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Controller Gap Control
Difference between Gap control vs PID

Can someone explain what a gap control is compare to typical P, I, D? Would appreciate a typical example.

By David Baird on 26 April, 2004 - 2:34 pm

Gap Control is two-mode control = on/off output switching between two setpoints hince the word "gap".
HVAC example: control device that monitors and operates the gap range and, when necessary, activates the building heating system below a predetermined temperature and the building cooling system above a second predetermined temperature.

PID Control is three-mode control = (Proportional-Integral-Derivative) control action allow the process control to accurately maintain setpoint by adjusting the control outputs. Proportioning control continuously adjusts the output dependent on the relative positions of the process and the setpoint.
HVAC example: A damper actuator must provide proportional damper control in response to
a 0 to 10 VDC control input from an electronic controller.

By Jonas Berge on 6 May, 2004 - 12:25 am

I don't agree that gap control is the same on/off control. Gap control is different and relates to PID.

The difference between regular PID and a PID with gap is that for the gap option there is no control action when the deviation between the setpoint and the process variable is within a certain limit (gap), i.e. a kind of
deadband. Optionally, it is possible to configure a different gain within the gap to get more or less action. An example of this may be boiler drum level and feed-water control where the cylindrical shape of the tank results in different gains around the 50% setpoint as opposed to at higher or lower levels. The gap control gives the PID a different gain around setpoint to ensure linear gains throughout the full operating range, i.e. it will not be
slow around setpoint and fast away from the setpoint.

We (SMAR) make a multiloop controller called CD600 whose advanced PID includes gap control. The manual gives a full explanation. Access it here:

Similarly, the SYSTEM302 controller manual contains an explanation as it related to FoundationT Fieldbus:

Jonas Berge
Learn fieldbus at your own pace:

By Anonymous on 14 June, 2004 - 4:23 pm

To me I thought GAP control is where you apply the PID action with a deadtime, or delayed action where you wait for your process to respond.

By Jonas Berge on 17 June, 2004 - 5:28 pm

In our controller mentioned below we call this wait-and-see scheme "PI Sampling". Like you said, it is quite effective on processes with dominant dead time.

However, I see it as a different thing from "Gap".

A good controller should offer both the "sampling" and "gap" features.

Jonas Berge
Learn fieldbus at your own pace:


I always thought that GAP control is on-off and relates to turning the PID control ON-OFF based on a certain criteria.

Do you agree?


By Jonas Berge on 17 June, 2004 - 5:24 pm

Don't agree completely. I see GAP as PID with different gains within a band around setpoint. The special gain can be e.g. 0 kind of meaning "off" or "manual".

Jonas Berge
Learn fieldbus at your own pace:

By Dick Caro on 18 June, 2004 - 2:08 am

How nice when you can say: You are both right.
The Foxboro 130M Series Gap Controller is available as an ON/OFF controller with a gap (Model 130M-N7). It is also available (Model 130M-N94) as a PI Gap Action Floating Controller.

The basic message is that when the PV is in the gap, there is no control action. See this web site:

Dick Caro
Richard H. Caro, CEO
CMC Associates
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +.978.764.4728
Fax: +1.978.246.1270
E-mail: <>
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Wireless Networks for Industrial Automation

By Darren Lock on 7 July, 2004 - 11:50 am

I was taught that there are two types of gap control. One is called differential gap and is an on/off control with a deadband persay. Ie. level control level gets up to the setpoint, the valve opens, but doesn't close until the level is below the setpoint. The other is called gap action and is a PID control. I'm not a big believer in using deadbands on PID control because whether you want the o/p to move within the deadband or not depends if the process is steady or not, so I prefer to tune that out.

I use gap action when I have 2 control elements. I will run the O/p of one pid to run one control element, and also to both the s/p and p/v of the gap action (P+I)controller, then limit the pv so that it cannot go below 20% or above 80%. This way the second control element will only move when the first one is about to reach the end of its control action.

A controller where the gain changes the further away from the setpoint you go is a non-linear controller, typical application would be something like ph control.