The RPM monitoring computer

Since I am having an electronic control system built for my engines, I though it would be a good idea to also have an RPM monitor built for the test stand I use to get things working properly. I contacted Russ over at Bad Brothers Racing to see what he could come up with that wouldn't cost me an arm and a leg to get done.

Using parts I already had was the idea to help keep the costs down on the sensor. Since I had a Basic Stamp Homework Board from Parallax, I figured this would be a good starting point. I also purchased a small LCD screen from Ebay, and a fiber optic sensor unit. What I shipped off to Russ was parts. What returned was a fully assembled and working RPM unit which is capable of reading speeds up to 120 thousand RPM and is fully re-programmable

Click here to see what Russ has been doing at Bad Brothers Racing

Custom built for ME!!!

As you can see here on the screen, the unit was custom built for me! It is always nice to have a bit of personalization like this. The programming inside the unit causes it to come up and display the start up screens with the above info, as well as what speed the unit is currently configured to detect.

Max RPM setting can be programmed for any engine

By knowing the max RPM the engine should run at, the program can display the engines speed in RPM, and percent of maximum operation. Since this would change and be different on engines with max speeds of say 80 thousand RPM and 120 thousand RPM, the program can be updated easily. This allows me to use the computer with any of my engines by simply hooking the unit up to my laptop and updating the software with a serial cable. Very cool stuff!!! Additional features can be added later too using this updatable programming.

Display during operation

The display shows the actual RPM in the upper left of the screen, and the percent of total safe engine operating speed in the upper right side. The bottom has a bar graph that extends from left to right as the speed increases. No bar, or one line on the left is 0%, while a full bar from left to right is 100%. This is handy as it allows me to get a visual indication of the RPM with just a quick glance even from a distance.

Fiber optic sensor

To get the RPM of the engine, a fiber optic sensor is used. I like using a fiber optic sensor as it doesn't require any contact with the engine, and has no moving parts. While you can make your own optical sensor with LEDs and photo transistors, the fiber optic sensor is all ready to go and you can find them pretty cheap on Ebay. I can also mount the sensor away from the engine, as the light travels through the tiny fibers to get near the compressor nut on the turbo. If something happens and the fibers do touch the compressor wheel, they can be re-cut and used again. If a sensor touched the compressor wheel it would most likely be damaged beyond repair!

Testing the computer

Here is the test rig I used to test the sensor. I mounted the fiber optic cable in a piece of tubing that goes through the side wall of the compressor housing and aims the fibers at the nut which holds on the compressor wheel. This should keep the cables in place with even the strong air currents passing by them. The fibers shine on the compressor nut, which has been painted half black and half white.

Close up of fibers

When the light from the fiber hits the white part of the compressor nut, the light is reflected back and the computer gets an on signal. When the light hits the black part of the nut it is not reflected back and the computer gets an off signal. This gives one on and one off signal for each revolution of the compressor wheel. By counting how many revolutions there are in one second, and them multiplying by 60, you get the RPM!

Screen in operation

The next few screens show the units output with the turbo being spun up by the shop vac. The output was slightly off as the fiber was receiving a "ghost signal" from a part of the compressor nut that should have been black. This part of the nut was allowing just enough of a reflection to make the unit have two on and off cycles per revolution, and the speed was reported as double of what it actually was. I fixed the problem and realigned the fibers and the problem went away.

Screen in operation

A few things about the screen. You can see the bar at the bottom start to go across from left to right as a percentage of the operating speed. The numbers look a bit fuzzy though. This is because the screen updates so quickly that the camera catches them in mid-update and you don't get all the letters at once! When you look at it with your eye though, the screen output is nice and clear and very easy to read.

Fiber sensor mount

I decided to change the mounting on my fiber sensor. I am now using a compression fitting that is drilled and tapped into the compressor inlet. This holds the copper pipe the fibers are mounted in much better than the original mounting method I was using.

Compression fitting

Normally, compression fittings do not let the tubing pass all the way through them. To make it work, I had to drill all the way through the fitting, but it is pretty easy since the material is soft brass. You can see here how the fitting threads right into the side of the compressor inlet.

Better focus

The focus on the compressor nut is better and the fibers are stable. If they move the least, it can cause false readings on the RPM sensor, so this is definitely the way to go.

Now that I have a working RPM sensor I can keep a close eye on the operation of my engines while they are on the test stand. I will continue to use analog gauges on the test stand for things like oil pressure and fuel pressure. Getting all of these readings puts me one step closer to having enough data to get the computer for the jet kart programmed.

Thanks for stopping by, and be sure to keep checking in for more updates!

Gary Richards

Return to Main Page