Mitsubishi Ac Servo Motor Encoder OSE105S2 OSE1O5S2 0SE105S2

The Component That Makes Positioning Possible

Servo motors get all the attention. Encoders do the actual work. Every time a CNC axis stops within microns of its target, every time a servo motor holds position under a cutting load, every time a drive detects that the shaft has drifted by a fraction of a degree — that information comes from the encoder. The Mitsubishi OSE105S2 is the incremental rotary optical encoder used on Mitsubishi's HC and HCR series servo motors, and understanding what it does explains why replacing it correctly matters as much as replacing the motor itself.

This is a flange-mounted incremental encoder that mounts at the rear of the servo motor, reading shaft position and velocity in real time and feeding that data back to the servo drive over an 8-pin connector interface. It outputs A, B, and Z phase signals — the quadrature A and B channels for direction and position counting, and the Z index pulse once per revolution for reference and homing operations. Resolution is 1,000,000 pulses per revolution, a figure that translates directly into the fine positioning granularity these motors are known for in CNC applications.

Manufactured in Japan to Mitsubishi Electric's precision standards, the OSE105S2 is the feedback backbone of a large installed base of Mitsubishi-controlled machine tools that are still in active production service worldwide.

An incremental encoder doesn't store position — it counts movement. Every time the shaft rotates, the optical disc inside the encoder passes a light source, generating electrical pulses on the A and B channels. The drive counts those pulses and tracks position relative to a known reference point. The Z-phase pulse marks one complete revolution and is used to establish or confirm that reference point at startup.

At 1,000,000 pulses per revolution, the OSE105S2 provides enough resolution to give the servo drive extremely fine angular discrimination — the difference between position counts is small enough that the closed-loop control system can react smoothly to tiny deviations rather than making coarse corrections. This is why HC-series motors equipped with this encoder feel smooth and responsive across the speed range, rather than exhibiting the stepping or hunting behavior that lower-resolution feedback produces.

The optical sensing technology makes the encoder resistant to electromagnetic interference — relevant in CNC machine environments where spindle drive noise, VFD switching, and other EMI sources are present and would otherwise corrupt position data on a magnetically-sensitive feedback system.

The OSE105S2 is associated with the older generation of Mitsubishi HC series motors that used a separately-mounted encoder rather than a factory-integrated one. Motors known to use this encoder include variants from the HC-SF, HC-SFS families, as well as HC-MF and HCR series motors. It also appears alongside motors designated for use with MR-J series amplifiers in those generations.

This contrasts with newer Mitsubishi servo motors — such as the HC-MFS or HF-SP series — which have the encoder integrated directly into the motor housing as a sealed unit. For those motors, the encoder is not a separately replaceable component. The OSE105S2's separate-unit architecture means that when an encoder fails on a compatible HC series motor, the motor body itself may be perfectly serviceable — only the encoder needs replacing.

The OSE105S2 is one of the more commonly sourced encoder spare parts for legacy Mitsubishi servo systems. Encoder failure typically presents as:

Before replacing the encoder, it's worth ruling out cable damage and connector contamination — the 8-pin connector at the motor rear end is exposed to coolant and debris in many CNC installations and is a common source of signal issues that mimic encoder failure.

When sourcing a replacement OSE105S2, verify the unit has been tested on a compatible Mitsubishi servo system, not just powered on. A genuine functional test confirms the optical disc integrity, signal output quality, and Z-phase pulse accuracy — none of which are verifiable by visual inspection alone.