On-board hardware consists of sensors, a computer with interface electronics, and battery power distribution. All mechanical control actuation is accomplished with standard RC servos.
The KVH Inc. Digital Gyro Stabilized Sensor System (DGS
) combines
inertial and non-inertial instruments to measure vehicle attitude angles.
A package of two gyros and an inclinometer measure
roll and pitch angles at 
Hz. with 
resolution.
A second package of two gyros and a magnetic flux compass measure heading
at Hz. with resolution. The KVH units weigh
kg. Internal signal processing provides noise-free outputs, but
introduces a 
sec. delay to the measurements.
Helicopter angular rates are measured by three standard RC piezo-electric
rate gyros.
Although the rate gyros have a higher bandwidth than the KVH units, the
outputs are noisy and include a significant drift.
The vehicle's height above ground is measured by a ultrasonic sonar,
which weighs approximately 
grams. The sensor uses a Polaroid
transducer and
custom-built timing/counting circuitry to provide altitude measurements
at 
Hz. with 
in. resolution and a 
in. maximum
range. The measurements are low-pass filtered (
Hz. roll-off) and
differentiated to estimate vertical speed. To avoid a destabilizing
interaction with the roll and pitch control loops, the altitude and
vertical speed estimates are compensated for pitch and roll angles and
rates.
Power is distributed to the sensor suite and flight computer through a
DC-DC converter powered by a 
volt Nickel metal hydride battery.
Servos are powered by a 
volt battery. The two batteries weigh
approximately 
grams.
The signal lines between the computer interface electronics and the altitude sensor are optically isolated for noise reasons. Signals between all other subsystems are delivered either directly or through level-shifters on a common ground. The KVH units interface directly to an RS-422 port on the flight computer and output a stream of ASCII characters encoding pitch, roll, and heading information. An interrupt routine on the flight computer monitors the serial port and converts the data to an integer format for use by the flight control software. The rate gyros interface to A/D ports on the flight computer, after passing through signal conditioning circuitry that consists of a low-pass filter and a range-matching op-amp to match the signal range to the A/D converter voltage range.
The altimeter, RC receiver, and RC servos are connected to an interface
board that is connected to the flight computer's 16-bit parallel port.
The interface board is controlled by a logic circuit built with a
Field-Programmable Gate Array (FPGA) chip.
The interface board and flight computer weigh 
grams.
The six channels from the receiver are interfaced to the FPGA through a voltage level-shifter chip. Channel 6 is reserved to select the control mode: manual or automatic. When channel 6 indicates manual control, the FPGA directly connects the output of each receiver channel to the appropriate servo signal line. When channel 6 indicates automatic control, the FPGA allows the computer to drive the servos through servo interface logic. The signals from the receiver channels are encoded using pulse-width modulation (PWM), a standard RC encoding scheme in which the information is contained in the width of the pulses. An interrupt routine on the flight computer, triggered at the end of each pulse, copies the values of the counters, which measure the pulse widths on each channel, into memory for the use by the flight control software, and then resets the counters.
Signals to the servos are also encoded with PWM. Three of the parallel lines
serve to identify a specific servo and actuate that servo.
In manual control mode, pull-up resistors force these three signals high,
copying the receiver signals directly to the servos.
When these signals are all low, the signals to all of the servos are held low.
Otherwise, the FPGA holds the signal to the identified servo high and the
signal to all other servos low. An interrupt routine on the flight computer
determines the desired pulse width for each servo and writes the servo's
identification number to the three parallel lines for the desired
length of time. After sequencing through all six servos, the flight computer
pauses to maintain constant the rate at which the servos are driven, 
Hz.
FPGA logic also controls the ultrasonic sonar, timing signals that
send the acoustic pulse and enable the sonar's echo-detection circuitry.
The FPGA drives the sonar at Hz. If an echo is received before
the next pulse is sent, the time between sending the pulse and receiving
the echo is latched onto nine of the parallel lines. Additionally,
the timing of the sonar system is used to synchronize the flight control
algorithm. Note that several of the other systems (i.e., KVH sensors,
ground to air communication, and servo commands) run asynchronously.