Smart nodes
As well as the growth in electronic control systems and communications
networks, which will lead to an increased number of microcontrollers in
the vehicle, there will also be a growth in the number of sensors and
actuators. In conventional systems sensors and actuators were connected
directly to the appropriate electronic control unit, usually by a
twisted pair. As the number of sensors and actuators increases, there
are several problems, which are faced. Handling their interrupts or
sampling their outputs, providing different interfaces for each sensor
/ actuator becomes problematic, as well as processing / sharing the
(usually analog) information which is exchanged between the control
unit and the sensor / actuator.
As the cost of electronics is coming down and the need for
simplification of such systems is increasing, it will become more
common to implement smart sensors and actuators. A smart sensor /
actuator need not include a microcontroller (but mostly will have a
small CPU) and will be connected to the ECU on a bus system. Much of
the growth in automotive microcontrollers, which is shown in Figure 1
will be a result of smart nodes.
Today's smart nodes are usually composed of a package containing a
microcontroller die, sensor die and sometimes an analog-interfacing die
(depending on how much functionality is included on the sensor die). It
is anticipated that it will be cost effective in the not-too-distant
future to integrate MicroElectroMechanical sensors (MEMs) and
microcontrollers on a single monolithic silicon chip. Such a concept
for a pressure sensor is shown in Figure 4. The sensing element, analog
interface and microcontroller functionality are all contained on a
single silicon die.
Figure 4 - Integrated single chip smart pressure sensor
Microcontrollers have been at the heart of safety critical systems for many years. Almost all of the safety critical automotive systems in which they have been used have provided a fail-safe function. In the near future, there will be an added requirement for fault-tolerant microcontroller based systems.
There is an important difference between fail-safe systems and fault-tolerant systems. Today's Antilock Braking Systems are fail-safe: if an electrical system error is detected, the ECU switches to a safe 'off' mode, allowing the foundation hydraulic brakes to operate without the faulty ABS system interfering. A fault-tolerant system must not only recognize that an electrical fault has occurred, but must continue to operate safely with the existing known fault. Antilock braking systems use redundancy to facilitate a fail-safe system. Typically the CPU at the heart of the system supervises the continual testing of all the major system components. The CPU can only validate these components however, if the CPU itself is known to be 'sane'. Hence a second, redundant CPU is used to validate the sanity of the first CPU. A redundant CPU can either be implemented as a second standalone microcontroller or as an error detection CPU with comparison logic on the same microcontroller.
Emerging automotive applications such as Brake-by-wire and Steer-by-wire systems will not be satisfied by a fail-safe system; they will require a fault-tolerant solution, as a hydraulic back-up mode will not exist. Therefore if a fault is detected, the system must continue to operate safely in a 'limp home' mode. Although simple redundancy of components and a voting algorithm can be used, this is normally a very expensive solution (voting requires at least three CPUs). There has been an increasing amount of work done in the field of time-triggered communications protocols (such as TTP/C), which will be used to satisfy fault-tolerant automotive system requirements. A cost-effective microcontroller based control system, which provides fault-tolerance will require control modules (like a TTP/C controller) integrated with microcontrollers to support the new fault-tolerant communications systems.
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