Servo Drive Basics

"We design and build servo drives for motion control systems."

That's an answer we might give when someone asks "What does ADVANCED Motion Controls do?"

For some people, that's all they need to know. For others, it might raise more questions than answers. We have to remember that not everyone has a background in motion control, or electronics, or even science and engineering for that matter.

So we're going to go over the servo drives basics at different technical levels so that anyone and everyone can have some understanding of what we do. You can stop whenever you feel comfortable.

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 and Motor sketch copy

Servo Drive Basics Explained to a 12-Year-Old

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

Servo drives are electronic devices made out of circuit boards, microchips, wires, and connectors. They are connected to electric motors to control the motor's spin. They can make the motor speed up, slow down, stop, or even go backwards at any time.

They accomplish this by controlling and directing the flow of electricity through the motor's wires. Without a servo drive, a motor might just spin uncontrollably or not spin at all.

Some servo drives control small motors, like in the elbow joint of a robot arm. Other servo drives control big motors, like in heavy machinery or the wheel of an electric vehicle. More powerful motors need more powerful servo drives.

Servo Drive Basics Explained to a High School Physics Student

Hopefully you're all still with us, but here is where it starts to get a bit more technical.

How do servo drives work and what goes into making the motor spin?

Driving the Motor

How does a motor spin?

An electric motor has two main parts: a rotor that spins and is attached to the shaft, and a stator that stays still and is attached to the frame. One of these components will have regular magnets, and the other has wire windings or "electromagnets" that can be turned on by running electrical current through them.

By turning the different windings on and off in succession, you can get a rotating magnetic effect. This pushes against the regular magnets and causes the rotor to spin.

Brushed motors have windings in the rotor and magnets in the stator, while brushless motors have the windings in the stator and magnets in the rotor.

Either way, the amount of current through the windings controls the amount of torque (how hard it turns), while the voltage controls the motor's speed (how fast it turns). By regulating the current and voltage supplied, the servo drive controls the motor shaft's torque, speed, and position.

3 phase brushless motor

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

Command and Control

Machine Controller

A large controller

But how do you tell the servo drive what torque, speed, and position to aim for? Easy. You just use a controller, which can be something as simple as a dial or complicated as a computer.

The controller sends a signal (a small, but specific pulse of voltage) to the servo drive's command input.  The servo drive then essentially amplifies the signal to the desired current or voltage for the motor.

Servo Drive Power supply

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

Of course, the law of conservation of energy tells us energy can't just be created out of nowhere.

So where does this amplified power come from?

The servo drive is connected to some sort of power supply unit (a battery or a device that plugs in) that provides a constant voltage. The servo drive then takes this supply voltage and sends power to the motor as necessary based on the command signal.

Feedback

If you're keeping track, so far we've talked about 3 main components that servo drives are connected to: The motor windings, the controller, and the power supply. But there's a 4th element that the servo drives are connected to that makes them so effective: The motor feedback device.

Speedometer feedback

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.

Incremental Encoder Feedback Device

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.

This is analogous to when you're using cruise control in an automobile and you start going up a hill. Without touching anything, your car computer makes all the necessary changes to keep your car going at the same speed despite the incline. In a servo drive, all this happens faster than humans can perceive, with thousands of adjustments per second.

Enjoying the read?

Are you the type that never stops learning?

Increase your motion control knowledge every month for free!

Newsletter Sign-Up 1

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 looked at servo drives as little magic boxes with electricity and wires going in and out of them. We know what they do, but not how they do it. Let's look at what's actually going on inside.

Negative Feedback Loops

Servo drives are often known as servo amplifiers because, at their core, that's what they do. They amplify a command signal. But it's the development of feedback-based control that makes them more sophisticated and useful devices.

As we mentioned, servo drives use feedback loops to correct for error.

In motion control and most other controls processes, errors are corrected using negative feedback loops.

In a negative feedback loop, the system output signal (from the measured value) is subtracted from the system reference input (the target value) to create the new input value (the error signal).

Take a look at this simple block diagram.

Simple Negative Feedback Loop

A simple negative feedback loop.

Say for example your target is 5 and your measured value is 3. The error signal will end up being +2. If your target is 5 and your measured value is 7, then the error signal is -2.

So if a system's output is too high, the error signal will be negative, and the system will respond in the negative direction to bring the output down. If the system's output is too low, the error will be positive, and the system will respond in the positive direction to bring the output up.

This process cycles through continuously, keeping the error as close to zero as possible.

This negative feedback is essential. If the feedback was positive (in other words, if you added the measured value to the target rather than subtracting it), a system going too fast would compensate by going even faster or a system going too slow would grind to a halt or eventually run in reverse.

Almost all servo drives are capable of closing the current loop, but others can also close the velocity loop and even the position loop. If a system has a servo drive that can only close the current loop, but the machine needs to also close the velocity and position loops then those additional loops will need to be closed by the controller.

Gains

The error signal goes through the system "gains", aka, the amplification step that takes the system input to produce the system output. In some systems, this is a simple proportional gain.

Let's think about this like a microphone and speaker setup. You speak into the mic, your voice gets amplified. If you're speaking to a crowd of 20 people in a school auditorium, then you might just have a small gain that makes you sound two or three times as loud as you really are. But if you're speaking to a crowd of hundreds of people at an outdoor event, then you're going to need a much bigger gain so that everyone can hear you, so you might have the system make your voice over ten times as loud. In either case, the louder you speak into the mic, the louder the sound that comes out will be.

Servo Drive Motor Feedback Loop with Gains

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.

So in one application's servo drive, you might have a proportional gain constant of 5 A/V where 1V input yields a 5A output, 2V input yields a 10A output, etc.

In another application, you might have a proportional gain constant of 10 A/V where 1V input yields a 10A output, 2V input yields a 20A output, etc.

PID control

For some processes, a proportional gain is good enough for control. But for most processes, such as robotics, more precise control is desired. One of the most widespread control schemes is (Proportional, Integral, Derivative) control.

In PID, you have the a proportional gain multiplied by the present error value, but also have the integral gain multiplied by the accumulation of error over time (integral) and the derivative gain multiplied by the change in error over time (derivative). This almost always results in a much more precise correction of error, reducing problems like overshoot and oscillation.

Servo Drive Motor Feedback Loop with PID Control

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

Servo Drive Basics Explained to the System Designer

Congratulations, if you've made it this far. Now let's look beyond a simple controller-drive-motor combination and look at how servo drives fit into a full motion control system.

Power Levels

Not all servo drives are created equal. Every servo drive has a nominal operating voltage and maximum peak and continuous current ratings. While many advancements have been made to increase power density (especially in recent years with our FlexPro drive family), larger servo drives are usually going to be more powerful and have a lower resolution of current control.

Maybe a 5 year old could understand this, but just in case, don't try to power a tiny 5 amp motor with a bulky 80 amp servo drive and don't try to run a heavy 80 amp motor with a miniscule 5 amp servo drive.

Basic Panel Mount Servo Drive

Panel Mount Servo Drive

Form Factor

Servo drives come in various shapes and sizes, some better suited for different applications.

Panel Mount

Panel mount servo drives have a metal baseplate and a plastic or thin metal cover that enclose the PCB.

Holes or notches in the baseplate are used to mount the drives to a flat surface using bolts or screws.

These are the traditional form of servo drives, typically used in machinery.

PCB Mount

FlexPro servo drive

PCB Mount Servo Drive

PCB mount servo drives are made without any case or coverings. They're designed to mount directly to another circuit board using pins or soldering, similar to how a sub-component might be attached.

These are very compact and offer reliable connectivity, but are less protected from the elements. These are often used in both fixed and mobile robotics.

PCB mount drives are sometimes plugged into a mounting card to offer more traditional connections with wires and cables while maintaining their compact nature. This can save the machine designer the trouble of designing a PCB with the exact pin connectors to match the drive.

Vehicle Mount Servo Drive

Vehicle mount

Vehicle mount servo drives are tightly enclosed by a thick plastic shell and a heavy baseplate. Screw terminal lugs are used to allow high current. As the name implies, these are used in mobile applications.

Regardless of the form factor, performance-wise they all work the same.  The difference is more to do with ease of installation for different industries and different applications.

Networks

In robots, mobile vehicles, machines, and other motion control systems, chances are there are going to be more than one axis of motion. That means more than one motor, which typically means more than one servo drive. The controller needs to send the commands to all of these servo drives.

There are two ways for the controller to handle this. With analog servo drives, a centralized control scheme is necessary where the controller is connected individually to each servo drive.

Machine Network Servo Drives

Having the servo drives on a network simplifies wiring. Data and commands can flow along a single network bus to each node, so the controller doesn't need a direct connection to each one.

With digital servo drives, however, a distributed control scheme is made possible through the use of a network. A network chains the servo drives together. Messages or data packets can be sent through the network and the servo drives will respond to the data that is addressed to them.

There are a variety of different network protocols. Real-Time networks such as EtherCAT or EtherNet/IP allow for incredibly fast response times, sending updates in less than a millisecond. Other networks like CANopen or ModBus are not as fast, but are easier and less expensive to implement.

Every ADVANCED Motion Controls digital servo drive model is designed for a specific network protocol, with many options available, including customs.

 

Other Types of Motors

Like cell phones have evolved far beyond just making phone calls, servo drives can do much more these days than just run servo motors.

Besides standard brushed and brushless servo motors, servo drives can also be used to control linear motors, two-phase and three-phase stepper motors, AC induction motors, voice coils and more.

Linear Motor diagram

A linear motor is electrically analogous to an "unrolled" brushless servo motor, so it is easily controlled by digital servo drives.

Even if your system has multiple motor types, it's very possible that you can control them all using the same or similar models of servo drive, simplifying your design.

I/O

I/O (Input/Output) functionality is used in digital servo drives to allow them to exchange high/low signals with other devices in the system. These devices can be temperature sensors, limit switches, pressure sensors, or even other servo drives.

Using I/O can be great for allowing the servo drive to control simple functions on the machine and take the load off of the controller and/or network.

Isolation

Either your servo drives or your power supply is going to need electrical isolation. Otherwise, you might end up with a floating ground that ends up frying your servo drives and other components in the system. You either need servo drives with built-in optical isolation or a power supplies with an isolation transformer. The exceptions to this rule are battery powered systems and systems with servo drives that are designed to take AC power directly.

Bonus: Servo Drive Basics for Philosophers

If a servo drive is a device that pushes current and voltage to a motor, then if you disconnect it from the system, is it still a servo drive?

 

by Jackson McKay, Marketing Engineer

Enjoying the read?

Are you the type that never stops learning?

Increase your motion control knowledge every month for free!

Newsletter Sign-Up 1

You might also be interested in...

Tech_Motion-Control_overview
Motion Control Overview
6 Cool Servo Drive features you might not know about
6 Cool Servo Drive Features You Might Not Know About
Brushless motor
Three Phase (Brushless) Motor
when do you need a custom servo drive featured image embossed
When Should You Get a Custom Servo Drive?
Velocity Mode Info Box
Velocity Mode