Measuring belt tension, automated.

[ChrisA]

Reading this forum and on Discord there seems to be a debate about how to measure belt tension. Many people don't like the method of plucking a belt and listening to the tone it makes. They say it is "inaccurate".

So, I looked at some engineering texts from the belt and pulley manufacturers. They give two methods for measuring belt tension:

1. Deflection. This is where you pull the belt until it is displaced a certain amount and measure the force used to pull. Then you know the force on one side of a triangle and you can solve for the fore on the other two sides. and
2. The "sonic method". where you pluck the belt and measure the frequency of vibrations.

Surprisingly they claim and sonic method is more accurate and more repeatable. The reason is that it is very hard (they say) to accurately measure belt deflection because the deflection is very small and measurement errors up to 20% are easy to make.

They sell instruments for measuring belt vibrational frequency that have input screens where you enter the belt span length and the kind of belt and the instrument has a microphone that you place very close to the belt. it does the math and reads out in tension

The "Voron Spec" for belt tension seems to be exactly the same as the A2 note on a piano or the open A string on a guitar. It seems a guitar tuner would be a handy thing to use.

One more idea. It would not be hard to build a sonic tension measurement instrument into the printer. The printer already has a Raspberry Pi and the Pi has a very good stereo audio input. If you would epoxy a small microphone to a wooden clothspin then clip the mic close to a belt and pluck it, software on the Pic could display belt tension. The hardware cost is very low. All the work is in the software.

Here is a "belt tension measurement device" already in the correct "voron standard" colors for $13. https://www.amazon.com/Korg-Contact-Microphone-Tuners-Black/dp/B07DZYCXNP. It is a "contact microphone" that picks up "sound" in solid objects.

DOn't expect me to have this working anytime soon. Maybe someone will beat me too it.

One more idea. Rather than a clamp. You could embed a microphone into the left and right belt tensioners and then lead these to the left and right audio channels. Rather than plucking, just pulse a stepper motor to excite the belt.

 

[Vorkosigan]

For me, I found that I didn't feel I could pluck consistently enough to be accurate, not so much that the idea of it was inaccurate, if that makes sense.

One more idea. Rather than a clamp. You could embed a microphone into the left and right belt tensioners and then lead these to the left and right audio channels. Rather than plucking, just pulse a stepper motor to excite the belt.

I would probably be interested in something like this.

 

[ChrisA]

For me, I found that I didn't feel I could pluck consistently enough to be accurate, not so much that the idea of it was inaccurate, if that makes sense.



I would probably be interested in something like this.

How you pluck the string does not influence the frequency of vibration. try it on a guitar. Playing louder does not influence the pitch. It is still an A if the instrument is in tune. You don't need a consistent touch. Yes it sounds maybe different if it is louder and there are overtones that might be different but the fundamental tone is the same.

I don't have the means to experiment now. The first step is to find a place to mount a microphone. that will pick up belt vibrations well. Then capture some data with different belt tensions.

The trouble with microphone held some distance away is they pick up ambient noise. You want to be just a couple centimeters from the string at most.

 

[NoGuru]

I like your ideas!

I built this little device and it works great. https://github.com/Diyshift/3D-Printer/tree/main/GT2 Belt Tension Meter
So far this has been the best method for me but like you mention, frequency is probably the best method.

 

[MagicToolbox]

I tried the 'pluck the belt and listen' method. I can't tell the difference between an 'A2' note and an 'H' note. I tried the app on the phone, there is a lot of ambient noise on the printer, and I couldn't continuously pluck the belt to really see where the peak was.

I printed out the tensiometer linked above and had all of my belts reading consistent values in just a few minutes.

 

[Dave32]

If you would epoxy a small microphone to a wooden clothspin then clip the mic close to a belt and pluck it

I bet with some software tweaks it could be done with the input shaper accelerometers that many Voron users already have. (ADXL345, etc)