Deep Dive: Absolute Encoder Servo Motor Principles & Zero Position Adjustment

Understanding Servo Motors and Absolute Encoders

Servo motors are the core of high-precision motion control systems in industrial automation. They rely on a feedback device, known as an Encoder, to provide the drive with the current position and speed of the motor. This information is then compared to the command values to optimize control performance as required.

What is an Absolute Encoder?

An Absolute Encoder is a position feedback device that can determine the exact position of a rotating shaft immediately upon power-up, without the need to move to a home position (Homing) like an Incremental Encoder. Incremental encoders provide a relative position, while Absolute Encoders provide an absolute position, which is unique for each angular position.

• Single-turn Absolute Encoder: Identifies the exact position within one revolution (0-360 degrees). When a full revolution is completed, the value resets.
• Multi-turn Absolute Encoder: In addition to identifying the position within one revolution, it can also count the total number of revolutions. This allows it to determine the exact position over a much wider range of motion, even after power cycling.

Operating Principles of Absolute Encoder Servo Motors

When a servo motor operates with an Absolute Encoder, the encoder converts the motor's angular position into a unique digital signal. Each position has its own specific code. This signal is then sent to the servo drive and/or PLC/Controller for precise position processing and control.

1. Position Encoding: An Absolute Encoder contains a disk with an encoding pattern (e.g., Binary, Gray Code) where each angular position generates a unique digital code.
2. Value Reading: Internal sensors within the encoder read this encoding pattern and convert it into digital data.
3. Communication: The position data is transmitted to the servo drive via specific communication protocols (e.g., EtherCAT, PROFINET, SSI, BiSS), providing immediate current position information upon system power-up.
4. Control: The servo drive uses this position data to compare with the target position and adjusts the current supplied to the motor to move it to the desired position with high accuracy.

Example position data from an Absolute Encoder:
Encoder_Position_Counts: 256789
Encoder_MultiTurn_Counts: 12
Total_Revolutions: 12.75

The Importance of Zero Position

Zero Position is the designated reference point that serves as the starting point for all motion in a mechanical system. It is critically important for defining the relationship between the physical position of the machine and the digital value read from the encoder. Correctly setting the Zero Position ensures that the system can move to the desired positions accurately and in alignment with the machine's mechanical structure.

How to Adjust the Zero Position of an Absolute Encoder Servo Motor

Adjusting the Zero Position is a critical step that must be performed carefully to ensure that the mechanical and control systems are synchronized. Generally, there are several methods for adjustment, depending on the brand and model of the servo drive.

Basic Adjustment Procedure (General)

1. Preparation: Ensure the machine is in a safe position and not subjected to unexpected loads. Temporarily disable the main control system (PLC/Controller).
2. Define Mechanical Position: Move the machine part controlled by the servo motor to the desired mechanical Zero Position precisely. Using measuring tools like a Dial Gauge may help confirm accuracy.
3. Enter Adjustment Mode: Access the servo drive's parameters via an HMI, Teach Pendant, or PC software.
4. Set Zero Value: Locate the parameter related to Zero Position Offset or Zero Position Latch and record the current encoder value as Zero.
5. Save Value: Save the set value to the servo drive's non-volatile memory to ensure it's retained after power cycling.
6. Test Operation: Exit adjustment mode and test the servo motor's movement to confirm that the set Zero Position is correct and the system operates as expected.

Adjustment via HMI / Teach Pendant

For some servo drive models, the Zero Position can be adjusted directly through the HMI or a connected Teach Pendant.

1. Enter Parameter Setting mode on the HMI.
2. Find parameters related to Zero Position setting (e.g., Pn000, Pn001, Pn002).
3. Move the motor to the desired Zero position.
4. Use the 'Zero Set' or 'Data Latch' function of the respective parameter to record the value.
5. Save the parameters to the EEPROM or non-volatile memory.

Example parameters for Zero Position (may vary by brand):
PRM. Pn000: Zero Position Offset Value
PRM. Pn001: Zero Position Set Command (Write 1 to set)
PRM. Pn002: Multi-turn Data Clear (Write 1 to clear multi-turn data)

Adjustment via Servo Drive Software

This is the most common and effective method, using proprietary software from the servo drive manufacturer (e.g., Yaskawa SigmaWin, Mitsubishi MR Configurator, Delta ASDA-Soft) on a computer.

1. Connect the computer to the servo drive via USB or Ethernet cable.
2. Open the software and establish communication with the servo drive.
3. Navigate to the parameter setting or Zero Position function screen.
4. Move the motor to the desired Zero position (you may use the Jog function in the software).
5. Execute the 'Zero Position Set' or 'Absolute Position Reset' command within the software.
6. Verify that the Encoder Multi-turn value is reset (if using a Multi-turn Encoder).
7. Save the settings to the servo drive.

Common Issues in Zero Position Adjustment and Troubleshooting

While Zero Position setting might seem straightforward, certain issues can arise.

• Zero Value Not Saved: Ensure that the value has been saved to the non-volatile memory (EEPROM) of the servo drive. If not saved, the value may be lost upon power-off.
• Motor Moves in Wrong Direction After Adjustment: This could be due to incorrect encoder rotation direction or motor phase settings. Check parameters related to rotation direction and motor phase sequence.
• Related Alarm Codes: If an alarm occurs after adjustment, consult the drive's manual for the specific alarm code to identify the root cause.

Common Alarm Codes (may vary by brand):
AL. E10: Encoder Communication Error
AL. E20: Absolute Position Data Error
AL. E30: Multi-turn Data Overflow
AL. 16: Zero Position Not Set (Yaskawa)

Additional Considerations

Selecting the appropriate Absolute Encoder for the application is crucial. Factors such as resolution, maximum speed, and communication interface should be considered. Furthermore, maintaining the encoder and its signal cables clean and free from damage helps extend its lifespan and preserve system accuracy.

• Cable Inspection: Regularly inspect encoder cables for signs of damage or wear.
• Contamination Protection: Keep the encoder and surrounding areas clean, free from dust, oil, or chemicals that could affect operation.
• Data Backup: Always back up all servo drive parameters to facilitate system recovery in case of issues.

Understanding the operating principles and Zero Position adjustment of Absolute Encoder Servo Motors is an essential skill for automation engineers. Correct settings lead to maximum performance and reliability of machinery. If you encounter complex issues or require further in-depth consultation, the expert engineering team at SP Automation is ready to provide technical assistance and guidance to resolve your problems and optimize your systems.