2253 Floating Control

A hybrid combination of on-off control and modulating control, also called incremental control. As with on-off control, there is a control range (cut-in/ cut-out). However, unlike on-off control, floating control systems have the ability to maintain a mid-position of the controlled device, instead of full-on or full-off. Between the cut-in and cut-out thresholds the controlled device merely holds its last position. The process variable is not actually under control within this range, and is seen to float with the load until it crosses a threshold to get another incremental nudge in the correcting direction. This control is not as tight as true modulating control, but is inexpensive and reliable. Equipment items from terminal units to 1000 HP Water Chillers are controlled in this manner with good success. Note that floating control is limited to processes that change slowly and do not require very tight control.

output is issued to regulate a process, and the magnitude of the output is directly proportional to the size of the error. This type of control is economical and reliable. A characteristic offset (residual error) is natural with this type of controller, and the size of the offset will increase with load. This offset is due to the inherent fixed-gain of

Figure 22.2 On-Off Control Mode Diagram
Figure 22.3 Floating Control Mode Diagram

the controller.

If the proportional control action is too sensitive (gain set too high), the controller's response will be excessive, and oscillation or hunting will occur. When this occurs, the controller output (and the equipment connected to it) oscillate up-and-down, open-and-closed, etc. and don't settle out.

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