As automation and robotics becomes more widespread across various industries in a bid to increase production in a cost effective manner, one factor is often overlooked until it is too late. That factor is the need for precise technology to power the production line, be it the careful assembly of PCBs by a robotic arm or the milling of intricate components on a high end CNC mill. At the heart of each precise movement is the humble servo motor.
Servo motors seem to be talked about by some modellers and mechanism builders, but rarely by others. In this guide we will explain what a servo motor is, how it works and help you select the correct servo motor for your model making or mechanism building project.
Understanding the Basics: What is a Servo Motor?
A servo motor is a rotary or linear electro-mechanical device whose purpose is to grant precise control over the positional or motion characteristics of a physical object. For a servo motor, a set of parameters such as angle, velocity, and acceleration are set.
Many people are familiar with how standard motors function. They are simply powered by electricity and continue to rotate indefinitely until they are stopped. Servo motors, on the other hand, are utilized within closed loop systems and are characterized by the inclusion of a feedback device known as an encoder. Controlling the servo motor is the controller, and it actively adjusts settings in order to maintain precise positioning, correcting for any minor inaccuracies - such as a deviation of a fraction of a millimeter.
The Core Difference: Open-Loop vs. Closed-Loop
To understand servos, you must understand the "loop":
Open-Loop (e.g., Standard AC Motors): You send power, and the motor turns. The system has no way of knowing if the motor actually reached the destination or if it stalled.
Closed-Loop (Servo Motors): The system sends a command, the motor moves, and the encoder sends back a "proof of position" signal. This constant "check-and-balance" is what provides incredible accuracy.
How a Servo Motor Works: The Feedback Loop
The "magic" of a servo motor lies in its ability to self-correct. This process involves three main stages:
- The Command: The controller (like a PLC) sends a signal to the servo drive.
- The Drive: The servo drive (amplifier) translates that signal into high-voltage electricity to move the motor.
- The Feedback: As the motor moves, the encoder tracks the shaft's position and sends data back to the drive. If there is a discrepancy between the commanded position and the actual position, the drive adjusts the current immediately.
Essential Components of a Servo System
A complete servo setup consists of more than just the motor itself. For high-performance industrial environments, you need four key parts:
- The Motor: Usually a brushless AC or DC motor designed for high efficiency.
- The Servo Drive (Amplifier): The "brain" that regulates the power delivered to the motor.
- The Controller: The "operator" (often a PLC or dedicated motion controller) that tells the drive what to do.
- The Encoder: The "eyes" of the system, mounted on the back of the motor to track every degree of rotation.
Servo Motor vs. Stepper Motor: Which is Better?
This is the most common question in motion control. While both provide precision, they serve different needs:
| Feature | Servo Motor | Stepper Motor |
|---|---|---|
| Control | Closed-loop (High accuracy) | Open-loop (Potential for lost steps) |
| Speed | Excellent at high speeds | Torque drops off at high speeds |
| Torque | Constant torque across RPM range | High holding torque at low speeds |
| Cost | Higher (Complex electronics) | Lower (Simple setup) |
| Applications | Robotics, CNC, Packaging | 3D Printers, simple indexing |
Quick Facts: Choosing the Right Servo
When selecting a servo motor, it is essential to consider key specifications in order to choose the appropriate motor for your application.
AC Servos are generally used in factory floors where High torque / High power is required. DC Servos are designed for portable applications such as small robots, battery operated devices, etc. where high precision is required in very small packages.
MOTOR SPECIFICATIONS SHEET Rated Torque: for normal running conditions. Peak Torque: for stop and start duty applications.
Damping: The damping of the test load and the motor should be comparable. Inertia Matching: The inertia of the test load should be comparable to the inertia of the motor rotor.
If you are planning to use the equipment in harsher environments, please check if the motors have IP65 or IP67 ratings. These motors are fully protected from dust and water ingress.
People Also Ask About Servo Motors
1. What is the difference between a servo motor and a stepper motor?
The other major difference between the two motors is control. In servo motors, the motor is closed-loop, meaning it has an encoder built in that reports back where it is at any given time, and the servo can correct for this if it has moved incorrectly. In stepper motors, the motor moves in steps (degrees or inches) but has no feedback control and, as such, can lose steps under heavy load.
2. How does a servo motor work?
Servo motors require the controller to send commands to tell the motor where to move to, and the servo drive then turns these commands into the appropriate amount of power to move the motor. In addition, the servo motor has either an internal or external encoder that keeps track of the actual position of the shaft of the motor, which the servo drive receives as feedback. The drive then adjusts the current to the motor until there is error or difference between commanded position and actual position.
3. Why is a servo motor used instead of a standard DC motor?
While a standard DC motor is designed for continuous rotation at varying speeds, a servo motor is designed for precision positioning. Servos provide high torque at high speeds and can hold a specific position indefinitely without drifting, making them essential for robotics, CNC machinery, and automated assembly lines where standard motors lack accuracy.
4. Do all servo motors have encoders?
Yes, by definition, a servo motor requires a feedback device to function within a servo system. While most industrial servos use optical or magnetic encoders to track rotation, some smaller or specialized versions may use potentiometers or resolvers. Without a feedback device, the motor cannot perform the self-correction that characterizes "servo" operation.
5. What are the main types of servo motors?
Servo motors are primarily categorized into two types:
BLDC/Servo Motors - AC Servo Motors are best suited for industrial applications where high torque, long life and over torque capability are required.
The motor type for this unit is DC Servo Motors. These motors are typically used for smaller, precision-heavy applications. Easy to control, they are ideal for use in battery powered/portable applications.
Conclusion
Servo motors are not Just A Motor. Servo motors need to be matched with the automation system they will control. A servo motor paired with a controller with closed-loop feedback enables high precision, high speed automation with unprecedented robustness and reliability.
Whether you are designing a new plant and starting from scratch with the motion control for serialization, or upgrading the existing system in an existing facility, investing in a high quality drive and encoder will enable your servo system to function optimally for years to come.
