SCRAP: System for Controlling and Recording Airbrake Performance

Summary

SCRAP is the control board for the 2023 IREC Payload. This board will control the experimental airbrake system the payload will be testing, and gather data on its effectiveness. The board will also have considerations for power distribution, arming, voltage regulation, and safeties to provide proper and safe operation of the payload as a whole.

Overview

Sensors & Logging

SCRAP's primary job is to record data pertinent to understanding the performance of the airbrake system. As such, it has a very robust sensor suite. This includes barometric altitude measures to 8", acceleration measured to 0.00024g, the ambient temperature measured to 0.2 °C, and relative humidity measured to 2%. In addition, multiple sensing elements are implemented to better quantify the health and mechanical performance of the payload. This includes a captive-shaft potentiometer for measuring motor movements and a current/voltage sensor (INA219) for determining the draw of the servo. All of this data is logged on an 8Gb flash memory chip that is formatted with a FAT file system, enabling the direct writing of a .csv, .txt, .xslx, or similar human-readable file. This file can then be transferred off of SCRAP to either an external SD card or over USB after landing.

Safeties

It is imperative that the servo does not begin its testing routing before it has exited the rocket. Attempting to open the airbrake while still constrained inside of the sabot could lead to damage to the payload or a non-optimal deployment of the payload and/or the rocket's recovery systems. As such, 4 pre-emptive and 3 reactive safeties are implemented to prevent errant operation and detect errant operation if it does occur. These are as follows:

Gravity Detect: The payload is loaded into the rocket inverted, and after deployment, the streamer should re-orient the payload in such a way that it will be falling "right-side-up". This acceleration due to gravity can be detected and be used as a metric for if the payload is properly oriented
Pull-Wire: Integrated with the passive streamer mechanism, a shunt wire or jumper is ripped out of the payload as it separates from the rocket.
Limit Switches: 4 limit switches on the 4 sides of the payload detect whether or not the payload is still in the sabot. The use of 4 switches located 90 degrees apart will counteract any vibration or flight forces.
Daylight Detector: An IR phototransistor exposed through the outer wall of the payload can be used to measure the intensity of sunlight on the payload, and therefore determine if the payload is inside of something or not.
Servo Power Toggle: To prevent errant operation due to an internal fault or a fault in the PWM control interface, the power to the servo is disconnected until it is determined safe for the servo to move.
Servo Power Monitoring: In addition to being recorded for later analysis, the power consumption of the servo is constantly monitored and if it rises above a predetermined threshold, it is assumed there is an obstruction.
Shaft Position Monitoring: In addition to being recorded for later analysis, the shaft not moving when it is commanded to do so can be interpreted as either a jam/obstruction or as damage to the electromechanical systems.

Power Systems & Arming

All power and arming needs for the payload will be managed through SCRAP. Power systems are divided into 3 main categories, explored in detail below. Of important note is that all power systems have considerations for being bypassed and recharged, an application the team has historically referred to as an umbilical. Every battery will be connected to an MCP73871, which will take a 5v input (intended to be from a USB rail), and use it to both power the system it is attached to as well as charge its battery. When the 5v supply is removed, the system changes over automatically to using the battery. The chip also implements a number of safeties to prevent brownout, voltage sag, battery strain, and over/under temperature issues. There are no plans for this to be implemented as a true umbilical on the rocket, rather simply as a means of having the payload fully integrated and sitting idle on an external supply, so it is always flight ready.

Logical: The logical power system is the primary power system of SCRAP. This power system is passed through an LDO to make a smooth 3.3 volts, which then feeds the microcontroller and all digital peripherals. The voltage and current consumed by the LDO is monitored with an INA219.

High-Power: The high-power system is intended for running high-draw items that are not critical to operation. To assist in monitoring and distributing power on this system, a load switch and INA219 are integrated onto each of the 2 outputs of the high power system. These outputs are also protected by polyfuses. The first output on the high-power system is for the servo, which is a simple direct connection to battery voltage. The second output on the high-power system is a 5v boost converter which can then be used to operate a camera mounted inside the rocket.

BigRedBee: Special care is taken regarding the BigRedBee's (BRB) power systems. Firstly, there are no electrical connections between the BRB and the rest of SCRAP. Any interface between the two is either handled through a photocoupler or a purpose-made magnetic isolation IC. Devices requiring regulated 3.3 volts that connect to the BRB have a dedicated LDO. The charging circuit is also independent from the other two, having its own USB port to allow for isolated grounds. Load switching for the BRB is handled by 2 load switches in parallel to reduce the risk of a failure. These are then controlled by a watchdog that will force the BRB on if there has not been a communication from the microcontroller in more than 1.6 seconds.

For long-term storage and transportation, SCRAP has considerations for screw switches that disconnect the batteries.

Since there is no energetic arming to take care of on this payload, novel arming methods can be explored with a lower risk to operational success. Of note, the team is considering the integration of an ESP32 to allow for WiFi health monitoring, arming, and short-range control of the payload. This serves as an important pathfinder for the team, as it serves to massively reduce integration complexity and may be the only viable option on future rockets that fly faster and therefore mandate a smooth, continuous surface with no holes for arming. At this time, there is no consensus on whether this ESP32 will serve as SCRAP's primary microcontroller, or will simply serve as a peripheral to an STM32 or similar.

Requirements

Measure altitude greater than 20,000 ASL with a >>10Hz refresh rate and low noise
Measure acceleration >>10Hz with low noise between 0 and <2G
Measure other critical factors regarding the payload's health and performance
Control the airbrake deployment/retraction
- The deployment/retraction state should also be measured with some sort of absolute position feedback, not solely relying on the servo's commanded location.
Log data for the duration of the descent with a sizeable factor of safety. The data log must also include timestamps and the positioning of the airbrakes at that time.
A means of moving the data from the internal memory to a computer. The current primary candidate is an SD Card slot
- This SD Card is not intended for in-flight use, only to dump data after the flight.
Handle all power distribution for the payload's other systems (GPS, Camera, etc.)
Provide a means of supplementing or bypassing the internal batteries during integration and potentially at the pad
- The 2023 payload will not have any arming switches accessible from the outside, so having a means of supplying power as the payload sits idle would be ideal.
- The tracker has a nominal input of 3.3v, so a purpose-made mux IC must be used. If that is the case, it may be worth using the same mux and 1S batteries for the rest of the payload (except the motor drive)
(Potentially) transmit live telemetry to the ground station
(Potentially) provide audible or visual aids for recovery and tracking
Add control interface for a camera
- Likely something small/cheap like https://www.adafruit.com/product/3202
- Could also consider a dash cam with a "record-on-startup" mode, that we enable with the flight board, like the Garmin Dash Cam Mini
(Potentially) provide a wireless monitoring and control interface
- If a wireless MCU is used, we can have a web interface for interacting with the payload
- Report battery voltages, turn on/off certain components, etc.

Component Selection

Altimeter: MS5607

Accelerometer: MMA8451

Ambient Temp/Humidity: SHTC3

Memory: W25Q256JV

Battery Monitor: INA219

1 on each battery input

Shaft Encoder: ?

We shouldn't just assume the servo is always at the right spot
A shaft-through-board encoder design can be accommodated if the board is kept small enough to mount parallel to the bottom of the payload.

Consider swapping to a lower voltage, locking cable servo; https://www.adafruit.com/product/4326

Microcontroller: ?

This is a simple and self-contained board, so it may be possible to use this as a way to test a novel MCU, like the RP2040
If we go for an ESP32 or similar, we can use BLE or WiFi arming easily at the pad and/or to get health reports from the payload.
Fallback options like the STM32F0 are well-known by the team
Probably avoid 8-bit solely due to speeds required

The need to effectively bypass 1S LiPo batteries means that a monolithic power switching IC is a better option over a jellybean approach.

Search: https://lcsc.com/products/Power-Distribution-Switches_969.html
The TPS2116 is a very attractive, low-cost option that handles priority and everything for $1
Can implement a GPIO-based switchover circuit with low-cost switches (Link), defining a default state through the use of a NOT gate and a pull resistor.
GPIO switchover also would allow us to change power supplies remotely without having to physically remove/attach the external power cable.

If we are dealing with 1S LiPos, we can integrate battery charging directly into the payload from the external power input

Using an MCP73871 would also handle load change-over automatically

A "pull pin" style arming system will be used to indicate to the payload it has left the rocket's tether. All that is needed for this is a simple IO header on a latching connector.

At the other end, probably will put a shunt XT30 or a 2.54mm Jumper

If power switching the BRB, integrate a watchdog and have it default to open

https://lcsc.com/product-detail/Monitors-Reset-Circuits_MaxLinear-SP706SEN-L-TR_C6860.html

Isolate any interface with the BigRedBee

i2c; https://lcsc.com/product-detail/Digital-Isolators_Analog-Devices-ADUM1250ARZ-RL7_C13839.html
ADUM1251 would also work