Transmitter Principles and Industrial Applications
发布时间：
2013-07-12
It is common practice to measure various electrical and non-electrical physical quantities, such as current (AD), voltage (VD), power (WD), frequency (FD), temperature (TT), weight (LD), position (PT), pressure, rotational speed (RT), and angle. All of these must be converted into receivable DC analog electrical signals before they can be transmitted over distances of several hundred meters to the control room or display devices. Such devices that convert the measured physical quantity into a transmittable DC electrical signal are called transmitters. In industrial applications, transmitters are typically classified into electrical transmitters and non-electrical transmitters.
It is common practice to measure various electrical and non-electrical physical quantities, such as current (AD), voltage (VD), power (WD), frequency (FD), temperature (TT), weight (LD), position (PT), pressure, rotational speed (RT), angle, and others. All of these must be converted into receivable DC analog electrical signals before they can be transmitted over distances of several hundred meters to the control room or display devices. Such devices that convert the measured physical quantity into a transmittable DC electrical signal are called transmitters. In industrial applications, they are typically classified as Electricity transducer and non-electrical transducers.
Transmitter Traditional DC electrical signals for output include 0–5 V, 0–10 V, 1–5 V, 0–20 mA, and 4–20 mA; currently, the most widely adopted method is to transmit analog signals using a 4–20 mA current. In industrial applications, the use of a 4–20 mA current for analog signal transmission is by far the most common practice.
The reason for using a current signal is its strong immunity to interference. Moreover, the internal impedance of a current source is effectively infinite, so the resistance of the connecting wires in series with the loop does not affect measurement accuracy; such a signal can be transmitted over several hundred meters on standard twisted-pair cable. The upper limit is set at 20 mA to meet explosion-proof requirements: the energy of the spark produced by switching a 20-mA current is insufficient to ignite flammable gas. The lower limit is not set at 0 mA because it allows for fault detection: under normal operating conditions, the loop current never falls below 4 mA; however, if the transmission line breaks due to a fault, the loop current drops to zero. A common practice is to use 2 mA as the threshold for triggering a open-circuit alarm.
Current transducer To convert a physical quantity into a 4–20 mA current output, an external power supply is required. The most common configuration is for the transmitter to have two power-supply leads and two current-output leads, totaling four wires; such a device is referred to as a four-wire transmitter. Alternatively, the current output can share one of the power-supply leads (either VCC or GND), thereby saving one wire; this configuration is known as a three-wire transmitter.
In fact, as many may have noticed, the 4–20 mA current loop itself can supply power to the transmitter, as shown in Figure 1C. In the circuit, the transmitter functions as a special type of load—its current draw varies between 4 and 20 mA depending on the sensor output. The indicating instrument only needs to be connected in series within the loop. Because such transmitters require only two external wires, they are referred to as two-wire transmitters. According to the industrial current-loop standard, the lower limit is 4 mA; therefore, as long as the measurement range is within specification, the transmitter will always receive at least 4 mA of supply current. This makes the design of two-wire sensors feasible.
In industrial applications, measurement points are typically located on-site, while display or control equipment is usually installed in the control room or control cabinet. The distance between these two locations can range from tens to hundreds of meters. Assuming a distance of 100 meters, eliminating two signal wires can reduce costs by nearly RMB 100. Consequently, two-wire sensors are being increasingly adopted in practical applications.
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