SELECTING THE CORRECT SERVO

Servos have a number of defining properties that make them suitable for different applications:

  • Torque is a measurement of the servos strength, how much pull or push it has. Torque is the product of force and the radius at which it acts. Larger planes need higher torque servos to move larger control surfaces. Servo size goes up with rated torque.
  • Dimensions depend on your application servos come in many different sizes.
  • Weight of a servo depends on several things. Recorded in grams, the weight of a servo should always be reported on the package it came in
  • Bearings are used to support the output shaft of a servo, another way to support the output shaft of a servo are brushes. Brushes are the cheaper of the two, but bearings last longer and operate a lot more smoothly. Small and cheap servos tend to have brushes, while the high end and large servos have bearings.
  • Gears from most hobby grade servos are nylon gears, while higher end servos use metal gears. Metal gears are heavier but can’t strip” Metal gears will wear over time, which can cause it to have free play in their rotation, gears can be replaced cheaply. Nylon servos are adequate for most flying. If you’re worried about crashing a model, or flying intense aerobatics, a metal geared servo is most likely the better choice.

    Servos

  • Speed measures how fast the servo can turn from one spot to another. Different models will need servos with different speeds.  For example: a trainer plane doesn’t need to change control surface positions quickly, but a helicopter does. High speed servos are more expensive than standard servos.
  • Digital / Standard To start with, a ‘digital servo’ is the same as a standard servo, except for a microprocessor, which analyses the incoming receiver signals and controls the motor. It is incorrect to  believe that digital servos differ drastically in physical design to standard ones. Digital servos have the same motors, gears and cases as standard servos and they also, most importantly, have a Feedback Potentiometer (Pot) just like their standard counterparts.Where a digital servo differs, is in the way it processes the incoming receiver information, and in turn controls the initial power to the servomotor, reducing the dead band, increasing the resolution and generating tremendous holding power.In a conventional servo at idle, no power is being sent to the servomotor. When a signal is then received for the servo to move, or pressure is applied to the output arm, the servo responds by sending power/voltage to the servomotor. This power, which is in fact the maximum voltage, is pulsed or switched On/Off at a fixed rate of 50 cycles per second, creating small ‘blips’ of power. By increasing the length of each pulse/blip of power, a speed controller effect is created, until full power/voltage is applied to the motor, accelerating the servo arm towards its new position.The downside to these advantages is power consumption. With power being transmitted to the servomotor more frequently, together with increases in power being supplied to the motor earlier, the overall power consumption must go up.
  • All RC kits and ARFs will specify the type and brand of servo required. Generally, you should adhere to these recommendations. When looking for a servo I always select a servo with a torque of 10% higher at the lower voltage required. This information is found in the servos specification sheet.  I found that Tower Hobbies  post the specification list for each of the servos they sell.

 

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