Guide to Servo Motor - How it Works
Some inventions have really made a difference in the way we use certain objects. One such invention is the servo motor. Fortunately for us, this little motor has been around for quite a while!
Many people are unaware of the many servo motor applications that exist. Items ranging from printers to toy cars make use of servo motors in one form or another. Servo motors in general are more popular with customers because of their compact dimension, fast dynamic response, and highly control-able characteristics.
While there are several other motor types to consider, servo motors are generally the function of the most common motor integrated, energy-efficient, with precision, and powerful. On a bigger scale, servo motors can be widely used in industrial applications and robotics.
Let’s discuss the servo motor in some detail so you can decide if it’s worth considering for the motion control projects or applications you need it for.
What is Servo Motor?
A servo motor is a closed-loop mechanism with a positional feedback mechanism to control rotational/linear speed & position. An electric signal controls the motor which regulates movement, specifying the ultimate command position of an output shaft.
How does a Servo Motor Work?
Have you ever picked up a small servo motor and wondered exactly how it works? How is it possible for such a small item to power objects as simple as toy cars or as complex as military robotics?
Well, the way a servo system works is actually not as complex as you might imagine. If you understand the basic components, you’ll get the idea.
Here’s a breakdown of the different components that make up a closed-loop system:
- Electric motor: A motor generates motion through the shaft.
- Feedback sensor: A sensor (encoder) used to feedback the angular/linear position, speed, or current value to the control unit.
- Control circuit: This series motor circuit provides control over the motor’s movement by using electric pulses.
- Drive gears: Gears increase or decrease the speed and torque of the motor.
Before we explain the process of how a servo motor works, it’s important to understand what a “closed-loop system” is. To simplify, a closed-loop system is a fully automated control system where the control action is dependent on the output.
Quite simply, the motor is attached to the feedback sensor. When the motor rotates, the sensor gives feedback information which allows the control circuit to regulate the amount of motion of the shaft and in which direction it needs to move. When the shaft has moved into the desired position, this is known as the command position.
As soon as the motor shaft is at the required position, the motor’s power is stopped. Electrical pulses send the required position via the signal wire. Since the motor speed is relative to the actual position versus the desired position, the motor will turn slower if it’s closer to the position and faster if it’s further away. The motor will therefore only run as fast as is required to accomplish the requested task, making it very efficient.
To make the servo motor work effectively, it’s recommended to use an external power supply. An average of 12/24 volt power supply can easily be used, which is similar to those used to charge a standard cell phone.
Larger servo motors that run industrial machines use gearboxes to achieve and maintain the correct output torque. Most servo applications operate at a gear ratio of 5:1, 10:1, or, even as much as 100:1.
Servo motors use electromagnetism to produce torque. Permanent magnets are found on the rotor while a stationary electromagnet surrounds the actual rotor. Since the servo is a vector driven induction motor, it’s powered by a rotating magnetic field and not a permanent magnetic field.
Servo motors are primarily more powerful than their cheaper counterparts. With their high-performance output and energy efficiencies, they are also very durable.
How to Control Servo Motor?
Controlling a servo motor is done by sending an electrical pulse of flexible width or pulse width modulation (PWM) through a control wire. Generally, the PWM signal will be controlled to rotate the motor at a certain Speed, Position, or output Torque. In common applications, the servo motor could only choose one controlling function. But the most common and widely used controlling type is position control.
In high precision applications, the servo motor's original point could be defined by using an absolute encoder. The controlling IC will send relevant PWM current to the motor decides the position of the shaft, and based on the feedback information provided by the encoder, the rotor will turn to the required position at last.
The DC Servo Motor by Assun Motor Pte Ltd, for example, uses the pulses for position control. For each pulse given, the motor will rotate at a certain angle. And for continuous pulses, the servo motor will rotate according to the speed calculated by pulse frequency, and speed could be up to 12K rpm.
So long as the frequency of the pulses is defined well, then the motor could either rotate at a stable speed or rotate according to pre-defined positions. The pulse frequency-time curve must be a smooth curve for the servo motor to find its targeted position precisely.
When it comes to controlling the actual speed of a servo motor, you should know that it’s not characteristically speed controlled. You’re actually just sending the servo continuous position signals. The servo then in turn tries to reach that position as quickly as possible. The speed can easily be reduced by sending a series of position signals with lower frequency.
Servo Motor Efficiency
Whether you’re using a motor for a simple hobby or a more industrial purpose, energy efficiency is the preferred way to go. Fortunately, servo motors are very energy efficient. But what exactly is that makes servo motors able to be so energy efficient?
For starters, the design and manufacture are different from those of stepper motors. Another crucial difference is the strength of the permanent magnets. Stronger magnets can easily decrease the motor length by 20% which then simultaneously increases the torque by at least 50%. Magnet motors generally have a much higher resistance to demagnetization.
Since torque density refers to the most rational amount of work an engine can potentially exert, a motor with a high torque will improve the servo motor efficiency. For the most part, efficiency is in the curve. Since the torque curve is used to regulate the operating speed, an increase in voltage will increase the operating speed range.
When it comes to operating speed, servo motors can be very efficient within a 40 to 90% range o stall torque. Simply put, servo motors are supposed to be over 85% efficient. This of course can be changed by the servo application that’s being used. So, as far as torque motors go, servos are definitely better.
Compared to other cheaper motors, servos are manufactured by winding more wire into the laminations. This means the space between the teeth is filled. As a rule, a higher slot fill delivers added torque with increased efficiency. It’s estimated that creating a motor in this manner increases the efficiency to at least 85%.
What is Servo Motor Used For?
Interestingly enough, servo motors can be used for a wide range of applications. Not only can they operate at a high speed, but they’re energy-efficient as well. For this reason, they’re used in many diverse industries for a variety of applications.
Some of the applications in which servo motors can be used are listed below:
- Industrial glue dispenser: Servo motors are widely used in the glue dispenser in industrial areas. The production line automation is highly reliable to servo motors.
- Industrial applications: Servo motors can be used to control industrial weaving and looming machines. They can also be used in knitting and embroidery machines that manufacture materials such as fabrics and carpeting.
- Printing presses: Some printing presses have servo motors that start and stop printer heads specifically on the print medium. It’s also the mechanism that moves the printer head on the paper.
- Automated door openers: Hospitals and supermarkets with automated doors are controlled by different size servo motors.
- Robotics: Every joint of a robot mechanism can have its movements activated by a servo motor. Servo motors enable this moving to take place.
- CNC machines: Woodworking machines such as lathes that cut and shape pieces of wood into table legs make use of a servo motor.
- Antenna positioning: Servo motors can be used on the elevation drive axis of telescopes such as those used by NRAO.
- Solar tracking systems: Angles of solar panels can be adjusted during the day so that each panel is consistently facing the sun. This ensures that maximum energy is absorbed during daylight hours.
- Robotic vehicles: Servo motors can be used in the robotic vehicles that are used for bomb detonation. The wheels contain small servo motors.
- Camera autofocus: Cameras have a highly complex and precise servo motor built into a camera lens. This serves to sharpen and rectify out-of-focus images.
- Conveyor belts: During product packaging servo motors move the conveyor belts that transport items to different stages of packaging and sorting.
Types of Servo Motor
Servo motors are available in a variety of different sizes and three basic types. They’re defined as the following:
Continuous rotation: Used for hobby applications.
Linear: More suited to complex applications.
Positional rotation: Most commonly used in everyday applications.
Are Servo Motors AC or DC?
The primary difference between AC and DC motors relates to the electrical currents and the different ways that each current works in each specific motor type. Let’s have a look at some of the basic differences between an AC servo and a DC servo.
For most household and domestic applications, the preferred choices are AC servo motors. AC servos have alternating current (AC) that functions in reverse order. A transformer device or power supply moves alternative current across huge distances at high voltages. An AC servo motor can facilitate changing voltages with greater comfort than a DC servo motor.
AC servo applications can be divided into two options, namely synchronous servo motors and induction motors. In general, in synchronous servo motors, the rotor speed relates to the stator. This means the two spin around in sync with each other. Items that ordinarily contain synchronous Ac servos include speed controllers, factory robots, and even alarm clocks.
DC servo motors contain a direct current (DC) with a positive as well as a negative current. Unlike the AC servos, the current flows in precisely the same direction. Current control is far easier than that of the Ac servo motor because only the armature magnitudes control the current.
DC motors are also known as brushless servo motors because they don’t have brushes. Controllers provide pulses of current to the motor windings which in turn control the motor speed and torque.
Advantages of Servo Motors
Since servo motors are so energy efficient and designed for power, they’re used in many different industries. There are also several other advantages to using a servo motor for your motion control projects.
Some of these advantages include the following:
- Torque control
- Small in size
- Smooth running
- High efficiency
- High power output relative to the size
- Added constant torque at greater speed
- Closed-loop control
- Quiet operation
- Very reliable
- High ration of torque inertia
- Exceptional accuracy due to the encoder
- High-speed performance
- Well suited to varying load applications
- High acceleration
- Vibration-free operation
More FAQs on Servo Drivers
Do Servo Motors Need Drivers?
Another plus point of servo motors is that most don’t require additional drivers. They have their own drive electronics and only require a PWM signal to control them. This means that servo drivers as well as servo motor control manage the operation in a closed-loop system.
Servo drivers or motion controllers are used to running the control loop. A closed-loop system also uses the feedback signal to alter the direction and speed of the motor to reach the required result. The motion control loops for torque, speed, and position. They function independently to perform the desired operation.
It’s possible for servo drives to control more than one or even a group of servo motors. Often, a project will require several servo motors to complete a specified task. The controller will have to manage more than one servo. An RC airplane is a perfect example of this.
Is a Servo Motor Analog or Digital?
Analog and digital motors don’t look any different. The primary difference between the two types is the way the signal is processed.
Let’s analyse the two in a little bit more detail:
- Analog: An analog servo usually has drives that operate a basic analog control circuit. These drives are found in both brushed and brushless DC motors. A traditional +/- 10V signal is used as the control input. Analog is also the standard type of servo and it controls the speed of the motor by sending on and off pulses. A minimum of 50 pulses is sent through an analog servo motor. However, when the unit is switched off or at rest, no voltage is transported through.
- Digital: Small microprocessors are used to receive and send required actions at high voltage pulses. Almost six times the amount of pulses are sent than an analog does. The faster the pulse, the more consistent the torque is. This ultimately results in faster and smoother response times.
To summarise, servo motors are used in many industries and have several advantages. They provide a solution to many mechanical problems and have features that make them powerful and efficient.
As you can see from our article, they can be used in anything from toy cars to high industry, complex robotics. It’s the ideal motor to use for your motion control projects, and because it’s so energy efficient, you’ll be saving on your electricity bill as well!
If you have any queries about our servo motors, do contact us.