PID Control

What is a Servo Motor? Motion Control Basics

Normal electric motors run continuously and are typically used as a source of motion power in machinery. However, a servo motor works in a different way by incorporating a control mechanism that allows it to rotate by a specific angle and stop at a desired position.

These capabilities allow the servo motor to be used in various unique applications ranging from joints in robotics to the control of radio-controlled aircraft to various manufacturing machines.

Below, we will learn the basics of servo motors, how they work, and the different types available.

What is the Definition of Servo?

The term ‘servo’ derives from the Latin word ‘Servus’, which can be translated literally to ‘slave’.

In mechanical and technology control mechanism, the mechanism that performs the control is called the master while the mechanism being controlled is the slave, or servo.

Thus, in technology, the term ‘servo’ generally refers to any item that can be controlled by a control mechanism.

Why is it Called a Servo Motor?

When applied to the word servo motor, it refers to a motor that can be controlled by a master mechanism and can be relied upon to operate exactly as commanded.

Thus, any mechanical and electric motors that are capable of controlling parameters like speed, angular position, and other measurable parameters, can be called a servo motor, regardless of how we achieve this control.

Servo motors have been around for quite some time and have been used in many different mechanical applications. They are typically small in size but can perform very fast rotations and many of them are very energy-efficient.

With these qualities, servo motors are often used in robotics, vehicles (including airplanes), mechanical toys for children, and various industrial applications.

How Does a Servo Motor Work?

The servo circuitry works by having a positionable shaft which is typically fitted with one or more gear. The basic principle of a servo motor is to have an electric signal of variable width sent to this servo circuitry, which will regulate the movement of the shaft and in which direction.

To fully understand how servo motors work, we have to understand what’s under the hood, and the basic set-up of a servo motor is pretty simple with just three elements:

  1. A DC motor
  2. A potentiometer
  3. A control circuit

We can control both the speed and position of the servo motors very accurately, but there are many simpler applications where only the position of the motor is controlled.

The DC motor is attached by gears to the control circuit, and as the motor rotates, the potentiometer’s resistance also changes. This is how the control circuit can accurately regulate how much movement should be performed by the shaft, and in which direction.

When the shaft is already at the desired position, power supplied to the servo motor is stopped automatically so the motor will stop turning.

Motors With Encoders

In newer servo motors, electronic encoders and sensors replace the potentiometers to ‘sense’ the position of the shaft, but the principle remains the same.

Command input is given according to the desired position of the shaft, and if the feedback signal differs from the desired input, an error signal is generated. This error is then amplified and applied as the motor’s input, causing the motor to rotate.

Similarly, when the shaft reaches the desired position, the error signal becomes zero so the motor will stay in its position.

Command Input

The command input for the servo motor is given in the form of electrical pulses, and the actual input applied to the servo motor is the difference between the applied signal (signifies desired position) and the feedback signal (signifies current position).

The speed of the motor is determined by the difference between this current position and the desired position, while the amount of power applied will determine the distance the shaft will ‘travel’.

How to Control a Servo Motor

Typically a servo motor turns 90° in either direction so the maximum movement is 180°. In most cases, a servo motor cannot rotate any further due to its built-in mechanical stop.

Typically there are three wires on the servo circuitry: positive, control wire, and ground. The servo motor is controlled (with the principles we’ve discussed above) by sending a pulse width modulated (PWM) electrical signal through the control wire.

Typically a PWM pulse is sent every 20 milliseconds, and the width of the PWM pulse will determine the power of the shaft.

  • PWM is a technique to control the speed and torque of a DC motor. It allows infinite control of the output voltage to feed into a motor by quickly turning the output fully on and fully off.
  • The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.
  • The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power.
  • Typically switching frequency is at the 500 Hz (T = 2 ms) to tens of kHz for a motor drive.
  • The term duty cycle describes the proportion of ‘on’ time to the switching interval or ‘period’ of time;
  • A low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.

For instance, we can set so that a 1ms PWM pulse will move the shaft counter-clockwise (-90 degrees), while a pulse of 2 ms will move the shaft clockwise at 90 degrees. To move the shaft at the neutral position (0 degrees), we can use a pulse of 1.5 ms width, as you can see in the image below.

Thus, we can summarize that we can command the servo motor to move by applying PWM pulses of appropriate width so that the shaft will move and hold its required position. When an external force is applied to change the position of the shaft, the motor won’t move.

Advantages of PWM (Pulse Width Modulation)

  • PWM works well with digital controls because of their on/off nature.
  • The control voltage can easily be adjusted by changing the duty cycle. (i.e. Only digital output ports are required. No analog output ports are required to achieve variable voltage control.)
  • Another advantage of PWM is that power loss in the switching devices is very low. When a switch is off, there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero.

Through this rapid switching, PWM can be used to simulate a sine wave.

Why Do We Use Servo Motors

We mainly use servo motors in applications that require rapid variations in speed and position, where other types of motors might get overheated quickly in these applications. Some implementations of servo motors include, but are not limited to:

  • Controlling the positioning and movement if radio-controlled airplanes.
  • Various robotics applications due to its smooth switching and accurate positioning.
  • Machine tools, factory automation, packaging, material handling, and many other industrial and manufacturing applications.
  • Hydraulic systems in the aerospace industry to maintain hydraulic fluid.
  • Many toys with precision movements.
  • Electronic devices such as DVD player to move the disc trays.

What Are The Different Types of Servo Motor?

We can differentiate servo motors into different categories based on their applications, but there are three main considerations when differentiating different servo motors:

  1. Types of current: AC or DC servo motors
  2. Types of commutation: brush or brushless motors
  3. Types of rotor: synchronous or asynchronous

1. Based on Types of Current

Whether the motor uses DC or AC current will also affect the ability of the motor to control speed.

With a DC motor, the speed generated is determined by the supply voltage (with a constant load). On the other hand, in an AC servo motor, speed is determined by the frequency of the voltage and the number of magnetic poles.

AC servo motors can withstand higher current and are more commonly used in applications that require a high number of repetitions and precision such as robotics and manufacturing applications.

2. Based on Types of Commutation

A commutator is a rotary electrical switch that will reverse the current direction between the rotor and the drive circuit periodically.

The commutator consists of two or more electrical contacts which are made of soft conductive material (hence the name ‘brushes’), which will ‘brush’ against the segments of the commutator as it rotates.

Servo motors can be mechanically commutated with brushes (with a commutator) or electronically commutated (brushless).

Brushed motors are generally more affordable and are also easier to operate, but brushless servo motors are more durable with a higher frequency and less noise.

Most AC servo motors are brushless designs, but there are some brushed AC servo motors that are implemented for their low-cost and simplicity. However, many DC servo motors are brushed with a permanent-magnet design.

In a brushless design, the physical brushes and commutators are replaced with electronic elements, mainly by using an encoder or Hall effect sensors.

3. Based on Types of Rotor

We can also differentiate servo motors into two different types based on the type of rotor/rotating field: synchronous or asynchronous.

This is especially applicable to AC servo motors, which are more often differentiated by the speed of their rotating synchronous or asynchronous rotor. DC motors, on the other hand, are more often differentiated by its brushed or brushless commutation.

We call a servo motor synchronous when the rotor rotates at the same speed as the stator’s rotating field, and vice versa, it’s asynchronous when the rotor rotates at a slower speed than the stator’s magnetic field. Asynchronous motors are also often referred to as an induction motor.

The speed of an asynchronous motor can be controlled using various control methods, typically by changing the number of poles and changing the frequency.

What Does a Servo Controller Do?

A servo motor controller, or also commonly referred to as the motion controller, mainly works by consistently ‘sensing’ or monitoring the encoder’s pulse signal while applying torque to the servo motor, effectively ‘controlling’ the motor.

The servo controller, therefore, is the heart of the whole servo system. One of the most common methods used in servo controllers is PID (Proportional-Integral-Derivative) control, which measures the difference between an actual value and the desired value of an output variable as an ‘error signal’.

The proportional value, in this case, is a simple gain value, while the integral value integrates the error signal over a period of time to gradually drive the error to zero. The derivative value stabilizes the system.

The simplest form of control performed by a servo controller is to hold a specific position of the shaft. When an outside disturbance causes the shaft to move off the desired position, the encoder will detect this change in position and create an error signal.

The servo controller then translates this error signal to a command current and drives the motor back to its original position.

Another more advanced role of the servo controller is to move the motor to a new position. In this case, the servo controller must command a specific acceleration, deceleration, and speed so that the motor can rotate along with the desired rotation profile.

Conclusion

We hope we’ve answered some of your questions with respect to what is a servo motor? Servo control and closed loop position/velocity feedback as a complex topic. We hope you have somewhat of a better understand how servo motors work.

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Fred Graham

Hi! Fred here, I am the founding member and site moderator here at PLCGurus.NET. I'd like to be the first to welcome you to the site. I have over 20 years in the Industrial Automation and Control Systems field. Be sure to Register Today!

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