When searching for a servo drive, you’ll be able to find thousands of options. ADVANCED Motion Controls alone offers over 200 servo drive models, and that’s without considering custom options. Selecting a servo drive requires careful consideration of the needs of your project and the options available. This guide covers the factors that decide the selection and the rules of thumb that apply to each.
In a hurry? Download the servo drive selection cheat sheet, or jump to the selection checklist.
What Are the Factors That Determine Servo Drive Fit?
Servo drives have different power ranges, sizes, network options, intelligence, form factors, digital features, and more. The factors below are the ones that decide whether a drive fits your application.
| Factor | What it decides | Rule of thumb |
|---|---|---|
| Application requirements | Power range, control type, operating modes | Define which elements of your design are flexible and which are set in stone |
| Motor and feedback | Whether the drive can commutate the motor and read its position | Choose the motor and feedback first, then match motor type, feedback protocol, voltage and inductance |
| Current rating | Whether the motor delivers the torque and velocity you want | Include an additional 25% of headroom above what you think is needed |
| Supply voltage range | Whether the drive runs without nuisance trips | Keep at least 25% headroom to both the undervoltage and overvoltage limits |
| Isolation | Whether the system survives first power-up | Isolation in the power supply or in the servo drive, at least one of the two |
| Operating mode | How your motor is controlled | Torque, velocity, or position, depending on what your device is doing |
| Control architecture | Centralized or distributed, analog or digital | Analog or digital for centralized control, digital for distributed control over a network |
| Network | How commands reach the drives | Ethernet-based for real-time sync, CANopen or Modbus for a traditional, inexpensive network |
| Form factor | Whether the drive fits inside your machine | Panel mount, PCB mount, or vehicle mount |
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.
How Application and Usage Needs Point to the Right Servo Drive
Your motion control solution should be based on the needs of your application, not the other way around.
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? The answers set your 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. Do you have to use a specific motor?
Is there a tight space requirement?
Are you married to a certain network communication?
If your parameters are 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.
The way the machine is used matters as much as the way it is designed. Problems discovered late, after production starts or after they get to the customer, are the worst. The rules of thumb in the sections that follow exist to help you avoid them.
Motor and Feedback: Match the Drive to the Motor
A servo drive runs one specific motor and reads its feedback device, so the drive decision follows choosing the right servo motor for your load’s torque, speed, inertia and duty cycle.
Rule of thumb: Pick the motor and feedback device first, then a drive that supports both.
- Motor type. Brushed motors commutate mechanically. Brushless and linear motors rely on the drive, using Hall sensors (trapezoidal) or an encoder (sinusoidal, for smoother torque at low speed).
- Feedback. The drive must read the device in its native format: TTL incremental, 1Vp-p sin/cos, BiSS-C or EnDat 2.2 absolute, or resolvers. With absolute encoders, the protocol has to match.
- Voltage. The motor’s back-EMF constant times its top speed must stay well below the supply voltage.
- Inductance. Low-inductance slotless and linear motors may need a series inductor to meet the drive’s minimum.
See which third-party servo motors and feedback devices have worked with our drives.
Current: Make Sure There’s Enough Current, but Don’t Get Too Much
Power range is perhaps the most crucial factor in servo drive selection. 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.
Make sure there’s enough current
You designed a delivery robot that has been installed at a lot of locations. Everything checked out during development and initial customer testing, then a customer calls because their robots keep getting stuck on an incline.
After taking some measurements you see that the servo drives are hitting their current limits.
Rule of thumb: Include an additional 25% of headroom to the current limit above what you think is needed for the application.
Why it’s important: Additional current capacity can ensure uninterrupted operation when the conditions are different from what you expected, and the application environment can be much different from the testing environment.
When it helps:
- When conditions change. In the scenario above a new location had a steeper incline than what the robot was tested for. On a flat surface, hitting the current limit could just mean slower acceleration. On inclines hitting the current limit could mean a stall.
- When machines get older. The friction can increase over time necessitating more power to do the same thing.
- When the load changes. Customers can increase the payload.
If you increase the current limit then make sure the motor can handle the additional current.
Don’t get too much power
If the current limit is left at the drive’s rating, an over-powered drive can push far more current than the motor is designed for. In that situation you should turn down the current limit to protect the motor, assuming the motor was appropriately sized to the application.
In most cases too much power won’t affect performance, but it can increase costs. If your system only uses 2A continuous and at most 6A peak then it really doesn’t make sense to have a servo drive that outputs 30A continuous and 60A peak.
A more powerful drive is also usually bigger, and a drive scaled for much higher current may not have the fine current control needed for the application.
A closer look at current scaling
The current loop is the innermost loop and is the foundation to a high performance system. When we design our servo drives we scale the current feedback to the rating of the servo drive.
If you take a 50A drive, the full range of output, in this case -50A to +50A, needs to be represented with a -10V to +10V signal, or in the case of a digital drive it needs to be represented with a set number of bits, 2^14 bits for example. For this example the scaling would be:
(±10V signal) / (±50A output) = (20V range) / (100A range) = 200mV/A for analog, or about 160 bits/A for digital
The higher the current rating on the servo drive, the more spread out the current feedback needs to be scaled.
As the feedback needs to cover a wider range you start to lose resolution for small current control. If you have an application that needs low current, you’ll get better current control by selecting a servo drive that is more closely rated to the application.
Supply Voltage: Allow for Voltage Fluctuations
Servo drives have a supply voltage range, so you need to make sure that you can properly provide that voltage for the servo drive. Our guide to selecting a power supply for a servo drive walks through the sizing in detail.
Rule of thumb: Whenever possible, you should select a power supply and servo drive combination that gives you at least 25% headroom between the power supply voltage AND both the undervoltage limit and the overvoltage limit of the servo drive.
Why it’s important: Voltage fluctuations occur for a number of reasons and cause nuisance trips for both under and overvoltage conditions.
When it helps:
- When the line voltage at the installation is different from the line voltage where you developed the machine. Differences can be especially pronounced when comparing voltages in different countries. Since many motion control systems use unregulated supplies, any changes in the line voltage will change the voltage that is supplied to the servo drive. For a servo drive that’s rated to 20-80VDC, an unregulated 24VDC supply could conceivably be too low in some places, and an unregulated 80VDC supply could be too high in some places.
- When regeneration is a factor. Regeneration is the moment when the motor changes from consuming power to becoming a generator. The regeneration voltage can go much higher than most people would expect, resulting in over-voltage shut downs and in some cases even damaged components. Regeneration is the most severe on vertical applications and during heavy deceleration.
- When batteries run low. With battery powered applications you should take into consideration the under-voltage limit and make sure the servo drive will still work when the battery runs low and you suddenly start to draw full power.
Isolation: Make Sure You Have Isolation
Use a non-isolated power supply with a non-isolated servo drive, and at first power-up the floating ground on the power supply creates a huge current that travels through the servo drive’s signal ground and through the controller’s ground as well, burning out the servo drive and the computer connected to it.
The simplest kind of power supplies consist of a full wave rectifier and a capacitor. They provide a lot of power at low cost. They’re also non-isolated, which means they have a floating ground where the voltage potential between the power supply ground and earth ground is different, usually by over 150V.
There’s a lot of power behind the difference, so a pull-down resistor is out of the question. These kinds of power supplies are perfectly fine to use with servo drives with one condition: the servo drives that connect to them need to have isolation between the power ground and signal ground.
With a non-isolated servo drive the power ground and signal ground are internally connected together.
The signal ground is also connected to the controller, network cards, computers, logic supplies and cable shielding, and with all of these connections there are bound to be multiple paths to earth ground.
Rule of thumb: To prevent problems at least one of these two things is needed:
- An isolation transformer between the AC line and the power supply’s power ground, AND/OR
- Optical isolation between the servo drive’s power ground and signal ground.
The information should be in the data sheet. Note this rule doesn’t apply to battery powered systems or for servo drives that take AC power directly.
Power supplies that don’t have transformers are less expensive and more powerful. They are safe to use if the servo drive has built-in isolation.
Servo drives without isolation can be made smaller and less expensive. They are safe to use if the power supply has an isolation transformer.
When it helps: Whenever the power comes from an AC source. Pay special attention when using power supplies that output voltage in the range of 100 – 200VDC. Power supplies rated below 100VDC usually use step-down transformers to lower the voltage, and these transformers also provide isolation. Servo drives rated to operate above 200VDC are usually isolated due to safety and noise concerns.
Operating Modes
What your device is doing determines 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 to compensate for any external disturbances, then torque mode is probably sufficient. Just about any servo drive you can find has torque mode.
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. Absolute position feedback devices can identify a rotor’s true position at any time, regardless of when the device was powered up.
The velocity 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. 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, whether you pick an analog or digital servo drive may not make much of a difference. In a multi-axis system it does.
If all your control is coming from one centralized source and sent out to each drive individually, 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 drives, the AxCent family, are designed for centralized control.
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 3rd order interpolation (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
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. Networks are also two-way, allowing the drive to send information back to the controller.
If your application is already set to use a certain network, then it’s best to stick with that for simplicity.
For a realtime network that will keep everything synced, you should pick a servo drive with ethernet-based network communication such as EtherCAT, Ethernet Powerlink, PROFINET, or Ethernet/IP.
If you want a more traditional and inexpensive network that’s still reliable, you may want to choose a CANopen or Modbus servo drive.
Size and Form Factor
If a drive won’t fit, then you can’t use it. For applications such as robotics or portable devices, finding a servo drive that fits inside is crucial.
You can’t always go with the smallest option and be in the clear. Size generally goes hand-in-hand with power: the larger the drive, the more power it can deliver. The push for more compact servo solutions creates some exceptions to the trend, such as ADVANCED Motion Controls’ micro-sized servo drives, but the general rule still holds true.
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, and they come in a wide range of power ratings.
PCB mount servo drives, also known as plug-in style drives, are the choice if you need a more compact, embedded solution. These drives typically plug directly into a control board or mounting card which has all the connections to the motor, supply voltage and feedback. They are most commonly seen in portable devices, robots, and smaller machinery.
Vehicle mount servo drives are for mobile robotics like autonomous guided vehicles. These drives are completely encased in a hard shell. Like panel mount drives, they have a large heat sink that is usually bolted to the frame of the device.
Form factor does not determine a drive’s performance, it only determines the way the technology is packaged.
When No Standard Drive Fits: Modified and Custom Drives
If no drive checks all the boxes, you may have to tweak your design. If redesigning isn’t enough or isn’t an option, 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 a custom servo drive solution. Custom solutions are often the most economical solutions when considering the integration problems that they can solve along with their application specific functionality. Today, custom and modified products each make up approximately one third of our business, the remaining third being standard products.
Servo Drive Selection Checklist
| Step | What to check | Target |
|---|---|---|
| 1. Define the application | What you’re making, single or multi-axis, the task, what is set in stone | A short list of candidate drives |
| 2. Motor and feedback | Motor type, feedback device and protocol, back-EMF at top speed, motor inductance | Drive supports all four |
| 3. Current | Continuous and peak current from the motor and load | 25% headroom, within what the motor can handle |
| 4. Supply voltage | Power supply voltage against the drive’s limits, including regeneration and low battery | 25% headroom to each limit |
| 5. Isolation | Power supply and servo drive data sheets | Isolation in at least one of the two |
| 6. Operating mode | Which loops the drive closes and which the controller closes | Torque, velocity, or position |
| 7. Control architecture | Centralized or distributed control | Analog or digital |
| 8. Network | Existing network, or realtime sync requirement | Ethernet-based, or CANopen/Modbus |
| 9. Form factor | Available space and environment | Panel, PCB, or vehicle mount |
Every application is different, so as the term “rule of thumb” implies, none of these rules are absolute, except one. Always talk to technical support. Online we usually respond within one business day. On the phone we have staff on hand during our normal business hours.
At ADVANCED Motion Controls, we sell you much more than servo drives. We will help you pick the servo drive that is perfect for your application. We have been designing and building servo drives since 1987, so please let us use that experience to help you.
