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View Full Version : ADC0831 -> Scaling readings to drive a servo problems.



Turnbull2112
02-15-2010, 05:21 AM
I have been at this for a couple of weeks, (about to throw it off my balcony!!). I am trying to get this thing running so I can sell some of my machines. Here is what I'm doing. I am reading a voltage from an ADC0831. I want to take the result from the ADC and scale it to drive a servo which usually takes 500 to 1000 to equal the servo's full 180* travel. I actually only need 90* or so but I can't get the thing to go further than 60*. The minimum result from the ADC is "51" and the max is "255" I need this to = 0* of travel through 90* + degrees of travel. HELP!! I am about to give up. Here is my program. I left the scaling part blank. Any help would be greatly appreciated. Again, I am not a student or anything, this is for a work problem I am trying to solve so if the solution is obvious to someone then please tell me what it is. THANKS!

' {$STAMP BS2}
' {$PBASIC 2.5}



CS PIN 0
Clock PIN 1
DataIn PIN 2
SERVO PIN 9

Cnts2Mv CON $139C 'x 19.6 (to mV)

result VAR Byte
mVolts VAR Word


Main:
DO

GOSUB Read_0831
mVolts = result */ Cnts2Mv


result = result */ 11
result = result + 500
PULSOUT 9, result


PAUSE 20
LOOP

'------------[subrtnes]----------------------------------

Read_0831:

LOW CS
SHIFTIN DataIn, Clock, MSBPOST, [result\9]
HIGH CS
RETURN

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Rob Turnbull
Audaci Favet Fortuna

sam_sam_sam
02-15-2010, 05:45 AM
Look in Stamp Works the link is here

http://www.parallax.com/Portals/0/Downloads/docs/books/sw/Web-SW-v2.1.pdf

EX28-ADC0831-Simple.BS2·


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Sam

Turnbull2112
02-15-2010, 05:52 AM
Yea, I got the original base for my program from there. The "result" is the value I'm trying to scale to drive the servo. The voltage measurement part is working fine. Thanks,

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Rob Turnbull
Audaci Favet Fortuna

sam_sam_sam
02-15-2010, 06:28 AM
·I know this feeling· ·(about to throw it off my balcony!!).···· Put down for a few days it will come to you how to fix this problem that you are· having

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Sam

Turnbull2112
02-15-2010, 06:46 AM
HAHAA!! My girlfriend just told me the same thing! It's already been put away. I'll let this post soak for a while and hopefully someone will throw me a bone here. Thanks again.

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Rob Turnbull
Audaci Favet Fortuna

Mike Green
02-15-2010, 06:46 AM
Change 'result' to a WORD value. It's 9 bits (or more) and you're trying to store it in an 8-bit byte.

sam_sam_sam
02-15-2010, 07:07 AM
Did you do EX26 and EX27 this might help as well

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Sam

Turnbull2112
02-15-2010, 08:19 PM
I am starting to make some progress now. I am still unable to get the 90* or even 180* throw that I'm looking for. It seems that I am not getting the "bottom end" 1.0ms pulse and the servo hits the top end of its travel at the max voltage from the ADC. Here is the program. Is my timing off? Should I use HEX with the */? HELP!! Thanks

' {$STAMP BS2}
' {$PBASIC 2.5}

CS PIN 0
Clock PIN 1
DataIn PIN 2
SERVO PIN 9

Cnts2Mv CON $139C 'x 19.6 (to mV)

result VAR Word
mVolts VAR Word

Main:
DO
HIGH 8
GOSUB Read_0831
mVolts = result */ Cnts2Mv

result = result */ 395
result = result + 150
PULSOUT 9, result

PAUSE 20
LOW 8
LOOP

'------------[subrtnes]----------------------------------

Read_0831:

LOW CS
SHIFTIN DataIn, Clock, MSBPOST, [result\9]
HIGH CS
RETURN

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Rob Turnbull
Audaci Favet Fortuna

Mike Green
02-15-2010, 10:06 PM
I suggest you use DEBUG to look at the intermediate results of your calculations like:

mVolts = result */ Cnts2Mv

DEBUG "1st ",result
result = result */ 395
DEBUG "2nd ",result
result = result + 150
DEBUG "3rd ",result

Turnbull2112
02-15-2010, 10:45 PM
I put that in there and the debug terminal spit out a bunch of gibberish. Do you have any idea what the calculation should look like to get the scaling to work? The "result" is reported as 51 through 255 in the debug terminal. What do I put in there to get the full range between 1.0ms to 2.0ms. I keep asking this question and no one has given me any answers related directly to the question. It seems like things don't add up from the stamp projects in the book to when I try to combine different projects. Whats the point of all of this if the only use for these things is to do the book projects?!?!? If there is someone out there on this site that wants to sell a few hundred BS2's then please give me something I can use, otherwise I am going to have to bail. Sorry if I sound grumpy but I am quickly going in that direction.

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Rob Turnbull
Audaci Favet Fortuna

Mike Green
02-16-2010, 12:19 AM
I'm sorry. I forgot the "DEC" before the "result". The DEBUG statement should be like: DEBUG "1st ", DEC result

That's what I get for switching back and forth among several different programming languages.

If you have a "result" range from 51 through 255, you want to transform that to a range of 500 through 1000 for use with the PULSOUT statement and a BS2.

For an approximate fit, try multiplying by 4 and adding 150. That will give you a range of 354 through 1170. (like PULSOUT 9,result*4+150)

For a more exact fit, you'd want: newResult = (oldResult-51) * ((1000-500)/(255-51)) + 500
or: newResult = (oldResult - 51) * 2.45 + 500

This comes out to: PULSOUT 9, ((result-51)*/627)+500

Turnbull2112
02-16-2010, 03:37 AM
MIKE!! You are the man! It is finally working. Thanks for you patience with me in this matter.

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Rob Turnbull
Audaci Favet Fortuna