Today, the value of electronic components in automobiles has accounted for 15-20% of the total cost of automobiles. In the future, as more safety electronics, fuel consumption and fuel emission control electronics, communication and navigation systems, infotainment systems, and other electronic systems that enhance comfort are embedded in vehicles, the ratio could be as high as 30-40%.
At present, 20-50 electronic control units (ECUs) are required to implement the above functions, and almost 70-150 sensors are used. These sensors are responsible for measuring a wide range of environmental data, including pressure, temperature, flow, speed, acceleration, and angle. They send measurements to the ECU for engine and environmental control, airbag triggering for improved comfort and safety. Sensor inputs such as ABS, electronic stability program/control (ESP/ESC), and brake assist systems rely on.
In these applications, the self-diagnostic capabilities of various systems are becoming increasingly important. For example, if it is possible to detect a sensor defect directly in the sensing component, the ECU can obtain reliable data and make the right decision. For systems that are closely related to security, system disabling and emergency startup are important.
Web application in the car
An analysis of automotive electronic control systems shows that the complexity of these assemblies is exponentially rising. Simple electronic control and regulation equipment has been replaced by more sophisticated IT systems. Among them, in addition to the actual hardware, software and two-way communication between ECUs has become a new focus.
For example, it is possible to access each individual ECU via the diagnostic CAN bus, ask for its status, read the error program code, or even refresh the program firmware. Today, sensors are often shared in many applications for cost reasons. This means that the measured values ​​of one sensor module will be processed by several ECUs.
A large number of applications in vehicles have turned into web applications. The common architecture of the past (ie, an ECU implementing an application) has been replaced by network functions shared by multiple ECUs.
Figure 3 is a functional tree of the trunk lid. Here, opening the trunk actually requires starting two ECUs. The remaining ECUs are used to perform display and control functions.
Any error will cause a system failure. There are six error modes that can occur when opening the trunk lid. It should be that an error caused the sensor to malfunction, which could result in more than a dozen different inputs in the ECU's fault memory. From the distribution of these error program codes, it is necessary to obtain more detailed sensor diagnostic information than ever before.
The communication protocol currently used by automotive sensors is still an analog output. This is a typical point-to-point connection—a sensor connected to an ECU and using voltage as its output signal. Although some improvements have been made, such as increasing resolution or increasing the diagnostic range (LDR, UDR, see Figure 4), the analog output is still at the heart of the technology since the 1990s.
The analog output only allows sensor signal transmission within the signal range (eg 10-90%) and switches the low diagnostic range (LDR) and high diagnostic range (UDR) to a fault condition via a switch. Therefore, it cannot transmit more detailed fault information.
The solution to this problem is to use digital communication between the sensor module and the ECU to transmit status information, time stamps, and error program codes in addition to sensor data. Unfortunately, the problems caused by the transition to digital communications are extremely complex, because the types of sensors vary widely and the architectures used by different sensor vendors vary (see Figure 5).
From a simulation perspective, a variety of sensors for all environmental variables are available on the market, and almost all ECU microcontrollers have analog inputs. Therefore, there is no big problem or big risk in developing new applications using existing components on the market or products that require only fine-tuning.
However, such a situation is not suitable for digital communication protocols. The standard protocols available must be used in a specific way. Currently available digital protocols include:
CAN: Overall too complicated, the sensor cost is too expensive;
LIN: supports only low transmission rates up to 19,200 baud;
External sensor interface (PAS4, PSI5): Developed for safety applications such as airbags, requiring 9V operating voltage and high current consumption;
SENT: It can only support one-way, and it is still in the standardization stage.
As a result, proprietary solutions are often used in applications that require digital communications. This means that each circuit manufacturer has its own proprietary protocol. ZACWire (Serial Digital Interface), which supports the ZMD31150 and ZMD, provides an open standard that provides communication security with flexibility in baud rate and end-of-line calibration.
The challenge for the next few years is to develop and implement a cost-effective, standardized digital interface that takes into account sensor systems and application requirements. The interface must meet the following three somewhat contradictory design conditions:
Circuit testing: maximizing communication speed in order to minimize test costs
Calibration: as simple and flexible as possible
Application: As fast, safe and compatible as possible, especially in conditions where the operating voltage is out of specification, EMC is high and maximum RF radiation is limited.
The application of automotive sensors in safety is increasing. For brake assist systems that can reduce the braking distance under dangerous braking conditions, a sensor is needed to measure the brake system pressure so that the ECU can detect the brake action from the driver. The sensor is the key to starting ABS, so the sensor must be 100% accurate. To ensure this, the self-test feature must be as comprehensive as possible.
If the Sensor Signal Conditioner (SSC) IC detects a sensor failure in the module (such as a sensor short circuit), or if the SCC fails due to an external fault, the ECU must be able to determine these issues. For example, ZMD31150 can be utilized to illustrate how to deal with the above issues. The ZMD31150 is an SSC for signal conditioning in automotive applications.
The diagnostic functions performed in the ZMD31150 (see Figure 6) will continuously monitor the sensor function and SSC.
Once a fault is detected, the diagnostic mode (DM) is initiated. An error flag will be set in the digital communication message or the analog output will be switched to the pre-programmed diagnostic range LDR or HDR.
There are two types of detectable faults, hardware and software errors. A hardware error is a failure caused by a hardware problem detected in the SSC. In this example, the signal conditioning is terminated and the DM is activated.
Conversely, the cause of software errors is not always so clear or continuous. They can be caused by external causes such as EMC interference or other electrical loads on the system board. For software errors, an error counter is used here to perform a '+' operation when the error occurs and a '-' operation when the error is no longer generated. When a software error is not detected, the software error message is filtered low and the sensor returns to normal operation mode. This practice is called temporary diagnosis of DM.
The Temporary DM in the ZMD31150 is an option to provide reliable error messages when errors persist. With additional information (such as redundant sensors or extensive checks), the ECU will decide whether the current application can continue to work reliably or must be shut down based on the error message.
If an inductive load (Schaffner Pulse 3a or 3b) is switched on and a fault is coupled to the supply voltage of the sensor system, the fault can also be coupled to the sensor, thus triggering a self-diagnostic function. But with a temporary DM, this situation has to occur several times before reporting errors to the ECU. Since the error counter filters the results, obvious error messages and corresponding misleading will be avoided.
For example, many drivers have experienced an abrupt signal on the dashboard, or the 'Check Engine' light is on, accompanied by a message to contact the repair shop. Sometimes the message no longer appears the next day, and the repairman replaced a module or sensor and found no problems. Proper software filtering can eliminate such annoying things.
Summary of this article
The use of sensor signal conditioning ICs can greatly simplify the development of automotive safety sensor systems. A self-diagnostic function that ensures 100% correct sensor output can only be implemented during the signal conditioning phase. For this reason, this function must be implemented on-chip. Components such as the ZMD Sensor Conditioning IC incorporate comprehensive self-diagnostics. By configuring the EEPROM, you can precisely define an error and define how the system reacts. The various execution procedures that respond to detected error events help to avoid obvious false error messages and thus increase the reliability of self-diagnosis.
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