What is Servo Drive: Basics and Working Mechanisms

Table of Contents

• Servo Drive Basics Explained to a 5-Year-Old
• Servo_Drive_Basics_Explained_to_a_12-Year-Old
• Servo Drive Basics Explained to a High School Physics Student
• Servo Drive Basics Explained to a College-Level Engineering Student

At the most basic level, servo drives are devices that control how motors move.

For some people, that’s all the explanation they need. But for others, that might raise more questions than it answers. After all, not everyone has a background in motion control, electronics, or even science and engineering.

That’s why we’re going to break down the basics of servo drives at different levels so anyone can understand. You can stop reading whenever you feel comfortable, or keep going if you want to dig deeper into the details.

Servo Drive Basics Explained to a 5-Year-Old

Servo drives are like small computers that send electricity to motors and make them spin round and round.

Servo Drive Basics Explained to a 12-Year-Old

Following so far? I would hope so. Let’s go into a little more detail.

Imagine you have a remote-control car. When you press the buttons, the car moves forward, backward, turns, and stops. Now, think about what’s inside that car helping it do all of this. One of those key parts is a servo drive.

A servo drive is a small, smart box made of things like wires, chips, and circuit boards. Its job is to control how a motor spins—how fast, how slow, or even if it needs to go in reverse. Without the servo drive, the motor would have no control. It could either spin wildly or not spin at all.

The servo drive works by controlling how much electricity flows to the motor, a bit like how a faucet controls how much water comes out. More electricity makes the motor spin faster, and less makes it slow down. If you want the motor to stop or go in reverse, the servo drive handles that too.

Servo drives are used in all kinds of cool things. Some control small motors in robots, helping move their arms or legs. Others work with huge motors that power big machines in factories. Whether big or small, a servo drive ensures everything moves the way it should, smoothly and precisely.

Servo Drive Basics Explained to a High School Physics Student

Let’s dive into the details of how servo drives work and what makes motors spin.

Driving the Motor

How does a motor spin? At the most basic level, servo drives are devices that control how motors move.

A motor consists of two main components: the rotor, which spins, and the stator, which stays still. The rotor is attached to the motor shaft, while the stator is attached to the motor frame. In a motor, one of these components will have magnets, and the other will have electromagnets (coils of wire that generate a magnetic field when current flows through them).

By turning different wire windings in the motor on and off in sequence, we create a rotating magnetic field. This rotating magnetic field pushes against the magnets, causing the rotor to spin. The difference between brushed and brushless motors comes down to where the electromagnets are located:

• Brushed motors have electromagnets in the rotor and permanent magnets in the stator.
• Brushless motors have electromagnets in the stator and permanent magnets in the rotor.

In either type of motor, the current through the windings controls the torque (how much force is applied to turn), and the voltage controls the motor’s speed (how fast it spins). A servo drive regulates both the current and voltage, allowing precise control of the motor’s torque, speed, and position.

In a brushless motor, electromagnets in the stator are powered on and off to rotate the magnetic rotor.

Command and Control

A large controller

How do we tell the motor what to do? That’s where a controller comes in. The controller sends a command signal (a small, specific voltage pulse) to the servo drive. The servo drive then amplifies this signal to provide the correct current and voltage to the motor.

An example of a power supply that plugs into a wall outlet.

The energy to power the motor comes from a power supply—either a battery or a plugged-in device. The servo drive pulls the needed energy from the power supply and sends it to the motor, adjusting the power based on the controller’s commands.

Feedback

Here’s where things get really interesting. A feedback device, like an encoder, is connected to the motor to monitor its performance in real-time. This device provides the servo drive with feedback on the motor’s actual speed, torque, and position.

Using this feedback, the servo drive constantly compares the motor’s actual performance to the desired values. If something changes—like an external force slowing the motor down—the feedback lets the drive know, and it quickly adjusts the current and voltage to keep the motor operating at the correct speed or position. This process happens thousands of times per second, making servo drives incredibly precise.

Drivers use the speedometer for feedback while on the road… or at least responsible drivers do.

As humans, we use feedback devices all the time. The speedometer on our car tells us how fast we’re going so we know whether to speed up or slow down. A cooking thermometer lets us know when our meat is close to being done. A pressure gauge lets us know when the tire’s on a bicycle needs more air or less. A customer feedback survey tells a company where they need to make adjustments. Feedback makes it a lot easier to take corrective action.

Most servo motors are equipped with some sort of feedback device, such as an encoder, that can connect directly to a servo drive.

Having a feedback loop allows the servo drive to make real-time corrections to the current and voltage it’s sending to the motor. This ensures that the motor spins with the desired torque, at the desired speed, and to the desired position regardless of interference.

An incremental encoder is a common feedback device used in motors to track the rotational movement.

For example, let’s say an external force starts acting on the motor shaft, causing it to slow down from the desired speed. The feedback signal from the motor indicates the true motor speed. The servo drive will then compare the true speed to the target speed and increase the power supplied to the motor to compensate until it reaches the proper speed.

Think of it like cruise control in a car. When you’re driving up a hill, your car’s computer senses the car slowing down and automatically adds more power to keep the speed constant. Servo drives work similarly, but at much faster rates. They can make real-time corrections to keep the motor spinning exactly how it’s supposed to, no matter what external forces or changes happen.

Servo Drive Basics Explained to a College-Level Engineering Student

Alright smarty pants, you want the good stuff? Here you go.

Up until now, we’ve examined servo drives as “magic boxes” that control motors by amplifying signals. Now, let’s break down what happens inside them, focusing on key concepts like negative feedback loops, system gains, and PID control.

Negative Feedback Loops

At the core of a servo drive is its ability to amplify a command signal and regulate motor performance using negative feedback loops. This method allows the drive to constantly correct for any errors between the desired output (reference input) and the actual output (measured value).

In a negative feedback loop, the error signal is the difference between the system’s desired state (target value) and the current state (measured value). This error is used to adjust the system to reduce the difference.

Take a look at this simple block diagram.

A simple negative feedback loop.

For example, if the target is 5 and the measured value is 3, the error is +2, so the system will increase its output. If the target is 5 but the measured value is 7, the error is -2, causing the system to decrease its output.

This error correction process cycles continuously, minimizing the error. Negative feedback ensures stability by making the system respond in the opposite direction to an error. If the system used positive feedback, it would overcompensate, making errors worse. For instance, a system moving too fast would speed up even more, leading to instability or system failure.

Closing the Loop

Servo drives can “close” different types of control loops:

• Current loop: Controls motor torque by regulating the current to the motor.
• Velocity loop: Controls motor speed.
• Position loop: Controls the motor’s final position.

Gains and System Amplification

Once the error is determined, it’s processed through system gains to produce the necessary correction. Gains act as amplifiers, determining how much the system will adjust based on the error. For basic systems, a proportional gain is sufficient.

Output=Kp​×Error

Where Kp is the proportional gain constant. For instance, if Kp=5 A/V, a 1V error signal results in a 5A output.

Think of gains like the volume control on a microphone. The size of the gain needed depends on the application, much like how speaking to a large audience requires a higher amplification than speaking in a small room. In servo drives, the gain amplifies the error signal to provide the necessary current or voltage to correct motor behavior.At the most basic level, servo drives are devices that control how motors move.

A basic feedback loop in a motion control system with a servo drive. The servo drive compiles the error signal and then “amplifies” it to get the servo drive output.

PID control

For more complex systems where simple proportional control isn’t enough, PID (Proportional, Integral, Derivative) control is often used. This is especially common in robotics and precision engineering where oscillations, overshoot, and lag are undesirable.

• Proportional gain (P): Directly corrects the current error.
• Integral gain (I): Addresses the accumulation of past errors, helping to eliminate steady-state errors.
• Derivative gain (D): Predicts future error based on its rate of change, helping to dampen oscillations and overshoot.

Each term in the PID control equation contributes to reducing error in a different way. The result is a more precise and stable system, capable of smoother performance in dynamic environments.

A basic feedback loop in a motion control system with a servo drive featuring PID control.

Conclusion

No matter where you stopped reading, we hope you now have a better idea about servo systems. At CNCPROFSSIONAL ONLINE, we often work with servo drives as part of CNC Professional Controller software that power the world’s most advanced motion control systems.

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