Why won’t this work…or will it?

[John HinkleyParticipant @johnhinkley26699]

While making slow progress on my Harold Hall grinding rest and making extensive use of the DROs on my mill, I got to wondering about a less cumbersome means of measuring cutting tool measurement in three axes.

Now, it occurs to me that modern video game controllers and even my iPad must contain some form of motion sensors and/or accelerometers probably in some form of chip, in order to detect movement of the device and even my (cheap) digital angle gauge can measure to a knat's of a degree. So, why couldn't these chips be attached to a mill table to measure displacement in 3-dimensions, with optional remote readout by wire or wirelessly?

Can someone with more electronic knowledge than I have please explain why this couldn't be acheived? Are the sensors not sufficiently accurate? I can't think that they'd be too expensive.

If it wouldn't work, I'm happy to be told why…If it would, how brilliant that would be!

Over to you boffins…….

John

[John HinkleyParticipant @johnhinkley26699]

[Russell EberhardtParticipant @russelleberhardt48058]

Unfortunately they use acceleration sensors rather than distance sensors. In principle, if you could measure the acceleration vs. time accurately enough and performed a double integral you could get a measure of distance but in practice the errors would be enormous.

Russell.

[John HinkleyParticipant @johnhinkley26699]

Russell,

Thanks for your prompt reply. I should have known it wouldn't work – or somebody would have done it by now! Oh well, back to the tried and tested method.

regards,

John

[jason udallParticipant @jasonudall57142]

Just think , want to resolve say a thou on 12 ".. thats one part in 12 000 …not imposible ( initial guidance platfoms achive that)….16 bit arithmatic resolves 1 in 64 000 ( yes I know 64 *1024 -1) and 16 bit is old hat ..

will have a nose and see what my android phone is capable off…

Shame you posted ..here you might have done your self out of a patent..

[Michael HornerParticipant @michaelhorner54327]

Hi John

Arn't you describing CNC?

In Mach3 You can type in your coordiates from the keyboard and the cutter will move to those points. The accuracy depends on how much you want to spend. A basic CNC machine is more accuate than I am on a manual machine.

Cheers Michael

[Another JohnSParticipant @anotherjohns]

I had a game-controller on my (LinuxCNC controlled) mill.

It did not "work" with me – I'm so used to turning knobs to machine that the game controller was confusing! I got a "mpg" rotary knob controller and all is fine for those times when I want to manually control machine movement.

Michael – As with Mach3, LinuxCNC can take keyboard entries and move the machine to specific locations. I'd think that any CNC system would also do that?

Another JohnS.

[jason udallParticipant @jasonudall57142]

I understood the op was suggesting the sensors as a alternative to scales /encoders

[Russell EberhardtParticipant @russelleberhardt48058]

Inertial navigation systems are used on fighter aircraft and missiles they use acceleration sensors, integrating the acceleration to get velocity and integrating the velocity to get position. These systems respond quickly but need input from GPS to correct the position frequently.

It would only take a tiny error in the measurement and maths for the system to think it's moving when in fact it is not. A similar error will occur when moving at a constant velocity. The error would then accumulate over time. Even with frequent homing, the errors would be much too great.

A typical chip (ADXL362) has a resolution of 1 mg on the 2g scale. That's one part in 2000 and the error will be much more as the nonlinearity is up to 0.5%. Remember that's just the acceleration not the position.

Good luck if you're going to try it!

Russell.

[jason udallParticipant @jasonudall57142]

I think that thats the crux…in this application noise in the sensor channel would be of the same order of magnitude as the rate of change of position. .and after two integrations errors would rapidly exceed the signal..
Another reason why ..I doubt that this technique would not have been explored if plausible

[Michael HornerParticipant @michaelhorner54327]

Hi Jason

He does mention a digital angle gauge. Not sure why he would find a DRO cumbersome! Might be a pig to fit but then that's it. Surely? Is he after something like a mini GPS? If so I think to get the accuracy it would be out of the price league of normal model engineers.

Cheers Michael.

[John HinkleyParticipant @johnhinkley26699]

Oh dear! This is a bit like Topsy – it just growed and growed. I have no intention of making a device of the type which I described in my original post – I have insufficient time and zero ability in that direction! It was just a random thought that occurred to me while I was winding the mill table backwards and forwards for what seemed like hours.

Had it been a "goer" I imagine the result would have been very small and needed one or two screws to attach not to mention hundreds of pounds/euros/dollars cheaper than CNC, nice though that would be.

Thanks for all the replies – like I said – back to the drawing board.

John

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi John ,

(1) Several ' backwards ' versions of more or less what you suggest have been devised .

These involve tracking devices on fixed parts of machine remotely detecting positions of a target on moving parts of machine .

(2) Most commonly these are laser devices but in more recent times digital cameras have been used to literally watch the target and calculate positions from changes in successive images .

Neither are commonly used on machine tools as such but are certainly used in scientific and experimental equipment .

(3) A different system uses an unpowered transponder on the moving parts and radio frequency detection from one or more fixed transmitter / detectors . Something like GPS but not quite the same .

(4) A problem with these systems is ' line of site ' – there is always a chance of something blocking the detection signals . Usually overcome by using multiple redundancy .

Regards ,

MikeW

[Anonymous]

A simple calculation may be illuminating. Let us suppose that we want to move, and track, the milling table at a moderate 300mm/min, or 5mm/sec. Let us assume that we have a 5mm pitch leadscrew, and that we accelerate to 300mm/min in one turn of the handwheel over 1 second. So, over 5mm and 1 second we've gone from zero velocity to 300mm/min, or 5mm/sec. Acceleration is the change in velocity divided by the time. So, using metres, we get 0.005m/s/s. But 'g' is about 9.81m/s/s. So our acceleration is about half a milli-g. Which according to Russell is less than we can resolve anyway.

Regards,

Andrew

[jason udallParticipant @jasonudall57142]

Andrew…"So our acceleration is about half a milli-g. Which according to Russell is less than we can resolve anyway."…

or as we used to say "down in the grass"

[John HinkleyParticipant @johnhinkley26699]

I've had a few more thoughts on this topic, so I thought that I would resurrect it! (It would appear that I don't have enough to do!)

After extensive research over a period of minutes, I discovered that computer laser mice – as opposed to optical – can resolve very small movements, typically 2000 dpi or better. What's to stop one using the laser gubbins in these to detect the movement of lathe/mill slides? An additional benefit would be the ability to connect wirelessly to the display computer. I know that a chap on HMEM has done a similar arrangement with an obselete Android tablet for the display, but he still uses caliper-type scales, hard-wired to an Arduino card and thence via a bluetooth transmitter to the display. If it would work, my system could be significantly more compact and, I think, accurate enough for the sort of work I intend to carry out. An added advantage of using the laser would be that it detects movement by comparing successive pulses reflected from a surface – virtually any surface – so no graduated scales required?

Any thoughts, all you electronics wizards?

John

[jason udallParticipant @jasonudall57142]

Hmmm

[jason udallParticipant @jasonudall57142]

“Laser” mice do on occasion “stick” that said I have seen print scanners using calibration grids ” watched ” by something simular looking down into base from the scan head…
“

Edited By jason udall on 12/08/2013 09:59:50

[Trevor WrightParticipant @trevorwright62541]

The standard length of a metre is measured by so many wavelengths of light…….

Trevor

[blowlampParticipant @blowlamp]

Maybe something like a mini GPS setup might work?

Martin.

[John HinkleyParticipant @johnhinkley26699]

Trevor – When I was at school – admittedly a long time ago when rods, poles and perches were still on the back of our exercise books – the wavelength of light varied, depending on the colour of the light. Good job, too, otherwise we wouldn't have had the wireless to listen to!

I did think that I might get shot down in flames with this idea, but it looks like it could be worth pursuing. I feel an ebay mouse purchase coming on!

John

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Trevor Wright on 12/08/2013 13:09:03:

The standard length of a metre is measured by so many wavelengths of light…….

Trevor

.

Not quite true, Tevor

… see here

MichaelG.

[Billy MillsParticipant @billymills]

Method 2 IS optical wavelenths Michael. For a very long time the metre WAS defined by optical wavelenths. Time measurements present the problem of the variation in velocity of the media, they only work without secondary effects in a vacuum. However there is no need for national standards accuracy on a home machine tool. For a home machine glass scale DRO's work very well, are not that expensive and cope with life quite well.

A cheap way of getting distance readings is to use a modified digital caliper or the ready made digital scales. Or you could do what people did BIC ( before integrated circuits) and use a dial gauge or two.

Billy.

[Michael GilliganParticipant @michaelgilligan61133]

Billy,

No argument with the practicalities, nor with the history … but the fact remains that the metre is now effectively defined in terms of time.

MichaelG.

P.S. My apologies; Trevor said measured, he did not say defined.

[Sub MandrelParticipant @submandrel]

Fascinating choice to use the time it takes light to cover a known distance. I'm surprised physicists would choose a standard they could never measure with absolute accuracy (due to the Heisenberg uncertainty principle), although I would concede it will have higher resolution than using optical methods.

Neil

[Author]

Posts