I have 2 no.s Micrologix PLCs used for PID control variable output to 7 no.s VFD drives.2 no.s are used as i have no spare analog cards for analog outputs.I have used a refernce signal of 4-20 mA( 500 ohm res. across analog input is placed for voltage ) for analog input and logic developed with appropriate scaling to engineering units.Now the program worked well with PID parameters tuned with the help of trends pens and output is almost linear with setpoint.But suddenly i saw 2 problems
1) With variation of PV from 4-20 mA refernce the output almost got stagnated either at lower limit or upper limit irrespective of error.When the error is more the control outpur still sat at lower limit( 50%)
2) The reference signal to these 2 PLC's is from same source and signal multiplier is used with appropriate tuning of outputs.
Now for suppose i am running 1 and 2 VFD from ML 1 and 3,4 from ML 2 will there be any problem in the execution of PID in these 2 PLC's?
P.S. : No conditional logic used for PID.
The snap is enclosed for reference.Please help me out.Thanks in advance.
1) With variation of PV from 4-20 mA refernce the output almost got stagnated either at lower limit or upper limit irrespective of error.When the error is more the control outpur still sat at lower limit( 50%)
2) The reference signal to these 2 PLC's is from same source and signal multiplier is used with appropriate tuning of outputs.
Now for suppose i am running 1 and 2 VFD from ML 1 and 3,4 from ML 2 will there be any problem in the execution of PID in these 2 PLC's?
P.S. : No conditional logic used for PID.
The snap is enclosed for reference.Please help me out.Thanks in advance.
