Sensor to measure movement of coloured object

Hi,

I am trying to measure the movement of a dark object on a light background with time. The movement is 1D over a distance of 300 mm. I need to be able to log the movement change to mm accuracy every 5 minutes or so. The movement is basically to try and track the compression (height change) of an object with time (research project). Based on there being color differences, i am thinking i can exploit this parameter to measure the movement.

I have thought about capacitive sensors, but there does not seem to be a suitable sensor over such a length. The nearest i could find was for a liquid level sensor for 100 mm:

http://www.sensortechnics.com/en/products/liquid-level-sensors-and-switches/capacitive-level-sensors/clc/

Does anyone have any ideas on sensors that might be able to do this with an Arduino type logger system?

Is this like a bladder or balloon being filled? 300mm is to us Texans about 12 inches. I think we can get there from here (there's more than one way to skin a cat). Little more detail may help.

Hi Jack,

No its not a balloon or bladder. Effectively I'm trying to measure a liquid/sludge interface as it settles/compresses. This occurs in a clear perspex tube. The slurry is black in color and the liquid is almost transparent.

I am thinking i can do this with a LVDT that has a weight that sits on the sludge as it changes in height (i.e. higher density than the liquid).

If you want mm accuracy over 300mm, that is of course 300 possible readings. To many readings for a dozen IR sensors.

Ok, how about this:
a pair of IR send, and sensors, mounted with the tube between them. The sensors are on a movable platform that can move up, and down, by either a servo, or stepping motor.

As the arduino moves the sensor up and down, it reads the IR, and determines at what point it is clear liquid, and logs it.

Does that make sense?

http://store.curiousinventor.com/300mm-softpot.html

This could be used fairly easily. so long as you can have method of pressing on it at the top level of your "slurry"

Jack, i see where you are coming from and thought of something similar, but the IR sensors wont penetrate the tube, or will they?

Alternatively, what about having a light bar on one side (e.g. floro tube) and on the other side a fiber optic tube with a photoelectric sensor at the top that measures more light as there is more compression (i.e. more light passing through the sample tube).

I still like the LVDT idea. I may even fabricate my own if i can create a float that can sink in water but float on the interface (density difference of 0.6 t/m3). This is a basic version of what i was thinking LVDT demo - YouTube

Justin, great idea and just the sensor i have been looking for. The problem is (apart from it not being in stock), is that I'd need a float to support the weight (or a weighted gimbal) to prevent more of the sensor going below the water interface. This might be a little difficult.

Any other ideas?

but the IR sensors wont penetrate the tube, or will they?

I thought the tube was transparent. If so, the IR will penetrate the tube.

Yes, you are correct, the tube is transparent (wall thickness of 2.5 mm). Can you provide me sensor details that you are thinking of please? I cant seem to find a suitable sensor/receiver.

Another idea (based on your approach) would be to try and use a laser/LED that can sweep up and down the tube with a fast stepper motor. One side is a transmitter, the other is a receiver. Now the advantage to this would be that i could actually measure the clarity of the water as the sample compresses (i.e. i could calibrate the sensor to try and read a turbidity value). What type of sensor can focus a high power light source and a receiver that could read the pentration of light through the dirty water?

Great stuff. I feel i'm getting closer to my goals with this research project.

Most any light source, and light sensor should work.
I assume this will be inside, and not in direct sun light.
Since we don't know the density of the sludge, vs the more clear liquid, you may want to experiment with a couple combinations.
I would start with an IR transmitter and IR detector first, such as Account Suspended
I assume you will have to mount then yourself since the tube is of some diameter. They should be mounted to move up and down together, not trying to read diagonally.