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Understanding Back-to-Origin Failure: Causes, Implications, and Mitigation Strategies


In modern technology and engineering, failures are an inevitable aspect of any complex system. A back-to-origin failure is one such failure mode that can have significant consequences in various fields, including electronics, robotics, transportation, and manufacturing. In this article, we will explore what a back-to-origin failure is, how it occurs, the implications of such failures, and potential mitigation strategies to minimize their occurrence and impact.

Definition of Back-to-Origin Failure


A back-to-origin failure, also known as a homing failure or return-to-zero (RTZ) failure, refers to a scenario where a system or component fails to return to its original or home position after a specific operation or movement. The home position is a predefined reference point, typically considered the zero point or starting position of the system. In such failures, the system may either halt unexpectedly, deviate from its intended path, or get stuck in an unpredictable state, compromising the overall functionality and safety of the system.

Causes of Back-to-Origin Failures


Back-to-origin failures can be triggered by a variety of factors, and understanding these root causes is essential for effective diagnosis and prevention. Some common causes include:

  • Sensor Malfunction: Sensors play a crucial role in determining the system’s position and detecting when it reaches the home position. A malfunctioning sensor can result in inaccurate or missing positional information, leading to homing failure.
  • Mechanical Jam or Binding: Physical obstructions or mechanical issues can prevent the system from reaching its home position smoothly. These obstructions may arise from foreign particles, worn-out components, misalignments, or improper lubrication.
  • Electrical Interference: Electromagnetic interference, signal noise, or power fluctuations can disrupt the communication between the control system and the components, leading to incorrect homing signals or failure to execute homing commands.
  • Software Bugs or Errors: In complex systems controlled by software, bugs or coding errors can cause improper homing sequences, invalid commands, or inaccurate position calculations.
  • Environmental Factors: Extreme temperature variations, humidity, or vibrations can adversely affect system components, leading to performance degradation and eventual homing failures.
  • Mechanical Overloading: If the system experiences unexpected mechanical loads beyond its design limits, it may fail to return to its origin due to damage or deformation of critical components.
  • Wear and Tear: Over time, the mechanical components of a system may experience wear and tear, leading to increased friction, reduced accuracy, and ultimately, back-to-origin failures.

Implications of Back-to-Origin Failures


The consequences of back-to-origin failures can vary depending on the application and system involved. Some of the key implications are:

  • Safety Risks: In safety-critical systems, such as medical devices, autonomous vehicles, or industrial robots, a back-to-origin failure can pose significant safety risks to operators, users, and bystanders.
  • Production Downtime: In manufacturing and industrial processes, a homing failure can halt production lines, leading to delays, increased operational costs, and potential loss of revenue.
  • Data Loss: In data storage systems, a homing failure can result in data corruption or loss, jeopardizing valuable information and business continuity.
  • System Damage: If a back-to-origin failure occurs during critical operations, it may cause damage to the system components, leading to costly repairs or replacements.
  • Loss of Precision: Precision machines, such as CNC equipment or 3D printers, heavily rely on accurate homing to ensure high-quality output. A failure to return to the home position can result in decreased precision and compromised product quality.
  • System Calibration: In systems with complex calibration procedures, a back-to-origin failure can invalidate the calibration, requiring time-consuming recalibration efforts.

Case Study: Back-to-Origin Failure in Robotic Arm Control


To illustrate the impact of back-to-origin failures, let’s consider a case study involving a robotic arm used in an industrial assembly line. The robotic arm is programmed to pick up components from a conveyor belt and place them onto a product assembly station.

Scenario:

During a routine production run, the robotic arm experiences a back-to-origin failure. The system fails to return to its home position after placing a component, causing the arm to collide with other components on the assembly line.

Implications:

  • Production Downtime: The collision halts the production line, leading to delays in product assembly and shipment.
  • Component Damage: The collision damages the robotic arm’s end effector and causes minor damage to some components on the assembly line, requiring replacements and repairs.
  • Safety Risk: Workers nearby narrowly avoid injury due to the unexpected motion of the robotic arm.
  • Calibration Loss: The back-to-origin failure invalidates the arm’s calibration, necessitating recalibration before resuming production.

Mitigation Strategies for Back-to-Origin Failures


Preventing back-to-origin failures requires a comprehensive approach that involves design considerations, maintenance practices, and robust control strategies. Here are some effective mitigation strategies:

  • Redundant Sensors: Implementing redundant sensors or multiple sensor types can enhance reliability and fault tolerance, enabling the system to cross-validate positional information.
  • Fault Diagnostics: Incorporating robust fault diagnosis algorithms allows the system to detect anomalies early and take corrective actions to prevent homing failures.
  • Mechanical Design: Ensuring proper mechanical design, including adequate clearances, alignment, and lubrication, can minimize the chances of mechanical jamming or binding.
  • Environmental Controls: Implementing environmental controls, such as temperature regulation and vibration isolation, can mitigate the impact of adverse conditions on system performance.
  • Software Validation: Rigorous software testing, including boundary testing and stress testing, can help identify and rectify potential software bugs before deployment.
  • Regular Maintenance: Adhering to a proactive maintenance schedule, including component inspection, lubrication, and replacement of worn-out parts, can extend system longevity and reduce the risk of failure.
  • Emergency Procedures: Establishing emergency shutdown procedures and fail-safe mechanisms can mitigate the consequences of a back-to-origin failure, preventing further damage or injury.
  • Training and Awareness: Providing proper training to operators and maintenance personnel about potential failure scenarios and appropriate responses can enhance the system’s reliability and safety.

Example 7-2 Taiwan DM4400M 3 axis machining center has unstable machining dimensions in the Z-axis direction, out-of-tolerance and irregular faults, that is to say, the Z-axis origin drifts irregularly, and the CRT and servo amplifier do not have any alarm display. The machining center adopts Mitsubishi M3 system, semi-closed loop control mode, and the AC servo motor and the ball screw are directly connected through the coupling.

According to the analysis of the fault phenomenon and the control method and connection method adopted by the machine, the cause of the fault may be that the connecting screw of the coupling is loose, causing the coupling and the ball screw or the shaft of the servo motor to slide. Check the connection of the Z-axis coupling and find that all six set screws of the coupling are loose. After the screws are tightened, the fault is eliminated.

Example 7-3 Taiwan DM4400M machining center, sometimes the tool change position is wrong during use, and sometimes it is a normal failure.

When the tool change position changes, the Z-direction machining dimension of the workpiece to be machined also changes accordingly, and corresponds to the change of the tool change position. No alarms are displayed. The machining center adopts Mitsubishi M3 CNC system.

The following methods are used to return to the reference point when the machine is turned on: the position encoder installed at the end of the servo motor has a certain number of equidistant grid points per revolution, and the distance between the two grid points is called the grid point interval. When starting the manual reference point return, the axis first moves rapidly to the reference point at the reference point return speed set by the parameter. Speed ​​movement, when approaching the reference point stroke block and leaving the reference point deceleration limit switch, the position of the first grid point detected by the encoder is the reference point return position. Since the machine has its own mechanical origin, it is required that the electrical origin should be consistent with the mechanical origin.

The offset between the mechanical origin and the electrical origin is called the reference point offset, which is set in the G28 parameter. When the reference point deceleration switch leaves the position when it is close to the reference point deceleration bumper and is not near the center of the grid interval, the reference point sometimes shifts. You can prevent the reference point from shifting by setting the parameter grid shield. The tool point is set by the second reference origin of the machine tool, and the second reference origin is determined by the first reference origin of the machine tool. Because the faults of the machine tool have some shifts and some shifts do not, it is suspected that there is a problem when the machine tool is turned on to manually return to the reference point. After investigation, it was found that the fixed plate of the z-axis returning to the reference point deceleration travel switch was not firmly fixed to the column, and was seriously loose, resulting in a drift of the origin.

Example 7P4 – A CNC lathe adopts FAGOR 8025 control system, X and Z axes use semi-closed loop control. After half a year of operation, it is found that the Z axis always has an error of 23mm each time it returns to the reference point, and the error is irregular, and the phenomenon remains after adjusting the control system parameters. It did not disappear, and the phenomenon still exists after replacing the servo motor. After careful analysis, it is estimated that the end of the lead screw is not tightened, and the phenomenon disappears after the nut is prepared.

Example 7-5 – Taiwan 4 Axis Machining Center TH6240. Using FAGOT 8055 control system, there is a big deviation in the C-axis accuracy during debugging, and no problems are found in the mechanical accuracy after inspection. After debugging by the technicians of FAGOR, it is found that the calculation of the servo parameters of the linear axis and the rotary axis is very different. After recalculation After the servo parameters are set, the C axis returns to the reference point, and the running accuracy is normal. For the debugging and maintenance of CNC machine tools, it is important to understand the PLC ladder diagram of the control system and the setting of system parameters. After a problem occurs, it is necessary to first determine whether it is a strong power problem or a system problem, a system parameter problem or a PIC ladder diagram problem. It is necessary to make good use of the system’s own alarm information and diagnosis screen, be careful, and most CNC machine tool failures can be prevented and eliminated in time.

In Conclusion


Back-to-origin failures are critical failure modes that can impact the performance, safety, and productivity of complex systems in various industries. Understanding the root causes and implications of such failures is crucial for implementing effective mitigation strategies.

In an increasingly automated world, where systems often operate autonomously, back-to-origin failures demand special attention to ensure safe and reliable operation. By adopting a proactive approach, incorporating redundancy, and implementing robust control strategies, engineers and operators can minimize the occurrence of back-to-origin failures and enhance the overall performance and longevity of critical systems. As technology continues to advance, it is imperative to remain vigilant and continuously improve the design and maintenance practices to meet the evolving challenges of complex systems and prevent potential back-to-origin failures.


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