Machining
One of the first applications of force control systems in robots were machining tasks such as grinding or blunting of sharp edges. Currently, robots are used in the following types of machining: grinding, polishing, chamfering, blunting, light milling. The implementation of such tasks requires the use of so-called position-force hybrid control [7–9]. The task of such a control system is to implement the desired trajectory of the tool movement along the edge being machined or on the machined surface and to exert an appropriate clamping force of the tool. Currently, industrial robotics has two principal hybrid position-force control strategies in robotic machining processes [10]. The first consists in maintaining the given interaction force at a constant speed of movement, whereby the path of movement is automatically adjusted to the shape of the contact plane (Figure 2). This method gives good results in applications such as polishing, when there is no significant loss in the machined surface, and the tool has a large active surface. The second control strategy [10] is based on the movement of the robot's tip along the programmed path regardless of the shape of the surface being machined (Figure 2). The variable value is the speed of movement, which depends on the resistance to movement. In this strategy, the pressure force is not a controlled quantity. It depends on the assumed motion path and the actual shape of the contact surface. Sometimes the basic control strategies are modified in commercial solutions in order to introduce some flexibility in their implementation [11].
Figure 2: Force control strategy for robotized machining.
In the field of robotic machining, an important and still valid issue is the development and implementation of control strategies that ensure the quality of the mechanical machining process of the part [12] despite the occurrence of unmodeled phenomena, caused by, for example, significant errors in the geometry of the parts [13] (associated with the uncertainty of their position relative to the robot) with local surface disturbances [14] or its flexibility [15].
Assembly TasksAnother of the basic applications of force control systems in robots are assembly tasks. In the first uses of assembly applications, pliable grippers with susceptibility were used to ensure high tolerance of the movement accuracy of a robot's arm. Such systems were gradually replaced or supplemented with systems with force control [16,17]. This approach allows the avoidance of blocking components during assembly, checking the clamping pressure of the assembled components to avoid overloading, or detecting a mismatch of components due to incorrect fitting- too tight or loose. The robotization of the assembly of flexible components, as described in [18,19] is a particular challenge. In such cases, force control is particularly important, because too high interaction forces between the assembled components lead to large distortions and prevent the correct process from running.
Monitoring of the Machining Process
Monitoring the machining process is important in the industrial production of parts with a high unit cost. Any irregularity in the production of the part causing its non-compliance with the documentation is a cause of significant financial losses. Process monitoring aims to prevent irregularities during its implementation and to correct or discontinue the machining process. Process monitoring involves observing (measuring) sizes in the process in a direct or indirect way. In the case of machining, the aim of which is to achieve the appropriate geometric dimensions or surface roughness, direct measurement of these quantities is not possible during the process. Therefore, signals related to phenomena accompanying the treatment process are monitored. These accompanying phenomena are, for example, vibrations, noise, cutting forces or heat generation (Figure 3). These phenomena can be quantified using signals such as accelerations, acoustic pressure, interaction forces or temperature. Irregularities in the implementation of the process cause a discrepancy between the levels of measured values associated with the accompanying phenomena and reference levels for those quantities determined for the correctly carried out process.
Figure 3: Machining process monitoring and control.
The problem of monitoring the machining processes carried out with the use of tools with high rigidity, such as drills, files or cutters, is known in principle. Processing with such tools results in accompanying phenomena that are very "explicit", and in the case of a disruption in the process the change in accompanying phenomena is significant and the disorder can be easily diagnosed. There are many papers in the literature that describe the problem of controlling the machining process using robotic force sensors. Numerous works describe the monitoring of the process of robotic polishing [20], grinding [21] or milling [22] using a signal from a force sensor.
Friction Welding
Friction stir welding is a method of joining materials without using consumable materials and without melting materials. In the process of friction stir welding, a cylindrical tool with a mandrel performs a rotary motion and at the same time is slowly moved along the joint area with simultaneous clamping. An industrial robot is responsible for the movement along the joint. As a result of friction of the tool on the joined components, heat is generated, and the surfaces of the joined components are brought to the yield point. The rotating tool causes the material to mix along the joint. After passage of the tool, the material cools down creating a permanent connection. Friction stir welding is used for joining components from the same materials, such as steels [23], aluminum alloys [24], metal matrix composites [25], but also materials of different types, such as steel with aluminum [26]. The friction welding process is very sensitive to the temperature in the joint area. The temperature is not controlled directly, but by three other parameters: tool feed speed, tool speed and tool clamping force. For this reason, robots used for friction welding are equipped with position-force hybrid control systems [7, 15].
The friction welding process can be disturbed by deformations of the welded components under pressure or inaccuracy of the path followed by the robot. For this reason, the works [27, 28] analyze the impact of these disturbances on the welding process and looks for ways to eliminate them. Another disturbance may be unevenness of the surface of the welded components, which can be compensated for by adjusting the clamping force. For this reason, the process of friction welding of components with complex curvatures is carried out using robotic stations [27, 29, 30], where it is possible to control the required process parameters and to implement complex trajectories, and not on CNC machines. Due to the advantages, this method is used in many industries: e.g. aviation [31], automotive [27], means of transport [32].
Robot-Human Collaboration
In recent years, the issue of cooperation in the human-robot system has become more and more important. Collaborative robots are commonly called cobots [33- 35]. The main area of application of this approach are assembly tasks [36, 37] and small parts handling and inspection tasks. This solution has a number of advantages, such as:
1. The possibility of tasks being carried out by humans that are extremely difficult to algorithm and program while other tasks are carried out by the robot.
2. The possibility of using the lifting capacity of the robot to lift heavy objects and the "ingenuity" of a human to maneuver the object - thanks to force sensor, the robot is able to detect the method of maneuvering an object desired by a human [38].
This approach, however, brings with it the need to ensure the safety of the human. The ISO/TS 15066 technical specification prepared in 2016, which complements the two-part ISO 10218-1 and 2 standards, defines guidelines for industrial applications in which human and robot collaboration occurs. Of the four types of cooperating installations, two of them require the use of force / moment sensors. The first is a system in which hand-guiding is used. The robotic system is controlled by the operator by direct manual control [39]. The robot recognizes the desired direction of movement by measuring the forces applied to the arm. The second case is a system in which the robot and the human are working at the same time, and the force and speed of the robot's arm movement are controlled and limited in such a way that the robot does not cause the human pain or injury while they are interacting with it.
