valve cards ?

why they use special valvecards ( if the translation is ok ) and not just two outputs and relay ?
thnx

Some valves are analog in nature. That is they open proportionally to a signal, usually a voltage or current. Some of these proportional valves actually have feedback on how far open they are (servo valves). These cards do the feedback control of the valve.

We used this one and we called it operational valves. This valves actually controlled the rotation of our compactor in wire rod mill. This valves have its own control cards with 4-20ma output. But we are just manually sets it speed reference by using potentiometer. Try to check the catalogue of Vickers.