Motors

The  parameters in the selection of the motor:

-- Desired torque 

-- Energy consumption

-- Size

-- Rated current

       

R/C Servomotors

         R/C Servomotors are used in model radio control cars, airplanes and helicopters to control the position of wing flaps and other light control mechanisms within similar devices. A servomotor includes a built-in gear train and is capable of delivering high torques directly. The output shaft of a servo does not rotate freely as do the shafts of DC motors and stepper motors, but rather is made to seek a particular angular position under electronic control.

Main characteristics: Restricted Motion, High Speed, High Torque, Controlled Position.

Advantages: Extremely Powerful for its size, High Accuracy and Repeatability, Low Power Consumption, Fast Response (Actuation)

          The output shaft of a servomotor does not rotate freely, but rather is commanded to move to a particular angular position. Control loop drives the motor to move the shaft to the commanded position. If he position is outside the range of movement of the shaft, or if the resisting torque on the shaft is too great, the rotor will continue trying to attain the commanded position.

           Servo is a classic example of a closed-loop feedback system. The potentiometer is coupled to an output gear. Its resistance is proportional to the position of the servo's output shaft (0' to 50'). This resistance is used by the control electronics to generate an error signal when the desired position isn't the same as the current position. If you send a servo a command to place self at 90' and the head is actually at 45', an error signal will cause the motor to move the head to the gears until the error signal is 0 (when the head has reached 90'). If the head had been 180', an error signal of opposite polarity would have been generated and the motor would have turned in the opposite direction to bring the head 'back' to 90'.

             A servomotor has three wires: Power, Ground, and Control. The power and ground wires are simply connected to a power supply (As with a DC Motor). Most servomotors operate from five volts (4.8V -7.2V). Servos are controlled using a system called Pulse Code Modulation (PCM). The Control lead is used to send the positioning signal. The control signal consists of a series of pulses that indicate the desired position of the shaft. Each pulse represents one position command. The length of a pulse in time corresponds to the angular position. Typical pulse times range from 0.7 to 2.0 milliseconds for the full range of travel of a servo shaft. Most servo shafts have a 180 degree range of rotation. The control pulse must repeat every 20 milliseconds. I Ps is Servo manufacturers usually specify pulse-widths in Ps, so it's hands to be able to convert between Ps and Ms. The servo's electronics work in 20 Ms blocks (50 of them every second). For each 20 Ms block, the servo needs to see a positive-going pulse that period (width in Ms) tells it where to position the head (output shaft).