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Friday, October 22, 2010

Antenna Deployment Subsystem Design

Back after a long time....

Here is what I designed for the antenna deployment subsystem which happens to be a preliminary stage of Studsat team recruitment in my college....

                Antenna Deployment Subsystem for a Cube Sat

Abstract:
          The core idea of this document is the proposal of an Antenna deployment system for a cube sat. Since my knowledge in the field of antennas is very limited, I have tried to bring up some concepts for the purpose of antenna deployment and you may feel it a bit impractical to realize. I have tried to focus on two major stages for this proposal. The first would be a mere illustration of the various deployment mechanisms in practice and their relevance for my proposal. The next stage will be a detailed illustration for developing the system and the design is as per my perspective of the ecology of the satellite and the launch conditions.
 Summary of the intended plan:
          To start off with the first phase, are several constraints like space, mass, power consumption and reliability. To overcome all these constraints, we have to design the antenna and the related subsystems to have minimum mass and occupy less volume. Also they must be able to withstand the high acceleration of the launch vehicle and the harsh conditions of the space. Now the actuators which cause the deployment of the antennas to its fullest spread must be able to mechanically hold the antenna in spite of the high G's experienced during takeoff and also must not consume any energy during this phase as some launch specifications require complete electrical shutdown.
            One type of actuator is the magnetic actuator which is a very simple concept and easy to implement. It consists of a permanent magnet holding the antenna strips during launch owing to zero electricity requirements and as soon as the satellite is thrown off the payload capsule, an electric current surges which in turn generates the required magnetic field to oppose that of the permanent magnet. Thus there would be an easy deployment of the antennas which comply with all the pre-launch requirements. This is also pretty good method which provides the feature of reusability during testing phase. Coming to the second type of actuator, there is the one-time-use melting wire actuator. This happens to be much more light compared to the magnetic actuator. Tests have proved that materials like nichrome which in its wire form of approximately 2mm diameter and 4mm length can melt down and break when voltages as low as 4V is provided with a current as less as 0.9A. In this type, a nylon wire holds up the antenna in a stowed position. The Nichrome coil is tightly wound across the nylon wire and as soon as the satellite is launched into space, a high power is delivered to the coil to melt down the wire and cut the nylon wire so that the antenna can resume its operations. The antennas would swing back to its actual position once the wire is cut due to the elastic inertial forces.
            Now moving onto the second phase, I would wish to express my design for the deployment system. Now when compared to the above two methods of antenna deployment, the melting wire actuator seems to be more suitable for the reason that it takes up less mass and volume and the intended target can be achieved at a lower electrical effort. For the design of this subsystem, I would wish to take into consideration the following factors. The first being the power consumption, the second being the size of the actuating circuit and the last being the method of actuation and initiation for other subsystems to start their functionalities as the antenna has been deployed.
            Let me assume that there is a separate subsystem control circuit allocated for just the antenna deployment. Taking power consumption into consideration, this subsystem needs the power only after the ejection of the satellite from the payload capsule. The main source of power is the energy harvested from the onboard solar cells and this might fetch up to 30mW/cm2 and hence a low power device would be suitable for this purpose. A microcontroller like MSP430 from TI would be optimal as it has higher performance, lower space (14 Pin SMD devices – TSSOP being the smallest) and suitable for small applications. This particular microcontroller works at 3.3V logic and can operate in voltages as low as 1.8V. The sleep mode current consumed by it is as low as 40nA and this would be optimal for the entire satellite's power requirements because MSP430 in my design would be inactive for the rest of the time after deployment and it is essential to ensure that it consumes almost no power. I would be using MSP430F2013 version of MSP430 as I have a practical experience in handling it on a debugger provided by TI called EZ430-F2013. Now coming to the design part,
1)                         Since no other system will be online in the satellite, it is up to the MSP430 to deploy the antenna and initiate all other systems. As there is no problem with the power requirements in the perspective of MSP430, the device will be instantly ready. The controller is assumed to derive power directly from a definite array of solar cells which ensures the peak voltage is below a specified limit and since the device internally has a regulator, there is no need for voltage regulation. We would generate a certain delay in the controller using 16bit timer before the controller could actually start its deployment work just to ensure that the solar cells are ready to output maximum power.
2)      After the controller, the next part of the circuit is the relay. I would wish to use a small MOSFET as it reduces the mass of the module. An 8V, 1A MOSFET would be sufficient to serve the purpose as the melting of the nichrome wire requires less than 1A at 5V.
3)      Now, as soon as the controller finishes a certain delay (delay is determined by trial and error method after simulating) it is made to actuate the MOSFET. The controller has one 8bit highly multiplexed port and definitely one pin can drive the gate of the MOSFET which would turn it on. So here I would be using a MOSFET to perform the job of switching or controlling the power flow between the solar cells and the nichrome wire. Since none of the device will be on till the antenna is deployed, we can ensure that maximum power is delivered to the actuator to perform the required melting of the wire.
4)      We shall ensure that the controller will drive the MOSFET for a certain duration which is known during testing and hence ensure that the antennas are successfully deployed. Once this job is done, the MSP430 now has to ensure that all other subsystems of the satellite are turned on.
5)      This process of actuation can be controlled via 7 other pins of MSP430. Another option is to have a common one time operable switch which can be activated by the MSP430 once the antenna has been deployed which in turn act as a chip enable for all other circuits onboard. After the deployment is successful, there is no further work for the deployment system and hence the MSP430 can be put into sleep mode which hardly consumes any power. The MOSFET also doesn't drive any power as the nichrome coil is melted and there is no complete electrical path.
In this way, an effective deployment system can be achieved at low power consumption and also less mass to transport the device.

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Nagaraja