Analog Signal - 4-20mA Current Control In/Out - Process Signal Controller

Analog Signal to Digital Signal Conversion in new Era.,,,

We live in an analog world and we often find ourselves in need of high-performance analog measurements as we push the limits of automation technology.

• 4-20mA Analog input/output Signal is used to read the process signal precisiously for long distance Control output/input.
• Minimum Current 4mA is used to identify the Cable broken or loop failed detection purpose.
• 20mA is used to get maximum process input/output signal.
• This signal current while travelling in long distance beside the 3 phase power cables can be distorted due to EMF (Electro Magnetic interference) and creates the accuracy error.
• In order to aviod this kind of (current) signal loss, can be prevented by using shielded the signal cable in addition with two wires can be twisted.
• The Signal wires once it reached to Control Panel can be easily converted to digital signal without any loss of accuracy. ref.fig-1  

fig-1

National Instruments-USA providing the excellent solution with updated technology to control the analog signal to digital signal to interface with PLC.

There are a few axioms that all automation engineers know:

1. Your outputs are only as good as your inputs. (Zero signal distortion)
2. The faster you close the loop, the more precise and efficient your control.

However, due to the generalized functionality of PLCs, getting high-speed, high-quality measurements, such as dynamic signal analysis, high-precision voltage, and current measurements, is often challenging without using specialized PLC hardware.

fig-1A

Further, 4-20mA Current based signal transmitter unit can be damaged due to Dynamic Static Charges produced by the process materials. This can be protected by stroung earthing the sensor connected to transmitter. In case of earth wire failure, the current signal sensor will transmit the high voltage static charges to the PLC. Due to the high spike voltage produced bye the process material can damage the current signal transmitter and PLC input or output module. ref fig-1A.

Latest invention to aviod such worst senorio, the current sensing signal transmitter unit has been optically isolated between the sensor portion and signal converting electronic control section. This galvanic isolation is protecting the equipment up to 4Kv spike voltage.


fig-2
 

 

Screening the two wire current signal wires, will protect the signal transmission from harmonics produced from the variable frequency drives or soft starters or any power electronics equipment. ref.fig-2.

fig-3

The 4-20mA current signal can be connected to the device in 2 wire loop power supply or external 24 vdc power supply connected in series with the transmitter device. The same unit can also be connected 3 wire provided with 24 vdc external power supply. ref.fig-3.

The 4-20mA current loop can be used to measure temperature and linear measurement and speed and liquied flow control and Gate value damping oscillation control and other various process input and output measurements. 

 

fig-4

 

A 10 Kohm potentiometer is used to tune-up Minimum and Maximum input signal to electronic board to convert 0 to 10K to 4-20mA current convertion. ref.fig-4