GR-1 Turbojet Project 3/6/04




	Posted on March 6, 2004 After getting the GR-1 in good running order, I set out to install a tachometer to see what kind of RPM the GR-1 is spooling. I had researched what it would take to install a tachometer and found that a high speed photo optic sensor can be used to detect the revolutions of the turbine shaft. By using a frequency counter, the revolutions can be converted into Hertz which are multiplied by 60 to display the true RPM.

I first had to find a photo optic sensor that used fiber optic sensor cables. I wanted to install the fiber optic cables into a steel tube that would be pointed at the compressor rotor nut. By painting the rotor nut half black and half white, the sensor can detect the change in reflectivity. After searching the internet for sensors, I found an Omron fiber optic sensor on eBay for $30. The sensor came with the fiber optic cables and was rated for 200 microsecond response time. The fiber optic sensor works with two fiber optic cables; one emitter and one receiver. The emitter sends certain light frequency down the fiber optic cable and is detected by the receiver cable. The cables can be used to detect an obstruction between two sensor cables or used as a reflective system.
The reflective system shines light on a object and if it reflects the light into the receiver cable it will indicate at the sensor. Some sensors use a coaxial fiber optic that has both emitter and receiver fibers in one cable. This type of sensor would of been perfect for my application but I could not find an affordable version of this sensor. I will make the Omron sensor work by installing both emitter and receiver cables into a steel tube pointed at the compressor nut. As the nut turns, the sensor will send a signal pulse for every rotation as the white paint reflects the sensors emitter light.




		The Omron E3X-A21 (N) 2m was a great find! It runs on 10-30VDC and can sense up to 5000 cycles per second.




		The inducer ring is drilled for the steel tubing to be welded in place.


	I welded 1/4” steel brake line into the side of the inducer ring and shaped the tubing to point at the rotor nut. I checked the clearances to allow for 3/16” away from the rotor nut. I intend to glue my fiber optic cables 1/16” past the end of the steel tube.




		The finished inducer ring with steel tubing welded in place and the sensor mounting bracket complete.





		The Omron sensor has great adjustability and is very user friendly.




			Model aircraft paint was used to color the compressor nut .

	I glued the fiber optic sensor cables into the steel tubing and test fitted the inducer ring onto the turbocharger. After adjusting the steel tubing a bit, the sensor was mounted onto the inducer ring. I wanted there to be the least amount of cable used as it is sensitive to bending and can be easily damaged. I protected the sensor cables outside of the inducer ring with a support wire and heat shrink tubing.



			The sensor tube is angled to point at the compressor nut


			You can see the emitter light reflecting off of the white paint on the compressor nut.



			The completed photo optic tachometer sensor assembly.



		The Omron sensor was configured to send a 12 volt pulse to a frequency counter every time the sensor is triggered by the white paint on the compressor nut. I use my multimeter/frequency counter to count the frequency in Hertz generated by the Omron sensor’s output. The value in Hertz can be multiplied by 60 to get RPM.

I now have a working tachometer to document my turbine RPM. I fired up the turbine to see what I was turning and found out that the lowest idle speed the GR-1 could sustain is about 30,000 RPM. I throttled up the GR-1 and had achieved 94,000 RPM at 13 PSI of combustor pressure. This information is very interesting to me as I now can adjust parameters and see its effects on the turbine. I have some more research to do with this new instrument and hope to graph the GR-1’s performance. I have got some research to do until then.

Till later,

Don Giandomenico

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