What is ACDS?
This subsystem is responsible for controlling (Attitude Control System, ACS) and determining (Attitude Determination System, ADS) the orientation of our satellite. Given that we need our LEDs to face Earth in order to be seen, we need to be able to control the direction that they are facing while on orbit.
Goals of EquiSat’s ACDS:
- Ability to keep one side of our satellite pointed at Earth
- No power consumption
- No need for complex algorithms
- Prevent tumbling and excessive rotation
There are two types of ACDS, active and passive. An active system determines its current orientation based on calculations from sensor data, then feeds that into a controller which moves the satellite towards the correct orientation. This will result in feedback from the sensors, and the cycle starts over. A passive system has no control and no sensor feedback, the control is “set” beforehand. You can learn more about the different types and different implementations for both of these methods here.
To meet all of these goals, we have selected to use a passive ACS. As a result, our ADS system is ride-along: all of the data it collects will be sent down to analyze, but will not be used to help control our satellite.
Attitude Control System
Our control system has two phases; transient response upon initial satellite deployment and steady state earth-pointing.
In order to keep our satellite reliably pointed at Earth throughout the life of our satellite, we need mechanism that will apply a torque on the satellite slowly throughout its orbit. To do this we are using a permanent magnet, which will keep our satellite aligned with the Earth’s magnetic field.
On the surface of Earth, Earth’s magnetic field lines run parallel to the surface. But as you move away from the surface, particularly at higher latitudes, the magnetic field vector points down towards the surface. Our permanent magnet aligns our satellite with the Earth’s magnetic field throughout our orbit, which keeps us pointed down at the Earth in the northern hemisphere, ensuring that our LEDs can be seen.
Unfortunately, the magnet can only keep our system aligned with a certain precision before the magnetic field from our own magnet would start to adversely affect our electronics. We have tried to find a good compromise between the two in selecting the parameters for this magnet.
When our cubesat is deployed from the PPOD, we will begin spinning slightly due to imperfections in the springs and mechanisms of the PPOD. Additionally, when we release our antenna from their coiled state, they will impart a moment on our satellite which will also cause a slight spin. It is important that we stop this spinning in order to allow both our radio transmission and LED pointing to work appropriately.
To do this, we use two pairs of hysteresis rods. These are long magnetic materials that will be placed perpendicularly to each other, and perpendicularly to the permanent magnet. Similarly to how our permanent magnet interacts with the Earth’s magnetic field, these too will impart a torque on the satellite based on the difference between their orientation and the Earth’s magnetic field vector through them. However, these materials are special in that they actually dampen the response while applying the torque. This effect is called hysteresis, and is generally defined as a response mechanism that depends on the previous state of the mechanism. In our situation, as the satellite is spinning, these rods will apply a damping torque to our satellite that slowly reduces our rotation over the course of a couple of days.
Attitude Determination System
Our ADS system is a ride-along system. Because our satellite has no control over our orientation, there is no need to actually have an ADS system, but we employ one anyway in order to learn more about how well our control system functioned. We will be collecting data from 6 IR sensors, 6 photodiodes, and our magnetometer to determine our orientation. Our on-board processor will not try to calculate our orientation based on this information. Rather, of this sensor data will be collected and stored. Then it will be streamed down to our ground stations by our radio. On the ground we will use this information to calculate our satellite’s orientation.