Options for tracking upsets in very fine movements

Question

I'm working on a small engraving device that sometimes hits a tougher part of the target material. Since the tip is very small and spins at a high speed, the engraver sometimes involuntarily slides around that harder part of material (see the image below). When this happens, I need to know that it happened, to what extent it had happened, and where the engraving tip is now -- otherwise I have to recalibrate the device and start over.

What could I do to obtain this information? The following things came to my mind:

1. Put a small camera next to the engraving tip and try to use the continuous image of the engraving to recognize the "upset" when it's happening. This is not a great solution though, as the splinters flying off the material will probably render most of the footage useless anyway.

2. Put an accelerometer on the tip holder, and then integrate the acceleration readings in periodic instances to find out whether there has been an upset since the previous instance.

The accelerometer option seems to me as the most plausible, but I am not sure if the present-day accelerometers are up to the task. I need to be able to do reasonably precise predictions based on acceleration data of the movements on the scale of tenths of a millimeter.

Would anyone happen to have an idea or an insight on this?

Answer 1

I have two-ish ideas.

The first is to make use of mechanical indicators. The same kind machinists and toolmakers use. Test indicators are tiny, very sensitive, fragile, use a lever, have low travel, and can measure deviations from zero down to tenths (1/10 of 1/1000th of an inch). The Dial indicators are larger, less sensitive, more sturdy, have high travel, use a plunger, and can measure down to 1/1000"...maybe 0.5/1000". Terminology to separate the two is fuzzy. "Test" usually specifically refers to the small, sensitive, lever type though.

Mitutoyo

Interapid

You mention debris and low required accuracy (also vibration) so the dial indicator with the plunger is probably better than the test indicator if it is to measure things while actually running.

The mechanical ones use gears and are like watches. Digital LCD readout versions also exist now and I don't know if they still use gears, or if they use something like hall-sensors or strain gauges. They cannot be built without experts in a precision machine shop but you can buy one along and manufacture parts to allow it to be installed into your setup the way you need it.

I don't know your setup but off the top of my head, direct contact using dial indicators with a roller contact, or a rotating guide around the spindle or engraving bit where the indicator can make contact with to make measurements while running. Or have levers that make contact with the bit on two-axis and apply the indicator to the other end of the lever to keep the indicator safe and far away.

Starett

The second idea is to use strain gauges. Install strain gauges somewhere where it can measure X-Y deflection. Then apply various deflections to an installed engraving bit using mechanical indicators to measure the deflection, map out deflection vs strain gauge readouts. I'm not sure if this will have to be done for every bit you use or if one works for all. Certainly the length of the bit is going to matter.

It certainly is a pain to try to do anything at the bit. I'd try to see if you can measure anything at a non-rotating part on the spindle. Even if it does rotate, it'd be easier than at the bit.

I think accelerometers are the least plausible.

There's also wacky ideas like fiber optic whiskers and fiber optic strain gauges. You'll want indicators to calibrate things still and I don't know how repeatable that is but these are extremely sensitive. You probably need to indicators to calibrate things here as well.

One last idea: Laser interfometry. Shining a combination laser-photodiode and measuring the received beat frequency. One for each axis. But aim, debris, eye safety, not to mention the two lasers interferring with each other. Also indeterminant distance relationship as your deflections are many times a wavelength.

Answer 2

From my memory, there is a distinct noise when a cutter/bit hits a knot (in wood) or an inclusion (in stone) caused by the lateral force on the bearing. You could use a microphone. But they would take some signal processing.

Another (better?) option would be to attach quadrature encoders to your motors (or read the encoders that may be attached to your motors), you could monitor if an x-axis or y-axis motor is pushed in an unexpected moment.

You may be able to do it without encoders, just back-emf (if using brushed DC motors, back emf will be easier to see than other types of motors). But an amplifier on each motor would be required to see each signal. Encoders would be way easier.