Floating Control

While searching this site for advice on tuning PID loops I came across an old (Jan. 2003) post by Tom Jenkins in which he said he tries to use floating control instead of PID control whenever possible (see below). He says that the topic has been discussed previously on the forum. I have searched for these threads but am unable to find them. I suspect they may have been deleted due to their age.

I would appreciate it if someone could point me to information on floating control as I am not familiar with it.

Thanks

I'm not sure what Ron or Tom posted, it was before my arrival here, but I've run into floating control in HVAC applications.

This link gives a fairly good description of floating control as I've encountered floating control in HVAC apps with an electric actuator driving a damper or a valve:

http://www.controltrends.org/tag/floating-control/

What they neglect to mention is that the controller is usually provided with a dead band zone such that when the process variable is within the zone, the output of the controller is a null output, meaning no output change (no output is not the same as "zero percent output" as in proportional control).

The deadband is desirable because otherwise the output would be changing continuously in one direction or the other, as it does in proportional control.

So a floating control controller output has only one state of three: either Off, one direction or the opposite direction.

For motorized actuators, that's OFF/CW/CCW or Off/forward/reverse.

Such motorized actuators typically are 3 wire - a common, power for driving in the forward direction; power for driving in the reverse direction.

When there is no power applied to either power lead, the motor holds its last position.

Some say that 'floating control" is the final control element "floating" at the last position.

Others claim that the "floating" is the process variable "floating" within the deadband zone.

I'm not sure which is correct, but the concept holds for electric partial turn rotary actuators used for dampers in HVAC.

Maybe this will help

Here is an excerpt from Process Instruments and Controls Handbook, Third Ed. McGraw Hill. It should help.

There are lots of variants with floating control. I use it almost exclusively on my process control systems, including flow and pressure control as well as more esoteric control loops. I find this technique easier to tune and more stable than PID. I usually employ time delays and neutral zone (tolerance) to promote stability.

It reminds me of Fuzzy control.

I wouldn't agree with the comparison. My limited understanding of Fuzzy Control is that it serves as a modification to PID by adjusting the tuning to incorporate additional considerations, control output limits, and learned responses. To me floating control actually resembles the venerable old house thermostat.

you are both correct.
float control is a normal P action in steps or floating, without INT or DIF.
Fuzzy is acting on several inputs and uses expertise from a database.