How to Select a Servo Drive

When searching for a servo drive, you'll be able to find thousands of options. ADVANCED Motion Controls offers almost 200 servo drives alone. But finding one that meets all the needs for your application can be a bit more complex. Servo drives have different power ranges, sizes, network options, intelligence, form factors, digital features, and more. Selecting a servo drive requires careful consideration of the needs of your project and the options available.  It may not be a simple decision, but it doesn't have to be a daunting task either. Let this blog serve as a guide to that process.

Prep: Define Your Application

Your motion control solution should be based on the needs of your application, not the other way around. As we always say, you focus on your machine, let us handle the motion control. Before you start the hunt for the right servo drives, you need to set the parameters for your motion control system, and that all depends on how you define your application.

You need to start by looking at the big picture: What is it that you're making? Is it a large manufacturing machine, a mobile robot, or a desktop device? Is it a single or multi-axis system? What kind of task is your system performing? This will answer some basic questions about your servo drive selection, such as power requirements, analog or digital, and necessary operating modes.

Next you need to determine which elements of your design are flexible and which are set in stone. This will further define the parameters for your motion control system, and ultimately, the parameters for your servo drive selection. Do you have to use a specific motor? Is there a tight space requirement? Are you married to a certain network communication? If there's literal or figurative wiggle room in your project's design, then you'll have more options available to you, but you might not know where to start. If your parameters are more tightly defined, there could be fewer options to select from, but there could instead be none at all. In that case, you may have to redesign or consider a custom solution.

Making All the Right Choices

Once, you have your parameters set, you can start looking at what different servo drives have to offer. As we said before, there are a lot of factors to consider. ADVANCED Motion Controls has a servo drive search tool that allows you to set your specs and present you a filtered selection of servo drives that meet them.

Power

Right off the bat, power range is perhaps the most crucial factor in servo drive selection. You need to find a servo amplifier that has the appropriate power rating for the motor you're pairing it with.

It's pretty simple, if your drive can't deliver enough power, your motor won't be able to turn, at least not with the torque or velocity you want it to. This is why it's so important to have the speed and/or torque requirements of your application defined. Using the properties of your motor and its load, you can determine the necessary power capabilities of your servo drive.

Conversely, you also want to make sure your servo drive isn't significantly over-powered for your motor. If you're pushing too much current, you can fry your motor easily in a severely unmatched system. Even if you run your drive at the low end of its operating range where it technically meets the motor rating, it's not recommended for precise control. It would be like trying to comb your hair with a garden rake – you can still do it, but it's just not meant for something that small, and you won't get the desired results.

It's also important to remember that while the drive supplies power to the motor, the drive's power needs to come from somewhere too. Servo drives have a supply voltage range, so you need to make sure that you can properly provide that voltage for the servo drive.

Every application is different and there are so many different power ranges for drives, so there's no sense discussing which applications need what kind of servo drive power. Just get the most precise estimate of your application's motor power needs, and start your selection from there.

Size and Form Factor

Is size really important? Sometimes. If you're making a large manufacturing machine or warehouse gantry system, then size of the servo drive probably doesn't affect your application either way. However, for applications such as robotics or portable devices, finding a servo drive that fits inside is crucial. Bottom line: if a drive won't fit, then you can't use it, so it's important to select one that does.

Unfortunately, you can't just always go with the smallest option and be in the clear. Size generally goes hand-in hand with power. The rule of thumb is usually the larger the drive, the more power it can deliver. However, the push for more compact servo solutions in the industry creates some exceptions to the trend, such as ADVANCED Motion Controls' micro-sized servo drives. A servo drive made this year may very likely have higher power than a larger servo drive made a few years ago, but the general rule still holds true.

Drives don't just come in different sizes, they come in different form factors too. This affects how a drive will be integrated into your system. Panel mount servo drives are the traditional servo drive form factor. They have a bracket shaped heatsink that is used to mount them to an electrical panel inside or outside of the device, hence the name. Panel mount drives come in a wide variety of sizes, and thus a wide range of power ratings. If the frame of your device allows it, then you might want to consider a panel mount drive.

However, if you need a more compact, embedded solution, you should consider a PCB mount servo drive, also known as a plug-in style drive. These drives typically plug directly into a control board or mounting card which has all the connections to the motor, supply voltage, feedback, etc. PCB mount servo amplifiers are most commonly seen in portable devices, robots, and smaller machinery. If you're tight on space, chances are a PCB mount drive will be your best bet.

For mobile robotics like autonomous guided vehicles, you might want to use a vehicle mount servo drive. These drives are completely encased in a hard shell, keeping them safe. Like panel mount drives, they have a large heat sink that is usually bolted to the frame of the device. For rugged applications that can accommodate a typically large-sized drive, a vehicle mount drive is an excellent choice.

Keep in mind that form factor does not determine a drive’s performance, it just determines the way the technology is packaged. There are PCB mount drives that can outperform panel mount drives and vice versa.

Operating Modes

This is again where you have to ask, "what is your device doing?" This is very important for determining how your motor should be controlled, and therefore what operating modes your servo drive needs to have. The most common operating modes are torque (current), velocity, and position mode.

If you want a motor that runs at a specified torque and doesn't need compensate for any external disturbances, then torque mode is probably sufficient. In some applications, the main controller is better-suited to close the velocity or position loops, which allows the drive to simply be run in torque mode. Just about any servo drive you can find has torque mode.

Alternatively, velocity mode is available on many analog and digital servo drives. A drive operating in velocity mode uses a feedback device to keep track of the motor speed, so it will send extra power to the motor if it is failing to meet the velocity target. This is useful for applications like conveyor systems, machine tools and mobile robotics where you want the same performance regardless of any change in load.

Position mode is available on many digital servo drives. It's used when the motor's position is critical to the operation, such as in manufacturing robots and 3D printers. Like velocity mode, position mode relies on a feedback device to track its movement over time. Absolute position feedback devices can identify a rotor's true position at any time, regardless of when the device was powered up.

As mentioned previously, the velocity loop and position loops can be closed by a controller instead, but drives with velocity and/or position mode can take this burden off of the controller or offer those levels of control in standalone applications that don't have an external controller.

Take time to consider what kind of control your application needs, then you'll be able to determine which operating mode your servo drive needs. If you need a servo drive that can switch operating modes for different tasks, AMC digital drives have on-the-fly mode switching as an option.

Centralized vs. Distributed Control (Analog vs. Digital)

In a single-axis motion system, your control layout is less of a concern; whether you pick an analog or digital servo drive may not make much of a difference. In a system with multiple motor axes, however, then you have a bit more to think about. Your choice of servo drive greatly depends on how you're setting up your motion control system, particularly in deciding between analog and digital.

Will it be alright if all your control is coming from one centralized source and sent out to each drive individually? If so, you could set up your system with either digital or analog servo drives as long as you have a powerful controller that can handle the logic. You also need to connect each servo drive individually to the controller, which usually requires lots of cabling and that all the devices be somewhat close together. AMC's analog servo drive family, AxCent™, is designed for centralized control.

However, if you want to distribute control throughout the system using a network, then you will need to use digital servo drives such as those in AMC’s DigiFlex® Performance™ and FlexPro™ drive families. Digital servo drives can take some of the processing load off of the controller by using their own internal processors and memory storage. Position control can be achieved by sending individual position points where the servo drive uses linear interpolation to define the path between the points (cyclic synchronous mode), or use 3^rd order interpolation to define the path between points (PVT and PV modes).  For simple motion paths or where the motion is always repeated, pre-configured moves on the drives can be triggered by simple commands from the controller or from another drive.

Network

As previously mentioned, digital servo drives allow for network control, allowing the controller to send its commands in one package along a "daisy chain" of servo drives rather than out to each one individually. This can greatly simplify wiring. Networks are also two-way, allowing the drive to send information back to the controller.

Different servo drives are designed for different networks, however. If your application is already set to use a certain network, then its best to stick with that for simplicity. If you aren't tied to a specific network for your motion control system, you have a bit more freedom to choose.

For a realtime network that will keep everything synced perfectly, you should pick a servo drive with ethernet-based network communication such as EtherCAT, Ethernet Powerlink, PROFINET, Ethernet/IP, and many others. If you want a more traditional and inexpensive network that's still robust and reliable, you may want to choose a CANopen or Modbus servo drive.

Need a Custom?

Let's say you've gone through all the criteria your servo drive needs to meet, and then you go to search and you can't find a single drive that checks all the boxes. Where do you go from there? In some cases, you may have to tweak your design. Sometimes a small adjustment is all it takes. Other times, redesigning isn't enough or isn't an option at all.

This is where you should consider a modified or custom servo drive solution. Modified servo drives can have various tweaks such as logic inversion, protective coating, or even silk-screening your logo on the case.

If a modification isn't enough, then you may need to seek out a custom servo drive solution. Custom solutions are servo drives designed and built solely with your application in mind. A custom servo drive can dramatically make a difference for your application and unlock its full potential, as it is specifically tailored for your device. Custom solutions are often the most economical solutions when considering the integration problems that they can solve along with their application specific functionality

ADVANCED Motion Controls is always ready, willing, and able to design and build custom servo drives for the right project. Today, custom and modified products each make up approximately one third of our business, the remaining third being standard products.

You are Not Alone

Does all of this seem overwhelming? Please don't worry. We are here to help. At ADVANCED Motion Controls, we don't just sell you servo drives; We work with you to design a motion control system that works for you. We will recommend motors, cabling, and controls, but most importantly, we will help you pick the servo drive that is perfect for your application. We have over 32 years of experience in the industry, so please let us use it to help you.

by Jackson McKay, Marketing Engineer