Yaskawa Servo Encoder Troubleshooting: Scope-Based Signal Test

Yaskawa Sigma-series servo motors use proprietary absolute and incremental encoders that are matched to specific amplifier/drive firmware revisions. Field replacement requires the encoder disc to remain indexed to the shaft or commutator; misalignment of even a few electrical degrees will produce commutation faults, runaway, or reduced torque. The procedures below describe how to verify encoder health at the motor connector with a dual-channel oscilloscope before committing to a full motor replacement or encoder swap. The target audience is a maintenance technician or controls engineer with access to the servo amplifier, the motor's connector pinout, and the Yaskawa product manual for the specific drive family (Sigma-II, Sigma-III, Sigma-V, or Sigma-7).

1. Yaskawa Encoder Families and Specifications

Yaskawa servo motors across the Sigma product lines use a small set of encoder hardware types. Identifying the exact encoder mounted on the motor is the first step in any test procedure because the channel count, protocol, and supply voltage all vary.

2. Channel and Pinout Configurations

The most common Yaskawa encoder cable conductors observed in the field are listed below. The pin-to-signal mapping is critical when building a test adapter because the cable shield and drain must remain terminated at the amplifier end to control common-mode noise.

Some Sigma-V and Sigma-7 motors combine serial data and incremental outputs on a single 9-pin or 17-pin connector, while older Sigma-II incremental units use a 7-pin MS-style plug. Always consult the specific motor's "Servomotor Specifications" sheet in the Yaskawa product manual before probing, and confirm against the drive parameter Pn002 (encoder type selection) before re-energizing.

3. Prerequisites and Test Equipment

The minimum toolset for in-cabinet encoder verification is:

• Two-channel digital oscilloscope, 100 MHz minimum bandwidth, with isolated channels. A Fluke ScopeMeter or equivalent handheld oscilloscope is adequate for 5 V TTL encoder signals, as documented in Fluke's rotary-encoder troubleshooting reference.
• 10x passive probes (100 MHz or higher), one per channel, with short ground springs (not alligator leads) to minimize loop area.
• Encoder test cable / breakout adapter wired from the motor connector to the amplifier pigtail. Use the same cable gauge and shield type as the production cable. A 1 m breakout that exposes the supply, ground, A, /A, B, /B, Z, /Z (and SD+, SD- on absolute units) is sufficient.
• 5 VDC bench supply capable of 200 mA continuous, current-limited to 500 mA.
• Hand-crank or low-RPM drill coupled to the motor shaft via a coupling or strap wrench. Do not rotate by grasping the encoder housing — the hub-to-shaft clamp can slip.
• Yaskawa servo amplifier with the motor connected and parameters Un00A (motor type) and Un010 (encoder type) accessible from the digital operator or SigmaWin+ tool to cross-check scope traces against the amplifier's reported position.

4. Safety and Pre-Test Checks

1. Lock out and tag out the upstream three-phase feed to the servo amplifier. Verify zero energy with a meter rated for the system voltage (200 VAC class for 200 V drives, 400 VAC or 480 VAC class for 400 V drives).
2. Disconnect the motor leads (U, V, W) from the amplifier output terminals if the motor will be hand-cranked, or insulate them and the shaft if the drive is powering rotation. Never back-drive a Yaskawa motor with the U/V/W leads open to a live amplifier: the amplifier's regeneration path can charge the DC bus and trigger an overvoltage regenerative fault (typically A.300 or A.320 depending on firmware).
3. Connect the bench 5 V supply to the encoder +5V and GND pins through the breakout. Measure supply current at idle: a healthy Yaskawa encoder draws 80–180 mA. A reading of 0 mA with the supply at 5.0 V indicates a short-circuited driver, an open internal bond wire, or a damaged connector. A reading above 250 mA indicates a short inside the encoder ASIC.
4. Verify ripple on the +5 V rail at the motor connector is below 50 mV peak-to-peak under load. Excessive ripple couples into the line-receiver thresholds and produces intermittent position faults (A.510 / A.511 / A.C20 / A.C21 on Sigma-V/7, depending on firmware version).

5. Channel-by-Channel Signal Test Procedure

5.1 Incremental A/B Channel Verification

Hand-rotate the motor shaft at approximately 60 RPM. With Channel A on the A+ pin and Channel B on the B+ pin, ground both probes at the encoder-side ground spring (not at the amplifier chassis).

• High-level voltage (V_OH): Must be ≥ 2.5 V for 5 V TTL. The typical Yaskawa AM26C31 or equivalent RS-422 line driver delivers V_OH ≈ 3.4 V at 5 V supply into the amplifier's 120 Ω differential termination.
• Low-level voltage (V_OL): Must be ≤ 0.5 V. Typical V_OL is approximately 0.2 V with a 20 mA sink.
• Quadrature phase error: Channel B must lead or lag Channel A by 90 electrical degrees ± 20°. The wider the deviation, the more likely the optical disc has shifted on the hub. A reading outside ± 20° will produce direction ambiguity and rough low-speed motion.
• Duty-cycle symmetry: Each channel should be 50% ± 10% high-time over one full cycle. Asymmetry points to a damaged reticle, contamination on the code disc, or a failing LED source.
• Waterfall / edge distortion: Rise time (10–90%) should be below 200 ns. Visible "waterfall" (the high-to-low transition taking more than 500 ns with ringing) indicates cable capacitance mismatch, a failing driver, or a contaminated disc.

5.2 Index (Z) and Commutation (C, U/V/W) Channel Verification

Switch the time base to 100 ms/div and rotate the shaft slowly enough to count index pulses by eye. Each mechanical revolution of a Yaskawa incremental encoder produces exactly one Z pulse. For absolute encoders, the C or U/V/W channel is a 1-pulse-per-pole-pair square wave, synchronized to rotor magnet position.

• Z pulse width is typically 90 electrical degrees (¼ cycle of A) for Yaskawa Sigma-II 13-bit encoders, and 1 full electrical cycle of A for Sigma-V/7 absolute encoders.
• Confirm that the Z pulse position relative to A/B does not shift on successive revolutions. Mechanical slop in the encoder bearing will manifest as jitter of more than ± 1 count when you reverse direction.
• For absolute encoders, monitor SD+ and SD- while the amplifier is in "encoder reset" or "origin search" mode. The amplifier transmits a query frame on RX+/RX- at startup, and the encoder must reply with a valid position frame within 1 ms. A missing reply raises Sigma-V/7 alarm A.830 (encoder communications error).

5.3 Absolute Serial (SD+/SD-) Channel Verification

Connect Channel A to SD+ (serial data) and Channel B to RX+ (serial request from amplifier), each with its own probe ground spring. Trigger on RX+ falling edge.

• The RX+ request line is idle high. The amplifier sends a short low-going request pulse approximately every 62.5 µs (16 kHz) during normal operation.
• The encoder responds on SD+ with a return pulse train beginning 5–20 µs after the request edge. Pulse-width modulated return data encodes 4 bits per request cycle; the protocol is proprietary to Yaskawa and is documented in the Sigma-V/7 product manuals (contact Yaskawa technical support for the latest revision).
• If you do not have a Yaskawa service tool (SigmaWin+), a healthy encoder is still identifiable by: (1) request pulses present on RX+, (2) reply pulses present on SD+ with monotonically changing duty cycle as the shaft rotates, and (3) no SD+ activity with the RX+ line held idle.
• Per the Fluke predictive-maintenance reference on oscilloscope-based encoder testing, the average voltage on a healthy serial line is typically 2.4 V (≈ midpoint of 0 V and 5 V at 50% duty) with peak voltages near 4.9 V on a properly terminated 120 Ω differential pair.

5.4 Analog 1 Vp-p Channel Verification

For the rare Yaskawa direct-drive motor with sine/cosine output, terminate A+ and B+ into 120 Ω differential. Measure the differential amplitude with the scope's math function (A+ minus A-, B+ minus B-):

• Signal amplitude: 1.0 Vp-p ± 10% at the motor connector, derated to 0.8 Vp-p minimum at the amplifier input to account for cable loss at 20 m.
• DC offset: 2.5 V ± 50 mV per leg.
• Phase A-to-B: 90° ± 5° electrical.

6. Acceptance Criteria Summary

7. Common Failure Modes and Field Symptoms

8. Verification and Documentation

1. With the motor reconnected to the amplifier and the motor U/V/W leads terminated, command a 1 RPM jog from the amplifier's digital operator or SigmaWin+ tool. Confirm the amplifier's Un00D (position feedback) increments by the expected count per mechanical revolution: 8192 for 13-bit, 131072 for 17-bit, 1048576 for 20-bit. Discrepancies of even 1 LSB indicate a counting error.
2. Run a bidirectional index search using the drive's auxiliary function (Fn001 or equivalent in the firmware menu). The amplifier should capture the Z pulse at the same mechanical angle within ± 1 count on repeated cycles.
3. For absolute encoders, cycle control power 10 times. Each cycle the amplifier must report the same multi-turn position within ± 1 turn. If the position drifts, replace the encoder backup battery or the encoder itself.
4. Save scope captures of A+/A-, B+/B-, Z+/Z-, and SD+/SD- to a maintenance database with motor serial number, encoder type, supply voltage, supply current, and test date. Calibrate any subsequent Yaskawa motor swap against the captured baseline.

9. Build Notes for a Field Test Adapter

If a Yaskawa-specific test cable is not available, the encoder portion of a Yaskawa motor cable can be adapted with the following reference design:

• Use a 17-pin (or 9-pin) Yaskawa encoder receptacle that mates with the motor.
• Break out the supply, ground, A+/A-, B+/B-, Z+/Z- to a 25-pin D-sub with each differential pair on adjacent pins and the supply/ground on twisted pairs.
• Add a 120 Ω termination resistor across each differential pair at the breakout end if the cable is shorter than 5 m, or at the receiving instrument end if the cable is longer than 5 m.
• For an inexpensive bench receiver, the Texas Instruments SN75175 quad RS-422 line receiver (5 V, 8-pin SOIC) provides 4 channels of differential-to-TTL conversion; pair it with a 68HC11 or a modern STM32 to display the position count and quadrature error. Commercial examples include the RIM Tach M100, but its $1,500 list price is rarely justified outside a fleet of 20+ Yaskawa servos.
• Always include a series ferrite bead on the +5 V supply line to the breakout to suppress high-frequency noise from the bench supply that would otherwise show up on the SD+ reply pulses.

10. When to Replace vs. Repair

Per Yaskawa service policy, absolute encoders in the Sigma-V and Sigma-7 families are not field-replaceable as a stand-alone part. The encoder is matched to the motor's commutation curve and serialized in the amplifier's parameters (Pn002, Pn205, Pn806, etc., depending on firmware). Practical field guidance:

• Incremental 13-bit encoders (Sigma-II): Yaskawa historically sold replacement encoder assemblies, and the housing is removable. Replace the assembly; re-align using the index mark, and verify Z-to-A relationship on a test stand before reinstalling into the machine.
• Absolute 17-bit and 20-bit encoders (Sigma-V/7): Field replacement is not supported. Replace the complete motor. Return the failed motor to a Yaskawa-authorized repair depot for encoder refurbishment. Do not attempt to swap absolute encoders between motors — the amplifier will reject the mismatched serial number and raise A.840 or refuse to enable.
• Battery-only failure: Replace the battery (typically a Yaskawa-supplied 3.6 V lithium cell, part JZSP-BA01 or equivalent) without disturbing the encoder. The amplifier's multi-turn counter must be re-zeroed (Fn010, Fn011, or the SigmaWin+ encoder clear routine) after battery replacement. Document the absolute position offset before zeroing if the machine coordinate system is critical.

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