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Full Closed-Loop Function of the A6 Servo Motor (RS Series)

D

Daniel Zhang

Last updated on Aug 4, 2026

1. Function Overview

A standard servo system uses an encoder mounted on the rear of the motor to form a closed loop: the driver issues motion commands and the encoder feeds back the motor's real-time position. However, from the perspective of the entire machine, this is still only semi-closed-loop control.

In other words, the motor-side encoder knows how many revolutions the motor has made and what position it has reached, but it cannot detect the actual movement of the load connected to the motor shaft. Traditionally, this gap is bridged through electronic gear ratios or other estimation methods.

With the full closed-loop function, you can install an external position-measuring device (such as a linear encoder / optical scale or magnetic scale) directly at the load side (e.g., worktable, ball-screw end). The servo driver then reads the true load position, eliminating mechanical transmission errors and achieving extremely high positioning accuracy.

⚠️ Note: Among the current A6 servo product line, only the RS series supports the full closed-loop function.

2. Wiring

On the driver's CN1 connector, pins 21, 22, 25, 23, 13, and 24 are used for the external encoder signal input.


Wiring Precautions

  1. Internal 5 V supply: The driver has a built-in 5 V output that can power the external (second) encoder. If you use this internal 5 V, connect the corresponding GND to PIN 16 (paired with PIN 15).

  2. Differential signals are mandatory: You must use a second encoder with differential (LVDS/RS-422) outputs.

    • A 5-wire single-ended encoder (VCC / A / B / Z / 0 V) is not recommended. Although in low-speed, interference-free laboratory settings a single-ended encoder can sometimes be interfaced through a signal-conversion/amplifier board, in real industrial full-closed-loop applications all major servo brands (Panasonic, Mitsubishi, Yaskawa, Siemens, etc.) do not support or recommend single-ended encoders as external feedback. Their noise immunity is extremely poor and pulses can be lost—a full closed-loop system cannot tolerate the loss of even a single pulse.

  3. Minimum signal requirement: The external encoder must provide at least A and B quadrature differential outputs.

    • The Z signal (index / zero-mark pulse) from the external measuring device is optional in most full closed-loop applications, provided you do not need hardware-level high-precision homing via the external scale. Normal operation, positioning, backlash compensation, and vibration suppression rely entirely on the A/B quadrature signals.

  4. Parameter C00.17 (wire-break detection):


    • This parameter checks whether the signal wiring is intact.

    • Factory default: C00.17 = 1 (detect A/B phase wire breaks only). If you are not using the Z signal, no change is needed.

    • If you are using the Z signal, set C00.17 = 2 (detect A/B/Z phase wire breaks), then power-cycle the driver for the setting to take effect.

3. Required Parameter Settings

After completing the wiring, follow these steps to enable full closed-loop control:

Step 1 – Switch the ABZ port to full closed-loop mode (Critical)

Set C00.1A = 2. By default the ABZ port functions as a divided-pulse output; it must be switched to full closed-loop mode.

Power-cycle the driver for the setting to take effect.

Step 2 – Enable external encoder feedback

Set C1B.00 = 1 (External Encoder mode). Power-cycle the driver for the setting to take effect.

Step 3 – Direction calibration (C1B.01)

  1. Enter the driver's JOG mode and jog the motor slowly in one direction.

  2. Observe the internal encoder pulse display C1B.10 and the external encoder pulse display C1B.12.

    • If both values increase or decrease together → keep C1B.01 = 0 (default).

    • If they change in opposite directions → set C1B.01 = 1, then power-cycle the driver.

Step 4 – Set external encoder resolution (C1B.04)

C1B.04 defines the number of external encoder pulses corresponding to one full motor revolution.

  1. Continue jogging the motor. Watch C1B.10 to confirm the motor has rotated exactly one full revolution.

  2. Record the change in C1B.12 (the external encoder pulse count).

  3. Set C1B.04 to the absolute value of that change.

  4. Power-cycle the driver for the setting to take effect.

Calculation formula:

Before rotation: C1B.10 = X1, C1B.12 = Y1
After one full motor revolution: C1B.10 = X2, C1B.12 = Y2

C1B.04 = 10 000 × (Y2 − Y1) / (X2 − X1)

(10 000 is the internal encoder pulses per revolution for this motor.)

The result must be positive. If it is negative, C1B.01 is likely set incorrectly—revisit Step 3.

Other C1B parameters can be fine-tuned during on-site commissioning and are not covered in detail here.

4. Precautions for External Encoders, Linear Scales, and Magnetic Scales

  1. 24 V encoders: If the external encoder requires 24 V, it must be powered by an external supply. Do not connect it to PIN 15.

  2. Cable type: Use twisted-pair shielded cable. Ideally, each differential signal pair should be individually twisted, with an overall outer shield.

  3. Cable routing: Signal cables from the magnetic scale / linear scale must be routed in separate cable trays from the servo motor power lines (U/V/W) and brake-resistor lines. Maintain a separation of at least 20 cm. Never bundle them together.

  4. Shield grounding: Connect the cable shield to ground at the servo driver end only (attach to the driver enclosure or the dedicated PE terminal). Grounding both ends can create a ground-loop current and introduce interference.