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Forum Index : Microcontroller and PC projects : CNC Speed Monitor
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OA47 Guru  Joined: 11/04/2012 Location: AustraliaPosts: 926 |
I would like to monitor the speed of the spindle in my CNC router. It has a motor capable of 20,000 rpm and I was wondering if any shedders could offer assistance with this project. I am considering using a mite with a display attached and contemplating either using IR transceiver with a reflector on the 52mm barrel fan at the top of the spindle or using the audio generated by the spindle. Any thoughts? 0A47 |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 858 |
Magnet + hall-effect? Bit more robust than an optical solution. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4233 |
Hall sensors are good ! If you place a (even small, such as 3mm) magnet you have to re-balance the spindle (definitely at 20000 rpm). Put 2 magnets at opposite sides, as good as you can balanced, use epoxy glue). Reflective optical sensor. You can mark half of the fan blades white, the other half black. A reflective sensor will distinguis between white and black. But it is a spindle of a CNC, so when dus settles in I would go for the hall sensor.. Volhout PicomiteVGA PETSCII ROBOTS |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6787 |
At that speed, personally, I'd go for optical - but only if you have access to the back of the shaft where you can enclose the sensor. Just paint half the shaft matt black. An approach that might be worth a try is to wind an air cored coil and stick it to one side of the motor. You *might* be able to pick up the field winding. Try it with a scope. You couldn't use the signal directly, it would have to be amplified, shaped and possibly divided down to something sensible. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 858 |
A more elegant solution |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6787 |
At 20,000 RPM you don't really want to be attaching anything to the shaft at all. Rebalancing the system is no easy job. If it's not perfectly balanced then, as a minimum, you are going to make the cut wider as the motor body oscillates. Ideally, if it's some sort of digital motor, you'd extract the speed signal from the motor electronics. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 858 |
It's a cutting tool, not a turbine. They're designed for radial loading. |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
At that speed attach nothing to the shaft, the vibration will be excessive. An IR remote control receiver and LED modulated at 38kHz make an excellent tacho as the receiver is designed to tolerate a lot of ambient light. One I made uses a NE555 for modulation and plugs on to a handheld scope. The frequency readout gives the speed and the trace shows the signal quality. Edited 2024-02-05 21:33 by phil99 |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6787 |
That's true, but any imbalance in the motor shaft will also cause imbalance in the tool - and, in fact, in the whole motor. If you are lucky it won't break the motor mountings or damage the X-Y axis bearings. I've seen the inside of a control panel mounted on a wall of a wind tunnel. You don't feel the vibration in that area but it unscrewed self-tapping screws and some components were only in position because at least some of their wires were still attached. You don't mess with vibration. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4233 |
That is actually a good idea. The IR sensors have build in AGC (automatic gain control) so they might even work fine as reflective sensors. On a half black/half white painted shaft Volhout PicomiteVGA PETSCII ROBOTS |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 858 |
This is concentric with the shaft. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6787 |
Oh, it's elegant ok. If it happens to fit the shaft and you are capable of re-balancing the shaft if needed, then yes. On the end of a 1500RPM mains synchronous motor with several kg of rotor om a variable frequency speed controller I wouldn't argue at all. On a little DC motor driving a gearbox and some wheels it's fine. Even at 3000RPM it would probably be fine. 20,000RPM is a different kettle of fish, especially when it has a precision cutting blade on the other end and you have to keep that blade cutting in the right place and to the right depth. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2914 |
Hi All, This is something I have been wondering about as well. I only have a cheapo 3018 (standard NOT pro) and there is no way I would want to attach anything to the motor. Ideas I have had before I saw this conversation were either of these. As the shaft is bright silver I could paint a black line on the side and use a photo transistor to read when it spun around and feed this into a ’mite pin and use the COUNT input. Or, if that didn’t work. Create a variable strobe flash that I could tune till the black line stayed ‘static’ then I would know the speed because I know the flash rate of my strobe (probably bright white LED). Interested in others views on this. Regards, Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 926 |
Thanks for the discussion, has anyone had any thoughts on using the audio from the spindle? 0A47 |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
The pitch will be proportional to the speed but mechanical resonances from the motor and supporting structure may cause issues at some speeds. Edit. Calibration may require another tacho method. Why build two? Big Mick, I tried a white LED and photo transistor but it was very fussy about ambient light and needed to be quite close to the shaft to reliably read the black stripe. White worked better as the shiny shaft reflects most of the light away from the sensor. The IR optical method (earlier post) works much better with a stripe of any colour. Best results are with white masking tape right round the shaft then colour one half with black marker pen. Clean the surface with solvent first to help the tape stay stuck at high speed. Edit. Or maybe instead of tape paint the shaft matte white first. Edited 2024-02-06 08:31 by phil99 |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 926 |
My biggest worry with an optical sensor is the cloud of dust that is produced when milling wood etc. I would at least have to detect the speed from the furtherest point away from the chuck which is a 52mm chromed plastic barrel fan. I would have to be careful not to put it out of balance. 0A47 |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 926 |
It seems that it is very hard to keep a piece of vinyl tape stuck on an object spinning at 20K rpm.  0A47 |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
Yes, also found vinyl tape useless. Not tried at 20k but masking tape sticks much better. Better still if the shaft is first wiped with a solvent such as metho. |
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| OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 926 |
I used isopropyl alcohol and I did need a dark colour as the fan is chrome plated. 0A47 |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6787 |
Can you see the spinning boss of the fan? That would be a good place, Or, depending on the number of fan blades, paint a mat black stripe on opposite blades or something. A simple reflective sensor might be fine, just looking at the blades. Use a reflective 38kHz modulated IR type and divide the pulse rate by the number of blades. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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