What are the basic elements of a servo system?

A servo system is a closed-loop motion control system built from three core components: an actuator (the muscle), a control device (the brain) a feedback element (the senses) and the cable (the nervous system). These elements work together to deliver precise, repeatable motion through continuous feedback and correction.

In industrial automation, the actuator is most commonly a servo motor, while the control device is typically a servo drive, motion controller or a combination of both. The feedback element may include a current sensor, encoder, resolver or even a vision system, depending on the application's performance requirements, the cable completes the servo system by carrying power, communication and feedback signals between the motor, drive and control elements. Cables with proper shielding and construction help maintain accuracy, reliability and overall system performance.

Understanding how these components function individually, and how they work together as an integrated motion system, is the foundation for selecting and optimizing servo applications.

What Does a Servo System Do?

A servo system continuously monitors and corrects servo motor movement to achieve a commanded position, speed or torque with high precision. When the motion controller or servo drive sends a signal to the motor to move to a specific position, the motor begins to move, and the feedback device signals to the motion controller or servo drive where and how fast the motor is moving. The motion controller or servo drive then reviews the feedback and determines if the motor has reached the commanded position. If not, the control device continues to signal the motor until it receives confirmation from the feedback device that the motor has reached the desired position.

This closed-loop motion control process operates much like the processes of the human body—a remarkable multi axis motion system of hundreds of muscles, multiple feedback devices, and a control system that makes swift adjustments. For example, when you reach for a glass of water, your brain sends signals to your muscles while your eyes and sense of touch continuously provide feedback on the glass's location and your hand's movement. Without conscious thought, your brain makes countless corrections until your hand reaches its target. A servo system performs this same continuous cycle of command, feedback and correction—but in microseconds. Modern industrial servo systems use sophisticated controllers, multiple feedback devices and high-speed processors to execute these adjustments with exceptional precision.

What Are the Main Components of a Servo System?

The basic elements of a servo system each serve a distinct, interdependent function. A breakdown of a typical industrial servo system includes:

• Servo Motor – The muscle of the system, available in a variety of technologies including brush or brushless, housed or frameless, and linear or rotary. The servo motor produces the torque required to accelerate and move the load.
• Servo Drive – This can be either the brain of the system or a portion of the brain. Simple servo drives may control torque and/or speed, while higher-level servo drives offer additional features and can be configured as positioners with programming capabilities. A drive matched to the servo motor controls the voltage and current the motor receives.
• Servo Controller – The brain of the system, which utilizes a programming environment to allow a variety of options for machine control, operation of inputs and outputs, and interconnection with a graphical user interface. Servo controllers can be standalone or integrated with servo drives.
• Feedback Device – The senses of the system, typically integrated into the servo motor. The feedback element can consist of an encoder, resolver, linear feedback device, tachometer or other sensor. Sophisticated control systems may include more advanced forms of feedback such as a vision system.
• Cabling – The servo system's nervous system, interconnecting feedback, communication, and power across the brain, muscles and senses. 

What Type of Feedback Device is Used in a Servo System?

A servo system's feedback device reports motor position and velocity to the control device in real time, enabling the closed-loop correction that defines servo performance. The most common feedback devices are encoders and resolvers, both of which are typically integrated directly into the servo motor.  

Encoders (incremental or absolute) convert shaft position into digital signals and are widely used across industrial motion control applications. Resolvers are analog devices valued for their robustness in harsh environments. More demanding applications may incorporate linear feedback devices for direct linear position measurement, tachometers for velocity feedback or vision systems for complex spatial feedback.  

The precision requirements of the application drive feedback device selection. The higher resolution feedback, the finer positional accuracy and tighter control loop performance. For a deeper dive, check out our white paper on selecting the right servo position feedback device. 

How to Select the Right Servo Motor and Drive for Your Application

Selecting the right servo motor and drive requires a structured analysis of system performance requirements. The amount of torque required to accelerate and move the load determines the size of the servo motor, which in turn constrains the servo drive selection. The following process covers the key decision points: 

1. Define load and torque requirements

Calculate the torque and inertia demands of the application, including acceleration, deceleration and continuous operating loads. This determines the minimum servo motor size.

2. Determine required precision, feedback and safety requirements

The application's accuracy, performance and safety requirements determine the type of feedback device and any functional safety capabilities needed in the drive system. High-precision applications typically require high-resolution absolute encoders.

3. Match the servo drive to the motor

Select a servo drive rated for the motor's voltage, current and communication interface. A drive and motor that are properly matched ensure stable closed-loop control and protect both components from operating outside their rated parameters. 

4. Assess coordination and controller requirements

The number of axes, the level of inter-axis coordination, and the required response speed determine whether a standalone servo drive with embedded positioning capability is sufficient, or whether a dedicated servo controller is needed.

5. Verify cabling and system integration

Confirm that feedback, communication, and power cabling are specified for the environment and system architecture. Cabling is the nervous system of the servo control system. Undersized or mismatched cables introduce signal errors that can degrade performance. 

Next Step: Starting the Selection Process

Putting this all together can be a considerable task. If you’re working through servo system selection for a new machine design, let the motion control experts at Kollmorgen help identify the right combination for your application requirements.  

You can also dive deeper into each stage of the process with these related resources:

• How to select a servo motor – learn how torque, speed, inertia, physical size and environmental conditions influence motor selection.
• How to select a servo drive – understand how voltage, current, feedback options and communication protocols determine the right drive for your application.
• Getting started: sizing and selecting servos  – explore a system-level approach to sizing motors, drives, feedback devices and cables for optimal machine performance.
• Selecting the proper cables for your system – learn how cable design, shielding and environmental conditions impact signal integrity, reliability and overall system performance.