An overview ofEMBEDDED SYSTEMS

IN AUTOMOBILES

By Amber Deep SinghEN. No-22/10

Dept. of Electronics and Communication

NIT Srinagar

CONTENTS

INTRODUCTION EMBEDDED SYSTEMS COMPONENTS CHARACTERISTICS OF EMBEDDED SYSTEMS APPLICATIONS OF EMBEDDED SYSTEMS EMBEDDED SYSTEMS IN AUTOMOBILES OVERVIEW OF ABS HISTORY OF ABS ABS COMPONENTS WORKING OF ABS OVERVIEW OF ACC PRINCIPLE OF ACC ACC COMPONENTS SENSOR OPTIONS PULSE DOPPLER RADAR DOPPLER SHIFT WORKING OF ACC IN DIFFERENT MODES CONCLUSION

Introduction. An embedded system is any device controlled by instructions

stored on a chip. These devices are usually controlled by a micro processor that executes the instructions stored on a read only memory (ROM) chip. The software for the embedded system is called firmware. Embedded systems are also known as real time systems since they respond to an input or event and produce the result within a guaranteed time period.

As it is evident from the above figure that the core of an embedded system is formed by a processor which processes the data from the input ports to arrive at a logical decision supported by memory where the firmware resides. The data from the sensors might be optical or an electrical signal so it needs to be converted to a digital signal to be processed by the processor. This is where the A/D converters come into play.

The processed data from the processor is conveyed to the the actuators controlling brakes, power trains, steering etc. after passing them through a D/A converter. This controlling mechanism operates in closed loops so that a feedback mechanism is always there to keep the system in a controlled state of affairs.

Characteristics of embedded systems They have the ability to run sophisticated algorithms. They provide real time operations. Many systems are multi rate i.e have the ability to

perform multiple operations at different rates. Low manufacturing cost. Low power consumption.

Applications. Vehicle systems for automobiles, subways, aircraft, railways and

ships.

Traffic control for highways, airspace, railway tracks and shipping lanes.

Process control for power plants, chemical plants and consumer products such as soft drinks and beer.

Medical systems for radiation therapy, patient monitoring and defibrillation.

Military uses such as firing weapons, tracking and command and control.

Manufacturing systems with robots.

Telephone, radio and satellite communications.

Computer games.

Multimedia systems that provide text, graphic, audio and video interfaces.

House holds systems for monitoring and controlling appliances.

Building managers that controls such entities as heat, light, Doors and elevators.

Embedded systems in automobiles

What is ABS?“An anti-lock brake system is a feedback control system that

modulates brake pressure in response to measured wheel deceleration, preventing the controlled wheels from becoming fully locked.” The motivation for ABS was that Under hard braking, an ideal braking system should provide the shortest stopping distances on all surfaces and maintain vehicle stability and steerability.

History of ABS1936: German company Bosch is awarded a patent for an

“Apparatus for preventing lock-braking of wheels in a motor vehicle”.

1936-: Bosch and Mercedes-Benz partner - R&D into ABS.

1972: WABCO partners with Mercedes-Benz developing first ABS for trucks.

1978: First production-line installation of ABS into Mercedes and BMW vehicles.

1981: 100,000 Bosch ABS installed.

1985: First ABS installed on US vehicles.

1986: 1M Bosch ABS installed.

1987: Traction control - in conjunction with ABS - used on passenger vehicles.

1989: ABS hydraulic unit combined with standard hydraulic brake unit.

1992: 10M Bosch ABS installed.

1995: Electronic Stability - in conjunction with ABS and TCS - for passenger cars.

1999: 50M Bosch ABS installed.

2000: 6 of 10 new cars on the road are ABS equipped.

2003: 100M Bosch ABS installed.

OverviewAnti-Lock Braking Systems (ABS) are designed to maintain driver

control and stability of the car during emergency braking. Locked wheels will slow a car down but will not provide steering ability. ABS allows maximum braking to be applied while retaining the ability to 'steer out of trouble‘ . The operation of ABS can slightly reduce stopping distance in some cases like on wet road surfaces, but it can increase the stopping distance in others, as may be the case in deep snow or gravel.

An ABS system monitors four wheel speed sensors to evaluate wheel slippage. Slip can be determined by calculating the ratio of wheel speed to vehicle speed, which is continuously calculated from the four individual wheel speeds. During a braking event, the function of the control system is to maintain maximum possible

wheel grip on the road - without the wheel locking - by adjusting the hydraulic fluid pressure to each brake by way of electronically controlled solenoid valves.

ABS Components Overview

Typical ABS Components:

Wheel Speed Sensors (up to 4) Electronic Control Unit (ECU)

Brake Master Cylinder, Hydraulic Modulator Unit with Pump and Valves

Vehicle’s Physical Brakes

Wheel Speed Sensor (WSS)

Magnetic field changes as toothed gear wheelrotates, resulting in a signal with frequency related to the angular velocity of the axle. The signal is then fed to an A/D converter so that the signal is changed to a digital signal so as to be processed by the ECU.

Electronic Control Unit

The signal from the WSS is proportional to angular velocity. By differentiating this signal, acceleration of each wheel is known. If a wheel is decelerating too quickly the brake pressure is modulated.

A fifth input to the ECU is from a brake pedal switch. This signal can shift program execution from a standby to an active state

Hydraulic Modulator Unit

The hydraulic modulator unit contains the ABS pump as well as solenoid valves for each brake line.

The fifth line - far right - is from the brake master cylinder, which is connected to the brake pedal.

Valves and Brakes

The valves modulate the brake pressure up to 20 times per second, effectively realizing the ideal tire slip percentage.

ABS ‘pumps’ the brakes much faster than any driver could.

How does ABS work?Basically, there are sensors at each of the four wheels (or in the case of the less sophisticated three-channel system, one on each of the fronts and only one for the pair of rears). These sensors watch the rotation of the wheels. When any one of the wheels stops rotating due to too much brake application, the sensors tell the car's computer, which then releases some of the brake line pressure that you've applied - allowing the wheel to turn again. Then, just as fast as it released the pressure, the computer allows the pressure to be applied again - which stops the rotation of the wheel again. Then it releases it again. And so on. With most ABS, this releasing and re-application - or pulsing - of the brake pressure happens 20 or more times per second.

Practically speaking, this keeps the wheel just at the limit - the threshold - before locking up and skidding. ABS prevents you from ever locking up the brakes and skidding - no matter how hard you apply the brakes. Obviously, this is going to mean much more steering control.

With ABS, all you have to do in an emergency is quickly squeeze the brake pedal as hard as you can and hold it there. And when I say hard, I mean HARD. Let the system do the finesse work for you. This may not be as easy as it sounds. After years of being told (and practising) not to press too hard on the brake pedal, this may not feel very natural.

What is ACC?Adaptive Cruise Control (ACC) is an automotive feature that allows a vehicle's cruise control system to adapt the vehicle's speed to the traffic environment. A radar system attached to the front of the vehicle is used to detect whether slower moving vehicles are in the ACC vehicle'spath. If a slower moving vehicle is detected, the ACC system will slow the vehicle down and control the clearance, or time gap, between the ACC vehicle and the forward vehicle. If the system detects that the forward vehicle is no longer in the ACC vehicle's path, the ACC system will accelerate the vehicle back to its set cruise control speed. This operation allows the ACC vehicle to autonomously slow down and speed up with traffic without intervention from the driver. The method by which the ACC vehicle's speed is controlled is via engine throttle control and limited brake operation.

PRINCIPLE OF ACCACC works by detecting the distance and speed of the vehicles ahead by using either a Lidar system or a Radar system [1, 2].The time taken by the transmission and reception is the key of the distance measurement while theshift in frequency of the reflected beam by Doppler Effect is measured to know the speed. According to this, the brake and throttle controls are done to keep the vehicle in a safe position with respect to the other. These systems are

characterized by a moderately low level of brake and throttle authority. These are predominantly designed for highway applications with rather homogenoustraffic behavior. The second generation of ACC is the Stop and Go Cruise Control (SACC) whose objective is to offer the customer longitudinal support on cruise control at lower speeds down to zero velocity . The SACC can help a driver in situations where all lanes are occupied by vehicles or where it is not possible to set a constant speed or in a frequently stopped and congested traffic. There is a clear distinction between ACC and SACC with respect to stationary targets. The ACC philosophy is that it will be operated in well structured roads with an orderly traffic flow with speed of vehicles around 40km/hour. While SACC system should be able to deal with stationary targets because within its area of operation the system will encounter such objects very frequently.

CONSTITUENTS OF AN ACC SYSTEM1. A sensor (LIDAR or RADAR) usually kept behind the grill of the vehicle to obtain the information regarding the vehicle ahead. The relevant target data may be velocity, distance, angular position and lateral acceleration.2. Longitudinal controller which receives the sensor data and process it to generate the commands to the actuators of brakes throttle or gear box.

SENSOR OPTIONSCurrently there are two means of object detection that are technically feasible and applicable in a vehicle environment.They are 1. RADAR

2. LIDAR

LIDAR (Light Detection and Ranging)The first acc system introduced by Toyota used this method. By measuring the frequency difference between a Frequency Modulated Continuous light Wave (FMCW) and its reflection.

Fig 1.Range estimation using FMCW-LIDAR

A company named Vorad Technologies has developed a system which measured up to one hundred meters. A low powered, high

frequency modulated laser diode was used to generate the light signal.

RADAR (Radio Detection and Ranging)RADAR is an electromagnetic system for the detection and location of reflecting objects like air crafts, ships, space crafts or vehicles. It is operated by radiating energy into space and detecting the echo signal reflected from an object (target) the reflected energy is not only indicative of the presence but on comparison with the transmitted signal, other information of the target can be obtained. The currently used ‘Pulse Doppler RADAR’ uses the principle of ‘Doppler effect’ in determining the velocity of the target .

PULSE DOPPLER RADARThe continuous wave oscillator produces the signal to be transmitted and it is pulse modulated and power amplified. The ‘duplexer’ is a switching device which is fast-acting to switch the single antenna from transmitter to receiver and back. The duplexer is a gas-discharge device called TR-switch. The high power pulse from transmitter causes the device to breakdown and to protect the receiver. On reception, duplexer directs the echo signal to the receiver. The detector demodulates the received signal and the Doppler filter removes the noise and outputs the frequency shift ‘fd’

Fig2. Block diagram of pulse Doppler radar

EFFECT OF DOPPLER SHIFTThe transmitter generates a continuous sinusoidal oscillation at frequency ‘ft’which is then radiated by the antenna. On reflection by a moving object, the transmitted signal is shifted by the Doppler Effect by ‘fd’. If the range to the target is ‘R’, total number of wavelength is ‘λ.’ in the two way-path is given by, n = 2R/ λ.

The phase change corresponding to each λ =2π

So total phase change, =2n . Φ π

=2(2R/ λ) π

So, if target moves, ‘R’ changes and hence ‘ ’ also changes. Φ

Now, the rate of change of phase, or the ‘angular frequency’ is

W=d /dt =4 (df/dt)/ Φ π λ.

Let Vr be the linear velocity, called as ‘radial velocity’

Wd = 4 Vr/ . =2 fd. π π

Fd=2Vr / λ .

But λ = ft, the transmitted velocity.

Fd= (2c Vr)/ ft

So by measuring the shift, Vr is found. The ‘plus’ sign indicates that the target and the transmitter are closing in. i.e. if the target is near, the echoed signal will have larger frequency.

Operational OverviewThe driver interface for the ACC system is very similar to a conventional cruise control system.The driver operates the system via a set of switches on the steering wheel. The switches arethe same as for a conventional cruise control system except for the addition of two switches to control the time gap between the ACC vehicle and the target vehicle. In addition there are aseries of text messages that can be displayed on the instrument cluster to inform the driver of the state of the ACC system and to provide any necessary warnings. The driver engages theACC system by first pressing the ON switch which places the system into the 'ACC standby' state. The driver then presses the Set switch to enter the 'ACC active' state at which point the ACC system attempts to control the vehicle to the driver's set speed dependent upon the traffic environment.

Control System InterfacesFigure 4 shows the information and signal flows between the different systems for ACC operation.

InitializationWhen the ignition key is in the off position, no power is applied to any of the systems. When the key is cycled to the on position, the ACC system initializes to the 'ACC off' state.

Engaging Cruise ControlEntering 'ACC standby' -Before active cruise control can be engaged the driver must first enter 'ACC standby'. This is performed by the driver pressing the ACC 'On' button. If no system faults are present, the ACC system will transition to the 'ACC standby' state.

Entering 'ACC active' – The driver enters the 'ACC active' state by pressing the 'Set' or 'Resume' button. If a prior set speed is present in memory, the system uses this prior value as the target speed when Resume is pressed, else, the current speed of when the Set button was pressed will become the target speed. The following conditions must be true for the system to enter 'ACC active' in response to the cruise switches:

Brake Switch 1 = brake not applied

Brake Switch 2 = brake not applied

Vehicle Speed >= 25 mph

When entering active ACC control, the vehicle speed is controlled either to maintain a set speed or to maintain a time gap to a forward vehicle, whichever speed is lower.

Operation during Speed Control Mode (ACC Speed Control)

Operation during this mode is equivalent to that of conventional speed control. If no forward vehicle is present within the Time Gap or clearance of the system, the vehicle's speed is maintained at the

target speed. The engine control system controls the engine output via throttle control to maintain the vehicle speed at the target speed.

Operation During Follow Mode (ACC Time Gap Control)

The ACC system enters follow mode or 'ACC time gap control' if the radar detects a forward vehicle at or within the clearance distance. During this mode of operation, the ACC system sends a target speed to the Engine Control Module and deceleration commands to the Brake Control module to maintain the set time gap between the vehicles.

deceleration control – The ACC system decelerates the vehicle by lowering the target speed sent to the Engine Control Module and sending a brake deceleration command to the Brake

Control Module. The maximum allowed braking effort of the system is 0.2 [g]. During brake deceleration events, the Brake Control Module activates the brake lights.

acceleration control – The ACC system accelerates the vehicle by increasing the target speed sent to the Engine Control Module. The Engine Control Module tries to maintain the target speed and can accelerate the vehicle at a rate of up to 0.2 [g] of acceleration.

adjusting the time gap – The driver can adjust the time gap via the 'Time Gap +' and 'Time Gap –' switches. Pressing the 'Time Gap +' switch causes the time gap value to increase and therefore the clearance between the two vehicles to increase. Pressing the 'Time

Gap –' switch causes the time gap value to decrease and therefore the clearance between the two vehicles to decrease.

reaction to a slow moving or stopped vehicle – Situations may occur such that the ACC system is not able to maintain the time gap within the deceleration authority of the system, 0.2 [g]. The clearance between the ACC vehicle and the forward vehicle may be rapidly decreasing or the minimum vehicle speed of 25 [mph] may be reached. Under these situations the ACC system enters 'ACC standby' and alerts the driver by displaying a "Driver Intervention Required" text message on the instrument cluster and by turning on an audible chime. If the brakes were being applied by the ACC system, they will be slowly released. At this point the driver must take control of the vehicle.

Transitioning Between Speed Control and Follow Modes

The ACC system automatically transitions between Speed Control and Time Gap (Follow) Modes. The mode of operation is determined by the lower of the set speed for Speed Control Mode and the target speed to maintain the gap between the ACC vehicle and a forward vehicle. Basically, if no vehicle is present within the clearance distance, the system will operate in Speed Control mode, else, it will operate in Time Gap mode.

Canceling Cruise Control OperationCruise Control operation may be canceled by the operator or automatically via the ACC system.

Either of the following conditions will deactivate ACC:

Brake pedal is pressed

'Off' button is pressed

Vehicle Speed < 25 mph

An ACC system fault is detected

ConclusionThe accidents caused by automobiles are injuring lakhs of people every year. The safety measures starting from air bags and seat belts have now reached to ACC, ABS and TCS systems. The researchers of Intelligent Vehicles Initiative in USA and the Ertico program of Europe are working on technologies that may ultimately lead to vehicles that are wrapped in a cocoon of sensors with a 360 –degree view of their surroundings. It will probably take decades, but car accidents may eventually become as rare as plane accidents arenow, even though the road laws will have to be changed, upto an extent since the non-human part of the vehicle controlling will become predominant.