Solar Door Panel

Team SodopaUniversity of Utah

Computer Engineering Senior Project David HurstTyson Hunt

Jeffrey Kelley

david . hurst 5@ gmail . com tyswitz @ hotmail . com Jeff.Kelley@utah.edu

Table of Contents

ContentsContents ........................................................................................................................................... 2 Introduction ...................................................................................................................................... 4 Functional Description ..................................................................................................................... 4 Microcontroller ............................................................................................................................... 5 Flexible Energy Management System ............................................................................................. 6 MCU Communication Protocol ....................................................................................................... 7 Display Device ................................................................................................................................. 9 Radio Communication ................................................................................................................... 10 Energy Harvester ........................................................................................................................... 11

TI BQ25504 Features (Texas Instruments, 2011) ...................................................................... 11 Energy Management ..................................................................................................................... 11 Software ......................................................................................................................................... 13 Bill of Materials ............................................................................................................................. 14 Teensy to XBee Adaptor Kit .......................................................................................................... 14 Material Descriptions ............................................................................................................ 14 Initial Tasking ................................................................................................................................ 16 Schedule ......................................................................................................................................... 16 Milestones ...................................................................................................................................... 17 Current Risks ................................................................................................................................. 17 References ...................................................................................................................................... 19

FiguresFigure 1- Functional Design of Solar Panel Display.......................................................................5Figure 2 - MCU Development Kit...................................................................................................5Figure 3 - MCU Functional Diagram (Energy Micro).....................................................................6Figure 4 - Datasheet Table for MCU Sleep Modes (Energy Micro)...............................................7Figure 5 - MCU Protocol Data Structure.........................................................................................8Figure 6 - Wiring Diagram for Kent Display (Kent Displays Incorporated)...................................9Figure 7 - Kent display power usage information (Kent Displays Incorporated)..........................10Figure 8 - Power Management IC (Texas Instruments, 2011).......................................................12Figure 9 - Website prototype.........................................................................................................13Figure 10 - Sechedule flow chart...................................................................................................16

TablesTable 1- Bill of Materials...............................................................................................................14Table 2 - Material Descriptions.....................................................................................................15

IntroductionDoor displays are used commonly throughout many business and schools, however, they are static and boring. Think of how nice it would be if someone could tap into their wireless network to update or change their door display that is powered from solar cells. Since the display uses a radio network it can communicate to a server which anyone can remote into from anywhere in the world. This will allow the user to change their name, set the display to their office hours, work hours, picture logo, or anything else the user would like. Using wireless communication through radio waves and being powered by solar panels, it is completely standalone, requires no maintenance, and best of all no recharging of batteries. This will provide flexibility to users who wish to change the display without even being in the same building.

This project consists of several major components. A cholesteric LCD (ChLCD), an energy harvester to capture and store solar energy, a microcontroller unit (MCU), and wireless radio will be wall mounted outside of an office, or conference room. This unit will operate completely standalone by managing how much power it uses. It will also notify the user of available energy, and prevent an update of the display from happening when there is not enough energy. All these components must use as little energy as possible in order for the chosen solar panel to supply enough energy. In case of catastrophic power failures during updates or radio communication, a backup battery will be activated.

Another major piece of this project is the base station radio that will hook into the user's computer and communicate with the wall mounted display nearby. The radio on both ends uses the 802.15.4 protocol, which has good range, relatively low power requirements, and is readily available. This base station acts as a bridge between the host computer’s usb port and the radio.

The software component of our project will enable the user to change what is displayed, preview any changes before making an update, and view information about the display module’s energy usage. The software will run a web server, and have a web page interface for the user to connect to, either locally on same machine, or over the internet from anywhere in the world. The user will also have the option to integrate the display device with a google calendar and enable automatic updates every day. This will be especially useful for conference rooms that have a daily schedule.

Functional Description Figure 1 below shows how each component of our system will connect.

Figure 1- Functional Design of Solar Panel Display

A solar panel will hook into our energy harvester and be converted to the correct voltage for our storage device, a supercapacitor. We plan on using a 5.5V 1.5 Farad capacitor which has the capability of storing about 20 Joules of energy. The energy harvester will boost the voltage to 5V and manage when to charge, and prevent discharging. It will also monitor the voltage and let the MCU know when the battery is full and empty, based on voltage values that we set. The energy harvester will let the MCU know when to wake up, when we have full power, and will also provide voltage values in order to track energy usage.

The supercapacitor will supply a 3.3 volt low dropout voltage regulator, which will power the MCU, Radio, and the display device. The MCU will manage the radio communication and display interfacing as described later in detail. Both the radio and display will use a SPI serial interface. A device connected to the user's computer via USB will act as a bridge to communicate with the display. The software running on the computer will send and receive data as it requires to update the display and gather information about the device. The server will be web accessible, and offer a portal that users will be able to control their device with, from anywhere in the world.

Microcontroller The microcontroller (MCU) is the main component of the device that interfaces with the display and radio. We will use the Energy Micro Gecko microcontroller, which has an ARM Cortex M3 microprocessor. This chip features four power modes and only uses 180uA/Mhz during CPU execution, along

Figure 2 - MCU Development Kit

with larger flash memories compared to its competitors. We plan on using the development kit which has a gecko EFM32 gecko MCU with 128kb of flash and run at 1 MHz. It allows each GPIO to be configured to drive strengths of 0.5, 2, 6, and 20mA. Energy Micro also provides an energy profiler for this device which will be invaluable for calculating exactly how much energy is consumed.

Figure 3 - MCU Functional Diagram (Energy Micro)

Flexible Energy Management SystemThese energy modes are the power consumptions for the gecko microcontroller as seen on the data sheet (Energy Micro). • 20 nA @ 3 V Shutoff Mode • 0.6 μA @ 3 V Stop Mode, including Power-on Reset, Brown-out Detector, RAM and CPU retention • 0.9 μA @ 3 V Deep Sleep Mode, including Real Time Clock with 32.768 kHz oscillator, Power-on Reset, Brown-out Detector, RAM

and CPU retention • 45 μA/MHz @ 3 V Sleep Mode • 180 μA/MHz @ 3 V Run Mode, with code executed from flash

The four energy saving power modes are shut off mode (EM4), stop mode (EM3), deep sleep (EM2), and sleep mode (EM1). The table below lists the peripherals that are available in each of the different modes. The table was taken from the data sheet for the development kit (Energy Micro). Due to components we will be using we will not use EM3 and EM4 modes. The RTC timer is the component that will wake up the device after a sleep which is only available in EM2. Also the user will have a switch that will wake up the device from sleep.

Figure 4 - Datasheet Table for MCU Sleep Modes (Energy Micro)

MCU Communication ProtocolCommunication will employ a simple packet structure that contains a header that will tell the device the command to be issued along with a payload size. The size can be used to interpret how much information to save and what is garbage. Refer to Figure 2 for a simple representation

of the data structure. The radio has a maximum packet size of 100 bytes, and any payloads larger than that will be split into different radio packets.

Figure 5 - MCU Protocol Data Structure

The commands the device support are grouped into 3 categories: screen updates, reporting energy usage, and reporting power usage. The update commands tell the MCU on the display side to update the screen with the data in the payload. The payload will contain the pixels to display on the screen in bitmap form. This may be a full or a partial update, based on how much of the screen needs to be updated. On the server side the update command will be interpreted as an acknowledgment that the device can receive an update if there is one available, and contains no payload.

Another category of commands are for reporting energy usage of the device. Options for this category include reporting energy usage since power on, lifetime power usage, and the energy usage of the most recent update. These metrics will be available for both the display, the radio, and the MCU itself. The MCU will handle storing and calculating these metrics, and reporting the correct numbers. Energy usage is calculated by reading voltage levels of the supercapacitors before and after the radio transmissions, and screen updates. Also the system can report the current amount of energy available to the device, as a percentage, and estimate the time it will take for the device to reach full charge.

The final category of commands is similar to energy reporting, but will instead report maximum, minimum, and average power usage. This command will also be able to specify which device to report metrics for. For example, the user can ask for the maximum amount of power the display, or the radio has used. These values will be approximations based on readings of energy values at different times. As the MCU makes energy readings, it saves timestamps, and estimates power usage based on the amount of energy lost between the last reading.

Below is a description of how the MCU will operate in order to save as much power as possible. The goal is to turn on the receiver as little as possible.

Device:1) Wake up every 1 hour.2) Send ready signal3) Turn receiver on for 5 seconds and wait.4) If response

I) Parse and complete commandII) Turn receiver off

5) Sleep6) Repeat

Server:1) Wait for ready signal2) Send acknowledgement once

received3) Send image data4) Receive power/energy data5) Send done signal6) Repeat

Display DeviceThere are a couple of low energy displays we considered using: E-ink displays, used in Kindles, and a passive ChLCD screen made by Kent Displays, Inc. We are using the Kent Display because it has good documentation, and we know how to interface with it. In case of availability, or other problems, a backup display is an 7.4” e-ink EW074AS184 e-paper Panel, sold by Pervasive Displays. However, no datasheet is available for this display which may cause problems. These displays have a resolution of 640x480 and are interfaced via a SPI connection at 3.3V. The displays are monochrome, so each pixel is one of two colors, and images are stored in bitmap format. The block diagram below shows what is provided by Kent. A SPI controller for the display, RAM to store the image, and drivers for the LCD are included with the display.

Figure 6 - Wiring Diagram for Kent Display (Kent Displays Incorporated)

Both displays are able to keep displaying their image even when completely removed from power. According to Kent Displays, their ChLCD consumes a maximum of 10uA of current in sleep mode. The power usage during a typical full-screen update cycle is shown below.

Figure 7 - Kent display power usage information (Kent Displays Incorporated)

The display will be a major consumer of energy. At near room temperature, the display will use an average of 150mW during a 2 second update cycle, and use a peak 1.5W of power. The energy storage device must meet these power and energy requirements.

Radio CommunicationThe radio chip we plan to use is a Texas Instruments CC2520 which uses a SPI interface along with general purpose input/output pins to connect to the MCU. The drivers are already partially written by the manufacturer but must be completed and adapted to the ARM Cortex Architecture. This device draws 15 mA in Rx mode, and 25 mA in Tx mode. Careful management of when to turn on the Rx part of the radio is key to low power usage, which will be handled by the MCU.

The server will communicate through an Xbee Pro Radio. The radio will be controlled via a Teensey 2.0 device. The Xbee radio uses the same frequency as the CC2520 (2.4Ghz). It operates at 40 meters and has a bandwidth of 250kbps. The Teensey will allow the radio to be operated through a USB interface allowing the server to directly send information over the radio to the display.

Energy HarvesterMany types of energy harvesting methods could be used, but we are targeting solar energy. This is because there will be a fairly steady source of light in the operating environment during usage. In conjunction with a solar panel, supercapacitors will be used to store energy when the device is in low power mode. Solar Panels present a challenge due to the variances in voltage level due to the amount and type of light available. In order to store energy in our supercapacitors, a steady voltage at their rated max voltage is necessary. Precautions must be taken to not overcharge the capacitors. Also, to make the system robust, information about how much energy is being stored and the status of the capacitors must be sent to the microcontroller. In order to solve these problems we will use a power management chip from TI, the BQ25504 Ultra Low Power Boost Converter with Battery Management.

TI BQ25504 Features (Texas Instruments, 2011)

● Ultra Low Power With High Efficiency DC/DC Boost Converter/Charger○ Continuous Energy Harvesting From Low Input Sources: ○ VIN ≥ 80 mV(Typical)○ Ultra Low Quiescent Current: IQ < 330 nA (Typical)○ Cold-Start Voltage: VIN ≥ 330 mV (Typical)

● Programmable Dynamic Maximum Power Point Tracking (MPPT)○ Integrated Dynamic Maximum Power Point Tracking for Optimal Energy Extraction

From a Variety of Energy Generation Sources○ Input Voltage Regulation Prevents Collapsing Input Source

● Energy Storage○ Energy can be Stored to Re-Chargeable Li-ion Batteries, Thin-film Batteries,

Super-Capacitors, or Conventional Capacitors● Battery Charging and Protection

○ User Programmable Undervoltage / Overvoltage Levels○ On-Chip Temperature Sensor with Programmable Over Temperature Shutoff

● Battery Status Output○ Battery Good Output Pin○ Programmable Threshold and Hysteresis○ Warn Attached Microcontrollers of Pending Loss of Power○ Can be Used to Enable/Disable System Loads

Energy Management The energy management chip we have chosen allows to us to choose 2 thresholds to control battery OK line. A high signal on this line will notify the MCU that there is enough energy to perform a display update. The first threshold is the voltage level considered when the IC is charging, and will be put high when it reaches the user programmed voltage. The other threshold is considered when there is no power available, and the capacitors are discharging. When it drops below a certain voltage, the battery OK line will drop low to let the MCU know that energy is limited. A certain amount of testing will be required to find what these values should be for our particular device. The battery OK line will be used as an interrupt to tell the MCU to wake up and be ready for updates, and also as a signal to control the backup battery.

This IC also has the ability to adjust the input voltage using an inductor on its voltage input pin. TI calls this feature Maximum Power Point Tracking (MPPT). Every 16 seconds, it will set the voltage to the most efficient level for harvesting depending on what type of source the energy is from. It is suggested to use 80% of the output voltage for solar panels, but we will vary this and find the most efficient level.

Another feature of the energy management IC is overvoltage and undervoltage protection. These values are set by resistors and also depends on whether voltage on the storage element is increasing or decreasing. Once the voltage at the battery exceeds VBAT_OV (Battery Overvoltage) threshold, the boost converter is disabled. The charger will start again once the battery voltage falls below the VBAT_OV_HYST (Overvoltage hysteresis) level. When there is excessive input energy, the VBAT pin voltage will ripple between the VBAT_OV and the VBAT_OV_HYST levels. The excess energy from the solar panels is minimal, and is dissipated as heat by the solar panels. The various voltage levels for an example setup is shown below in Figure 3.

Figure 8 - Power Management IC (Texas Instruments, 2011)

SoftwareWe will be building a server to interface our display device with personal computers and mobile devices. The first step in this process is to make an HTTP server that we can use to give users a website to interact with and handle the data they provide back to us. Once we have the image the user wishes to put on the display we will need to make it fit the display by resizing it and making it into black and white. This image will then be sent to the radio for transmission to the device hanging on the door and metrics will be returned that will be added to the web page that the server gives when contacted. We will add functionality to accept HTML files and relevant pictures or URLs of existing sites and expand support for mobile device uploads to the server.

Our GUI will be a JavaScript and HTML5 web page. This page will include a form for submission of a file through HTTP. The server will then parse the information into commands that the MCU understands. The server will also parse information received from the display device and display the appropriate power and energy metrics. It uses a Teensy 2.0 as a USB bridge to the radio which then transmits to the display.

Figure 9 - Website prototype

Another feature the user will have from the GUI is to enable automatic updates from google calendar. This will automatically send daily updates to the display showing the daily agenda. This will allow the user to have the display update itself without having to manually update it.

Bill of MaterialsTable 1- Bill of Materials

Component Vendor Contact info Part Number Price Qty

Teensy ++ PJRC www . pjrc . com TEENSY $16 1

Teensy to XBee Adaptor Kit

PJRC www . pjrc . com KIT_XBEE_ADAPTOR $6 1

XBee DigiKey www . digikey . com XB24-AWI-001-ND $19 1

1E-Ink (backup)

Pervasive Displays, Inc.

Contact Link EW074AS184 / EW074AS183

$50 1

ChLCD Kent Displays

TEmanuele@kentdisplays.com

015574130050167 $109 1

MCU DigiKey +47 23 00 98 00 or website

EFM32-G8XX-STK $72.88

1

Assorted Caps/Resistors/Connectors

Mouser ~ ~ N/A N/A

Energy Harvester

TI Contact Link BQ25504 $49 1

PCB Sunstone, Inc.

www.sunstone.com ~ N/A 1

Super Capacitor Panasonic (via mouser)

Contact Link S5R5H155 $4.14 2

Solar Panel Solmaxx 6V 3Watt Thin Film $28 1

Material DescriptionsTable 2 - Material Descriptions

Component Picture Basic Info

Teensy 2.0 (PJRC) Visit Website

ChLCD (Kent Displays Incorporated)

Visit Website

MCU (Energy Micro) Visit Website 128kb Flash, 3 SPI, RTC, power and onboard management unit.

Super Capacitor (Mouser) Visit Website 1.5Farad @ 5.5Volts

TI BQ25504 (Texas Instruments)

Visit Website

XBee Radio (Digi) Visit Website

Initial TaskingEach task will have a different person over that task even though each member will contribute to the entire project. The current project leads are the following.

Tyson● MCU, Display, and Radio communication.

David ● Energy Harvester, Power management, PCB & Packaging

Jeff ● Server, Web Interface, Google Calendar integration, USB interfacing via Teensy 2.0

ScheduleThe schedule is shown below. We plan on getting each component working individually by the end of summer and then start integrating everything starting the Fall semester of school in August.

Figure 10 - Sechedule flow chart

MilestonesThe major milestone are shown in the schedule section shown by the top part of the figure.

● June: Website Deployed● July: PCB Assembled and Tested● July: LCD Working● August: Two way radio communication● August: Power management capability● August: Display power usage in GUI● September: Display and GUI fully integrated● October: Energy harvester and display integrated● November: Device fully functional

Current RisksThe current risks are outlined below.

The radio’s range and throughput is a minor risk. The radio needs to be close enough to the server to pick up the signals and the current radio doesn’t have a long distance for communicating between two radio’s. The radio operates at 250kbps which is enough bandwidth for our purposes. The problem arises due to the power constraints of the device. If we wish to do a full screen update for the display and send statistical information of the power management it could take a long time. This will cause lots of power drain which we may not be able to afford. To address this risk we must wait for our display to arrive and look at how to update the screen and research further into how much power it takes (through experimentation) to update it.

Keeping to our low power constraints will increase the time needed to get a working prototype This risk goes along with the radio’s risk. Virtually all problems come from this constraint. We didn’t realize how hard it would be to find components that are low power that will also be able to accomplish our desired tasks. We expect to see many problems to develop due to not having enough power. To address this we have to cut back on the routines of sending or receiving data or add a little extra power with a small battery. Cutting back on updates and data would mean our project would not have as many features as we would like but we could still implement them and show them working with a battery but disable them when running on solar panels.

Capacitor discharge is a risk due to the system always needing a minimum amount of power to operate without shutting off. Every time we use energy it decreases the charge on the capacitors. We will need to make sure that the capacitors are never allowed to use so much that the system will fail. One way this is addressed is to use a backup battery system but even then this will not fix the issues if the capacitors can’t hold a charge long enough to operate the MCU alone.

Currently experimenting with the radio driver on a ARM Cortex M3 architecture has been a little difficult. It takes up more space than we had anticipated so this has increased the problems with finding a good MCU that has enough space to store the driver and be able to store data to update the screen. This is the reason for choosing a Energy Micro gecko which has 128kb with low power. There are very few low energy MCU’s with larger flash sizes. Since the radio driver uses about 64kb and the kent display requires 32kb it will leave 32kb for coding.

References

Digi (n.d.). XBee® ZB ZigBee® Modules. Making Wireless M2M Easy. Retrieved April 24, 2012, from http://www.digi.com/products/wireless-wired-embedded-solutions/zigbee-rf-modules/zigbee-mesh-module/xbee-zb-module

Energy Micron (2011, November). EFM32g Reference Manual. Eink. Retrieved April 24, 2012, from www . energymicro . com

Kent Displays InCorporated (2010, April 5). 1/4 VGA Cholesteric Display Module with SPI - Compatible Interface. Kent Displays. Retrieved from http :// www . kentdisplays . com / services / resources / datasheets /25085 c _320 x 240_ SPI _ datashee t . pdf

Mouser (n.d.). EEC-S5R5H155. Mouser Electronics. Retrieved April 2012, from http :// www . mouser . com / ProductDetail / Panasonic - Electronic - Components / EEC - S 5 R 5 H 155/? qs = sGAEpiMZZMsCu 9 HefNWqphhDW 1 I 6 X 6 cazxL 9 wSMAdug %3 D

PJRC (n.d.). Teensey USB Development Board. Electronic Projects Components Available Worldwide. Retrieved April 24, 2012, from http :// www . pjrc . com / teensy / index . html

Texas Instruments (n.d.). Ultra Low Power Boost Convert with Battery Management for Energy Harvester Applications. bq25504. Retrieved from http :// www . ti . com / lit / ds / symlink / bq 25504. pdf