Chips in Space: Let’s look inside ARISSat-1 (part 2)

Welcome to Part 2 of our dive into each of ARISSat-1’s subsystems, where I will focus on the solar and battery power systems that are managed by the Power Supply Unit board discussed last week. It’s going to take a total of three parts to cover everything, and next week’s blog post will wrap up the remaining subsystems.

ARISSat-1 has been in operation for two weeks now. Last week the battery failed, causing the satellite to go silent during eclipses. However, once its solar panels are back in the sun, the systems power up and it begins operation. The Mission Elapsed Time (MET) sent in the telemetry is reset to zero each time the satellite falls into eclipse. I explain more about the battery below. All other subsystems are working nominally.

X, Y and Z Axes Labeled

In previous blog posts, I showed you the cross-sectional drawing of ARISSat-1. Above, I am showing you the axes labeled. In the telemetry transmitted down, several of the subsystems are labeled -X, +X, -Y, +Y, -Z and +Z. The axes are often associated with the solar panels, MPPTs and cameras. This way you will know which way is up (and for those that have read Ender’s Game, remember, the enemies’ gate is down).

Solar panel on the - X axis

Solar panels
The six solar panels were donated to the project by NASA. These are space-rated solar panels that were left over after the Small Explorer (SMEX) satellite program ended. Each panel measures 19-inches by 10.5-inches and consists of 50 cells. In full sunlight, each panel can produce about 50 volts open-circuit and more than 19 watts of electrical power. A panel is mounted on each of the six surfaces of the space frame.

Maximum Peak Power Tracker (MPPT)

Maximum Peak Power Trackers (MPPTs)
The six MPPTs are intelligent, SEPIC, switching power supplies that will either boost up the solar-panel voltage (when the sun is low) or buck down the solar-panel voltage (when the sun is high) to the battery voltage of approximately 28 volts. The minimum operating voltage is 15 volts, and the maximum is 100 volts. The peak operating point of the panels is specified as 45 volts. So far, telemetry is showing that the panels are running between 15 and 46 volts. Each MPPT uses a PIC16F690 8-bit MCU as its SMPS controller. The algorithm running in the PIC16F690s quickly shifts the operating point of the SEPIC up and down, to hunt for the peak power point. As the satellite was expected to tumble, the MPPTs must hunt very quickly and then track the fast-moving peak power point. Each panel is expected to have six seconds between sunrise and sunset.

Silver-Zinc (AgZn) type 825M3 battery

Battery
The battery was donated by RSC Energia. This is a type 825M3, and is the same exact type used to power the Russian Orlan space suits. It internally consists of eighteen rechargeable, Silver-Zinc (AgZn) cells and is specified for 14 ampere hours at 28 volts.

The battery has failed much earlier than we expected. This particular battery is not constructed to be a long-life battery. It is only rated for five charge cycles. We knew this going in. Since this was the battery we were given, we did our best to prolong its life through shallow charging. Now the battery is not holding a charge, and thus goes silent and resets all of its circuitry during eclipse. When ARISSat-1 returns to sunlight, the satellite begins operating after the 16-minute safety timers expire. The Mission Elapsed Time (MET) telemetry is reset to zero each time. This can be seen by monitoring the telemetry.