Sensor for grease application

Does anyone know of a reliable way to detect air bubbles in 1/8" plastic tubing grease line? I'm using a robot to grease parts and need to know that grease is flowing as well. A camera inspecting the greased parts does not work. Customer will not allow any additives to grease to make detection with a camera reliable.

Thanks in advance
Bill D

Check out the HPQ-D sensor from Yamatake. The application I'm familiar with is acids and solvents flowing in Teflon lines (clear liquids). It's designed to detect gaps in the fluid. Small bubbles will go undetected, but larger gaps is your biggest concern. Maybe not perfect, but this is probably the best you can do considering your contraints.
Cost is about $500.00 for head and amplifier. Everything is made out of Teflon or similar to make it acid/solvent proof, so it may appear a bit pricy.

Thanks, I was looking at this type of sensor but when I talked to the company they said it was not a good fit for this application. I'm going to try a analog capacitive sensor but the vender does not feel this will be reliable either.

Nothing is a good fit for your application. Sensing a void in grease is tough. If you can't have an additive, then the only other things you can put in it are light or heat. Sensing flow by looking at heat transfer is the method used for gas and liquids, but it wouldn't do so well detecting a bubble. There is a time delay in heat transfer, and would only work while the grease is flowing.
Light is the only reasonable way to do it. If this was mission-critical, I would insist on a camera.
A more practical way is light refraction. How dark is the grease? Will red or IR light pass through it? If so, then the Yamatake should work. The sensor expects light to bend as it passes through the liquid. No liquid = no bend.

If the grease is dark, then use a through sensor. In the normal state the light is blocked. If there's a void or even a good sized bubble, light will pass through.

I am sure that I have seen Keyence demonstrate photo sensors designed to switch on changes in light intensity when they are looking through translucent objects (the object in the demonstration was the salesman's hand). You could contact them if you think that this could fit the bill.

Andybr

I have no idea whether this will help or not, but I found it while googling something completely unrelated and remembered reading this post.

Steve

Your capacitance vendor is truthful - capacitance is highly unlikely to work. It would have to sense the differnce between two extremely low dialectrics, grease, a petrochemical, which has a very low dialectric and air, the other component which is ther reference dialectric of 1. The difference between the two is probably too minimal to be reliable or repeatable.

That ultrasonic sensor Steve Etter linked to looks intriguing. If yougo with it, please post how it works out.

Dan

If the grease will not allow light thru the specifed size bubble at 1/8", then you are not going to get vision to work. You would have to make the thickness of the viewing tube such that it was smaller than the maximum bubble size allowed. I can envision a flattened tube, very transparent, with a DC high contrast light source and a color vision system with "blob" tools to be quite feasible, however expensive.

You would need top notch lighting, some sort of enclosure probably, and a fast processor depending on the rate of flow of grease and blobs of air.

But so, you've now detected a bubble in the grease, what next? Stop production? Open a vent valve? Add a little more grease to the process? None of those are final solutions are they? I could be wrong, just Shored Bitless in the Land of Bugs Weeds and Toll Roads.

Sounds very challenging...one of those problems that makes you look at the whole application to see if eliminating the problem might be superior to sensing it and reacting.

Why air there bubbles in the 1st place and how can we prevent them?

Not having seen your application, one can assume you have already thought long and hard about that, and probably have tried solutions there.

Shirley, there is an air trap suitable for grease on the market? Maybe you can roll your own that fits your end of arm tooling.

If not, would it be possible to detect a sudden pressure (or volume) change when the air escapes at the point of delivery?

Without knowing the length of the tube, the type of supply pump and how it is fed and distributed, and the pressure, I can't really go much farther with my vision of how to detect the bubble.

Hmmmm...

What are you using to pump the grease? Because of the viscosity of the grease I'm betting that you are using a positive displacement pump. So put the sensor on the pump. You know how much grease you applied because you know how much the piston stroked. Put an orfice inline to cause a pressure drop between the grease pump and the grease applicator, then a pressure transducer will tell you that that the grease pump piston is actually pushing grease and not air. Thus, if you have piston stroke and a pressure drop across the orfice, then you have applied grease. Any stroke without pressure indicates that you are pumping air.

If you use a hand pumped grease gun and it gets air locked it goes soft in the pump action, so I would back Alaric in his concept of a pressure transducer.
Probably straight after the pump.

If I may present an opposing observation without appearing to be disagreeable....When the grease gun (positive displacement pump) goes soft it is because the PUMPING chamber has no grease......grease makes the plunger SEAL so without it, you aren't pumping AIR, you aren't pumping anything.........not the same situation as a small air bubble in the delivery line.

This is evident when the grease gun "feels" perfectly normal, but you still hear a crackling noise as the grease goes into the fitting. That "crackling noise" is from escaping/collapsing air bubbles.

Pressure is pressure in a TUBE or LINE like the OP is describing. I doubt if sensing PRESSURE will be reliable enough to detect BUBBLES.

I know we're just brainstorming here and I don't intend this post to sound critical.

Stationmaster

I like Paul's concept.

OkiePC said:

Why air there bubbles in the 1st place and how can we prevent them?

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Why not just heat the grease in the first place? Thats how those knuckle dragging grease monkey's do it. It makes the air bubbles go away before it gets to the pump. I'm speaking from a knuckle dragger turned sparky perspective.

Stationmaster said:

If I may present an opposing observation without appearing to be disagreeable....When the grease gun (positive displacement pump) goes soft it is because the PUMPING chamber has no grease.....not the same situation as a small air bubble in the delivery line.

This is evident when the grease gun "feels" perfectly normal, but you still hear a crackling noise as the grease goes into the fitting. That "crackling noise" is from escaping/collapsing air bubbles.

Pressure is pressure in a TUBE or LINE like the OP is describing. I doubt if sensing PRESSURE will be reliable enough to detect BUBBLES.

I know we're just brainstorming here and I don't intend this post to sound critical.

Stationmaster

Click to expand...

Each of those little cracklings sounds is accompanied by a dip/spike in pressure as it is released. Using a transducer would require exacting design in its location and monitoring.

If the force applied dissipates the bubble there shuld a dip, follwing by a spike in the pressure depending on how it is controlled (piston? screw pump?, other?)

Is that enough to sense based on pressure? That depends on how strong and how controllable the pumping force is along with other factors already brought up. If it could work, it would be low maintenance and dependable using that scheme.

Or, maybe they'll invent an audio analyzer card for the PLC so we can listen for bubbles...hmmm....

Stationmaster said:

Pressure is pressure in a TUBE or LINE like the OP is describing. I doubt if sensing PRESSURE will be reliable enough to detect BUBBLES.

Click to expand...

Thats precisley why I suggested pressure DROP across an orfice near the grease dispenser combined with pump stroke.


Another idea: If the part is not large and heavy, weigh the part with a load cell before and after its greased.

OkiePC said:

Or, maybe they'll invent an audio analyzer card for the PLC so we can listen for bubbles...hmmm....

Click to expand...

I was just sitting here (honestly) thinking the EXACT same thing.

I was also musing about "vibration analysis".............hmmmmmmm.....

Stationmaster