Download A Compact Wireless Modular Sensor Platform Ari Y

A Compact Wireless Modular Sensor Platform
Ari Y. Benbasat and Joseph A. Paradiso
Goals & Philosophy
Hardware Instatiation
The master board is responsible for the data collection and
transmission to the central basestation and is included in every project.
• 22 MIPS processor with 12-bit ADC
• 916MHz transceiver running at 115.2kBps
• The processor pins are broken out to the interboard interconnects
This board draws 35mW under normal operation.
To simplify the rapid prototyping and testing of wireless sensor
systems, it was decided to design a modular sensor platform.
Overall, the goal was to allow the user to treat sensing as a
commodity, i.e. allowing an application to trivially incorporate
different kinds of measurement. There were three keys to
achieving that goal:
•Encapsulation Knowledge: A single pane of a modular
system can
•represent the best practices in a given field
•save a large amount of design time
•allow for easy upgrades.
E.g., an RF single pane with a proper HF transceiver and
antenna layout
A six-axis IMU is provided to measure motion.
• Acceleration via two dual-axis accelerometers
• Angular velocity via three gyroscopes
• Four-way static tilt sensor for single-bit acceleration measurement
This pane draws 65mW.
•Reducing repetition of circuit design: The creation of
individual panes containing one or more such circuits can
eliminate much of the drudgery of the design process:
•Most systems involve reuse of known circuit blocks usually
with only slight changes.
•E.g. serial line converters, power regulators and
microcontroller support circuitry.
An ambient sensing board provides a range of methods of detecting
audible and visible signals.
• Narrow and wide cone phototransistors
• Pyroelectric heat sensor
• Microphone (and microcontroller for processing)
• VGA quality cellphone camera
This board draws 100mW (including the camera).
•Simplifying prototyping: Rather than proceed directly to a
final layout, this platform makes it possible to build a
prototype to:
•collect the relevant data
•provide a valuable proof of concept
•help detect flaws in the design
•provide a basis to begin the construction of necessary
interface and analysis software.
A board is provided for inputs from a number of different tactile and
pressure sensors combinations:
• Four single-ended force-sensitive resistors
• Two back-to-back FSR bend sensors
• Two piezoelectric sensors
• Loading-mode capacitive proximity sensor
This pane draws 65mW.
Given the above goals, we consider three key philosophies in
the individual board designs:
A matched pair of sonar receiver and transmitter boards provide for
distance measurement.
• Single omnidirectional transmitter
• Two pickups placed a fixed distance apart
This configuration allows both displacement and relative angle to be
calculated.
The transmit and receive modules each draw approximately 120mW.
•Individual panes should be combinations of circuitry
that cannot or should not be separated
•E.g. Six-axis inertial measurement unit
•It must be as easy as possible to combine and
recombine the panes into new applications
•As many data/signal/power lines as possible
•Connectors must be structurally strong
On-board data storage is provided.
• 1Gbit flash memory chip
• Controlled by a microcontroller via SPI
This board draws 40mW.
•Expandability is key to future utility
•Footprint and height should be reasonable
•Monopolization of interconnects should be avoided
Sample Applications & Lessons Learned
Microcontroller
Data
We consider lessons learned in two categories:
State Analysis
Sensor 1
Sensor Controller
Sensor 2
State
Enable
Algorithm Selector
Transceiver
Sensor N
Data Compression
Processed
Data
The Wearable Gait Lab, designed by Stacy
Bamberg, applied our platform to develop a
prototype inexpensive wireless wearable
system for the analysis of the motion of feet.
The system consisted of:
•Master board (data collection / transmission)
•IMU (motion capture)
•Tactile (connected to insole
•Sonar boards (ranging)
•Power regulation board (3.3V, 5V, 12V)
The wireless nature of this system allows for
real-time feedback to the patient during
daily wear which is not possible with a fixed
gait lab.
This application exploited the modularity of
the system to examine the benefits of
capacitive and sonar sensing that were not
considered in the initial design.
Storage
This platform is currently being used to prototype
Real-time Adaptive Sensor Systems. These
techniques should reduce the sensor system's
power drain by varying:
•The sampling rate of the sensors
•The choice of sensors
•Analysis of the incoming data
such that the amount of data collected by the
system is reduced without affecting the
amount of useful information collected.
This work is being tested using a prototype stack
consisting currently of the master and IMU boards.
Future work will be based around the ambient
board and possibly other boards as well.
This application exploits the completeness of
each sensing modality as expressed by the stack
panes. E.g., the IMU board provides both low and
high power (and accuracy) sensors for measuring
acceleration.
Electrical
•Timing details become quite important as
multiple boards are connected.
•Long sensor startup times can cause problems
for polling-based systems.
•Digital ouputs measurement times can add up
•Power supplies and regulation also suffer for
heterogeneous devices.
•More voltage levels mean more regulators,
leading to increases in RF noise.
•Voltage level clashes between devices (eg
sensor and ADC) also occur.
Mechanical
•Strength of individual connectors is key in
wearable applications.
•Must consistently provide both mechanical and
electrical contact.
•Parts rated for large numbers of insertion cycles
are a necessity.
•Possible orientations of the boards should not be
limited
•Daughter boards and connectors should not
interfere.
• “Top” boards only if absolutely necessary.