# Engineering Method and Tool for the Complete Virtual Commissioning of Robotic Cells
## 3 Proposed Virtual Commissioning Approach
### Description of the Signal Exchange Mechanism
For each mapped signal, the value in RS is read and the corresponding variable in TC is updated. An optimization strategy is used to avoid useless operations. The last registered value is stored in the mapping info object, and it is compared with the new value in the next cycle in order to skip the writing operation if the value has not changed. This strategy allows the communication overhead to be reduced and makes the signals synchronization faster. ```

**Algorithm 1** Update variables in TC

## 4 Case Study: Robotic Assembly Cell

The approach described in Section 3 has been applied to a robotic assembly cell for gearboxes. The virtual model of the cell was realized in RS starting from a CAD representation of the real implemented prototype. The developed case allowed the VC capabilities of the proposed approach to be tested in a complex scenario made of two robots and various additional systems. In particular, the interaction mechanism between the PLC and the virtual representation of the cell devices is reported here in detail. Finally, an extract of the PLC code is shown and discussed, as well as the capability of the VC approach to improve the overall performance of the system. ### Description of the Robotic Assembly Cell

The robotic assembly cell includes two ABB industrial robots working in the same workspace. The aim of the cell is to mount fasteners and bearings on a basement resembling the case of a gearbox. The basement is placed on a Cartesian positioning table that is translated by two linear axes. The table can arrange the basement in different positions under an electric press, that is activated when pins or bearings must be pressed in their seats. Custom-designed punches are used depending on the object that must be pressed: they are brought in the right position by a robot and locked on the press due to its fast tool changer interface. The two robots in the cell are equipped with different tools. IRB 1600-10/1.2 (RobotI) mounts a tool changer, which is used to lock the appropriate gripper depending on the object that has to be picked and then placed in the basement. Five Schunk grippers are provided belonging to two different families: the PNG-plus 80-1, which includes parallel grippers, and the PNZ-plus 80-1 family, which is a three-finger centric gripper. In particular, two parallel grippers are used to pick and place pins and handle punches, while three centric grippers are used to pick up bearings. IRB 2600-20/1.65 (Robot2) mounts an electric, torque-controlled screwer from Bosch. Rexroth. It picks the fasteners from their holder and screws them on the basement. The system is coordinated by a Beckhoff Embedded PC CX5140 PLC equipped with Intel Atom quad-core processor. An EtherCAT fieldbus connects the devices in the cell and is responsible for the I/O signals connections. A list of the main hardware components present in the cell is given in Table 1.

The virtual model of the cell was created starting from a 3D representation realized in SolidWorks, including both the models used to design the components and models provided by the manufacturers of the devices (Figure 4). Then, the geometry was imported into RS. This is then followed by a standard workflow, which includes the definition of movable devices, known as mechanisms, used in the specific case for the press, positioning table and grippers. ### Communication Pattern between PLC and Cell Devices

The role of the PLC is to coordinate the activity of the robots and the devices included in the cell. According to the proposed approach, in the RS virtual model, a dedicated SC is created for each physical device. The SC represents the behavior of the physical asset being represented recurring to the functionalities provided by RS. However, the interface of the SC in terms of exchanged signal must be equal to the real device in order to permit a real VC of the cell.