Servo motor basics - What are they?

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:

• Electronic assembly (AC or DC motor; the control circuit; the potentiometer/sensor)
• Motor shaft
• Servo drive
• Drive gears
• Amplifier
• Encoder/resolver/sensor which provides feedback
• Power supply
• Cooling system
• Cables and connectors
• Bearings
• Casing

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.

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Inner workings of servo motors

Let's take a closer look at each of the components mentioned above and how they work:

• Electronic assembly:
  
  AC or DC motor: This is the core of the servo motor, consisting of either Alternating Current (AC) or Direct Current (DC). The motor type (which will be discussed later on in the article) determines what the motor is used for.
  Control circuit: This is essentially the "brain" of the servo motor. It receives signals from the controller, processes these signals, and then determines the position, speed, and torque of the motor shaft.
  Potentiometer: This is commonly found in smaller and less complex servo motors. It acts as the position sensor. The sensor provides feedback on the position of the shaft. Other types of feedback sensors are discussed below.

• Motor shaft: This is the part of the servo motor that rotates. It is connected to the mechanism that's being driven (like a robot arm).
• Servo drive: This component is the interface between the control system and the motor. It receives signals and converts them into electrical currents to power the motor. It regulates the power based on feedback received from sensors like the potentiometer, encoder, or resolver.
• Drive gears: The gears increase the torque and are also responsible for exact motion. The gear assembly is connected to the motor shaft. They ensure that movement is smooth and accurate - key for high-precision applications.
• Amplifier: The amplifier is often part of the servo drive. It boosts control signals from the control circuit and provides power to the motor.
• Encoder: Like the potentiometer, the encoder is part of the feedback system. It provides feedback signals on the speed and position of the shaft. Encoders can be incremental (provides relative position information) or absolute (provides unique position information for each shaft angle).
• Resolver: The resolver is an analog electro-mechanical device that also provides feedback on the motor shaft's position. These are commonly found where servos are used in harsh conditions since they're much more robust than encoders and potentiometers.
• Power supply: The power supply will depend on the type of servo motor in question(AC or DC). It must be able to provide enough current and voltage for the motor's operation.
• Cooling system: A cooling system may be required, depending on the use and power output of the motor.

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How do servo motors 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.

1. Receive the command signal

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.

2. Servo drive processes and amplifies the signal

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.

3. Motor activation

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.

4. Continuous feedback

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.

5. Maintain position or await new command

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.

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What can servo motors be used for?

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:

• Robotics: Servo motors are used to power robotic arms and grippers, like Standard Bot's ROI. The precision of a servo motor ensures that the robotic arm moves with accuracy.
• Automated guided vehicles: Servo motors can control the wheels of robotic vehicles, especially those used in industrial or warehouse settings. The motors provide the required torque to move the robotic vehicle and achieve speed control.
• Computer numerical control (CNC) machining: CNC machines have servo motors that control the position and speed of the cutting tools.
• Conveyor systems: Servo motors are used to control the smooth operation of conveyors. They're crucial for tasks like packaging and labeling.
• Hobbies: Many hobbyists make use of hobby servos for remote control cars, model airplanes, and small hobby robots.
• Pharmaceuticals and food services: Servo motors are used in these environments where high pressure, temperatures, corrosive materials, and hygiene standards must be maintained.
• Cameras: Cameras with an auto-focus feature use servo motors to position the lens.
• Medical equipment: Servos are used in medical devices like MRI and CT scanners, and X-ray machines because of their precision. They're also used in robotic surgical systems where precise movements are crucial.
• Textile industry: Servos are used to control the movement of looms and knitting machines.
• Printing press: High-speed printing presses use servo motors to ensure high-quality printing and color control.
• Telecommunication: Servos are used to accurately position antennas and satellite dishes used in telecommunications.
• Renewable energy systems: Servos are used in wind turbines and also to move solar panels that automatically track the sun.
• Virtual reality: Servo motors are used in flight simulators, driving simulators, and virtual reality systems to accurately translate controller movements to the system, enhancing user experience.

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Best practices for using a servo motor

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.

1. Understand their use

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.

2. Proper installation

A servo motor must be installed properly. This includes mounting, securing, and aligning the motor to prevent mechanical or operational issues.

3. Environmental considerations

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).

4. Maintenance

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.

5. Electrical compatibility

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.

6. Avoid misuses and understand limitations

Common misuses include:

• Overloading the servo motor
• Not using it in the correct environment (e.g. a high-impact environment without adequate shock absorption)
• Not using an appropriate feedback mechanism

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.

Introduction to servo motor control

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.

Wiring

They typically have three wires:

• Positive: This wire is connected to the positive side of the power supply and is usually red.
• Negative: The negative (or ground) wire connects to the negative/ground side of the power supply. Usually, it is black or brown.
• Control: The control wire (also called the signal wire) is used to send command signals to the motor. It can be orange, yellow, or white.

Pulse width modulation

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:

• 1 ms pulse: Shaft moves to -90° (anticlockwise direction)
• 2 ms pulse: Shaft moves to +90° (clockwise direction)
• 1.5 ms pulse: Shaft moves to 0° (center position)

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.

Inside the servo motor

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.

Power requirements

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.

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Types of servo motors

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:

1. DC servo motor: A DC servo motor uses direct current. This servo motor requires a bit more maintenance than their AC counterparts, but they are able to quickly respond to signals.
2. AC servo motor: An AC servo motor uses alternating current. They can handle current surges much better than a DC motor. AC servo motors can deliver higher torque and better control, but their response times are slower. AC motors are commonly used in robotic arms and CNC machines where heavier loads are involved.

Next, let's look at the three basic types of servo motors based on their functioning and uses:

1. Positional rotation: This is the most commonly used servo motor. The shaft can rotate 180° (90° in either direction), making use of stops to prevent it from turning further. Positional rotation servos are often found in robotic arms, as well as in the limbs and legs of humanoid robots.
2. Continuous rotation: These are similar to positional rotation servos; however, the output shaft can turn indefinitely in either rotation and is not limited to 180°. The command signal does not indicate the position of the shaft, but rather the direction and speed of the rotation. These types of servos can be used on satellite dishes or on robot vehicles to drive the bots.
3. Linear: This is a positional rotation servo motor that has additional gears that change the circular motion to a back-and-forth one. They are quite rare and are mostly used in large-model airplanes as actuators.

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How do servo motors work in robotics?

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:

• Robotic arms: The motors move the joints, arms, and end-effectors to perform specific tasks.
• Humanoid robots: Servo motors are used for balance and movement.
• Mobile bots: The servo motor is used to power the wheels and steering mechanism.
• Hobbyist robots: Servos are commonly found in DIY robots and robotic kits sold to hobbyists.
• Medical bots: As mentioned, servo motors are found in medical devices and also in robots used for surgery.
• Exploration: Robots used for underwater and space exploration, as well as search-and-rescue bots have servo motors.
• Cobots: Collaborative robots - called Cobots - use servo motors to work alongside humans safely without causing injury or collisions.

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FAQs

What's the difference between a servo motor and a stepper motor?

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.

What are the disadvantages of servo motors?

• Servo motors are complex machines, which makes them quite expensive.
• Because of their complexity, they require trained staff for installation and maintenance.
• Servos are not very robust. They are sensitive to harsh environments with high temperatures, humidity, and dust.
• Servos may be very precise, but they struggle at higher speeds when compared to other motor types.
• Servos don't always run very quietly, especially when working at high torque.

How do you power a servo motor?

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.

Are servo motors suitable for heavy-duty industrial applications?

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.

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Wrapping up

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.

Considering the importance of servo motors, the future of robots will likely hinge on the technological advancements and innovations for these powerful and versatile machines.