In the process of industrial production control, especially in continuous production processes, there is often a requirement to collect and control physical quantities such as voltage, current, pressure, speed, flow rate, etc., which change continuously over time. In the field of control, these physical quantities that change continuously over time are called analog quantities.
When using analog quantities, we first need to understand several types of devices:
sensors, transmitters, A/D converters, D/A converters, etc.
Sensors
are the primary components for achieving automatic detection and control. The existence and development of sensors have endowed objects with senses such as touch, taste, and smell, gradually bringing them to life.
Commonly used sensors include temperature sensors, pressure sensors, humidity sensors, etc.
Transmitters
are converters that transform the output signals of sensors into signals that can be recognized by controllers (or convert non-electric inputs from sensors into electric signals and amplify them for remote measurement and control). Sensors and transmitters together constitute the monitoring signal source for automatic control.
A/D converters
are circuits that convert analog signals into digital signals, known as analog-to-digital converters (abbreviated as A/D converters). The function of A/D conversion is to convert continuous-time and continuous-amplitude analog quantities into discrete-time and discrete-amplitude digital signals. Therefore, A/D conversion generally involves four processes: sampling, holding, quantization, and encoding. In practical circuits, some of these processes are combined, for example, sampling and holding, quantization, and encoding are often implemented simultaneously during the conversion process.
D/A converters
are also known as digital-to-analog converters, abbreviated as DACs. They are converters that transform discrete signals in the form of binary digital quantities into analog quantities based on standard quantities (or reference quantities). Their function is to convert digital quantities into analog devices.
Analog Input (AI)
(i.e., current or voltage signals ranging from 4-20mA or 0-10V, which are input into the DCS and transformed to recover their original values)
Analog Output (AO)
(AO refers to the DCS outputting 4-20mA or 0-10V signals to control the operating parameters of equipment)
Common analog input/output signals:
4-20mA, 0-10mA, 1-5V, 0-5V, 0-10V
Essentially, they represent voltage and current variations.
Common voltage ranges include: 0~10V, -10~+10V, 0~5V, -5V~+5V, with the most commonly used being the 0~10V voltage signal.
Common current ranges include: 0~20mA, 4~20mA, -20mA~+20mA, with the most commonly used being the 4~20mA current signal.
Application Fields
Analog has numerous application fields, such as building automation, environmental monitoring, factory automation, and intelligent transportation;
Environmental Monitoring: IIOxidationcarbon concentration monitoring, oxygen concentration monitoring, smoke concentration monitoring, water quality monitoring, etc.
Why are analog signals converted into 4-20mA current signals, rather than 0-20mA current signals or 0-10V voltage signals?
(1) 0-10V voltage signals are susceptible to external electromagnetic interference, especially when the cable length is long.
(2) With 0-20mA current signals, it is impossible to determine whether the 0mA current signal is caused by a cable break or whether the pressure itself is 0kPa and the output is a normal 0mA.
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