An incandescently boring 2 minute video showing the high accuracy tachometer I made for my low accuracy milling machine.
Tedious details for nerds:
Uses a Hall effect sensor (latching type) to generate a square wave signal from a magnet attached to the milling machine’s draw bar. The sensor is taped to the dust cap that covers the drawbar and thus does not interfere with changing the tool holders or chuck.
The brains of the unit is a Parallax P8X32A multi-core microcontroller (aka "Propeller"). Electronics enthusiastss might think that this is overkill for a tacho and I’d agree, but I do still plan to add more features to this, and using a Propeller chip will allow these to be implemented without compromising on pin count or timing precision.
There are a couple of minor glitches in the firmware for this device. You may notice that once the RPM drops belo a certain value, the last known speed tends to remain in memory and get displayed later on momentarily once the speed picks up again.
Currently the accuracy of the RPM measurement is never more than 0.5 RPM away from the true value. This is true over the range of 60 RPM up to approximately 1,000,000 RPM as far as I can tell.
Obviously the 4-digit display is only useful up to 9999 RPM. My milling machine will not go any faster than 1975 RPM when it is free-running with no tool in the taper. I am using the value "9999" to indicate that the unit is unable to establish the true speed.
It is *NEVER* safe for the LED display to say that the machine is running at 0000 RPM. This is a safety measure.
The RPM readout is never more than 1 revolution out of date – this is due to the algorithm used – it counts the number of CPU clock cycles between positive going edges on the HE sensor. There is a 1 second timeout on the half-cycle of the square wave. SO if in doubt, the readout always defaults to 9999 – which is the failsafe error value.
There will be more details about this build in the next few weeks. I plan to add X and Y axis position indication as the next phase of the project.
Hope you found this somewhat interesting. 🙂