Introduction
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Are current loops PC?
When communicating with people, it is easy to get the wrong information. When transmitting information, it is essential that we as engineers make sure that does not happen. For example, the term “PC” in our society has become synonymous with being Politically Correct. In the context of current loops, PC stands for Process Control.
Process control is used in industrial control systems to interact with the process. The information that is transmitted might be a tank level, a motor speed, a temperature or the pH of a chemical process at a specific location. Whatever measurement you can imagine can be transmitted from one point to another using process control techniques.
Process control is how manufacturing is done. It is a part of power production, toy and tool creation and chemical and pharmaceutical plants. A small process control system found in the home is the thermostat. It senses the temperature and controls a furnace or AC unit to control the air temperature. It does not typically use a current loop to transmit and control the process. More about that next.
Why 4 to 20mA?
A common way to transmit data is to convert the signal into a current. Specifically, into a 4 to 20mA signal. To help understand this let’s consider an example. If there is a tank, and it does not matter if the tank holds 100 gallons or 1 million gallons, we can sense its level and transmit that information on a current. If the tank is empty we send 4mA. If it is full we send 20mA. If it is 50% full we will transmit 12mA.
Using this method has some advantages. The first is that a signal of 4-20mA is easy to detect. Smaller currents become more expensive to sense and more prone to noise. If a process is zero we still have a signal (4mA). This allows us to differentiate between a zero signal and a broken wire.
Historically, process control was accomplished using pneumatic sensors and actuators before electronics became feasible. Signals between 3 and 15psi were used, for the same reason described above. When electronics came on the scene the transition followed similar conventions.
How does a current loop work?
There are 5 parts of a current loop. The first is the sensor. This is the device that converts a physical parameter into an electrical signal. It is a transducer. The second part, is the transmitter. This is another type of transducer. It converts the sensor signal into a 4-20mA signal. For a current loop to function, it must have a power source. The power source provides the voltage needed to make the loop work. If the voltage is too low for the resistance in the system, the loop will not work correctly. For example, it might work fine for low currents but then saturate at higher currents. If the voltage is too high, power is wasted. Choosing an appropriate voltage level for the system is critical.
The forth part of the loop is the physical loop itself. This is a wire that starts and ends at the transmitter. The voltage drop along a wire is usually not a concern, unless the loop distance is greater than about 1,000 feet. Resistance at connection points is almost always greater than the resistance along the wire itself.
The last part is the receiver. The receiver reads the current by converting it into a different format. It might convert it into a digital format to be sent to a distributed control system or to be saved in a database for later analysis. It might be part of a control loop that takes that parametric reading and controls an actuator (like a motor or a valve) to keep the process at a desired setpoint.
Is the receiver in the middle of the loop?
It might seem strange, but the receiver is not at the end of the loop but in the middle. The loop is literally a loop of wire that starts and ends at the same location, the transmitter. Somewhere along that loop will be the receiver. It must read the current. The most common way of doing this is with a sense resistor. A small value resistor is inserted into the current loop and the voltage across the resistor is measured. This is not the only way to read the signal, but it is the most commonly used method.
Where is the control?
So far, we have discussed getting information from a process. That may be all you want, to know what a particular parameter is in a process. More useful is a parameter that can be manipulated or controlled. As a process control engineer, you will sit at a DCS or Distributed Control System. The parameters from the process are sensed in the field, converted to an electrical signal and sent to you over a current loop. The controlling signal is also sent using a current loop, but the transmitter is located at the DCS and the controlling parameter is sent out into the field. This might position a valve or adjust the speed of a motor. It might increase or decrease the pump rate of acid or base into a process to control the pH.
Backbone of manufacturing
While many of us may never come across a 4-20mA current loop, it is the backbone of the manufacturing process and it touches almost every product we use. If the current loop is the backbone, then the PLC (programmable logic controller) is the heart. This is a digital computer used in the manufacturing process to read and write signals and control how fast they change and when to change them.
Final thoughts
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