Pic based Wind Speed meter

[Bobbo_ZA]

Hi Bob,

I totally agree, I have been playing with the manual AD routines and find it much more stable than the ADCIN function ,:-), the only question is which 18F to use, currently i have a 16F876A with a LCD connected and would like to connect some temp sensors and perhaps another pressure sensor, so im not 100% sure which 18F chip would be better suited. Any advice?

Kind Regards
Rob

[BobEdge]

Hi Rob,

That was written for a 18F1220 18 pin device. I also use 18F2420 28 pin, and 18F4550 40 pin. It depends which of the internal peripherals you want to use. Microchip have a chip selector tool on their site. You tell it what you need, and it will sugest which chips are suitable. Though it did not work last time I tried, but that was probably due to our totally slow, & unreliable internet at work.

I love how you can get clock speeds of 40 or 48MHZ using 10 or 20MHZ resonator, and using the HS PLL Enabled option. You can pretty much ignore how long calculations etc. take. It's so fast that it usualy makes no difference to your results.

The only things that get in the way are LCD, EEPROM, or serial comms, etc that always take a long time.

Regards
Bob.

[Bobbo_ZA]

Hi All,

Been a slow coding month, although I have been thinking and reading on how to do it, think i have settled on a disk option using a Phototransistor and counting the triggers every 1.5s using something like:

Count PORTB.2,1500,RPM

Depending on the number of gaps in the disk i can then do the maths to calculate the RPM and then tweak it / calibrate it to give me a rough wind speed, :-).

Going to start working on the mechanical parts of it this week and will post pictures and code once its done, :-)

[Bobbo_ZA]

Hi All,

Well, the count didn’t work as expected, seems the count function will only work for >100-150 rpm measurements, :-(. Even an event measured in 1 second, multiplied out the LCD reads it as 60rpm, ;-(

So far my code is as follows:

INCLUDE "DT_INTS-14.bas"     ' Base Interrupt System
INCLUDE "ReEnterPBP.bas"     ' Include if using PBP interrupts
INCLUDE "Elapsed_INT.bas"    ' Elapsed Timer Routines
DEFINE OSC 4 '4 MHz Osc
DEFINE LCD_COMMANDUS 2000
DEFINE ADC_BITS 10
DEFINE ADC_CLOCK 3
DEFINE ADC_SAMPLEUS 10
T1CON = %00000000 '1:1 Prescaler
TMR1H = 0  ;Clear registers
TMR1L = 0

TRISB = %00000001 'Set PortB.0 as input for Hall sensor
ADCON0 = %11100001
ADCON1 = %10000000  'Right Justify, VDD as vref
Hall var word 'VAR for hall sensor
Period var word
P1 var word
W2 var word  

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler    INT_INT,  _HallCount,   PBP,  yes
   ;     INT_Handler   TMR0_INT,  _TimePeriod,   PBP,  yes
        INT_Handler   TMR1_INT,  _ClockCount,   PBP,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
OPTION_REG = OPTION_REG & $80 | 1  ; Set TMR0 Prescaler to 256, leave RBPU alone
@    INT_ENABLE   INT_INT     ; enable external (INT) interrupts
'@    INT_ENABLE  TMR0_INT     ; enable Timer 0 interrupts
@    INT_ENABLE  TMR1_INT     ; Enable Timer 1 Interrupts  
wsave   VAR BYTE    $20     SYSTEM      ' location for W if in bank0
'wsave   VAR BYTE    $70     SYSTEM      ' alternate save location for W 
GOSUB ResetTime              ' Reset Time to  0d-00:00:00.00
GOSUB StartTimer             ' Start the Elapsed Timer
Main:
    IF SecondsChanged = 1 THEN  
       SecondsChanged = 0
       LCDOUT $FE,$C0, DEC Days,"d-",DEC2 Hours,":",DEC2 Minutes,":",DEC2 Seconds
    ENDIF
    LCDOut $FE, $80, "Period = ", DEC P1 DIG 3, DEC P1 DIG 2, dec P1 dig 3
GOTO Main
'---[INT - interrupt]---------------------------------------------------    
HallCount:  
         @ bcf T1CON,TMR1ON ; Switch off timer
        period.lowbyte = TMR1L  'Record Result
        period.highbyte = TMR1H
@ clrf TMR1H  ; Reset registers to zero
@ clrf TMR1L
@ bsf T1CON,TMR1ON  ;Start timer
period = (period / 5) * 2  'Gives period value in microseconds
p1 = period
@ INT_RETURN

I have left in the timer interupt running as it shows that the pic is running for now.

The output to the LCD now changes depending on the speed of the rotations, however I'm not sure if my prescaler is setup right (4mhz osc) and if the calc to get the microseconds is correct for my osc (thanks BobEdge).

[Bobbo_ZA]

One correction to the code, the hall sensor is on RB0 not in PortB.0.

RB0 is pulled high and the hall sensor pulls it low when the magnet passes (would like to still change this around but assume it wont make a difference except to power consumption).

[Bobbo_ZA]

Still stumped with the maths to try get the numbers is giving me into a usable number, anyone that can advise?

ALso, does anyone have any examples of where / how to capture the TMR overflow?

[BobEdge]

Hi Bobbo,

Did you download the PIC timer calculator linked above?
Using this we can see what is happening much more easily.

Lets take a look, you have a 4MHz osc, and the timer uses fosc / 4, and your using tmr0 in 16 bit mode.

So a prescaler of 1:2 gives us a timer clock frequency of 500,000Hz so microseconds value is result x 2 BUT! we can see the interrupt frequency is 7.63Hz. This means that you need at least 7.6 pulses / second otherwise the timer will overflow.

Lets try 1:8 prescaler. Clock freq is 125,000Hz so period = result * 8, so less resolution, but interrupt frequency is 1.9Hz, so you only need at least 2 pulses / second to give a valid result.

You need to decide what to do here, you may want 4 or 8 or even 16 pulses / revolution to give more accurate results over a wider range of RPM.

The timer 0 interrupt you have called _TimePeriod. You need to have a service routine that just tells you that the timer has overflowed, and therefore the wind speed is too low to measure.

Converting microseconds to wind speed will be tricky, since the higher the wind speed the lower your result. Thinking about it, it may be easier to count the number of pulses in the time it takes for the timer to overflow. This would need many pulses / rev, but the relationship between pulses & wind speed would be proportional and easier to scale.
For example : prescaler of 1:16, preload timer with value 3036, this gives interrupt at 1Hz. So when timer interrupt happens, stop timer, scale pulse count, and display windspeed, zero the pulse count, set timer to 3036, start the timer, and return from interrupt. When not in interrupt simply increment the number of pulses in a loop. Forget preloading the timer, 1Hz is not required, since the result will need to be scaled anyway so knowing the exact time is not needed.

Hope this helps.

Bob...