What Is A Servo Motor – The Ultimate Guide

The word “servo” is a Latin word that means a servant or a slave. Since its inception, the servo motor has been popular as an auxiliary drive. Its main role is to boost the main drive system.

However, advancements in technology have made it possible for servo motors to work as main drives with precision and excellent performance.

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What Is A Servo Motor

This is a translational motor that supplies a torque force to any mechanical system after receiving power from a servo amplifier. They apply this torque to mechanical systems such as brakes or actuators.

Servo Motor
Servo Motor

They operate using a closed-loop control system to offer accurate control of the system’s velocity, acceleration, and precise angular control. In this closed-loop control system, the current output is taken into account and then adjusted to fit the required status.

This output current is what will determine the action that will be taken to control the system. Basically, the shaft’s position and any of its motion are regulated using a positive feedback system.

What separates this shaft from other regular motor shafts is – it can attain angles, positions, and velocities that other shafts cannot. This is because it combines the regulator motor with sensors to achieve this positional feedback.

You can either use an analog or digital electric signal from a controller to control the servo motor. This is what will tell the level of movement the motor will achieve to give the shaft its final command position.

Benefits Of A Servo Motor

We have many benefits of a Servo Motor in its application. Some of the notable ones include:

  • Even with their small size and lightweight, the output power they produce is significantly large.
  • They have a very accurate position control when compared with other types of motors. Their speed control is also highly precise, reliable, and consistent.
  • They have an excellent efficiency rating. Under light loads, they are estimated to be about 90% efficient.
  • Their operations are generally very smooth and quiet while working with commendable efficiency
  • Because of its improved torque-to-weight ratio, they are able to rapidly accelerate heavy loads. This factor also makes them suitable for use in applications with limited space.
  • They are energy efficient since they only consume power when needed.
  • They are versatile since they have a wide range of both operating speed and torque. This means that they can be used in many applications.
  • They are very durable

Components of A Servo Motor

The structure and design of these motors are a bit complex compared to other types of motors. Here are the components that you can find inside a servo motor:

Servo Motor Components
Servo Motor Components
  • Motor: depending on the source of power rated for the motor, it can either be an AC or a DC motor. This motor is what generates the mechanical power that sets the output shaft in motion.
  • Sensor: there are many sensors that can be used in these motors including resolvers, potentiometers, or even encoders.  You can also use an external device as a sensor. Their main function is to analyze the output shaft speed, position, and torque. They then relay the same to the controller using a feedback signal for further action.
  • Controller: this is normally a circuit that is either analog or digital. The sensors send a feedback signal to the controller which analyses it using a set point signal. It uses the result to regulate the current or voltage of the motor within some tolerance using a control signal.
  • Encoder (Feedback System): this is an optical device whose function is to record and transmit the number of times the shaft rotates. It also monitors and conveys the position at which the shaft is located at any given time.

How A Servo Motor Works

Servo Motor Working Principle
Servo Motor Working Principle

A servo motor basically works using input and output signals. There are two types of input signals that are sent to the controller to begin with:

  • Set-point signal
  • Feedback signals

The set-point signal can either be analog or digital and shows the output shaft’s required torque, speed, and position. Also, the analog or digital feedback signal relays the actual torque of the output shaft, its speed, and position.

The controller will then analyze both signals and come up with an error signal showing the difference between the two. A control algorithm such as a PID (X) will then process this error signal and come up with a control signal.

Here, this control signal decides the amount of current or voltage that the motor will require at that exact point. At this point, this signal is then converted into the appropriate voltage or current using a power amplifier.

At the same time, current/voltage then drives the motor based on the control signal received. This will alter its torque, speed, and position. In turn, it will transmit back to the controller a new feedback signal.

This process will continue indefinitely to the point that the error signal is negligible or goes to zero. This will imply that the required set point has been achieved by the output shaft.

Features Of A Servo Motor

Let us have a look at some of the prominent features:

  • Versatile: these motors can be used in many applications in a wide range of industries
  • Closed-Loop Control System: they control the mechanical systems by interpreting the feedback signals they receive
  • High Speed: these motors can rotate very fast and at extremely high speeds. A small motor can offer a very large torque to the system.
  • Precision: this motor has a seamlessly efficient mode of operation. It is this type of accuracy that makes it the best choice of motor used in CNC machines for precision cutting.
  • Encoder: this basically means that this motor has the ability to produce a digital signal by deriving it from either a rotary or linear motion.
  • Shaft: this is the component responsible for transmitting the output power from the motor. It drives the load since it is connected to another transfer mechanism such as a coupling.

Types of Servo Motors

When classifying them, we have to group them according to factors such as their power supply, design, or their feedback system.

Let us analyze the main types and their classifications:

· AC Servo Motors

AC servo motors are powered using an alternating current. They have an encoder that is responsible for sending feedback signals from the Servo Motor to the controller.

Usually, they have a complex design that enables them to attain higher torque levels. These motors have a variety of precision applications like industrial automation and CNC machines.

AC Servo Motor
AC Servo Motor

We can further classify these motors into three specific categories namely:

Positional Rotation AC Servo Motors

In this type of motor, there is a 180° rotation of the output shaft, basically a half-circle displacement. Any other motion beyond this is restricted by physical stops within the gear mechanism.

This is to prevent any damage to the rotational sensor. We use it mostly in robotics, remote controlled cars among many other applications.

Continuous Rotation AC Servo Motors

What differentiates this from a positional rotation motor is its ability to indefinitely turn in any direction. It interprets the control signal to be the motor’s rotational direction and speed.

Various commands allow it to rotate either clockwise or anticlockwise, at the same time varying its speed. We mainly use it as a drive motor applicable in mobile robots or in a radar dish.

Linear Servo Motor

It functions more or less like the positional rotation servo but with extra gears. These gears transform the circular output into a back-and-forth motion.

Such motors are not readily available. We mostly use them in complex designs of model airplanes as actuators.

· DC Servo Motors

As the name suggests, the power supply for these motors comes from a DC source. You can achieve control over the motor by adjusting either the field or armature currents.

Its response time to command signals is very fast majorly due to the minimal reactive inductance of its armature. We mostly use them in CNC equipment and many other devices.

DC Servo Motor
DC Servo Motor

Let us now look at how further we can breakdown the DC Servo Motor classification:

Series DC Servo Motor

This motor is known to require a very large torque for it to start. This means that its level of current consumption is usually high.

It also has a very low-speed control with turnaround achieved by alternating the polarity of the voltage generated by the field. You can do this using split series field windings.

Split Series DC Servo Motor

These motors are rated with a split field. They are able to work independently as energized field-controlled servo motors with excellent damping characteristics.

Shunt Control DC Servo Motor

It is composed of two field windings on the stator together with armature windings on its rotor. These windings are connected in parallel and a DC source is supplied.

Permanent Magnet Shunt DC Servo Motor

This DC servo motor can be permanently excited if the field has a stable magnet.

How Servo Motors Are Controlled

Servo Motor controllers are the only way we can achieve control over the Servo Motors. This is made possible using Pulse Width Modulation which transmits an electric pulse to the motor.

Besides, this PWM is kind of a wave having characteristics such as maximum and minimum pulse with a constant rate of repetition. The rotor will be displaced to the required position after the lapse of the pulse duration in split seconds.

Both analogue and digital servo motors can receive the PWM but they process the information and signal differently. Let us individually see how each responds to PWM:

Analogue Servo Motors

To control them, they first need to receive a voltage signal generated by the PWM. In the resting position of this motor, the PWM is normally switched off.

When a signal is transmitted, the reaction time in producing a torque is usually very slow. This response time is clearly not recommended in advanced applications.

Analogue Servo Motor
Analogue Servo Motor

Digital Servo Motor

The high-frequency voltage signals in this motor are received and interpreted using a small microprocessor. Usually, the pulses transmitted by this motor are almost six times those sent by the analog one.

Normally, the high-speed transmission of these pulses results in a steady torque that receives a rapid response from the motor. This results in more power emitted by the motor.

Digital Servo Motor
Digital Servo Motor

Control Algorithms Used By Digital Servo Motor

The Servo Motor Controller can make use of a number of control algorithms when running the motor. The most common ones you will find in regular applications are:

Proportional-integral-derivative Control (PID)

This algorithm regulates the received signal by factoring in its proportional, integral and derivative proportions. It uses a feedback based system to compare it to the error signal.

It will repeat this process until the error signal is zero or negligible. This algorithm usually produces a rapid and precise response.

Fuzzy Logic Control

The algorithm used in this control system is normally rule-based. It uses pre-defined linguistic variables together with fuzzy sets to tune the control signal received from the motor.

We mostly apply this control algorithm in servo systems that are non-linear. Additionally, servo systems that deal with uncertain responses use this algorithm too.

Adaptive Control

This control algorithm is self-tuning and reacts to any changes in the condition of the motor.

It will regulate the control parameters with reference to such changes. It is mostly applied in servo systems placed in areas having a lot of disturbance.

Ways of Generating and Sending Pulse Wave Modulations (PWM) To a Servo Motor

We have several methods that you can use to generate a PWM signal to be fed to the Servo Motor. The most common of them are:

  • By using an analog sensor such as a potentiometer. You can vary the resistance of the potentiometer and use it to control your servo as an input device.
  • Using Micro-controllers such as Arduino. They contain PWM outputs that you can use to transmit signals to your motor.
  • By using digital input devices such as a joystick. They can detect motion in terms of their magnitude and direction along the x and y axes.
  • By using a specific driver or servo controller

Servo Motor Drives – Learn How to Drive and Control Motors

Servo Motor Drives are the gadgets we use in the control and operation processes of servo Motors. They are responsible for converting any input signal they receive from the control system into mechanical motion by the motor.

It does this by sending an electric current to the servo motor that converts it to motion equivalent to the sent command. This drive infinitely analyses the feedback signal of your servo system application.

It will adjust any error signal and ensure that the deviation is rectified back to or near zero. Normally, this response to the feedback system ensures that the motions from the signal command are very accurate with unnecessary motions eliminated.

The precision of these drives enables them to regulate the shaft position accurately even with disturbances or friction present. That is why we widely use them in precision applications such as robotics and CNC Machining among other automation processes.

Servo Motor Drives
Servo Motor Drives

Limitations of Servo Motors

Even with such performance and efficiency, the Servo Motor still has its downside. Some of its demerits include:

  • For the feedback loop to remain operationally stable, the motor will require regular adjustments
  • It requires a protective circuit to cushion it from sudden breakdowns to avoid inconveniences in its application
  • It is expensive to set up compared to other motors such as stepper motors. This is because of the feedback system it uses

Interfacing a Servo Motor With Micro-controllers

Interfacing a Servo Motor with a micro-controller unit (MCU) is actually a very easy task you can carry out. Basically, your Servo Motor has three connecting wires that are attached to them.

Servo Motor Interfacing with Microcontroller
Servo Motor Interfacing with Microcontroller

Two of these wires are for the power supply (positive and negative) while the third is the signal line that you will connect to the MCU. You should note that the color coding for various servo motors is different depending on the manufacturer.

This means that you have to go through the motor data sheet before making any connections. You can supply your servo motor with +5V from any supply rail.

However, monitor the amount of current drawn by the servo motor to regulate consumption. You should design a servo shield if you want to use more than two servo motors in the same MCU unit.

Proper Cooling Of a Servo Motor to Prevent Overheating

Heat dissipation is not really a major area of concern when it comes to Servo Motors but it is equally important. Most Servo Motors dissipate their heat using conductive cooling.

This is the process by which the motor transfers the heat through its casing and mounts onto the host machine. Some motors also come with heat sink sizes specified on their data sheet from manufacturers.

The heat sink sizes can tell you the ability of the motor to have a continuous torque. Additionally, the rate of heat dissipation literally increases or decreases the load capability of the motor.

You can also use forced air cooling to reduce the Servo Motor heating. You simply attach an electric fan around the motor to blow air over it.

The forced air will also decrease the heat transfer from the motor to the machine. This will improve the motor efficiency since it will operate at a much higher load point.

Another not-so-common method is liquid cooling. Here, liquid cooling agents such as ethylene glycol are circulated around the motor to absorb the heat.

Servo Motor Cooling System
Servo Motor Cooling System

Precautions When Installing Servo Motors

For your Servo Motor to operate optimally, you need to take certain precautions during its installation. Consider the following:

  • Your application area should contain no shocks, vibrations, or electrical noise. These greatly affect the performance of the motor together with its drivers and controllers.
  • The installation environment should be non-static
  • When you have exposed circuits in your installation, keep it well guarded or enclosed to prevent human contact with live electricity
  • All connections to the motor should be done with the power turned off.
  • If you intend to do any inspection of your motion control system, give it at least 5 minutes to rest before beginning. This is to let the electrical energy in the capacitors to discharge
  • At no point should you run your servo motor system where there is flammable gas, contaminants such as dust, debris and oil.
  • Servo motors installed outdoors should be well covered to protect them against environmental extremities. However, ensure that the cover provides adequate ventilation to allow cooling.

Choosing the Right Servo Motor

When you are sourcing a servo motor for your application, there are several factors that you have to take into account. We can analyze some of the common ones namely:

· Load Requirements

You have to ascertain the requirements of your application in terms of the acceleration, speed, and torque it needs. This will assist in selecting the best power rating and servo motor size.

· Feedback Mechanism

Go for a type of motor sensor such as an encoder or potentiometer that best suits your application. This will enable your motor to achieve maximum accuracy and resolution.

· Environmental Conditions

You should keep in mind various environmental factors around the application area that may affect the motor. Examples are dust, the surrounding temperature, and humidity that reduce the motor lifespan

· Control Signals

The control system you intend to use must generate signals that are compatible with your servo motor for efficient response. Ensure you ascertain whether your motor uses analog or digital signals.

· Budget

For you to get a good return on investment, you should strike a balance between the motor’s cost and its reliability and performance. You should also factor in the maintenance requirements.

· Torque

Most Servo Motors come with data sheets that contain their torque-speed curve. This will help you settle for a motor that fits your application’s peak and continuous torque with the motor achieving maximum speed.

· Speed

You should ascertain the distance of travel your load should travel and at what speed. This will help you know the motion profile of the Servo Motor you want to go for with the specific speed ratings in RPM.

Servo Motors Vs Stepper Motors

Though they may have an almost similar structure and application, we still have variations between these two motors. We can have a look at some of the outstanding differences;

  • The control system of a servo motor is basically a closed-loop system as opposed to  that of a stepper motor which is an open-loop control system
  • The shaft speed, position, and torque in a Servo Motor are determined by the feedback system. Stepper motors achieve the same by using command prompts.
  • Servo Motors tends to be more costly compared to Stepper motors. This is due to the difference in the complexity of the closed-loop and open-loop control systems.

Troubleshooting And Maintaining A Servo Motor

Regularly servicing and maintaining your Servo Motor systems ensures that they achieve performance.  Some of the common errors you are bound to experience in your application and their fixes include:

· Positioning Errors

Normally caused by the sending of an incorrect input signal, a damaged sensor, or loose servo motor connections. Ensure that all connections are well placed and check on the working condition of the sensor to rectify the error.

· Overheating

Usually caused by poor ventilation that leads to improper cooling of the motor or the motor being overloaded. Another cause is over-current on the motor.

Properly ventilate the operation area and ensure that the motor is working within its rated power and torque range. You should occasionally switch off the motor and let it cool for some time.

· Vibrations

Strange noises and vibrations come about due to loose mounting or worn-out bearings. Inspect your motor for any mechanical issues and replace worn out components.

Also, ensure that all components are properly aligned and that the motor is properly fastened in place.

· Low or No Torque

This may be due to power supply problems or shorted windings. Verify the power supply source and amount of power available.

Additionally, the torque output can also be affected by the motor magnets having demagnetized. This will require you to replace the motor.

· Dust/Contamination

A dirty environment can make the motor get contaminated with things such as dust and oil leading to a malfunction of the bearings or breaks. Ensure that the working environment is always clean and regularly clean your motor.

Applications of A Servo Motor

Some of the most common applications we have are:

  • Vehicle wheels – It is applied in the control of the robot vehicle wheels. The torque it produces is strong enough to perform operations like starting and stopping, moving, and even regulating the speed.
  • Manufacturing industries – The manufacturing and processing industries is in the running of conveyor belts that carry goods.
  • Solar panel systems – They are used to adjust the angle in which the solar panels face the sun.
  • Textile industry – These motors are used in knitting and weaving machines to control their motion.
  • Photography – They are incorporated into the camera to adjust the lens to take out-of-focus photos
  • Robotics – It provides the robot arms with a wide angle of movement using precise actuation
  • Automobile industry – We use this motor in the power steering control and in automatic cruise control.
  • Medical industry – It is used in operating various medical equipment such as ventilators and pumps or even modern surgical robots. They ensure that the medical staff carry out safe operations with precision and accuracy.
  • Printing industry – The servo motor is used as part of the drive system used in the machines that print papers. These servos can be custom-made to suit the diverse requirements of individual systems.

FAQs

1. What are Stall Torque and Continuous Torque in a Servo Motor?

The Servo Motor stall Torque is the torque required to bring the motor shaft to a halt. On the other hand, the Servo Motor’s Continuous Torque is the indefinite torque produced by the motor.

2. What Sensors do Servo Motors have?

Rotary encoders and potentiometers are the main sensors widely used.

3. How do you know that a Servo Motor is Efficient?

We can determine the efficiency of a Servo Motor by observing its conversion of electrical energy to mechanical energy.

Its efficiency can be affected by factors such as the control algorithm applied, the design of your motor, or its operating conditions.

4. How do you Calculate the Torque of a Servo Motor?

We can calculate the Torque of a Servo Motor in two different ways, either mechanically or electrically.

In the mechanical way, we find the power by multiplying the angular speed with the torque.

When we estimate the torque electrically, we find the power by multiplying the current by the voltage. You can use the below formula to precisely get the torque:

Torque = Torque Constant Rated Current

5. Define Stall Current In A Servo Motor

This is the total current that the Servo Motor draws after it has stalled and the rotors have no motion. This is normally caused by the load torque of the motor being more than the shaft Torque.

6. Is the Rotation of a Servo Motor Uni-Directional?

No, a Servo Motor can rotate both ways. However, the direction is largely dictated by the control mode you are using.

Your controller can easily do this without altering the servo drive reference pulse or even the voltage polarity.

Conclusion

Having a clear understanding of the Servo Motor and its working principle is really important if you’re dealing with motion control systems. This will give you an easy time when designing, installing, and maintaining such systems.

These motors have an edge in their application areas due to their precision in controlling the position, acceleration, and velocity.

The information you have gathered here will enable you to make informed decisions when choosing a servo motor. It will also go a long way in helping you optimize your servo motor system performance.

More Resources:

Troubleshooting Hydraulic Power Pack – Source: TARGET HYDRAULICS

Hydraulic Power Pack – Source: TARGET HYDRAULICS

Servo Motor Basics – Source: Science Direct

Servo Motor – Source: Wikipedia

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