AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

What Are Servos on Airplanes and How Do They Work?

Servos are small, modular actuators developed by the radio control (RC) industry to remotely control or move various parts and surfaces to the desired angular position of the operator. They can even be found in airplanes for moving aircraft wings, boats for their rudders, cars for their steering linkages, or parachutes for their release mechanisms. The servo is a device that can be programmed with different settings to achieve a desired result. They have garnered popularity for their ability to optimize size, speed, and torque while also costing less than other mechanical or electrical components, making them an ideal generic actuator used by aircraft manufacturers across industries.

The servo is responsible for converting electrical signals into physical movement. For example, when you push a button on a transmitter, it sends an electrical signal that tells the servo what physical movement to make, enabling the object to move to the desired angular position. This article offers a high-level overview of servos, including typical applications, standard specifications, and how servos work.

What Is an RC Servo?

Servos are the lifeblood of any remote control system. They take commands from receivers and physically move, using power from a battery or an electrical outlet to better control their movements. Most servos have rotary outputs that can go past 180 degrees in movement, though specialized variations offer linear motion (going back and forth) or can turn 360 degrees or more. The servo can maintain an object's position by monitoring the input and applying power accordingly, making it a vital component for many applications. For instance, if you try to enact force against the servo, it will push back to hold the set position.

Servo connectors are becoming more standardized so that most servos can be used with different transmitters. The vast majority of servos use 3-pin connectors with 0.1"-spaced pins and identical assignments, but some models require a different plug or a different color scheme to work correctly.

Servos are an essential part of any remote control system, and they are typically used as the link between the operator and the object, such as within aircraft, cars, or other machines that need some extra help to function correctly.

The average radio control servo is an all-inclusive machine with many components. The outer casing houses the motor and feedback potentiometer (pot). A high-quality servo's gears are ball raced to reduce friction when the shaft is rotating.

The servo's output shaft is splined and exits at the top, on which you will find the servo arm attached with just one screw. This robotic muscle is connected to the aircraft's control surface through a linkage, and any servo arm movement is directly transferred to the relevant object, making required adjustments possible.

The electrical wiring of a servo is at its base. There are three wires: positive, negative, and signal types, but some RC servos also have four wires. In three wire servos, the central wire is always positive.

How Servo Works

When you actuate the transmitter by moving a stick or flicking a switch on a control, a radio signal is sent to the receiver. Once received, a voltage signal is sent out to the relevant servo. For example, the pulse is usually 50Hz for an analog servo. The amplifier translates the incoming signals and sends them onto a potentiometer (pot) that of which is simply an adjustable resistor. The central rotating shaft of the pot has one gear cog resting on it that connects with another set to drive your servo motor's output shaft–which extends up from the cog atop the pot.

The pot is a variable resistor, meaning its value changes based on how far it has been rotated. The amplifier constantly monitors this voltage fluctuation, comparing new signals with the present ones as they are received. If these voltages do not match up, the pilot most likely has sent a fresh signal via the transmitter. As a result, the amplifier powers the servo motor to move in a particular direction.

This motor movement also rotates the pot, causing it to produce a new signal, which is again monitored by the amplifier. When the motor rotates, so too does its output shaft. This has a linear effect on servo arm movement due to pulse modulation and error correction techniques in use during the operation of this system.

Servo Specifications

Torque – The most crucial consideration to make for any servo is its torque. It is a value calculated in ounces, and larger servo models offer much more torque than their smaller counterparts. At the same time, an expensive servo made of exotic materials can provide high torque while still delivering smooth performance for your application. A standard size servo can provide 40 to 100+ ounce inches of torque, and a giant servo is capable of offering more than 500-ounce inches of torque. On the other hand, a small servo can only provide less than ten-ounce inches of torque. The servo torque can help you determine the type of servo you require for the application.

Speed – Measuring a servo's speed is slightly different from conventional methods for other components. In general, the speed of any rotating object is calculated in rotations per minute. However, in the case of a servo, since it cannot make one complete rotation, speed is measured by considering the time required for the servo to move sixty degrees. Servos can be fast or slow, depending on their type. For example, a typical servo speed is around 0.15 seconds per 60 degrees of movement. A movement speed of less than 0.1 second is relatively quick, and more than 1 second will take longer to move completely through an angle position change cycle.

Weight and Size – The weight of a servo is strongly correlated to its output power and torque. Generally, a high speed or high torque servo will often weigh more. Whereas a giant servo often weighs five ounces or more, small servos feature a weight less than or equal to one half of an ounce. All servos generally have the same shape, and the size depends on the output power and weight. However, the servo can be designed as needed for exceptional cases such as retractable landing gear or for fitting in thin wings.

Servo Applications 

Servos are finding their way into more diverse applications such as robots, and hobbyists have found various new applications. For example, some people can modify traditional RC model vehicles to embed a microprocessor into them to log data or enhance control to make innovative use of these servo motors. Also, they can effectively actuate arms, walking devices, or graspers with a minor modification of the potentiometer. There is no feedback signal being sent by the servo motor to control angle adjustments on its hinges, and this allows for much simpler designs that often offer improved performance.


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