Design Question part ][

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Kam 20 years ago

Hi all,

Thanks for all the replies! Now for a followup question or 2!

Okay, I want to build my "how much has it opened" thingy. I want to put a strip of metal on one side with teeth on it (teeth size TBD), and on the other side a Rotary Encode with a gear attached that fits on the teeth. I am talking about the smallest/cheapest I can find. My max slide will be 6 inches with a 1/64th max resolution.

Now I do have some Rotary Encoder questions;

#1. On my last thread, someone replied that I can use a "n" TURN Rotary Encoder. Does this mean that this encoder will turn "n" full times, and present a unique "value" for each stop in the turn?

#2. Looking at digikey's catalog, there are a few "Mechanical Rotary Enclosers", and some say "Horizontal Mount with Switch", does the "with Switch" means that this is an encoder, and a push button switch? I don't want to take this for granted!

#3. It also says for the same encoder, "Resolution 16 CPR" and "80 CPR", does the CPR mean "Clicks Per Rotation".

#4. The real difference between the "16 CPR" and "80 CPR" is the resolution, yes? So if I put a gear on this, with a single rotation of one inch, using a 80 CPR will yeald me a resolution of 0.0125", where

1/64th is 0.015", so I can exceed my 1/64th with this?

I plan on using a PIC to sense the Encoder and present the opening value.

So to sum it up, I need a (assuming geared for 1") "6 turn 80 CPR Rotary Encoder". Such an animal?!...Time to look thru Mouser and Digikey!

Thanks all

~Kam (^8*

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Tim Wescott 20 years ago

No, they suggested a multi-turn potentiometer with an accompanying ADC, not a rotary encoder. This is not a bad suggestion at all.

Rotary encoders are continuous devices -- they turn and turn and turn. That's the up side. The down side is that if it's a quadrature encoder you don't know where the heck it is, only how far it's traveled. That means that every time you turn the power on you need to home the mechanism, which generally means slowly driving into a home switch until you notice a transition.

You can also get absolute encoders, but they're more rare and expensive

-- I don't believe you'll find one in the Digi-Key catalog.

Probably.

"Cycles per revolution", also known as "lines" for the way an optical encoder is made. There won't necessarily be any clicks. With a good quadrature encoder you can get four steps per cycle if you know what you're doing.

Yes and no. A 16 CPR quadrature encoder will give you a resolution of

1/64th of a turn, 80 CPR will give you a resolution of 1/320th of a turn.

But your average gear may give you more than 1/64th of an inch backlash, unless you use a very fine tooth gear in which case you'll have to locate your rack and pinion very precisely, or use an anti-backlash pinion.

Do you know how to interpret the output of a quadrature encoder?

Just a 80 cycles per revolution encoder.

But I do think you'll do better with a string pot, or a gear and a multi-turn pot. ADCs are easier to read than encoders and you won't have to home the mechanism.

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

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Mike Young 20 years ago

I think a small rubber wheel has low enough slip for this application. Me personally, I would opt for a simpler solution; maybe read the exposed length of a nichrome wire. 6" of 10 AWG nichrome wire is about 1k ohm. You should be able to wire the whole house from a single appropriated toaster.

You probably want to answer the first question of reading quadrature encoding with a PIC. Microchip I think has a few app notes.

Also try US Digital. Much cheaper, and much more friendly about answering questions. I'm curious to know if they try to talk you out of it.

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