Temperature sensors

Lagomorph

Hi,
I'm trying to make a touch sensor pad using a grid of temperature sensors that would be covered with a thin, heat-conducting rigid sheet. I need these sensors to be able to pick up the temperatures on the sheet above them when the sheet is touched with the fingers and give the readout to the microcontrollers that will be polling them. This will mostly be used in room remperature conditions.

Do any of you know of a good temperature sensor for this kind of application?

Thanks in advance.

Danger

How discriminatory does this thing have to be? Most touch pads that I know of work by capacitance.

Lagomorph

I want this thing to accept multiple inputs, so I don't think I can use capacitance.

What do you mean by discriminatory? As in how fine do I want the grid of coordinates that it outputs? I figure that multiple sensors should detect a temperature change around the area on the top sheet that was touched. Then, I should be able to find the weighted average between all the readings and find a coordinate anywhere between the sensor positions, as precisely as I want/can.

If you mean what kind of resolution I'm looking for in the sensors, I'm not entirely sure. I don't know how much heat a touch can produce. I'm a bit worried that the sensors would have trouble picking up quick touches by cold hands. Anyway, I figure it would need to pick up pretty small fluctuations in temperature pretty precisely, so I'm not sure. I know I don't need a large range, but I don't know any specifics. Hence, I'm seeing if anyone has any suggestions.

brewnog

Why does it need to use temperature as a signal?

Danger

I was primarily asking about 'resolution'. Even with weighted averages of multiple sensors, I doubt that you could get much more accurate than 1/32" or so. The other problem that I can see is that it would have to measure a temperature change, not just an increase. It's entirely possible (not likely, perhaps, but possible) that the ambient temperature might be higher than that provided by a touch. If you're using it outdoors in Las Vegas, for instance, you would have to detect the coolness of the finger.

FredGarvin

The easiest, I think, would be thermocouples. They are simple and relatively inexpensive, plus the circuitry is pretty well understodd and widely available. Bare wire TC's are pretty responsive and could be tack welded to the bottom of your rigid plate. It will not be an instant reading, bit I can't think of any temperature sensor that operates that fast.

Cyrus

This sounds very tricky. You are going to need to find something that has a high thermal diffusivity. This is the ability of a material to respond quickly to changs in thermal environment. If you're not mindful of this, when you touch the foil it is going to take a long time before the temperature changes. And afterwards, its going to take a long time for the temperature to become that of the free stream environment once again.

If it's something you want to give a quick touch-touch-touch, your going to need very low time constants on your sensor.

chroot

The heat capacity of even small bare-silicon thermal sensors is very large; coupled with the relatively low thermal conductivity of human tissue and plastics, you're looking at large time constants. This concept is flawed. I suggest you look into capactive touch sensors instead.

- Warren

Cyrus

This is another very good point, appart from the time constant, resolution is going to be a significant issue. If you want to detect a rapid touch, you are going to have to measure a very very small change in temperature. I.e., small local change in temperature where the sensor is placed, and thus high resolution with a low threshold.

This was a very nice post Danger.

FredGarvin

Where in the OP did it mention anything about time response as a design criteria?

Danger

Thank you, cyrus; I consider that an extreme compliment coming from someone with your knowledge.

Fred, the response time wasn't mentioned. It seemed to be implied that it should be quite rapid (within a second or so). Nobody wants to stand there with his finger on a particular spot for a couple of minutes while the system sorts things out.

Cyrus

Your brain is a dictionary of scientific terms, you know that? You always seem to use the precise terminology. It is impressive. I myself need to work on that skill.

Danger

You do just fine, yourself. Don't sweat it.
I simply try to use the most applicable terms to cut down on confusion. Sometimes the terms that I use are in conflict with professional usage, which has resulted in misunderstandings a couple of times. I always try (not always successfully) to remember the error and not repeat it. I guess it's just a hold-over from my professional science/science fiction writing days. A newspaper or book audience doesn't have the luxury of requesting clarification, so I tried to minimize ambiguity.
It can have some really weird consequences. When W and I were at the hospital a few weeks ago when our niece's baby died, the nurse/counsellor thought that I was a doctor because of the questions that I asked. She was a bit weirded out to learn that I never finished high-school.

NoTime

Just a thought.
You might want to consider an IR sensor like those used in motion detectors.
IIRC, the sensor is an array with a lens system to focus the image of the covered area on it.

Danger

That's not a bad idea at all, NoTime. I can still see problems with it, but not as many as other techniques would have.

NoTime

Thanks, Danger.
Looks like a good reserch problem.
While not conclusive, I don't know of any off the shelf solution here.

FredGarvin

Another approach could be a set up that uses multiple Peltier circuits. In the "untouched" mode the controller regulates the temperature of the board, call it the main reference temperature, by applying a specific current to each junction. Then, once the plate is touched, the controller measures the current differential required at those junctions being warmed by the hand to try to keep the plate at the reference temperature. The increased current at each location would then correlate to temperature rise.