Doosan Robot Force Compliance With DR_TOOL Coordinate

Understanding Force Compliance Control

When working with robotic systems, especially in applications requiring precision and delicate interaction, force compliance control becomes crucial. This control method allows the robot to adjust its movements based on the external forces it encounters. In essence, it enables the robot to "feel" its environment and react accordingly. This is particularly important in tasks such as assembly, where parts need to fit together snugly, or in polishing, where consistent pressure is essential. Force compliance ensures that the robot doesn't apply excessive force, which could damage the workpiece or the robot itself. By monitoring and responding to external forces, the robot can perform tasks with greater accuracy and safety. This level of control enhances the robot's adaptability and makes it suitable for a broader range of applications.

To achieve effective force compliance, the robot needs a reference coordinate system. This system acts as the basis for measuring and controlling forces. The default reference is often the robot's base coordinate system, but sometimes, controlling forces relative to the tool coordinate system (DR_TOOL) is more appropriate. The tool coordinate system moves with the robot's end-effector, providing a more intuitive frame of reference for certain tasks. For example, if the robot needs to apply a specific force along the tool's Z-axis, using the DR_TOOL coordinate system simplifies the control process. This approach ensures that the force is applied in the intended direction, regardless of the robot's orientation. The ability to switch between different reference coordinate systems is a key feature of advanced robotic control systems, offering greater flexibility and precision in complex applications.

Furthermore, force compliance control enhances the safety of robotic operations. By limiting the forces exerted by the robot, the risk of damage or injury is significantly reduced. This is especially important in collaborative robot (cobot) applications, where robots work in close proximity to humans. Force sensors integrated into the robot's end-effector provide real-time feedback, allowing the control system to make adjustments on the fly. This dynamic response is critical in preventing accidents and ensuring a safe working environment. The sophistication of force compliance control systems has advanced significantly, with capabilities such as impedance control and admittance control providing even finer levels of manipulation. These advanced techniques allow the robot to mimic mechanical impedance or admittance, making its interaction with the environment more natural and intuitive. As a result, force compliance is a cornerstone of modern robotics, enabling robots to perform intricate tasks with precision, safety, and adaptability.

The Challenge: DR_TOOL Coordinate Reference

The core challenge addressed here involves using the DR_TOOL coordinate system as a reference for force compliance control on a Doosan A0509s robot. The user, working with this robot model, encountered an issue while attempting to implement force control in the Z-axis using the check_force_condition function. This function is designed to monitor force conditions along a specified axis, with the option to set a reference coordinate system. By default, the reference is set to DR_BASE, which is the robot's base coordinate system. However, the user's goal was to control the force relative to the DR_TOOL coordinate system, which moves with the robot's end-effector.

Despite setting the reference to DR_TOOL within the function call, the robot continued to move based on the DR_BASE coordinate system. This discrepancy suggests a potential issue in how the reference coordinate system is being handled within the robot's control software or the function itself. The DR_TOOL coordinate system is particularly useful when the desired force direction is relative to the tool's orientation. For instance, if the robot needs to apply a constant force perpendicular to a surface, regardless of the robot's position, using DR_TOOL simplifies the control logic. The fact that the robot did not respond as expected indicates a need to investigate the function's implementation and the robot's firmware to identify the root cause of the problem.

This issue highlights the complexities involved in robotic control systems, where various coordinate systems and control parameters interact. Understanding these interactions is crucial for achieving precise and predictable robot behavior. The user's attempt to switch to the DR_TOOL coordinate system demonstrates a clear understanding of the benefits this reference frame offers in certain applications. The inability to achieve the desired behavior underscores the importance of robust debugging and troubleshooting techniques in robotics. By carefully examining the code, the robot's configuration, and the behavior of the control system, it is possible to identify and resolve such issues, ultimately enabling the robot to perform its tasks with the required accuracy and precision.

Investigating the check_force_condition Function

The check_force_condition function is central to the issue of force compliance control with the Doosan A0509s robot. A detailed examination of the function's code reveals its intended behavior and helps pinpoint potential areas of concern. The function's primary purpose is to check force conditions along a specified axis, ensuring that the force remains within defined limits. It takes several parameters, including the axis to monitor (e.g., DR_AXIS_Z), minimum and maximum force thresholds, and a reference coordinate system. The code includes extensive error checking to validate the input parameters, such as ensuring the axis is valid and the minimum force is not greater than the maximum force.

One key section of the code handles the reference coordinate system. If no reference is explicitly provided, the function defaults to _g_coord. Otherwise, it uses the provided ref value. The code then checks the type and value of the reference to ensure it falls within the acceptable range (DR_BASE to DR_TC_USER_MAX). However, there's a specific condition that may be relevant to the user's issue: if the monitored axis is a rotational axis (DR_AXIS_A, DR_AXIS_B, or DR_AXIS_C), the reference is explicitly set to DR_TOOL, regardless of the user's input. This suggests that the function may have a built-in limitation or assumption about when DR_TOOL should be used.

The code also includes a section that appears to be specific to ROS2 (Robot Operating System 2) integration. This section constructs a CheckForceCondition.Request message, sets the axis, minimum, maximum, and reference values, and then calls a ROS2 service to perform the force condition check. The result of this service call determines the function's return value, indicating whether the force condition is met. This ROS2 integration adds another layer of complexity, as the issue could potentially lie in the ROS2 service implementation or communication between the robot's control system and the ROS2 environment.

By carefully analyzing this function, it becomes clear that several factors could be contributing to the user's problem. The explicit setting of the reference to DR_TOOL for rotational axes, the ROS2 integration, and the overall logic of the function all warrant further investigation. Understanding the nuances of this function is essential for resolving the force compliance control issue and achieving the desired robot behavior.

Possible Causes and Solutions

Based on the provided information and the analysis of the check_force_condition function, several potential causes for the issue of force compliance control with DR_TOOL as the reference coordinate can be identified. One primary cause could be an oversight or limitation within the function's logic. As noted earlier, the function explicitly sets the reference to DR_TOOL for rotational axes. While this doesn't directly explain the issue with the Z-axis control, it highlights a potential area where the function's behavior might not align with user expectations. It's possible that a similar, less obvious condition is affecting the translational axes as well.

Another potential cause lies in the interaction between the robot's control system and the ROS2 environment. The check_force_condition function utilizes ROS2 services, which means the issue could stem from the service implementation itself or the communication between the robot's control system and the ROS2 service. For instance, the ROS2 service might not be correctly interpreting the DR_TOOL reference, or there might be a mismatch in coordinate system conventions between the robot's control system and ROS2.

A third possibility is a configuration issue within the robot's control system. It's conceivable that the DR_TOOL coordinate system is not properly defined or calibrated, leading to incorrect force control behavior. This could be due to incorrect parameters in the robot's configuration files or a problem with the robot's internal coordinate system transformations.

To address these potential causes, several solutions can be explored. First, a thorough review of the check_force_condition function's code is necessary to identify any logical errors or limitations. This review should focus on how the reference coordinate system is handled and whether there are any conditions that might override the user's specified reference. If issues are found, the function should be modified to ensure it correctly implements force control with DR_TOOL as the reference.

Second, the ROS2 service implementation should be examined to ensure it properly handles coordinate system transformations and force control logic. This might involve debugging the service code, checking for errors in the service logs, and verifying that the service is correctly interpreting the robot's state. If problems are identified, the ROS2 service should be updated to address the issues.

Third, the robot's configuration should be checked to ensure the DR_TOOL coordinate system is properly defined and calibrated. This might involve using the robot's programming interface to inspect the coordinate system parameters and verifying that they match the physical setup. If any discrepancies are found, the configuration should be corrected to ensure accurate force control.

By systematically investigating these potential causes and implementing the corresponding solutions, the issue of force compliance control with DR_TOOL as the reference coordinate can be resolved, enabling the Doosan A0509s robot to perform its tasks with the required precision and accuracy.

Conclusion

In conclusion, achieving force compliance control with the DR_TOOL coordinate system on a Doosan A0509s robot presents a multifaceted challenge. The initial issue reported highlights the complexities involved in robotic control, where the interplay between software functions, coordinate systems, and external environments can lead to unexpected behavior. By meticulously examining the check_force_condition function, potential causes such as logical limitations, ROS2 integration issues, and configuration discrepancies were identified.

The solutions proposed involve a comprehensive approach, including code review, ROS2 service debugging, and robot configuration verification. This underscores the importance of a systematic methodology when troubleshooting robotic systems. Each component, from the control software to the physical setup, must be scrutinized to ensure seamless operation.

Ultimately, the ability to control forces relative to the tool coordinate system is crucial for many robotic applications. It allows for more intuitive and precise control, especially when the task requires interaction with the environment. Resolving this issue not only enhances the capabilities of the Doosan A0509s robot but also contributes to a deeper understanding of robotic control principles. By addressing these challenges, we pave the way for more sophisticated and adaptable robotic systems in the future.

For further information on robotic force control and coordinate systems, consider exploring resources like the Robotics Institute at Carnegie Mellon University.