Sensor Network Hardware Platform Design Andreas Savvides Embedded Networks and Applications Lab ENAL

1
Sensor Network Hardware Platform Design
Andreas SavvidesEmbedded Networks and
Applications LabENALABhttp//www.eng.yale.edu/en
alabYALE EE CS DepartmentsApril 27, 2005
Research Supported by
2
Hardware Platform Design

• Platforms in applications and deployments
• Computation requirements in applications and
  design
• Platforms vs. application needs
• Hardware design and interface issues
• Experiences with the platform development process
  
• Emphasis topic Hardware characterization
• Power characterization is discussed in SPOTS
  papers/posters
• I will pick on antenna behaviors in 3-D scenarios
• Algorithms and platforms should change together
• HW platforms can still change the way we think
  about algorithms

3
Hardware Sensing Platforms
HW Platforms
Experiment with unknown environments
Shrink the HW
NIMS Nodes _at_UCLA
UC Berkeleys Spec Node Smartdust
Intelligent Integrated Sensing Network Platforms
4
Hardware Platform Priorities
HW Platforms
Experiment with unknown environments
Shrink the HW
Understanding unknown sensing phenomena
Power Cost Reduction
NIMS Nodes _at_UCLA
UC Berkeleys Spec Node Smartdust
Intelligent Integrated Sensing Network Platforms
5
Platforms vs. Application Needs

• Each application has different computation,
  memory and interface requirements
• Wide range of applications requirements
• Surveillance
• Medical care
• Structural health monitoring
• Traffic management
• Tracking fires
• Environmental exploration
• Child motion monitoring
• Hard to create a single platform for all
  applications
• Links to SPOTS Platforms - Pages 429 431 of the
  proceedings

6
Opportunities for New HW at Different Levels

• Processor core
• New instructions
• Support for different power modes
• Peripherals
• Need new custom peripherals
• Often running as different HW treads
• Sensors
• Create new sensing modalities
• Move computation and intelligence inside the
  sensor
• Still many tradeoffs and engineering challenges
  to address

7
When should you attempt to build a new platform?

• If you have a specific problem in mind for which
  existing platforms wont suffice
• If you plan to create a hardware component for
  which you need tight control of the hardware
• If cost and size become a limiting issue
• Need to consider
• What is the benefit of having own platform?
• Is this going to enable or handicap your research
  effort?

8
Plan your priorities

• What is your design objective?
• Avoid building new HW for the sake of building
• Target a specific feature or application
• Power consumption vs. proof of concept
• Which is more important to you?
• Proof of concept
• Over-design vs. under-design
• If the algorithm is known, size and power become
  the focus
• If the algorithm/application is not known you
  need to relax the constraints

9
Before you begin to build a sensor node

• Are the tool chains available?
• Make sure you have all the tools you need to
  complete the cycle available
• Flash programmer
• Compiler
• Debugger JTAG tools
• Is the processor chip you are using mature?
• If not, then dont use it unless you have
  collaboration with the manufacturer
• Get the development kit first and try to write
  software before you start
• Does the radio you are using have software
  support/tools?

10
Design Tools and Component Selection

• Try to use well established packages, ORCAD for
  instance
• Typically available from the CAD tools suite
• Easier to find/share component footprints
• This is one of the most time-consuming and
  error-prone part of the process
• Make sure you select the right components
• Components come in different packages
• Components have different cost and power
  consumption
• Good idea to purchase all the components before
  the prototype PCB is sent to fabrication
• If you plan to build large numbers, talk to
  people who did it before first

11
Design Considerations

• Design for manufacturability
• Take into account that you need to build more
• Plan for an economical way to do it
• Capitalize on the fabrication cycle
• Most companies have reduced rates for 4 week runs
• Assembly houses may have specific requirements on
  assembly
• Find out about this before you begin
• Talk to the assembly house before you finalize
  your design
• Some PCB boards and components will require
  fiducial points for machine assembly
• Some manufacturers may be able to suggest
  alternative components
• Put testpoints for debugging and power
  characterization during operation
• Be careful with radios they have specific PCB
  requirements

12
Developing your PCB

• Look around for existing designs first
• Investigate parts
• Availability, packaging, power consumption cost
• Get your tools together for the whole process
  first
• Schematic capture and review process
• Layout
• Double check your component footprints
• Talk to the manufacturer some places charge
  less for 1 phase board
• Odd number of layers does not save you money
• Make sure you follow the manufacturer directions
  for radio laout
• Make sure you wire the board for test
  measurement
• Plan for testing

13
SmartKG iBadge Platform (NESL/UCLA)

• One of the most highly integrated sensor
  platforms
• Hard to build very small components 0201
  components, difficult to machine assemble
• Production and assembly costs is a limiting
  factor
• Lots of educational value!

14
Study Case Building the XYZ

• Work with Cogent Computer
• Small single board computer company in Rhode
  Island
• Already has expertise and interest in embedded
  ARM
• Collaboration with OKI Semiconductor
• Make sure that all the peripherals are available
• Talk to Chipcon to make sure they would have an
  IEEE 802.15.4 MAC available
• Design prototype according to our specification
• Second pass design with Cogent Computer
• Identify inexpensive components
• Make the design easier to manufacture
• 1 side, 6-layer board
• Placement done to accommodate hand and machine
  assembly

15
Example XYZ Mobility Ultrasound Board

• Align components to make low production assembly
  and debugging more efficient
• Makes hand assembly or low end machine assembly
  easier

16
Lessons Learned

• Dont bother soldering everything by hand
• Look for places esp. local shops that can help
  you
• If the layout is too complex, outsource to an
  expert
• Cost is the same if you consider the lost time
  and the possibility of bugs
• Pace yourself
• Long, organized planning period
• Fabrication assembly cycle (2 to 6 weeks)
• Have a support strategy for the system
• How are you going to make more, distribute it,
  test it, use it etc.
• Plan for iterative implementation and
  customization
• After some field deployment you will probably
  need to make some changes
• Verify the software and programming cycle before
  you finalize the hardware design

17
Lessons Learned

• Go for the mainstream design tools
• Design for manufacturability and testability
• Be aware of what if already available
• Look into the community to see if there are
  pieces you can reuse
• Reconsider picking platform development as a
  research topic if other companies are doing it
• Ember OKI have IEEE 802.15.4 implementations on
  radio chip
• re-implementing the same MAC w/o a longer term
  plan will have short half-life

18
After Fabrication Completion

• Have a test strategy in mind for SW HW
• Write diagnostic code to check each subsystem
• Diagnostic code should become part of the runtime
  environment
• Treat your new platform as a new device.
  Characterize it!
• Characterize power consumption at different modes
• Characterize platform in a realistic environment!
• Push the platform to the limits, know where
  things break down
• Post your data, this is would be the most
  valuable asset to the community
• Example Antenna Characterization for CC 2420
• PCB design affects the antenna
• Characterize radio and antenna properties in 3D!

19
Chipcon CC2420 Radio Power Levels
RSSI_VAL Computed by the radio over 8 symbol
periods (128us) RSSI_OFFSET Determined
experimentally, based on front end gain (around
-45dBm) Approx. Range at power level 6 in an
office corridor 30ft Antenna Length 2.9cm
20
Radio Calibration for TX and RX

• Each radio chip is different

EPr29.94dBm s2.7dBm
40cm
10 Different Transmitters
10 Different Receivers
EPr26.375dBm s2.88dBm
40cm
21
Orientation variations at ground level

• Repeat experiment for 4 different nodes, same
  receiver
• TX Power -15dBm
• 8 different positions, 4 orientations for each
  position

22
Indoor Path Loss Measurements
Floor measurements in a 24 x 20ft lounge no
obstacles
Same power level using suboptimal antenna
?3
23
Indoor Path Loss Measurements
Floor measurements in a 24 x 20ft lounge no
obstacles
24
Monopole Antenna Radiation Pattern
Side View
Top View
Communication range
Symmetric Region
Antenna orientation independent regions
Communication range
25
RSSI at Different Antenna Orientations

• At the bad orientation, antenna has to be at
  similar height to get proper results

26
3-D Radio Connectivity
27
Link Asymmetry in 3D-scenarios
of one-way links
28
RSS Asymmetry at Different Power Levels

• Asymmetric Links

29
Platforms in Undergraduate Curriculum Setting
up a lab
Capstone Project
EENG 460a Networked Embedded Systems S.
Networks
EENG 449 Computer Systems

• Embedded and Real Time OS
• Radio Technologies and MAC
• Routing for small devices
• Sensor network applications
• Self-Configuration
• Data Storage
• Mobility and Actuation

• Expect to have a research
• caliber project
• Undergraduates participate on
• research papers

• Computer Architecture
• Embedded Processors
• Assembly Language

Most important assets 1. Develop HW intuition
early on 2. Have fault diagnostic code for the
device
30
Conclusions

• Building HW is a great learning experience and
  adds to the diversity
• Useful to uncover new ideas and concepts
• More insight, more prudent researcher
• Close consideration with software design is
  crucial
• HW changes faster than SW
• One of the biggest challenges
• Radio technology
• There is a large domain of problems for which the
  radio may not be sufficient
• Need to become more critical of radio
  capabilities in applications
• Try out different radios!
• Data traces and benchmarks are still missing
• Need better ways of reporting power and
  performance
• Utility value in terms of the application should
  be factored in