Little Black Boxes

Servos are the muscle power in our models. All servos do the same job, they’re sent a signal from the receiver and translate the transmitter stick movement into an equivalent action at their output arm. This motion is used to drive any number of functions, from a control surface to retracting undercarriage.


CONSTRUCTION

Servos generally consist of the following main parts held within their black plastic case.

  • Electronic amplifier. This receives a signal from the on-board radio receiver and translates it into a command to send to the servo motor. The motor will then run, turning the gearbox until the output shaft reaches the position dictated by the position of the relevant control on the transmitter.
  • Motor. A small DC motor that turns in response to a voltage that’s applied to it from the servo’s electronic amplifier. Some high-end servos are starting to appear with small brushless DC motors fitted.
  • Gearbox. This multiplies the relatively small torque available from the motor to a more useful figure. The final output gear connects to the output arm of the servo which moves in a 60-degree arc and allows the connected pushrod to transmit the motion to the item being controlled. 
  • Output shaft / bearing. Small cheap servos often run the output shaft in the plastic material of the servo case. In larger or more powerful servos the output shaft is usually supported in single or twin ball race bearings to help resist the sideways loads that result in transmitting the operating force to the control surface.

ANALOGUE OR DIGITAL?

This question simply refers to the way in which the input signal to the servo is processed. A digital servo will tend to process the input information faster and more often than an analogue servo, which gives several advantages:

1. Increased holding power. Here the servo position data is refreshed more often so the servo will hold position more accurately.

2. Faster response. Again, because the data is processed more quickly the servo output arm will respond to the transmitter commands faster.

3. Smaller deadband. Deadband is where the transmitter stick must be moved a small amount before the servo will respond.

Although these features are desirable, like most things in life nothing is for free. The trade-off comes in the increased power digital servos normally require. 


SERVO SELECTION

Whilst we’ve never had it so good in terms of the sheer number and variety of servos available, this can bring problems in trying to decide which servos to use. For many models simple analogue servos will be more than adequate, however here are a few points to consider:

  • Torque. The amount of torque required to move a control surface is very difficult to establish. What we do know is that larger, faster models with large control surfaces require more torque from a servo than smaller, slower types. Up-rate the servos if there’s any doubt about their ability to cope. It’s not possible to have too much torque but it is possible to have not enough.
  • Size. Whilst a micro servo might have enough torque to operate a control surface it might not be physically tough enough. Fitting a slightly larger servo might add a few grams but the larger gears etc. will mean that the unit is better able to handle the rough and tumble.
  • Servo speed. High speed servos are more suited to specialist applications like 3D type flying where the ability of the servo to move extremely rapidly from one end of its travel to the other can be very useful in certain manoeuvres. High-speed servos are often quite expensive and are only worth the extra cost if the model warrants it. A 0.06s / 60° servo would be wasted on the rudder of a vintage mode. Likewise, a standard speed servo controlling the tail rotor of a 90-powered 3D helicopter is likely to disappoint.

METAL OR NYLON?

Gears that is. Many servos are now available with metal (usually brass) gears. The obvious advantage is that they are less likely than resin or nylon to strip teeth if the output arm is knocked. Note, however:

  • There’s little to choose either way between nylon or metal gears when selecting servos for a standard sport model. As the output torque requirements of a servo increase, however, so metal gears become more desirable. 
  • In the smaller servo sizes the main penalty for choosing metal gears is weight. A typical 9-gram servo might weigh over half as much again.
  • Whilst 9g servos have sufficient torque to drive control surfaces in small i.c. powered models, the extra vibration can be detrimental to nylon gears; the teeth really are very small. 
  • As well as metal (brass) and nylon (resin) gears some servos are available with titanium or Karbonite (a proprietary composite material) gears. 

SERVO SIZE

Servos come in all shapes and sizes. Often these are defined by a weight figure. This weight might be representative of the general physical size of servo under consideration but it can vary considerably. 

Sub micro (5g). Typically these measure 25 x 22 x 12mm and have an output torque of around 0.5 to 0.8kg.cm. They usually come with nylon gears and are suitable for indoor models and very lightweight outdoor aircraft.

Micro (9g). These are around 29 x 23 x 12mm and weigh anything from 8 grams up to around 16 grams depending on the specification. Output torque is between 1.6 and 2.6kg.cm. They can be obtained with metal gears and output shaft bearings and are often used in small to medium electric models, gliders and small i.c. powered designs. At this size metal gears are recommended if the servo is to be used in an i.c. powered model. Nylon gears in this type of servo are small, relatively fragile and may not stand up to the vibration from an i.c. engine.

Mini. Mini servos are around 34 x 29 x 12mm and weigh from 16g up to around 30g. Output torque is between 2.5 to 3.5kg.cm. Again, metal or nylon gears are available. These servos produce as much torque as standard size servos did several years ago and are used in models of all types. Being relatively thin they are particularly well suited to driving ailerons in slim wings. They can also offer a useful weight saving over standard servos.

Standard. So called ‘standard’ servos are around 40 x 40 x 20mm and weigh anything from around 30g to over 60g. Torque output can also vary enormously from around 3.5kg.cm to well over 12kg.cm. Gears are usually nylon at the lower end of the torque range with brass and even titanium gears being available as the torque output increases. There’s usually a servo suitable for most sport models up to and including some quite large airframes with powerful petrol motors.

Specialist. There are many types of servos available for more specialised applications. For indoor models, tiny lightweight units of 2g and less are available, as are ultra-thin high torque digital jobs that are particularly useful in slender moulded glider wings. 


TORQUE AND SPEED

Torque is a measure of a servo’s power and tells us what load it will drive. Usually measured in kilogram.centimetres it is a function of force and distance. The force is the load the servo will drive and the distance is the how far the pivot point is from the load. The concept is best illustrated by a practical example. If we imagine a servo with a torque rating of 1kg.cm this means it will lift a bag of sugar (1kg) by means of an arm 1cm long. If we double the length of the arm to 2cm the available force will halve to 0.5kg.

The speed of a servo is usually defined as the time it takes for the servo to rotate through 60 degrees. Again this varies depending on the specification but will usually range from around 0.25 seconds in a low-cost standard servo down to around 0.06s / 60 degrees in a high spec unit.


BATTERY REQUIREMENTS

Since almost the dawn of proportional radio, four-cell rechargeable batteries with a nominal voltage of 4.8 volts have been used to power the on-board radios and servos. Such batteries are still very popular today and highly suitable for many sport models. However, servos have become more powerful over the years with greater torque output and this, coupled with the larger number of servos often used in a model, means a higher current will be drawn from the battery. 

As has already been mentioned, digital servos use more current than analogue units so where digital servos are being used the battery’s voltage, capacity and ability to supply enough current (without suffering an excessive drop in output voltage) should be carefully considered. Remember that the current for all the servos on board must travel through the switch and switch harness, so make sure this component can cope, too.

Much of the above can also be applied to the Battery Elimination Circuit or BEC which often forms part of the ESC in electric powered models. Unfortunately many budget ESCs tend to over-specify the current that the on-board BEC can supply.

If there’s any doubt about the ability of the on-board power supply being able to cope with the current demanded then expert advice should be sought. If the power supply fails because it has been overloaded then it is highly likely that the model will crash with all the implied safety issues.

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