Mini Jet Engine Control - Measuring High Temp and Speed

[elec_mech]

Hi All,

A buddy is interested in some help making his own controls for a small jet engine. While the one he'll be using is normally for RC planes, he plans to put it on an RC boat. I'd like to ask if anyone's had any experience interfacing high temperature sensors and high speed rotary sensors. He told me he doesn't need fast measuring, roughly taking a measurement once a second should suffice, so I thought the PICAXE might work.

Here are the gating items as I see them at the moment. I'm waiting on more info from by buddy on specifics, but these were the things he mentioned that concerned me immediately.

High temperature sensor - up to 800 degrees C. A quick bit of research turned up a K thermocouple and a PICAXE link to reading a thermocouple here. So I might have lucked out here.

High speed rotary sensor - something capable of measuring up to 150000 (150k) RPM. In a perfect world, I think a sensor that could output either serial data or an analog voltage would be great so the PICAXE doesn't have to attempt to read 2500 pulses per second, however, I don't know if a) such a sensor exists or b) if the PICAXE is indeed capable of counting that many pulses in such a short time. Would a hall effect sensor with a PICAXE perform such a feat?

Any suggestions or words of wisdom are welcome.

 

[rossko57]

An alternative way of looking at 150,000RPM is as 2,500RPS ... a 2.5kHz signal if there were a once per rev trigger. You'd have to look at the specs of your intended hall effect to see if it could handle that. Is there any lump of metal you could actually read from though? Think I'd be looking for optical means.

An alternative way of measuring RPM is rather than count revs, time a single revolution. 0.4mS ... sounds like a job for PULSIN

EDIT a quick google suggests model turbine have ECU control and an existing RPM sensor. You might look to see how to get the info you need from the existing ECU

 

[PieM]

For K thermocouple, I use MAX31855 , with SPI output. Only 3 lines to read positive and cold-junction compensated temperature. You find it on breakout board Here

 

[elec_mech]

... a 2.5kHz signal if there were a once per rev trigger. You'd have to look at the specs of your intended hall effect to see if it could handle that.

An excellent point. I found a hall effect that had a max frequency of 10kHz, so I imagine 2.5kHz won't be hard to do. I was originally thinking 150kHz, before I realized 150k RPM translates to 2.5kHz as you mentioned. Whew.

I also came across the pulsin command - I think you're right, that looks more than capable. At 4MHz, it can read 10us - I need a max of 400us, so we should be good there. So you think an optical sensor would be a better way to go with the pulsin command? Would an (IR?) LED with a phototransistor be ideal or something else?

I did ask my buddy about what is used now - he's done some research and the existing ECUs, as I understand it, are sold with the rotation sensors, but there is no way to buy just the sensors. He's trying to get away from paying several hundred USD for an ECU and make one instead. I need to research this further myself.

For K thermocouple, I use MAX31855 , with SPI output. Only 3 lines to read positive and cold-junction compensated temperature. You find it on breakout board Here

But can a PICAXE read an SPI? I did somehow get a 20M2 to work with a MAX7221 which uses SPI, so I suppose I can make this work. I didn't realize there were thermocouple ICs with serial out, so I'll look for an I2C version as well which works a little easier with the PICAXE.

Thanks for the advice everyone! This just changed from a mountain to a molehill for me.

 

[Goeytex]

A Picaxe 20X2 can do hardware SPI (No bit banging like the 20M2). See HSPISETUP / HSPIOUT / HSPIN in Manual 2.

 

[PieM]

But can a PICAXE read an SPI?

No need of hardware SPI. Bit banging is very easy with MAX31855. I'll give you a program ...

 

[rossko57]

Commercial designs seem to use both opto and magnetic detectors. If you can organise a magnetic one to specs, they do hold the advantage of being much more immune to dirt, oil - or water spray; e.g its what they use in car engines, rather than optos.

I think what you pay for with commercial units is development costs; I wonder if developing your own control system from scratch will rapidly get expensive in written off turbines ?

Waterborne turbjets can be problematic, maybe research "Ekranoplan" machines for tips.