Sun Sensor Subsystem Requirements and
Functional Description
A lot of blood, sweat and tears went into this subsystem. The successful
sensor employed photocells (photo resistors) that change resistance with
incident light. However, Sarah and I first thought using solar cells would
be a neat idea. We decided to follow the lead of one of the 1997 Kiwi projects,
Odyssey
KiwiSat. The premise was that for two solar cells oriented 120 degrees
apart, the voltage differential is proportional to the sine of the angle
of the incident light. We attempted to use a simple op amp-filled circuit
to read the voltage differential, but there were many obstacles. The biggest
problem was the LM318
buffer. It seemed to reduce input voltage from 2.3 V to 2.0 V, thus defeating
the purpose. We made the difficult decision to abandon the solar cells,
but the attempt lives on in the use of the LM324
op amps as comparators. The figure below demonstrates the layout of the
solar cells as they might have appeared had they actually worked.
Subsystem Requirements
The goal of this assignment is to design and build the sun sensor subsystem
for YODA. These are the requirements for the subsystem:
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The output should in some way represent the sun angle with an accuracy
of 15 degrees. The total field of view for the sensor should be 120 degrees
in one plane.
-
Interface requirements: The sun sensor subsystem must have the following
pinout in a male DB connector:
-
Ground
-
+ 5 V
-
+ 8 V (if needed, otherwise unconnected)
-
Battery unregulated voltage (if needed, otherwise unconnected)
-
- 5 V (if needed, otherwise unconnected)
The pinout of the output signal should be defined by the teams. Be sure
to document your pinout format and what the signal should be for each angle.
The sensor must be readily accessible from the outside (we should be
able to use a flashlight to simulate the sun).
Functional Description
The final approach involved using the photo resistors in a voltage divider
circuit. Each sensor is powered by 5 Volts and connected in series with
a 1k resistor (see Electrical Layout). The
voltage is measured across the resistor with respect to ground and compared
to a reference voltage, Vref, by the LM324
op amp. Below is the qualitative analysis which provided the information
to size the resistor and Vref.
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Qualitative Photo Resistor Analysis
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Light
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Sensor Resistance
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Majority of Voltage Drop
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Off
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High (30k - 100k)
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Photo Resistor
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On
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Low (700 Ohm)
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1k resistor
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Our design rationale was that it would be easier to shine a light on the
sensor than to "turn off" ambient conditions. We chose Vref
to be 2.5 V since it was halfway between the applied +5 V and ground. It
would make sense to have low voltage (0 V) correspond to dark conditions
and high voltage (5 V) correspond to illumination.
When the sensor is not illuminated, its resistance will be high, so
in order to have a low voltage drop across the resistor, the latter's resistance
must be low. Conversely, in the case of illumination, a high voltage drop
will occur across the resistor if its resistance is high. To reduce the
effects of ambient light, we want the sensor to almost always think there
is no light, so a low resistance is desired. However, the resistance cannot
be below approximately 700 Ohm or else no amount of light will trigger
the sensor.
Links to Subsystem Documentation
YODA