Fig. 2.17 A CAN bit sequence

The CAN-H wire switches between 2.5 and 3.5 V and the CAN-L wire switches between 2.5 and 1.5 V. When CAN-H and CAN-L are at 2.5 V, there is no voltage difference between them and this represents computer logic 0. Computer logic 1 is created when there is a 2 V difference between the two wires as happens when CAN-H is at 3.5 V and CAN-L is at 1.5 V.

2.12 Prototype network systems

In order to provide further insight into the way in which vehicle systems are networked it will be helpful to consider the following details of a concept vehicle that was developed by LucasVarity. Figure 2.18 gives an impression of the system.

Several references to networked systems, e.g. traction control, stability control etc., have already been made. The system shown in Fig. 2.18 is suitable for virtually any vehicle, including trucks and buses. The system comprises four subsystems.

1. The Lucas EPIC electronically-programmed injection control system, which is a computer controlled engine management system for diesel engines, similar to the one described in section 1.11.

2. The Lucas flow valve anti-lock braking system. On this advanced prototype vehicle, the ABS system is provided with a second solenoid valve at each front wheel that permits independent application of the brakes by using the ABS pump to supply pressure.

3. A clutch management system (CMS). This replaces the normal clutch pedal linkage with a computer controlled, hydraulically actuated system. The manual

Fig. 2.18 The LucasVarity advanced prototype vehicle

gearshift is retained, but there is no clutch pedal. The driver still lifts the foot from the accelerator pedal when changing gear. The advantage of this is that the driver gains two-pedal control without the fuel consumption penalty that is associated with automatic transmission. The driver also retains full control over gear change operations.

4. Adjustable rate dampers are fitted. The damping rate is adjusted by the computer (ECM) to provide optimum damping during rapid steering input, braking and acceleration.

Master controller

Each of the above systems has a CAN interface which permits them to be connected to the master controller. A network of twisted pair cables connects each of the above subsystems to the master controller and this allows the transfer of sensor information and control signals with reliable safety checking and minimal wiring. The master controller thus receives information from the subsystems via the CAN bus (cables).

The master controller is directly connected to a switch pack (for cruise and damper control), two accelerometers and an inclinometer (for hill detection). This means that the master controller 'knows' the complete status of the vehicle and the driver's requirements. The vehicle status information is processed by the master controller to generate control signals which are sent to the subsystems. These 'master' signals over-ride the normal operation of the subsystems to operate another tier of systems known as the integrated systems. In the event of CAN failure, each subsystem defaults to stand-alone operation.

The integrated systems

The four subsystems, i.e. EPIC, ABS, damper control and clutch management, are integrated (made to work together) to provide seven additional functions of vehicle management. The computer programs that do this controlling are executed by the master controller. These seven integrated systems are:

• traction and stability control

• cruise control

• engine drag control

• damper control

• centralized diagnostics.

Traction and stability control

The ABS wheel speed sensor signals inform the ABS computer of wheel to surface conditions. At low speeds, or when only one wheel spins (such as pulling away with one driving wheel on ice and the other on dry tarmac), the brake is automatically applied to the spinning wheel. At higher speeds, or when both wheels spin, engine power is reduced to eliminate wheel spin. These two strategies combine to give improved traction and acceleration, and safer cornering at higher speeds.

Cruise control

The vehicle speed sensor information is used by the engine control (EPIC) to maintain a constant vehicle speed that is selected by the driver. The cruise control switch pack provides commands for setting the desired cruise speed and for switching on and off as required.

Power shift

The power shift function automatically reduces engine power during gear changes. This means that the driver no longer has to lift the foot from the accelerator pedal and need only move the gear lever to effect a gear change. This reduction of engine power, via the controller, overcomes the difficulty of synchronizing the accelerator and gear lever movements. It is also possible to provide for 'blipping' of the throttle to give smooth downward gear changes.

Engine drag control

This eliminates wheel locking due to engine braking on very slippery surfaces, and improves anti-lock braking performance. The reduced engine drag is achieved by increasing engine power slightly to maintain the correct level of wheel slip for maximum retardation and stability. In extreme cases, the clutch can be disengaged to remove the inertia from the engine driveline. This allows the wheels to respond more quickly to the anti-lock brake control and gives improved steering capability and reduced stopping distances.

Hill hold

Hill hold uses brake actuation to apply the rear brakes automatically when coming to a halt on a hill. When re-starting on the hill, information from the inclinometer sensor, the EPIC system, the ABS controller and the clutch management controller is used to determine the point at which the brakes should be released to give a smooth pull away, with no roll back.

Damper control

The damper control system uses data from the CAN data bus to control the damping rate settings. The dampers are switched to 'firm' setting for optimum response to rapid steering input, braking and acceleration. On returning to 'normal' cruising, the 'soft' damper setting is selected for improved ride comfort.

Centralized diagnostics

The centralized diagnostics uses the master controller to monitor all of the networked systems. Data bus information is interpreted in order to detect and respond to faults in the subsystems and the network communications. By this means, fail safe operation is achieved and correct fault recognition is made. The diagnostic section is provided with an interface which permits an interrogation tool, the Lucas Laser 2000, to access diagnostic information. Future developments of this system are anticipated and these will include electronic power assisted steering, electronic braking, and active anti-roll bars.

2.13 Summary

The details of data bus communication that are outlined in this chapter are highly specialized and it is work that is normally of most concern to designers. However, vehicle repair technicians do encounter the terminology and it is helpful to have an insight into some of the details. Fortunately, diagnostic equipment manufacturers need to take care of the aspects that affect the suitability of their equipment for diagnostic tests on the vehicles that it is made for. It is probably in the area of equipment selection that a vehicle repair technician is most likely to feel the need for familiarity with serial data terminology. Equipment specifications sometimes contain references to some of the material contained in this chapter and it should help when arranging for a demonstration of an instrument's capabilities, which should always be a step in the process of making a decision about purchasing an item of equipment.

2.14 Review questions (see Appendix 2 for answers)

1. Serial data transmission:

(a) requires a separate wire for each bit that is transmitted between the computer and a peripheral such as a fuel injector?

(b) is faster than parallel transmission of the same data?

(c) is transmitted one bit after another along the same wire?

(d) is not used in vehicle systems?

2. In a multiplexed system:

(a) a data bus is used to carry signals to and from the computer to the remote control units?

(b) each unit in the system uses a separate computer?

(c) more wire is used than in a conventional system?

(d) a high voltage is required on the power supply?

3. In networked systems messages are divided into smaller packages to:

(a) prevent problems that may arise because some messages are longer that others?

(b) avoid each computer on the network having to have an interface?

(c) make the system faster?

(d) save battery power?

4. When DTCs are stored in an EEPROM:

(a) the DTCs are removed when the vehicle battery is disconnected?

(b) an internal circuit must be activated to clear the fault code memory?

(c) they can only be read out by use of a multimeter?

(d) they can only be removed by replacing the memory circuit?

5. A traction control system:

(b) is not networked because it does not need to work with other systems on the vehicle?

(c) is not used on front-wheel drive vehicles?

(d) can only operate on vehicles equipped with a differential lock?

6. The computer clock is required:

(a) to permit the time of day to be displayed on the instrument panel?

(b) to allow the time and date to be stored for future reference?

(c) to create the electrical pulses that regulate the flow of data?

(d) to generate the voltage levels required for operation of a data bus?

7. In the CAN system:

(a) a 'twisted pair' of wires is used so that the correct length of cable can be placed in a small space?

(b) a 'twisted pair' of wires is used to provide the two different voltage levels and minimize electrical interference?

(c) the 'twisted pair' of wires carries the current that drives the ABS modulator?

(d) it is used only for the engine management system?

8. The RAM of the ECM computer is:

(a) the part of memory where sensor data is held while the system is in operation?

(b) only used for storing of fault codes?

(d) not a volatile memory?

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