Averaging sensor

  • Thread starter Joe Jansen/ENGR/HQ/KEMET/US
  • Start date
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Thread Starter

Joe Jansen/ENGR/HQ/KEMET/US

I am looking for a sensor that can read the surface of a fluid, and average the level. The application is as follows: We have a pan about 8 in. by 12 in. by 3 in. deep. It is filled with a fluid held in suspension. Because of this, we need to agitate the fluid to keep the suspended material from settling out. We have parts that are indexed into position and then dipped by raising the pan to the parts. The dip depth is a critical measurement, requiring accuracy to 0.1mm. We stop the agitator when the parts arrive at position, but the surface motion on the fluid is still enough that the ultrasonic sensor that we use to read depth is jumping around somewhat. What I would like to try to find is a sensor that will read an area of perhaps 3in. by 3in. and give me an 'averaged' distance reading. I have tried using some debouncing and averaging algorithms in the PLC, but with only a single point reference from the sensor, this isn't working out real well. Any advice, including telling me I am just crazy, is appreciated. Thanks! --Joe Jansen
 
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David W. Spitzer

Joe, Consider using a capacitance level transmitter with a metal plate above the liquid as the sensor. This may require some experimentation. Regards, David W Spitzer, PE Copperhill and Pointer, Inc. http://www.icu.com/spitzer 845.623.1830 (phone/fax) Enhancing productivity by: - solving measurement and control problems - providing process control engineering guidance - evaluating instruments and assessing new technologies - presenting training seminars for people who use and make instruments
 
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Joe, Have you tried moving the sensor away from the soup? Since the ultrasonic beam diverges fairly rapidly, you might be able to get a better average reading just by doing this. Willy Smith Numatics, Inc. Costa Rica
 
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Alfonso Padilla

Joe, If you said the dip depth is a critical measurement requiring accuracy to 0.1 mm, but on the other hand you have an agitated fluid (agitation was removed a very shot time before), the crests and valleys may exceed by far yor required accuracy. Having an averaged measure may cause your dipped parts to be unevenly coated. I guess the pan upraising may also add to fluid motion and surface ondulation I suggest you first check the surface ondulation amplitude using those “jumping” readings you obtain from your ultrasonic sensor and compare with the 0.1 mm accuracy you require. The result may lead to other approaches to the problem. David’s or Willy’s suggestions may work, depending on the results of your search. Keep me posted Alfonso Padilla QPS Control & Automation Mexico [email protected]
 
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Steve Myres, PE

You might try using a tank within a tank, where the upper tank is the one into which your parts are dipped. The liquid is constantly spilling over the lip of the upper tank and returning by gravity to the lower tank, then returned to the upper with a pump. This pump might give sufficient agitation, or additional prop mixing might be required in the lower tank. We use this type of arrangement in continuous (strip) processing. The lip of the upper tank, probably wants to be chamfered where the mixture cascades over it to maintain the best surface. You may need to be careful how you introduce the liquid into the upper tank, preferably in a laminar fashion (maybe through a perforated plate in the floor of the upper tank).
 
Hello Joe. To my understanding. You need to maintain the level because - as the pan rises into product the "lift height" is based on the fluid level in the tank. In other words something reads the level and then lifts the tank to and around the product based on the surface level? So - there is no inlet or level control - it is simply based on the top surface - correct? 1)What is the fluid temperature? 2)What material will work eg. SS? 3)Is this stuff bad? 4)Is the top of the pan completely open? 5) How do you fill it? 6) Is the agitation internal or external of the pan? I think you have a couple of custom options but please confirm the above. Bob Hogg www.almegcontrols.com
 
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Brian Kukulski

An averaging technique that I have used numerous times with good success is to use the PLC PID block. Now for a mimimal number or registers, one has a tunable filter/averager. In your case input the actual level as the setpoint and tie the output back to the PV. Tune it up and now your smoothed level is the PV.
 
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Kent L. Gerhardt

Joe, Another option that we use in large vessels that are on moving structures (offshore oil and gas) is to place an open-ended pipe vertically in the fluid with the top just above the surface of the liquid. Fluid entering the tube from below u-tubes to the same level as the fluid outside. The walls limit the "wave action" on the surface allowing more accurate level measurements. Does your setup allow a small tube to be attached to the pan such that the level sensor would look at the level inside the tube? It's not high-tech but it might be worth a try as it is cheap. Please let me know what you do find for a solution. My curiosity is piqued. Thanks, Kent
 
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Joe Jansen/ENGR/HQ/KEMET/US

>To my understanding. You need to maintain the level because - as the pan rises into product the "lift height" is based on the fluid level in the tank. In other words something reads the level and then lifts the tank to and around the product based on the surface level? So - there is no inlet or level control - it is simply based on the top surface - correct? < For the most part. Actually, the lift is controlled by a stepper motor and geared down significantly. From the home (lowest) position, we start lifting up with the pan. The sensor is watching the surface level rise (with the pan). When a set point is reached, the motor is stopped. The carriers that hold the parts are then locked into place, and the stepper motor lifts the pan an additional distance (about 4 mm) to perform the dip. The reason that we can get 0.1mm accuracy is that while the carriers are locking down and everything else is occuring, our surface motion continues to decrease. This leads us to look for ways to increase our non-agitation time. The problem is that this stuff settles out of suspension fairly rapidly. We are dealing with pretty small parts (building surface mount capacitors) so the mixture needs to be maintained. We are running some trials right now on stopping the agitation sooner. 1)What is the fluid temperature? About 100deg F 2)What material will work eg. SS? Yes, we are using teflon coated stainless for the reservoir itself. The agitator consists of a sheet of stainless that is 'perferated' such that it is a grid of holes. This sheet is raised and lowered to agitate. It is always submerged, but the lift and drop action causes the agitation. 3)Is this stuff bad? Yea, you don't want to get any on you.... 4)Is the top of the pan completely open? other than where the bars go down to connect to the agitator, yes 5) How do you fill it? In the back of the pan is a small, secondary reservoir. The fluid is pumped up to the secondary res. and spills over a weir into the main pan. Once level is achieved, it is pumped back out of the secondary container into a holding tank where a propeller mixer keeps it blended. 6) Is the agitation internal or external of the pan? both (see above) >I think you have a couple of custom options but please confirm the above. > >Bob Hogg www.almegcontrols.com< We are trying a couple of things, one of which is stopping the agitation at the beginning of the index, rather than the end. This gains us just under 5 seconds of more 'settle' time. It was also recommended that I try having a small tube in one corner of the pan and read the level from that, thus reducing wave action from the entire pan. Thanks to all for assistance... --Joe Jansen
 
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Kent, and Joe, We use to call that a stilling well. If your vessel is pressurized, both ends of the vertical tube have to be connected to the vessel. Alternatively the entire tube may be installed inside the vessel. Vitor Finkel [email protected] P.O. Box 16061 Tel (+55) 21 285-5641 22221.971 Rio de Janeiro Brazil Fax (+55) 21 205-3339
 
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