4-20MA signal controls electric control valve
Thread Content
The steam temperature and pressure control system on the cardboard production line is equipped with a Delta PLC as well as analog input and output modules. The analog input (04AD) module is connected to 7 flap-type level sensors and 3 pressure sensors. The analog output module (04DA) is connected to three electric control valves. The three voltage control valves have a power supply of 220V, with an input signal of 4-20MA. The length of the cable from the analog output module to the electric control valve is 200 meters. The DC switching power supply has a capacity of 350W at 24V (it is used to power the flap level sensor and pressure sensor). There is now a problem: the 4-20MA signal generated by the PLC cannot be used to control the electric control valve. After the signal is connected, the electric control valve moves slightly and then stops (similar to a slight tremor). It is normal for the change in the PLC’s 4-20MA output to be measured using a multimeter. I used the Fluke signal generator to test the electric control valve again, and the valve functioned properly with an opening range of 0-100%. I moved the control cabinet back to the company to test an electric control valve, and it worked properly; however, there is only one electric control valve available in the company. I would like to ask now: when using a PLC output of 4-20MA to control a device located about 200 meters away, will such a long cable length affect the 4-20MA signal? Is there a solution? Do we need to add something after the PLC output module?
For a current signal, 200 meters should be no problem; I wonder if the wires were damaged during wiring...
You should measure whether the 4-20MA signal output by the PLC at the electric control valve end is correct. As mentioned above, there might be an issue with the signal cable. Additionally, it is advisable to check the requirements of the PLC output module regarding the load, as well as the requirements of the signal input terminal of the electric control valve concerning the input impedance, to determine whether the two are compatible.
Does the electric valve have a built-in servo amplifier? Where is the valve position feedback signal from the electric valve connected to? Is there a manual operator between the PLC and the electric valve? Have the PID parameters of the PLC been set? The original poster didn’t mention any of these; how can netizens determine the problem then?
You have already ruled out all faults other than the cable; what else is there to ask? It is likely due to some loose connection issue, which causes the signal from the PLC to fluctuate
Yesterday, I went to the site to measure the current of the PLC output module; with the electric control valve not connected, I had the PLC change the output signal value, and using a multimeter, I confirmed that the 4-20MA signal was normal. Connect the electric valve in series with a multimeter to measure it; the current, which was originally 20 MA, dropped to only 5 MA. Finally, after installing an isolation module for 4-20MA, everything worked properly.
It’s fine when the PLC is connected to only one electric control valve. Problems will arise if three electric valves are connected. It now seems that the allowable load impedance output by the PLC module is too low. (500 euros). The isolation module is powered separately by 24V. The allowed load impedance is 800 ohms.
The cable issue cannot be ruled out; are the 3 cables public cables?
I mean, is it a single cable that’s used to connect them?