Digital control heads to autobahn

InTech, Oct 2001 by Lamoreaux, Matthew G

Automakers back new drive-by-wire control bus technology

Cruising down the freeway at 70 miles (116 kilometers) per hour, you might not think too much about fault-- tolerant critical control-but the auto industry is poised to cut the cables connecting your accelerator and brake pedal. Even your steering wheel will no longer be connected to your wheels.

That's right: Your life will depend on a sophisticated digital control network in the form of drive-by-wire systems.

When German-based Audi AG said it was backing time-triggered protocol (TTP), a specification developed by the Technical University of Vienna and promoted through the TTP Forum (www. ttpforum.org), the manufacturer likely decided the future of your car's critical control system.

BMW, DaimlerChrysler, Motorola, and Philips Semiconductors jumped in soon afterward. They announced they would develop a competing standard for an advanced automotive communication system called FlexRay (www.flexraygroup.com). It will also support a time-- triggered network architecture.

These activities should come as little surprise to industrial users. In many respects, automotive manufacturing has led the way in developing technologies that later find their way into other industrial applications. For instance, General Motors commissioned the development of the programmable logic controller in the late 1960s. Industrial control developers later made use of controller-area network (CAN), popularized by the auto industry for a number of onboard control applications. CAN now forms the base of industrial control networks such as DeviceNet and CANopen.

Now, the auto industry leaps forward again with highly sophisticated networks that will replace mechanical steering, braking, and accelerating.

Does TTP or FlexRay lead?

In the field of real-time control systems, there are two fundamentally different principles of how to control the activity of a computer system: time-- triggered control and event-triggered control. In a time-triggered system, all activities execute at certain points in time. Accordingly, all nodes in time-- triggered systems have a common notion of time, based on synchronized clocks. In contrast, in event-triggered systems all activities execute in response to external events.

Industrial firms participating in European research projects, such as X-by-Wire and Time-Triggered Architectures, worked cooperatively to refine the TTP technology. It integrates all services needed to design fault-tolerant, real-time systems with minimal effort.

Inside the TTP specification, however, one will notice it uses the time division multiple access (TDMA) media-access scheme, which is also used in the ControlNet industrial networking specification. Event-triggered schemes such as carrier-sense multiple access with collision detection form the basis of popular networks such as Ethernet, CAN, and DeviceNet.

But FlexRay safety bus proponents recently rejected TTP recently for safety-critical systems, according to EETimes.com.

It doesn't appear, however, that this is a competitive bus war. Both sides reportedly agree on the need for a unified standard that uses a time-triggering architecture considered key to reliable operation of such safety-critical systems as drive by wire, adaptive cruise control, collision avoidance, and active suspension. Event-based CAN buses typically used for power train and other automotive controls are not considered reliable enough for drive by wire. With a maximum data rate of 1 megabit per second (Mbps), CAN buses face speed limitations as well.

FlexRay supposedly offers fault tolerance and determinism needed for safety-critical applications and higher speed than CAN buses. Where TTP uses TDMA media access, FlexRay will use what it calls flexible TDMA time slots for data. The system targets a maximum of 64 nodes and operates at data rates of up to 10 Mbps. It also employs an independent bus guardian, which eliminates the potential of the "babbling idiot mode," in which a faulty sensor transmits a stream of meaningless data and locks up the controller.

One key area in which TTP and FlexRay diverge is speed. TIP typically runs at 2 Mbps over copper wire; FlexRay runs at up to 10 Mbps. But unlike TTP, which is already available, the FlexRay specification will sample by 2002 or 2003, say members of the FlexRay consortium, according to EETimes.com.

Time-triggered advantages

The TTP Forum claims several important advantages a time-triggered architecture has over event-triggered systems.

Because interactions among components are based on a predetermined synchronization, developers can build up the components of a software system independently and integrate them at a late stage of software development. The smooth integration of components facilitates management of the ever-increasing complexity of embedded real-time systems.

Time-triggered systems also provide predictable behavior. The progression of time, based on a periodic pattern, controls system behavior. That makes it possible to predict the temporal system behavior, and that eases system validation and verification time and expense.