Techniques for measuring RPM of centrifuge

[Will Beaufoy]

From talking to some people yesterday, it seems we will be painting a white dot at the end of one of our centrifuge arms, and using the reflected light to judge its speed. We initially thought of using a strobe, but then it was suggested to just fix a reflective sensor where the light from the dot would be reflected to, and count it this way.

Does anyone have a spare sensor of this kind, or a strobe light if they think the strobe method is better?

[930913]

A small laser pointer and an LDR?

If it's possible to attach a small magnet, a bike speedometer may suffice.

Is this for a one off calibration, or for constant reading during normal use?

[Adrian Godwin]

A laser tachometer is about £10 on ebay (eg
http://www.ebay.co.uk/itm/270913948041).

These come with a few reflective squares of scotchlite-style
retroreflective material, and work reasonably well as long as the
speed isn't too low.

-adrian

[Simon Howes]

I made a speedo for my cnc mill *really* cheaply one day in a pinch. Basically we'd hacked the pulleys to make it much faster but we didnt have any idea of what speed it was now running (because of belt thickness etc) as the pulley was quite small the variances were large and I wanted to know how fast it was spinning.

Solution? Get a thin metal rod, drill a hole in it.
Get an ir led and ir led sensor - it my case I only had ir leds - but thats okay, they work as sensors too!

Get a square of any material that will block the light. Drill two holes through it at 90 degrees - one the thickness of the rod the other the thickness of the leds.

Clamp the rod in the chuck of the spindle.
And you now have an interruptor  sensor. Measure the waveform on an oscilloscope and you can calculate the speed.

It was a nice 3600rpm in the case of my mill :)

[Will Beaufoy]

This is for one off calibration - but it would be cool to have a tachometer for future use, so think I'll get one of those ebay ones. Cheers for the tip!

[Peter "Sci" Turpin]

I'll say those little tacho eat batteries. I have one. I think they draw
power whenever they're connected, so best to remove the batt when it's
not in use.

On 11/10/2012 15:50, Will Beaufoy wrote:
> This is for one off calibration - but it would be cool to have a
> tachometer for future use, so think I'll get one of those ebay ones.
> Cheers for the tip!
>
> On Thursday, October 11, 2012 12:30:26 PM UTC+1, Simon Howes wrote:
>
> I made a speedo for my cnc mill *really* cheaply one day in a pinch.
> Basically we'd hacked the pulleys to make it much faster but we
> didnt have any idea of what speed it was now running (because of
> belt thickness etc) as the pulley was quite small the variances were
> large and I wanted to know how fast it was spinning.
>
> Solution? Get a thin metal rod, drill a hole in it.
> Get an ir led and ir led sensor - it my case I only had ir leds -
> but thats okay, they work as sensors too!
>
> Get a square of any material that will block the light. Drill two
> holes through it at 90 degrees - one the thickness of the rod the
> other the thickness of the leds.
>
> Clamp the rod in the chuck of the spindle.
> And you now have an interruptor sensor. Measure the waveform on an
> oscilloscope and you can calculate the speed.
>
> It was a nice 3600rpm in the case of my mill :)
>
> On Oct 11, 2012 8:11 AM, "Adrian Godwin" <artg...@gmail.com

> <javascript:>> wrote:
>
> A laser tachometer is about £10 on ebay (eg
> http://www.ebay.co.uk/itm/270913948041

> <http://www.ebay.co.uk/itm/270913948041>).

[Will Beaufoy]

Cheers for the tip