We all make mistakes. Even people who've been working with servo drives for over a decade make mistakes. I've even seen it happen countless times in our own labs. Heck, I'll admit I've made mistakes while setting up servo drives on a non-zero number of occasions. So it shouldn't come as a surprise that our customers also run into trouble once in a while and ask for help.
To make your next servo drive installation easier we asked over 200 motion control engineers what are the most common issues users have when installing servo drives. Here are the top four areas where things can go wrong.
- Not Reading the Datasheets and Hardware Manuals
- System Troubles
Not Reading the Datasheets and Hardware Manuals
This seems obvious, but 30% of our respondents stated their most common issue was customers simply not reading through the datasheets and hardware manuals. Not good.
There are many specific wiring and configuration errors that can result from not reading the instructional materials, and we'll explore those in more detail later, but let this be a general piece of advice:
PLEASE READ THE MATERIALS WE GIVE YOU.
You can find each product's datasheet and appropriate hardware manuals on its product detail page.
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55% of respondents cited issues that fall under the broad umbrella of wiring. Wiring mistakes are incredibly common and often result in the system malfunctioning or not running at all. A simple wiring error can be the catalyst that pushes an engineer to the edge of their sanity.
Some instances of wiring error are procedural. A customer might not see that they need to hook up a separate power supply for drives that require separate logic power, or they might not pull inhibit pins to ground on drives with inverted inhibit logic.
Situations like these can be avoided by, again, reviewing the datasheet and hardware manual.
"Sanity check the basics. Logic power (if needed) and wiring. Check that power is being applied. You may feel silly, but you will feel even more silly if you spend an hour banging your head against the wall then come back to check the basics, and it was in fact a basic issue."
- Nash Dingman, ADVANCED Motion Controls
But sometimes you follow all the steps and something still isn't working. It's possible you mixed up two wires or miscounted on the pin board. These types of wiring mistakes can be especially frustrating because finding where you went wrong can be difficult. If you've double-checked the hardware manual, triple-checked the datasheet, and banged your head on your desk for at least 7 minutes, it might be time to bring in another set of eyes to look at your setup.
"Have somebody else checking wiring for you, sometimes it is hard to catch our own wiring mistakes."
- Nicolas Cantin, Electromate
We discussed missed connections, but what about connections that just aren't done well? This can be something as simple as a poorly crimped pin, untightened connector screw, or anything else that results in a poor or failed electrical connection. About 20% of our respondents said that loose connections were among the most common sources of error they've encountered with customers.
I ran into this problem with one of our ball toss units. We have a really cool eye-catching demo where we toss a 1 inch ball bearing through a spinning ring then catch it (you can check it out in the video). At a trade show one of the linear motors started to overshoot its position and slam against the frame. The problem was intermittent and totally unpredictable, it would work for a few minutes then SLAM! It would crash. Not only frustrating for us, but totally embarrassing.
We couldn't figure out what was going on so we shut down the demo and took it back to the office.
We ended up tearing the thing apart trying to find out why this was happening. We tried adjusting the encoder resolution, we tried adjusting the current limits, we tried for weeks to fix the problem. It turned out the motor phase cables were just connected too loosely. A few screw turns on a Phoenix connector would have saved us weeks of stress.
"I'll be right back; I need to go scream and cry in my office for a few minutes."
The moral of this story, even if everything looks good at a glance, give all your cable connections a light tug to make sure they're all secure. If your system is working sometimes but not all the time, there's a good chance you have a loose wire.
Wrong Motor Phasing
Speaking of motor phases, make sure you have your motor phase cables plugged in in the correct order when using a three-phase servo motor, because yes, that's really important. Using the incorrect motor phase combination can result in the motor performing with low torque or even not running at all.
With an analog drive, you will have to manually check each of the six combinations to make sure you find the one that works best.
And yes, check all six. I don't care if you think you found it on the third try, check all six. There's often a combination that appears to work decently but isn't the best. Check. All. Six.
For digital drives, the job is a bit easier. You can simply use DriveWare or ACE's phase detect for the first time setup.
Poor EMC Practices
Another type of wiring problem that is often overlooked is poor EMC (electromagnetic compatibility) practices. Poor EMC practices aren't likely to prevent a system from running, but they can result in EMI (electromagnetic interference), also known as electrical noise, that hinders performance.
There are lots of steps to be taken to reduce EMI, but our respondents cited these as ones that customers often forget.
- Positive and negative leads of the same circuit should be run wrapped in twisted pairs so that they reduce electromagnetic fields by canceling each other out.
- Your drive and entire electrical system should be properly grounded.
- Your communication cables should be shielded for most of their length.
You can find additional practices for reducing EMI that you can follow in the video below.
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Installing a servo drive isn't like adding RAM or a new graphics card in a PC. Servo drives need to be configured to your system in order to work correctly, and many people don't realize this.
"They expect to pull them out of the box, plug them in, and have them work in their application. We all know that this isn't practical or possible"
- Jim Meek, Automation Support Group
1/3 of respondents cited issues that fall under the category of configuration.
At least 99% of the time, the default gains of a digital servo drive aren't going to be what your system needs because every system has different inertial properties and torque/velocity/position requirements.
At the very minimum, a servo drive needs to be current loop tuned in order to get motion in your system. For digital drive current loop tuning is a necessary step before performing Auto-Commutation in DriveWare or ACE. Don't just assign random gains either; use a scope as outlined in the quick start guide (once again, read the instructional materials please), and work methodically.
On the flip-side, you also don't want to over-tune your drive. Tuning the current loop too aggressively can overwork your system and cause excess audible noise.
Don't try to tune the loops out of order either. Always tune the current loop first, then the velocity loop, then the position loop.
Not Running AutoComm
I just mentioned that many users attempt to run AutoComm (Auto-Commutation) before they tune the current loop, but what's arguably worse is not running AutoComm at all. If your drive is poorly commutated, you're going to have a bad time, and you're rarely going to do a better job manually commutating than you would with AutoComm.
If you're using a digital drive, please use AutoComm in ACE or DriveWare after you've tuned the current loop. It will make your life a lot easier, I promise.
Incorrect Motor Parameters
For digital drives, don't forget to enter the correct motor parameters into the configuration file in DriveWare or ACE.
Some motors will be recognized by the drive and the parameters will be filled in automatically, but it's still important to double check and make everything is right.
This includes making sure you're using the correct encoder resolution, especially if the motor is not using the encoder it came with.
Motor manufacturers typically make the datasheets for their motors available online. These datasheets should contain all the correct motor parameters. Big surprise, we suggest reading them.
A servo drive can only work as well as the system it's in. The drive might be connected and powered up properly, but it's not going to you much good if your system isn't ready for it.
Motor and Drive Mismatch
You don't use an RC car to tow a bus and you don't use a jet engine to blow out birthday candles. Servo drives should be sized (in terms of power) appropriately with the motor they are driving.
"As the boundaries of motion control have shifted from a centralized approach (smart controller and dumb amplifiers) to a more decentralized approach where motion planning and tuning is taking place on the drive, more attention is needed on the sizing. Good knowledge of the load, inertia, and planned trajectories is necessary to make sure that the drive has the bandwidth to control and settle the load as needed. "
- Brian Prescott, Motion Control Products
This one should be obvious, but we see instances of customers trying to purchase or run drives that are significantly underpowered for their applications. If your drive isn't rated to output enough power, you're not going to achieve the desired torque or speed requirements.
The issues with using an overpowered drive are less obvious, but still problematic. Trying to run a low power motor with a high power drive is not only a big waste of money, but it's difficult too.
Imagine you're running a 150 amp servo drive at 4 amps, you're going to have difficulty getting precise control, especially if you're using an analog controller. The slightest change of input command will correspond to a dramatic change of output.
Motors have current and voltage ratings that should be compatible with those of the selected servo drives. Too little power and you won't be able to get the motor moving. Too much power, and you'll burn the motor out.
For more advice on picking the right servo drive for your motor, read our 5 Rules of Thumb When Selecting a Servo Drive.
Not Enough Inductance
Contrary to common belief, inductance is not the enemy. Some people will brag to us that they got their inductance down to almost zero and we hiss through our teeth because that's not what the goal should be. In fact, most servo drives have a minimum inductance requirement.
If the motor inductance is too low, the current loop becomes unstable and difficult to control. The response will oscillate and cause the motor to overheat. If your motor or system needs more inductance, you may need to add inductors to the motor phase cables.
Compare the motor inductance on the motor datasheet with the minimum load inductance on the servo drive datasheet and make sure they're compatible. If you have a choice between two motors try to favor the one that meets the minimum load inductance requirement, otherwise you may need to add additional inductors to the low-inductance motor. This increases weight, takes up room and increases cost.
That all said, you don't want to over-do the inductance either, as this can cause the current loop to be sluggish.
Reach out to our tech support if you have concerns so we can evaluate your system. Current loop stability and efficiency is determined by a number of factors including motor inductance, switching frequency and bus voltage.
Trying to do it all at once
If you have a complex mechanical system, it's inadvisable to install a servo drive fresh out of the box and attempt to do the configuration and tuning with everything connected. Doing it this way is not only difficult, it can also be dangerous. You don't want a poorly tuned robot arm to start spinning around uncontrollably.
Instead, disconnect the motor from the rest of the system and run it on its own with the servo drive. Yes you will likely have to re-tune once you connect the motor to the mechanical system again, but you will be at a much better and safer starting point. Poor tuning is inconvenient, but it's far better than having your whole system in a runaway condition or spinning the wrong direction.
Mistakes are going to happen during servo drive configuration, and those mistakes are going to be much less consequential when your motor is disconnected from the rest of the mechanical system.
We never want to discourage people from asking questions and seeking assistance. Here at ADVANCED Motion Controls, we'll always be here to support you. That's why we have our Support Portal and a technical support team available during all operating hours.
That said, we hope this guide will help you avoid some of these common mistakes the next time you're setting up a servo drive, and perhaps you won’t even our help.
If there are three main takeaways from this blog, they should be the following:
- We're all human, and even motion control experts make mistakes.
- Seriously, for the love of all that is good and decent in this world, read the #$!&ing datasheets and hardware manuals!
Look, we get it. Manuals aren't exactly riveting to read, but come on, we don't make them for our own enjoyment.
The hardware manuals give you installation instructions for different servo drive groups, while the datasheets give the pinouts, switch settings, and other key information for individual drive models. Together, they should provide just about everything you need to know about how to get your drive wired into your system.
We make them available to you for a reason, and we've spent a lot of time working on them to make them as helpful as possible. It will make your setup and installation process much easier, I promise.
JUST READ THEM. I MEAN IT.
- Reach out to our technical support for assistance. They're always happy to help 🙂
by Jackson McKay, Marketing Engineer