RPM BOX functions??? June 27, 2023 06:15AM
Can anyone confirm how the rpm boxes actually work? Does it actually count the number of times that the magnet crosses the sensor or is it reading the voltage generated by the magnet crossing the sensor and translating that into rpm? The ISSPro part is only 3 wires, is does not have a power input. Is there anyone "in the know"?
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Re: RPM BOX functions??? June 27, 2023 06:25AM
It counts the number pulses per minute. 1 pulse equals 1 revolution.
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Re: RPM BOX functions??? June 27, 2023 10:24AM
That was my original thought but that ISSPro sensor that is the standard is a 2 wire pick up and when asking ISSPro engineering questions, it was said that the pick up does not send a signal, it acts like an alternator and generates "voltage" when the magnet passes across it. So, do we know at what voltage the "boxes" consider a signal? That was the path that I was going down until I took into consderation the "rumors" of the competitors that were found to have resistors in their pickups that caused false/slower readings. Which lead me to think that perhaps the "programming" was measuring the voltage and in a sense calibrated to say "4volts produced equals 2750 rpm" Which is basically how all the pressure sensors work in the data loggers. Can/would any of the "box" manufactures be able to verify either one? Have experienced some "questionable" set ups and looking for ways to write better rules to prevent rule infractions.
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Re: RPM BOX functions??? June 27, 2023 11:59AM
Remember that old Sears Craftsman riding lawn mower you used to work on with a Briggs and Stratton engine? It had a big magnet on the flywheel and whenever it passed by the coil it would create a voltage. Damn, I can still feel it. That is exactly what the ISSPRO R8906 Tach Pickup is doing.... creating a voltage every time the magnet passes by the coil. You can actually see this voltage using a voltmeter if you are so inclined. The reading will be zero until the magnet passes by the coil and then there will be a sudden reading (a spike) and then it will go back to zero.

Now, how does it go from a voltage spike to the reading on your tach? Well the little microchip in your gauge, or ECU, whatever little black box you have is programmed to do two things:

A) the MP sets a trigger threshold that says, "Hey voltage spike, you need to be this tall before I will recognize you as a legitimate spike". So, if you set the threshold at 1 volt then every spike larger than 1 will count and every spike not as tall as 1 will not. The reason for doing this is to filter out the baby noise spikes that are floating around things like starters or water injections pumps. Google "voltage triggers" and you will see there's a whole lot of science behind obtaining, conditioning, and counting them.

Cool the MP secondly counts how fast these legitimate spikes are happening every second and then makes a conversion to RPM based on how many magnets you have.
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Re: RPM BOX functions??? June 27, 2023 12:56PM
Regarding your concern about "questionable setups".... there might be some that are adding a resistor in order to lower the voltage spikes. This can be a useful tool for filtering out noise. The problem though that if you do it too much it will cause all the spikes to drop below the threshold and then you have no reading. Some clubs have a rule that having a Zero reading at the sled box will get you DQ'd the same as having a higher reading.

The other thing you need to watch out for is that it is possible to design and build a box that will read a frequency (how fast the spikes are coming) reduce it by a factor/percentage and send out a replacement signal with a lower or higher frequency. There are some of these on the market currently for altering a vehicles tach or speedo when a different engine is installed. This is why some clubs say that there must be a a continuous uninterrupted wire from the sensor to the receptacle on the back of the vehicle.

If it were me, I'd add the some verbiage saying something like, "Signal from tach pickup sensor shall not be conditioned or modified"
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Re: RPM BOX functions??? June 27, 2023 02:52PM
Limited RPM You are chasing ghosts. The box DOES NOT convert a given voltage to a rpm. It simply counts the number of pulses, Yes Electrical pulses, per minute and then spits out the given rpm. The pulses can be any range of voltages depending on how strong or close the magnet is to the pickup coil. The box can handle a range of "Voltages" But does not convert the "Voltage " to rpms.
As for putting a resistor in the line will only decrease the signal level potentially below the low threshold of the box to read. I know what you are thinking, that you may want to replace the pickup with a variable voltage source which will do nothing but give a 0 rpm reading regardless of voltage, not to mention if you hit the box with too high of a voltage you might damage the box. I recommend using the magnet and coil as it was intended to be used and life is good. The box will read accurate rpms. Problem solved.
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Re: RPM BOX functions??? June 27, 2023 07:56PM
First of all, MLP and FYI, your explanations are brilliant. As I read the posts above yours I started immediately to think about how I would try to explain what's going on there, but then I read your contributions and they're just exactly what I wanted to write.

Maybe something to add, as for an general overview. There are 3 different common principles for tacho or speedo applications in barnstorm motorsports around. Either it is hall effect base, requiring a magnet on the object to look at. Or it is either a VR (variable reluctance) or inductive sensor, which only require distinctive metal surface differences (e.g. toothed wheel, pins or bolts etc).
As already stated above, most of these sensor need particular signal conditioning to make it readable for the downpath logic / mircrocontroller. In the case of of inductive sensors, this effort seem the lowest hanging fruit, it's quite simple and reliable to get a suitable squarewave shaped signal, what can be dialed in to the interrupt pin of a microcontroller, starting its ISR for processing it into a RPM value.

Hall sensors sometimes need a bit more attention for signal conditioning, and VR sensors even more, because the original output of them differ the most from a typical squarewave shape. However, it is suitable anyway, and used in automotive industry for decades. The only things you've to consider, if are doing it with a manually created sensoric chain, is to take care of your noisy low volt values, and the destructive high volt peaks. For the low volts, you can use typical filtering methods, for the high volts it seems worth to try a Zener diode to limit that onto a harmless level.
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