12.03.2026 by Viktor Siebert
Preventive overhaul of a Yaskawa AC servo motor SGMM-B5SF13 with encoder UTOPI-20ANS due to unstable running behavior
Initial situation and fault symptoms
The examined servo motor is a Yaskawa AC servo motor from the SGMM series with the model designation SGMM-B5SF13. A position feedback device type UTOPI-20ANS encoder is installed in this motor. The motor operates in the system together with a servo drive type SGDF-B5CS.
This assembly is typically used in small machining machines, automated production systems or compact robotic systems. The motor power is relatively small, but the requirements for positioning accuracy are high.
The operator reported unstable axis behavior at low speeds. In addition, occasional position deviations occurred during operation. In some cases the fault appeared after the reference run. In other cases the disturbance occurred during a slow positioning process.
It was noticeable that the axis initially ran stable again after a machine restart. After some operating time the disturbance appeared again. This behavior often indicates mechanical wear or changes in the signal quality of the position feedback.
Since the motor works with an incremental encoder UTOPI-20ANS, stable mechanical motion is especially important. Even very small irregularities in the rotor movement can lead to position deviations.
Incoming inspection and first diagnosis
After arrival in the workshop a complete incoming inspection was carried out.
The first step consisted of a visual inspection of the motor. Housing, connection cable and mechanical interfaces were checked. No external damage could be detected.
In the next step the mechanical movement of the motor shaft was checked. A slightly rough running behavior could be felt. This behavior often indicates early wear in the mechanical bearing system.
Electrical base measurements were then performed. These included an insulation measurement and verification of the phase resistance values. The measured values were within the expected range.
The motor was then operated on a test bench using a suitable servo drive SGDF-B5CS. The reported behavior could be reproduced. Especially at low speeds unstable running behavior appeared. At the same time small irregularities were visible in the signal quality of the encoder UTOPI-20ANS.
This allowed the fault area to be limited to the mechanical motion of the motor and the feedback system.
Technical analysis
Servo motors of the SGMM series are constructed in a very compact design. The mechanical structure is precisely manufactured and operates with very small tolerances.
With increasing operating time wear can occur in the mechanical system. The rotating support structure is permanently exposed to mechanical loads.
If slight wear develops there the rotor running behavior changes slightly. These changes are often hardly visible mechanically but directly influence the position feedback.
The encoder UTOPI 20ANS installed in the motor continuously detects the position of the motor shaft. The signals are evaluated by the servo drive and compared with the commanded position.
If mechanical irregularities occur small fluctuations appear in the signal pattern. The servo drive interprets these deviations as position errors or unstable control behavior.
The cause effect chain can therefore be described as follows.
Mechanical wear in the rotating support leads to minimal running deviations. These deviations influence the signal quality of the encoder UTOPI 20ANS. The servo drive detects position deviations and reacts with unstable control behavior or alarm messages.
This behavior is a typical aging effect in older servo motors of this size.
Repair measures and restoration
The servo motor was completely disassembled and all functional assemblies were inspected.
Wear was detected in the rotating support structure. These assemblies were therefore replaced.
The feedback system with encoder UTOPI-20ANS was then inspected. Signal quality, mechanical alignment and electrical connection were checked.
The feedback system was cleaned and readjusted. Afterwards the signal quality was verified under simulated operating conditions.
All mechanical components were thoroughly cleaned and then reassembled with precise alignment.
Additional preventive measures were also carried out. These included inspection of the motor cable, shielding and connector condition.
These measures are particularly important for older machines because many disturbances develop from a combination of mechanical wear and electrical influences.
Final functional test
After completion of the restoration the motor was tested on a servo test bench.
The test bench consisted of a servo drive SGDF-B5CS, a stabilized power supply and measurement systems for analyzing speed, current consumption and encoder signals.
The test started with verification of the power on and power off behavior. Afterwards a reference run was performed.
Different speed ranges were then tested. Particular focus was placed on operation at low speeds because control deviations are most visible in this operating range.
Medium and high speed ranges were also tested. Thermal stability and current consumption were monitored as well.
All tests were stable and reproducible. The encoder UTOPI-20ANS delivered a clean and consistent signal.
Conclusion
The servo motor showed typical wear effects in the mechanical system which caused instability in the position control.
By refurbishing the mechanical assemblies and verifying the position feedback with encoder UTOPI-20ANS the motor could be completely stabilized.
After restoration the motor operates reliably again and can continue to be used in the machine.
Information about the mentioned Servopack and Servomotor:
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Technical Specifications
| Parameter | Value |
|---|
| Manufacturer | Yaskawa |
| Device type | AC servo motor |
| Model designation | SGMM-B5SF13 |
| Series | SGMM |
| Power | approx. 5 W |
| Input voltage | approx. 48 V |
| Output voltage | depending on the servo drive |
| Rated current | approx. 1.5 A |
| Control type | Servo control |
| Feedback | Incremental encoder UTOPI 20ANS |
| Cooling | Self cooling |
| Protection class | approx. IP40 |
| Ambient temperature | approx. 0 to 40 °C |
| Mounting | Machine integration |
| Origin | Japan |
| Product status | partly discontinued |
Operating environment and possible applications
Servo motors of the SGMM series are often used in compact automation systems.
Typical applications include small CNC machining machines, automatic assembly lines, pick and place systems and compact robotic systems.
Many machines equipped with these motors originate from production years between approximately 1995 and 2010.
Due to their compact design these motors are often installed in machine areas with limited cooling. This can lead to elevated operating temperatures.
Stable power supply, good cable shielding and controlled thermal conditions are important for reliable operation.
Functional description
The servo motor converts electrical energy into a precisely controlled rotational movement.
The servo drive SGDF-B5CS controls torque, speed and position.
The encoder UTOPI-20ANS installed in the motor continuously provides position signals. These signals are evaluated by the servo drive.
The control system compares the commanded position with the actual position and corrects deviations immediately.
In addition the system monitors several protection functions. These include current monitoring, temperature monitoring and encoder signal monitoring.
These functions are safety relevant because they help prevent damage to the motor and the machine.
Alarm messages and troubleshooting
| Alarm code | Description | Possible cause | Recommended action |
|---|
| A.02 | Parameter breakdown | corrupted parameter data | check parameters |
| A.04 | Parameter setting error | incorrect configuration | verify parameter settings |
| A.10 | Overcurrent | blocked axis or mechanical resistance | inspect mechanical system |
| A.31 | Position error pulse overflow | excessive position deviation | inspect feedback and mechanics |
| A.51 | Overspeed | abnormal speed condition | check control parameters |
| A.70 | Overload | excessive mechanical load | check machine load |
| A.80 | Encoder error | encoder signal fault UTOPI 20ANS | inspect encoder and cable |
| A.C2 | Encoder output phase error | incorrect encoder signal phase | check feedback signal |
| A.C3 | Encoder phase A B disconnection | signal interruption | check encoder wiring |
| A.C4 | Encoder phase C disconnection | encoder signal failure | inspect feedback system |
Assembly overview
| Assembly | Functional designation | Function | Notes for inspection or repair |
|---|
| Power section | Motor winding system | Generates motor torque | insulation measurement |
| Mechanical section | Rotor and support structure | Motor rotation | running behavior inspection |
| Feedback system | Encoder UTOPI-20ANS | Position feedback | signal quality verification |
| Connection interface | Motor cable | Power and signal transmission | cable inspection |
| Housing structure | Protective enclosure | Mechanical protection | visual inspection |