Good day everyone. I'm relatively new to LOGO. I have an application where I am using an analogue ramp generator to slowly increase the output AQ, from 4 ma to 20 ma. I'm using this to drive a valve positioner. I have the system programmed and simulated, all works well. When I test the system measuring the output with my meter, again all works well as expected. My problem comes when I connect the positioner directly to the AQ current o/p. My zero is elevated to 4.86mA, not 4 mA. This is leaving me with a problem as my valve will not fully close.

I have an amplifier function block between the ramp generator and the AQ output. I have changed the o/p to 0-10v, and changed the output device also. This works fine both on my meter and also when I hook up a valve.

can anyone suggest a solution? I am going to install a signal isolator between the AQ output and the valve positioner, but I'm just wanting to know if this should be necessary?

thanks Gerry

I've added my program for review

valve three is set up for 0 - 10v, just as a trial.

valve 1 and 2 is 4 - 20mA