Autosport Labs

Hi all,

Here's one of my plans for sensors for the Race Capture. For those who don't know, a tire pyrometer array is a set of infrared thermometers pointed at the tire tread, you may have seen the commercial systems that mount to a car's wheel wells, or the little sensor bars over the tires on some FSAE cars. This lets you log temperatures across the tire tread *as you drive,* an extremely powerful suspension tuning tool.

For my particular system the parts I'm looking at using are:

x12 MLX90614 infrared thermometer - 3 per tire, you can do more if you can afford it, more is better.
x2 CP2120 SPI to I2C bridge - for connecting the thermometers to the sensor expansion port. Again assuming 16 or less sensors here.

I need 4 infrared thermometers with temperature ranges suitable for the brakes, so far I've only found a variant of the MLX90614 that goes up to 125C which for brake discs, is just getting warmed up.

I am definitely interested in this... this would be huge for tuning suspension setup!

robert

The only IR pyrometer head I can find that reads in roughly the right temp range (that isn't hellishly expensive) is this one from Omega:
http://www.omega.com/ppt/pptsc.asp?ref=OS136&Nav=temj09

It's still nearly $200 though, and only reads to 1000F, which might be slightly low for a heavy race car on a brake-heavy track.

You'll definitely be able to get some useful data from measuring caliper temp (which doesn't need to be done with a non-contact sensor, of course), although that will be more useful for longer-term trends. For instance, how long does it take for the brakes to reach steady-state temp on various tracks, am i likely to boil the fluid, etc. It won't tell you about peak rotor-pad interface temp, which is more critical for determining shorter-term characteristics like where you are on the friction curve of the pads, and how much margin you have before you start to get brake fade.

I was just checking out the MLX90614's page on Sparkfun and it seems the number I was looking at was the sensor operating temperature, and the sensing temperature is actually up to 382C:

https://www.sparkfun.com/products/9570

Datasheet:

http://www.sparkfun.com/datasheets/Sens ... 14M005.pdf


So the MLX90614 should work for the brake discs as well.

BTW the cheapest place I've found them for sale is Future Electronics (copy link below)

This is a great idea. If you can do it as a CAN bus, where each corner of the car is a CAN bus node, that will make it easier to get the data to reliably propagate around that car, eventually landing at race capture's CAN node (up coming).

Let's keep this dialog rolling! Are you comfortable doing PCB design? I (we) would love to help do any design work on this thing.

I've never done any PCB design before but I don't think much will be needed here, just some to mount the thermometer sets to and maybe a breakout board for the bridges, really just for neatness and convenience. The thermometers have PWM and SMBus signalling so something to interface one of those to CAN would be needed.

Thought I'd update, I'm buying parts now and it looks like I'm going to go with the MLX90614ESF-ACC-000-TU sensors, the operating temperature goes up to 85C rather than the safer 125C, but none of the gradient-compensated ones rated for automotive use do. A Recom R-535.0PA with a 2.6kohm adjustment resistor installed can power the sensors.

Also only one SPI to I2C bridge is needed, I'll probably go with an SC18IS600 since it's available from the same place. A Murata 7803SR-C can put out 3.3v power from 12v input for this.

'nother update: I'm now trying to decide whether to use a custom PCB printing service or make the boards myself - I don't have any of the equipment needed currently. I could probably find some friends who would be able to do it, so something in between may be an option. But I haven't started the circuit diagram yet so I'm getting ahead of myself a bit I admit this is my biggest electronics project yet, and one of few. I hope my software experience will carry over to the hardware world.

I'll make one board for each trio of tire thermometers, the brake disc thermometer will wire into this board. I'd like to find a way to make this assembly sealable to protect it when not at the track, maybe some kind of tube that screws on over the tire sensors and a bottlecap over the brake sensor. Another option is attaching each sensor group to an easily detachable assembly and using an "umbilical connector" for each one that could be easily sealed.

Next there will be one board for the "support equipment" - SPI to I2C bridge and power supplies. This board will accept 12V power and the expansion port SPI interface on one side, and wires from the 4 sensor groups on the other.

Decided I'm going to lose the infrared brake temp sensors on my setup for 3 reasons:

1. Cost: Those gradient-compensated infrared thermometers aren't cheap.

2. Usefulness: Brake temps aren't terribly useful for me to log. It would end up being one of those neat things I get bored with quickly. If my brakes are getting too hot that's something I can tell while driving, and unlike tires, live brake temps aren't a whole lot more useful than pointing a temp gun at the rotors when the car comes in. My car is light and low-powered and will soon have a brake vent system, brake temps aren't a big deal on this.

3. Durability: The brake temp sensor would be exposed to much more heat and dust than the tire sensors, and if I go with some kind of protective cover system, would be a massive PITA to cover and uncover, being right next to a searing hot brake rotor.

I may still go with contact sensors on the calipers if I can find something sealed that will stand up to the environment - those would be cheap, no trouble and provide me with some entertainment

Had a clever idea today, instead of fitting a row of sensors like everybody else, why not have one that can quickly sweep across the tire, giving the resolution of a very large number of sensors? I'm thinking about it.

Here are some parts I could use:

4x OpenServo board (adds I2C control and position sensing, among other things, to a standard-size servo)
4x Hitec HS-425B servo (cheap standard-size high-speed servo, faster is better)
^You could just go with 4x OpenServo v3 instead of the separate servo and board, but they're all sold out
1x Dimension Engineering SportBEC RC power supply (convenient 6v power source that can handle the load)

Just worked out that the distances involved make i2c communication from the sensors to the Race Capture box impossible, I'm now looking at using 4x NXP P87C591 to translate between I2C and CANbus, and the upcoming CANbus communication module.

Ran into another problem, the NXP P87C591 is one-time-programmable, the reflashable variant is the P83C591 and it seems to be out of production. But I did find the STM32F105VCT6 ARM-based microcontroller which looks like it should be able to do the job.

You know what you could do that could be much simpler, is to use one of the 4 Race Capture PWM outputs to directly control the servo motors. Or use each of the 4 PWM outputs to individually control 4 servo motors. That way, you won't need to build as much external circuitry!

Good idea, I didn't want to tie up too many of the built-in PWM outputs but splitting one to drive all four servos simultaneously would be worth it.

I might still go with the fixed multiple sensors though, the servo idea wouldn't save much money and it would take up more room above the tire. The advantage is greatly improved resolution.

@Gameboy, any updates on this?