The FrSky Xact series coreless servos utilize all CNC machined aluminum protective case and metal gears, these servos are 3.7V to 8.4V wide voltage capable and offer high speed and configurable torque performance. Also, the Xact series servos offer multiple form factors and configurations to fit for different model types and precision usage.

Working with the F.Port 2.0 protocol, the telemetry data (like real-time current and voltage) of Xact servos can be achieved and transmitted back to the radio by connecting either F.Port 2.0 capable receiver or flight controller. All the preset servo options of speed and torque can be configured directly on an ACCESS radio through the host device of F.Port 2.0 line, and the configurable servo steering as well as travel degree are additions now can be expected on this series.



Selecting a motor for servo usually needs checking the move profile as well as the torque requirements. This gets done to be able to know the mechanical demands of the system like acceleration and maximum velocity. The moment you choose the motor, the next thing would be to choose the drive.

Choosing servo drivers may appear like just matching current and voltage output to the requirements of the motor. However, different factors should be considered if the driver must work well within the whole servo system.

Do you need a driver?

Note that servos come with their in-built drivers. What the servo needs are to have as much position signal and power as needed. Note that the servo drive or amplifier is needed to power the servo motor. This drive is a vital component that holds the ace to the performance of the servo system.

When a servo gets compared to the straight power amplifiers, they offer different benefits such s better speed, positioning, and control.

The role of drivers in a servo

Servo drives can get controlled using digital and analog inputs. The function of the servo driver is to translate low power command signals from the controller into high current and voltage. Depending on the intended use, the servo drive can assist in coordinating and regulating the desired speed, position, and torque of the motor.

Usually, servo motors are known to be able to account for the expected errors via a device which gets uses negative feedback to relay signals via its control loop. With motion control, what the feedback device does is to evaluate the connection between the actual position of the mechanism and the control input.

When you comprehend the connection between the wanted value and the actual value of the shaft's position, the drive will relay a signal to the drive for action within the motor.

Servo system diagnostics

For you to better understand how servo drives truly function, you need to understand their role as it applies to servo systems. Note that a servo system relies heavily on four main components which are: motor, controller, Servo drive, and feedback device.

  • Motion applications

Currently, servos are used in automation, robotics, and processing manufacturing semiconductors.

Several motion control applications deploy analog servo drives which are genuine technology. Even though they may not possess as many whistles and bells as the newer drives, analog drives often do not need a processing time and are often less expensive than the digital ones. Analog servos get configured manually need a central controller in multi-axis applications.

In certain multi-axis applications, servo controllers can communicate with the digital drives with the help of network commands. Also, digital drives give an improved performance and configuration capabilities than what analog and pneumatic devices offer. Given the added benefits of the devices, conventional servo drives give improved diagnostic benefits.

Several digital servo drives come with even more advanced features that can help them store up motion indexes as well as sequences in their memory. This gets done without the need for a dedicated controller.

Currently, most sectors need Fieldbus networks which have become the standard for motion control. With the help of Ethernet networks, manufacturers are now able to operate in time-sensitive applications. Even though each network comes with its benefits, there are certain products designed for specific Fieldbus networks.

For this reason, you need to get the right product if there is a standardized network protocol for your facility.

What is inside the servo driver?

For you to properly understand how the servo works, you need to know what lies beneath the hood. The servo is made up of the potentiometer, DC motor, and the control circuit. The motor gets attached with gears to the control wheel. With the rotation of the motor, the resistance of the potentiometer will vary. Hence the control circuit will be able to regulate the degree of movement and the direction of the same.

When you notice that the motor shaft is at the desired position, the power supplied to the motor gets stopped. If this doesn't happen, the motor gets turned in the right direction.

Last thoughts

Servo motors do need drivers and you have to understand the working principles of the drivers to optimize your experience. The applications of servos across different sectors make it imperative to keep studying them and making improvements where needed.



Several people would believe that servos are plug and play just like any other electronic device. However, this isn't always the case. Depending on the way the machine generates mechanical energy from DC or AC, the controllability will vary. The improvements in technology in the 21st century have afforded designers more tools to control the motors. This has given rise to devices referred to as motor controllers.

What motor controllers do is allow operators to change the behavior of their motor which in turn gives increased flexibility when using the same machines. In this article, we will explore the servo motor controller and seek to understand the principles behind it.

What are servo motor controllers?

Servo drives, servo motor controllers, and servo motors are terms that are used interchangeably when referring to the same type of motor controller. The systems are made up of components that permit designers to minimize the error that exists between the motor's output and the operator's command. This gets done by factoring in the disparity between the output and input and relaying the same information back to the electronic controller which cuts down the gap until it doesn't exist again.

The feedback is in a closed-loop mainly because it navigates from the output back to the input in a continuous manner in a loop that is separate from other components. Systems that operate in this manner are often known as negative feedback systems. The main components of the majority of servo motor controllers are the motor, the motion control unit, and the power supply.

How do servo controllers work?

Note that the control unit is simply a subassembly that accommodates the user interface, amplifier, and programmable controller. The power supply then links to every one of the components that need power and the motor gets connected to the feedback sensor and helps drive the application. The use of each component is explained here:

  • Command module

This component represents the user interface for this operator as well as the first half of the controller. This enables the user to give directives such as “rotate 300 times in 3 minutes” and any specific needs. The feedback gets compared with the output and becomes the final output.

  • Programmable amplifier and controller

What the programmable controller does is to interpret the given data and convert it into electrical signals which get sent to the motor to carry out certain functions. Also, it considers the feedback which is given by the encoders to lower system error. What the amplifier simply does is to increase the signal so it will be strong enough to move the motor. The different servo motors vary based on their position controllers.

  • Feedback encoders

These are sensors that are placed on the motor's output shaft and help the relay shaft position back to the motor control unit. These in turn help provide the needed feedback to cut down errors. The sensors are often Hall Effect Sensors even though other technologies exist.

  • Compatible motor

One vital aspect of servo motor controllers is that different types of motors function in servo motor regulations. Different DC and AC motors work efficiently and include brushless, brushed, induction, synchronous, and reluctance. Hence, when searching for a servo motor, ensure you search for a system that will give you precision feedback regardless of the motor type used.

What are the types of motor controllers?

The way via which the output of a servo motor output is controlled gets dictated by the way its positional controller relays commands through electrical signals. Usually, the frequency of the current of a servo motor controller gets modified to change the output rotation. This gets done in different ways and depends highly on the nature of the motor used.

However, the two most notable motor control means include pulse-width modulation and proportional, integral, and derivative control.

Last line

There has never been a better time to understand what servo controllers are and the way they work. The immense benefits offered by servo controllers are numerous considering the fields of endeavor wherein they find the application.

The operational principle of servo controllers is not exactly rocket science as they are simple. With this article, you should know how the components of the servo controller such as the compatible motor, feedback controllers, amplifier, and the likes work.



Servo controllers get used for controlling servo motors. When handling servo systems, you need to understand the difference in the definition of a servo system, servo motor, and servo controllers. When choosing a servo system for a given application, you need to ask the supplier what it is they are offering.

What are servo controllers?

Servo controllers represent the core of servo systems. The ideal servo system has feedback, a motor, and a controller. The control circuitry has a motion controller that helps generate the motion profile of the motor. It also has a motor drive that helps supply power to the motor based on the commands from the motion controller.

Note that servo systems are closed-loop systems that have some benefits over the open-loop systems. Some of the benefits include improving transient response times while cutting down steady-state errors and system sensitivity to load parameters.

Servo controllers carry out two core tasks including improving a system's disturbance rejection and tracking some commanded input. One of the methods of control is PID control. This means proportional integral derivative control. PID control stands for a combination of proportional control, derivative control, and integral control. A PID control functions on the error signal which is the distance between an actual value and the real output variable. This drives the error to zero.

The proportional value can get viewed as a simple gain value. What the integral value does is to integrate the error collated over an extended time. When this gets done, it moves the error to zero. The derivative value ensures that a system that uses a proportional and integral term gets stabilized.

Uses of Servo Controllers

The core function of the servo controller is to take in a control and deliver a certain amount of current to a motor. The motor will then spin and the amount that it turns will get viewed using the encoder.

The core function of the servo motor controller is to help close the loop that is on the system by viewing the encoder signal and deploying a torque to the motor to control it. The easiest form of this is to keep a fixed position. In such an instance, if the disturbance leads to the motor to move off from a position, the encoder will detect this positional change and create an error signal. The error signal then gets translated into a commanded current through the controller to drive the motor back to its original position.

A more advanced scenario is when the controller attempts to move the motor to a different position.

What should you consider before getting a servo controller?

  1. Type of motor

The first thing you should know before getting a servo controller is to consider the type of motor you want to control. Is the motor a DC or AC? If it is a DC motor, do you want it brushed or brushless? This information will help you decide on the type of communication which the motor requires and if it can get accommodated by the controller.

  1. Number of axes

Also, how many axes of motion does such an application have? Is it a multiple or single axis of control type? Servo controllers help control certain simple axis applications and for more complicated motions such as in robotics.

  1. Number of channels

The next thing to consider is the number of channels of I/O that you need. Do you need special inputs beyond those for feedback signals such as position and speed? Also, ensure that the controller can accommodate the relevant feedback device like encoder signals, SSI, resolver signals, tachometer, and Hall sensor signals.

  1. Controller set up

One other vital factor which gets overlooked sometimes is the controller setup. Is your controller easy to program and set up? Does the programming get conducted through a keypad or via a computer screen? You also need to factor in the available links for communication. Do you have basic RS232 or RS485 links? Does the controller come with bus interfaces for certain networks such as DeviceNet, CAN, or Ethernet?

Final thoughts

Servo system controllers help maximize the potentials of your servo motor. The mechanism of operation of servo motors is quite easy to comprehend and use. This system has different uses depending on which side of the technological divide you belong to.



Servo motors have existed for a while now and find application in different ventures. They come in a very small size but possess a lot of energy. These outstanding features ensure that they can be used to navigate radio-controlled and remote-controlled robots, cars, and planes. Servo motors are also used in robotics, industries, and food services. But what do servos do?

Note that the servo circuitry is constructed within the motor unit and comes with a positional shaft that has a gear. This motor gets controlled using an electric impulse which dictates the shaft movement.

What does a servo motor do?

Servos find great use in RC airplanes, robots, elevators, and for controlling grippers. Servo motors are quite small and get designed with in-built control circuitry and also have a unique power for their little size.

In the pharmaceutical and food services sector, the tools get designed for use in harsher climes. Here there is a higher possibility of corrosion because of constant washing at high temperatures and pressure to meet best hygiene standards.

What does the servo use to do its work?

For you to better understand the mode of operation of a servo, you have to carefully look beneath the hood. Within it, you will find a rather simple set-up made of a control circuit, DC motor, and potentiometer. The motor is joined using gears to the control wheel. The moment the motor begins to rotate, there will be a change in the resistance of the potentiometer. Hence, the control circuit can determine the amount of movement and in which direction.

Once the motor shaft is placed in the position that is desired, there will be a stop in the power supplied to the motor. If this does not happen, the motor gets returned to the ideal position. The desired position gets sent using electrical impulses via the signal wire.

Note that the speed of the motor is closely related to the difference between the desired position and the actual position. Hence, if the motor is close to the position desired, it will turn, if not it will turn quickly. This situation is proportional control. This refers to a situation where the motor will only run as hard as needed to put in place certain outstanding functions.

How do servos do what they do?

Servos get controlled by relaying electrical impulses of different widths or PWM via a controlled wire. Usually, there is a maximum pulse, minimum pulse, and repetition rate. Servo motors can turn up to 90 degrees in both directions thus giving a maximum of 180 degrees movement. The neutral position of the motor is referred to as the position wherein the servo possesses the same amount of potential rotation in both anti-clockwise and clockwise directions.

Note that the PWM which is sent to the motor will determine the shaft position. The servo motor expects one pulse every 20 milliseconds and the pulse length will tell the maximum distance of the motor. For instance, a 1.5ms pulse will make the turn to the 90 degrees position.

What are the types of servo motors?

There are two distinct types of servo motors the DC and AC. The AC servo can deal with increased current surges and they are useful in industrial machinery. Note that DC servos are not created to handle high current surges and are often best suited for smaller applications. On a general note, DC motors are often less expensive than AC. The servo motors have gotten created to handle continuous movements thus making it easy for the robot to keep moving. They come with two distinct ball bearings located on the output shaft which help give lower friction as well as improved access to the potentiometer.

Servos are three: the positional rotation servo, linear servo, continuous and rotation servo

Final thoughts

You may not need to know the modus operandi of a servo, the more you know, the more access you will have to its potentials. If you are still asking what a servo does, chances are high that you haven't gone through this piece. Regardless of whether you are an engineer or tech enthusiast, knowing the nitty-gritty will put you in a better place.



Servo motors function based on a principle where there is a continuous flow of current to get to a certain position, torque, or velocity. The actual amount of current which will get delivered to the motor gets determined through the servo controller. The basis is on the supplied information from the encoder. We consider it as it deals with the actual condition of the system. The controller then relays the current commands to the servo drive which then provides the motor with the much-needed current to make adjustments to the differences that exist between the command values.

When dealing with accuracy within a linear system, we call the positioning the repeatability and accuracy of the mechanical drive. No matter the level of accuracy of the mechanical system, if the servo does not get the ideal amount of current, it will not produce the right amount of torque. Especially the torque to propel the mechanical system to the position intended.

What is servo gain?

Servo gain involves making adjustments to the gains towards the motion controller direction to cut the response time of the servo system, overshoot, and settling time. The core function of the servo tuning is to cut down the error that exists between the real value attained and the position commanded.

The most prominent control loop type used in servo gain is the PID loop. The P stands for proportional gain, the "I" stands for integral gain while the D represents derivative gain. A gain is a ratio between the input and output and within a servo control loop, the gains play a huge role in determining the way the controller seeks to amend the errors spotted by the feedback mechanism.

PID gains get used in servo tuning to help find out how much the system attempts to correct the errors that exist between the actual position and commanded position.

The proportional gain amount in the control loop plays a huge role in the level of restoring force that should get applied to be able to cancel out the error that exists between the actual value and commanded value. The proportional gain is then multiplied by the error and then creates the contribution to the output for the later period. "Proportional" in this context gets used because the restoring force amount is proportional to the amount of error at any time.

What integral gain does is to propel the system to zero error after the move. The integral gain value grows with time, which is why it has the name. But due to the increase in the integral gain at the close of the move, it may lead to the system oscillating or overshooting. And in a case where the gain is quite low, the system will have a slower time of response. Integral gain gets used in a situation where the system is subject to static torque loads.

Derivative gain is proportional to the change rate of the error. It gets used along with proportional gain to cut down overshooting while giving damping. But, when the derivative gain is too high, it may lead to a reduction in the response time and lead to oscillations.

Feed-forward gains for very dynamic applications

The PID gains illustrated earlier are corrections to the behavior of the system with time. In the case of a very dynamic application, or for improved responsiveness, the feed-forward gain works outside the feedback loop and gives a detailed and proactive approach to the error correction by predicting the needed commands to meet zero error. Feed-forward gains get grouped either as acceleration feed-forward or velocity feed-forward.

Because feed-forward commands function outside of the feedback loop, they often do not lead to instability or oscillations. Velocity feed-forward functions during the constant velocity part of the move profile. What it does is to lower the error and help improve the response time by multiplying the velocity feed-forward gain value with the derivative of the position command.

Last line

Acceleration feed-forward functions during deceleration and acceleration portions of the move and can remove the overshoot caused by the velocity feed-forward. The second derivative of the position command multiplies the acceleration feed-forward by the acceleration feed-forward gain value.



Servo Motors are AC synchronous brushless motors that have an in-built positional feedback mechanism. Servo motors get used for closed-loop motion control systems wherein speed, angular position, and torque can get controlled with accuracy.

The rotor used by a servo motor uses permanent magnets and can get infinitely positioned between the magnetic poles on the stator. It gets done by varying the current and voltage between the windings.

Servo motors are suited for precision to applications wherein accurate control of position, motor speed, and torque is needed.

What are the advantages of Servos?

Servos are deployed in different industries and have a lot of outstanding advantages. Some of the advantages include high efficiency, high output when compared with the size, and increased torque at a constant speed. Also, they have closed-loop control, a quiet mode of operation and they are quite reliable. Furthermore, there is an increased ratio of inertia to torque, high speed, and acceleration. There is also torque control, smooth running, high accuracy, compact size and they can get used for different purposes.

Servo motors find application in different spheres of human endeavor. The advantages outlined indicate that servo motors are finding increased use in industries to replace the regular stepper motors and AC motors and the hydraulic pneumatic systems. There are several applications where servo motors have a unique advantage.

The first is in robotics where there is a need for power, high accuracy, and speed while the size and weight give an advantage. Also, it finds application in machine tools where there is a great need for accuracy. Handling and conveyer systems also need it as they require high accuracy and torque.

Other applications include simulation applications where torque, speed, and smoothness are required. Also, they find application in renewable energy where efficiency and performance are highly needed. The printing press sector is another vital area where speed, accuracy, and reliability are vital.

Uses in demanding and harsh conditions

Servo motors as a result of their unique design have a lot of advantages when applied in certain demanding and harsh conditions. Some of these conditions include:

  • Defense applications where there is a high level of shock loads, as well as temperature ranges, are viewed while performance is needed.
  • They are also used in the beverage and food sector where motors need to cope with temperature variations and washdown.
  • They are also used in oil and gas and subsea applications where there are high temperatures and pressure.

The construction that makes servos unique

Servos are constructed with a closed-loop mechanism that incorporates positional feedback. This gets done to be able to regulate the linear or rotational position and speed. Servos are a combination of four distinct components:

  • Gear reduction unit
  • Normal DC motor
  • Control circuit
  • Potentiometer

What the DC motor does is to help connect with a gear mechanism which in turn gives feedback to position sensors which mostly is the potentiometer.

It gets connected to the central shaft that informs at different times the angle via which the motor's shaft is available. Right from the gearbox, the motor output delivers through the servo spline directly to the servo arm. This gearbox gets formed through gears that may decrease or increase the overall torque and speed.

The average servo motor deploys plastic gear while the power servo motor deploys metal gear. The control circuit gives room for control of the motion of the motor by relaying electric impulses.

The motor has three wires: are red, black, and white wires.

What are the advantages of AC Servo over DC Servo?

AC motors have always retained some level of popularity for decades and have remained so till today. Some of the unique advantages of AC servo over DC servo includes the following:

  • Efficiency

AC motors give maximum efficiency for different applications because of their speed to torque features. It makes it easy to feasible to operate applications with decreased wear, and heat.

  • Durability

AC motors get built to last longer in even complex scenarios. 

  • Quietness

AC motors are generally quiet when operating and the quietness makes them suitable for several applications.

  • Flexibility

One other key feature of the AC motors is that they are flexible which means they can quickly go into motion with just a switch.


The advantages of Servos are numerous, hence, you need to know each one as it applies to you. Understanding the applications will better position the user to leverage the potentials servos offer.


Related Posts

Customer Reviews

FrSky Xact Brushless Series BLS5400H Series
High-end servo technology Review by Wilhelm
I have the FrSky Xact Brushless Servo BLS5402H.
I use it with my TANDEM X20S transmitter in combination with FrSky RB30 and 2 ARCHER RS receivers.
That‘s really high-end servo technology!
Perfect servo design, fast speed with high torque performance!
This servo supports telemetry feedback, current, voltage, and temperature via FBUS mode.
(Posted on 12/17/2021)

FrSky Xact Brushless Series BLS5400H Series
Quality servo Review by Dennis
I have a BLS 5403H to test. The servo oozes quality. It is powerful, smooth, quick and quiet. The case is beautifully made, no slop in the gears and centres beautifully. I am yet to play with telemetry in my X20s. Will definitely buy some more for my bigger models.
(Posted on 11/16/2021)

FrSky Xact Brushless Series BLS5400H Series
Quality servo Review by Dennis
I have a BLS 5403H to test. The servo oozes quality. It is powerful, smooth, quick and quiet. The case is beautifully made, no slop in the gears and centres beautifully. I am yet to play with telemetry in my X20s. Will definitely buy some more for my bigger models.
(Posted on 11/16/2021)

FrSky Xact Brushless Series BLS5400H Series
Awesome value. Review by Romeo
I got two sets from FrSky 5402H x 3 for cyclic with 1x 5401H for tail and 5403H x 3 for cyclic with 5401H for tail I installed in 700 size scale helis. I have only a few flights on them so far but wow I love them and at that price they cost almost half the ones they replaced. They are a perfect match to the X20 or my ISRM X12s using ETHOS. (Posted on 10/1/2021)

FrSky Xact Series Glider Servos H5701 for F5K | D5701 for F3K DLG
Incredible small yet powerful micro servo with great speed with a Hall Sensor very lightweight with precise centering i expect this will last another great servo from FrSky (Posted on 9/3/2021)

FrSky Xact Series Glider Servos H5701 for F5K | D5701 for F3K DLG
I am surprised of the size of this servo!I use it for my DLG's. Highest servo technology in a tiny aluminium case.This servo is one of the best value you can get! (Posted on 9/3/2021)

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