Design question...

Digital calipers would use linear encoders -- without going into details they take the features of an encoder wheel and put it on the stationary part with a reader in the head.

The "slot car controller" idea you're talking about is a potentiometer, and yes it would work. John Popelish's suggestion for a string pot is a good one -- 1/64" accuracy with a home made one would be a trick, but may be possible. It'll certainly be easy to mount.

You may also want to look into DROs (Digital Read Outs) such as are often found on machine tools. On the plus side they are often retrofitted, so there may be something out there that you could use. On the minus side they are designed to be accurate to .001 or .0001 inch, and they are sold to experienced machinists. So they may be too expensive and hard to mount mechanically.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

There are many ways to accomplish this with resolution considerably better than 1/64th. Being "NOT a seasoned electronics person" works well to narrow down the range of possibilities. Devise a string and pulley arrangement that rotates a 10-turn potentiometer, available at surplus for about $3. Connect that to a digital voltmeter, available from Harbor Freight on sale for well under $5.

A 1/2" dia pulley would rotate 7.63 turns with 12" of string travel. If you excite a 10-turn pot with 1.573 volts then the digital voltmeter would read 0 to 1.20 volts, giving resolution of 1/100 inch. Others may suggest ways to realize a 1.573 volt supply using an LM317 or whatever. It's quite simple but it does involve soldering 3 or 4 parts together and I don't know just how "NOT" you are with elex.

Hi all,

I'm trying to build a little gadget for my daughter, and I'm lost! Firstly, I am NOT a seasoned electronics person, so the answer might be a simple one!

What I'm building:

A sliding "window" (approx 0 ~ 12 inches max opening/sliding). I want to measure how much has opened, to a resolution of 0.15mm or 1/64th. I want to be able to open the window and capture the opening amount. Once I know how to "read" this value, I can program my display to show it.

Any advice would be greatly appreciated!

~Kam (^8*

Hello Kam,

1/64th is a pretty tough spec. I guess the best bet might be to find an older ultrasonic distance meter and hack it. The old ones have a lot of discrete parts in there so finding a place to tap off before the display circuitry is easier. But for that job you would have to become a seasoned electronics person :-(

Then there are rotary encoders. For this precision you'd need two. One with a large wheel for the coarse position and another with a small wheel that revolves many times over those 12 inches for the precise measurement. With steel cable similar to what is used to pull up garage doors (but much smaller gauge) maybe you could eke out the desired precision. The smaller gauge stuff can possibly be found in bicycle shops.

If you don't want to hack anything, ultrasonic distance meters already have a nice LCD display ;-)

Regards, Joerg

Joerge, et al.,

I was just googling and found 6" digital calipers. They clain to have

0.001" rez. I wonder if they are using the rotary encoders like you mentioned.

Now for a non-electronics person question (ie. me!), could I not magically detect a voltage drop/addition like how the speed control on the wired slotcar remote is?, you know, it has a long flat bar with another contecting bar sliding up and down it (speed control)...

They res does not have to go up or down in 1/64ths, but it has to be constent, so I can figure the resolution I need...

Thanks again...

~Kam (^8*

Joerg wrote:

position an optical mouse over the edge of the frame, Last price for a wired Optical mouse $ 15.00 Else re-invent the wheel ! Yukio

"I can program my display to show it" ... if you know enough about programming then you might be able to hack up something with a dsp chip (a very basic PIC microcontroller, an ultrasonic sensor proximity sensor and other regular components of a circuit ...

Do you already have a way to make the window slide, you can use stepper motors for this ... will be an interesting project I'm sure!

6" digital calipers with resolution to .001" can be bought for $9.99 on sale at Harbor Freight.

The 12" travel of your window can be halved to 6" by means of two strings, and two pulleys of any size. One end of each string would be fixed to the windowframe. They'd wrap around pulleys affixed to the (stationary) caliper and then return to the window. You need two strings and pulleys so the window can pull the caliper in either direction, like little block & tackle arrangements.

I'd recommend 20 lb test Spectra musky fishing line because it has very little stretch, nearly like wire but much more flexible.

These calipers have their own display, which can be made to display either inches or mm and can be set to zero at any point. Some of them have a tiny connector with signals that can be read by a P.C. There are websites that describe how this works.

Note: these calipers do not respond well to getting wet, as from rain.

We will shortly have some premium grade 12 inch linear potentiometers up on eBay. But they probably will be priced too high for your ap.

And some even better super precise and contactless magnetostrictive pots up to six feet long!

Asking for resolution approaching ten bits is way pushing anything analog. But a spring return 4" wheel and a string on a better grade potentiometer might work.

--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073
Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml   email: don@tinaja.com

Please visit my GURU\'s LAIR web site at http://www.tinaja.com

If you use steppers to "remote control" the window opening, you already know the number of steps and thus the position ahead of time.

Easy PIC project.

While closed loop is often best, steppers allow open loop operation which is much simpler.

--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073
Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml   email: don@tinaja.com

Please visit my GURU\'s LAIR web site at http://www.tinaja.com

Yes I'm a programmer by trade. My issue is not the window moving etc, but capturing how much the window has opened. In my minds eye, I need something just like a digital caliper. I would like to build it (I know I can buy then cheap!) with my daughter etc. Makes for a fun project!

~Kam (^8*

snipped-for-privacy@gmail.com wrote:

Hello Kam,

Tim has explained it already. Let me add one thing: Due to the fact that it is a relative measurement there is (usually) no absolute zero on the slider. This is why they have a zero or reset button. Usually they don't easily lose track but they could and you'd have to hand-zero them. If that is ok you might be able to find a longer digital caliper at places like Grainger. But be prepared to write out a rather large check.

John's string pot is a good idea as well. However, even the most expensive 10 or 20 turn pots I have bought for lab equipment don't quite fulfill you precision spec of 1/64" over 12" which would be around 0.1%.

Regards, Joerg

Maybe I missed it, but did Kam say why on earth he needs that high precision in such a toy?

--
Terry Pinnell
Hobbyist, West Sussex, UK

I got some linear scales for a small milling machine. I assumed these would be mousewheel type rotary encoders, but on examining them, they would appear to use a capacitive scale, like an electrostatic analog of Inductosyn. Digital output, too, but sadly far to slow to use for CNC.

Paul Burke

Easy to rectify! Tap your heels together three times and slap down your plastic..

It wouldn't be hard to make a linear pot - I just looked at McMaster-Carr, and they have a 5' length of .125" diameter fiberglass rod for $2.13. Just wind a bunch of, say, #36 nichrome wire on it. For a slider, you could use a piece of shim stock or cut a piece from a coffee can lid. ;-)

I've also seen a rack and pinion and 10-turn precision pot, and toothed belts, also with a 10-turn precision pot.

Have Fun! Rich

Hello Terry,

No, he didn't. Maybe for the same reason I cut floor tiles to within

1/20" tolerance where tile setter pros would say I'm crazy. I just assume that since he wants to go through all this pain he really needs it.

Regards, Joerg

Hello Paul,

Drool. I wish I had one on these.

Regards, Joerg

Wouldn't you like an extra 1000 sq-ft in your garage ?:-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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