Proportional fan control. I suck at math!

[polymer52]

I have a solar collector controller and I need to build a proportional fan controller for it and I need some help with the math. I need it to be proportional from 90 to 120 deg F and off below 90 deg.
I'm using an LM34 temp sensor that outputs 10mv/degree F and I want to use a PIC with a hardware pwm to drive a DC fan. I haven't chosen the pic yet.
Below 90deg F I need a 0% duty cycle (off), at 90deg F (.900 volts at the pic) a 50% duty cycle and at 120deg F and above (1.20 volts at the pic) I need 100% duty cycle..
I know you guys are geniuses so I'm hoping someone can come up with a formula for this.
Thanks.

[rsocor01]

Hi,

If I understand correctly your question, when you get .90V out of the temperature sensor that corresponds to 90deg F and 1.20V corresponds to 120deg F. A very simple linear equation crossing the origin (0,0) would be

Temperature = 100 * Voltage

Use this simple equation for Temperature in the range between 90F and 120F. I hope this helps.

Robert

[polymer52]

Hi,
If I understand correctly your question, when you get .90V out of the temperature sensor that corresponds to 90deg F and 1.20V corresponds to 120deg F. A very simple linear equation crossing the origin (0,0) would be
Temperature = 100 * Voltage
Use this simple equation for Temperature in the range between 90F and 120F. I hope this helps.
Robert

I need the fan to come on at 90 deg and output a PWM signal of 50% at that temp. Then increase the duty cycle of that PWM signal as temperature (voltage) increases to a maximum of 100% at 120deg and above.

[malc-c]

I need the fan to come on at 90 deg and output a PWM signal of 50% at that temp. Then increase the duty cycle of that PWM signal as temperature (voltage) increases to a maximum of 100% at 120deg and above.

So basically, assuming the PWM part has 255 steps, when temp = 90 the PW would = 0, and when its 120 the PW = 255 ?

So that's a 30 degree range represented by a 255 step. 255 / 30 = 8.5

That would mean that for every degree the PW is 8.5

You would then need to write your code so that if temp < 90 PW=0 and if temp >120 then PW =255 to give you your range. I would then use a variable to give you the 30 degree steps you need, something like range value = temp - 90, and then use this for the steps, sorta PW= range value X 8.5

This may not be the ideal way, and I'm sure it could be simplified, but it's the way I would approach the problem. You need to choose a PIC and set the resolution of the PWM you wish to use, the more steps the smoother the fan control will be

EDIT:

just re-read your post, you want the fan to run at 50% at 90 degrees, if so then wouldn't that be a simple case of a statement like
If temp >90 and <120 then PW=127
If temp > 120 then PW=255

Hope this helps

[alec]

Hi Polymer
try this - I used a 12F683
Take care
Alec


'************************************************* ***************
'* Name : fan controller_forum *
'* Author : Alec Noble *
'* Notice : Copyright (c) 2010 Alec Noble *
'* : All Rights Reserved *
'* Date :1/3/2010 *
'* Version : 1.0 *
'* Notes : *
'* : *
'************************************************* ***************
@DEVICE=PIC12F683
define OSC 8
OSCCON = %01110001 ' Internal 8MHz osc
CMCON0 = 7 ' Comparators off
WPU = 0 ' Internal pull-ups = off
'GPIO = %00000000 ' All outputs = 0 on boot
TRISIO = %000000001 ' GPIO.0=data in, GPIO.2=PWMout, GPIO,1,3,4,5 unused

'******************** set up ADC ************************************
ANSEL = %00010001 'gpio.0 analog, others digital, ADC clock fosc/8
DEFINE ADC_BITS 10 ' ADCIN resolution (Bits)
DEFINE ADC_CLOCK 1 ' ADC clock source (Fosc/8)
DEFINE ADC_SAMPLEUS 11 ' ADC sampling time (uSec)
adcon0.7 = 1

'********************* define variables *******************************
sensor var word
temp var word
span var byte
duty var word
'********************** intitialize variables *************************
duty = 0
temp = 0
span = 0

'********************** Program starts here **************************
main:
HPWM 1, duty, 5000 'set PWM output
adcin 0, sensor 'read temperature sensor
sensor = sensor * 500
temp = div32 1023 'convert ADC to degrees
span = temp - 90 'get range above 90 degrees

if temp < 90 then 'less than 90 degrees PWM off
duty = 0
endif

if temp > 90 and temp < 120 then
duty = 130 + (span * 4) 'between 90 and 120 start with 50% and linearly
endif 'raise rate up to 120 degrees

if temp > 120 then 'above 120 degrees 100%
duty = 255
endif

pause 100 'pause to avoind to frequent cycling
goto main 'do it again

[Byte_Butcher]

I think Malcolm had it... it's about 8.5 PWM steps per degree, over a 30 degree span.

So, something like this?

If temp < 90 then 
	duty = 0
else
	duty = (temp - 90)  * 85 /10   'subtract 90 from temp, and multiply * 8.5
endif

steve

[polymer52]

EDIT:
just re-read your post, you want the fan to run at 50% at 90 degrees, if so then wouldn't that be a simple case of a statement like
If temp >90 and <120 then PW=127
If temp > 120 then PW=255
Hope this helps

I think I have settled on the 12F683. It has an 8mhz internal osc, A/D converter and hardware PWM so it doesn't have to be constantly strobed.
Also the 2k words program memory is a nice touch. I can be as sloppy with my code as I want and still have room to spare.

Getting closer..Anything below 90deg fan is off so:
If Temp<90 then HPWM=0
Then at 90 deg fan comes on at 50%:
If Temp=90 then HPWM=127 (50% of 255 leaving 127 remainder). 'Minimum fan speed

Here is the part I don't understand. From 91 to 120 the fan speed needs to vary according to temperature so:
From 91 to 120 each degree needs to widen the HPWM by 4.23 (remaining 127 / 30 deg) speeding up the fan incrementally by temp.

Finally
IF Temp>= 120 then HPWM=255 'Full fan speed at or above 120 deg

The actual PBP command is: HPWM Channel,Dutycycle,Frequency
so at 90 deg on channel 1, 50% duty cycle and 1khz rate the command
would be HPWM 1,127,1000. I can replace the 127 with a variable.

[mackrackit]

I can replace the 127 with a variable.

I f you are asking then yes. You can use a variable.

[Byte_Butcher]

Ahh, that's right, the minimum on speed is 50%... (duty = 128)

So then you've got to increase duty by about 4.2 for every degree between 90 and 120 ...

How about something along these lines:

If temp < 90 then duty = 0

If temp > 120 then duty = 255

If temp > 89 AND temp < 120 then duty = 128 + ((temp - 90) * 42 / 10)

steve