Root Cause Analysis & Closed-Loop Rectification of Intermittent Encoder Signal Loss Fault

This case targets intermittent encoder signal loss failure of NC servo feeder supporting a 160T servo punch press for precision new energy terminal stamping. The equipment configuration: 160T servo punch with servo crank drive, NC-600 servo feeder, material width 0–180 mm, designed maximum feeding speed 12 m/min, positioning accuracy requirement ±0.015 mm. The customer runs two shifts, 16-hour daily production. The fault first appeared 14 months after equipment commissioning, showing random emergency stop, HMI encoder communication alarm, feeding dimension drift. The customer’s internal maintenance team checked servo parameters and restarted the equipment several times, the fault temporarily disappeared but recurred after a few hours of production, which seriously affected delivery schedule.

Our case study team collected all alarm logs, on-site test data, maintenance operation records and repair reports, carried out layered root cause analysis, sorted out fault induction chain, evaluated the effectiveness of corrective actions, and formed standardized maintenance specifications and acceptance criteria, which can be reused for similar servo feeding encoder faults on other customer production lines. This case focuses on the hidden failure mode of weak signal transmission system under long-term vibration environment, which is a typical and easily overlooked failure in servo feeding synchronization control.

The fault has obvious condition correlation: it occurs more easily when the punch runs above 80% rated speed; it rarely appears under low-speed jogging mode. After alarm triggering, if the operator resets and restarts without changing anything, the equipment can run normally for 20 minutes to 3 hours before the next alarm. The customer sampled 30 groups of feeding dimension data during fault occurrence, the maximum feeding error reached 0.047 mm, exceeding the drawing tolerance of the terminal parts. The servo drive feedback showed that the encoder position value jumped randomly, and the communication cycle error exceeded the system threshold. Static measurement without equipment vibration could not reproduce the fault, which greatly increased the difficulty of preliminary diagnosis.

Our engineer team classified the possible failure paths into four categories: encoder hardware failure, servo drive communication port abnormality, signal transmission interference and mechanical connection failure. A set of controlled verification tests were designed one by one. The first test: swap the encoder with the spare encoder temporarily, the fault still existed, so encoder body damage was ruled out. Second test: replace the servo drive communication terminal and re-download the program, the abnormal alarm still appeared, eliminating drive and program factors. Third test: fix the coupling with locking glue and temporarily bind the cable to avoid friction, the fault occurrence frequency dropped sharply. Combined with oscilloscope waveform capture results, the research team locked the two root causes: shielded cable mechanical abrasion and coupling loose caused by cyclic vibration fatigue.

The original cable was laid along the machine frame without protective sleeve. The edge of the steel frame had sharp burrs. Under the continuous vibration generated by punch stamping, the outer jacket and shielding braid of the cable rubbed against the burrs for a long time and broke partially. The shielding layer cannot form a complete Faraday cage. The electromagnetic field generated by the punch servo motor, solenoid valves and frequency converter would couple noise into the signal wire. When the noise amplitude exceeds the signal identification threshold of the drive, pulse signals will be lost, triggering communication alarm. The intermittent characteristic comes from vibration: when vibration is slight, the broken shielding layer is still in contact; under strong vibration, the shielding gap opens, interference invades.

The encoder connects the feeding roller shaft through a flexible coupling. The set screw of the coupling only used ordinary fastening without thread locking treatment. Repeated impact vibration of stamping circulation produced micro-slip at the screw contact surface. The clearance between the driving shaft and encoder detection shaft gradually accumulated. When the clearance exists, the rotation angle detected by the encoder cannot synchronize with the actual rotation of the feeding roller, leading to position feedback lag and jitter. When the clearance is small, the error is within the allowable range; once the vibration makes the clearance instantaneously enlarge, the position feedback jumps.

The customer’s original maintenance checklist for the servo feeder only covered roller pressure, oil lubrication and air pressure check. Items related to encoder wiring, coupling fastening and signal shielding inspection were not included. There was no regular cable routing aging inspection mechanism. The maintenance personnel lacked systematic training on weak signal anti-interference knowledge, so they could not identify this type of hidden fault at the early stage of development.

The corrective actions are divided into immediate repair measures, on-site verification measures and long-term preventive control measures.

1. Immediate repair: Remove the old damaged signal cable, adopt high-flexibility double-shielded encoder cable, lay the wire through flame-retardant corrugated sleeve, fix the cable clamp at intervals of 300 mm, keep a distance ≥150 mm from all power lines and hydraulic valve lines, single-point grounding of the shielding layer at the drive side to avoid ground loop interference.
2. Mechanical rework: Clean the inner hole of coupling and shaft surface, apply medium-strength thread locker on set screws, use torque wrench to fasten according to the specified torque value. Check the coaxiality between encoder and feeding roller shaft, adjust the mounting bracket to reduce additional radial stress.
3. Parameter calibration: Reset encoder home position, carry out feeding length calibration, optimize the position filter parameter of the servo drive properly to suppress high-frequency noise without reducing response sensitivity, match the synchronization curve with the 160T servo punch main shaft.
4. Verification test: Segmented test, including low-speed jog test, medium-speed continuous running test, full-rated speed 4-hour aging test, manually tapping the cable and mounting bracket to simulate vibration, observe whether position jump and alarm appear.
5. Management improvement: Add encoder system inspection items into the customer’s PM checklist, compile the anti-interference wiring guide, deliver technical training for maintenance staff, set alarm log regular review mechanism.

After the whole rectification work was completed, the acceptance test was carried out jointly by our engineer and the customer’s production and maintenance supervisor. Within 8 hours of continuous full-load production, no encoder communication alarm popped up. 50 groups of feeding positioning data were sampled randomly, the repeatability was stably controlled within ±0.014 mm, meeting the product drawing tolerance requirement. The customer continued to track the equipment operation status for 30 days after service, and no similar fault recurred. The batch scrap rate caused by feeding dimension deviation dropped from 1.27% before maintenance to 0.11%, achieving the expected improvement target.

Servo encoder signal faults belong to hidden weak-electrical faults in stamping automation lines, which are easy to be misjudged as encoder or drive damage, leading to unnecessary spare parts replacement cost. For all NC servo feeders matched with servo punch presses, the following general preventive rules are summarized:

• All encoder, proximity switch and optical fiber weak signal cables must use shielded cables and take isolation routing measures; moving parts must use drag chain or flexible corrugated protection.
• Couplings for position detection components must use thread locking treatment after installation, and coaxiality check is required during installation and regular maintenance.
• The HMI alarm log shall be exported and analyzed regularly. Intermittent communication alarms are early warning signals, which shall not be simply eliminated by restarting.
• When retrofitting equipment or adding peripheral solenoid and power equipment, re-evaluate the electromagnetic interference risk of the existing signal wiring.

表格

Q1: What is the difference between single-point grounding and two-end grounding of encoder shielded cable? Which one should be adopted for punch workshop? A1: Two-end grounding is easy to form ground potential difference loop, generate induced current on the shielding layer and introduce new interference. In stamping workshops with many high-power equipment, single-point grounding at the drive side is the recommended scheme, and the encoder side shielding layer is suspended.

Q2: If the feeding system uses a rotary encoder, can we increase the filter parameter infinitely to avoid signal jump? A2: Excessively large filter parameters will increase the position feedback delay, reduce the synchronization response speed between feeder and punch spindle, and may cause feeding asynchronization and material pulling during high-speed operation. Filter parameters need to be balanced between anti-interference and dynamic response.

Q3: For new equipment installation, what acceptance items should be added for encoder signal system? A3: Acceptance shall include shielding continuity test, waveform sampling under full-speed operation, cable routing spacing inspection, coupling torque and coaxiality inspection, and simulated vibration test to verify signal stability, instead of only checking static feeding accuracy.

Q4: How to distinguish encoder signal loss caused by electromagnetic interference from mechanical clearance signal jitter? A4: Waveform observation is the core discrimination method. Interference often shows irregular burr pulses on the waveform; mechanical clearance usually presents periodic position jitter synchronized with the rotation cycle of the feeding roller. Combined with vibration simulation test, the root cause can be quickly distinguished.

Q5: What spare parts should customers reserve for the encoder signal system of servo feeder? A5: It is recommended to reserve matched high-flexibility shielded cables, coupling assemblies, set screws and spare encoder. It is not necessary to stock a large number of encoders, but signal transmission accessories are the high-frequency failure parts under long-term vibration.

Q6: Can this analysis conclusion be applied to other models of servo feeder matched with pneumatic punch and mechanical punch? A6: The anti-interference wiring principle and coupling vibration fatigue failure mechanism are universal. But the synchronization control logic and filter parameter range are different for different punch driving types. When copying the maintenance scheme, parameter setting needs to be adjusted according to the host drive type.