Digital and analog signals are the most frequently encountered concepts in power systems. Whether you're learning about PLCs or relay protection, these two input/output methods are involved. What are digital signals? What are analog signals? After reading this article, you will clearly understand these concepts.
I. Concept of Switching Quantities Switching quantities are on/off signals, passive signals, and resistance is measured as 0 or infinite by resistance testing methods. They mainly refer to input and output quantities, and are auxiliary contacts of a device, such as the auxiliary contacts of the relay in a motor temperature controller (which change position after the motor overheats), the auxiliary contacts of a valve cam switch (which change position after the valve is switched), the auxiliary contacts of a contactor (which change position after the contactor operates), and thermal relays (which change position after the thermal relay operates). These points generally transmit power to the PLC, and the power supply is usually provided by the PLC or integrated protection device. They do not have their own power supply, so they are called passive switching quantity contacts, or PLC switching quantities.
It can also be an active signal, professionally called a step signal, which is 0 or 1, and can be understood as a pulse quantity. Multiple switching quantities can form a digital quantity.
Analog quantities refer to continuously changing physical quantities, such as voltage, current, pressure, speed, flow rate, etc. Analog signals are signals whose amplitude changes continuously with time. Typically, voltage signals are 0~10V and current signals are 4~20mA. Data can be acquired using the analog quantity module of a PLC, and after sampling and quantization, they can be converted into digital quantities.
Digital quantities, as commonly referred to, are signals composed of "0"s and "1"s, typically encoded signals with a predictable pattern. For switching signals, a closed contact is considered "1," and an open contact is "0," thus being acquired as a digital signal. Analog quantities can be quantized using threshold values; values less than the threshold are "0," and values greater than or equal to the threshold are "1."
II. Difference 1. Digital quantity is defined as: a discrete signal that changes discontinuously in both time and value.
Analog signals are defined as signals that change continuously in both time and value.
2. Switching signals: These reflect status signals (such as switch on or off).
Analog quantities: These reflect electrical measurement values (such as current and voltage).
III. Examples
The diagram above shows a typical device that can output switching signals. When the pressure is high, contacts C and B close, outputting a high pressure signal; when the pressure is low, contacts C and A close, outputting a low pressure signal.
With such signals, the local pressure signal can be transmitted to a remote electrical control cabinet for automatic remote control. Here, C and B are switching quantities, and C and A are also switching quantities. Therefore, a switch contact is a switching quantity, characterized by being either on or off at any given time. On is 1, representing the presence of a signal; off is 0, representing the absence of a signal. This is what is known as a switching signal.
Although a pressure gauge can transmit pressure signals over long distances, it only transmits a signal indicating whether there is pressure; it cannot determine the actual real-time pressure value.
The device shown in the image above is called a pressure transmitter. Inside the pressure transmitter is a circuit board, which is connected to a pressure sensor F.
Its working principle is that the pressure sensor F transmits the detected pressure to the circuit board C. After the detection signal enters the circuit board, the circuit board converts and calculates the pressure signal into a current signal, which is output from points A and B. The right side of the diagram shows a schematic diagram of the conversion process. It can convert a 0-10 kPa pressure signal into a 4-20 mA current signal, output from points A and B. We then say that the outputs from points A and B are analog signals. The characteristic of analog signals is that their values are continuously variable within a certain numerical range.
4. Q&A Q: Why are analog signals converted into 4-20mA current signals instead of 0-20mA current signals or 0-10V voltage signals? A: (1) 0-10V voltage signals are easily affected by external electromagnetic interference, especially when the cable is very long.
(2) If a 0-20mA current signal is used, it is impossible to determine whether the 0mA current signal is caused by a broken cable or the normal 0mA output when the current signal is 0mA.
