IR Photodiode project

[dknguyen]

Should be fine. Don't forget that thermopiles, just like thermocouples measure the temperature difference between the target and themselves. That means if you want absolute temperature then you need a local absolute temperature sensor to determine the temperature of the thermopile itself, or have two thermopiles right next that are the same temperature (possibly by mounting them on the same block/pad of aluminum or copper) and have one point at a known temperature target. Or you could point at different targets to measure the temperature difference between two remote targets.

If all you're interested is in the temperature rise above ambient and the thermopile is far enough away from the peltiers to not be heated up, then you probably don't need to do any of this.

 

[shane ren]

Ya I think all I will need is the rise above ambient, so are you saying have the thermopile back at a distance where it will be relativly close enough to room temperature or the temperature of its surroundings? Seeing as thermopiles do seem to generate a lot of noise due to temperature flucuations I was thinking of using an instrumentational amplifier as it will have infinite input impedance, would this be a suitable op amp? If so the two inputs into the circuit are v1 and v2, would these be hooked up to ther thermopile and thermister within the thermopile in order to obtain the difference in temperature?

 

[shane ren]

If I was using the thermopile up close, I could box it off with suitable material so that the temperature off the peltier would be less likely to affect it?

 

[jpanhalt]

If you put it in an insulated box, it will get warm from the peltier. How much will depend on its heat mass. I would probably do the opposite -- perhaps put it in a small, vented box connected by flexible ducting and a fan to keep it at ambient.

 

[dknguyen]

Ya I think all I will need is the rise above ambient, so are you saying have the thermopile back at a distance where it will be relativly close enough to room temperature or the temperature of its surroundings? Seeing as thermopiles do seem to generate a lot of noise due to temperature flucuations I was thinking of using an instrumentational amplifier as it will have infinite input impedance, would this be a suitable op amp? If so the two inputs into the circuit are v1 and v2, would these be hooked up to ther thermopile and thermister within the thermopile in order to obtain the difference in temperature?

Yeah. If you only care about above ambient then keep it far enough away from the heat source that it wont heat up itself or stick on "heatsinks" or even just giant blocks of copper to stabilize the temperature at ambient. For example, if you want to use fans to keep the thermopile at ambient, you should be aware that blowing air will cause temperature fluctuations on it's own so you might want to mount the thermopile on a large block of metal or a heatsink. Just something with lots of thermal mass to buffer against small pertubations in temperature.

You could use an instrumentation amp. I don't think it's necessary though. You do want an op-amp with high input impedance, low bias currents, and low-offset voltage though. Which one has highest priority depends on how much you want to amplify the signal by. Auto-zero op-amps are quite good for this.

DO NOT use a thermistor just connected in series with the thermopile. In fact, pretty much never do this unless they are identical sensors. Thermopiles are basically thermocouples and are quite linear voltage output sensor. Thermistors are super non-linear resistive output sensor. If you do use a thermistor, you have to take the reading separately, linearize it, and then subtract that calculated temperature from the thermopile temperature (probably in an MCU). You can't just combine the output voltages together.

But what you can do is connect two identical thermopiles in anti-series onto the same metal block, or at least very close together (so they are kept at the same temperature) and have one point at the target and the other point at a distant point in the room where the FOV grabs much of the room. Just make sure no one walks in front of it. You could also point the second sensor at anything else to find the temperature difference between the peltier and that thing.

 

[shane ren]

I like the concept of your last idea, however I am struggling to get my head around the idea. What do you mean by anti series? I tried looking this up and couldnt find anything electronics related. Another question is how would I mount the thermopiles to a a metal block?
Just going back to the instrumentation amplifier, you know the way there is a v1 and a v2, would these be the two thermopiles if I was to use your method on it?

Also while reading about how to read the temperature I came across thermocouples and how they have certain voltage values which apply to each temperature reading depending on the type of thermocouple, are thermopiles considered the same seeing as they are made up of thermocouples? Or would I just calibrate it by by heating an enclosed area with a calibrated thermometer, and at each degree record its voltage?

Thanks in advance, you have been a great help to me.

 

[KeepItSimpleStupid]

Anti-series: Two 1.5 V batteries in series would be 3 V; anti-series = 0V

 

[dknguyen]

I like the concept of your last idea, however I am struggling to get my head around the idea. What do you mean by anti series? I tried looking this up and couldnt find anything electronics related. Another question is how would I mount the thermopiles to a a metal block?
Just going back to the instrumentation amplifier, you know the way there is a v1 and a v2, would these be the two thermopiles if I was to use your method on it?

Also while reading about how to read the temperature I came across thermocouples and how they have certain voltage values which apply to each temperature reading depending on the type of thermocouple, are thermopiles considered the same seeing as they are made up of thermocouples? Or would I just calibrate it by by heating an enclosed area with a calibrated thermometer, and at each degree record its voltage?

Thanks in advance, you have been a great help to me.

Thermopiles and thermocouples have a + and - pin. Anti-series means connect them in series with either - to - or + to + so their voltages cancel each other (automatically determine the temperature reading difference between the two, assuming they are identical components and are of the same temperature). Which way you choose will determine how the temperature subtraction willw ork.

NOTE: In both of these cases, if your amp is a unipolar output, then you need to either bias things mid-supply so you can get both positive and negative temperature difference readings (you are subtracting temperature differences after all), or else you must always have one sensor be at least as hot as the other or else you will get a zero reading whenever you would expect a negative temperature difference. This is actually true too if you want to use just a single thermopile to measure something colder than itself.

If you connect two thermopiles in anti-series, you can just treat it as a single thermopile when connecting it up. If you use a diff-amp, you could also connect the two thermopiles to separate inputs of the diff-amp, but have one of the thermopiles be connected in reverse.

No need to calibrate: look at your thermopile sheet. It has a temperature vs voltage output curve. Thermocouples OTOH, don't have this because there are several standardized types of thermocouples with charts for each type.

Attaching one thermopile to a metal block to stabilize it's internal temperature against external fluctuations, or attaching more than one thermopile to a metal block to make sure they have the same internal temperature is just an ideal. It's not always necessary and might be more effort than it's worth for your project. Off the top of my head...possibly large holes drilled into an aluminum plate that the thermopile can can be pressed fit into sounds the simplest. Or maybe some small holes for the thermopile leads drilled into an aluminum plate that the thermopile mounts into though there may be some bad electrical effects of metal being close to the leads, not to mention potential shorts unless you take additional precautions.

 

[shane ren]

So I am just after being told again that I cannot use the thermopile as it is too a like a similiar project, so therefore he has given me a pir sensor https://ie.rs-online.com/web/p/infrared-sensors/1245982/?sra=pstk to use, this is all new to me now after doing all my research on thermopiles. I know you did mention using this earlier, to simplify it could I just use the sensor and connect it to a non inverting amplifier or a another amplifier, then the output of this connected to either a high or low pass filter to remove unwanted noise. Will I have to use the chopper if we are using a stepper motor to move the sensor around to 16 different locations? Would the stepper be the same thing as using the chopper?

I mentioned that I wanted to use two thermopiles and I had it so set in my head that I would have one measuring the room and one measuring the peltier in order to get the temperature difference. The pir sensor doesnt seem to work in that way. I am just looking up the notes which you had linked to pyro sensors

 

[dknguyen]

So I am just after being told again that I cannot use the thermopile as it is too a like a similiar project, so therefore he has given me a pir sensor https://ie.rs-online.com/web/p/infrared-sensors/1245982/?sra=pstk to use, this is all new to me now after doing all my research on thermopiles. I know you did mention using this earlier, to simplify it could I just use the sensor and connect it to a non inverting amplifier or a another amplifier, then the output of this connected to either a high or low pass filter to remove unwanted noise. Will I have to use the chopper if we are using a stepper motor to move the sensor around to 16 different locations? Would the stepper be the same thing as using the chopper?

I mentioned that I wanted to use two thermopiles and I had it so set in my head that I would have one measuring the room and one measuring the peltier in order to get the temperature difference. The pir sensor doesnt seem to work in that way. I am just looking up the notes which you had linked to pyro sensors

If you can directly control the stepper via the MCU and feed the stepper's position into the MCU then you technically don't need a chopper. There is a catch however. Since the PIR measures the rate of temperature change, moving from one area of interest to the next will only give you the temperature difference between those two areas. If you are wanting to measure with respect to ambient, you are going to have to throw something at ambient temperature into the mix so the PIR can view a known temperature at some point.

Also note that the orientation of the movement matters since the PIR you linked is a dual element sensor where the elements are connected in anti-series. Therefore, it only picks up heat sources moving across the sensing elements individually. Heat sources move in a direction such that they move over both elements simultaneously cancel each other out and won't give a reading.

PIRs are also piezoelectric and thus sensitive to vibration so keep that in mind. Tapping one can generate a lot of voltage. Enough to damage other components sometimes.

Make sure to get a IR thermometer so you can actually calibrate the PIR. No simple temperature vs output voltage curves for PIRs.

You also cannot use the voltage reading directly like you can with the thermopile since the piezo only outputs AC. You are going to have to examine the output voltage curve between chopping or during the movement between temperature targets to figure out the temperature difference between them. In that sense, chopping is probably a lot easier since you can just sample the peak value while blocked and unblocked and come up with a temperature based on how fast things are being chopped. There's also the issue that if no chopper is used, you might be spinning the thermoelectric elements so fast that the air flow is influencing their temperature.

 

[shane ren]

The stepper part of the project is up to another guy and I don't think he will have that working at all, so I am just trying to get my part of the project to work so that I can pass. So if I used a differential amplifier with one of the inputs being the pir sensor and the other being a calibrated thermometer at room temperature would this work? I would like to avoid using the chopper part of the program as with the time left I think its far too advanced for me to understand,

 

[dknguyen]

The stepper part of the project is up to another guy and I don't think he will have that working at all, so I am just trying to get my part of the project to work so that I can pass. So if I used a differential amplifier with one of the inputs being the pir sensor and the other being a calibrated thermometer at room temperature would this work? I would like to avoid using the chopper part of the program as with the time left I think its far too advanced for me to understand,

No, like I have said previously, you can't just combine voltages from different sensors willy nilly since even if the the voltages both represent temperature, they probably won't be on the same scale. In this case, the PIR sensor output doesn't even represent temperature while the calibrated thermometer does. The PIR output represents It represents how fast a heat source is moving in front of the sensor.

In order to measure temperature with a PIR, you will need to have it move back and forth between the target and a known temperature, and the speed that you move it back and forth must also be taken into account since the faster the movement, the higher the output will be.

If you're in damage control mode, grab a brushed DC motor making a chopping wheel for it and apply the same regulated voltage to it all the time so it always spins at the same speed, whatever that speed may be. Then blindly calibrate temperature to output voltage (probably peak voltage since that's probably the easiest most straightforward thing to do if you're chopping fast enough) to temperature. Look at the output waveform on the oscilloscope with the PIR looking through the chopper and figure out out where the minimums and maximusm are and how far apart they are in time from each other so you know when to sample it in the MCU.