padu

Hi,

I'm starting an altimeter project. Right now I'm using a 7segx3 display, but in the future I want to display the altitude using a needle, just like the mechanical types.
I'm using one of the motorola pressure sensors and a PIC (still don't know if the pic's ADC will do or if I will need a better ADC).

The fundamental question is:
How digital information can be displayed through an analog needle?

I'm looking for a solution that is easy to implement (few parts), cheap and reliable.


Cheers

transistance

through the digital information your ADC has processed use your PIC's pulse width modulation to control a servo that rotates the needle to that certain degree it should display on the meter, maybe?

only thing you will have to purchase (since you already are going to have a PIC+ADC or PIC only) would be a sub-micro servo - since i imagine you wont have too much space behind your meter, else you can go for a cheaper but larger servo.

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padu

I had originally thought about a stepper motor that is sized like the motors that spin CD's on a CD reader, but I thought it would be too much of a hassle.

The servo would be very simple, as I've interfaced with those a lot in the past, but the size aspect is concerning. Another concern is how smooth the movements are. I don't know if they will be smooth with a servo.

picasm

If you use a PIC that has a hardware PWM module (eg. a 16f628) then you can easily vary the pulse width ratio of an output.
This can be smoothed by a capacitor if required to create a varying voltage which could be measured by an analogue voltmeter meter.
Info on configuring pwm is here:
http://www.piclist.com/tecHREF/microchip/pwm.htm

There is an interesting open source project that uses analogue meters and PWM for showing various information at:
http://www.sharebrained.com/chronulator/
It uses a different make of microcontroller, so would need a bit of conversion.

padu

on another thought: how does those gauges in modern sportbikes (I have an 04'gixxer 600) work? I bet the RPM/speed is encoded to a sort of MCU then displayed on the instrument cluster. How that works?

transistance

I prefer to keep a simpler hardware form. If I can do it on an IC, I won't use external devices, which a stepper motor requires.

A servo will have a very smooth motion since it has numerous amounts of gears in it. The problem with the servo is how responsive it is going to be, yet you would have the same or larger amount of response issue with a stepper motor in my opinion.

When it comes to size, do not worry about it. There are unbelievably tiny servos that are commercially available now.

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JimB

So I assume tha the pressure sensor output is already an analogue.


Why digitise an analogue signal, just to convert back to analogue?
Why not just use a few op-amps to do the scaling and offset and drive the analogue meter directly?

JimB

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padu

Yes, ratiometric per input voltage.

Because this is my first prototype. The final goal is to create a logger that saves the samples to a CF (or similar) card.

The point is: which analogue meter?

KMoffett

What's your altitude range? The mechanical types I'm familiar with (aircraft) usually have two needles...one for hundreds of feet and one for thousands, mechanically sync'd by gears...like an analog watch. Is this what you're thinking? Servos (R/C), only travel in a 90º or 180º arc.

Ken

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padu

The mechanical altimeters I'm used to and trying to replicate are the skydiving ones (yes, I'm one of them).

They have 360 degrees motion. Calibrate zero ft with a dial and most of them to as high as 12000ft, but they will read 1000ft at 13000ft for example.

So, if I use a servo, I'll need to modify it in order to rotate more than 180 degrees... but that introduces other problems (it is not a servo anymore)

KMoffett

I know there are hacks out there for 360º "continuous" rotation of R/C servos. I did a quick Google and came up with these:

http://www.servocity.com/html/hs-785...rotations.html
But the 3.5 revolutions for 200 steps of "servo control" results in 6.3º per step...only 57 steps for one rotation...not to good for an altimeter display
http://www.kaper.us/basics/Bas_360_R.html
Methods 1 and 2 would actually give you 360º rotation, and the resolution would only go from ~1º/step to ~1.2º/step...not bad.

Ken

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"To invent, you need a good imagination and a pile of junk."
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padu

Nobody to comment on what's behind the RPM gauge on my bike? Can't I use something similar to that?

KMoffett

It looks like a 270º analog meter...but that's a guess. Seems like those are only common outside the USA or in automotive displays.

Ken

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"To invent, you need a good imagination and a pile of junk."
Thomas A. Edison (1847 - 1931)

JimB

Exactly! That analogue meter.

JimB

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Experience is directly proportional to the value of the equipment ruined.

KMoffett

The problem with these are that they have very deep cans behind the meter face. From what I see of skydiving altimeters, they are very flat. Would this be a problem padu?

Ken

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"To invent, you need a good imagination and a pile of junk."
Thomas A. Edison (1847 - 1931)