Today we want to introduce you to our GCU. GCU stands for Glider-Mounted Control Unit, which hints toward the placement of the PCB. It is mounted on top of the glider near its nose. The GCU will contain everything needed to measure the health data (temperature, voltage), GPS and transmit RF signals.
It is split into three modules:
- the IMU (Internal Measurement Unit)
- the PTU (Packet Transmission Unit)
- and the PSU (Power Supply Unit)
As the GCU will be fixated on top of the glider, another small PCB on the bottom side of the glider is needed to provide the GCU and therefore the batteries with power during its storage inside of the rocket. These small PCBs will be fixated on the bottom of the glider and will be connected to the GCU via wires through the glider structure. To make sure the small PCB is contacted correctly the gilder is pressed onto the ejection mechanism which has spring contacts protruding from its surface. This way the spring contacts are compressed and won’t loose contact to the small PCB during the vibration of the start. After the glider has been ejected the GCU will be provided with power by the batteries.
The GCU will be controlled by a microcontroller from the Siliconlabs Gecko family. These microcontrollers are optimized for a low power consumption and are therefore the perfect fit for us. The IMU will have two types of sensors. Firstly, a 9-axis-sensor with magnetic field sensors, gyroscopes and accelerometers in one chip and secondly a sun sensor that has been used in a previous REXUS mission. The sun sensor is made up of 6 photodiodes to calculate the gliders position towards the sun. In addition to those two sensors, there will be temperature sensors on the PCB as well.
What you can see in the picture below is the first iteration of the GCU PCB design. By now it has changed quite a bit. For example it now is far more rectangular and the PCB space is used more efficiently.
To make sure we will start the transmission of data only after we have been ejected, a pull up resistor is used to determine if the ejection has taken place. After the ejection has been detected the GCU will enter its flight mode and will start to transmit data using its antennas. The data will then be detected by the antennas on the ground and will then be processed by our Ground Station.
We have now two complete versions of the final iteration of the GCU. We ordered the mulitlayer PCB online but soldered it in our soldering lab. You can see the two PCBs in the oven below as well as one mounted on the glider but without the electronic parts.