Temperature sensor for automotive brakes

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The topic says it all really :P

Reason is that I have some serious trouble with my brakes, and it would be interesting to get live data telling the calliper temperature on all four corners(And it would look really cool).

Also some design tips would be great. I guess the sensor would be glued to the calliper with thermal glue, but not sure if I should put a MCU down there too and use RS232 up to the display, or if I should route cables straight to the sensor from the display unit.

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I got an IR thermometer at Harbor freight for $9. They call the sensor a thermopile. I guess if you put one down near the brakes the lense would get all muddy. Maybe it needs a little windshield wiper.

Imagecraft compiler user

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A common technique the racers would use is thermochromic paint. The would put the paint on different parts, do some laps then check the paint. The hi- tech solution is to use infrared measurements. TI and Melexis have some nice sensors that should suit. Being non- contact you can measure the disc temperature. There's also thermocouples.

For the low level signals, you'd want your electronics close to the sensor. Being automotive, you'd probably use CAN rather than RS232

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Given that the discs themselves can run up to red-heat, the only practical way of measuring *their* temperature is via infra-red radiation. Complex, messy, and probably not what you need to measure.

But the rest of the system runs much cooler (apart from the surface of the pad, which isn't accessible anyway) and I suspect you're more concerned about heat soak into the callipers and brake fluid.

A standard silicon diode drops its junction voltage by about 2mv/degree C and is cheap, easy to implement, and repeatable. It works up to at least 150C (I've tested that) which is DOT4 boiling point and I suspect way beyond that (use glass packages!). A DC gain stage mounted close to the calliper but far away to avoid the worst of the heat will improve noise, if required.

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OP wrote:
Reason is that I have some serious trouble with my brakes

Engage brakes firmly, then release them and run the car on a flat road for a while (50km/h, a minute or so) without using brakes (use only gear reduction or up-hill and ignition-off to stop) and if their callipers are above ambient temperature then you have a problem.

Car brakes have tremendous power. They are able to stop a 1500kg vehicle from 100km/h on a distance of under 50m (deceleration of ~7,7[m*s^-2]). That gives 320kW and they are designed to be treated that way. But they will not be able to withstand much lower, but continuous power so be careful with that.

Mountainous Norway - not a friendly place for cars with brakes with serious trouble.

No RSTDISBL, no fun!

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A normal car can retard at 0.7G max or so. A Bugatti Veyron manages 1.6 or so with its air flap.

F1 cars do 5G or so. But they already have 1G of just aerodynamic drag :)

So, first time F1 drivers are not that impressed by the acceleration, cornering speed, but are by the extreme amounts of braking force :)

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Sounds like an interesting project.

You might start with something like: Spark Fun MLX90614 IR Thermometer . This board includes a Mega328. Or start with just the sensor and a Tiny at each wheel.

Kartman mentioned can bus. I've never used can bus, but its yet another technology on my list of items to learn more about... I've used RS-232 to route signals around vehicles without difficulty, but I tend to use slow baud rates and shield cables.

This sensor is designed for -40'C to 125'C, which is obviously not centered on your range of interest, but would be a starting point for investigation. Melexis might have others in the line up more suited to your anticipated temp range.

Obviously a lot would depend upon exactly what part of the brake system you were actually aiming at.

Remember one of the harshest factors involved in vehicle projects is the vibration, especially when you are dealing directly with the wheel assemblies, and not inside the vehicle's interior where the suspension system has already smoothed things out.

You've probably read some of the other Threads discussing vehicle based projects. Vehicle electrical systems are very noisy, with bi-polar spikes and some significant "high voltage" spikes. Google "Load Dump" for further info on this. The point being, as you begin to experiment be sure to have a very well protected power supply or you will risk damaging your electronics.

JC

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Thanks for all the tips and suggestions :)

The disk temperature is not really useful as mentioned, but the calliper is. I know for sure that I have passed 270C (Dry boiling point of DOT5.1 fluid) and the fluid was as fresh as it could be.

I have found one sensor rated to 300C, which should be enough, https://www1.elfa.se/data1/wwwroot/assets/datasheets/07308414.pdf I guess using constant current and just reading the voltages should be good enough. That way I could get away with two wires for each corner too. Thoughts?

Should probably have mentioned that it was on track, on road I have no problems with normal driving or doing an emergency stop.

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JC: Yeah I have been looking on those IR sensors, but as mentioned, how to keep them clean? I found one of the melexis sensors rated to 380C, and that leaves plenty of headroom at least, pluss no contact so could just mount it with screws instead of glue.

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That looks like a good option.

I'd put a micro near the sensor, as suggested above.

Please post a photo of how you mechanically attach the sensors, as I think that will be an interesting part of this project.

Select your cable harness, and the connections to the PCB, very carefully. The vibration and repetative flexing can destoy the wires and connections very quickly.

JC

Cross post...

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So basically you're interested in the fluid temperature inside the caliper. I'd think the only way is to drill a hole and insert a thermocouple in there so it sits directly in the fluid.

Good luck sealing it though :)

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Better start making the design then, i`ll probably post it here for reviews later on, and feel free to give more of your thoughts.

jayjay: hehe, good idea ^^ Would have been a cool project to actually get measurements inside the calliper without drilling holes though, guess power would be the main issue.

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I wonder if you could use high-temperature epoxy and a tapered hole (narrower on the outside!) in a plug with a thermocouple through the middle, to replace the bleed valve? I wouldn't want to use it in anger anywhere that didn't have some good run-off areas...

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Could be possible, but i`m not gonna be the crash test dummy on that one :)

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There are plenty of 150bar (working) pressure rated thermocouple fittings from people like Parker etc. You can easily measure the fluid temp directly inside the caliper this way, we do it all the time. BUT, you would be liable in a big way if your modification to the brake system resulted in a failure and accident! (Out test mules run on stricktly controlled test facilities without general public access.......)

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DocJC wrote:
You've probably read some of the other Threads discussing vehicle based projects. Vehicle electrical systems are very noisy, with bi-polar spikes and some significant "high voltage" spikes. Google "Load Dump" for further info on this. The point being, as you begin to experiment be sure to have a very well protected power supply or you will risk damaging your electronics.
How to select power line polarity protection diodes (for automotive) by Soo Man Kim (Vishay).

"Dare to be naïve." - Buckminster Fuller