Hi Michael,
Thank you for your detailed explanation, I
was under the same impression but for some reason was unable to get my
simulation to work. I appreciate your time.
Kind Regards
JAMES GALEA
MRH 90
Software Engineer
AUSTRALIAN AEROSPACE LIMITED
65-75 Pandanus Ave
Brisbane Airport, QLD 4007
Ph: +61 7 3637 3679
Fx: +61 7 3637 3955
Mob: +61 401 089 963
Em: address@hidden
Web: www.ausaero.com.au
From:
address@hidden
[mailto:address@hidden On Behalf Of Michel GORRAZ
Sent: Tuesday, November 11, 2008
7:20 PM
To: address@hidden
Subject: Re: [Paparazzi-devel] IR
Sensor Board
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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