Spindle Tachometer & Load with MESA 7i76

30 Mar 2017 20:58 #90605 by bymccoy

I'm upgrading a small VMC to LinuxCNC, with a MESA 7i76 for IO.

Any advice on spindle tachometers? Would a prox switch work at the speed/pulse frequency? I'm basically looking for closed feedback on speed control.

As I don't know how the spindle driver has been set or if the speed control is truly linear, it kind of makes sense that if I'm going to go to the effort of measuring the speed for setup, that I may as well fit a tachometer for permanent feedback. Plus I can get real world feedback in the HMI which is always nice.

Will the 7i76 inputs be up to the sampling frequency? Supposedly the spindle has a max RPM of 5k, so about 84Hz max pulse sample rate.

Now this also raises another IO question, as to spindle load. If it use a CT or find an analogue output from the spindle driver (a Sprint 400 single direction/single quadrant), what's the best way of interfacing it? As I'm aware there's no analogue inputs in the 7i76.

30 Mar 2017 21:06 #90606 by bymccoy
Checking the Sprint 400 manual, there's a current output...

0-5v scales to 0-100% current at 1K impedance.

Not sure what 100% current refers to... it's relative, but to what...

30 Mar 2017 21:29 #90608 by PCW
A single proximity switch can work for threading but its not optimal and will not work well
for closed loop control except at high speeds

If possible encoder is better and allows rigid tapping

The 7I76 has 4 analog inputs (these would be low resolution but
probably adequate for a spindle load meter)
31 Mar 2017 00:45 #90620 by bymccoy
Suppose question there, is how you would fit an encoder to a spindle... Was only thinking a prox switch, as it can be aligned facing the spindle. There's a small threaded hole (about 5mm I think) which could be used as the trigger point. Wouldn't necessarily want to stick a magnet etc on it, as don't want to unbalance the shaft.
31 Mar 2017 11:51 - 31 Mar 2017 11:52 #90638 by andypugh

bymccoy wrote: Suppose question there, is how you would fit an encoder to a spindle.

One way is to use an existing gear in the lathe head and a tooth detector.
Or you can attach a custom-made target wheel to the spindle, and use optical sensors.
Or you can belt-drive a commercial encoder from the spindle.

I have done all 3 at various times.
Last edit: 31 Mar 2017 11:52 by andypugh.
The following user(s) said Thank You: bymccoy
31 Mar 2017 13:48 #90642 by bymccoy
Thanks Andy (as always ;)

With the design of the spindle, the optical target would work best. The motor pulley is small, so no space to sense the teeth, plus both that pulley and the spindle have flanges, so can't get the edge of the teeth. A belt driven could work, if I could pick up he existing belt drive. That's a slight maybe!

So, any recommendations as to optical sensors and how to make the targets? I do have a vinyl printer, so could print squares onto a strip of clear vinyl. No idea if it'll stay in place on the spindle?! Spindle diameter is about 40-50mm, so might get down to 15degree increments, so 16 pulses per rotation, and should be able to fit the sensors and target. Nicely, I'm fitting a powerdrawbar, so there'll be a plate of steel about 40mm from the spindle, giving me ample opportunity to mount sensors and wiring securely.

31 Mar 2017 16:10 #90655 by andypugh

bymccoy wrote: So, any recommendations as to optical sensors and how to make the targets?

I tried a machined aluminium target and a reflective sensor once, and it wasn't an unqualified success, so I would definitely suggest the transmissive type.

Something like these should work.

The ones pre-wired with cable are likely to be more convenient.
If you have two short ones and a long one it makes it easier to detect the index.
You probably want to be careful about what output device the detectors have, some are easier to connect to interface hardware than others.
Alternatively, if you filter those results to only see the 2-channel ones you will see some that do quadrature in the device (but they will tend to need a very specific slot spacing). Note that most of the quadrature ones are very tiny indeed.

The target can probably be easily made with a laser cutter, possibly out of a CD, if you don't have a milling machine. or your Vinyl idea is reasonable (laser-printing on to acetate and sticking to a transparent substrate is also a possibility)
02 Apr 2017 17:38 #90775 by bymccoy
It seems quite a challenge to find a big enough pre-made target, and not sure if I can get the print resolution if I make my own. Found some very nice sensors (US Digital brand) that would work very well at good resolutions (and include indexes too).

I'm wondering if easier to use a shaft quad encoder, and belt drive it. Might be easier fitting a tooth pulley wheel than making a custom plastic disc - just need to find a suitable size aluminium pulley wheel bore the inside out to the spindle shaft size. Maybe leave as compression fit, or try to fit some grub screws. Will need to measure up accurately to decide.
02 Apr 2017 17:43 #90776 by andypugh
You don't need that much resolution. 100 slots is fine.
You could probably get something laser-cut fairly cheaply.
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