IN DEGREE PROJECT MECHANICAL ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2016

Electronic throttle control in a small two-stroke engine

FREDDI HAATAJA

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

Electronic throttle control in a small two-strokeengine

Degree Project in Mechatronics, Second Cycle

by

Freddi Haataja

Master of Science Thesis MMK 2016:131 MDA 533KTH Industrial Engineering and Management

Machine DesignSE-100 44 STOCKHOLM

.Examensarbete MMK 2016:131 MDA 533

Electronic throttle control in a small two-stroke engineFreddi Haataja

Approved: Examiner: Supervisor:

2016-06-20 Lei Feng Baha Alhaj HasanCommisioner: Contact Person:

- Henrik Eklund.

Abstract.

This thesis covers the design and prototyping of an electronic throttle control(ETC) system for a small two-stroke engine. The thesis work is being realizedin collaboration between a cooperating company and KTH. The throttle controlsystem is verified with a physical combustion engine.

The development process of the control systems is being done mostly in-house inthe cooperating company. The company has developed electronic control of fueland ignition system. In addition to the ETC throttle control system this thesiswork also develops a control system of the angular velocity of the combustionengine by controlling the intake air to the small two-stroke engine.

Today the air supply to the engine is controlled by a valve mechanically con-nected to the trigger/gas pedal operated by the user. The focus of the thesis isto investigate the advantages and drawbacks of implementing an electronicallycontrolled air supply to a small two-stroke combustion engine.

This master thesis discuss the detailed system design requirement of an ETCsystem for a small two-stroke engine. The result of this master thesis showsthat it is possible to have velocity control on a small two-stroke engine, alsominor improvements in acceleration of the small combustion engine is shownwhen using the ETC system.

Keywords

Electric throttle control, electric throttle body, throttle positioning sensor, in-ternal combustion engine, decoder.

.Examensarbete MMK 2016:131 MDA 533Elektronisk styrning av gasspjall i en liten

tvataktsmotorFreddi Haataja

Godkant: Examinator: Handledare:

2016-06-20 Lei Feng Baha Alhaj HasanUppdragsgivare: Kontaktperson:

- Henrik Eklund.

Sammanfattning.

Detta examensarbete omfattar design och prototypbygge av ett elektroniskt gas-reglagesystem (ETC) for en liten tvataktsmotor. Examensarbetet utfors som ettsammarbete mellan ett sammarbetande foretag och KTH. Gasreglagesystemetverifieras med en fysisk forbranningsmotor.

Utvecklingsprosessen av styrsystemet bedrivs mestadels internt hos det sam-marbetande foretaget. Foretaget har utvecklat elektronisk styrning av bransleoch tandsystem. Utover det elektroniska gasreglagesystemet har detta examen-sarbete ocksa behandlat utvecklingen av ett styrsystem av vinkelhastigheten hosforbranningsmotorn genom att styra insugsluften till den lilla tvataktsmotorn.

Idag styrs lufttillforseln till forbranningsmotorn av ett spjall som ar mekanisktansluten till avtryckaren som manovreras av anvandaren. Fokus for examen-sarbetet ligger i att undersoka for- och nackdelar med att implementera ettelektroniskt styrt luftspjall till en liten tva-taktsmotor.

Examensarbetet behandlar det detaljerade designkravet hos ett ETC systemfor en liten tvataktsmotor. Resultatet av detta examensarbete visar att det armojligt att ha hastighetsstyrning pa en liten tvataktsmotor, aven sma forbattringari acceleration pavisas nar ETC systemet anvands.

Nyckelord

Elektroniskt styrt gasspjall, elektronik forgasarkropp, gasspjallsgivare, forbranningsmotor,decoder.

Foreword

First and foremost i would like to thank my company supervisor Henrik Eklundfor his support through this project. With interesting ideas and constructivecriticism Henrik have helped me in my progress of this master thesis. Secondly Iwould like to thank Baha Hasan for being my university supervisor who alwaysanswers my most confusing questions.

I would also like to thank the cooperating company for letting me do my masterthesis with them. I would like to give special thanks to Fredrik Hellquist whohas contributed with the ability to do tests and measurements of the combustionengine.

I would like to thank Per von Wowern and Nora Faredal for all the extraordinarysupport they contributed with during the most challenging moments of the thesiswork.

Glossary

Table 1: GlossaryAbbreviation Explanation

DC Direct CurrentETC Electric Throttle ControlETB Electric Throttle BodyFreescale FRDM-KL25Z Development board with an ARM core microcontroller chipHW HardwareKTH Kungliga Tekniska Hogskolan, Royal Institute of TechnologyICE Internal Combustion EnginePI Proportional Integral controllerPID Proportional Integral Derivative controllerppr Pulses Per RevolutionPWM Pulse Width ModulationSPI Serial Peripheral InterfaceSW SoftwareTPS Throttle Positioning SensorWOT Wide Open Throttle

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Contents

1 Introduction 51.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2 Problem description . . . . . . . . . . . . . . . . . . . . . . . . . 61.3 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.5 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.6 Verification and Validation . . . . . . . . . . . . . . . . . . . . . 81.7 Research method . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Frame-of-reference 92.1 Dynamics of the combustion engine . . . . . . . . . . . . . . . . . 92.2 The shape of the throttle plate . . . . . . . . . . . . . . . . . . . 92.3 Dynamics of the throttle actuator . . . . . . . . . . . . . . . . . . 102.4 Sampling period . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.5 Resolution of the TPS . . . . . . . . . . . . . . . . . . . . . . . . 112.6 PWM frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.7 Encoder types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.8 DC motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.9 Fuzzy logic controller . . . . . . . . . . . . . . . . . . . . . . . . . 122.10 Anti-windup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.11 Unintended acceleration . . . . . . . . . . . . . . . . . . . . . . . 132.12 ETC system setup example . . . . . . . . . . . . . . . . . . . . . 132.13 Select throttle gain . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3 Implementation 153.1 Stakeholder requirements . . . . . . . . . . . . . . . . . . . . . . 153.2 Detailed subsystem requirements . . . . . . . . . . . . . . . . . . 163.3 Testing the combustion engine . . . . . . . . . . . . . . . . . . . 163.4 Torque test on the throttle . . . . . . . . . . . . . . . . . . . . . 183.5 DC-motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.6 Encoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.7 Decoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.8 H-bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.9 A potentiometer as trigger/gas pedal . . . . . . . . . . . . . . . . 213.10 Development platform . . . . . . . . . . . . . . . . . . . . . . . . 213.11 External electrical components . . . . . . . . . . . . . . . . . . . 223.12 The electronic subsystem . . . . . . . . . . . . . . . . . . . . . . 22

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3.13 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.13.1 Software of the throttle controller . . . . . . . . . . . . . . 243.13.2 Software of the velocity controller . . . . . . . . . . . . . 24

4 Results 264.1 Controlling the throttle . . . . . . . . . . . . . . . . . . . . . . . 264.2 Throttle controller integrated into the combustion engine . . . . 284.3 Controlling the velocity of the combustion engine . . . . . . . . . 30

5 Discussion and conclusion 365.1 Performance of the throttle controller . . . . . . . . . . . . . . . 365.2 Performance of the velocity controller . . . . . . . . . . . . . . . 365.3 Back to the research question . . . . . . . . . . . . . . . . . . . . 36

6 Future work 38

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List of Figures

1.1 A product development process Ulrich and Eppinger [3]. . . . . . 8

2.1 Principal sketch of a throttle mechanism [9]. . . . . . . . . . . . . 102.2 Sampling a step response [4]. . . . . . . . . . . . . . . . . . . . . 112.3 Voltage and current curves of a PWM signal [4]. . . . . . . . . . 122.4 Block diagram of a ETC system [11]. . . . . . . . . . . . . . . . . 13

3.1 Traditional throttle used to manually control the engine speed. . 173.2 Mechanical connection to the throttle. . . . . . . . . . . . . . . . 193.3 The LS7366 counter click [14]. . . . . . . . . . . . . . . . . . . . . 203.4 Infineon H-bridge development board [8]. . . . . . . . . . . . . . 203.5 The trigger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.6 The Freescale FRDM-KL25Z microcontroller. . . . . . . . . . . . 223.7 The electronic subsystem of the ETC. . . . . . . . . . . . . . . . 233.8 Block diagram of the throttle control system. . . . . . . . . . . . 243.9 Block diagram of the velocity control system. . . . . . . . . . . . 25

4.1 Angle control of the throttle angle. . . . . . . . . . . . . . . . . . 274.2 Step response from the throttle control system. . . . . . . . . . . 284.3 Electric throttle control. . . . . . . . . . . . . . . . . . . . . . . . 294.4 Step response from the ETC and the combustion engine. . . . . . 294.5 Velocity control system. . . . . . . . . . . . . . . . . . . . . . . . 304.6 Filtered velocity signal. . . . . . . . . . . . . . . . . . . . . . . . 314.7 Filtered and transformed velocity signal. . . . . . . . . . . . . . . 324.8 Starting sequence of the combustion engine. . . . . . . . . . . . . 334.9 Stop sequence of the combustion engine. . . . . . . . . . . . . . . 344.10 Increasing and decreasing the throttle angle in small steps. . . . 35

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Chapter 1

Introduction

The introduction chapter gives the reader a description of the thesis and themain reasons to why the thesis is started.

1.1 Background

The cooperating company is a Swedish company that is manufacturing outdoorproducts. Development of handheld tools powered by small two-stroke combus-tion engines is done within the product category Handheld. The developmentprocess of the control systems is being done mostly in-house in the cooperatingcompany. The company has developed electronic control of fuel and ignitionsystem. This thesis work will develop a control system for the intake air to asmall two-stroke engine.

Today the air supply to the engine is controlled by a valve mechanically con-nected to the trigger/gas pedal operated by the user. The focus of the thesis isto investigate the advantages and drawbacks of implementing an electronicallycontrolled air supply to a small two-stroke combustion engine. During the thesiswork an air control system will be developed, a physical prototype be built andcode created for the embedded target. Also the air control system will be testedin the physical engine.

During the thesis the electronic control of the intake air will be done by con-trolling the throttle, so called electronic throttle control (ETC). The goal is tofind optimal control of the intake air during different operating conditions. Acontrol system needs to be constructed and an actuator, electric throttle body(ETB), will be built. The throttle control system is restricted to use only fewtypes of sensor data e.g. user trigger position (the gas pedal), engine speed andthrottle angle.

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1.2 Problem description

The main task of the thesis is to develop control algorithms for the air supplyand evaluate how these algorithms can improve the engine characteristics. Theelectronic control of the air valve could provide advantages such as better abilityto start the engine, smoother idling speed, limited maximum speed etc. Of par-ticular interest is to investigate transitions between different operating modes.Since the operating modes in a hand held two stroke engine is very differentfrom e.g. a car, it is likely that different (compared to a vehicle) air controlalgorithms must be used.

A two stroke engine of a handheld tool has normally a few discrete operatingmodes (no continuous change of operating conditions):

1. Start

2. Idle (valve almost completely closed).

3. Partly opened throttle without load.

4. Partly opened throttle with load.

5. Wide open throttle (WOT) without load.

6. WOT with load. (Since the valve is fully opened there is sharply restrictedpossibility to adjust the air amount)

A major challenge for the control algorithm will probably be to switch betweenthe operating modes. The research question is

Is there a control structure that can be used to sustain the smalltwo-stroke combustion engine as a stable system regardless of rapidchanges of operating conditions such as: workload, gas pedal position,start?

The difference between zero workload and full workload imply dramatic changesof the dynamics of the system to be controlled. All non-zero workload casesbring disturbances. It might be required to use a smart control structure orsome kind of mode-based control structure to be able to control a small two-stroke combustion engine. The combustion engine is behaving like differentdynamic systems depending on if the combustion engine is being used in loadconditions or not and if the load is varying.

Several design decisions need to be done when connecting a DC-motor as anactuator to the throttle such as:

• Should there be an additional position sensor for the throttle actuator?The existing throttle position sensor (TPS) updates the position with therevolution of the internal combustion engine (ICE).

• Should the spring connected to the throttle, used to close the throttle, stillbe mounted? If the current decreases in the engine (both due to intendedor unintended reasons) these springs would close the throttle limiting theairflow to the engine. The springs would apply toque in opposite directionwhen opening the throttle but apply torque in the same direction as the

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controller when closing causing slower response in one direction and annegative overshoot in the other direction [18].

• The system requires a control algorithm that considers both throttle angleand engine speed.

The purpose of the prototype is to test control algorithms for a small combustionengine. The ETB can be designed in various ways, e.g. integrated into the bodyof the two-stroke engine or even built onto the current carburetor.

1.3 Purpose

The purpose of this thesis will be designing and building a prototype of an ETCsystem and developing control algorithms for the throttle control. The actuatorwill be built by the student in cooperation with the company. The controlalgorithms will be developed by the student, supervised by one supervisor fromthe cooperating company and one supervisor from KTH.

Electronic throttle control contributes to extended control of the engine. Bettercontrol allows possibilities for modified performance of the engine in differentoperating conditions, but also additional features such as idle speed control.

1.4 Method

The actuator will be built in cooperation with the company. The main focus isdirected to the building process of the actuator and the software of the necessarycontrollers.

Algorithms that might be used are for example a normal feedback structure witha P, PI or PID controller, fuzzy controller, cascade controller or a mode basedcontroller. The study will be used to determine differences in performance pa-rameters between different control structures and compared to the mechanicallycontrolled throttle.

1.5 Limitations

The amount of control structures handled in this thesis is restricted to not morethan two control algorithms due to time limitations of the thesis work. Thespecific choice of control structures to handle is error feedback for the throttlecontrol system and a cascade structure for the velocity control system. Thethrottle control system is the one of these two structures that will be comparedwith the mechanically controlled throttle but both control structures will betested.

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1.6 Verification and Validation

The initial control algorithms will be tested in conditions where the combustionengine is not running to prevent possible hazardous events.

For verification the cooperating company would like to see a working prototypeaccording to the aim of this thesis. The ETC will be tested with a runningcombustion engine in a test cell.

The verification tests will be done in such manner that it shows if the systemfulfills the specified requirements of the system.

1.7 Research method

A qualitative study will be done to determine which control structure is bestsuited for the ETC. Several control structures will be tested. The study willbe used to determine differences in performance parameters between differentcontrol structures and also compared to the mechanically controlled throttle.In this way the control structures are developed in parallel in the design blocksand then tested according to Figure 1.1.

Figure 1.1: A product development process Ulrich and Eppinger [3].

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Chapter 2

Frame-of-reference

In this chapter the author analyzes the literature reviewed.

2.1 Dynamics of the combustion engine

The small two-stroke engine has high power/weight ratio and much less rota-tional inertia than many other engines. The rise time for a small two-strokeengine is therefore comparatively short. The rise time for a small two-strokeengine is less than the rise time for a private car [10]. Faster dynamic systemsusually require faster actuation. The rise time in this case is the time requiredfor an combustion engine to go from idle speed to maximum speed withoutexternal loads.

2.2 The shape of the throttle plate

The shape of the throttle plate is of great importance. The throttle plate couldbe unsymmetrical around the throttle plate axis as shown in Figure 2.1. Thethrottle plate is unsymmetrical around the throttle plate axis if the distance a isgreater than distance b in Figure 2.1. If the area of the throttle plate is biggeron one side of the throttle plate axis a torque will arise on the throttle plate.This is due to the airflow causing high pressure air pressing the throttle plateon one side and low pressure air on the back side of the throttle plate [9].

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Figure 2.1: Principal sketch of a throttle mechanism [9].

Also Figure 2.1 shows an example of how to mechanically connect an actuatorto a throttle.

2.3 Dynamics of the throttle actuator

In a case of a cascade control structure the the cascade control requires the innerloop to be 5 to 10 times faster than the outer loop [1]. In this thesis the innerloop is controlling the throttle and the outer loop is controlling the velocity.

2.4 Sampling period

The sampling period is of great importance for a controller to work properly.A too low sampling frequency could cause instability problems while a too highsampling frequency could increase the CPU usage in such extent that the controltasks risk to miss its deadlines.

The sampling period should be set so that the are 4 to 10 samples per rise time[4]. In Figure 2.2 the dark blue curve is an example step response and the greencurve is the sampled step response when using zero order hold. The graph on

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the left hand side with sufficient samples per rise time and the graph on theright hand side with few samples per rise time.

Figure 2.2: Sampling a step response [4].

2.5 Resolution of the TPS

Small changes of the throttle plate affect the combustion engine. One exampleof the resolution of the TPS can be seen in the U.S. Patent by Harrison [6]where the resolution in normal case is 1/16 degree for each step.

2.6 PWM frequency

When controlling an inductive load, for example a DC-motor, the PWM fre-quency should be kept high in order to keep leakage current low and to avoidnoise. The voltage, blue curve, and the current, red curve, of a PWM signalis depicted in Figure 2.3. High PWM frequency avoids the current to deviatefrom the momentary mean current value. Lower PWM frequencies could causeoscillations of the DC-motor shaft.

The PWM period of the actuator should be 10 times shorter than the electrictime constant of the DC-motor [4].

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Figure 2.3: Voltage and current curves of a PWM signal [4].

2.7 Encoder types

There are many types of rotary encoders: absolute encoders, incremental en-coders, magnetic encoders and optical encoders to name a few. A high resolutionmagnetic encoder have 32 pulses per revolution while a typical optical encoderhave more than 1000 pulses per revolution [2].

2.8 DC motors

The most common type of DC motors are the permanent magnet brushed DCmotors. These motors use permanent magnets to create the stator field whichprovides a constant magnetic field of the stator. This in turn makes the DCmotor to respond very quickly to voltage changes which makes the permanentmagnet DC motor feasible for control applications [15].

2.9 Fuzzy logic controller

Adaptive cruise control is a cruise control system with an additional featureof detecting potential collisions and if so automatically applying the brakes.Fuzzy logic is well suited to adaptive cruise control and thus engine control[16].A fuzzy logic controller uses linguistic variables and the output of a fuzzy logicdepends on more than one input. Adaptive controllers could be required toproperly control the small two-stroke engine.

2.10 Anti-windup

Often in control applications PI-controllers are used instead of P-controllers.When using PI-controllers avoiding integral windup is a common requirement.One common method for anti-windup is called Back-calculation and tracking[17].

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2.11 Unintended acceleration

When talking about controlling the velocity of an engine one often control thevelocity of a vehicle. Mixing with the throttle could result in unintended ac-celeration. When the combustion engine is accelerating without an accelerationcommand from the human user. One example of unintended acceleration is theToyota Unintended Acceleration case [11]. The case study of Koopman alsobrings up real time scheduling and how important it is that the ETC systemmeets its deadlines [11]. Missed deadline could result in failure.

2.12 ETC system setup example

A block diagram of the ETC system is shown in Figure 2.4. An accelerationcommand is read from the accelerator pedal. The acceleration command is usedas a reference value in the closed loop throttle controller. The throttle controllermay get sensor information from other sensors and also other subsystems, suchas the fuel injection and the ignition coil, may get information from the throttlecontroller.

Figure 2.4: Block diagram of a ETC system [11].

2.13 Select throttle gain

Another advantage of an ETC system is the ability to adjust the gain of thethrottle controller. This could give the system or the user the possibility toadjust the control parameters. For example changing the control parametersdepending on the weather conditions [13]. In eco mode the ETC could makethe movements of the throttle carefully while in sports mode the ETC could

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move the throttle more rapidly. Also the ETC could compensate for higheraltitudes where the air pressure is lower [13].

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Chapter 3

Implementation

This chapter brings up the system requirement analysis, the detailed design, theimplementation and the integration phase of the project.

3.1 Stakeholder requirements

1. Requirements for the throttle controller

(a) The ETC shall be able to open the throttle to full extent.

(b) During a WOT state the throttle shall not oscillate.

(c) The rise time of the throttle shall be as fast or faster than the tradi-tional throttle operated by a human.

2. Requirements for the velocity controller

(a) There shall be no risk for integral windup while using the velocitycontroller.

(b) The ETC should be able to operate in all angular velocities betweenidle speed and maximum speed of the combustion engine.

(c) When the trigger is pressed to its maximum position the ETC shallignore the velocity controller and instead open the throttle to fullextent.

Additional note: there are no restrictions on size and weight of the ETC systemduring the thesis work.

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3.2 Detailed subsystem requirements

1. Requirements for the ETC

(a) The rise time of the throttle should be 10 times faster than the risetime of the combustion engine.

(b) The PWM period to the actuator should be 10 times shorter thanthe electric time constant of the DC-motor.

(c) The sampling period of the ETC should be 10 times shorter than therise time of the throttle.

(d) The ETC should be able to detect at least a 1/16 of a degree ofchange in angle.

3.3 Testing the combustion engine

An initial test is being done in a test cell to collect information about thedynamics of the two-stroke engine. Information such as the idle speed, maximalspeed and rise time of the engine are measured.

As a system the combustion engine is considered as an input-output stablesystem. The throttle angle is considered as the input signal while the angularvelocity of the engine is considered as the output signal. As an input outputtest a step response is generated when the throttle is moved from fully closed tofully open as fast as practically possible with the traditional throttle system anda human operator representing a step. The throttle angle and the step responseis depicted in Figure 3.1.

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1 2 3 4 5 6 7

Time [S]

Am

plit

ude

Throttle anlge

Engine speed

Figure 3.1: Traditional throttle used to manually control the engine speed.

The rise time of the combustion engine is considered to be the time it takes forthe engine to go from 10 % above idle speed to reach 90 % of the maximumspeed during the acceleration process in the step response. The rise time couldbe defined with slightly different measuring intervals of the acceleration processbut the rise time is valid as a performance parameter as long as it is measuredin the same way for all test cases.

Every individual engine is different from another. In manufacturing, for someengine models, each engine is tuned individually with the idling screw to getthe idle speed in the desired interval [7]. The need of tuning the idling screw foreach engine indicates that it might be difficult to control the angular velocityof a combustion engine and indicates the importance of precision of the ETC.There are different ways to adjust the idle speed. Another way is to adjustthe idle speed with a digital ignition system and electrical fuel system. Oneof the reasons of why the cooperating company are interested in the ETC isthe possibility to control the air flow as well which give additional, improved,possibilities to control the combustion engine.

However, the ETC system is only tested on one and only one engine as this thesisis considered to be qualitative research. In this thesis, the author assumes thatthe results of integrating the ETC to other engines will affect the other engines in

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more or less the same way as the engine being tested on. For other combustionengine models the control parameters might, of course, have to be different.The combustion engine used in this thesis is a specific model chosen by thecooperating company.

3.4 Torque test on the throttle

Measurements on the traditional throttle gives the required torque to overcomethe forces from the springs. A torque test is performed on the opening mech-anism of the throttle. The purpose of the torque test is to find the torquerequired to open the throttle, the torque required to overcome friction forcesand the force from the springs connected to the throttle.

The torque test is done by applying variable mass on a well-defined distancefrom the axis of rotation of the throttle. The test is done on different distanceswith various mass. The mass is increased until equilibrium is achieved betweenthe applied torque and the torque from the springs.

The measured torque do have some variations between test cases and the max-imum difference is 2 mNm. The differences in measured torque are mainlycaused by static friction close to the state of equilibrium. An average torque iscalculated from the measured values. This average measured torque is used asa guideline on how strong the actuator should be.

3.5 DC-motor

There are various types of electrical motors that could suit as an actuator.Considering the requirement on the resolution of the position controller it islikely that, for example, a stepper motor would require complex software to beable to take the smallest steps needed. The electrical motor chosen in this thesiswork is a permanent magnet DC motor.

The size of the DC-motor are determined by the help of the required speedaccording to the cascade controller, the required torque according to the springconnected to the throttle and the required gear ratio in order to increase reso-lution of the encoded throttle angle.

Without any restrictions on size of the actuator it is fairly easy to find a suitableDC-motor on the market. Adapting to potential requirements in size could forceone to change the spring connected to the throttle to a weaker one. In this thesisthe original spring that is connected to the throttle is used.

3.6 Encoder

A high resolution incremental optical encoder with 5000 pulses per revolution(ppr) is used to measure the throttle angle. The angular velocity of the combus-tion engine is very sensitive to the throttle angle [7]. To be able to control the

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combustion engine speed the actuator needs to be really accurate and thereforethe encoder needs to be accurate enough to detect even the smallest changes ofthe throttle angle, e.g. 1/16 degree as a TPS used in the case of Harrison et al.[6].

The encoder is connected to the DC motor shaft. The encoder is placed on theleft hand side of the gear while the throttle is place on the right hand side ofthe gear according to Figure 3.2 giving 5.3 times larger angular change of theencoder shaft than the angular change of the throttle shaft which increases theangular resolution by 5.3 times (exact 69/13). Placing the encoder on the DCmotor shaft gives a resolution of 26500 pulses per revolution (approximately26538.46 ppr) which fulfils requirement 1d.

There are different types of encoding, e.g. X4 encoding if one are countingboth rising and falling edges. However, only the X1 encoding are being used inthe current setup of the implementation leaving possibilities to further increaseresolution if needed. This is considered as a safety margin for resolution.

Figure 3.2: Mechanical connection to the throttle.

Placing the encoder on the other side of the gear, relative to the throttle, alsohas its cons. The backlash in the gear might introduce an angular error. Adifference of the angle after the gear ratio is considered. However, the throttleis connected to a spring that always affect the throttle to constantly have arelatively small torque in the closing direction. The maximal spring torque ismuch smaller than the maximal torque from the actuator. The actuator canfully control the throttle and the spring torque lowers the error of the backlashin the gear. The error due to the backlash in the gear is therefore assumed tobe low and in this thesis assumed to be negligible.

3.7 Decoder

There is a risk that the high resolution encoder generates a pulse train withhigher frequency than the microcontroller can handle. Therefore the decodingof the signals are being done by an external counter. The chosen counter isa LS7366 which has a working frequency of 40 MHz. The signals are beingdecoded, counted and sent by serial communication, serial peripheral interface(SPI), to the microcontroller. The decoder is ordered already mounted on adevelopment board named Counter click, manufactured by Mikro Elektronika[14] . The Decoder is shown in Figure 3.3.

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Figure 3.3: The LS7366 counter click [14].

3.8 H-bridge

In this thesis an H-bridge is used as a power amplifier when controlling theDC-motor. There are restrictions in both voltage and current when choosingan H-bridge. The H-bridge should be able to deliver enough power to the DC-motor. Also the PWM period should be at least 10 times shorter than theelectric time constant of the DC-motor [4].

These restrictions leave quite few alternatives when choosing an H-bridge. In theETC system the DC-motor is connected to two half bridges named BTN8982TA.The half bridge can handle high enough voltage and current for this specificapplication and a switching frequency of 30 kHz [8].

The two half bridges combined are used as one full H-bridge that handles a PWMsignal with a PWM period 9.3 times faster than the electric time constant ofthe DC-motor. The H-bridge is mounted onto a development board by themanufacturer and the H-bridge board is depicted in Figure 3.4.

Figure 3.4: Infineon H-bridge development board [8].

The microcontroller creates the PWM signal by using a timer that counts eachtick of the target board itself. As the PWM frequency can be 30 kHz [8], the

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PWM period is set to be 33 microseconds long. With a clock frequency of 48MHz this gives 1584 ticks during the PWM period, which in turn leaves 1584different voltage levels from the H-bridge. This means that the DC-motor canbe actuated with 1584 different speed levels (if the static friction is zero).

In order to actuate on high resolution of the throttle angle also high quantizationlevel of the voltage is required. This is why the software allows the H-bridge tohave so many voltage levels. The quantization level of the voltage is consideredto be high, but the required quantization level is not known for this system.

3.9 A potentiometer as trigger/gas pedal

The trigger is a button that is used as a gas pedal. The trigger contains apotentiometer. A potentiometer that outputs a linear value from low to highdepending on how much the trigger is pressed. The trigger is shown in Figure3.5.

Figure 3.5: The trigger.

The signal from the trigger passes a low pass filter right before it is receivedat the analog input pin on the controller board where the low pass filter isused to eliminate high frequency noise. Without the low pass filter, the triggervalue is deviating from its original value and could lead the noise further via thecontroller to the position of the actuator. The components of the low pass filterare chosen to create as low cut off frequency as possible without restricting thedesired behaviour of the throttle control system.

3.10 Development platform

The development board used in this thesis work is a Freescale FRDM-KL25Zmicrocontroller. The development board is chosen by the cooperating company.The Freescale board is shown in Figure 3.6. The KL25Z microcontroller has aclock frequency of 48MHz.

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Figure 3.6: The Freescale FRDM-KL25Z microcontroller.

The control loop is programmed to sample with a constant frequency. Thesampling frequency should be chosen in relation to the fastest dynamics in theclosed loop characteristics of the feedback [4].

3.11 External electrical components

The microcontroller cannot handle all tasks that could be given to it. Oneexternal signal inverter and a logic level converter board are also added to theelectronic subsystem.

The signal inverter inverts a PWM signal created from the microcontroller.Both the inverted and the original PWM signals are used to control the H-bridge that drives the DC-motor. A logic level converter circuit is used toconvert the SPI channels between the microcontroller and the decoder. Theseexternal components along with the rest of the electronic subsystem is shownin Figure 3.7. The electronic subsystem board runs on both 5 volt and 3.3 volt.Decoupling capacitors are added on both the 5 volt side and the 3.3 volt sidefor a more robust electrical environment.

Using the trigger to generate a reference value makes it difficult to producerepeatable tests and evaluating the performance of the throttle control system.Therefore a button is added that generates a step in reference value when it ispressed.

3.12 The electronic subsystem

Most of the electronic components are mounted onto the same board. The boarditself is designed and made by the author. The electronic subsystem is shownin Figure 3.7.

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Figure 3.7: The electronic subsystem of the ETC.

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3.13 Software

The ETC is used as two different systems. The first system is a throttle anglecontroller giving the same feature as the traditional throttle. The second systemis an angular velocity controller of the combustion engine.

3.13.1 Software of the throttle controller

The throttle angle control system samples the throttle angle in a feedback loopand compares it to the reference angle that is being read from the trigger. Thedifference between the signals are used to create a control signal for the actuatorwhich is depicted in Figure 3.8. The throttle controller is a P controller tunedto achieve the subsystem requirement 1a.

Figure 3.8: Block diagram of the throttle control system.

The throttle control system is simulated in Matlab/Simulink including a modelfor the actuator. The moment of inertia of the throttle is calculated and includedinto the Simulink model. Also a spring force and a friction model are includedinto the model. In the simulated environment the throttle controller is tuned.The throttle controller is also tuned in reality and the parameters from thesimulation are used as guidelines.

3.13.2 Software of the velocity controller

The angular velocity control system samples the velocity of the combustionengine as a feedback signal. A reference velocity is being read from the triggerand the difference between the reference velocity and the actual velocity is usedto determine a signal to control the velocity. The velocity control signal is,together with the current throttle angle, used to determine the angle controlsignal for the throttle. One can recognize the throttle control system workingas an inner loop when the velocity control system is considered as a cascadecontrol structure. The control structure of the velocity controller is depicted inFigure 3.9. The throttle controller uses exactly the same parameters as when itis tuned in the throttle control system.

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Figure 3.9: Block diagram of the velocity control system.

To fulfil the stakeholder requirement 2a one could use anti-windup technique forthe integrating part of e.g. a PI or PID controller. However, in the test cases ofthis thesis the controllers used are P-controllers (one as the throttle controllerand one as the velocity controller) and thereby the stakeholder requirement 2ais fulfilled.

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Chapter 4

Results

The results chapter presents the outcome from tests on the ETC system itselfand from tests where the ETC is used as a subsystem of the two-stroke engine.

4.1 Controlling the throttle

During testing, the ETC is connected to the trigger and the throttle. An inputsignal is registered from the trigger as a reference value and the actual angle isregistered from the encoder. The result is presented in Figure 4.1.

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1 2 3 4

Time [S]

Am

plit

ude

Reference angle

Actual angle

Figure 4.1: Angle control of the throttle angle.

In Figure 4.1 the trigger is pressed, which is shown as the blue curve, and thethrottle, the red curve, is moving accordingly. The throttle control system worksas intended. The trigger is pressed to its maximum position and the throttle isopening to its full extent, thereby the stakeholder requirement 1a is fulfilled.

A step is generated in the reference value to find the rise time of the electricalthrottle. The rise time of the step reference angle is zero as the reference valuemoves from low to high in one single loop iteration. The step response of thethrottle angle is shown as the red curve. The results of the step is depicted inFigure 4.2.

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1 2 3 4 5

Time [S]

Am

plit

ude

Reference angle

Actual angle

Figure 4.2: Step response from the throttle control system.

4.2 Throttle controller integrated into the com-bustion engine

In this section the ETC is connected with the throttle body. The throttlebody is installed in a small two-stroke engine. The small two-stroke engine isproperly connected to a water brake in a test cell. However, the water brake isnot activated and the braking torque is zero.

Integrating the electric throttle control with the two-stroke engine does work.The throttle is moving when the trigger is pressed. The movement of the throttleaffects the air flow to the combustion engine. When the throttle engine increasesalso the engine speed increases. The results are shown in Figure 4.3 where thetrigger value is shown as the blue reference angle, the throttle angle is shown inred as the actual angle and the engine speed is shown with a yellow color.

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1 4 8 12

Time [S]

Am

plit

ude

Reference angle

Actual angle

Engine speed

Figure 4.3: Electric throttle control.

Another step in reference is generated, but now with the ETC integrated intothe combustion engine and the results are depicted in Figure 4.4.

1 3 5

Time [S]

Am

plit

ude

Reference angle

Actual angle

Engine speed

Figure 4.4: Step response from the ETC and the combustion engine.

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4.3 Controlling the velocity of the combustionengine

The velocity controller is deployed in the microcontroller, the combustion engineis running with the ETC system integrated and the results are depicted in Figure4.5. The trigger is read as the reference speed. When the trigger is pressed theengine speed increases according to the requested speed signal. The throttleangle is not opening depending on the reference speed but depending on thedifference between the reference speed and the engine speed. The throttle isactuating the combustion engine so that the engine speed changes according tothe reference speed. This is the function of the velocity controller which is alsoshown in 4.5.

1 8 16

Time [S]

Am

plit

ude

Reference angle

Throttle angle

Engine speed

Figure 4.5: Velocity control system.

The engine speed in Figure 4.5 looks noisy due to big cycle to cycle variations.The engine speed is increasing rapidly in case of a powerful power stroke. Theengine speed signal is filtered with a low pass filter and the result is shown inFigure 4.6.

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1 8 16

Time [S]

Am

plit

ude

Reference speed

Throttle angle

Engine speed

Filtered engine speed

Figure 4.6: Filtered velocity signal.

In Figure 4.6 it is clear how the low pass filter is affecting the original enginespeed. Eventual phase delays caused by the filtering process is taken into ac-count and the filtered signal shows a clearer picture of the engine speed.

The filtered engine speed in Figure 4.6 is multiplied by a scale factor and pre-sented as a transformed version of the engine speed in Figure 4.7.

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13 15 17 19

Time [S]

Am

plit

ude

Reference speed

Throttle angle

Engine speed

Filtered and transformed engine speed

Figure 4.7: Filtered and transformed velocity signal.

Further more, in Figure 4.7 the filtered engine speed is scaled from its originalamplitude to a more suitable amplitude when comparing the reference speedwith the engine speed. Also the scaled engine speed is moved in amplitude tocompensate for the idle speed of the combustion engine which otherwise lifts thesignal to a level above zero. Figure 4.7 shows a clear indication that the velocitycontrol system works as intended as the ’filtered and transformed engine speed’follows the ’reference speed’.

A starting sequence of the combustion engine, with the velocity controller inte-grated, is shown in Figure 4.8.

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1 4 8 12

Time [S]

Am

plit

ude

Throttle angle

Engine speed

Figure 4.8: Starting sequence of the combustion engine.

Starting the combustion engine when it is integrated with the velocity controllercould be problematic without additional software. The throttle could move toan inappropriate angle as soon as the ETC gets power. One could think ofadditional software that handles the starting sequence. For example software toconstantly keep a small throttle angle while starting.

However, extra software is not needed for the combustion engine to start andan interesting behaviour can be seen in Figure 4.8. The throttle is closing assoon as the engine speed is rising. This shows that the the velocity controllerbehaves as expected.

The stop sequence of the combustion engine integrated with the velocity con-troller is shown in Figure 4.9.

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1 2 3 4

Time [S]

Am

plit

ude

Throttle angle

Engine speed

Figure 4.9: Stop sequence of the combustion engine.

During the stop sequence in Figure 4.9 the combustion engine loses its abilityto ignite the fuel and the engine speed decreases. Also during the stop sequencethe throttle angle increases due to the velocity controller which also indicatesthat the velocity controller behaves as expected.

In the last test the throttle angle is opened 5 degrees at a time, step by step,causing almost a ramp but with the shape of a staircase. The throttle reachesthe wide open throttle position and starts closing. The closing process is doneby steps of 10 degrees at a time. The throttle angle and engine speed is recorded.The results are presented in Figure 4.10.

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30 50 70 90

Time [S]

Am

plit

ude

Throttle angle

Engine speed

Figure 4.10: Increasing and decreasing the throttle angle in small steps.

As a response to a ramp of the throttle the engine speed should increase steady.Instead the engine speed reaches a medium speed quite easily and requires15 degrees of opening of the throttle before the engine speed can continue toincrease. This is a special behaviour of the engine that before this thesis workwas difficult to show. Now, with a controllable throttle these kind of tests arepossible to perform.

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Chapter 5

Discussion and conclusion

This chapter discusses the results presented in chapter 4.

5.1 Performance of the throttle controller

Figure 4.3 clearly shows that the engine speed is affected by the throttle controlsystem. The throttle control system of the ETC is working which is a bigmilestone in this thesis work.

From Figure 4.4 the rise time of the combustion engine is measured. The resultof the measurement might be difficult to predict but more obvious when proven,the rise time of the combustion engine is increased by a few milliseconds com-pared to the same combustion engine rigged with the traditional throttle. Eventhe smallest increases of acceleration of a combustion engine is a huge finding.

5.2 Performance of the velocity controller

The second milestone of this thesis is to get the velocity controller working. InFigure 4.5 it is possible to see a correlation between the reference speed and theengine speed. The velocity controller seems to work as intended as the enginespeed responds as expected to the reference speed when the combustion engineis running.

It is important to note that the static error between the reference speed andthe engine speed does exist but cannot be distinguished in Figure 4.7 becauseof the transformation.

5.3 Back to the research question

When the ETC is integrated with the combustion engine the two specified setupsof the ETC system in this thesis are both working setups. There should be

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setups that do work better or worse but the research question can now partlybe answered. A cascaded control structure can be used to control the velocityof a small two-stroke engine.

This thesis has developed a hardware platform where it is possible to further testother software setups of the ETC system, therefore this thesis has contributedwith a huge step in the right direction with the practical work and collectedinformation to two specific setups of the ETC for a small two-stroke engine.

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Chapter 6

Future work

Continuing the work can be done by testing and analyzing other setups ofboth controller and control structures. The P controllers used in this thesisare sufficient to keep the combustion engine as a stable system but one couldfor example change the P controller in the throttle controller to a PI or PIDcontroller. One could also change the whole control structure to a controllerbased on fuzzy logic or trajectory planner techniques.

The parts of the signals seen in Figure 4.5 that looks like noise is not noisebut cycle to cycle variations. The signal has its shape due to speed variationsbetween different cycles of the combustion engine and the throttle moves ac-cordingly in order to control the engine speed. In future work one could includean investigation on how the behaviour of the ETC system could be improvedwhether the engine speed signal was treated with signal treatment methodsbefore it is used in the controller of the ETC system.

During this thesis work the engine with an installed ETC was not tested incondition with an external load on the engine. This kind of test cases can bedone in future work. However, external load on the combustion engine wouldnot directly hinder the throttle from moving as the ETC is strong enough toover come eventual changes of the air flow through the throttle body. This canbe seen in Figure 4.3 as the static error of the throttle is not visible affected bythe air flow to the combustion engine. If the throttle would be distinguishablyaffected by the air flow the static error between the reference angle and thethrottle angle would be affected.

Furthermore in future work one could also decrease the size of the ETC system ifone would like to permanently include the ETC in the small two stroke engine. Ifone would like to permanently include the ETC in the combustion engine also aninvestigation whether the benefits of the ETC weights against the disadvantagescould be done.

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