Advanced Automotive Fault Diagnosis
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LÔØI NOÙI ÑAÀU, ledklok
- Điều hướng trang này:
- Figure 2.8 Open loop system Inputs Control Outputs Figure 2.9
- Figure 2.10
- 2.8 On- and off-board diagnostics 2.8.1 On-board diagnostics
| 2.7.5 Block diagrams
Another secret to good diagnostics is the ‘block diagram’ approach. Most systems can be con- sidered as consisting of ‘inputs to a control which has outputs’. This technique means that complex systems can be considered in manageable ‘chunks’. Many complex vehicle electronic systems can be represented as block diagrams. In this way several inputs can be shown supplying informa- tion to an ECU that in turn controls the system outputs. As an example of this, consider the oper- ation of a vehicle alarm system (Figure 2.10). In its simplest form the inputs are the ‘sensors’ (such as door switches) and the ‘outputs’ are the actua- tors (such as the siren). The ‘control’ section is the alarm ECU. The diagnostic approach is that if all the sen- sors are providing the correct information to the control and the actuators respond when tested, Inputs Control Outputs Figure 2.8 Open loop system Inputs Control Outputs Figure 2.9 Closed loop system Door switch Voltage sensor Control switch Movement sensor ECU Warning light Siren Figure 2.10 Block diagram 20 Advanced automotive fault diagnosis then the fault must be the control unit. If a sensor does not produce the required information then the fault is equally evident. 2.8 On- and off-board diagnostics 2.8.1 On-board diagnostics On-board diagnostics refers to the systems on the vehicle carrying out some form of self-monitor- ing. The more complex automobiles become, the greater the number of electronic systems and the more difficult it is to register the actual condition in case of a defect. Many connecting cables and adapters are required to achieve this. Data about the different systems and their working together is needed to allow a system specific diagnosis. Modern elec- tronics with self-diagnosis supports the techni- cian by registering actual values, comparing them with the nominal values, and diagnosing faults that are stored for repair purposes. Internal to an ECU, a checksum of the program memory is calculated. Then a read and write test of the random access memory (RAM) is performed. Other elements such as A/D (analogue/digital) converters are also checked within this test cycle. During the operating time of the vehicle, the ECUs are constantly checking the sensors they are connected to. The ECUs are then able to determine whether a sensor has a short circuit to ground or battery voltage, or if a cable to the sensor is open circuit. By comparing the measured values and the stored data, an ECU is able to determine whether the measured values exceed or are still within the tolerance required. Combining information pro- vided by other sensors allows the ECU to monitor for plausibility of the sensor signals. Measuring the current normally taken by their circuits is used to carry out a check on actuators. Powering the actuator and observing the reaction of the system can test the function of an actuator in some cases. If discrepancies to the nominal values are diag- nosed, the information is stored in an internal fault memory together with other parameters, such as engine temperature or speed. In this way, defects that appear intermittent or only under certain con- ditions can be diagnosed. If a fault occurs only once during a set period of time, it is deleted. The fault memory can be read later in the workshop and provides valuable information for the technician. When a defective sensor is detected, the meas- ured values are replaced by a nominal value, or an alternative value is calculated using the infor- mation from other sensors to provide a limp- home function. With the help of an appropriate code reader or scanner, a technician can commu- nicate with the ECUs, read the fault memory and the measured values, and send signals to the actuators. Another task of self-diagnosis is to indicate a defect to the driver. A warning light on the dash- board is the most common method used to do this. Regulations concerning exhaust emissions mean an extension of self-diagnosis is desirable. The control units will soon have to be able to control all exhaust gas functions and components and to clearly indicate a defective function or the exceeding of the permissible exhaust limits. Chapter 5 covers this subject in detail. tải về 7.07 Mb. Chia sẻ với bạn bè của bạn:
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