Hydraulic actuators are fairly rare and I don't believe I've seen a positioner on one. That said, the unit would serve the same purpose of feedback to the control loop and the ability to not rely on benchsetting the actuator. I use to not spec postioners until I put in a "tight" seal (low packing loss) valve. The hystorisis (sic) was terible, the valve jumped everywhere. The positioner cured it.
Hydraulic servos are pretty common in the power generation industry for modulating duty on governors, MSV's and reheat valves. They're also used on valves off the tubine itself in similar applications where a modulating valve is required with very high force, good accuracy, and long service intervals.
One of the more common servo valve manufacturers is Moog. Their servo valves are often seen married to third party cylinders from the likes of Rexroth, Vickers, Parker, etc, although they also produce their own cylinders.
Feedback to close the loop is usually from an LVDT or a magnetostrictive transducer on the cylinder, while the servo controller itself could be either an OEM product embedded within a larger control system, e.g. Emerson's QVP card for their WDPF DCS system, or a stand-alone item from the likes of Moog.
If you are looking for a self-contained package more akin to a traditional valve positioner, have a look at Rexa's offerings. If you give it electrical power and a demand signal it behaves much as a positioner. Failure modes to mimic OAF / CAF / stayput can be designed in as required.
Backing off slightly from servo valves, you can use proportional valves to get the same result at less than half the cost.
Servo valves have a faster reponse time and lower hysteresis than proportional valves. But if the system is closed loop and the position of the hydraulic cylinder is known, a good proportional valve will do the job.
It will just achieve the correct position slightly slower.
- Control signal is 4-20 mA.
- Valve position feedback to positioner is 4-20 mA.
- Output is 2x 24 VDC (1 solenoid to open / 1 to close)
- Power supply is 24 VDC
We use something similar from the BTG days which is reasonably good at its job. It lacks the finesse of the precision servos on the turbines but costs significantly less too.
