Servo Drive Systems: Feedback Devices, Encoders, and Basic Commissioning

Servo Drive Systems: Feedback Devices, Encoders, and Basic Commissioning

What Is a Servo System?

A servo system is a closed-loop motion control system that continuously compares a commanded position, velocity, or torque with actual measured feedback and corrects any error. Unlike an open-loop stepper motor that simply steps a fixed number of pulses and hopes for the best, a servo drive knows exactly where the motor shaft is at all times. This precision makes servo systems the standard choice for industrial robot joint drives, CNC machine axes, and high-speed packaging equipment.

Every servo system has three main components: a servo motor (typically a permanent-magnet AC synchronous or brushless DC motor), a feedback device mounted on the motor shaft, and a servo drive (amplifier) that closes the control loops in real time.

The Three Control Loops

Servo drives nest three control loops inside each other, from outermost to innermost:

Position loop — the outermost loop. The motion controller commands a target position in encoder counts. The drive compares this to actual position feedback and generates a velocity command to reduce the error.

Velocity loop — the middle loop. The drive compares the commanded velocity (from the position loop) to the actual velocity derived from encoder feedback. It generates a torque command to reduce velocity error.

Torque (current) loop — the innermost loop. The drive regulates motor phase current to produce the commanded torque. This loop runs at the highest update rate — often tens of kilohertz — because current must be controlled very tightly.

Each inner loop runs faster than the outer one. A fault in the feedback device breaks all three loops simultaneously, which is why encoder wiring integrity is a critical safety and reliability concern.

Incremental Encoders: A, B, and Z Channels

An incremental encoder is an optical or magnetic device that produces digital pulse trains as the shaft rotates. It has three signal channels:

• Channel A — a square wave that produces one pulse per encoder line.
• Channel B — a second square wave shifted 90° out of phase with A. The drive detects whether B leads or lags A to determine direction of rotation.
• Channel Z (index) — one pulse per full revolution, used to establish a repeatable reference point during homing.

Resolution is stated in lines per revolution (LPR) or pulses per revolution (PPR). Most drives use quadrature decoding, counting both rising and falling edges of A and B. A 2,500-line encoder therefore yields 10,000 counts per revolution (CPR) in quadrature. Higher CPR gives finer position resolution.

The drive converts counts to velocity by measuring count frequency or by dividing the number of counts collected in a fixed time window: velocity (rev/s) = counts per second ÷ CPR.

Resolver vs. Incremental Encoder

A resolver is an analog feedback device — essentially a rotary transformer — that outputs sine and cosine voltages proportional to shaft angle. A resolver-to-digital converter (RDC) inside the drive converts these analog signals to a digital position value. Resolvers are rugged, operate in high-temperature and high-vibration environments, and are common on FANUC servo motors. They are inherently absolute within one revolution.

An incremental encoder loses position information when power is removed. Absolute position must be re-established each time power is applied, which is why drives with incremental encoders require a homing routine on startup.

Encoder Battery Backup and Absolute Position

Many industrial robots use a multi-turn absolute encoder that stores position across power cycles using a small backup battery in the robot controller or servo amplifier. On FANUC systems, if this battery voltage drops below threshold or is disconnected with the controller off, the absolute position data is lost. The robot then displays a zero-return (mastering) alarm and cannot run until homing (mastering) is performed. This is why encoder battery replacement is a scheduled preventive maintenance task — never remove the battery while the controller is off unless you are prepared to re-master every axis.

FANUC Servo Amplifier Architecture

Each FANUC robot joint axis is driven by a dedicated servo amplifier channel. The servo amplifier receives position commands from the robot controller CPU over a high-speed serial bus, reads encoder (or resolver) feedback from the motor, and closes the velocity and torque loops independently. The feedback path runs: motor encoder → amplifier feedback terminal → velocity feedback calculation → current command → inverter bridge → motor phases. Annotating this path on a block diagram is a core FANUC CERT technician skill.

Safety Before You Power Up

Before connecting or disconnecting any encoder cable or drive wiring:

• Follow OSHA 29 CFR 1910.147 lockout/tagout procedures. De-energize the drive, discharge the DC bus (bus voltage can remain hazardous for several minutes after power-off — verify absence of voltage with a rated meter), apply your lock and tag.
• Encoder cables carry low-voltage signals but run near high-voltage motor power cables. Maintain separation to avoid noise coupling.
• Consult NFPA 70E shock and arc-flash boundaries before opening any live panel. If energized work is unavoidable, use an energized-work permit and appropriate PPE.
• Follow ANSI/RIA R15.06 / ISO 10218 safeguarding requirements whenever working within a robot work envelope — robot servo drives can respond to commands faster than a person can react.
• Never defeat drive enable circuits or safety relays to force motion during commissioning.

Basic Commissioning Sequence

1. Verify motor nameplate data matches drive parameter settings (motor type, encoder PPR, current rating).

2. Connect encoder cable shield at the drive end only to avoid ground loops.

3. Power up in speed (velocity) mode with a low maximum speed limit set in parameters.

4. Perform a jog move using the drive keypad or commissioning software; observe actual velocity on the drive status display and compare to commanded value.

5. Verify direction: commanded positive velocity should match positive encoder count direction per your axis convention.

6. Execute a homing sequence (seek Z-pulse or hard stop) to establish position reference.

7. Increase speed gradually, monitoring for oscillation (indicates tuning is needed) or overcurrent faults (indicates load or wiring issues).

Record all commanded vs. actual velocity data on your lab worksheet — this comparison is the core evidence that your closed-loop system is functioning correctly.