Selecting Analog Sensors: 0-10 VDC, 4-20mA, and 0-20mA
This guide explores the fundamental concepts behind selecting analog sensors with voltage and current outputs for Programmable Logic Controller (PLC) systems. We will delve into the distinctions between these signal types and provide clear guidelines for their application.
What You Will Learn:
Understand the core concepts of voltage and current sensor feedback.
Differentiate between voltage and current input signals in industrial applications.
Explore the reasons for preferring 4-20mA signals over 0-20mA signals.
Learn why current signals are often favored over voltage signals for analog feedback.
Gain straightforward guidelines for selecting analog output straight position sensors.
Understanding Analog Output Position Sensors: Voltage vs. Current
While a wide scope of advanced sensor interface types like fieldbus protocols (e.g., Profibus, EtherCAT) and synchronous serial interfaces are available, direct position sensors with basic analog outputs (0-10V, 4-20mA) still account for a significant portion (approximately 66%) of all straight position sensors sold.
When selecting a basic analog output position sensor, your choice typically narrows down to a straightforward voltage signal (e.g., 0 to 10 VDC) or a basic current signal (e.g., 4 to 20 mA or 0 to 20 mA). Let's examine the characteristics of each.
Selection of 0-10 VDC Sensors in PLC Systems
The 0-10VDC signal is a widely recognized and commonly used sensor interface in industrial automation.
Advantages of 0-10 VDC Sensors:
Widespread Compatibility: It is a ubiquitous standard, readily accepted by almost every modern PLC or industrial controller.
Ease of Troubleshooting: Its direct voltage measurement often makes it simpler to troubleshoot with a standard multimeter.
Simplicity: Conceptually, it's a straightforward signal type.
Disadvantages of 0-10 VDC Sensors:
Vulnerability to Electrical Noise: All analog signals are susceptible to electrical interference. 0-10V signals are particularly vulnerable to noise induced by nearby devices such as motors, relays, and "noisy" power supplies, which can degrade signal integrity.
Susceptibility to Voltage Drop: Over long cable runs, the 0-10V signal can experience significant voltage drops due to wire resistance. This leads to inaccurate readings at the PLC input, as the voltage arriving at the controller will be lower than that at the sensor output.
Selection of 4-20mA or 0-20mA Sensors in PLC Systems
Current-based signals like 4-20mA and 0-20mA are generally more robust for industrial environments.
Advantages of 4-20mA/0-20mA Sensors:
Enhanced Noise Immunity: Current signals offer superior immunity to electrical interference compared to voltage signals. This is because current loops are less affected by voltage fluctuations or noise picked up along the cable.
Reduced Signal Loss over Long Distances: Current signals are less susceptible to signal degradation and voltage drops over long cable runs. The current remains relatively constant throughout the loop, ensuring accuracy at the receiver end regardless of wire resistance (within the loop's compliance voltage limits).
Wider Controller Acceptance: Increasingly, modern industrial controllers and PLCs are designed to readily accept current signals, recognizing their inherent advantages.
Specific Advantages of 4-20mA over 0-20mA:
The 4-20mA signal provides a significant advantage over the 0-20mA signal due to its "live zero" or "bumble condition area" capability.
Built-in Fault Detection: With a 4-20mA signal, a 0% measurement (e.g., zero position) is represented by a 4mA current. If the signal ever drops to 0mA, it unambiguously indicates a fault condition, such as a broken wire, sensor failure, or power loss. This allows for immediate fault detection and system safety.
Clear Zero Indication: The sensor is still actively providing a signal even at its lowest measured value. This is a critical distinction from a 0-10V sensor, where 0V could mean either a true zero measurement or a complete sensor failure, making fault detection more ambiguous.
Disadvantages of 4-20mA/0-20mA Sensors:
Slightly Higher Cost: Historically, 4-20mA sensors could be marginally more expensive than their 0-10V counterparts. However, this cost differential is continuously decreasing as more sensor manufacturers integrate current-output capabilities as a standard feature.
More Complex Troubleshooting (initially): While robust, troubleshooting current loops can sometimes require a different approach or specialized tools compared to simple voltage checks.
Why Use Current Signal Instead of Voltage Signal?
In summary, current signals (like 4-20mA) are generally preferred over voltage signals (like 0-10V) for analog feedback in industrial environments due to their:
Superior Noise Immunity: Less affected by electromagnetic interference.
Reliability Over Distance: Minimal signal degradation over long cable runs.
Built-in Fault Detection (especially 4-20mA): The "live zero" allows for clear distinction between a true zero measurement and a system fault.
