Tacho install on DP

I know that a number of members have installed these and other type of tachometers (especially on lathes) but haven't seen that many tachos on DPs, or haven't seen any posts that use the following implementation.

The tacho is an ebay cheapie that cost about $12 like this one.
https://www.ebay.com.au/itm/4-Digita...53.m2749.l2649
It uses a Hall Effect sensor in a stationary position held close to a rotating part of any machine on which a small permanent magnet is mounted.

I've had this tacho for a couple of years but was trying to work out a way of powering it (they require between 8 and 24V DC) without using a dedicated power adapter, when I was reading one of my VFD manuals and realized that the VFD itself had a 24V DC / 100 mA power out capability. A quick mock up showed that the Tacho and sensor drew about 25mA which means it could run direct from the VFD.

Then I needed to find an out of the way place for the sensor and magnet on my DP. Putting the magnet on a small diameter rotating part of a machine means magnet has limited contact and is more likely to come off, and on a DP there's very little exposed rotating shafts anyway.
On closer inspection I decided to have a crack at doing it by locating the magnet on the spindle pulley and the sensor mounted on the pulley/belt cover of the DP as shown in the photo below..

To locate the spot for the hole in the lid of the DP pulley/belt cover I turned down a suitably sized short stub of 10 mm mild steel rod and gave it a sharp point on the end. Then I stood the MS stub on the pulley rim with the pointy end up, closed the cover on top and tapped the cover with a mallet and it made a nice little dent on the inside of the cover.

H is the Hall effect sensor and M is the magnet
The magnet sticks pretty well by itself to the top of the pulley but I added some contact glue to provide a bit more grip.

Attachment 456709

The tacho unit itself (T) sits on top of the VFD box like this.
"S" shows the back of the sensor poking out fo the Pulley/belts cover.
Attachment 456710

Couple of things to watch out for;
The sensor only works if the magnet is facing in a specific direction.
The sensor and magnet need to be quite close to work - the LED on the back of the sensor will indicate when you are close enough.
The Tacho unit comes with a 5 way ribbon cable but only 3 wires are needed.

Most VFDs have a 24VDC/100mA output as standard. Almost all VFDs have a 10V output for digital switches but these are usually limited to 20mA so these tachos would not work on this output.

If anyone would like to try this its quite safe as you are only working with 24V.
However, you need to be careful when connecting up the sensor to the Tacho - if you put the +24V direct onto the sensor output it will let out a little "cruck" and short puff of smoke (:upset::club::weeping2:) and cook the sensor.
How do I know ? - Yes I did it!!!!
If this happens, don't stress, it's only a $3 mistake (replacement sensors on ebay cost $3 including shipping) so buy 2 while you are at it.

Timely bob

As per thread in mwf, I'm about to install the same item to a milling machine.

For some reason had decided the sensor needed to be horizontal. Complete block in the thought process. Maybe because I never close the belt cover. [emoji848]

I'll have to check the 24v from my vfd - I was about to dig out an old plug pack..


Russ

One thing I thought of last night was that to balance the the magnet will require a similar mass piece fo steel to be located in a diametrically opposite position on that pulley. I only tested the drill up to about ~500 rpm and could not detect any noticeable vibe but later this morning I will run the drill up above that and I bet it will vibrate unless the balancing mass is added. An alternative is to drill out a shallow hole and insert the magnet into the hole.

This morning I examined the vibe and there was definitely something happening at 500 rpm and even as low as 250 rpm.

To assess this quantitatively I employed my Vibe meter (Mobile phone with Vibe app) and checked the vibe on the DP table with the balanced and unbalanced pulley.
The vibe meter measures the x-y-z components of the vibe. The "z" component is the vertical which is much smaller than the other two so I just used the X-Y to get a vector sum to create a 1 single numerical value.
I should have measured the vibe without either the magnet or the counterweight but that would have involved ungluing the magnet which I really dod not want to do.

Anyway what it shows is that the balanced pulley has about 1/3rd of the X-Y vibration as the unbalanced pulley so its really worth doing. Some further improvement could be obtained because my counter weight was only about 90% of the weight of the magnet. I'll put it on the todo list.





Attachment 456760
One other thing I noticed was when weighing the magnet on a small set of digital scales was the mass of the magnet was ~3.5g, whereas the almost exact same dimensioned counterweight was ~1g.
Hang on I thought - I know that these REE magnets do contain some exotic elements but even so that would make the density of the magnet material about 27g/cc which is way more than the densest known material on earth.
It did not take long to determine that the magnet really disturbs the scales. To get around this I distanced the magnet from the balance using a plastic 35mm film canister and sure enough the weight of the magnet went down to ~1g.

By the way I can really recommend this Vibe App (Vibration by Diffraction Limited Design LLC) It's excellent for assessing vibrations on machinery and vehicles.
The basic version costs $5 and the advanced version (data logging etc) costs $15. I think its a Mac devices only but there are similar apps available for other platforms
All you do is lay the phone or iPad on a horizontal surface of a running machine and hit the sample button.
The vibration is reported as x/y/z component which helps in identifying the orientation the vibe is happening.
you can control sample rate, delay, length(time) etc.

A while back i built a pid controlled speed control for a high speed motor application. The way i solved the balancing was to place four magnets on the "wheel". Precisely spaced of course...that also gave me better accuracy re speed. Used a HE sensor, pic micro etc:)

Quote:

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Originally Posted by ericks2

A while back i built a pid controlled speed control for a high speed motor application. The way i solved the balancing was to place four magnets on the "wheel". Precisely spaced of course...that also gave me better accuracy re speed. Used a HE sensor, pic micro etc:)

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Good idea. The refresh rate on these cheap Tacho's is not that great so they don't read all that well when the speed is being changed. They're fine when the speed has stabilised.