The analog circuit interface industry commonly employs voltage ranges of 0-5(10)V or current ranges of 0(4)-20mA as methods for transmitting analog signals. These approaches are also frequently utilized by programmable controllers. While both voltage and current signals serve as mediums for analog signal transmission, they differ in several aspects. Below is a brief overview of their differences and typical applications.
Voltage signal transmission, such as 0-5(10)V, involves sending an analog voltage signal from the transmitter to the receiver via a long cable. However, this method is prone to distortion due to factors like output impedance, cable resistance, and contact resistance, all of which contribute to voltage drops along the path. These losses result in transmission errors, which depend on the input bias current of the receiving circuit multiplied by the respective resistances involved. For minimal error, the receiving circuit must have a high input impedance. If an operational amplifier (op-amp) is used as the input amplifier, it’s critical to ensure its input impedance is sufficiently high—typically less than 1MΩ. High-impedance circuits, especially those at the input stage of amplifiers, are vulnerable to electromagnetic interference, which could lead to significant inaccuracies. Thus, when using voltage signal transmission, there is often a trade-off between minimizing transmission errors and reducing susceptibility to electromagnetic interference.
In summary, voltage signal transmission is suitable when electromagnetic interference is low or the cable length is short, provided the receiving circuitry is appropriately designed.
Current signal transmission, such as 0(4)-20mA, offers distinct advantages in environments with strong electromagnetic interference and longer transmission distances. Unlike voltage signals, current signals remain stable regardless of cable resistance or contact resistance, as the current source ensures the desired current flows irrespective of these factors. This makes current signal transmission less susceptible to hardware-related distortions. The receiving circuit’s low input impedance prevents electromagnetic interference from affecting the current signal, as the current source is grounded. This characteristic ensures robustness against external noise.
To conclude, current signal transmission is ideal when dealing with strong electromagnetic interference, long transmission distances, or situations where reliability is paramount. Current loops are commonly implemented in two-wire and three-wire configurations. Given the prevalence of the two-wire system, we will focus on this approach, often referred to as the current loop method.
Key Characteristics of the Current Loop:
- Simple Implementation: If the operating current of the signal transmitter and any connected circuits remains constant, the operating current and signal current can share the same cable. A load sampling resistor is used to measure the voltage drop across it, providing a usable signal. It’s important to ensure the operating voltage is sufficient to accommodate the voltage drop within the loop.
- Cost-Effective: Compared to digital signal transmission requiring an AD converter, the current loop method only necessitates a single cable, a load resistor, and a voltmeter. This results in lower costs, particularly when high precision is required.
- Error Diagnosis: One notable advantage of 4-20mA current signal transmission is its ability to provide automatic error detection. For instance, if the system outputs a zero signal (4mA) but receives a signal greater than 0mA and less than 4mA, the system likely has a fault. A zero signal indicates either a broken cable or a problem with the receiver. If the current exceeds 20mA, it suggests an overloaded signal or an issue with the input.
- Long-Distance Transmission: The transmission distance depends on the driving capability of the transmitting terminal, the resistance of the cable, and the load resistance at the receiving end. If additional instruments are included in the transmission line, the load resistance must account for the input impedances of these devices. Often, these instruments connect directly to the 4mA power supply, making the current loop cost-effective without needing an external power source. Circuit design must therefore consider the load capacity of the current source loop.
A basic example of two-wire current signal transmission includes an adjustable current source and resistor comprising the signal sender and receiver (measuring the voltage drop across the load resistor). Figure 1 illustrates this setup.
In Figure 1, the transmitter assumes the role of producing a desired signal current (IOUT = 4-20mA) linked to the measured value. The resistor RL serves as the receiver, measuring the voltage drop VA across it or directly using an ammeter in series to obtain IOUT. In practice, the transmitter combines multiple functional circuits, often acting as a signal measurement converter in the sensor field. This includes a sensor, a power supply for the sensor, and a current source. Beyond transmitting the sensor signal, the current loop can also function as an isolation amplifier for the output stage of PWM-modulated pulse-width signals or serve as a simple voltage output source.
Figure 2 shows a complete current signal transmitter circuit. Here, the sensor signal or PWM-modulated pulse-width signal varies from zero to full scale (FS). Consequently, the voltage-controlled current source must generate a zero-point current of 4mA and a full-scale current of 20mA (a difference of 16mA).
In industrial control systems, control devices (control rooms) are often located far from the measurement signal transmission circuit. Using the power line as a signal transmission line enables the transmission of signals with just two wires, reducing costs while simplifying the circuit. This is known as the two-wire current transmission method.
As illustrated in Figure 3, a working power supply is provided by the control device (control room) to power multiple transmission circuits simultaneously.
In conclusion, current loop technology offers numerous benefits, including simplicity, cost-effectiveness, error diagnostics, and long-distance transmission capabilities, making it a popular choice in industrial settings.
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