Environmental Engineering Reference
In-Depth Information
4.6.3 Future communications: TTCAN
CAN are event triggered protocols covered under ISO specification 11898 for data
link layer communications. Future by-wire technologies require time triggered
communications protocols so that closed loop control is consistent and free of
network latency issues. As communications traffic increases on the vehicle bus, due
for instance to more modules being added or more functionality from existing
modules, there will be an attendant increase in bus contention and message latency
using event triggered protocols. To mitigate this shortfall, time triggered CAN
(TTCAN) has been proposed that can schedule messages as either event driven or
time triggered without excessive software overhead and minimal additional cost.
As more and more of the vehicle's subsystems are linked, there is need for fast
sensor data sharing and real time performance in the associated controls.
For hybrid propulsion systems, data that must be shared amongst the various
system modules are key position (start, run, accessory), accelerator pedal position,
brake pedal position, gear ratio and lever position (P, R, N, D, L), steering wheel
angle, plus longitudinal and lateral acceleration and yaw rate. It is also necessary
that the communications channel link with the instrument cluster (human-machine
interface) for feedback to the operator on system status. As an illustration of how
TTCAN works consider linking the sensors necessary to provide the chassis func-
tion of anti-lock braking system (ABS). With ABS, the wheel speed sensor data
must be shared with the vehicle speed sensor data in the driveline, plus accel-
erometer data and yaw rate data so that wheel slip can be managed via appropriate
application of the service brakes and engine torque (via electronic throttle control).
Linking this sensor data to steering wheel sensor data provides the possibility to
further enhance vehicle stability by including vehicle pitch, roll and spin into the
mix with wheel traction control. TTCAN's controller would trigger the appropriate
sensor for its data and share these data with the affected modules.
Figure 4.51 illustrates where TTCAN fits into the spectrum of on-board com-
munications protocols. TTCAN is a hardware layer (OSI layer 1) extension to CAN
that synchronizes the network bus so that messages can be transmitted at specified
points in time, thereby avoiding the main shortcoming of event triggered CAN, and
that is loss of bus access due to message collision.
Network bus access
Event triggered
Time triggered
10-125 kb/s
250 kb/s 500 kb/s 1 Mb/s 5 Mb/s 10 Mb/s
CAN
TTCAN
Flexray
Class A & B body
functions.
Class C powertrain
functions
Chassis functions (steering,
braking, stability) and
powertrain control
High speed, deterministic,
communications for safety
critical systems and powertrain
Figure 4.51 TTCAN in the network spectrum
