Electronics and Propulsion System in RC Flying - Part 2 - Servo

2. Servo
  • Servos are the end units in a radio control chain.
  • They are used to move the aircraft's control surfaces, the motor throttle (in IC engines) and to actuate other devices such as retractable landing gears etc..
  • There are two types of servos – Digital and Analog Servos.
  • A servo consists of basically a motor, gearbox, feedback potentiometer and an electronic control board inside a plastic case.
  • Outside are the servo arm and the servo cable and plug.
  • The servo arm is often a plastic piece with holes on it for attaching push rods  or other mechanical linkages.
  • There are linear and rotary servos, the most famous are the rotary servos whose arm rotates about 45 degrees left and right from its center point. 
The picture below shows some servo hardware attachments, such as mounting screws,rubber pads, and different sorts of servo arms.

  • RC servos convert electrical commands from the receiver back into movement.
  • A servo simply plugs into a specific receiver channel and is used to move that specific part of the RC model. This movement is proportional meaning that the servo will only move as much as the transmitter stick on your radio is moved.
  • All RC servos have a three wire connector. One wire supplies positive DC voltage – usually 5 to 6 volts. The second wire is for voltage ground, and the third wire is the signal wire.
  • The receiver controls the servo through this wire by means of a simple on/off pulsed signal.
  • Servos basically come in 3 different sizes (micro, standard, and giant or 1/4 scale) to accommodate the type of RC models they are being used in. 
  • Other than physical size, the next item that all RC servo specifications indicate is speed and torque. 
  • Speed is a measurement of the time it takes the servo to rotate a certain number of degrees.
  • This has been standardized in most specifications to 60 degrees; In other words, the time it takes the servo wheel to turn 60°. The smaller the number, the faster the servo is.
  • Torque determines the maximum amount of rotational force the servo can apply.This specification is measured in ounces per inch (oz-in) or in kilograms per centimeter (kg-cm).
  • Both speed and torque specifications are usually given for the two common voltages used for receiver battery packs 4.8 to 6.0 volts .
 Digital Servos vs. Analog Servos
  • First off, there is no physical or main component difference between a digital servo or analog servo.
  • The servo case, motor, gears, and even the feed back potentiometer all have the same functions and operations in both types. 
  • The difference between the two is how the signal from the receiver is processed and how this information is used to send power to the servo motor.
Analog Servo Operation 
  • An analog RC servo controls the speed of the motor by applying on and off voltage signals or pulses to the motor. This voltage is constant (the voltage of the receiver battery pack, voltage regular, or BEC to be exact - 4.8 to 6.0 volts).
  • This on off frequency is standardized to 50 cycles a second. The longer each on pulse is, the faster the motor turns and the more torque it produces. 
  • In analog servo, a short power pulse every 20 milliseconds doesn’t get the motor turning that quickly or allow it enough time to produce much torque.
  • This is the draawback with all analog servos; they don’t react fast or produce much torque when given small movement commands or when external forces are trying to push them off their holding position.
  • This area of slow sluggish response and torque is called deadband.
Digital Servo Operation
  • A small microprocessor inside the servo analyzes the receiver signals and processes these into very high frequency voltage pulses to the servo motor.
  • Instead of 50 pulses per second in analog servo, the motor will now receive upwards of 300 pulses per second.
  • The pulses will be shorter in length of course, but with so many voltage pulses occurring, the motor will speed up much quicker and provide constant torque.
  • The result is a servo that has a much smaller deadband, faster response, quicker and smoother acceleration, and better holding power
  • You can test this very easily by plugging in a digital servo and an analog servo to your receiver.
  • In analog servo try to turn the servo wheel off center, notice how you will be able to move it slightly before the servo starts to respond and resist the force - it feels a bit spongy. 
  • Now do the same thing with the digital servo. It feels like the servo wheel and shaft are glued to the case – it responds that fast and holds that well.
  • Now nothing is perfect and this increase in speed, torque, and holding power does come with a small disadvantage. Power Consumption!
  • Digital servos are power hungry. All those hundreds of power pulses per second use up more battery power than an analog servo . 
Core less & Brush-less Servo Motors 
  • Most low cost and standard servos (analog or digital) use what is called a 3 pole electric motor. This is just a standard 3-pole wire wound DC motor – the most common type of DC motor in existence.
  • One step up from the 3 pole is the 5 pole servo motor. As you can imagine, two more wire winding will give a 5 pole motor quicker acceleration and more torque on start up.
 Core-less Servo Motors: 
  • A standard 3-pole wire wound servo motor uses a steel core with wires wound around the core, this core is then surrounded by permanent magnets.
  • As you can imagine, the core and all that wire weighs a fair bit. When voltage is applied to turn the motor, it has to first overcome this weight to get things turning – it is slow to accelerate. Once up to speed, it also continues to turn for a while when the voltage is removed – it is slow to decelerate.
  • In a Core-less design, the heavy steel core is eliminated by using a wire mesh that spins around the outside of the magnets. This design is much lighter resulting in quicker acceleration and deceleration. The result is smoother operation, more available torque, and faster response time. 
RC Servo Bearings, Metal Gears, & Water Resistance 
Bearings: 
  • You will notice when you do servo shopping, specifications list of many servos shows bearings and the number of bearings – usually 1 or 2.
  • These bearings are used on the main servo output shaft instead of a simple bushing.
  • The advantages of having ball bearings on the output shaft in a servo – less friction and slop. 
Metal Gears & Metal Output Shafts:
  • With today's high torque and high speed digital servos, metal gears are getting more and more common.
  • They are a popular choice for several reasons, but strength is the obvious one. There are two downsides to metal gears however.
  • They weight is little more than plastic or nylon gears and they wear out a little faster. 
Water Resistant/Dust Proof RC Servos 
  • Some servos are sealed to prevent water and dust from seeping inside. The case have gaskets, there is an o-ring around the output shaft, and there is silicone sealant where the wires exit the servo case.
 Classification of Servos
Servos are generally classified as per weight 
  • Giant - weights around 100gr (3.5oz)
  • Standard - 45gr (1.6oz)
  • Mini - 20gr (.70oz)
  • Micro - 8gr (.28oz)
  • Pico - 5.5gr (.18oz)
  • Wes Technik - 2.1gr (.08oz)
  • Falcon Servo - 1.7 gr 
Servo – Manufactures Data

Hobby King S0361 3.6g / .45kg / .12sec
Specifications:
Operating Voltage: 4.8~6.0V
Operating Speed (4.8V): 0.12sec/60°
Operating Speed (6.0V): 0.10sec/60°
Stall Torque (4.8V): 0.30kg.cm
Stall Torque (6.0V): 0.45kg.cm
Idle current: 4mA  Running current: 120mA
Hold current: 350mA   Weight: 3.6g (.13oz)
Connector: Standard JR Style
Type: Analog Micro

Turnigy TGY-50090M Metal Gear 9g Analog
Specs: Weight: 9g
Size: 23.1mmx12.0mmx25.9mm
Torque: 1.6kg.cm (4.8v)2 kg.cm (6.0v)
Speed: 0.08 sec/60deg (4.8v)0.07 sec/deg (6.0v)
Voltage: 4.8-6.0v
Type: Analog Miini
Gear Train: Metal
Ball Bearing: No
Lead Length: 240mm
Plug: JR/Futaba
In the above example of Manufactures data, highlighted details are the specifications you should look and compare.
How to select a Servo for your model.
  1. When you are going to buy servos, carefully go through the manufactures data.
  2. Check whether it is analog or digital servo. (digital means more cost).
  3. The choice is as per your model. If the model you are going to make is an EDF Jet better select digital servos. If it is a basic trainer analog servo will work fine.
  4. Check the working voltage. ( High voltage servos are also avialiable ).
  5. Check the torque and weight as per your model. (Big / high speed models requires high torque servos).
  6. Check the gear train - metal or plastic. ( metal servo cost is more ).
  7. Other specifications such as ball bearing, water resistant, coreless, etc.. are also important, but for beginner models you can compromise on this for saving cost.
  8. Finally select a servo which suits your budget and requirement.