What is 4-20 ma?

This article will be geared towards the curious, I am not going to try to train you how to work with the subject materiel.
But you will be able to converse intelligently after reading this.

BASIC DEFINITION

4-20ma can be either an input or output from the perspective of a PLC, (Programmable Logic Controller)which was covered in an earlier article.
The PLC specialty cards necessary are referred to as Analog cards(both input and output types).

MA stands for milliamp, a measurement of current flow. 4-20 is the lower and upper limits of this defined scale.
One example is the use of this scale to order a motor to run at a specific speed, with 4ma representing stopped and 20ma representing full speed.

Precision pneumatic valves

HARDWARE

They do require specialized electronic hardware, on both ends. On the PLC side this usually takes the form of a plug in card(Analog) which can be purchased
from the PLC manufacturer. In the case of the example above, The VFD (Variable Frequency Drive) (which was also covered in an earlier article),
usually has special terminals for the 4-20 wiring.

But lets be clear here, this standard has far greater usage than just telling a motor how fast you want it to run.

USAGE

Some examples of 4-20 inputs to a PLC are:
Temperature sensors
pressure sensors
CO2 sensors
Humidity sensors
Potentiometers
Flow sensors

Some examples of 4-20 outputs from a PLC are:
Exact valve positioning for desired gas, air,or hydraulic pressure
Motor speed
Remote display of value.

HISTORY

The 4-20 MA convention was created in the 1950’s as a means of electrical control for the popular 3-15 PSI pneumatic control signal standard, which was serving basically the same purpose but with all pneumatic logic. When electronics became cheap and reliable enough and electrical controls became more prominent, the transition started taking shape.
Even to this day some equipment that require exact valve positioning still use the 3-15 pneumatic standard,Industrial Boilers come to mind.
I to P converters (Current to pressure)are used to allow modern PLC’S to control these old but reliable devices. Electrical control with pneumatic operation of valves is the standard, but typically the valve is on, or off.( Cylinder completely extended or retracted).

However, these converters and associated equipment allow for exact positioning.

ADVANTAGES

A big advantage comes in the form of noise immunity and accuracy.
The reading on one end will always match perfectly the reading on the other end, this is because the current flow is constant everywhere on the wire, regardless of length.
Whereas if you were using say zero to ten volts, there would be voltage loss, the extent of which would depend on the length of the wiring. This would be due to the resistance of that wiring. It’s worth mentioning the HART protocol here. This is a communication scheme that rides on the 4-20 ma signal. It allows passage of all types of information between the device and the controller. It is also worth mentioning that the reason the standard is not 0-20ma, is to supply an indication of malfunction. If the processor sees zero milliamps then it knows something is wrong, like open wiring.

The exact positioning world does have competition in the form of servos, which are motors with controllers specifically designed for the task, all electrical. But this is very expensive equipment. I just don’t see the pneumatic type going completely away any time soon.

DISADVANTAGES

In a system with multiple 4-20 devices, there is a lot of wiring involved, And a lot of specialized and expensive hardware.
The fact is, 4-20 is slowly going away.
It is being replaced by network enabled devices, These are devices with Industrial Ethernet(or another type of network) interfaces as opposed to 4-20 interfaces.
You get rid of all the wiring, because everyone is on the same comm line with unique addresses, Only 2 wires leave the panel and allow communication to many devices, and Most PLC’s these days have built in Ethernet ports.
However with only one or two devices it may still be cheaper to go 4-20, so it may still be around a while.

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