The PPM period was around 22.5 ms, and the conversion to PWM was trivial: the time of the PWM high state was the time position of the PPM pulse for that servo. The period of 20 ms (50 Hz) comes from the days where the signal was encoded in PPM ( pulse-position modulation) format to be sent over the air. 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. Most RC servos move to 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 central position (neutral position). (This is different from the PWM used, for example, in some DC motor speed control). Modern RC 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 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. For example, in many RC servos a 1.5 ms pulse will make the motor turn to the 90° position (neutral position). The width of the pulse will determine how far the motor turns. The typical RC servo expects to see a pulse every 20 ms, however this can vary within a wide range that differs from servo to servo. This is a form of pulse-width modulation. In modern RC servos the angle of mechanical rotation is determined by the width of an electrical pulse that is applied to the control wire. Different servos will have different constraints on their rotation, but the neutral position is always around 1.5 milliseconds (ms) pulse width. Given the rotation constraints of the servo, neutral is defined to be the center of rotation. The parameters for the pulses are the minimal pulse width, the maximal pulse width, and the repetition rate. Small hobby servos (often called radio control, or RC servos) are connected through a standard three-wire connection: two wires for a DC power supply and one for control, carrying the control pulses. The PWM signal might come from a radio control receiver to the servo or from common microcontrollers such as the Arduino. Servo control is a method of controlling many types of RC/hobbyist servos by sending the servo a PWM ( pulse-width modulation) signal, a series of repeating pulses of variable width where either the width of the pulse (most common modern hobby servos) or the duty cycle of a pulse train (less common today) determines the position to be achieved by the servo. At the end of this function, this analog value is returned.A diagram showing typical PWM timing for a servomotor Then the analog value for the servo is calculated by dividing the pulse wide by 1,000,000 to convert the value to us per second and multiplied with the frequency and 4096 for a 12 bit resolution. First the pulse with is calculated between 0 degree and 180 degrees depending on the minimum and maximum pulse width with the map function. Now we create the function pulseWidth to calculate the analog value for the motor depending on the desired angle. Therefore we do not need a loop function. To find the optimal value for MAX_PULSE_WIDTH, set the angle to 180īecause we only want to parameterize the servo motor, we need this one iteration from the setup function and have to change the value of MIN_PULSE_WIDTH to get the 0 degree position.To find the optimal value for MIN_PULSE_WIDTH, set the angle to 0.Function to specify the angle: pulseWidth(angle) where angle can be between 0 and 180.The numbers are printed on the board starting with 0 up to 15. On which position of the PCA9685 is the servo connected. ![]() In the setup function the PWM object that we created previously is activated with the frequency of 50Hz. Also we control the servo motor with the setPWM function of the pwm object. You find this information on the data sheet of your motor. Nearly all servos use a frequency of 50Hz. ![]() In this step you need to tweak the parameters MIN_PULSE_WIDTH and set the parameter MAX_PULSE_WIDTH to a fixed value. To find the right I2C HEX address you can use the I2C HEX code scanner in this article. Then we create an ServoDriver object called pwm with the corresponding I2C HEX address, that is in my case 0x40. If you do not know how to install a library you find here a tutorial.įirst we include the libraries Wire.h to use the I2C communication and the Adafruit_PWMServoDriver to manage the CA9685 16-Channel Servo Driver board. To control the PCA9685 we use the Adafruit_PWMServoDriver library.
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