Design question...

[Kam]

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*

 

[Joerg]

Hello Kam,

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.

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

 

[Kam]

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*

 

[John Popelish]

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*

Probably the simplest circuit to measure such a distance is a string
pot. You wrap a string around the shaft of a multi turn potentiometer
(possibly with a small cylinder attached to the shaft to take a longer
length per turn, so the 10 turns has just a bit more length capability
than you need) and apply a fixed voltage across the full resistance.
Then the voltage from one end of the resistance to the wiper
represents distance. You also need a spring or weight tensioning
arrangement or double winding (one end of the string unwinds and the
other winds up as the window slides) to keep the string tight.
Digitizing the voltage and "capturing" that is another problem.

If you use an A/D converter, you are talking about 12*64=768 points or
about a 10 bit converter.

 

[Tim Wescott]

(Top posting fixed, or at least moved)

Joerge, et al.,

I was just googling and found 6" digital calipers. They lain 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
constant, so I can figure the resolution I need...

Thanks again...

~Kam (^8*

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.

 

[Joerg]

Hello Spehro,

Some of us have these "basement" things, for example, a so-called
"4,000-square foot house" with two stories might have an *extra* 2,000
square feet (not counted in the square footage by law) for such
purposes, minus a wee bit of space for physical plant elements.

That was the biggest disappointment after moving from Europe to
California. No basements and no attics out here. Sigh. In hindsight we
are happy because you just cannot become a packrat that way. The house
may be bigger than the ones in Europe but there is not that much storage
space.

Also, I know some folks (retired, generally) with shops that are
larger than their houses. Naturally, zoning becomes a factor in such
deviant lifestyle choices.

Zoning laws are regulations that never made sense to me. That is one of
the reasons for our high fuel consumption and for obesity. You have to
drive everywhere.

Regards, Joerg