0-10V versus 4-20 mA: Why Do We Prefer the 4-20 mA System?

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In the world of linear position sensors, analog reigns supreme.  Sure there are all kinds of other sensor interface types available; digital start/stop, synchronous serial interface, various flavors of fieldbus, and so on.  But linear position sensors with analog outputs still account for probably two-thirds of all linear position sensors sold.

When choosing an analog-output position sensor, your choice generally comes down to analog voltage (e.g., 0 to 10 V), or analog current (e.g., 4 to 20 mA).  So which should you choose?

0-10V versus 4-20 mA

When it comes to sensor interface signals, 0-10V is like vanilla ice cream or, if you prefer, a Chevy Cavalier.  It’s nothing fancy, but it gets the job done.  It’s common, it’s straightforward, it’s easy to troubleshoot, and nearly every industrial controller on the planet will accept a 0-10V sensor signal.  However, there are some downsides.  All analog signals are susceptible to electrical interference, and a 0-10V signal is certainly no exception.  Devices such as motors, relays, and “noisy” power supplies can induce voltages onto signal lines that can degrade the 0-10V sensor signal.  Also, a 0-10V signal is susceptible to voltage drops caused by wire resistance, especially over long cable runs.

A 4-20 mA or 0-20 mA signal, on the other hand, offers increased immunity to both electrical interference and signal loss over long cable runs.  And most newer industrial controllers will accept current signals.  As an added bonus, a 4-20 mA signal provides inherent error condition detection since the signal, even at its lowest value, is still active.  Even at the extreme low end, or “zero” position, the sensor is still providing a 4 mA signal.  If the value ever goes to 0 mA, something is wrong.  The same can not be said for a 0-10V sensor.  Zero volts could mean zero position, or it could mean that your sensor has ceased to function.

In some cases, 4-20 mA sensors can be slightly more costly compared to 0-10V sensors.  But the cost difference is becoming increasingly smaller as more sensor types incorporate current-output capability.

What is a 4-20 mA Current Loop?

In order to understand what a 4-20 mA direct current (DC) loop is and how it works, we will need to know a little bit of math. Don’t worry; we won’t be delving into any advanced electrical engineering formulas. In fact, the formula we need is relatively simple: V = I x R This is Ohm’s Law. What this is saying is that the voltage (V) is equal to the current (I) multiplied by the resistance (R) (“I” stands for Intensité de Courant, French for Current Intensity). This is the fundamental equation in electrical engineering.

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Consider the simple DC circuit above, consisting of a power supply and three loads. A current loop requires voltage to drive the current. This is provided by the power {PART 1} 12 supply, with the voltage of the supply labeled as Vtot. Current then flows through the loop, passing through each load. The voltage drop at each load can be calculated from Ohm’s Law. The voltage drop V1 across R1 is:

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Every element in the loop either provides voltage or has a voltage drop. However, the current, I is the same everywhere in the loop. This is the critical principle of the 4-20 mA loop. Current is the same in all places throughout the loop. It may be difficult to understand why the current remains constant, so consider your home’s water system as a comparison. There is a certain amount of pressure in the water pipes pushing the water towards your house. Voltage, in a similar fashion, acts as a pressure, pushing current through the circuit. When a tap inside your home is turned on, there is a subsequent flow of water. The flow of water is analogous to the flow of electrons, or current. The ability of the pressure to push the water through the pipes is limited by bends and restrictions in the pipe. These restrictions limit the amount of flow in the pipe, similar to how a resistor limits the current. The flow through the pipe, and likewise the current through the wire, remains constant throughout the system, even though pressure, and likewise voltage, will drop at various points. This is why using current as a means of conveying process information is so reliable.

Components of a 4-20 mA Loop

Now that you have an understanding of how and why current is used, you can begin to understand what exactly the loop is for.

  1. Sensor

First, there needs to be some sort of sensor which measures a process variable. A sensor typically measures temperature, humidity, flow, level or pressure. The technology that goes into the sensor will vary drastically depending on what exactly it is intended to measure, but this is not relevant for this discussion.

  1. Transmitter

Second, whatever the sensor is monitoring, there needs to be a way to convert its measurement into a current signal, between four and twenty milliamps. This is where a transmitter will come into play. If, for instance, a sensor was measuring the height of a fifty foot tank, the transmitter would need to translate zero feet as the tank being empty and then transmit a four milliamp signal. Conversely, it would translate fifty feet as the tank being full and would then transmit a twenty milliamp signal. If the tank were half full the transmitter would signal at the halfway point, or twelve milliamps.

  1. Power Source

In order for a signal to be produced, there needs to be a source of power, just as in the water system analogy there needed to be a source of water pressure. Remember that the power supply must output a DC current (meaning that the current is only flowing in one direction).

  1. Receiver

Finally, at someplace in the loop there will be a device which can receive and interpret the current signal. This current signal must be translated into units that can be easily understood by operators, such as the feet of liquid in a tank or the degrees Celsius of a liquid. This device also needs to either display the information received (for monitoring purposes) or automatically do something with that information. Digital displays, controllers, actuators, and valves are common devices to incorporate into the loop. These components are all it takes to complete a 4-20 mA current loop. The sensor measures a process variable, the transmitter translates that measurement into a current signal, the signal travels through a wire loop to a receiver, and the receiver displays or performs an action with that signal.

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HART Communication Protocol

HART -HIGHWAY ADDRESSABLE REMOTE TRANSDUCER

HART Communication is a backward compatible enhancement for 4-20mA instrumentation that enables remote, two-way digital communication with smart microprocessor-based field devices. HART devices support two simultaneous communication channels on the same wire—the 4-20mA “current loop” analog communication channel and the HART digital communication channel.

The 4-20mA analog communication channel is significant because it ensures compatibility with legacy systems and fast transport of control variable information to/from the process connection and controller.
Continuous, simultaneous and complementary real-time use of both communication channels provides a high level of control security and loop integrity far beyond what is achievable by using either channel alone.

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The Smart Part of HART

All HART-enabled field devices, regardless of manufacturer, contain 35-40 data items of rich information for improving plant operations and managing assets. The inherent intelligence of these devices allows them to perform internal diagnostic checks and communicate information regarding their status continuously.

Standard HART commands make it easy for systems to access the real-time data in HART devices with valuable device status information being part of every response packet from the device.

Integration with control systems allows both communication channels to be used for multi-variable process data and real-time detection to any problems impacting the device or the integrity of the 4-20mA current loop.

Detailed Device Diagnostics

In addition to standard indicators for device status and process variable quality, the HART Protocol provides an efficient mechanism for control systems to access detailed device diagnostics. Up to 136 device-specific diagnostic parameters can be accessed with in a single HART command.

Control Loop Validation

Reliable, continuous communication is critical to making good control decisions. Quality, integrity, and accuracy of the 4-20mA current loop signal are also essential for good control. HART Communication enables control systems to continuously monitor and validate the integrity of the 4-20mA current loop. Real-time integration with control, safety and asset management systems delivers tremendous benefits as previously undetectable device or loop integrity problems are detected within seconds of occurrence.

Getting Connected

Today, most process automation system suppliers offer a variety of HART interface solutions to support integration with their control systems. Many have intelligent remote I/O subsystems and system interfaces for direct connection to HART devices. Most allow the HART data to be used in real-time for operator display, alarm and control functions. Third-party I/O systems and interface products are available to support integration with legacy control systems that might not be easily upgraded for HART Communication. Gateway interface solutions are also available for linking HART devices to systems based on network protocols such as Ethernet, Modbus, and Profibus.

Real-time HART integration with plant control, safety and asset management systems unlocks the value of connected devices and extends the capability of systems to detect any problems with the device, its connection to the process or interference with accurate communication between the device and system. The justification is two-fold; (1) cost reduction due to improved operations and increased efficiencies, and (2) cost avoidance as early warning to impending problems enables the high cost of process disruptions and unplanned shutdowns to be averted.

HART is the global standard for smart process instrumentation communication. With its huge installed base, ease of use, global acceptance, and the support of major process automation suppliers, HART Communication will continue to lead the world market well beyond the next decade—creating an ongoing demand for development of new HART-enabled products and advanced applications.

HART has the potential to be the fieldbus you already have.

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