need help with adc

new to using pbp language

using pic18f452
using port A to do conversion
using strain guage sensor
lcd on port b
keypad on port c

question:
need help writing code to do adc for strain guage sensor
to output weight on the lcd

strain guage conditioned to 0-5v
weight desired 0-300 or 0-400lbs

any help would be greatly appreciated
sorry i don't really know the processes for writing threads yet since i am new
thanks for any help

Code:

---

    DEFINE  OSC 4              ' Set Xtal Frequency
    DEFINE  ADC_BITS 10        ' Set resolution of conversion
    DEFINE  ADC_CLOCK 8        ' Set clock source (x/FOSC or FRC)
    DEFINE  ADC_SAMPLEUS 50    ' Set sampling time (in uS)

    ADFM    var    ADCON1.7    ' ADC result format select bit
    PCFG3  var    ADCON1.3    ' ADC port configuration bit
    PCFG2  var    ADCON1.2    ' ADC port configuration bit
    PCFG1  var    ADCON1.1    ' ADC port configuration bit
    PCFG0  var    ADCON1.0    ' ADC port configuration bit

    PCFG0 = 0                  ' Configure AN0 as analogue input, others digital
    PCFG1 = 1
    PCFG2 = 1
    PCFG3 = 1
    ADFM = 1                    ' Right justified result in ADRESL and ADRESH
                             
    adcin 0,adc_in              ' Place the conversion of channel0 into adc_in

---

For 0-300 lbs output = adc_in * 100 / 341
For 0-400 lbs output = adc_in * 100 / 256

This will give you an output within 1 lb.

thanks a lot

Theese calculations will overflow when adc_in is greater than 655.

For 0-300 lbs output = adc_in * 100 / 341
For 0-400 lbs output = adc_in * 100 / 256

It would be better to use the "*/" operator for these calculations.

For 0-300 lbs output = adc_in */ 75
For 0-400 lbs output = adc_in */ 100

If you want proper rounding of the result you can use.....

For 0-300 lbs output = ((adc_in */ 750)+5)/10
For 0-400 lbs output = ((adc_in */ 1000)+5)/10

Oops, missed that one. I personally would use this:

d = 100
dummy = adc_in * d
adc_value = DIV32 256

Substitute 341 for the 256 is using the 0-300 lbs scale. The adc_value variable will contain the lbs.

thanks a lot guys
but more probelms occured...
after testing the program and testing the voltage received using a multimeter
the actually voltage range is approx 0.44 volts +/- .009 volts and the max load we were able to obtain is approx 4.99 volts +/- .009 volts.

can you please explain to me how the equations so i can understand the analog to digital conversion + plus can if possible, derive a new equation to fix this problem.

here's the source code:

Define LOADER_USED 1
Define LCD_DREG PORTB
Define LCD_DBIT 4
Define LCD_RSREG PORTB
Define LCD_RSBIT 0
Define LCD_EREG PORTB
Define LCD_EBIT 1
DEFINE OSC 4 ' Set Xtal Frequency
DEFINE ADC_BITS 10 ' Set resolution of conversion
DEFINE ADC_CLOCK 8 ' Set clock source (x/FOSC or FRC)
DEFINE ADC_SAMPLEUS 50 ' Set sampling time (in uS)
'---------------------------------------------------------------
' INTCON.7 = 0 ' Enable PORTB pullups
' ADCON1 = 7 ' Make PORTA and PORTE digital
Low PORTB.2 ' LCD R/W low (write)
TRISA = %11111111 ' Set PORTA to all input
ADCON1 = %10000010 ' Set PORTA analog and right justify result
'---------------------------------------------------------------
d VAR word
dummy var word
adc_in var word
W VAR WORD
adc_value var word

ADFM var ADCON1.7 ' ADC result format select bit
PCFG3 var ADCON1.3 ' ADC port configuration bit
PCFG2 var ADCON1.2 ' ADC port configuration bit
PCFG1 var ADCON1.1 ' ADC port configuration bit
PCFG0 var ADCON1.0 ' ADC port configuration bit

PCFG0 = 0 ' Configure AN0 as analogue input, others digital
PCFG1 = 1
PCFG2 = 1
PCFG3 = 1
ADFM = 1 ' Right justified result in ADRESL and ADRESH

loop:
adcin 0,adc_in ' Place the conversion of channel0 into adc_in
d = 100
dummy = adc_in * d
adc_value = DIV32 256
W = adc_value
pause 500
Lcdout $FE, 1
Lcdout $FE, $80, #W," lbs"
pause 500
goto loop

anymore help would be appreciated

thanks a lot

Ok, here's one way to do it.

The general formula would be ....

result = (adc_in - CalLO)*Range/(CalHI-CalLO)

.... where

adc_in = input from ADconverter
Range = Maximum load
CalLO = adc_in when load is zero
CalHI = adc_in when load is "Range"

You could calculate and "hardcode" the last two, but the best solution would be to make a small calibrationroutine that gathers the readings. I'll use 0.44v as zero load and 4.99v at maximum load(300lbs).

Code:

---

Range        CON 300
adc_in        VAR WORD
CalLO        VAR WORD
CalHI        VAR WORD
div_val        VAR WORD
result        VAR WORD
dummy        VAR WORD

CalLO = 90        ' calculated calibrationvalue (0.44/5*1024)
CalHI = 1022        ' calculated calibrationvalue (4.99/5*1024)

div_val = CalHI - CalLO
dummy = (adc_in - CalLO) * Range
result = DIV32 div_val

---

If you want rounding you could do .....

Code:

---

Range        CON 3000
adc_in        VAR WORD
CalLO        VAR WORD
CalHI        VAR WORD
div_val        VAR WORD
result        VAR WORD
dummy        VAR WORD

CalLO = 90        ' calculated calibrationvalue (0.44/5*1024)
CalHI = 1022        ' calculated calibrationvalue (4.99/5*1024)

div_val = CalHI - CalLO
dummy = (adc_in - CalLO) * Range
result = DIV32 div_val
result = (result + 5) / 10        'rounded result

---

/Ingvar

Thanks a lot Ingvar
You are the best
I'll try that code out

You can put the scheme?

scale isn't accurate...

the code is logical and it makes sense, but the scale itself doesn't weight correctly

can it be that i have a faulty sensor?

is there a way to correct this?