Start
• Mavric-II
Board
• Sensor
Board
• LCD
Panel Board •
Sensor & Thermocouples •
Two-Axis
Accelerometer
• Radio
•
Radio
Packet Stategy •
Data
Logging
• Adaptable
System •
Sources and PDF
RADIO
PACKET STRATEGY
The
heart of the system is a packet strategy that keeps
the data timely, accurate, and organized. The formula
race car team set the real-time requirements to
be nearly instantaneous with the pit crew’s (a sample
every 0.5 s). Most of the measurements didn’t require
a lot of sampling (e.g., temperature). The time-critical
measurements (e.g., wheel speeds) simply had to
be accurate enough so the pit crew wouldn’t notice
any time lag.
Our
packet strategy is depicted in Table
1 (on page 44). The data and command packets
are shown in Figures 4a and 4b (on page 44). The
packets are distinguished by the code in a packet
type look-up table. The strategy was based on packets
being correctly transmitted and received once every
0.125 s. Both axes’ acceleration and all four suspensions
are sampled eight times in 1 s. The wheel speeds
are accurately sampled once per second. RPMs and
temperatures are sampled from the engine four times.
The four temperatures are sampled twice per second.
The 0.125-s intervals ensure that accurate data
is constantly sent to the PC without flooding the
wireless serial connection.
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(Click
here to enlarge)
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Figure
4a—Data packets are sparse and flexible. The
ID byte immediately indicates what will follow
is data. The payload is self-contained. The
packet schedule allows you to assemble the full
packet from any number of sensors without having
to reinvent the packet structure. The total
packet size is 1 + m reports × (1 ID byte +
2 bytes), where m is the number of sensors reporting,
and n is the number of bytes of data or bytes
of payload. b—We kept the command packet as
simple as possible to keep the overhead low
for sending a command to the car unit. This
packet also accommodates messages to the LCD
with a variable-sized field to contain the text
string. The total packet size is equal to n
bytes of data divided by the settings. |
A
ping to the base end’s PC is transmitted after the
eighth packet and a reply is expected within another
second. The ping-pong handshake tells both units
if the connection is up, if they are in sync, and
if there is an error. An error or dead connection
leaves the car unit searching for the base unit
before sending more data. This prevents the data
from getting out of sync because only in-sequence
data is sent. This strategy allowed the pit crew
unit to use stock Visual Basic serial port functions
to control the radios without extra connections
to the radio.
