How to analyze the measurement accuracy and reliability of a diffusion silicon input level transmitter measurement system
The diffusion silicon input level transmitter is widely used in electric power, chemical industry, water plants, sewage treatment, environmental monitoring, and other projects. However, the operational reliability of the system needs to be further improved. It often shows false alarms, wrong alarms, severe drift, and other phenomena in actual application. It will affect the whole machine that can not usually work, which brings many inconveniences to system maintenance, calibration, and use.
This paper starts with the idea of product reliability design and tries to design a liquid level measurement system with a diffusion silicon input level transmitter to provide some new ideas and methods for the reliability design of small intelligent instruments. The quantitative design of the reliability of the system equipment involves the demonstration of the system reliability index, the establishment and calculation of the model, the estimation and allocation of the reliability index, etc.
1. System function and structure
1.1 System functional design
A diffusion silicon input level transmitter measurement system should have basic functions such as measurement, calibration, display, range setting, migration, unit selection, alarm, communication, information processing, and control. In terms of functional division, the information processing is divided into two parts: low-end information processing and high-end information processing, where the low-end information processing includes functions such as display, calibration, range setting, migration, unit selection, alarm, and low-end communication; the high-end information processing is a developable platform based on liquid level data, which can be repositioned and developed by the user according to actual needs.
Liquid level measurement has different principles and methods according to the other aspects of the measurement object, environment, accuracy requirements, etc. In the contactable height, for liquid density, consistent liquid level measurement using pressure measurement simulation is simple, reliable, and has high measurement accuracy, which can meet most applications. Pressure measurement methods are highly reliable from the principle, and pressure sensors have developed rapidly in recent years.
The pressure sensor has been dramatically improved in terms of range, linearity, and stability, is smaller and less expensive, outputs millivolt voltage signals, is flexible in use, and is a design base component with good technical specifications and reliability.
1.2 System structure design
In terms of functional design, the available structure of the system should include a pressure measurement module, a signal conversion module, a signal transmission module, an information processing module, a display alarm module, and a control module. The pressure measurement module and the signal conversion module are designed to form the pressure transmission unit from the hardware module division. The information processing module, the display alarm module, the control module, and the signal transmission module include the secondary instrumentation unit. High-end information processing uses a microcomputer as the development platform.
2. Hardware design for secondary instrumentation
The hardware design of the secondary instrument mainly includes power supply design, A/D conversion design, microcontroller peripheral circuit design, display, alarm and relay control design, remote communication design, etc. The primary consideration of the power supply design is that the transformer should have sufficient voltage withstand and power derating design, and the transformer shield should be grounded. The analog and digital parts of the secondary instrument, which controls the electrical appliances, are different from the secondary coil of the transformer power supply to prevent the impact of large pulse currents and power supply fluctuations. The layout of the entire power supply in the circuit board should be far away from the signal line part. The selection of the A/D converter chip should focus on the stability and reliability of the product, which should undergo strict aging and screening before leaving the factory. The design of the peripheral circuit of the A/D should pay special attention to the stability of the reference voltage.
The microcontroller peripheral circuit is designed with basic programming, but the hardware and software design of its communication port is related to the data exchange method of the A/D converter chip. Thus, the level measurement does not require a high measurement frequency. The data exchange between the microcontroller and the A/D converter chip can be done through the serial port query so that the program structure is clear and easy to write debug.
2.1Reliable design of the liquid level transmitter
The level transmitter is used as the primary front-end instrument of the system, which directly affects the reliability of the measurement and control system, so it must be designed to ensure its reliability and stability. The design of the transmitter mainly includes mechanical protection structure design and electrical design of two major parts. Structural design on the central issues to be addressed is the choice of sensor structure, anti-corrosion design, anti-clogging design, sealing design, anti-condensation design. The electrical design mainly includes the design of transmitter power protection, sensor power supply design, signal extraction, amplification, conversion circuit design, etc. The following analysis of the technical aspects affecting reliability is addressed as follows.
3. Transmitter mechanical protection structure design
The structure chosen for the diffusion silicon input level transmitter sensor is the input level transmitter sensor, which will feel the measured liquid pressure at one end and the power and signal leads at the other. The sensor structure is cylindrical and has a sealed tapered pipe thread at one end for easy sealing. In addition, the selected sensor must have adequate overpressure protection.
3.1 Sealed design
The electrical chamber and the feeling end of the level transmitter should be completely isolated, the circuit components should be well potted after commissioning and sealed with tapered pipe threads, and when we connect, sealing fillers such as epoxy resin should be applied to the threaded connection. The compatibility with the liquid to be measured should be considered when selecting the sealing filler.
3.2 Corrosion-resistant design
Input level transmitter use environment determines the housing and lead must be compatible with the measured medium, ordinary measured liquid can choose 1Crl8Ni9Ti, it should be noted that the transmitter housing and lead and the measured medium compatibility design to consider the influence of temperature, some metal and liquid compatibility with the temperature change. Anti-clogging design into the level transmitter sensor probe immersed in the measured medium for a long time, impurities can easily block the pressure transmission hole, resulting in measurement distortion. In the sensor’s design, the probe should be protected by the end cap, the liquid is introduced through the pressure-transmitting hole, and the position and angle of the pressure-transmitting hole should be suitable.
3.3 System reliability guarantee
The components used in the component selection system shall have high reliability, all elements affecting the stability of the system may not have high accuracy, but shall have high stability, the accuracy of the system can be guaranteed by debugging and calibration, the components shall have sufficient derating design according to their use requirements and have considerable stress shock resistance margin.
Anti-interference design is essential to ensure the accuracy and reliability of the measurement. Considering the transmitter link, filter capacitors should be set at each signal node in the circuit. In addition, the transformer shield of the secondary instrument should be earthed, and the signal acquisition circuit should be laid out on the PCB away from sources of interference such as power supplies and relays.
The reliability of a system comes from the reliability of the structure in the system design and the regular operation of all relevant factors in the system. Therefore, a complete understanding of the system’s working principle, careful consideration of the various influencing factors, control from the source, and grasp from the process is an effective method and fundamental way to improve the reliability of the system design.