The valve offset parameter
How does Alicat do it?! How can a proportional control valve hit a setpoint from zero so quickly and accurately?
Part of the answer is our valve offset setting. It’s like having a sprinter poised in the starting blocks, versus standing flat-footed, when the starter pistol sounds.
Valve offset is an adjustable control parameter which affects the amount of drive power applied to a closed valve. When we’re building a controller for a customer—and nearly all of our instruments are built-to-order—this parameter is set based on customer—provided inlet and outlet pressure values; by tweaking a small amount of valve drive supplied to our normally closed valves, we are able to tune the controller to hit its setpoint without overshoot or delay. With optimally tuned PID and valve offset parameters, an Alicat controller will be able to achieve its setpoint as quickly and stably as possible.
Here’s how it works: Alicat’s proportional control valves start out closed—at a zero flow setpoint—with little to no valve drive applied. When the valve is told to open by the PID control algorithm, the amount of passively applied valve drive in part determines how quickly the valve reaches the appropriate position for the commanded setpoint. With no passive valve drive applied, the valve will take more initial drive change to open, which can cause a delay in lifting off a zero flow value. If too much passive valve drive is applied to a closed valve then the valve will lift off too quickly, and the controller may overshoot its setpoint before flow is stabilized at the commanded rate. In essence we can control how hard the valve is clamped down on its seat when it’s closed, so that when it needs to open to allow pressure through the flow body it is in a better position to hit the setpoint on the dot.
Here’s a video on how to recognize if your valve offset parameter could use some tweaking — and how to change it.
Having trouble getting to your setpoint the way you want to? Here’s a video showing easy troubleshooting steps—or contact our apps engineers for expert help.