Wheelspeed and Direct Magnetic reluctance Brake assembly.
engine speed measurement or Hall effect device
(Vehicle nav.
Systems)

Electronic fuel injection (EFI)

1. Allows precise and fast control of fuel injected
2. By control of the ‘on-time’ period of the solenoid operated injectors (spray
nozzle) and plunger.
3. Delivery pipe fuel pressure is maintained constant by a fuel pressure
regulator
4. Opening and closing times of between 0.5 and 1 ms.
5. Engine operating speed of 6000 rpm (10 ms revolution time)
6. Injector on-time can be controlled between 1 and 10 ms

Power driver application

1. Multi-point or sequential fuel injection, with one fuel injector near the intake valve
(or valves) of each cylinder.
2. At a device level, a fuel injector IC package
3. Provides the high solenoid drive current required
4. Incorporates both over-voltage and short-circuit protection,
5. Fault reporting diagnostic routines also included

Two types of EFI System ----- Speed-density EFI

1. Inlet manifold absolute pressure (MAP) sensor has an important role
2. Fuel injection opening period or pulse width is related directly to the mass of air
flowing into the engine as fuel-air ratio must be maintained constant in steady-
state operation
3. The mass of air-flow is related to the manifold absolute pressure by the equation

Where,
Vd = the displacement of the cylinder,
nv = the volumetric efficiency or the fraction of Vd actually filled on each stroke, [=
f(speed)]
pi = manifold absolute pressure,
R = a constant and
Ti = the intake air temperature.

Mass air-flow EFI

1. Direct measurement of the quantity of air drawn into the engine (using an air-
flow sensor (AFS)).
2. Simple flap-type,
3. Hot-wire and Karman vortex devices
4. Direct measurement is better than feed-forward control in speed density EFI
(Factors like variation in volumetric efficiency, engine displacement due to speed
 and internal deposits need to be taken care of). Both of these forms of EFI may
be improved
5. Exhaust gas oxygen sensor for closed-loop control of the air–fuel ratio.
6. If engine is to be controlled precisely air–fuel ratio must be controlled to within
1%.
7. Only possible with closed-loop control

Closed-loop control of air–fuel ratio

The objective of low exhaust-gas emission levels

1. Maintain the air–fuel ratio at 14.7:1 [stoichiometrically / chemically perfect]

2. Three-way catalytic converters to control emission

In a closed loop system

1. The fuel injection period computed by air intake measurement is modified
2. Based on measured exhaust gas oxygen (EGO) content.
3. Injection period modification factor between 0.8 and 1.2.
4. EGO tells whether  < 1 or  > 1
5. Closed loop system has a limit cycle frequency between 0.5 to 2 Hz

Electronic clutch control

1. To relieve pressing of clutch during gear change

2. Throttle cable of accelerator pedal replaced by closed loop control system

– Accelerator pedal position sensor and servomotor

– Connected to an ECU for the gear change process

 Block Diagram of an Automatic Clutch and Throttle system

• Control of clutch engagement and disengagement

• Improved safety
• Prevention of engine starting when in gear
• Inappropriate gear change
Integration of Transmission and Engine ECU


transmission ECU signals the engine management ECU

• cut off fuel injection and
• Signals TECU to allow gear change
• During gear change down
• TECU energizes signals E-ECU
• Changes ignition timing a few degrees to reduce engine torque,
• Signals TECU to allow gear change
• In the end both systems return to independent operation.
 Integration of engine management and transmission control systems

• the multi-way switch reports position of the selector lever

• If the lever is not in either Park or Neutral when starting, operation of the
starter motor is inhibited

• a warning buzzer sounded

• The hold switch (push-button switch on the selector lever)

• instructs the ECU to hold the transmission in a current gear ratio

• useful in descending a hill.

• The stoplight switch

• When the brakes are applied and transmission is in a lockup condition,

• the lockup clutch is then disengaged.

• The overdrive inhibit signal (O/D) (from a separate cruise control unit)

• prevents the transmission from changing into overdrive (fourth gear) if

• cruise control is activated and

• vehicle speed is more than a certain amount below the set cruising
speed.

• The Automatic Transmission Fluid (ATF) thermosensor

• modify the line pressure at temperature extremes

• To account for changes in fluid viscosity.

• Atmospheric pressure

• If above 1500 m (engine develops less power at high altitudes)

• the automatic gear change points are modified to suit the change in
performance.
 Power train Control System

Also includes

– Exhaust gas recirculation system (circulating exhaust into intake to reduce max
combustion temp, and hence NOx)

Controlled by powertrain ECU

Engine temp, load, speed
– Evaporative Emission Control System (to circulate fuel vapour into intake and
prevent leakage into atmosphere)

Chassis Control Systems

• Anti Lock Braking system

• Electronic Damping Control system

• Power Assisted Steering System

• Traction Control Systems

Anti-lock braking systems (ABS)

• The vehicle skids, the wheels lock and driving stability is lost so the vehicle cannot
be steered;
• If a trailer or caravan is being towed it may jack-knife;

• The braking distance increases due to skidding;

• The tyres may burst due to excessive friction and forces being concentrated at
the points where the locked wheels are in contact with the road surface;
Variation of the coefficient of friction (µ ) with slip ratio
• Induction type wheel-speed sensors on the wheel assembly or differential

• couple magnetically to a toothed wheel known as an impulse ring.

Antiskid braking system (ABS)
• All electronic signals come to the electronic controller (ECU)
• The ECU controls the hydraulic modulator
• To control the Brake line pressure in Brake master cylinder

Wheel-speed and braking pressure during ABS-controlled braking

• If wheel decelerates beyond a certain level, curtail brake pressure (1)

• If wheel decelerates further, reduce brake pressure further (2)
• If wheel accelerates, increase brake pressure (3)
Traction control systems

• Prevent drive wheels from wheel spinning during starting or

accelerating on a wet or icy surface.
• Avoid reduction of either steering response in front-wheel-drive (FWD) /
vehicle stability on rear-wheel-drive (RWD) vehicles.
TCS operates
• Tto maximize adhesion to the road surface during acceleration
• Same sensors as in ABS
• The actuation uses fuel, ignition and driven wheel braking action
Traction Control Systems (TCS)
• To achieve reduction in driven wheel torque during wheel spin.

• Maintain the acceleration slip of the driven wheels equal to the mean
rotational velocity of the non-driven wheels + a specified speed
difference known as the slip threshold.
• Driven wheels are kept at a faster speed than the non-driven wheels

• The vehicle accelerates at a constant rate proportional to the

difference in the two speeds. (if difference is not in limits (slip
threshold), traction needs to be controlled)

• Control depends on road surface conditions or adhesion coefficient.

• On dry road surfaces, maximum acceleration at slip rates of 10 to 30%.

• On glare ice, maximum traction between 2 and 5 percent
• So TCS systems designed for a slip rate range between 2 and 20%.
Adhesion force coefficient µA as a function of acceleration λA(Jurgen, 1995)

• on loose sand or gravel and in deep snow the coefficient of adhesion increases
continually with the slip rate

• TCS systems incorporate slip-threshold switches to allow the driver to

select a higher slip threshold or switch off the TCS
• The control objectives of TCS are modified by vehicle speed and curve
recognition.
• Both of these variables can be derived from the speeds of the non-driven
wheels.
• coefficient of adhesion or friction decided on the basis of acceleration rate and
engine torque

• The slip threshold is raised in response to higher friction

coefficients to allow higher acceleration rates

• Curve recognition or cornering detection also affects the control strategy

for TCS.

• This strategy employs the difference in wheel speeds of the non-driven wheel
speeds as a basis for reductions in the slip set point to enhance stability in
curves.

• High vehicle speeds and low acceleration requirements on low coefficient of

adhesion surfaces imply a control strategy of progressively lower slip
threshold set points as the vehicle speed increases, gives maximum lateral
adhesion on the surface.

Electronic damping control

• The primary function of a shock absorber

– control vehicle movement against roll during turning and pitch during
acceleration or braking.
– Requires hard suspension
• secondary role
– To prevent vehicle vibration caused by a poor road surface.
– Requires a soft suspension
• Electronic damping control (EDC) used to attain these twin objectives
• altering the characteristics of spring and oil-filled damper
arrangement
– difficult and expensive
• Simple option - Suspensions with at least three settings; ‘soft’, ’medium’ and
‘firm’
• OR electronically controlled suspension systems using air, nitrogen
 gas and hydraulic oil as a suspension agent.

• sensors used

– Vehicle speed,
– Engine r.p.m.,

– Brake system pressure,

– Steering angle,

– Chassis and wheel acceleration,

– Throttle position,
– Vehicle load and

– Even road surface condition

• Road condition - implied by processing signals from front and rear height
sensors rather than direct measurement.
– If the height sensor signals a small high frequency but a large low
frequency amplitude
• a heaving or undulating road surface
• Does not require a softening of damper.
– A large high frequency component would suggest
• a rough road surface and
• Softening of damper action.
• Conflicts with damper requirement to prevent rolling during
cornering.
• If the vehicle corners on a rough surface this must be resolved by
the ECU.
 Electronically controlled damping system

Electronically controlled power-assisted steering (PAS)

 Hydraulic bridge circuit for electronically-controlled power steering showing flow paths

Electronically controlled hydraulic PAS

• The ports of a solenoid valve are connected across the rack and pinion
steering hydraulic power cylinder.
• with increasing vehicle speed the valve opening is extended
– reducing the hydraulic pressure in the power cylinder

– increasing the steering effort.

• bridge-like restrictions for control of the power cylinder are formed by the paths
through the pump to port connections of a rotary valve
• The valve is connected directly to the steering wheel and
– A small movement of this controls the high pressure hydraulic fluid to reach
the power cylinder/solenoid valve.

Electric PAS
• input to the rack and pinion steering system is from a motor/reduction gearbox
• Motor torque is applied directly to either the pinion gear shaft or to the rack shaft.
• The steering effort range is greater than with hydraulic systems,
• Installations are cheaper and reliable.
• Power is only consumed when steering wheel moves, (unlike hydraulic system)

• a torque sensor on the column shaft

• The electric motor coupled to the worm wheel mechanism through a reduction
gearbox.
• The load torque TL on the steering column is the load presented by the worm
mechanism and the rack and pinion assembly to which it is attached.
• The amount of motor torque is proportional to the motor current IM.
• in a simple armature controlled d.c. motor the average current is given
V  k  N
IM  M
R

– where R is the armature resistance,

– N is the speed of the motor and VM the motor voltage,

• the set point motor voltage depends on how much control effort is
required from the d.c. motor.

• When a driver turning a steering wheel at a constant rate, say in cornering.

– The d.c. motor, must turn at a speed proportional to this rate.

– Controlling term --- motor voltage = k x N

• at high vehicle speeds the assistance given to the driver must decrease in
proportion to speed
– i.e., Decrease motor current or voltage as vehicle speed increases.

– Motor voltage component = kT x Tm (Tm is output from Torque sensor)

– Inverse function of vehicle speed

• Add both components to get appropriate control

Air-bag and seat belt pre-tensioner systems

• systems consist of
– Crash detection sensors (typically piezoelectric) with a signal conditioning
amplifier

– A microcontroller distinguishing between crashes and normal vehicle

dynamics,
 – Igniter triggering for the pyrotechnic inflator

• Used for air-bag deployment and seat belt tightening.

• The allowable forward passenger travel with an air-bag system is 12.5 cm
– with seat belt tensioning systems it is about 1 cm.

• Approximately 30 ms are required to inflate air-bags and

• time required to tension a seat belt with a retractor = ~10 ms.
• triggering must be done by the time forward displacement is reached
minus the activation time of the respective restraining device.
• Often multiple sensors and sensor mounting positions
• When airbag is triggered

– ECU turns on the firing current switches,

– allows current through the igniter,

– initiates a gas generation reaction inside the inflation module.

– Capacitance based power maintained even if battery is disconnected

Air-bag electronics block diagram