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From Error to Precision: How does force control compensate for imperfect environments?
Force control has become increasingly important in machine building, industrial, and service robotics environments. The main purpose of this control technology is safety. It can effectively prevent accidental collisions between people and machinery, thereby reducing damage and injuries. In many cases, the motion of a robot may be blocked by obstacles, which makes the application of force control crucial.
Force control avoids damage to equipment and workpieces and reduces the probability of injury to personnel during operation by adjusting the contact force between the machine and the environment or workpiece.
Traditional motion control adds manipulation variables to correct position errors when the path is wrong. However, such practices may bring unexpected consequences and may even cause damage to the machine or instability of the working environment. Therefore, force control systems were developed to avoid these potentially dangerous situations by limiting the maximum force of the machine.
In machining tasks, unevenness of the workpiece often causes problems. For example, when position control is applied, the tool may dig too deeply into the surface or lose contact with the workpiece. In this case, the application of force control technology is particularly important because it ensures uniform material removal through stable contact force.
Application Scope
Force control applications can be divided into significant contact tasks and potential contact tasks. In contact-significant tasks, the contact between the machine and the environment or workpiece is a core component of the task, which usually involves mechanical deformation and surface processing. In tasks where contact is a potential problem, the machine should be able to avoid generating excessive contact forces in a dynamic environment.
Force control is widely used in mechanical manufacturing operations such as grinding, polishing and deburring, as well as force controlled processes such as controlled joining, bending and pressing rivets into pre-made holes.
Force control can also be used to scan unknown surfaces. The contact pressure can be maintained at a relatively constant level, allowing the scan head to be moved using position control. The application of this method can help describe the surface geometry in detail and further improve the processing accuracy.
Technological Development
Force control technology can be traced back to 1980, when John Kenneth Salisbury of Stanford University proposed the concept of active stiffness control. Over time, force control has undergone intense research and development, especially with the advancement of sensor technology and control algorithms, the application of force control has become more and more extensive.
Modern machine controllers are able to perform one-dimensional force control in real time with a cycle time of less than 10 milliseconds, indicating that the technology of force control has long been mature.
In force control, accurate measurement of contact forces is crucial. While traditional direct measurement methods focus on using force/torque sensors to obtain the current contact force, another economical option is to obtain this data indirectly by estimating the motor current during motion control. This approach not only reduces costs, but also reduces the risk of sensor failure.
Control Concept
The various control concepts used in force control mainly include direct force control and indirect control. While the goal of direct force control is to set the desired contact force with a definite value, indirect control usually increases the flexibility and responsiveness of the machine by regulating the impedance of the machine. In actual implementation, these two control methods are often complementary, and the best control technology is selected based on the current environmental conditions.
In the future, force control technology will likely play a greater role in more fields, such as medical robots, service robots and their collaborative applications. Fully automated force control ensures safety and stability when collaborating with humans and other machines. With the continuous advancement of technology, this control method will assist more complex application scenarios and thus improve overall work efficiency. Whether facing dynamic environments or various technical requirements, whether machines can successfully overcome these challenges remains to be tested by time.
