🚀 go-pugleaf

> I'm unsure of how accurate it would become.
> Where are the primary places where the accuracy
> could be lost ? Maybe in the 555 circuit ?

I think that you can have a big problem with parasitic capacitor.

And I think, that disk with holes, the much metter idea.

On Wed, 18 Jun 2003 11:53:17 +0200, "Iman Habib"
<imanhabibREMOVETHIS@eml.cc> wrote:

>Hi there.
>
>I have just started out on a small project and at this moment I am at the
>point where I need a linear and rotational sensor with high accuracy
>and the sensor should not be prone to wear down after a short while.
>And the sensor would somehow be connected to my PIC chip
>that would be continuously reading from the sensor and storing the
>most recent value in the memory and later sending it of to wherever
>I need it.
>
>First I was thinking about the obvious choices.
>A variable potentiometer. But high accuracy pots with long usage
>lifetime costs way to much.
>A IR diode & IR transistor in an light isolated slider tube. But that would
>not give me linear results directly. And I think the that there would be
>an accuracy problem.
>
>After some thinking I came to the conclusion that capacitors would be
>an excellent choice. The idea was mostly due to the digital calipers
>I have. It has an accuracy of 0.01 mm and I read somewhere that
>the sensor is essentially a linear capacitor strip running inside the
>calipers slider. And I thought the actual electronics shouldn't be that
>expensive as the caliper itself costs about 30$.
>
>Also capacitor would  give me a more linear values than anything else
>as long as the surface geometry of the variable capacitor would
>increase linearly.
>
>Essentially what I want is a primitive form of a capacitor meter.
>
>Now the problem is that I have never done this before and I am not that
>sure of how it would be done and I thought some of the guys here
>may have come across similar problems.
>
>I imagine that I could use a multivibrator (555 based circuit) and generate
>and square pulse depending on a variable capacitor. And on the PIC
>circuit just increment a number from 0 to N depending on the time it takes
>between the pulse peaks that I get from the 555 circuit. Thus the resolution
>of the meter would be up to the operating frequency of the PIC (?).
>
>Would this be the way to go ?
>
>I'm unsure of how accurate it would become.
>Where are the primary places where the accuracy
>could be lost ? Maybe in the 555 circuit ?
>
>Could anyone here give me some pointers and
>help out a newbie here... please....
>
>thanks
>//iman
>
>ps. sorry for the crosspost.

What sort of resolution do you need?, as that may well dictate the
options available.
A capacitor meter using a 555 is fairly easy, and has been used in
several capacitor meter adaptors in SC and EA projects over the years.
1% accuracy is easily achievable.  Search the back issues and you'll
find some projects. 

Regards
Dave :)
---------------------------
(remove the "_" from my email address to reply)

On Wed, 18 Jun 2003 11:53:17 +0200, "Iman Habib"
<imanhabibREMOVETHIS@eml.cc> wrote:

<snip>
>
>After some thinking I came to the conclusion that capacitors would be
>an excellent choice. The idea was mostly due to the digital calipers
>I have. It has an accuracy of 0.01 mm and I read somewhere that
>the sensor is essentially a linear capacitor strip running inside the
>calipers slider. And I thought the actual electronics shouldn't be that
>expensive as the caliper itself costs about 30$.
>

Don't your calipers have a long rack and pinion gear like the old
dial calipers?  I assumed the digital encoding was strictly rotary.
If it uses capacitors, it is probably in the same way that you would
use a slotted disk with photo-optical methods:  strictly for counting.
That will give you best accuracy and linearity.

If you want to avoid gearing, tehn all you need is a linear grating
of some sort.  If you want to get really fancy, you can interpolate
between the teeth to get better resolution.  With an optical
system, you have 2 sensors with a slight offset, so that one is
being uncovered while the other is being covered by a tooth.
Similar idea will work with a capacitive system, where you are
looking at the capacitive coupling between the rotor tooth and
two sensor plates.   More excitiation passes through to the
sensor closest.



Bob Masta
tech(AT)daqarta(DOT)com
 
            D A Q A R T A
Data AcQuisition And Real-Time Analysis
 Shareware from Interstellar Research
          www.daqarta.com

On Wed, 18 Jun 2003 12:24:03 GMT, the renowned no_spam@aol.com (Bob
Masta) wrote:

>On Wed, 18 Jun 2003 11:53:17 +0200, "Iman Habib"
><imanhabibREMOVETHIS@eml.cc> wrote:
>
><snip>
>>
>>After some thinking I came to the conclusion that capacitors would be
>>an excellent choice. The idea was mostly due to the digital calipers
>>I have. It has an accuracy of 0.01 mm and I read somewhere that
>>the sensor is essentially a linear capacitor strip running inside the
>>calipers slider. And I thought the actual electronics shouldn't be that
>>expensive as the caliper itself costs about 30$.
>>
>
>Don't your calipers have a long rack and pinion gear like the old
>dial calipers?  I assumed the digital encoding was strictly rotary.
>If it uses capacitors, it is probably in the same way that you would
>use a slotted disk with photo-optical methods:  strictly for counting.
>That will give you best accuracy and linearity.

The calipers use a crude (crude probably isn't the best word) fingered
capacitor arangement, perhaps 0.1" (give or take half an order of
magnitude) between "plates", and interpolate between to give 0.0005"
or 0.01mm resolution. There's no rack and pinion to pick up dirt like
the old dial caliper, they are mechanically just like the vernier
style. 

Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

On Wed, 18 Jun 2003 14:27:55 GMT, Spehro Pefhany <speff@interlog.com>
wrote:

>The calipers use a crude (crude probably isn't the best word) fingered
>capacitor arangement, perhaps 0.1" (give or take half an order of
>magnitude) between "plates", and interpolate between to give 0.0005"
>or 0.01mm resolution. There's no rack and pinion to pick up dirt like
>the old dial caliper, they are mechanically just like the vernier
>style. 

Thanks, Spehro.   By the way, congratulations on winning
Third Prize for your Chimney Sentinel in the Circuit Cellar /
Microchip Mad Dash for Flash Cash contest!   Saw the
writeup in the June Circuit Cellar.   Good show!


Bob Masta
tech(AT)daqarta(DOT)com
 
            D A Q A R T A
Data AcQuisition And Real-Time Analysis
 Shareware from Interstellar Research
          www.daqarta.com

On Thu, 19 Jun 2003 17:13:18 GMT, the renowned no_spam@aol.com (Bob
Masta) wrote:

>On Wed, 18 Jun 2003 14:27:55 GMT, Spehro Pefhany <speff@interlog.com>
>wrote:
>
>>The calipers use a crude (crude probably isn't the best word) fingered
>>capacitor arangement, perhaps 0.1" (give or take half an order of
>>magnitude) between "plates", and interpolate between to give 0.0005"
>>or 0.01mm resolution. There's no rack and pinion to pick up dirt like
>>the old dial caliper, they are mechanically just like the vernier
>>style. 
>
>Thanks, Spehro.   By the way, congratulations on winning
>Third Prize for your Chimney Sentinel in the Circuit Cellar /
>Microchip Mad Dash for Flash Cash contest!   Saw the
>writeup in the June Circuit Cellar.   Good show!

Thanks!

Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com