Beginners Guide Part 3 - Servos and controls

Servos and controls


Servo motors receive signals from the receiver and turn them into work. They are in RC models similar to what the muscles are in your body. The servo is most often used for controlling aerodynamic surfaces that then change the air flow on airplane wing or tail and thus the flight direction of the model itself. By aerodynamic surfaces, we mean the steering and directional control surfaces like ailerons, elevator, rudder, flaps, air brakes, etc.

For models with internal combustion engines, one of the servo should be assigned for throttle control. It is clear that some smaller and lighter servo can be used for this purpose since its strength is not the most important, but it is precision. The servo and the carburetor on the engine are adjusted so that the full throttle on the stick makes the engine reach its maximum speed, while with the stick at the opposite end, holds motor at idle.

Servos can also be used to control the wheels on the model in order to control the direction of movement during taxiing. Usually the same servo is used to run the steering wheel combined with rudder control.
Some other applications for the servo in the flying models are servos for big towing gliders, who have the task of unlocking the glider from the model of the aircraft that pulls them up and raises with them.

There are a large number of servo types in the market, which differ in size, strength, quality and price,but almost all of the servos have the same design. There is a miniature electric motor that triggers a reduction mechanism, which ends with a control arm. There is also a potentiometer or similar electronics that controls the arms range of motion.

The servo connects to the receiver via cable with 3 wires. In the past, various manufacturers have used different connectors but today it is mostly standardized. Beginners in RC modeling usually come up with this question: Can I use one manufacturer's servo with the receiver of another manufacturer? Answer: Today, in 99% of cases, yes. The next question is: Is it good to mix the servos of different manufacturers in the same model? Answer: There is nothing particularly bad about this, you can see in the models of more experienced modelers mixed types of servos. The servo is a servo. Possible problems may arise around energy consumption in larger models with more than 4 servos. Such models are not for beginners, so there is no need to discuss about that now.

Today , servos are mostly  categorized in these three categories: standard, mini and micro.

Standard servos are often supplied with the RC transmitter and receiver. These are robust servos with a typical weight of 40-50 grams and a power (ie. torque) of 3 to 4 kg/cm. Some of the known models are Hitec HS-311, Futaba S3003, etc. Four such servos will suit eg.. a trainer model with full flying weight of about 2 kg, powered by an internal combustion engine of 6.5 cubic centimeters.

For slightly smaller models, typically about one kilogram of flying weight, optimal are "mini" servos, weighing 15-20 grams and have a power of about 2.5kg/cm such as Hitec HS-81.

For small models, so-called park-flyers,which are most often powered by an electric motor,"micro" and "sub-micro" servos are suitable. Their weight is about 10 grams, and the power is 1.2 kg/cm (Hitec HS-55). A few decades ago, these small servos were quite expensive due to the delicate mechanism, but today it is no longer the case. The most popular micro-servos mentioned are Hitec HS-55, HITEC HXT900 and its derivatives (Towerpro TG9, various Turnigy and HobbyKing 9 gram servos ...)

The servo connection with the model itself is fitted with two or four screws and servo tabs. In some cases the screws may be impractical, such as with foam models. It is not uncommon for servo to glue it in model with foam safe glue (hot glue, 5 min epoxy). But - what if the servo breaks down and needs replacement? Servo glued with hot glue is easiest to remove, use a hair dryer (for glue to soften a little bit) around the servo, and just pull it out. Servos does not break often but it does happen. The more servos and other components you have in the model, the higher the chances of something breaks down.

With some models you should also consider motor vibration. These vibrations can, in some cases, lead to improper operation of the servo mechanism or its damage. That is why the servo is mounted with the help of the rubber pads, so the servo body does not directly touch the fuselage of the model and does not receive vibration from it.

Control links

Finally, let's talk about control links and rods. This should be given maximum attention because if the control connection release or fail during flight it will lead to damage or loss of the model.

Generally, control links can be divided into rods and cables. In the case of the cables, usually seen in larger models, two are required on both sides of the servo arms ("closed loop control"). If you use rods, only one is enough.

The control connection between the servo and control surfaces usually consists of two clevises and one lever. If it is a metal rod, then the clevises can be omitted - the rod is bent at the ends in the shape of the letter Z (Z-bend) and such insert in one of the holes of the servo arm.

In the early days of RC modeling, the control rods and clevises were regularly metal and soldered together. One or both clevises  would have a screw and nut for fine tuning of the control link length. For example, the connection could be shortened or extended by some 5 mm.

Nowadays, a large number of modelers, use nylon clevises and carbon rods, which are then combined with some adhesive. Such a control link is not flexible in length but is light and inexpensive. Another way to connect is by using stoppers at the ends of the control rods. They have much greater flexibility to adjust the control link length.
I usually use a combination of 2 methods, on the one hand I use the Z-bend and the other stopper.

Types of Servos

The basic question is what are the proper servos for my model?
What is the difference between them?
Can every servo be connected to any type of receiver?

1. METAL GEAR vs. PLASTIC GEAR servo


The most important difference is that those servos with metal gears are often stronger and can withstand shocks more than plastic ones. Most commonly these servos are marked with "MG" (Metal Gear) in their names.
However, they also occasionally cause harmful interference when connected to certain receivers.
Also, those "MG" servos have an additional problem , some cheap manufacturers cheat by pointing that they are metal geared and very often they put one or two plastic gears between the metal ones, so it is some kind of a mixture, and not full MG servos.


2. BALL BEARING vs. BUSHING servo


Each servo has an output shaft that passes through the servo housing. The servo works much more accurately and easier if it has ball bearing.
If not, over time, the casing will be worn out and some scratches appear on the casing, and that can lead to problems in servo arm movements. It's a recommendation to buy servo motors with bearings. Most commonly these servos are marked with "BB" (Ball bearing) in their names.

3. DIGITAL vs. ANALOG servo


Can I connect a digital servo on my receiver?

Both digital and analog servo can be programmed on each receiver.

So where's the difference?

Similarities:
Both digital and analog servo motors have majority of components same. This means both of them can have the same electric motor, same gears and the same potentiometer. There are no differences there.

Differences:
What is different is how to process the incoming signal (servo move commands).
The analog servo motor has a specially designed motor control chip and the digital servo has a microprocessor and control amplifier.
When the signal is given, the analog servo sends the motor pulses (50 impulses per second). At digital servo, the microprocessor sends 300 impulses per second to the motor and the motor moves faster.

Both types have its good and bad sides.

Digital servo centers servo arm perfectly, keeps its position and responds faster. Unfortunately, some digital servos will not last longer than analog ones because they constantly "push" the electric power to the servo motor regardless the position of servo arm and its load, so they easier malfunction or burn the motor.
Analog servos are less accurate, more accessible and more durable than digital ones.

Digital servo motors spend much more electricity and if you have several of them in your model, be sure to supply them with enough battery power.

Digital servos require an electronic power switch for good micro-processor performance, as well as cable protection in the form of ferrite rings to reduce interference.
Digital servos give us a full torque at all angles while the analog does not.

4. STANDARD MOTORS vs. CORELESS MOTORS in servos


Common electric motors have a stator and rotor. The rotor is wound to the metal core. Because of this, the rotors are heavy and inert. The rotor is secured on both sides.
Coreless motors are designed on the same principles as common ones, but they are assembled differently. The rotor is lightweight. The threads are made in a metal-free cylinder and are attached only at one end of the rotor.
Because they are much lighter (no metal in the middle) they react much faster on commands from stick, they are less inert, and slow down much faster, also they are more precise, and can generate more torque for the same size of servo as one with common motor.

HIGH SPEED vs. HIGH TORQUE servos

For most of the people, faster and more powerful servo is better. In fact, high-speed servo motors are as good as one can react quickly.
Large torque is important for larger models or models with large control surfaces where large forces (such as 3D models) are created.
The speed of the servomotor is expressed in seconds to move servo arm for 60 degrees. There are servos of 0.05s - 0.2s and maybe more for 60 degrees of movement. Here it should be noted that not all manufacturers give exactly at what voltage on the servo is claimed speed. It is not the same whether it is 0.12s for 4.8V or for 6V voltage.

Almost all servo sizes are roughly the same speed as they are loaded, while their power is different. Therefore, servos with lower power are slower and servos with higher power are faster at the same load.

Also, the torque depends on the voltage supplied. The motor is more powerful at higher voltages.
The torque is measured in kg/cm or oz/in . The ratio is as follows: 1 kg = 35.27 ounces, 1 inch = 25.4 mm.

How to measure the torque of the servo?

The servo must be fixed and 1 cm long servo arm attached to shaft . Then you can check how much weight this servo can hold without bending or breaking the gears. So if it can withstand 2.63 kg then it's a 2.63 kg/cm servo or 36.1 oz/in.

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