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Embedded Control of SolidState Lighting Systems Arjuna Madanayake, Post Doctoral Associate Ganesh Doluweera, Instructor and PhD Candidate Electrical and Computer Engineering Schulich School of Engineering University of Calgary Overview • • • • • Introduction Solid State Lighting Some fundamentals Basic embedded systems Putting it all together The Real-World Engineering Problem: Lighting Approximately 1.6 billion people, 1/4 of the world’s population, have no access to electricity and the vast majority live in rural areas in developing countries (IEA,2004). • Home lighting is often associated with electricity • Those who have no access to electricity, rely on fuel based lighting sources • Eg.: Kerosene wick lamps, candles etc. Typical environment inside a non-electrified home after dark. • Kerosene lamps are by far the most common fuel based lighting source 3 Smoke and fumes from wick lamps are a serious health concern. Typical kerosene wick lamp, in a village called “Somlavari Thanda” in Andhra Pradesh, India 14th July 2004 Photo credit: Light Up The World (LUTW), www.lutw.org 4 Homemade Kerosene Wick Lamp Leye County Village, Guangxi (China, 2005) Photo credit: LUTW 5 An unsafe way for achieving portable light Accidental upset of kerosene lamps cause fires and burn injuries Kerosene Bulb Lamp and Bamboo Torch Knuckles Range, Sri Lanka - June 2002 Photo credit: LUTW 6 Kerosene Lamps Expensive Even with government subsidies kerosene consumes 10% to 25 % of a villager’s annual income. ($2-$15 per month) Low Levels of Illumination Reading with kerosene lamps, besides being unhealthy, is a difficult task and creates a significant barrier to education. Inefficient The efficacy of a typical kerosene is 0.02 lm/W – 1/500 of an incandescent lamp Scarcity and remoteness of suppliers The burden of physically acquiring kerosene keeps people away from educational and productive activities. 7 Environmental and Health Impact of Kerosene Lamps VOC Volatile Organic Compounds • Eye infections • Respiratory illnesses NOx, SOx Nitrogen and Sulphur Oxides • Carcinogenic • Lung and eye infections • Respiratory illnesses CO2 Carbon Dioxide • Global warming • CO2 emission from fuel based lighting 244 tCO2/year (3.25 % of global CO2 emission). CO Carbon Monoxide • Lethal (Mills, 2002) 8 Introduction to WLED Lighting • What is a “Solid State Lamp” ? • What is a White Light Emitting Diode (WLED) ? • Why are WLEDs a big deal ? Solid State Lighting • Solid state lighting (SSL) use white light emitting diodes (WLED) • Invented in late 1990s • WLEDs have gone through a dramatic progress since then Different types of WLEDs 10 Properties of WLEDs (White Light Emitting Diodes) • Low Power DC operation – 0.1W - 5W • Efficient – present state of the art 80 - 100 lm/W • Long Life (50,000 + hrs) – virtually a one time capital investment. No replacement bulbs or components. • Physically Robust – LEDs can withstand severe shock and vibration. • Reliable & Safe Lighting – LEDs are very much safer and cheaper than kerosene. • Environmentally Friendly – WLEDs, unlike fluorescent tubes, do not contain mercury, are safer to manufacture and free from end-of-life problems. LEDs also reduce the amount of wood used for lighting. 11 Solid State Lighting - Trends Manufacturers’ Expectations State of the Art 80-100 lm/W 12 SSL Home Lighting Systems • A typical rural house can be illuminated to an adequate level using 2-3 WLED and 5W of electrical generating capacity • Needs energy generation & storage systems ~5W • Due to the DC operation - preferred choice for battery operated systems with RE based home lighting systems 13 Need for Embedded Control of SSL Systems • By “embedded controls” we mean a tiny computer and program that ensures the correct operation of the SSL lighting system: 1. To drive the WLEDs at required voltage and current. 2. To control energy flow to- and –from the energy storage device. 3. To controlling the rate of consumption of scarce energy by methods such as lamp dimming and automatic shutoff when the batteries are low. White Light Emitting Diodes (WLEDs) are awesome ! • Is a promising solution to lighting requirements of the future • Great for solar and wind power applications • Lasts way longer than incandescent bulbs and CFLs •Very rugged and unbreakable (you can break it if you try hard enough..) Some facts about WLEDs • White light, great for reading and studying • Uses a blue light emitting diode (LED) with a special chemical that converts blue light to white light • Needs about 1 Watt (W) of power • 1W = 3.6 V * 350 mA using P=VI • To get 1W of power consumption, we make sure that the WLED draws about 350 mA of direct current (DC) and that the voltage drop across its terminals is about 3.6 Volts. Blue light Yellow light from phosphor From the datasheet of Luxeon III WLED From Philips Lumileds. How white light is produced from blue How do you measure the light output from a white LED ? • The SI unit for luminous flux (light) is called “Lumen”. • The device for measuring the amount of flux from a lamp such as a WLED is called an Integrating Sphere. • The “Integrating sphere” collects the light from the lamp and measures it using a sensor called a “photopic”. • Typically, the measured Lumens is available on a digital display. Photo: Advanced Solid State Lighting Laboratory at the Univ. of Calgary Voltage-Current Behavior of a WLED • WLEDs are very similar to Current LEDs or diodes when I (mA) considering IV-curves. • A certain minimum voltage is required before current starts to pass through a WLED – this is called the Threshold Voltage • For input voltages greater than the threshold, the current starts to rapidly increase (exponentially) 350 mA 1W Point VThtreshold 3.6 Terminal Voltage Volts (V) Energy Sources For WLED Lamps • • • • Energy is scarce and expensive Solar energy, pico hydro, pedal power WLEDs lead to efficient lighting WLEDs can get “More bang for your bucks” • But WLEDs are not cheap. But they will be in the future! • Huge market opportunities Energy storage devices • Storing electrical energy is a very big problem • A typical way to store energy is to use a rechargeable battery • Batteries can be Lead-acid (Pb-H2SO4), Nickel Metal Hydride (Ni-MH), or LithiumIon • A new technology is based on “Supercapacitors”. I-V Curve of a WLED • The first part of this project requires you to find the IV curve of the WLED using a Power Supply, Voltmeter, Ammeter, Variable resistor, and WLED. WLEDs Variable R DC Power Supply Measuring The I-V Curve • Basic setup • Log both voltage and current • Start from 0 V. Increase to 4V in 0.1 V steps 5V-12V DC Power Source Variable resistor + 0 Amps WLED If using Multimeters, make sure you use the correct connections when measuring voltage and current V DC Voltmeter A - + ANODE Milli Ammeter For measuring The current through The WLED Current flow WLED Is a 2-terminal device - CATHODE WLED Resistance • DC resistance is defined by R(V) = V/I and depends on the “bias point”. Bias point refers to the voltage (V) and current (I) value the WLED is operated at. • AC (small-signal) resistance is defined by R(V) = dV/dI and equal to the reciprocal of the slope (tangent) of the I-V curve. Biasing a WLED • WLED needs a constant current source (CCS) • A CCS will adjust it’s output voltage so that the required current is maintained. • Our CCS will need to maintain 350 mA through the WLED. • Our design will be a simpler, voltage controller using a method called Pulse Wide Modulation (PWM). Pulse Width Modulation (PWM) • Look at the below square wave Vo Tx Time (s) T Period = T, Frequency = 1/T, Duty Cycle K = Tx/T The average value, that is DC value, is simply VAV = VoK By changing the duty cycle, we can change the average (DC) value This leads to a method called PWM … PWM Driver Circuit • PWM is very cool, because we can start with a constant DC power supply Vo and obtain any DC output voltage between 0 V and Vo by simply changing the Duty Cycle of a Square Wave Generator. DC input Vo PWM Circuit PWM DC Output Square Wave DC Value VOUT= VoK PWM Control Input sets duty Cycle K to some fraction between 0 and 1. Voltage-Driven WLED • Using PWM to set the bias point of a WLED WLED DC input Vo PWM Circuit PWM DC Output Square Wave DC Value VOUT= VoK 1W Point VOUT Embedded PWM Circuit • The main part of our project is the come up with ways to do PWM using a microcontroller integrated circuit (IC) and software control. • There are many ways to do this, some of which are quite advanced. • We will do it the easiest, and simplest way! • Simple but it works! Getting Started With Embedded Systems • What’s a micro-controller ? It’s a tiny self-contained computer you can program in C or BASIC. It has some input and output pins. A micro-controller can communicate with other devices, do measurements of voltages and currents, and switch circuits on and off. More on Micro-controllers • Used extensively in cars, aircrafts, audio equipment, DVD and CD players, TVs, washing machines, microwaves, you name it! • Very flexible to program. • Great for controlling various electronic devices and gadgets. • Cheap. Small. Reliable. What Does a Micro-Controller Look Like ? Close up view of an INTEL 8742 micro-controller [Wikipedia http://en.wikipedia.org/wiki/Microcontroller] Programming Tools for Microchip PIC Devices • We will use a microcontroller called PIC12F675 from a company called “Microchip”. • We will use a programming tool from Microchip called MPLAB ICD2 and a FREE C-compiler called PICCLite. • The programming device needs Windows XP and a USB port. • The device driver needs to be installed before it can work. Ask your TA: How can we locate our PIC Chip on this connector ? PIC 12F675 Basics • • • • Eight Pin Device Needs a regulated 5V DC power source. Has internal analog-to-digital (A/D) converters. An A/D converter can be used to measure a voltage that is applied to one of the “Analog Input” pins of the chip. • The measured voltage can be used in your program. • External circuits and devices can be switched on and off by setting output pins to logic 1 or 0 within your program, which shows up on the chip’s pins as 5V and 0V. • Normally, we will use a transistor as a switch to interface logic outputs to analog circuits. Review of Basic Circuit Design • Transistor as a switch Vcc LOAD Control current input Iout= BIin B = current gain Iin Basic circuit Basic Circuit Design • Darlington Drive connected to PIC Vcc Control current input WLED Iout= B1B2Iin 2N2222 Iin PIN 2 Of PIC BDARLINGTON = B1B2 current gain R Solar Photo-Voltaics • The IV Curve of a solar panel: • Operates like a current-source in region A and, as a voltagesource in region B CURRENT I ISC A • Open circuit voltage is about 19.5V typically, for 12V systems, and about 9 V for 6V systems. B • Current control is achieved by connecting a shunt transistor to create a short-circuit (Region A) VOC V VOLTAGE Solar Charge Control • • • This diode We use current-shunts for prevents our controlling the amount of charge current flowing into the storage device shunt from (battery or supercapacitor). draining (and damaging) the battery! Setting Vcontrol = HIGH in software will short-circuit solar panel through the Darlington + transistor. By setting Vcontrol = HIGH we can prevent the battery from overcharging or overheating. solar panel current shunt + Battery - LM7805 5V Voltage Regulator Vcontrol PIC C HERE ! • We now see how the PIC micro-controller can be programmed. • We will use a version of C. • Alternatives include Assembly language, machine code, embedded Java, BASIC, and Matlab. • We will only go through the basics here. More advanced coding will require more detailed coding examples. Grand order of things in PIC C.. • We will need to follow a basic format in order program the PIC microcontroller in C. • This example to toosimple to work! We will look at a real example in the next slide … #include <pic.h> void some_function (void) { // do something now !! } void main (void) // main function { // execution starts here. some_function (); //call function } PWM Code - Duty Cycle 75% #include <pic.h> Ignore void main (void) for now! { unsigned int index; unsigned int dt; INTCON = 0; // disable interrupts TRISIO=0; OSCCAL = _READ_OSCCAL_DATA(); dt=75; // sets duty cycle of PWM wave to 75% while (1) { for (index=0;index<dt;index++) { GPIO=0x20;} for (index=dt;index<100;index++) { GPIO=0x00;} } } We need this line to make the internal CLOCK get going at about 2 MHz These two lines implement the PWM algorithm by setting an output (PIN 2) high 75% of the time Accessing the A/D Converter • Embedded systems rely on the ability of reliably measure voltage on an external circuit. • Our PIC micro-controller has an in-built analogto-digital (A/D) converter, which makes measurement very straightforward. • We will learn to use the 16-bit A/D converter in the PIC. A/D Access in PIC • • • • #include <pic.h> Time to get our hands dirty. Look at this code ! You can use it in your projects without modification. Let’s go through it step-by-step and see how it works … unsigned int adc_read (void) { unsigned int result; GODONE=1; This line combines two 8-bit words into a single 16-bit word while(GODONE); Value = 256*WHigh+WLow } It takes time for the A/D converter to complete a conversion. We wait until the A/D has gone about it’s business and has a value ready for us to use result=(ADRESH<<8)+ADRESL; return result; Embedded Control Algorithms START Solar charge control flowchart: Measure VBattery using PIC A/D Converter PV Shunt Off Y Is VBattery < 5.8 RETURN N PV Shunt Off Example algorithms cont.. • WLED intensity selection algorithm Do { Vdd = Get_Battery_Voltage_using_A/D_Converter (); If (Vdd>=4.4) Then DUTY_CYCLE = V_bright / Vdd ; // full brightness If (Vdd>4.0) and (Vdd<4.4) Then DUTY_CYCLE = V_dim / Vdd ; // semi brightness If (Vdd<=4.0) Then DUTY_CYCLE=0; // Switch off lamp } while(1); Web References • http://www.technologystudent.com/pics/pic1.htm • http://www.lutw.org