Technical Bulletin: Understanding the INC142-005 Encoder Quadrature Signals (A/B/Z)

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Tools Required

• oscilloscope
• multimeter
• teach pendant

Article

Overview

The NexBot Robotics INC142-005 Incremental Encoder is a critical component for achieving high-precision motion control in NexBot R-20, R-50, and S-5 series robots. Installed on joints J2 through J5, this encoder provides the robot's motion controller with real-time rotational position data. This document provides a technical explanation of the encoder's Quadrature (A/B) and Index (Z) output signals, which are fundamental to its operation.

Understanding these signals is essential for maintenance personnel and integration engineers when diagnosing positioning errors, verifying system integrity, or integrating the component into custom applications. The INC142-005 features a resolution of 8192 Pulses Per Revolution (PPR), meaning it generates 8192 distinct pulses on both its A and B channels for every full 360-degree rotation of the motor shaft.

Principle of Quadrature Encoding (A/B Signals)

Incremental encoders use two output channels, typically labeled 'A' and 'B', to provide both position and direction information. These two signals are square waves that are electrically 90 degrees out of phase with each other. This phase-shifted relationship is known as quadrature.

• Position Tracking: The robot controller counts the rising and falling edges of both the A and B pulses. With 8192 PPR, the controller can resolve 4 x 8192 = 32,768 distinct positions or 'counts' per revolution, providing extremely fine control over the joint's angle.

• Direction Detection: The controller determines the direction of rotation by observing which channel's signal leads the other.
• Clockwise (CW) Rotation: Channel A's signal will lead Channel B's signal. When observing the waveforms, the rising edge of A will occur before the rising edge of B.
• Counter-Clockwise (CCW) Rotation: Channel B's signal will lead Channel A's signal. The rising edge of B will occur before the rising edge of A.

An incorrect reading of direction can lead to significant control system errors, making signal integrity paramount. Noise, cable degradation, or improper shielding can corrupt the phase relationship and cause the controller to misinterpret the direction of movement.

The Z-Index Pulse (Reference Mark)

In addition to the A and B channels, the NXB-SNS-INC142-005 provides a third channel, 'Z', known as the index or reference pulse.

• Function: The Z channel produces a single pulse for every full 360-degree rotation of the encoder shaft. This pulse occurs at the exact same rotational position during each revolution.

• Application: The primary use of the Z pulse is for homing or establishing a zero-reference position. When a robot is powered on, its controller knows the relative position of each joint based on the encoder counts, but it does not know the absolute position. During a homing sequence, the joint moves until the controller detects the Z pulse. This event synchronizes the controller's internal count with a known physical position, ensuring accuracy and repeatability between operating cycles.

Losing the Z pulse due to a connection issue will typically result in a homing failure error on the robot's teach pendant.

Signal Specifications

The INC142-005 is designed for robust performance in industrial environments.

• Operating Voltage: 24VDC. Ensure a stable, clean power supply is provided as voltage fluctuations can impact signal quality.
• Output Protocol: The encoder supports both TTL (Transistor-Transistor Logic) and HTL (High-Threshold Logic) signal levels, making it compatible with a wide range of motion controllers.
• TTL (5V logic): Provides fast switching speeds suitable for high-speed applications. It is more susceptible to electrical noise over long cable runs.
• HTL (often 24V logic): Offers superior noise immunity due to its higher voltage swing, making it ideal for typical industrial environments with significant electromagnetic interference (EMI). The NexBot robot controllers are configured to use the HTL interface for maximum reliability.
• IP Rating: The encoder's IP67-rated housing protects it from dust ingress and temporary immersion in water, ensuring reliability in harsh conditions.

Verification and Troubleshooting

When troubleshooting positioning faults or 'axis following error' alarms related to joints J2-J5, the encoder signals are a primary point of investigation.

Required Tool: Oscilloscope

Procedure:

1. Safety First: Power down the robot and follow all applicable lockout/tagout (LOTO) procedures before accessing electrical connections.
2. Access Connections: Carefully connect oscilloscope probes to the A, B, and Z signal lines at the controller input terminal or a designated breakout point. Use a ground probe connected to a common signal ground.
3. Power On and Move Axis: Power the robot back on and, using the teach pendant at a very low speed (e.g., 1-5%), manually jog the suspect joint.
4. Observe Waveforms:

• A and B Signals: You should see two clean square waves. Verify they are 90 degrees out of phase. As you change the jog direction, you should see the lead/lag relationship between A and B reverse.
• Z Signal: You should see a single, sharp pulse for each full rotation of the joint.

Common Faults:

• No Signal: Check for power (24VDC) at the encoder connector. Inspect the cable for damage or loose pins.
• Jittery or Noisy Signals: This often points to EMI. Check that the cable shielding is properly grounded at the controller end. Ensure the encoder cable is not routed alongside high-power motor cables.
• Missing Z-Pulse: Indicates a fault within the encoder's optics or a broken wire for the Z channel. The encoder (NXB-SNS-INC142-005) will likely need replacement.

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