There are many factors in the failure of instrumentation equipment. From the point of view of the cause of the failure, the fault can be divided into hard fault and soft fault. Hard faults mainly refer to components that cause faults, internal leads of integrated circuits, circuits, printed circuit board (PCB) jumpers, and so on. Soft faults mainly refer to changes in the internal logic state of the instrumentation and the design of system software and application software. Some faults are either hard faults or soft faults, such as communication faults.
Faults can also be classified into static faults and dynamic faults. Static faults are those that have a stable error output for a given input and a good repeatability of the fault symptom. Dynamic faults are random or intermittent faults that occur when the signal changes. The former is easier to observe, detect, study and exclude than the latter. Fault Cause Open or Open Circuit In terms of logic state characteristics, such fault is a fixed fault. The input circuit of the output and feedback signals of the instrumentation device is disconnected or open.
Power failure This type of fault is a static fault. The cause of the power failure is caused by the open power line or ground wire, incorrect wiring or poor contact. It may also be due to the input voltage of the instrument or the power supply component of the circuit board exceeding the allowable deviation, and the output of the power supply component itself. The voltage exceeds the allowable deviation (such as an abnormal increase or decrease in voltage).
Passive device failures such as open circuit of the resistor end cap loose, open or open capacitor, capacitance value or resistance value changes, resistor burned. Changes in the resistance value may cause malfunctions such as ambiguity of the logic value, and variations in the capacitance value may cause malfunctions such as poor decoupling, variations in the oscillator frequency, and failure to start the motor or the like.
Power supply decoupling This type of fault is mainly caused by the interference waveform (or signal) generated being superimposed on the normal waveform. Large-capacity filter capacitors and high-frequency ceramic capacitors can be used to suppress this interference.
Interference with this fault is due to the fact that the signal on one line is coupled to the other line by induction. The size of the crosstalk signal is proportional to the line spacing and signal frequency. If the strong electricity and the weak electricity are mixed in the same cable, the weak electric signal is bad or even the signal channel is burned.
Application software design failures are generally fixed faults. If the design of the database is unreasonable or improperly modified, the logical design idea does not meet the process requirements. Such failures or defects occur more frequently in engineering practice.
The design failure of the instrumentation equipment is caused by the designer's inconsistency, or the instrumentation equipment is harsh on environmental conditions, process quality, and component quality. Such failures or defects occur more frequently in engineering practice.
Auxiliary equipment or devices Unreliable devices or devices have poor reliability due to various reasons, such as unreliable node or contactor performance, mechanical failure, and the like.
The operating environment of the instrumentation equipment does not meet the requirements. Such a fault is that the use environment of the instrumentation equipment does not meet the requirements of the instrumentation equipment and its components for the use environment, such as not meeting the environmental temperature and humidity requirements.
The PCB fault is caused by short circuit or open circuit of the PCB power supply, wrong jumper address, especially the high integration IC chip causes transient failure or permanent damage due to manufacturing defects, overheating and other reasons.
Human error occurs when the chip or board PCB or IC is incorrectly installed and used, causing damage to the PCB or IC. Especially IC, you must be especially careful.
Poor contact is a common failure that can cause headaches for maintenance personnel. The symptoms of the malfunction are similar to those of the open circuit, but they have certain contingency, and the initial stage of the fault is extremely difficult to find. Common causes of poor contact are: loose plugs, poor soldering, oxidation of the contact surface, weak terminal wiring (sometimes due to large environmental vibrations), and elastic degradation of the contact reed. The most important thing is to make sure it is secure when it is first installed.
Poor insulation such faults are easily overlooked. Poor insulation caused by poor surface treatment on the board will cause signal strings and leakage, making the board unstable or even unable to work properly.
Poor shielding can cause related faults such as signal distortion or distortion.
Component aging and poor quality Any equipment has a normal service life. After this period, all components will enter the attenuation period, and the failure rate will increase greatly. Poor quality components can also cause high failure rates.
Computer communication failure Computer communication failure is actually a failure that occurs during data transmission.
1. The failure of data transmission is manifested as no data transmission or data transmission is disorderly and unrecognizable. The main reasons are: transmission or receiving device circuit failure, modem failure, signal channel failure, etc.
2. The error manifests itself as an error in the individual code. The main reasons are: external interference and channel quality are not high.
3. The main causes of signal distortion are: the output of the transmitting end, the input impedance of the line and the receiving end are not matched, and the receiving, receiving or transmitting are interfered.
4. The main causes of accidental failures are: poor contact such as solder joints, connection plugs of the interface, looseness due to vibration and other reasons. The basic strategy and method of fault finding The process of fault finding is a process of determining the cause of faults and a test analysis, logical reasoning and verification confirmation of fault points. The basis is based on past experience and knowledge, and observation and analysis of phenomena. And test, verify and confirm. Basic Strategy for Fault Search The basic strategy for fault finding can be expressed as the following process: Recording observations and test phenomena and data—analysing phenomena and organizing data—logical reasoning—in-depth testing and verification—reanalysis and collation—retesting verification... The fault was finally diagnosed.
Basic Approach to Fault Finding In order to quickly identify the nature and location of a fault, effective methods and procedures are generally required. Fault finding generally consists of three steps: fault checking, fault diagnosis, and troubleshooting. Fault Checking A fault check is a comprehensive understanding of the state of the instrumentation (or components) and its fault performance in order to find clues to the fault.
Routine inspection This is a routine inspection, but it is very necessary. Generally divided into two steps:
a) Overall inspection. This is mainly based on the observation of the fault, such as visual inspection (such as whether there is falling off, burning, abnormal smell, mechanical damage and power insurance, etc.).
b) Functional check. In the power-on state or online check the function of the instrumentation, this can better grasp the overall performance of the fault. This has good guiding significance for further examination and diagnosis.
After the static check is energized, observe and test the basic state of the instrument equipment (such as indicator light, power supply voltage, etc.), and then further check the key points of the instrument equipment. Most of the faults are exposed during the static check. The diagnosis of the fault is to simplify the diagnosis of the fault of the instrument. First, it needs to be divided into several subsystems, and then the subsystems are separated and checked in turn. For the fault diagnosis and diagnosis of instrumentation equipment, the following methods or strategies can generally be adopted:
a) Inject the signal. When the previous stage cannot produce normal output due to fault or subsystem separation, an additional signal is required to perform dynamic characteristic check on the subsystem. If multiple signals are required to be added at the same time, the influence between signals should be reduced as much as possible to better analyze the cause of the fault.
b) Disconnect the loop. For the subsystem with feedback link, in order to facilitate the determination of the fault location, the feedback loop should be disconnected (information source ). Note that in some cases the factor system or loop needs to add the appropriate level or signal at the break of the link.
c) Search forward or backward to narrow the suspect area. When signal injection is required, the strategy should be taken from the signal injection end to the signal output direction until it is abnormal; when the known signal is not normal, the strategy of searching from the output end to the input end can be used until the signal is normal. .
d) Intermediate separation. In order to speed up the fault search, the subsystem can be split in half.
e) Comparison method. To identify and diagnose a fault, it can be compared to the condition of a working subsystem or circuit, or by referring to the service manual.
f) Replacement method. When it is suspected that the circuit board and components are faulty, the circuit board and components that have been confirmed to be replaced are replaced by subsystems to aid diagnosis. Once the troubleshooting has been diagnosed, it is not too difficult to repair and troubleshoot as long as it is not a fundamental defect in the design. Pay attention to the necessary isolation measures when performing multiple fault repairs. After the repair work is completed, a careful inspection is required. Then verify the repair.
When the instrument equipment fails, whether it is a system-level fault or a component-level fault, the basic judgment and analysis of the fault is required first, that is, the fault preview. Firstly, the eyesight inspection is carried out. If the visual inspection is not closed, it is necessary to analyze whether the local components can be replaced or the parts that are not closed can be repaired and replaced according to the conditions of the visual inspection. If the visual inspection is completed, Then carry out power check, check the power supply circuit or power supply components to ensure the normal operation of the power supply, so that the next step of maintenance can be carried out. Then, decide on the strategies and methods for troubleshooting.
Summary As long as we master the principle and structure of instrumentation, carefully summarize and strengthen the exchange of practical experience, and at the same time master the correct fault detection and troubleshooting methods, we will be able to better analyze and solve any faults in the instrumentation.
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