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4/27/05 IPSN/SPOTS 2005
Sensor Network Hardware Platform Design
Andreas SavvidesEmbedded Networks and
Applications LabENALAB
http://www.eng.yale.edu/enalabYALE EE & CS Departments
April 27, 2005
Research Supported by:
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
Hardware Sensing Platforms
HW Platforms
Shrink the HWExperiment with unknown environments
UC Berkeley’s Spec Node & Smartdust
NIMS Nodes@UCLA
Intelligent Integrated Sensing Network Platforms
4/27/05 IPSN/SPOTS 2005
Hardware Platform Priorities
HW Platforms
Shrink the HWExperiment with unknown environments
UC Berkeley’s Spec Node & Smartdust
NIMS Nodes@UCLA
Intelligent Integrated Sensing Network Platforms
Power & Cost Reduction Understanding unknownsensing phenomena
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
When should you attempt to build a new platform?
• If you have a specific problem in mind for which existing platforms won’t 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?
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
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 don’t 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?
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
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!
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
Lessons Learned
• Don’t 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
4/27/05 IPSN/SPOTS 2005
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
4/27/05 IPSN/SPOTS 2005
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!
4/27/05 IPSN/SPOTS 2005
Chipcon CC2420 Radio Power Levels
Level TX Power(dBm) Power Consumed (mW)
1 0 31.322 -1 29.7
3 -3 27.36
4 -5 25.02
5 -7 22.5
6 -10 20.16
7 -15 17.82
8 -25 15.3)mW(
1mW
P(mW)20logP(dBm)
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 = 30ftAntenna Length 2.9cm
[dBm] TRSSI_OFFSE RSSI_VALPRX
4/27/05 IPSN/SPOTS 2005
Radio Calibration for TX and RX
Each radio chip is different
E[Pr]=29.94dBmσ=2.7dBm
40cm
40cm E[Pr]=26.375dBmσ=2.88dBm
10Different
Transmitters
10Different
Receivers
4/27/05 IPSN/SPOTS 2005
Orientation variations at ground level
Node ID E(RSSI_VAL) σ(RSSI_VAL) 0x0019 25.75 1.159
0x0008 27.48 1.46
0x0022 28.1 1.16
0x001F 30.92 0.98
Across all nodes 28.0625 2.15
Repeat experiment for 4 different nodes, same receiver:• TX Power -15dBm• 8 different positions, 4 orientations for each position
4/27/05 IPSN/SPOTS 2005
Indoor Path Loss Measurements Floor measurements in a 24 x 20ft lounge – no obstacles
-100
-90
-80
-70
-60
-50
-40
0 5 10 15 20
Distance (feet)
RS
SI(
dB
m) Same power level using suboptimal antenna
η=3
4/27/05 IPSN/SPOTS 2005
-100
-90
-80
-70
-60
-50
-40
0 5 10 15 20
Distance (feet)
RS
SI(
dB
m)
Indoor Path Loss Measurements Floor measurements in a 24 x 20ft lounge – no obstacles
),0(X
exponent losspath -
d distance reference aat losspath )PL(d
power, transmit P
Xd
d log 10)PL(dPRSS(d)
2
00
T
0100T
N
4/27/05 IPSN/SPOTS 2005
Monopole Antenna Radiation Pattern
Side View Top View
Communication rangeSymmetric Region Antenna orientation
independent regions
Communication range
4/27/05 IPSN/SPOTS 2005
RSSI at Different Antenna Orientations
At the bad orientation, antenna has to be at similar height to get proper results
0 5 10 15 20 25-45
-40
-35
-30
-25
-20Best Orientation: 135 degrees
Distance(feet)
6.5ft3.5ft1.5ft
0 5 10 15 20 25-50
-45
-40
-35
-30
-25Worst Orientation: 180 degrees
Distance(feet)
6.5ft3.5ft1.5ft
4/27/05 IPSN/SPOTS 2005
01
23
45
6
0
1
2
3
4
50
0.5
1
1.5
2
2.5
7
8
9
6
27
10
5
28
12
X coordinate
42
33
4
30
35
29
11
13
14
3
34
32
37
36
17
31
Connectivity at Power Level 7
41
25
15
1
2
38
39
18
26
24
16
Y coordinate
40
19
22
23
20
21
Z c
oo
rdin
ate
3-D Radio Connectivity
01
23
45
6
0
1
2
3
4
50
0.5
1
1.5
2
2.5
7
8
9
6
27
10
5
28
12
X coordinate
42
33
4
30
35
29
11
13
14
3
34
32
37
36
17
31
Connectivity at Power Level 6
41
25
15
1
2
3839
18
26
24
16
Y coordinate
40
19
22
23
20
21
Z c
oo
rdin
ate
01
23
45
6
0
1
2
3
4
50
0.5
1
1.5
2
2.5
7
8
9
6
27
10
5
28
12
X coordinate
42
33
4
30
35
29
11
13
14
3
34
32
37
36
17
31
Connectivity at Power Level 4
41
25
15
1
2
3839
18
26
24
16
Y coordinate
40
19
22
23
20
21
Z c
oo
rdin
ate
4/27/05 IPSN/SPOTS 2005
Link Asymmetry in 3D-scenarios
1 2 3 4 5 6 7 820
22
24
26
28
30
32
34
36
Power Level ( 1 - Maximum power level )
One way Links
% o
f on
e-w
ay lin
ks
4/27/05 IPSN/SPOTS 2005
1 2 3 4 5 6 7 820
25
30
35
40
45
50
55Percentage of Assymetric Links
Power Level (1 - Maximum power)
>=2 >=3 >=4 >=5 >=6
dBm diff. 2
dBm diff. 3
dBm diff. 4 dBm diff. 5 dBm diff. 6
RSS Asymmetry at Different Power Levels
Asym
metr
ic L
inks %
4/27/05 IPSN/SPOTS 2005
Platforms in Undergraduate Curriculum – Setting up a lab
EENG 449Computer Systems
Capstone Project
EENG 460aNetworked
Embedded Systems& S. Networks
• 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
4/27/05 IPSN/SPOTS 2005
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
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