Rejecting sensor noise in tank level control

  • Thread starter Antony Hilliard
  • Start date

Excellent suggestion! Part of the reason I have been so reluctant to add new equipment is that we expect to find this problem in new installations where the tanks are practically unmodifyable. They will be double-bottom tanks below deck and adding level sensors will be a very painful process.

If I had the unit more easily accesible I would gladly try out some of the mechanical approaches. Adding new equipment aboard ship is much more difficult than it would be ashore.

Signal processing is also compounded by the combined effects of waves and ship course corrections - while steering through narrow Alaskan passages the tank's disturbance signal is certainly not sinusoidal!

I will post the results of trying this speed-master approach.

Thank you so much to everyone who posted, I had no idea that the response would be this enthusiastic!


Here are some possible solutions:

1. Gap Action Floating Control. Provides standard PI control action when
the process measurement rises
above high gap limit or falls below the low gap limit and floats in between
tending to drift toward the middle of the gap ( in this case say +/- 10 cm).
This URL give a description of a pneumatic gap controller( sorry don't know
off hand of an electronic controller that has this type of control).

3. A gain scheduler controller or adaptive/adjustable gain controller -
This type of
controller can have a low gain around the set point and a higher gain
outside the "gap" around the setpoint. Most modern digital controllers have
this function.

Bill Mostia


John G. Boland

Hi Antony Hilliard,

On 5 July 2003 you wrote:
> I'm having some serious trouble with flow/level control in a
>large tank. The tank is fed constantly, the tricky bit is that
> it's aboard ship and as the boat rolls, the level indicator
> changes value, and the PI-controlled pump ramps up and
> down to compensate.

And on 22 July 2003:
> I will post the results of trying this speed-master approach.

Well??? Any luck? Hope so! Inquiring minds would like to know...


John G. Boland, president
[email protected]
Strateg!c Method$ Corporation
One Parker Square Suite 408
2525 Kell Boulevard
Wichita Falls, Texas 76308
940.723.1478 fax

Antony Hilliard

One year later:

Well, the solution was to eliminate the integral portion of the control. As I understand it, a single-in-single-out tank level control system is self-integrating by nature. Having an integral control component on top of that made it unstable.

A very low KP factor and time averaging of the input signal did the trick. Thank you to everyone who replied and my apologies for waiting so long to get back to this thread.

(We're working on our next project - this time the tanks are 240m3 volume and the in/out flowrate 50m3/hr...)