With the price is expensive.
(3) Relay isolation. Since there is no electrical connection between the coil and the contact of the relay, the coil can be used to receive the signal and then transmit the signal through the contact, which can effectively solve the problem that the strong electric power and the weak electric signal are in contact with each other, and complete the interference isolation.
(4) Wiring isolation. Isolation is achieved through the layout of the board, mainly between the strong and weak.
1.4 Printed circuit board anti-jamming design
A printed circuit board is a carrier of circuit components that provides electrical connections between components. The quality of the printed circuit board design will directly affect the anti-interference ability of the system. When designing a printed circuit board, the following principles are generally followed:
(1) When wiring the crystal oscillator, try to be close to the pin of the central processing unit, ground the case and fix it, and finally isolate the clock area with the ground wire. This method can avoid many difficult problems;
(2) Under the condition that the system performance requirements are met, the central processing unit should use a low frequency crystal oscillator as much as possible, and the digital circuit should be as low as possible;
(3) For input and output port resources that are not used by the central processing unit, they cannot be left unconnected and should be connected to the system power supply or ground. The same is true for other chips;
(4) The wiring between the high-frequency components should be shortened as much as possible, and the components with input and output functions should be kept as far as possible, and the components that are easily interfered should not be too close;
(5) The current loop cannot appear in the low frequency and weak signal circuits. If there is no circumvention, the loop will be made as small as possible to reduce the induced noise.
(6) The 90° fold line should be eliminated during system wiring to prevent high frequency noise emission;
(7) The input and output lines in the system should not be parallel as much as possible, and a ground line should be added between the two wires, which can effectively prevent the occurrence of feedback coupling.
2 software reliability design
2.1 Digital Filter Design
At present, power meters have widely applied various metering chips, and the central processing unit and the metering chip communicate through serial peripheral excuses or universal asynchronous transceivers to obtain parameters for power system operation. If the bus is disturbed during the communication process, or the metering chip is in an abnormal state, the central processor will get the wrong data. Therefore, it is very important to add filtering processing to the software program. For the ordinary power parameters, the mean method can be used. When calculating the effective value, five to six data are collected, the maximum value and the minimum are removed, and then the average value is obtained; for the electric energy data, the unit time can be estimated according to the rated operating environment of the meter. If the power data is abnormal, the software can discard the data. In addition to this, there are a median method, an arithmetic mean method, a first-order low-pass filter method, and the like. Practice has proved that the use of software filtering can maximize the reliability of each read parameter.
2.2 Data redundancy design
In order to improve the reliability of the system, the setting parameters of the system and the calibration parameters can be multi-backup design. When a group of data is disordered, another set of backup data can be enabled. In order to ensure the security of data and increase the probability of data surviving in the wrong operation, several sets of data should be stored in a distributed manner.
2.3 Redundancy design of data verification and operation
When the central processor writes the setting parameters or the calibration parameters to the storage space, it may be disturbed, causing the erroneous data to be written into the storage space. However, the central processing unit cannot judge whether the written data is correct or not. . In order to ensure the normal writing of data, when designing the software program, the data to be written is processed as "checksum", and the "checksum" is also written into the storage space, when each write operation is completed. After that, a read operation is performed again, and the read data is subjected to a "checksum", and a comparison is made with the writing of "checksum". If the two data are inconsistent, the write operation is resumed until the data is correctly written. If the set number of rewrites is exceeded, the write operation error display is performed.
2.4 Software Trap Design
Software traps are an application form of instruction redundancy for the capture of programs "running". When the noise signal interferes, the system program will leave the normal running track. In order to stabilize the "running" program, the designer designs a trap in the program. The so-called software trap is to force the captured program to a specific address through a boot command, and error processing of the disordered program. For disturbing and confusing programs, multi-byte instructions are the most dangerous because the wrong pointer can "run" between multiple byte instructions, thus running deeper agnostic instructions. Compared with multi-byte instructions, single-byte instructions can smooth out the pointers in the disorder, let them run in the normal order, and the disorder can be effectively suppressed. According to the above principle, the software trap can form a program. Generally, in order to improve the capture rate of the "flying" program, two empty operation instructions can be added before the boot instruction, and the specific form is as follows:
--NOP-- --NOP-- JUMP ERROR
The JUMP ERROR in the program is to transfer the "flying" program to the error handler. Software traps are best used in large blocks of read-only memory space unused in the program, unused interrupt vector areas, "breaks" in the program area, and at the beginning and end of the table.
2.5 Software Watchdog Design
The "watchdog" is a combination of soft and hard to prevent the program from infinite loop. The hardware foundation of the "watchdog" is a counter that runs independently and has a timing period of T. The reset pin of the central processor is connected to the timing output pin of the counter and the central processor control counter is cleared. During normal operation of the system, the “watchdog†will be cleared during the time interval less than T, and the timer will not overflow. However, when the system is disordered and is in an abnormal working state, the timing logic of the central processing unit is scrambled, and the counter cannot be cleared in the period T, eventually causing the counter to overflow, and the "watchdog" generates a reset signal to be transmitted to The central processor resets it. This design can make the system get rid of the temporary interference and enhance the reliability of the system.
3 Other considerations
3.1 Component selection and control
Components are the basic unit that makes up the power meter. The reliability level of power meters depends first on the reliability level of the components. The main components include: central processing unit, measuring chip, digital tube, LCD screen, electrolytic capacitor, varistor, current transformer, voltage transformer, crystal oscillator, chip capacitor, chip resistor, optocoupler, battery, etc. Ankerui has strict requirements on the selection of components: the selected components must have reliability indicators, component purchases must be model specifications, fixed purchasers, fixed procurement channels, and purchased components must be inspected. Incoming inspection, pre-use inspection.
3.2 Margin Design
The margin design makes the working stress of the component under working properly lower than the rated value of the component, thereby achieving the purpose of reducing the basic failure rate and improving the reliability of component use.
3.3 Redundant design
The redundant design uses one or more identical units to form a parallel form. When one of them fails, the other units can still make the system work normally. The main signal lines and cables should be connected with high reliability. If necessary, redundant technologies can be used for switches, connectors, etc., such as using parallel or redundant contacts.
3.4 Reliable production process
Reliable production processes focus on electrostatic protection and moisture protection. The introduction of an electrostatic protection system in the production environment can effectively avoid damage to metal oxide semiconductor field effect transistors and integrated circuit chips. When the humidity of the environment is large, water molecules can penetrate into the interior of the material, forming a leakage path between the conductors, reducing the insulation resistance of the component and the isolation withstand voltage. At the same time, excessive dry environment can also cause the material to become brittle and produce static electricity and other adverse effects. Ankerui has introduced a series of control measures in the production environment, such as the use of dehumidification systems, air conditioning systems, etc., as far as possible to make the instrument production environment at a stable level, to avoid adverse external factors. In addition, some product boards are painted at the factory to further prevent moisture and improve the reliability of the power meter.
3.5 High temperature aging treatment
The hidden dangers and performance defects in the welding and assembly process of components in power meters can be exposed in advance by means of high temperature aging. After the processed products, normal electrical parameters are tested, and the variable and failed components are screened and removed, and the potential problems are eliminated before the product is sold, thereby ensuring that the manufactured products can stand the test of time.
3.6 Maintenance
In order to facilitate the post-maintenance principle, the component design layout should take into account the later maintenance work. Software design should fully consider the possible errors of the system, and list all possible errors. When the system prompts the error, the design and maintenance personnel can capture the error, solve the problem, and improve the reliability of the system operation.
4 Reliability test method
4.1 Hardware Reliability Test
According to the anti-interference design of the system hardware, the main test contents of the hardware are: electrostatic discharge immunity, radio frequency electromagnetic field immunity, fast transient pulse group, radio frequency field induced conduction disturbance immunity, surge immunity, attenuation Shock wave immunity, radio interference suppression, etc.
In addition, Acry has introduced high accelerated life testing and high acceleration stress screening equipment to accelerate the discovery of problems and improve the design and manufacturing process of power meters by applying higher stresses than those present in the environment.
4.2 Software Reliability Test
Embedded software reliability testing is generally performed using black box testing techniques. The general process for software reliability testing is to clarify reliability goals, develop test plans, conduct development operations, enter test preparation phases, perform reliability tests, analyze and evaluate, and provide reliability test reports based on test data.
5 Conclusion
This paper combines the experience of Ankerui to produce power meters for many years, explains the importance of reliability in power meter design, analyzes the types of interference sources and the causes of the use of the instrument, and focuses on the hardware and software for interference. Finally, it introduces a series of verification experiments after the end of product design, which proves that the design of the instrument is feasible and ensures the high reliability of the product.
Article source: "Electric World" No. 6 of 2015
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