|deadmanv1||Original design using I2C|
|deadmanv1.1||similar to v1, but uses simpler serial-only communications|
This device is designed to melt through a synthetic support rope using nichrome wire. It will listen for a heartbeat signal (deadman timer reset) from the communications module, received from mission control. If the heartbeat signal stops arriving within a certain timeout period, then the cutter automatically fires. The cutter should also fire in response to direct cut command.
December 12, 2010 - Carl reprogramming new chip with: Cutlength upped to 20s, never repeats. Changing serial to normal TTL, not inverted
The device should be made as small and lightweight as possible.
The device will not bear, nor transfer, any load. A single rope line will be run through the device, and when it cuts, both rope ends must reliably entirely exit from the device upon being cut. If both rope ends do not exit, the device will fail to cut down the balloon.
The device will be loosely mounted to allow the line to shift slightly. Batteries should be secured to device. Auxiliary power input connection, or removeable battery securement (such as cable ties) to replace battery, should be provided for bench and cryo testing the device. Solder connection to battery terminals is fine.
This module will not be mounted in an insulated payload box. All electronics for this flight program should be designed for operation at -55C ambient temperature. For the cutdown heater wire, please create a small insulated chamber only around the heater element to minimize airflow both and weight.
Interface will consist of serial communications via three wires, with logic levels of 3.3v: GND, TX Out from Cutdown, RX In to Cutdown
Serial data is NORMAL, (not inverted) TTL polarity.
For flight SpeedBall-1, the Cutdown module will only communicate directly with another module via standard 3.3v serial lines, inverted data. I2C will be implemented in future versions. The Cutdown module should be designed with firmware upgradability in mind to accomodate these in the future.
The Cutdown Module needs to respond to several ASCII serial commands, followed by a CRLF:
|!CUTDOWNNOW||CUTDOWNNOW<CR><LF>||Activate cutdown system immediately|
|!R||R<CR><LF>||Reset Uplink Dead-Man Timer.|
|!T###||T015<CR><LF>||Set Dead-Man timer interval length to ###, AND reset timer|
Each command is echoed back on serial to confirm command was received by cutdown.
CutDown NOW should immediately activate the cutdown system.
Resetting Dead-Man's Switch should reset the Cutdown Module's programmable interval before cutting.
The timer should be adjustable via software command, and the last setting should be retained through power-cycling by storage in Non-volatile memory.
Timer adjustment will be via software command argument, in the form of a 0-255 value. 0 should be the shortest interval, and 255 the longest. These should roughly approximate actual minutes, so 0x0F would equal ~15 minutes.
The actual timer intervals will be calibrated while in a cryogenic test by the White Star team.
Cutting will be accomplished with a coiled nichrome wire, wrapped around a plastic load rope. It will be energized for 20 seconds to melt the rope.
If no timer reset commands are received via serial before the timer expires, the device will countdown and cut after the expiration of the last T time received. The PIC controller onboard then goes into an infinite loop. The cutdown will no longer respond to commands.
Power will be provided from a dedicated separate battery pack, using Energizer Ultimate Lithium batteries. It will be maintained at -20C+ temperature to allow for sufficient cutdown amperage.
Note: This differs from cutdowns in the past that have used 2CR5 battery packs. This deviation is a result of testing many battery packs at -55C. The 2CR5 packs did not work at -55C, whereas the AA Lithiums did. It is intended that the packs may be left out of insulated areas on future flights.
This is the only module that will be self-powered. This is for safety/redundancy. It should have enough power to run the onboard microcontroller for 96 hours, and still cut down the balloon at the end of that time.