Hi hhumble,
Sensing the speed of a turbocharger could be a very difficult task. My main concern is that the high temperatures involved could fry any sensor in a short period of time. From my limited experience in working with speed pickups used in diesel engines (engine RPM that is), I can offer some possible suggestions.
Do not interface your speed pickup sensor directly at the turbine wheel where the maximum temperatures will be seen. There is no need to count the number of blades on the turbine wheel. On diesel engines, a magnetic pickup (“mag-pickup for short) is mounted on the bell housing of the starter gear and the sensor “counts” the teeth on the flywheel as the engine spins. I think a similar approach would work for your application. Instead of counting blades, you could mount your mag-pickup at the coolest part of the turbo shaft (assuming the temperatures there will not melt the sensor) and trigger the sensor off some single part of the shaft (keyway or whatever). Now you have one digital pulse from the sensor per revolution (MUCH easier, no need to beat yourself up). I have worked mainly on the software side of these applications, but I know the Electrical Engineers feed the pulse signal from the mag-pickup into an electrical hardware circuit called a phase-locked-loop (PLL). The PLL processes the signal and increments a dedicated counter once for every pulse from the mag-pickup. This frees up the ECU or microcontroller from operating in a dedicated mode where it’s trying to run code to count input signals, convert the count to RPM and do something with the value obtained, all while performing the other tasks required to run the application. By setting up the microcontroller to trigger on a DEDICATED interrupt schedule (e.g. every 20 milliseconds the microcontroller stops whatever it’s doing and runs this routine) you can read the elapsed counter value at the start of the interrupt, and since you know the micro is reading this counter EVERY 20 milliseconds (i.e. a standard timebase), you can easily use a simple lookup table to convert counts to RPM. This may be the only practical approach in acquiring pulses at the speeds you’re looking at. My best suggestion would be to find an Electrical person with some experience in PLL’s and see if you could get some assistance with a prototype circuit. What you are trying to do has been done before in other applications (they do measure the shaft speed of turbine engines), but I’m not sure the extreme temperatures will cooperate in your application. Best of luck!
hole right through the whole housing, crossing through the outer edge of the compressor inducer. A light source in one end, and a receiver in the other will count the light blocked and passed by the passing blades, and will give higher contrast for the sensor.