PROMPT - Project for Recording, Observing, and Measuring Parachute Tube

PROMPT Components List

The PROMPT is a remote sensor daughterboard that will be placed at the base of the parachute bay, to record metrics regarding the operation of the parachutes. The data from this module will be sent back to the main sensor board over RS485. Data will be measured at 100Hz from right after launch to 30 seconds after parachutes deploy. The data measured includes:

• The pressure of the parachute bay 
• The temperature inside the parachute bay
• The temperature in the booster
• Whether or not the payload has been deployed

Temperature Sensing:

Since this board needs to monitor 2 remote temperature sensors, choosing a relatively low-cost and compact temperature sensing solution is important. Thermistors are immediately out of the running since the motor's temperature can reach up to 212C (per NFTA 1125), far beyond the range of most NTCs. So, a choice is left between RTDs and Thermocouples. While RTDs would be more accurate/precise, they are more expensive and available in fewer packages than a thermocouple would be. Based on this, thermocouples seem to be the direction the team will go in. Whoever finds this sentence first and sends me a message about it gets a cookie. To compensate for the lower precision, a J-type or similar thermocouple can be utilized instead of the much more common K-type thermocouple.

The temperature sensing inside the parachute bay will be accomplished using a temperature sensor screwed directly into the bulkhead between the avionics and payload bays. For the booster, the temperature sensor will be on a set of breakaway wires through both bulkheads. The most likely approach that will be taken to implement this is to put a barrel jack on the bulkhead of the booster, which is then connected to with a barrel plug. This will provide the greatest chance of a proper pull-out, but may not provide a reliable connection and may introduce errant resistance, skewing measurements. 

Comments/Questions:

• Bad therm0c0up|e - therm0cuples read magnitude 0f v0ltages between 2 different metals which is proportional to the temp. Since it is reading voltage magnitude the wire has to be special, having resistances, even small ones will affect voltage magnitude. This is bad because the breakaway wires will have the same effect, which is ultimately hindering the readings. Mary Dertinger (RIT Student) 
  • Correct. Trying to measure millivolts when your wires/connectors add multiple ohms of resistance will skew your readings. Since RTDs are resistance-based devices measured in the tens of kiloohms, the added resistance of a wire and connector is not that bad.

Pressure Sensing:

The pressure inside a standard parachute bay should be between 8 and 15 PSI when the parachutes deploy. In case that value is incorrect, a significantly overrated sensor was chosen. The selected sensor also has considerations for the attachment of a measuring tube, to allow the sensor to be situated away from the bulkhead, therefore reducing wiring complexity. This measurement tube will be run through the bulkhead and to PROMPT wherever it is mounted.

Comments/Questions:

• Breakout is not in stock
• Chip is in relatively low stock
  • Correct, but not that big of a deal. Pressure sensors intended to be both board-mounted and also connected to a tube are relatively uncommon and not often used. That, paired with the fact that they are physically large, means suppliers rarely stock many of them. The same thing can be seen on a lot of oddball large devices like shaft encoders.
• Expensive chip
• why the weird tube thing jk  
• Sensor should be placed close to the bay, shortest tube possible

Data Reporting:

PROMPT will be connected to the backplane through the standard RS485 interface provided on the sensor board. This interface also provides a 5-volt supply for PROMPT. In the event that an RS485 connection cannot be established, PROMPT will log its data locally.

Comments/Questions:

• Louis Fleisher (RIT Student) correctly ID'ed that there is not really a need for local logging, since if there is no 485 connection that very likely means that there is no power coming to the board either. However, it is a very small cost to add it now which gives us more flexibility down the road. It also opens up the possibility of independent operation in the future. 

Power Systems:

PROMPT is primarily intended to operate off of the regulated 5v provided from the backplane through the sensor board's connection. To allow for PROMPT to be utilized independently, the power system should be rated to allow for the direct connection of a 1S LiPo battery. This will include all the requisite battery protection circuits, such as reverse-polarity protection and over-current protection. Current consumption is expected to be <100mA, closer to 30-50mA. A jellybean priority mux circuit will be implemented to force a selection between the 5v and battery connections. The output of this mux will then go through a boost converter to regulate it to a clean 5 volts. 

Comments/Questions:

• Louis Fleisher (RIT Student) and Mary Dertinger (RIT Student) correctly ID'ed that there is no need to boost to 5 volts, since most devices are 3.3 volts. There is still more to look at in this section though...
• Battery power does not need to be raised to 5V to power RS485 chip because if the board is on battery power then it likely is not able to communicate with Backplane anyways. 
• Use a Linear Regulator (LDO) to lower battery voltage to 3.3V because it will create a low noise rail. 

Human Interface:

2 debug LEDs will show the state of PROMPT at a glance. A CP2102N is integrated on the board to act as a UART-USB bridge, providing both a debugging interface and a means of downloading data from the board if it is being operated independently of the backplane. 

Comments/Questions:

• USB t0 UART thing

Where are  LEDS seLecti0n

Component Selection:

0-60PSI pressure: ABP2LANT060PG2A3XX

• Used by Sparkfun
• i2C interface
• Not on JLCPCB

Comments/Questions:

Thermocouple Amplifier: MAX31855 

• Used by Adafruit
• 3-Wire SPI interface
• Cannot be used with grounded thermocouples
• JLCPCB: C52028

RTD Amplifier: MAX31865

• Used by Adafruit
• Compatible with PT100 RTD Sensor
• SPI Interface 
• JLCPCB: C118474

Comments/Questions:

• Me no like thermocouple. 
• RTD is friend. 

RS485 Transceiver: MAX487

• Slew-rate limited variant of MAX485, commonly used with Arduinos
• UART interface
• JLCPCB: C9953

Comments/Questions:

Microcontroller: 

• Unknown, probably STM32F446RET6

Comments/Questions:

• Pr0bs fine

Memory: W25Q128JVSIQ

• Same as most other devices on the backplane
• JLCPCB: C97521

Comments/Questions:

• a|s0 pr0bs fine what we are using 0n rest 0f backpain

Boost Converter: TPS61023

• Widely used by Adafruit and similar
• Input range from 0.5 to 5.5v
• Output adjustable from 2.2 to 5.5v
• 3.7A max current
• Pass-through mode when VIN > VOUT
• JLCPCB: C919459

Comments/Questions:

• Use with |eanear dr0p d0wn