Design Concept of the Power Subsystem of the Standard Bus OPUSAT-KIT For 1U-CubeSat

March 30, 2017

Contributed article by
Dr. Yosuke Nambu, Assistant Professor, Osaka Prefecture University

This article will introduce power system design of OPUSAT-KIT to contribute to further CubeSat development.

OPUSAT-KIT, developed based on the design of the OPUSAT Cubesat (Fig. 1), is a standard bus system for 1U-CubeSat. It has the essential basic functions for CubeSat such as a communication, a power supply, etc. When installed, the transceiver, battery, and electric circuit board make up 2/3 U size (Fig. 2). Users can develop their own CubeSat by installing mission equipment in the remaining 1/3 of the space. OPUSAT-KIT is developed based on OPUSAT (nickname:kosumozu), a 1U size CubeSat developed by Osaka Prefecture University and launched in H-IIA rocket No. 23 in 2014.

Since CubeSat uses many commercial grade parts, the power subsystem requires a function to cope with abnormal levels of radiation. For example, microcomputers used in space are at risk of destruction by bit inverting and overcurrent induced by cosmic radiation. For OPUSAT and OPUSAT-KIT, the former can be controlled by power supply reset and the latter can be controlled by an electronic fuse. The following will focus on the power reset of the microcomputer.

The necessary functions for power reset of the microcomputer are monitoring and power supply interruption. The design will be completely different dependent on timing, criteria, and point of power supply interruption. For OPUSAT-KIT, more resistant device is configured to monitor low devices, this is based on an order of radiation tolerance (watchdog timer > Bus On-Board Computer(BOBC) > Mission On-Board Computer(MOBC)). (Fig.3) Specifically, for BOBC, a pulse signal (“I’m alive”) will be sent to Watchdog timer every second continuously. If the pulse signal cannot be received for 3 seconds, the watchdog timer judges that the microcomputer is abnormal and sends a signal to the FET switch to shut down the total power supply of the satellite. The important point is to shut down the total power supply of the satellite. There is a past case in which a satellite, designed to use a reset pin of microcomputer to reset, didn’t recover from abnormal conditions. Although monitoring of MOBC is similar, it interrupts only power supply to mission system in emergency. The summary is shown below.

1. BOBC is monitored by the watchdog timer and when an error is detected, all power supply will be shutdown.

2. MOBC is monitored by the watchdog timer and when an error is detected, only the power supply to the mission subsystem will be shutdown.

The feature of the power reset of the microcomputer is that it needs to be designed to encompass not only hardware but also software. For example, as the monitoring method is adopted, the software needs to be equipped with a function to send a signal (“I’m alive”) every second continuously. FET switch has a function of power shutdown, however, the order for the shutdown, monitoring and judgement is all done by software. This is a restriction for software design. Even though, the reset method mentioned above is the best method overall.

The optimal design of the power reset of the microcomputer needs ability to comprehend the satellite as a system and definite and practicable hardware and software which have functions to ensure soundness of the satellite operation. This article ends with, and for many this is like preaching to the converted, emphasizing the importance of understanding CubeSat as a system.

OpusatFig 1. OPUSAT

Opusat boardFig 2. Electric Circuit Board of OPUSAT-KIT

Opusat systemFig 3. Schematic Diagram of OPUSAT-KIT Microcomputer Reset Circuit