01

As an “old hand” who’s been working with servo systems on-site for many years, I’m often urgently called in by clients for “consultations.” The situation on-site is usually something like this: The operator points at the equipment and says, “Engineer, lately this machine starts shaking whenever it’s running, or it can’t quite lock onto the right position—could you take a quick look and see if there’s any mechanical looseness?”

And the first thing I usually do isn't to pick up a wrench—it's to head straight for the electrical control cabinet and open the panel of the servo drive.

Because, For all the “instabilities” in the equipment, the servo drive has likely already alerted you to the answer via an alarm. It’s like the device’s “black box,” recording the most critical pathological information just before a failure occurs.

02

Why should we first check the servo drive alarm when dealing with “instability”?

A servo system is a closed-loop control system that continuously compares... “Instruction” and Feedback the difference. Once this difference exceeds the “tolerance range” set internally by the drive, it will immediately take action: first by triggering an alarm; and if the problem persists or becomes severe, it will promptly sound an alarm and shut down the system to prevent further damage to the equipment and products.

The “instability” exhibited by the equipment—such as jitter, abnormal noises, overshoot, and large positioning errors—is precisely the external symptom of this closed-loop control being disrupted. The driver’s alarm codes serve as the primary clue for diagnosing exactly which link in this “closed loop” has gone awry.

03

The Alarm Culprits Behind Several Common “Instability” Phenomena

Here are several typical situations I’ve repeatedly encountered throughout my career:

1. Mild high-frequency jitter accompanied by a “hissing” abnormal sound.

Possible alarm/parameter:

Overload alarm; encoder feedback count is unstable (there may not be an explicit alarm, but it can be detected through real-time monitoring).

Experience Interpretation:

Overload Alarm: When the mechanical load suddenly increases—such as insufficient lubrication in the guide rails, wear on the lead screw, or an excessively tight synchronous belt—the motor’s output torque continuously exceeds its rated value. The drive detects an abnormal current and keeps attempting to correct the situation, resulting in jittering. Eventually, this triggers an overheating or overload alarm. The root cause often lies in the mechanical system.

Encoder feedback anomaly: The encoder cable is interfered with, the connector is loose, or the encoder itself is damaged, causing the position signal fed back to “drop frames” or become “disordered.” When the driver receives “unreliable” feedback, it will frantically adjust its output in an attempt to catch up with that “illusory” target. As a result, the motor emits abnormal noises during high-frequency oscillations.

Reference solution:

✅ Check the smoothness of mechanical transmission.

✅ Tighten the encoder connector and ensure that the shielded cable is reliably grounded.

2. The positioning of the endpoint is inaccurate, and the error location varies each time.

May call the police:

The following error is too large.

Experience Interpretation:

This is one of the most classic alarm mechanisms. The driver sets a “maximum allowable tracking error”—the maximum difference between the commanded position and the actual feedback position. When the load suddenly increases, the gain setting is too low, or the commanded speed is too high, the motor “can’t keep up” with the command, causing the error to accumulate beyond the limit and triggering an immediate alarm. As a result, the motor either fails to stop precisely where it’s supposed to or shuts down directly in response to the alarm.

Reference solution:

✅ Appropriately increase the position loop gain of the driver.

✅ Check whether the instruction speed and acceleration are reasonable.

✅ For long-term issues, it’s necessary to reflect on whether the motor torque has been selected too small.

3. The machined arc is not perfectly round, and the surface has streaks.

Possible alarm/status:

Usually there is no hard alarm, but the current inside the driver fluctuates dramatically.

Experience Interpretation:

This is often caused by a mismatch in the dynamic responses of the two axes. For example, when the X-axis and Y-axis are used to interpolate a circle—one axis responds quickly while the other responds slowly; naturally, the resulting trajectory will not be a perfect circle. At a deeper level, the root cause could be inconsistent servo gain parameter settings for the two axes or differences in mechanical transmission backlash.

Reference solution:

✅ Use servo tuning software to perform frequency response analysis and automatic gain adjustment for the dual-axis system.

4. As soon as it’s enabled, it triggers an alarm—even blows up the module.

May call the police:

Overcurrent, ground fault.

Experience Interpretation:

This is the most troublesome—and yet the most critical—situation to be vigilant about. The reason is very straightforward:

Damage to motor or cable insulation—particularly with drag-chain cables subjected to prolonged bending—can lead to wear on the cable jacket and cause phase-to-phase or phase-to-ground short circuits. As soon as power is applied, a surge of high current instantly triggers the drive’s protective mechanisms.

Incorrect UVW phase sequence of the motor: Highly likely to occur after maintenance.

IGBT power module damaged.

Reference solution:

✅ Be sure to disconnect the power! Use a megohmmeter (megger) to measure the insulation resistance of the motor and cables.

✅ Confirm that the power cable phase sequence is connected correctly.

04

As a field engineer, please develop the following habits:

Confirm alarm code:

Write down the exact alarm code—don’t just say “the alarm went off.” Refer to the technical manual for the corresponding brand’s servo drive—this is the most reliable and official “medical record.”

Understand the meaning of the alarm:

The manual will provide detailed explanations of the alarm’s triggering conditions, possible causes, and recommended troubleshooting steps. For example, an “overvoltage alarm” might not only be caused by issues with the external power grid, but could also result from improper selection or damage to the braking resistor.

Troubleshoot from simple to complex:

First external, then internal: Start by checking external factors such as the power supply, cables, connectors, and so on.

First mechanical, then electrical: Disconnect the motor from the load and run the motor at no load. If it operates normally, there’s a 99% chance the problem lies on the mechanical side.

First check the parameters, then the hardware: Verify that the servo parameters are correct (especially those related to the encoder, inertia, and gain). Only after confirming that the parameters are accurate should you suspect hardware damage.

Make good use of tools:

All servo drives come with debugging software that allows you to monitor current, speed, and position curves in real time. These curves serve as “diagnostic tools,” enabling you to visually pinpoint the exact moment a problem occurs and gain a clear understanding of what the system has been through.

05

The alarm from a servo drive is by no means a source of trouble—it’s actually the device’s most loyal “guardian.” It tells you, in the most straightforward way, about the abnormalities occurring inside the system. Next time your equipment starts to “lose its stability,” put down the wrench, take a deep breath, and calmly decipher the “alarm language” of the servo drive.

If you understand it, you’ll become a “prophet” of equipment failures; if you ignore it, you might just end up the next “firefighter” rushing in to put out the blaze. Of course, choosing a servo system with high stability can help you solve over 90% of your problems—saving you both time and effort, and even sparing you from having to work overtime.