Bradley D., Russell D.W. (eds.) Mechatronics in Action: Case Studies in Mechatronics

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96 D. Seward

S10

S13

0.00

1.00

2.00

3.00

4.00

5.00

6.00

Injector time

(micro-sec)

Engine rpm x 1000

Throttle %

Fig. 6.7 3D surface fuel map

Fig. 6.8 Engine management system architecture

Mechatronics and the Motor Car 97

6.5 Anti-lock Braking System (ABS)

Anti-lock Braking Systems (ABS) are sometimes called anti-skid brakes. The

systems use mechatronic technology to individually control the four brakes on a

vehicle, which would clearly be impossible for a human driver. The main purpose

is to control the stability of a vehicle and maintain steering response either during

very hard emergency braking or for normal braking under wet or icy conditions.

The braking distance required to bring a vehicle to a stop can also be reduced, but

this is really a secondary bonus effect of the system.

The ABS employs a mechatronic system comprised of wheel speed sensors,

electro-hydraulic valves and a central processing unit. The system constantly

monitors the rotational speed of all four wheels and when it detects one wheel

slowing down faster than all the others, it reduces the braking force at that wheel

by blocking or even reversing the flow of hydraulic fluid to the wheel. It is thus a

system which if it malfunctioned, is capable of immobilising the entire brake

system of the vehicle. For this reason, it must clearly be regarded as safety critical.

6.5.1 Background to the Theory of Braking [2]

When a vehicle moves at constant speed, the distance moved by the vehicle in one

revolution of a wheel is, as expected, equal to the circumference of the wheel.

However, when the vehicle is accelerating or braking, this simple relationship

does not apply. There is relative movement between the tyre tread and the road

surface known as “slip”. When braking, it is known as “brake slip”. Under normal

breaking, the amount of brake slip increases linearly as the braking force

increases. This is known as the stable region. However, under hard breaking when

the brake slip reaches a value of about 20%, the braking force stops increasing and

thereafter actually decreases. This is the unstable region.

When a wheel is fully locked (brake slip = 100%), the effective coefficient of

friction between the tyre and the road is reduced to about 80% of its maximum

value. Wet road conditions exaggerate the effect.

ABS endeavours to keep the slip in the stable region for all wheels at all times.

It does this by monitoring the relative speeds of all the wheels and comparing

them with an average reference value obtained from diagonally opposite wheels.

When it detects a wheel slowing down faster than it should, it momentarily holds

or releases the brake pressure on that wheel.

When a wheel enters the unstable region, the peripheral deceleration increases

rapidly and this can be detected by the ABS controller. The normal peak

coefficient of friction between a modern tyre and the road is about 1.0. This means

that a vehicle can decelerate (and accelerate) at a maximum of about 1 g. Of

course, if there was no brake slip, the perimeter of the wheel would also decelerate

at about 1 g. However, a wheel can lock within only 110 ms. For a car travelling at

98 D. Seward

60 mph, this represents a deceleration of about 20 g and hence can easily be

detected by a fast processor. This is shown in Figure 6.9.

Deceleration of

wheel perimeter

1.0 g

Time

ABS

zone

Wheel

locked

Brake pressure

Braking

torque

Road friction

torque

130 ms 240 ms

Acceleration Braking

Friction (μ)

1.0

Brake slipTyre slip

ABS

zone

Traction

control zone

100%

Wheel

locked

Wheel

spinning

Fig. 6.9 Brake slip

ABS systems achieve the above by means of complex software logic (rules). A

typical sequence for dry breaking is described below:

Phase 1 – The wheel peripheral deceleration moves beyond the defined threshold

(–a) and the ABS valve is instructed to “maintain brake pressure”.

Phase 2 – The wheel perimeter velocity continues to reduce until it falls below a

reference value which is based on extrapolation of previous vehicle speed. The

valve shifts to “pressure release” mode.

Phase 3 – The brake pressure drops and the deceleration rate of the wheel also

drops until it again rises above the threshold value (–a). The brake pressure is then

maintained.

Phase 4 – The wheel actually starts to accelerate again and eventually reaches a

certain threshold (+A).

Phase 5 – The brake pressure is allowed to increase again until the acceleration

again falls below the threshold (+A).

Phase 6 – The brake pressure is maintained until the acceleration drops to the

lower threshold (+a).

Phase 7 – The wheel is now “underbraked” so the pressure is slowly ramped up

until the deceleration drops below (–a).

Phase 8 – The cycle is repeated from phase 3 until normal breaking is detected.

Mechatronics and the Motor Car 99

6.5.2 ABS Components

Hydraulic Control Valves – One for Each Wheel

These are 3-position valves that employ an electrical solenoid to move between

positions. The first position allows fluid to pass between the brake master cylinder

and the wheel brake cylinder in the normal way (see Figure 6.10). The second

position (50% of maximum current) prevents movement of fluid between the

brake master cylinder and the wheel brake cylinder. The third position (maximum

current) connects the wheel brake cylinder to the fluid return line and with the aid

of a pump, can reduce the brake pressure at the wheel.

(1) Damper (2) Throttle

(3) Pump (4) 3/3 valve

(5) Accumulator

Fig. 6.10 ABS hydraulic components [2]

Wheel Speed Sensors

The wheel speed sensors consist of an electromagnetic proximity sensor fixed in a

stationary position to the wheel hub. The sensor is positioned about 1 mm from a

rotating toothed ring which is connected to the moving wheel shaft. Each time a

tooth passes the sensor, a signal is sent to the ECU.

100 D. Seward

The Electronic Control Unit

This is configured as shown in Figure 6.11 and comprises the following major

subsystem:

Input circuit – conditions the signals from the wheel speed sensors and forwards

them to one of two microcontrollers.

Microcontrollers – Two identical but separate integrated circuits. Each one

connected to two diagonally opposite wheels. They process the wheel speed

signals to decide on the appropriate ABS action. Commands are sent to an output

circuit.

Output circuits – In response to commands from the microcontrollers, two-stage

output circuits use power transistors to amplify the signals to provide enough

current to energise the solenoids on the hydraulic valves.

(1) Wheel-speed sensors (wheel frequencies) (2) Battery

(3) Input circuit (4) Digital controller (5) LSI circuit {1}

(6) LSI circuit {2} (7) Voltage stabilised memory

(8) Output circuit (9) Output circuit (10) Output stage

(11) Solenoid valves (12) Safety relay (13) Stabilised battery voltage

(14) Indicator lamp

Sensors ECU Actuators

Fig. 6.11 ABS control unit [2]

Mechatronics and the Motor Car 101

6.5.3 ABS Diagnostics

Partly because of the safety critical nature of ABS, the system contains extensive

diagnostic functionality which operates at several stages of the vehicles operation:

• When first switching on the car ignition, the ABS controller will send signals to

the four three-way valves and the wheel speed sensors. By monitoring the

current consumed, the controller will be able to predict whether the hydraulic

valve spools have stuck or if either a short or an open circuit exists in a

component.

• In general, if a defect is discovered, the ABS system will be disabled and a

warning light will be illuminated on the dashboard.

• When first pulling away, the output from the four wheel speed sensors is

monitored and a reasonableness check applied.

• All defects and spurious behaviours are recorded in a fault memory within the

controller for downloading when the vehicle is serviced.

6.6 Conclusions

Consideration was given to three automotive mechatronic systems. The first two,

electronic ignition and electronic fuel injection, are integrated within the engine

management system. They provide examples of how the superior performance of

mechatronics displaced traditional (and relatively unreliable) electrical and

mechanical systems. The third, the anti-lock braking system (ABS), is an example

of mechatronics offering a new opportunity to improve the controllability and

hence safety of vehicles. This is typical of the mechatronic process. It first

supplants traditional technologies and then offers vastly increased functionality

because of the flexibility afforded by software control.

References

1. Schwarz H et al., (1995), Automotive Electric/.Electronic Systems, Robert Bosch GmbH

2. Siegert E et al., (1999), Driving safety systems, Robert Bosch GmbH

3. www.kunhadi.org/main/?q=node/74

4. www.vdo.com/-+archive+-/e-corner/technology-behind-ecorner/technology-behind-e-

corner.htm

5. www.gill.co.uk/products/digital_ignition/Introduction/6_4stroke.asp

6. Banish G, (2007), Engine Management – Advanced Tuning, Brooklands Books Ltd.

7. www.familycar.com/Classroom/ignition.htm

8. www.iop.org/EJ/article/0305-4624/10/1/304/ptv10i1p20.pdf?request-id=5559c3fd-a4b0-

4320-865b-c8bf9f4ad8eb

102 D. Seward

9. www.zparts.com/zptech/articles/mal_land/ml_sucarb2/images4/SUcarb_111601b.htm

10. Hartman J, (2003), How to Tune and modify Engine Management Systems, Motorbooks

International

11. www.motorola.com/mediacenter/news/detail.jsp?globalObjectId=767_523_23&page=

archive

Chapter 7

Multi-mode Operations Marine Robotic Vehicle

– a Mechatronics Case Study

Daniel Toal, Edin Omerdic, James Riordan and Seán Nolan

Acronyms and Abbreviations:

AUV Autonomous Underwater Vehicle

DGPS Differential Global Positioning System

DOF Degree of Freedom

DVL Doppler Velocity Log

GAPS Ultra Short Base Line made by ixSea

GPS Global Positioning System

GUI Graphical User Interface

HT Horizontal Thruster(s)

INS Inertial Navigation System

INSS Irish National Seabed Survey

LLC Low Level Controller(s)

MMRRC Mobile and Marine Robotics Research Centre

MPPT Ring Multi-Purpose Platform Technologies for Subsea Operations

NI-PSP National Instruments Publish-Subscribe Protocol

PHINS Inertial Navigation System made by ixSea

RIO Reconfigurable Input-Output

ROV Remotely Operated Vehicle

SVP Sound Velocity Probe

UDP User Datagram Protocol

USBL Ultra Short Base Line

UUV Unmanned Underwater Vehicle

VR Virtual Reality

VT Vertical Thruster(s)

VUL Virtual Underwater Laboratory

University of Limerick, Ireland

104 D. Toal, E. Omerdic, J. Riordan, and S. Nolan

7.1 Introduction

This chapter details the development of a novel, multi-mode operation marine

robotics vehicle designed using mechatronic principles for operational flexibility

in high-resolution near-seabed surveys from shallow inshore waters out to the

continental shelf edge. The vehicle can be operated in surface-tow mode or as a

thrusted pontoon. With the buoyancy module released, the vehicle becomes

neutrally buoyant and is operated as a survey class remotely operated vehicle

(ROV) depth rated to 1,000 m.

Special features of the system include:

• deployment interoperability for small inshore boats and larger research vessel;

• fault tolerant thruster control;

• novel high frequency short range sonar;

• onboard computer control enabling real-time disturbance reaction;

• topside augmented reality system support.

The background which motivated the vehicle development is presented,

including the parallel development of vehicle models and simulations with

concurrent physical vehicle design construction and testing. The associated

development of the virtual reality/augmented reality instantiation in tandem with

the physical vehicle design and construction has had many benefits for the overall

system design, optimisation, flexibility and robustness.

Based on the experience gained in previous projects, the challenges met, the

solutions developed and the inherently high costs associated with marine

technology and offshore operations [1–4], work started on the design and

development of a flexible multi-mode of operation survey class thrusted

pontoon/ROV as part of the MPPT Ring project [5]. This is aimed at developing

multi-purpose platform technologies for subsea operations, which will in turn

serve as a generic solution in addressing the following challenges:

• easy integration of survey equipment, efficient planning and mission

simulation;

• training of ROV pilots;

• fault-tolerant control of ROVs;

• enhanced operator environment and survey tools during mission execution;

• offline analysis of acquired data.

The MPPT Ring (see Figure 7.1) is implemented as a hardware/software

platform and web service to enable product demonstration both offline (e.g., in a

lab or trade show) and online (for run-time survey application).

This duality of operations opens new frontiers for the applications of modern

control, modelling and simulation tools in marine technology development. It

provides a framework for researchers to develop, implement and test advanced

control algorithms in a simulated virtual environment under conditions very

similar to the real-world environment. Recent advances in computer technology

Multi-mode Operations Marine Robotic Vehicle – a Mechatronics Case Study 105

have made it possible to perform data acquisition, processing, analysis and

visualisation with high speeds previously considered infeasible. A review of

activities in the field of hardware-in-the-loop simulators is given in Ridao et al.

[6].

Fig. 7.1 MPPT Ring

7.2 MPPT Ring System Overview

7.2.1 Main Features

The main features of the MPPT Ring are:

• signal-level compatibility between simulated and real-world environments;

• 3D real-time visualisation of navigation data;

• real-time vessel and sonar simulators;

• advanced, flexible fault-tolerant control system with autotuning capabilities;

• open architecture for rapid control prototyping and hardware-in-the-loop

development;

• set of aiding tools for ROV pilot;

• survey planning and operation tool.

Bradley D., Russell D.W. (eds.) Mechatronics in Action: Case Studies in Mechatronics - Applications and Education

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