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thumb Servo control from a radio control receiver to the servos is done by sending each servo a PWM (pulse width modulation) signal, a series of repeating pulses of variable width. Small radio control servos are connected through a standard three-wire connection: two wires for a DC power supply and one for control, carrying the pulses. The parameters for the pulses are a minimum pulse width, a maximum pulse width, and a repetition rate. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has exactly the same amount of potential mechanical rotation in the clockwise direction as it does in the counter clockwise direction. It is important to note that different servos will have different constraints on their rotation but they all have a neutral position, and that position is always around 1.5 milliseconds (ms). ==Pulse duration== thumb The angle (of mechanical rotation) is determined by the width of an electrical pulse that is applied to the control wire. This is a form of pulse-width modulation, however servo position is not defined by the PWM duty cycle (i.e., ON vs OFF time) but only by the width of the pulse. The servo expects to see a pulse every 20 ms, however this can vary within a wide range that differs from servo to servo. The width of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral position). The "RC PWM" used in RC servo control acts very differently from the PWM used in any other system. (In particular, the confusingly similar-sounding direct PWM DC motor speed control works entirely differently). Most RC servos move to exactly the same position when they receive a 1.5 ms pulse every 6 ms (a duty cycle of 25%) as when they receive a 1.5 ms pulse every 25 ms (a duty cycle of 6%) -- in both cases, they turn to the center position (neutral position). The low time (and the total period) can vary over a wide range, and vary from one pulse to the next, without any effect on the position of the servo motor. With many RC servos, as long as the refresh rate (how many times per second the pulse is sent, aka the pulse repetition rate) is in a range of 40 Hz to 200 Hz, the exact value of the refresh rate is irrelevant.〔 Ron Lund. ("Servo Frequency and Center Pulse Width Information" ) 〕〔 Bob Blick. ("Servo pulse to PWM converter" ) "The rate at which pulses are sent to the servo is relatively unimportant" 〕〔 (Society of Robots: Servos ) 〕〔 Pololu. ("Servo control interface in detail" ) "servo control signals... the frequency of the pulse train does not affect the servo position if the pulse width stays the same" 〕〔 ("Introduction to Servomotor Programming" ) 〕〔 ("Understanding PWM" ) 〕〔 ("servos" ) 〕〔 4QD-TEC. ("Pulse Width Position Servo" ) 〕〔 ("Servo Control" ) 〕 The period of 20 ms (50 Hz) come from the days where the signal was encoded in PPM (Pulse Position Modulation) format to be sent over the air.〔http://serge.laforest.free.fr/pcm/PCM.htm〕 The PPM period was around 22.5 ms, and the conversion in pwm was trivial : the time of the pwm high state was the time of the PPM Pulse for that servo. Most RC receivers send pulses to the RC servo at some constant frame rate, changing only the high time. However, it is possible to command an RC servo to move over its entire range with a function generator set to a constant 10% duty cycle by changing only the frequency (frame rate).〔 ("Driving the Servo Motor" ) 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Servo control」の詳細全文を読む スポンサード リンク
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