Like other instrumentation equipment, smart meters may also malfunction or even be damaged during use. Once such problems occur, they should be resolved as soon as possible and the instrument can resume normal operation. Of course, after the introduction of the microcomputer in the smart meter, although the function of the meter is greatly enhanced, it also adds certain difficulties to the diagnosis and troubleshooting, including the diagnosis and processing of the microcomputer hardware and software. This requires that the use and maintenance personnel of smart meters must have certain knowledge of fault diagnosis, overhaul and maintenance of intelligent equipment.
First, common fault types
The types of faults of smart meters can be generally divided into two categories: hardware faults and software faults.
1. Common hardware failures
(1) Logic error
Logic errors in the instrument hardware are usually caused by design errors, process errors during processing, or other factors in use. Such errors mainly include: wrong lines, open circuits, short circuits, phase errors, etc., where short circuits are the most common and difficult to eliminate. The smart meter often requires a small size in the structural design, so that the wiring density of the printed circuit board is high, and foreign matter in use often causes a short circuit between the leads to cause a malfunction. Open circuit faults are often caused by poor quality metallized holes in the printed circuit board or poor contact of the connectors.
(2) Component failure
There are two main reasons for component failure: First, the components themselves are damaged or poor performance, such as: resistance, capacitor type, incorrect parameters, integrated circuit damage, device speed, power consumption and other technical parameters do not meet Requirements, etc.; second, component failure due to assembly reasons, such as: capacitor, diode, triode polarity error, integrated block orientation installation error.
(3) Poor reliability
There are many factors that the system is unreliable. For example, metallized holes and poor contact of the connector may cause the system to be good or bad, and cannot withstand vibration; internal and external interference, excessive ripple factor of the power supply, and excessive load on the device. The logic level is unstable; in addition, the unreasonable routing and layout can cause poor system reliability.
(4) Power failure
If the smart meter has a power failure, it will cause damage to the device after power-on. The fault of the power supply includes: the voltage value does not meet the design requirements; the power lead-out line and the socket do not correspond; the short-circuit between the power supplies of each gear; the power of the transformer is insufficient, the internal resistance is large, and the load capacity is poor.
2. Common software failures
(1) The program is out of control
When the fault phenomenon is run in a breakpoint continuous mode, the target system does not operate according to the specified function or has no result. This is due to the transfer of the program to an unexpected location or a loop somewhere. The reasons for this type of error are: the transfer address is incorrectly calculated in the program, and the working register conflicts. When using a real-time multitasking operating system, the error may be in the operating system, and the correct task scheduling operation is not completed; or in the high priority task program, the CPU is in a dead loop.
(2) Interrupt error
1 does not respond to interrupts. The CPU does not respond to any interrupts or does not respond to an interrupt. This erroneous phenomenon is a prescribed operation that does not execute the interrupt service routine during continuous operation. When the breakpoint is set in the interrupt entry or interrupt service routine, the breakpoint is not encountered. The cause of the error is: the initial value of the interrupt control register (1E, IP) is set incorrectly, so that the CPU does not have an open interrupt or does not allow an interrupt source request; expansion of the on-chip timer, serial port and other special function registers There is an error in the I/O port programming, causing the interrupt not to be activated. An interrupt service routine does not use the RETI instruction as the instruction to return to the main program. Although the CPU has returned to the main program, the internal interrupt status register is not cleared and thus does not respond. Interrupt; an external interrupt request is invalidated due to a hardware failure of an external interrupt.
2 loop response interrupt. This type of failure is that the CPU cyclically responds to an interrupt, preventing the CPU from executing the main program or other interrupt service routines normally. Most of these errors occur in external interrupts. If an external interrupt requests an interrupt in level-triggered mode, then the interrupt service routine does not effectively clear the external interrupt source (for example, the 8251's transmit interrupt and receive interrupt cannot be cleared when the 8251 is disturbed), or the interrupt is caused by a hardware fault. Always active, at which point the CPU will continuously respond to the interrupt.
(3) Input/output error. Such errors include cluttered input operations or no action at all. The cause of the error is that the output program is not well coordinated with the I/O hardware (such as address errors, written control words, and specified I/O operations, etc.); there is no synchronization in time; there are still faults in the hardware.
In short, software faults are relatively hidden and easily overlooked. It is generally difficult to find, and usually requires testers with rich practical experience.
Second, the basic method of fault diagnosis
Since the introduction of the microprocessor into the meter greatly enhances the function of the smart meter, it also increases the difficulty of diagnosing the fault and troubleshooting. First of all, it is difficult to judge whether the instrument fault belongs to software fault or hardware fault. This work requires maintenance personnel to have rich microprocessor hardware knowledge and certain software programming technology to correctly judge the cause of the fault and quickly eliminate it.
Although the fault diagnosis can be performed by using the self-diagnosis program, any diagnostic program must be operated under certain circumstances, such as the environment where the power supply and the microprocessor are working properly. When the fault of the system has destroyed the environment and the self-diagnosis program itself cannot run, it is naturally impossible to diagnose the fault. In addition, the results listed in the diagnostic program are sometimes not unique, and it is not possible to locate the specific part or chip. on. Therefore, if necessary, it should be supplemented by manual diagnosis to be effective. Here are some basic ways to diagnose a fault.
1. Tap and hand pressure
When the instrument is used, it often encounters the phenomenon that the instrument is running well. This phenomenon is mostly caused by poor contact or virtual welding. For this case, “knock and hand pressure method†can be used.
The so-called tapping is to gently tap the board or part through a rubber hoe or other tapping object to see if it will cause an error or a shutdown failure. The so-called hand pressure is that when the fault occurs, after the power is turned off, the plugged components and the plug socket are re-pressed by hand, and then try to eliminate the fault. If it is found that the casing is normal, it is best to re-insert all the plugs and try again; if the instrument is normal after hand pressure, then try to remove the contact of the pressed parts or plugs; if the above method is still unsuccessful, Then choose another method.
2. Using the sensory method
This method uses vision, smell, and touch to detect faults and identify areas of fault. In some cases, damaged components can change color, blister, or burnt spots; burnt devices can produce some special odors; faulty chips can become very hot.
In addition, sometimes a virtual weld or a weld joint can be observed with the naked eye.
3. Pulling and unplugging
The so-called "plugging and unplugging method" is a method of judging the cause of the fault by plugging and inserting some plug-in boards and devices in the smart meter machine. If the meter returns to normal after removing a plug-in or device, the fault has occurred here.
4. Component exchange test method
This method requires two instruments of the same model or sufficient spare parts. Replace a good spare with the same component on the faulty machine to see if the fault is eliminated to find the faulty component or faulty board.
5. Signal comparison method
This method also requires two instruments of the same model, one of which must be in normal operation. Use this method to have the necessary equipment, such as a multimeter, oscilloscope, etc. According to the nature of the comparison, it can be divided into voltage comparison, waveform comparison, static resistance comparison, output comparison, current comparison and so on.
The specific method is to let the faulty meter and the normal meter run under the same conditions, then detect the signals of some points, and then compare the two sets of signals measured. If there is a difference, you can conclude that the fault is here.
This method requires the maintenance personnel to have considerable knowledge and skills.
6. Lifting temperature method
Sometimes, the meter will work longer or it will fail when the working temperature is high in summer. It is normal to shut down the check, and it will be normal after stopping for a while, but it will be faulty after a while. This fault is caused by the poor performance of individual integrated circuits or components, and the high temperature characteristic parameters are not up to the index requirements. In order to find out the cause of the malfunction, the temperature rise and fall method can be used.
The so-called cooling, that is, when the fault occurs, use a cotton swab to wipe the anhydrous ethanol in the area where the failure may occur, to cool it, and observe whether the fault is eliminated. The so-called warming is to artificially raise the ambient temperature, such as bringing the heated soldering iron close to the suspected part (note, do not raise the temperature too high to damage the normal device), and see if the fault occurs.
7. Riding the shoulder method
The “shouldering method†is also called “parallel methodâ€. Install a good integrated circuit chip on the chip to be inspected, or connect good components (resistors, capacitors, diodes, transistors, etc.) in parallel with the components to be inspected to maintain good contact. If the fault is caused by an open circuit or poor contact in the device, this method can be used to eliminate it.
8. Capacitor bypass method
When a circuit produces a strange phenomenon, such as a display on the display, the "capacitor bypass method" can be used to determine the part of the circuit in question. For example, connect the capacitor across the power supply and ground of the integrated circuit; connect the transistor circuit across the base input or collector output to observe the effect on the fault. If the capacitor bypass input is inactive and bypasses its output, the fault disappears and the problem occurs in this stage of the circuit.
9. Change the original state method
In general, do not touch the components in the circuit before the fault is determined, especially for adjustable components, such as potentiometers. However, if a reset reference measure is taken in advance (for example, position mark or measured voltage value or resistance value before untouched), it is allowed to be touched if necessary, and the fault may be eliminated after the change.
10. Fault isolation method
The “Fault Isolation Method†does not require comparison of the same type of equipment or spare parts, and is safe and reliable. According to the fault detection flow chart, the segmentation and enveloping gradually narrows the fault search range, and then cooperates with the signal comparison and component exchange methods, and generally finds the fault.
11. Using tool diagnostics
Test, analyze and judge integrated circuit chips, resistors, capacitors, diodes, transistors, thyristors and other components with maintenance tools and test equipment. The main contents of the test observation are: signal waveform, current, voltage, frequency, phase and other parameters, and fault diagnosis is performed based on the obtained information.
12. Direct experience method
After a certain period of maintenance practice, the maintenance personnel are familiar with the instrument system used, and have accumulated rich experience to know what features are in the part, what is the normal phenomenon, and what is the abnormal phenomenon.
When the system fails, it is often possible to find out the fault and quickly eliminate it by direct observation.
13. Software Diagnostics
"Software Diagnostics" is also an effective troubleshooting method for smart meters. Usually smart meters are equipped with automatic fault diagnosis, which is implemented by pre-programmed software programs.
Third, the fault handling method
Some methods of fault diagnosis are introduced above, but the exact part of the fault diagnosis can only be said to be a large part of the repair work, and the remaining 10% of the tasks are repair work. Even if this small part of the work is not taken seriously, it will not reach the expected goal or even fall short. This article focuses on some of the knowledge of smart meter repairs that are also applicable to the repair of any electronic product.
1. Remove the replaced components
If it has been diagnosed that a component has been damaged, or if it is suspected to be defective, remove it from its original position. This work is relatively easy for two terminals or integrated circuits with sockets, but for those integrated circuit chips or multi-head components directly soldered on printed circuit boards, such as transistors, relays, resistors, etc. Not easy. To remove such components, the following methods are generally available.
(1) Use a "plastic pipette", that is, a soldering iron without a soldering iron. The process is: first use the soldering iron to heat the solder to be removed until it is melted, then align the vacuum tube with the hot solder, quickly remove the soldering iron, and relax the spring on the vacuum pump, so that the solder can be sucked into a tube. room.
(2) After smoothing it with a large injection needle for veterinary use, while heating the solder with a soldering iron to melt it, quickly insert the needle into the terminal to separate the solder from the terminal.
(3) First use a copper wire band to contact the solder, then heat the copper beam near the solder joint, the copper beam heats up quickly, and transfers the heat to the solder, the solder melts and enters the copper wire under capillary action. The drawstring, the solder is sucked away.
(4) A special tool called "starter" and "fusion head" is used. To solder the chip, simply insert the "starter" on the chip and use the "fusion head" to heat the back of the printed board. After the solder is completely melted, press the button above the "lifter". At this time, the spring piece produces an upward elastic force on the chip, and the chip will bounce off the board.
2. Remove welding residue
After removing the components, there is inevitably residual solder in the board holes. At this time, it can be heated to melt it, and then the toothpick or small nail is quickly inserted into the hole to be cooled and then pulled out, so that the hole can be kept open for later insertion of components. Another way to remove the solder that is stuck in the solder hole is to use a micro drill to drill the hole, but when using this method, be sure to remove all the debris from the hole. A magnifying glass can be used for careful inspection.
3. Repair the board
Before soldering new components, first check the wires or solder tabs that are not detached from the board. If the wire is broken, it should be re-welded to connect the wire. Use the 18# or 20# wire. It is best to use the printed line with adhesive on the back to reattach it to the damaged place, scrape off the oxide layer on both ends of the new printed line, so that it can be welded to the old line, and then remove the excess tin particles and drill. Pass all lead terminal holes that are filled with garbage.
4. Check for replacement components
It is necessary to check the replacement components before soldering. This requires maintenance personnel to have a high theoretical knowledge and testing technology, with the help of commonly used test instruments for resistance, capacitance, diodes, transistors, integrated circuit chips, etc. test and judge.
It is also possible to use a simpler method of inserting the terminals of the chip and other components into the corresponding solder holes, and plugging them with a toothpick, and then powering up. If the function is normal, you can remove the toothpick and solder the components.
5. Welding
There is no doubt that hand soldering is one of the least noticeable and easiest to operate errors in electronic repair. Many people not only have poor welding skills, but also often use the soldering iron. Welding is not simply a matter of joining two metals together. Its correct meaning is to melt and combine the two metals into a solid electrical connection that is mechanically joined together. The time and temperature requirements are very strict in this process. When the soldering iron is used correctly, manual soldering is usually completed in 1.5 s or less.
In order to clean the greasy, dusty or oxide layer at the weld, a good quality cleaning flux should be used. The key to successful welding is the soldering iron. To choose a soldering iron that has a working temperature that matches the board to be repaired, an electric soldering iron that is too low or too high is not correct. The tip should be as large as possible, but slightly smaller than the component being soldered. In order to prevent the soldering iron from being oxidized and corroded, a layer of tin is applied to the soldering tip at any time during use, which can accelerate the heat conduction and prevent it from being oxidized. The used soldering tip is always black and dirty, and has corroded pits. Its thermal conductivity is not so good. It should be rubbed with sandpaper and then re-ironed with tin to be used again.
Wiping with a wet sponge when the horn is not cooled is a erroneous method of wiping off the protective layer so that the surface of the horn is exposed to air and oxidized. The best way is to apply some new tin.
6. Commissioning
After the repair is completed, some parameters should be re-commissioned and tested, so that the performance of the instrument is consistent with the original product after the repair.
Only then will the entire maintenance task be completed, otherwise it will have to be repaired.
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