Control mechanisms on Modern Cars – The control system in modern cars plays a very important role. In modern cars, not only machines that have sophisticated control systems but also safety systems and even entertainment systems are getting a touch of today’s advanced technology. It is not surprising that the control system has become the main attraction for automotive lovers to learn lately. Even for some workshops, this may be one of the main focuses that they must learn in order to improve the abilities of their mechanics.
Control mechanisms on Modern Cars
In this section we will discuss control mechanisms. To understand this easily, there are a few things we need to understand first. First, we have to understand network topology, then we also have to understand network organization, and lastly is about OSI reference model. After we have read this at least, then we can read and understand the discussion of control mechanisms more easily.
Event Control – Control mechanisms on Modern Cars
In an event-driven bus system, messages are transmitted as soon as an event that triggers the transmission of a message has occurred (Fig. bellow part a). Examples of such events are:
▶ Pressing a button on the air conditioning system control panel
▶ Operating the hazard warning flasher switch
▶ Incoming message that requires a reaction (e.g. information from rpm sensor to speedometer needle motor)
▶ Expiration of a fixed time period (time frame, e.g. 100 ms), after which messages are transmitted cyclically
Since the stations are not synchronized with each other, situations, where several stations wish to access the bus simultaneously, are unavoidable. In order to allow a message to be transmitted without falsification, only one station at a time can transmit data on the bus. Collision avoidance mechanisms are available for preventing or solving bus conflicts.
If a node wishes to transmit a message whilst the bus is occupied, the transmission is delayed (Fig. part b). A station that is ready to transmit must then wait until the transmission that is currently in progress has been completed.
Since bus access is subsequently renegotiated, the transmission may be delayed yet again. These delays become problematic if the bus becomes overloaded by a large number of network subscribers that wish to transmit messages. In this case, messages may be lost if the transmitter abandons the transmission due to excessive delays.
Event-driven bus systems are suitable for reacting to asynchronous (unforeseen) events as quickly as possible. In an ideal case, they reduce the delay between the occurrence of the event and the message transmission (latency time) compared to time-driven systems. However, the latency time can vary considerably depending on the network loading.
▶ High level of flexibility and capability of retrofitting new nodes in the network
▶ Good response time to asynchronous external events
▶ Bus usage depending on event frequency in line with requirements
▶ No network loading by unused events, since only events that have actually occurred trigger a transmission
▶ Static bus occupancy, non-deterministic (i.e. not possible to prove that a message was transmitted at the right time)
In the most recent developments in dynamic driving systems such as brakes and steering, an increasing number of mechanical and hydraulic components are being replaced with electronic systems (x-bywire). Mechanical connections such as the steering column are becoming superfluous, and the functionality thereof is being taken over by sensors and actuators. The reliability, safety and failure tolerance requirements of these systems are extremely high. This means:
▶ Messages must be received on time
▶ The latency time of critical messages must be extremely small
▶ The system must have a redundant design
▶ The failure of a node must affect the rest of the system as little as possible and
▶ It must be possible to achieve a safe operating status from any fault situation
X-by-wire systems require close networking by the various components. The external increase in complexity places new demands on the safety, failure tolerance and availability of the communication system. The demands that are made of the electronic and network architecture therefore also increase. Reliable, fault-tolerant network architecture is required so that data is transmitted with guaranteed transmission characteristics, and electronic system malfunctions are handled in the most efficient way.
System architectures for real-time applications meet these requirements because their behavior is predictable and verifiable because of the way in which they are constructed. In these protocols, time windows within which a node is permitted to transmit are assigned to the control units in the communication network (nodes) during network planning (Fig. bellow). In order to comply with the time window, the nodes must be synchronized as precisely as possible.
All transmissions are processed sequentially in accordance with the network planning (without collisions). Once each node has transmitted its message, the cycle restarts with the first transmitter. This makes it possible to determine how chronologically up-to-date the data is at any time. Since missing messages are detected immediately, time-triggered concepts are more reliable than event-driven systems.
If a fast data rate is required in a time-triggered system, the time delay between the occurrence of an event and the transmission of the data can be so small that the system complies with strict real-time requirements.
The bus can be protected from unauthorized access by a bus guardian. The bus guardian prevents a defective node from interfering with network communication by transmitting messages outside the relevant transmit window.
These characteristics make it possible to create redundant, fault-tolerant systems in which transmission errors can be remedied and faults in the network can picked up by network nodes that can provide the functionality without errors.
▶ Deterministic system
▶ Punctual data transmission
▶ Reliable detection and isolation of defective network nodes
▶ The overall system must be planned for
▶ The capacity for expanding the communication system must be planned in
▶ Good response time to asynchronous external events
If a communication system allows independently developed subsystems to be integrated in an overall system, it is said to support composability. An important criterion when doing this is that the properties that have been assured for the functionality of a subsystem are not adversely affected by adding other subsystems. If this has been ensured, the checking of system functionality is restricted to subsystem checking that can be carried out by the constructor of the subsystem.
If a communication system supports composability, changes can be made to a control unit without affecting the functionality of other control units. It is therefore not necessary to recheck the entire system after integrating a modified control unit – it is sufficient to check that the individual subsystems are operating reliably. Composability, therefore, reduces the time and cost of integrating new subsystems. This is the only way to increase the complexity of the electronics in the vehicle.