The system design was begun in summer '95 by Paul DeBitetto and Bill Hall with help from Bob Butler and MIT student Long Phan. It took advantage of numerous lessons learned in the implementation of the previous year's helicopter system. Major design changes were:
The navigation system uses an extended Kalman filter designed by Eric Johnson with help from Paul DeBitetto. The filter adjusts the weights placed on the various measurements (GPS, IMU, Altimeter, Compass) according to the situation. For instance, if the altimeter indicates a low altitude, it is given a higher weight than the GPS system's altitude measurement. Likewise, the GPS system's measurements are weighted according to satellite constellation and tracking mode; if the GPS has differential lock, its measurements are given more weight than they are in single point (non-differential) mode. Experience operating the system uncovered some quirks in the GPS unit's operation and more realistic CEP numbers than those reported in the specifications.
The power regulation system was designed and built by MIT student Christian Trott. It uses a combination of switching and linear regulators to regulate the main battery's voltage. Supplying a total of approximately 35 watts from a 16.8 volt NiMH pack, the power board supplies power at 15v, 12v, 5v, -12v, and -15v. To facilitate "live" switching of battery packs, the board has two diode isolated battery connectors. The power system in conjunction with the on board processor also has the capability of monitoring the voltage of the main battery.
The main computer interfaces to the compass and altimeter through a custom parallel interface designed by Christian Trott. The interface is controlled by a Stamp processor which reads and operates the sensors and sends their values over the parallel interface. Christian Trott and MIT student Dmitri Brant designed and built the interface between the Stamp processor and the compass, and Long Phan interfaced the altimeter to the Stamp chip.
The RC receiver uses a Field-Programmable Gate Array (FPGA) designed and built by Bob Butler. The FPGA is interfaced to a signal inside a Futaba FM receiver. It reports the commanded stick positions of the test pilot's transmitter to the onboard computer and accepts servo position commands from the onboard computer through an RS-232 interface.
The receiver interface also implements a primitive "fail-safe" capability. If the received RF signal doesn't meet certain criteria (indicating transmitter malfunctioning, out of range, or off), then control automatically goes to the on-board computer. If the transmitter signal is no good and the commands from the on-board computer don't meet certain criteria (indicating computer malfunctioning), then the servo actuators are commanded to a preset "fail-safe" position (all neutral).
Long Phan built the packaging for many of the subsystems. The housings for much of the electronics are lightweight balsa frames covered with copper foil and screen mesh that provide both shielding and ventilation.
Several vibration-sensitive components are mounted on rubber shock mounts that allow over half an inch of travel and prevent virtually all high-frequency vibration from reaching the components. Paul DeBitetto invented these mounts after failing to find a commercial mount that worked well enough. They were an important part of the helicopter's success.
Alex Lob, owner of Alex's Hobbyworks in Watertown, Massachusetts assembled and tuned the helicopter, in addition to performing the role of safety / test pilot. In many of the early tests, he flew several of the helicopter's controls while the computer ran the others, allowing the team to get one loop working at a time. We definitely couldn't have done it without him!
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