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The Electronic Speed Controller (ESC)
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 The ESC controls the DC motors rotational velocity. PWM’s Duty Cycle controls and alters the DC motors rotational velocity. The PWM could be generated by the Flight Controller, instead, in order to reduce the Flight Controller’s work load, each of the four ESC’s has a dedicated PIC16F629 microcontroller that generates the PWM. Depending on the ADXL335 accelerometers inclination the flight controllers outputs a HIGH or a LOW to the appropriate ESC. A HIGH will increase the ESC’s PWM Duty Cycle, in turn increasing the DC motor rotational velocity. A LOW will reduce the PWM’s Duty Cycle to its normal setting and reduce the DC motor rotational velocity. Taking into consideration cost and this being the initial project, will be using brushed DC motors and assuming all goes according to plan will later upgrade to Brushless DC motors.
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Pulse Width Modulation (PWM)
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PWM is often used to drive Brushed or Brushless DC motors. The basics are explained as follows.
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Suppose a voltage is applied to a LED via a switch. Suppose the switch is ON then OFF at a two seconds interval. If the switch is ON for only a fraction of the two seconds interval the LED will be at its least bright, if the switch is ON a little longer during the two seconds interval the LED will a little brighter and if the switch is ON for the full two seconds interval the LED will be at its brightest. Th interval is usually referred as the Period and the time that the switch is ON during the period is referred to as the Duty Cycle.
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 Current to the DC motor is being applied for 25% of each period. Assuming the rail volts is 10v the average DC voltage is 2.5v.
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 Current to the DC motor is being applied for 50% of each period. Assuming the rail volts is 10v the average DC voltage is 5.0v.
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 Current to the DC motor is being applied for 75% of each period. Assuming the rail volts is 10v the average DC voltage is 7.5v.
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