Because it needs a precisely controlled solenoid current with specific shaping, not just a voltage. The card provides current control, dither, ramps and deadband compensation. Without them a proportional valve behaves like an expensive on-off valve - sticky, hysteretic and incapable of fine movement.

A small high-frequency oscillation superimposed on the solenoid current that keeps the spool moving slightly so static friction never takes hold. It substantially reduces hysteresis and dead band, and its amplitude and frequency are adjustable because the optimum depends on the valve and the fluid.

Jumps the current across the valve's spool overlap so that a small command produces a small movement rather than nothing. Without it, commands below the overlap threshold do nothing at all and the actuator responds only once the signal exceeds it - which makes fine control impossible.

Because solenoid coil resistance rises as the coil warms, so a fixed voltage produces falling current and a drifting valve position. Controlling current directly keeps valve position stable regardless of coil temperature, which matters on any machine running long enough for the solenoid to heat up.

They limit how fast the command signal can change, so the actuator accelerates and decelerates smoothly instead of responding instantly. That reduces shock loading on the machine and the pressure spikes that damage components, and it is often the difference between a machine that feels controlled and one that slams.

On-valve cards remove wiring between card and valve, simplify commissioning and are pre-matched to the valve. Separate DIN-rail modules keep the electronics in a cleaner environment and are easier to access and replace. The choice usually follows the machine environment and the control cabinet layout.

One that closes a position, pressure or velocity loop itself, taking feedback from a sensor rather than relying on the PLC to run the loop. It offloads fast control from the machine controller and generally gives better dynamic performance, since the loop runs close to the valve.