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Command & Data Handling Subsystem  

Subsystem Requirements

The Command and Data Handling subsystem is a critical component of the satellite providing the link between the payloads and the ground station. The CPU will allow us to process the data from the sun sensor and temperature sensor and output that data as a voltage that may be transmitted by the communications subsystem to a ground station. The CPU will also be used to interpret uplink signals sent from the ground station that toggle which sensor is to be read and to trigger the payload.

The Command and Data Handling subsystem will meet the following requirements:

Functional Description

The Command and Data Handling subsystem consists of four basic elements:
  1. One which processes the 8 digital signals from the sun sensor and through a series of CD4066 quad switches returns a single resistance (R1) which is seen by the 555 timer.
  2. The CPU also receives the voltage across the temperature sensor, performs a series of comparisons to reference voltages, and returns a three bit number corresponding to hot (100), ambient (010), or cold (001). This three bit number controls another 4066 switch and again returns a resistance which is seen by a separate 555 timer.
  3. The lights-and-whistles payload is controlled by yet another 4066
  4. The sun and temperature sensor readings are then passed to the communications system where they are transmitted to the ground station.
The C&DH system interprets a signal sent from the ground station to toggle between sending sun or temperature sensor data. The CPU is designed to trigger  operation of the payload when the former receives a pulse from the ground station.
 
 
Measurement
R1 (kOhm)
Frequency (Hz)
Period (msec)
Resistance at Ground Station
Sun Sensor
Bin 1
10
545.45
2
150.6
2
15
458.6
2.2
129.3 
3
22
375.0
2.6
109 
4
33
291.5
3.4
86.6 
5
22+15=37
269.66
3.8
81.7 
6
33+10=43
242.42
4.2
74.1 
7
47
227.13
4.4
70.2 
8
22+33=55
201.68
4.9
63.8 
Temp. Sensor
Hot
47+22=69
168.62
4.8
61.5 
Normal
47+33=80
149.38
5.4
55.7 
Cold
100
123.71
6.6
 
The CPU manages this information with the use of a couple of simple IC's. The 8 signals from the sun sensor are connected to the control pin of two CD4066's. This circuit represents eight possible paths (two chips times four controls) each crossing a different resistance. The inputs for each path are tied to pin #8 of a NE555 timer and the outputs are tied to pin #7. The control signals  from the sun sensor are either high (3.8V) if the sensor is illuminated or low (0V) when shaded; a high signal will close the associated switch and allow the distinctive resistance to be seen by the 555 timer.

The temperature data must be digitized before inputting to the CD4066. This is accomplished by using a series of comparators. Two reference voltages, one slightly above the cold sensor output voltage (Vref1 = 2.906 V) and another slightly higher than the output voltage at ambient conditions (Vref2 = 3.1045V) are used in comparison to the normal sensor voltage, Vtemp. Through a series of logic (7408 AND) gates, a high signal will result corresponding to a hot, ambient or cold reading. These voltages are manipulated in the same manner as in the sun sensor. The Hot, Normal, and Cold voltage signals are fed into the CD4066 control pins where a high signal throws a switch allowing a unique resistance to pass to a second 555 timer (see Note 1).

The pulse generated by the 555 timers are connected to the inputs of a 7432 OR gate (see Note 2) whose output is passed to the Communications subsystem. The CPU toggles between sending sun or temperature data through a data select 4066 switch. The data select is controlled by flip-flop signals (Q and Q_bar) from Comm and the associated inputs of the 4066 are connected to +5V regulated. One output is connected to Vcc for one 555 timer and the second output is connected to Vcc for the other 555 timer. Since the flip-flop provides a Q signal and its inversion, Q_bar, only one 555 timer will have power at a time (see Note 3). Success!

Finally, the payload is controlled by a fifth 4066 switch whose control pins are connected to the inverted pulse from the IR detector. See the Payload section for more details.

Notes:

  1. The fourth control  on this 4066 remains unused and must be grounded.
  2. We had to use a CMOS OR gate to get a clean pulse. It seemed like the TTL chip introduced a lot of noise.
  3. We originally wanted to flip-flop power between the entire subsystems (sun and thermal) but instead flipped power between 555 timers. One problem we noticed was that it seemed like the 4066 could not provide enough power to the sun sensor. That is, the output of the 4066 dropped to around 4.3V even though it should have been +5V regulated. You can check the working version at Amy's and Santiago's page.

Links to Subsystem Documentation

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