Have you ever thought about how a robotic vehicle commonly used in military application with bomb detention is controlled or how metal cutting and forming machines provide precise motion for milling, lathes and bending for metal fabrication or how an antenna positioning system control the precision in azimuth and elevation?
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As you will learn within this lesson, servo motor applications are most commonly used in closed loop systems where precise position control commonly found in industrial and commercial applications.
Together with the recently RealPars published blog post, what is a Stepper Motorand How it Works, and this lesson, you will learn about motion control using different types of motors available, primarily stepper and servo motors.
In this lesson we will discuss what a servo motor is and how it works, so let’s first determine what a servo motor is and examine some unique features of the types of a servo motor and its application.
Let’s begin, with the servo motor basics. Servo motors are part of a closed-loop system and are comprised of several parts namely a control circuit, servo motor, shaft, potentiometer, drive gears, amplifier and either an encoder or resolver.
A servo motor is a self-contained electrical device, that rotate parts of a machine with high efficiency and with great precision.
The output shaft of this motor can be moved to a particular angle, position and velocity that a regular motor does not have.
The Servo Motor utilizes a regular motor and couples it with a sensor for positional feedback.
The controller is the most important part of the Servo Motor designed and used specifically for this purpose.
The servo motor is a closed-loop mechanism that incorporates positional feedback in order to control the rotational or linear speed and position.
The motor is controlled with an electric signal, either analog or digital, which determines the amount of movement which represents the final command position for the shaft.
A type of encoder serves as a sensor providing speed and position feedback. This circuitry is built right inside the motor housing which usually is fitted with gear system.
Types of Servo Motors are classified into different types based on their application, such as the AC servo motor, and DC servo motor.
There are three main considerations to evaluate servos motors. First based on their current type – AC or DC, and secondly on the type of Commutation used, whether the motor uses brushes and the third type of consideration is the motors rotating field, the rotor, whether the rotation is synchronous or asynchronous.
Let’s discuss the first servo consideration. AC or DC consideration is the most basic classification of a motor based on the type of current it will use.
Looking at it from a performance standpoint, the primary difference between AC and DC motors is in the inherit ability to control speed.
With a DC motor, the speed is directly proportional to the supply voltage with a constant load.
And in an AC motor, speed is determined by the frequency of the applied voltage and the number of magnetic poles.
While both AC and DC motors are used in servo systems, AC motors will withstand higher current and are more commonly used in servo applications such as with robots, in-line manufacturing and other industrial applications where high repetitions and high precision are required.
Brushed or brushless is the next step. A DC Servo Motor is commutated mechanically with brushes, using a commutator, or electronically without brushes.
Brushed motors are generally less expensive and simpler to operate, while brushless designs are more reliable, have higher efficiency, and are less noisy.
A commutator is a rotary electrical switch that periodically reverses the current direction between the rotor and the drive circuit.
It consists of a cylinder composed of multiple metal contact segments on the rotor. Two or more electrical contacts called “brushes” made of a soft conductive material such as carbon press against the commutator, making a sliding contact with segments of the commutator as it rotates.
While the majority of motors used in servo systems are AC brushless designs, brushed permanent magnet motors are sometimes employed as servo motors for their simplicity and low cost.
The most common type of brushed DC motor used in servo applications is the permanent magnet DC motor.
Brushless DC motors replace the physical brushes and commutator with an electronic means of achieving commutation, typically through the use of Hall effect sensors or an encoder.
AC motors are generally brushless, although there are some designs—such as the universal motor, which can run on either AC or DC power, that do have brushes and are mechanically commutated.
And the final classification to consider is whether the servo motor application will use a synchronous or asynchronous rotating field.
While DC motors are generally categorized as brushed or brushless, AC motors are more often differentiated by the speed of their rotating synchronous or asynchronous field.
If we recall from the AC-DC consideration, that in an AC motor, speed is determined by the frequency of the supply voltage and the number of magnetic poles.
This speed is referred to as the synchronous speed. Therefore, in a synchronous motor, the rotor rotates at the same speed as the stator’s rotating magnetic field.
However, in an asynchronous motor, normally referred to as an induction motor, the rotor rotates at a speed slower than the stator’s rotating magnetic field.
However, the speed of an asynchronous motor can be varied utilizing several control methods such as changing the number of poles, and changing the frequency just to name a couple.
The working principles of a DC servo motor are the construction of four major components, a DC motor, a position sensing device, a gear assembly, and control circuit.
The desired speed of the DC motor is based on the voltage applied.
In order to control the motor speed, a potentiometer produces a voltage which is applied as one of the inputs to error amplifier.
In some circuits, a control pulse is used to produce DC reference voltage corresponding to desired position or speed of the motor and it is applied to a pulse width voltage converter.
The length of the pulse decides the voltage applied at the error amplifier as a desired voltage to produce the desired speed or position.
For digital control, a PLC or other motion controller are used for generating the pulses in terms of duty cycles to produce more accurate control.
The feedback signal sensor is normally a potentiometer that produces a voltage corresponding to the absolute angle of the motor shaft through the gear mechanism. Then the feedback voltage value is applied at the input of error comparator amplifier.
The amplifier compares the voltage generated from the current position of the motor resulting from the potentiometer feedback and to the desired position of the motor producing an error either of a positive or negative voltage.
This error voltage is applied to the armature of the motor. As the error increases so does the output voltage applied to the motor armature. As long as error exists, the comparator amplifier amplifies the error voltage and correspondingly powers the armature.
The motor rotates until the error becomes zero. If the error is negative, the armature voltage reverses and hence the armature rotates in the opposite direction.
The working principles of an AC servo motors are based on the construction with two distinct types of AC servo motors, they are synchronous and asynchronous (induction).
The synchronous AC servo motor consist of stator and rotor. The stator consists of a cylindrical frame and stator core.
The armature coil wound around the stator core and the coil is connected to a lead wire through which current is provided to the motor.
The rotor consists of a permanent magnet and this differs with the asynchronous induction type rotor in that the current in the rotor is induced by electromagnetism and therefore these types are called as brushless servo motors.
When the stator field is excited with voltage, the rotor follows the rotating magnetic field of the stator at the same speed or synchronized with the excited field of the stator, and this is where the synchronous type is derived.
With this permanent magnet rotor, no rotor current is required so when the stator field deenergizes and stops, the rotor also stops. These motors have higher efficiency due to the absence of rotor current.
When the position of rotor with respect to stator is required an encoder is placed on the rotor and provides feedback to the servo motor controller.
The asynchronous or induction AC servo motor stator consists of stator core, armature winding and lead wire and the rotor consists of shaft and the rotor core winding.
Most induction motors contain a rotational element, the rotor or squirrel cage.
Only the stator winding is fed with an AC supply.
Alternating flux field is produced around the stator winding with the AC supply. This alternating flux field revolves with synchronous speed.
The revolving flux is called a rotating magnetic field (RMF). The relative speed between stator rotating magnetic field and rotor conductors causes an induced electromagnetic force in the rotor conductors according to Faraday’s law of electromagnetic induction. This is the same action that occurs in transformers.
Now, the induced current in rotor will also produce an alternating flux field around itself. This rotor flux lags behind the stator flux.
The rotor velocity is related between the rotating stator flux field and the rotor rotates in the same direction as that of the stator flux.
The rotor does not succeed in catching up the stator flux speed or not synchronized, hence where the type asynchronous is derived.
Servo Motor Applications are applied in many industrial and commercial systems and products such as with robotics where a servo motor is used at every “joint” of a robot to perform its precise angle of movement.
The camera auto focus uses a servo motor built into the camera that corrects precisely the position of lens to sharpen the out-of-focus images.
And with antenna positioning systems where servo motors are used for both the positioning of azimuth and elevation axis of antennas and telescopes such as those used by the National Radio Astronomy Observatory.
This concludes the blog post, what is a Servo Motor and How it Works. I hope you have learned what’s required to move forward in creating your own motion control project.
We at RealPars hope that you found it interesting, and that you will come back for more of our educational blogs.
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The RealPars Team
Servo motors have been used for many years in a variety of applications. These compact machines pack a punch, delivering lots of power while remaining energy efficient.
These properties make them great for use in robotics, remote-controlled cars, and airplanes, as well as the manufacturing industry.
This article will answer the question: "How does a servo motor work?" and mention some applications and best practices for using these powerful motors.
Before diving into the inner workings of servo motors, let's take a look at the basics.
A servo motor is a closed-loop control system. It consists of various components that work in unison to power the motor. The main components include:
Servo motors are completely self-contained, rotating different parts of the motor with incredible efficiency. What sets these machines apart is that their motor shafts can be moved to a certain angle, velocity, and position (an ability that regular motors lack). This is done by the controller.
The servo motor is controlled by an analog or digital electrical signal, which determines the movement and position of the shaft.
The encoder, resolver, or potentiometer then provides feedback on the speed and position.
All of the above are enclosed in a case that is fitted with the gear assembly.
Let's take a closer look at each of the components mentioned above and how they work:
As mentioned, servo motors are very efficient, which is why they've been adopted in so many different industries.
They are able to control an object's position (linear or angular), acceleration, and speed with unwavering precision. For this reason, they are perfect for tasks that regular motors wouldn't be able to perform, such as factory automation and robotics.
Here's a step-by-step guide to the servo motor working principle.
The servo motor receives a low-power control signal from the controller. This signal indicates the desired position, speed, acceleration, and torque of the motor shaft.
The servo drivers receive the low-powered electrical signal. It processes the signal, determining how much power must be applied to the motor.
The amplifier then amplifies the low-powered signal to a high-power current and voltage, which is used to drive the motor. This power is continuously monitored through feedback sensors to ensure precision.
The amplified signal from the servo drive is fed to the motor to activate it. The shaft starts to move, driven by electromagnetic forces inside the motor.
If the servo motor has drive gears, the movement of the motor shaft is transmitted through them. Gears can reduce speed, increase torque, and achieve exact control.
As the motor rotates, sensors (like encoders, resolvers, or potentiometers) monitor the shaft position and speed. A feedback signal is sent to the servo drive, providing real-time information on the machine's performance.
The drive uses the feedback signal to compare the actual shaft movement to the command signal. If there are any differences, it adapts the power output.
This closed-loop control system allows servos to achieve incredibly high precision and efficiency.
Once the servo drive has determined the correct speed or the desired position has been achieved, it can maintain this state.
When a new command is received, the entire process is repeated.
From remote-controlled cars to pharmaceutical applications, servo motors can be used in a variety of applications and industries.
The most common servo motor uses include:
Although servo motors are fairly easy to use, it is a good idea to know when and when not to use them, as well as what to look out for.
It is important that someone using servo motors understands their application, including their speed, precision, and torque. This will ensure that the correct servo motor is selected for the task.
A servo motor must be installed properly. This includes mounting, securing, and aligning the motor to prevent mechanical or operational issues.
It is vital to consider the operating environment within which the servo motor will function. The motor itself may require special casings or additional protection if used in harsh environments (like if there is a lot of dust, high temperatures, or high humidity).
As with any electrical or mechanical device, servo motors require regular maintenance. Although these machines are generally very easy to maintain, they still require regular checking for any wear and tear or updates to the servo drive.
Before using a servo motor, it is important to ensure that it is compatible with the power supply and any other related control systems. Proper wiring and grounding must be used to ensure safety and to get the best performance out of the machine.
Common misuses include:
There are also limitations to servo motors. They create heat, and if the cooling system isn't working, the motor can break. They should also not be pushed beyond their capabilities in terms of speed and load weight.
A standard servo motor provides positions within a 180° range. That means it can turn 90° in either direction from the center position. This range is expressed as either +90° or -90°, depending on the direction it turned.
They typically have three wires:
Servo motors are controlled using pulse width modulation (PWM). This means that the position of the shaft is determined by the width (length) of the electrical signal that is sent through the control wire.
Standard motors receive electrical pulses continuously - about one every 20 milliseconds (ms). The duration of each pulse tells the motor what to do:
Once the pulse is received, the servo motor moves to the required position and stays there. Should an external force try to move the shaft, the motor will resist the force and maintain its position. Continuous pulses allow the motor to hold its specified position.
This article has already discussed the components found within a servo motor. The main components are the AC or DC servo motor, sensors, and gears.
The sensors provide feedback to the servo drive, which helps the motor and gears determine position, speed, and torque. The feedback allows the motor to know where and how it should move to maintain its position or move to a new position.
As mentioned, these machines require a power supply. Most servo motors work with a +5V power supply. However, different motors can draw different amounts of current, especially when pushed to their max or when more than one servo motor is used at the same time.
If two or more servos are used, the power supply must be adequate to handle the current they'll draw. In some instances, a separate power supply or servo shield (a device used to manage multiple servo connections) may be required.
In terms of the motor itself, there are two main types. There are also three additional types of servo motors based on their application, which are discussed below.
The two main types of servo motors based on current are:
Next, let's look at the three basic types of servo motors based on their functioning and uses:
Servo motors have various applications within robotics, some of which were already discussed above.
Their compact size and high accuracy make them ideal for use in robots. They also allow for perfect repetition of tasks; a requirement of most robots in the manufacturing industry.
Servos are used in robotics for:
Servo motors use electric signals and feedback to allow for meticulous control. Stepper motors do not make use of any feedback sensors, moving in fixed steps. Stepper motors are simpler but offer less precision and control. Stepper motors are commonly used in cost-effective systems where dynamic control and movement are not a top priority, like slot machines, printers, and motion-activated lighting.
Servo motors are usually powered by an electrical power source, like a battery. The exact voltage required will depend on the servo motor. Generally, it is 5V, but it can range from 4.8V to 6V. Larger servo motors will require a higher voltage to operate.
Yes, servo motors can be used in heavy-duty industrial applications. These powerful industrial-grade servos are designed to deliver high torque and precision. This makes them ideal for application and use in complex industrial machines, including robotics, CNC machining, and manufacturing.
Although a servo motor can be quite complex, it is a valuable part of the robotics industry. By understanding each part and its functioning, it is much easier to grasp exactly how a servo motor works.
There are various servo motor applications - in robotics and other industries - thanks to their accuracy and precision. The feedback from the closed-loop system ensures that a servo motor always remains precise and responds accurately to signals from the controller.
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