Open-Loop Control

Open-loop control is controlling something without directly measuring the result. For instance, the temperature knob on your barbeque grill controls the heat, but the actual temperature is not measured.

In web handling, many controls operate open-loop.

Draw or Ratio control is open-loop. Speed is changed by a draw percentage or ratio. This affects tension, but the actual tension is not measured or known. The operator’s experience is required to set the draw correctly.

Gauge or caliper control may be operated open loop. Die bolts are adjusted manually. Caliper is measured in the lab long after the web is produced.

Torque control may be used to establish tension. The torque is known, but the actual tension is not known because friction and diameter vary.

Web width may be controlled open loop. The operator uses tools and a tape measure to get the web width correct and centered.

There are many more examples. Any parameters that may be set by operators or maintenance personnel to dial in the process is probably an open loop control. That may be air pressure, hydraulic pressure, spring tension, and temperature.

Open-loop control may be compared with its more expensive and complicated cousin, closed-loop control, in which the process is measured and action taken.