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filjoa
Joined: 04 May 2008
Posts: 260
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| Configure ADC with rotation sensor |
 Posted: Fri Jan 13, 2017 1:25 pm |
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Hi
After so long without programming, for professional reasons I'm back ...
But now it is time to take a course, so I come to ask for your help.
I have a positioning sensor "ri360p0-qr24m0-eliu5x2-h1151" and wanted to read with the values on a PIC18F458
As the values vary between 0 and 10V (0º-360º), how should I configure my ADC to print the terminal in degrees?
As the PIC works at 5V, I have to make a resistive divider for values ranging from 0 to 5V (0º-360º), I had thought to use pin A3 with a reference value to facilitate calculations.
Can someone help me figure out how to make these calculations?
| Code: |
#include <18F458.h>
#device adc=10 //adc com 10bits
#fuses HS,NOWDT,NOPROTECT,NOLVP,BROWNOUT,STVREN
#use delay(clock=20000000)
#use rs232(baud=9600,parity=N,xmit=PIN_C6,rcv=PIN_C7,bits=8)
void main(void)
{
int16 readadc, value;
int8 i;
setup_adc_ports(AN0_AN1_VSS_VREF); // colocar referencia em A3 escala 0-1023
setup_adc(ADC_CLOCK_DIV_2);
while (TRUE)
{
printf("Angle: %4lu\r", value);
}
}
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asmboy
Joined: 20 Nov 2007
Posts: 2128
Location: albany ny
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| Posted: Fri Jan 13, 2017 1:43 pm |
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So take stock.
You have a 10 bit DAC and 360 degrees.
So 360/1024=.3516 BEST CASE degrees per DAC count.
Sadly this is a ratio of 2.8444.... which is never going to give you
whole degrees - since 3 counts =1.0547 degrees.
How FAST does the encoder turn in your application?
I see it is rated to 12,000 RPM. thats 200 RPS
Why on earth are you using a BRUTALLY expensive voltage
output encoder and how well do you understand the way
rotation speed, PIC Clock frequency and only 10 (slow) bits +/- - 2 bits
is going to make it hard to know anything about the encoder position with high accuracy unless it only moves at a dead CRAWL.....
Why not use a pulse output encoder and a PIC with fast DE-encoder hardware built in ? |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19529
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| Posted: Fri Jan 13, 2017 1:46 pm |
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Start again.
Use the 4-20mA output. This is better for noise immunity anyway, and allows you to choose the voltage being generated.
Then connect this to a 110R resistor, so you get 2.2v for 20mA.
Use a 2.5v bandgap reference on Vref. Need this for stability.
Then read the ADC. Subtract 180, and divide by 2. Result is in degrees.
How does this work?.
The range from 0 to 360, gives 4 to 20mA. 16mA span.
16mA * 110R = 1.76v
The ADC running off 2.5v gives 2.5/1024v V/step. = 0.00244v
1.76/0.00244 = 720. 720/2 = 360.
The 4mA gives 0.44v. 0.44/0.00244 = 180.
So subtract 180, then divide by 2, and you have degrees.
This is a classic example of designing the hardware to simplify the maths. |
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filjoa
Joined: 04 May 2008
Posts: 260
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| Posted: Fri Jan 13, 2017 4:38 pm |
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hi
thanks for this help...
My head is full of rust on programming
I understand all
best regards |
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