Re: [Paparazzi-devel] IR Sensor Board

[Michel GORRAZ]

Hi,
Two sensors on the same axis are connected "head-to-tail". So, only the temperature (voltage) difference is amplified. The problem is that the output voltage of this configuration (before the amplifier) can be negative.
So a "virtual ground" is implemented (Vcc/2) and becomes the ground of the sensor board (but NOT the autopilot board's ground).
With this trick, the amplifier supply is +Vcc/2 and -Vcc/2 (sensor board ground point of view) but it's outputs got a Vcc/2 offset (autopilot's ground point of view)

Let's see how it works on roll axis with 4µV per °C sensor's sensitivity, amplifier gain x820 and Vcc = 3.3v:

- when the aircraft is level:

• temperature difference between sensor right & left is 0°C, so amplifier input is 0V (sensor board pov)
• amplifier output is 0V (sensor board pov)
• => IR signal is Vcc/2 + 0 = +1.65V (autopilot pov)

- when the aircraft is banking right:

• temperature difference is, for example, (+25°Cearth.right) - (-10°Csky.left) = +35°C
• amplifier input is +35°C x 4µV = +140µV (sensor board ground pov)
• amplifier output is +140µV x 820 = +114.8mV (sensor board ground pov)
• => IR signal is Vcc/2 + (+114.8mV) = +1.7648V (autopilot ground pov)

- when the aircraft is banking left with same angle:

• temperature difference is now (-10°Csky.right) - (+25°Cearth.left) = -35°C
• amplifier input is -35°C x 4µV = -140µV (sensor board ground pov)
• amplifier output is -140µV x 820 = -114.8mV (sensor board ground pov)
• => IR signal is Vcc/2 + (-114.8mV) = +1.5352V (autopilot ground pov)

Hope this will help...

@+
Michel

2008/11/10 James Galea <address@hidden>

Hello Everyone,

I have recently discovered the paparazzi project and was interested in designing an IR sensor board for stability augmentation. I was having a look at the schematics that are currently on the wiki and was a little confused at the configuration. Please correct me if I am wrong but it seems that the current setup is utilizing a non-inverting op amp configuration with a Vcc/2 offset applied to both the V+ and V- inputs of the op amp. What I don't understand is how the output will swing between Vcc and GND about the Vcc/2 point, the reason for asking is that we are dealing with DC signals from our sensors with the offset also being DC, this DC offset will also be amplified therefore saturating op amp by swinging to Vcc if (sensor voltage > Vcc/2) or GND if (sensor voltage < Vcc/2). I did a simulation in PSPICE and achieved the same result. Can someone please explain the configuration as I must be missing something.

 

Any help would be greatly appreciated.

 

Kind Regards

James  

 

 

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