What's the Difference Between Closed Loop and Open Loop Control

ACI made a decision a few years ago to manufacture our electric to pneumatic interfaces with Closed Loop control. They discontinued the open loop pneumatic interfaces primarily because the closed loop offers a better controlled system. ACI closed loop interfaces have been used to replace many other open loop interfaces that were failing from excessive cycling. Commonly the BAS controller in an open loop system periodically gives a refresh command signal to the interface to maintain a specific pressure on the branch line. The closed loop interfaces from ACI react at the transducer to compensate for pneumatic variances on the branch line. If the branch line is severed the closed loop interface will remember the last commanded position and recall the desired pressure output upon reconnecting.

Analogies:

• Open Loop. An automobile with an absolute throttle position at 40 MPH will bog down going uphill but then speed up considerably when going downhill.
• Closed Loop. The automobile will automatically compensate for any steep grade and keep a consistent 40 MPH speed.

The ACI pneumatic closed loop interface accepts a control input signal (voltage, current, pulse width modulation, or floating point) and converts this to a proportional pressure output signal. The interface will continue to maintain the desired output pressure until the input command signal changes. If the control input signal does not change and the branch pressure is not proportional to the pre-calibrated range, the transducer will self correct the pneumatic output to achieve the desired result. If you are having any difficulties with a current system, we suggest that you try one of these interfaces and observe if it corrects the performance of your system.