Download A Small Semi-Autonomous Rotary-Wing

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capable of driving a motor. The 217D from AstroFlight, Inc. was used for the
quadrotor [51]. This ESC is rated for a continuous current of 40 Amps, weighs one
ounce, and costs about $20. This versatile ESC can be powered by battery packs
that have 6–16 nickel cadmium cells or 2–5 lithium polymer cells. These controllers
have three sets of connections that are shown in figure 3.7. One is to receive the
input signal, one is for a battery to drive the motor, and one is to output the
voltage specified by the input signal to the motor. The connection from the input
signal is the same as a servo connector and has three pins: a positive voltage, a
reference ground, and the signal. The connections with the motor and the battery
to drive the motor both have positive and negative terminals.
Rechargeable
Battery Pack
Servo Signal
Electric
Speed
Controller
20 ms
Motor
Figure 3.7. A diagram of connections to the ESC.
The voltage supplied to the motor is varied by turning the motor on and off
very quickly at a frequency of 2.8 kHz (a frequency faster than the time constant
of the motor helps keep the motor speed constant). This is the same technique
used by the microcontroller’s PWM channels. For example, for the motor to run