Not always identical: digital twins of machine tools

There are two very different application scenarios for digital twins in the lifecycle of machine tools: One is determined by the requirements of the machine builder, and involves application during the design, engineering, commissioning, maintenance, and servicing of machines. The second application scenario concerns the machine tool operation, particularly the creation and verification of reliable CNC programs. It is important to be aware of and understand these differences when digitalizing production and purchasing new machines. For this, it helps to be familiar with the basics of how digital twins are designed.

A machine tool is a complex mechatronic entity, and its digital twin is complex, too. Like the real machine tool, the digital twin needs to be segmented into modules: a virtual image of the CNC equipment, a virtual image of the machine behavior, and a virtual image of the mechanics.

The digital twin concept potentially covers all phases of a machine’s lifecycle. This ranges from the develo pment, engineering, and commissioning of the machine, to its use in manufacturing a wide variety of workpieces, to servicing and maintenance: all of this is monitored, planned, checked, and optimized on a virtual level with a digital twin.

Requirements for the digital twin differ greatly depending on the machine’s life phase (i.e., develop - ment, use, service). The machine operator uses a digital twin to ensure, on a virtual level, that CNC programs are fault-free and will deliver the desired machining result on the first go. This saves the machining time otherwise spent to run in the parts program step-by-step. It also allows the operator to ensure that a new NC program will not cause any collisions between a tool or clamp and the machine or workpiece.

In short: during production, the digital twin is there to ensure that the machine will run reliably and is relieved of process steps that serve no purpose. The fact that the machine itself will function as intended is taken for granted, and not checked using the digital twin.

Electrical engineers, software departments, and machine builders’ service technicians, on the other hand, are interested in exactly that: they use the digital twin to ensure that the machine will function in line with its specifications. Testing the interplay between the CNC application and actuators, sensors, and the mechanics is the key here. All conceivable operating situations are played out in advance, and special occurrences are taken into account in the CNC application. Following approval and delivery of the real machine, service technicians use this digital twin to track down malfunctions and offer solutions without having to travel to the customer.

The area of focus for the machine builder is shortening the development periods and time-to-market. This is achieved by shifting development stages to the virtual sphere, saving the creation of mechanical prototypes, and maximizing the operating safety and availability of the machine. Unlike the machine tool operator, the machine builder does not have to create fault-free CNC programs, but makes use of triedand- tested, quality- assured parts programs for prototype tests that need to be run.

These two fundamentally different perspectives shape how the digital twin is used — that is to say, the software and hardware environment in which it is deployed. These differ depending on how the digital twin is to be used, what options it must offer the user, and which are not needed..

When it comes to parts programming and production planning, the core task of the digital twin is to ensure that CNC programs are fault-free. The virtual image of the CNC equipment within the digital twin is used to simulate the execution of CNC programs. There are two different approaches to doing this available on the market. The first option emulates the function of the CNC — that is to say, it simulates the CNC in the machine software.

The second option uses the CNC’s real system software. For Sinumerik, this is performed by the virtual numerical controller kernel (VNCK). The Sinumerik VNCK contains the same system software as a Sinumerik CNC. As a result, the virtual Sinumerik CNC in a digital twin behaves in exactly the same way when executing parts programs as the Sinumerik CNC behaves in the real machine. The reliability that a part which has been tested and optimized using a digital twin will be machined in precisely the same way on the real machine as it was in the simulation — with the same process safety, speed, and machining quality — is especially high with VNCK-operated digital twins of machine tools. It means that the goal of producing an in-spec part straight away, even in one-off production, is achievable even with complex workpieces.

Aside from the VNCK, the virtual image of the machine’s mechanics is important for the practical aspects of working with the digital twin: it visualizes each and every moment of the production process, meaning that it can be monitored on the PC. In addition to the optical display of machine and tool movements, the digital twin is responsible for reliably identifying and flagging collisions between the tool and part, clamping, and machine com ponents — which ultimately involves visualizing the machining itself through removal simulation. The NX Virtual Machine software is one of the leading systems in the visualization of machine movements as well as material removal, and this software also supports seam less integration of the Sinumerik VNCK

The third aspect of a digital twin — the simulation and manipulation of the behavior of individual machine components, actuators, and sensors — has only minor significance in CNC production planning. All that is required here is a realistic virtual machine control panel (MCP) that operators of the virtual twin can use to operate the simulated machine and manipulate execution of the CNC program.

The behavior of any machine is determined by the interplay between a large number of different actuators and sensors. Let’s take the example of a tool changer: The spindle must adopt the tool changing position, a slide opens a flap, the spindle clamp has to be opened, a gripper clutches the previous tool, moves the new tool into the spindle, the clamp is closed, the gripper is retracted, the flap is closed. In development, commissioning, and servicing, a digital twin not only simulates the behavior of all the involved machine parts, actuators, and sensors in the virtual sphere, but must also allow for specific adjustments in their interaction.

Unlike the simple NC kernel, there is no virtual image of the CNC equipment that completely encompasses the CNC, PLC, and drive control functions. Therefore, machine builders need an interface between the virtual and the real world, to control their digital twins with real CNC and drive components using a test rack.

Implementing the Simit and NX Mechatronic Concept Designer programs offered by Siemens is one solution to this. Simit realizes the connection of hardware signals and fieldbus with the actuators and sensors of the digital twin. The NX Mechatronic Concept Designer essentially ensures that the mechanical components of the machine mapped out as a 3D model move as intended, for example along a guide rail or around a joint — or do not move at all, if they are fixed together. Thanks to these software products, a machine’s CNC equipment is put into operation without real prototypes. The machine’s mechanics and electrical equipment are represented by the digital twin. When a customer reports a fault, this digital twin allows machine service departments to track down the problem, analyze it, and offer detailled advice to technicians on-site. The compatibility of software updates or function expansions is verified without affecting the availability of the machine itself. The collision avoidance and machining simulation features that are so important in production play a minor role here.

Strictly speaking, a “digital twin” operated by means of hardware-in-the-loop is not a full virtual twin of the machine tool — seeing as a real CNC is used to control it. The simulation computer “only” provides the virtual image of the machine’s mechanics and simulates the behavior of all machine parts in detail.

Use of digital twins in development and in production planning are by far the most important forms of application. However, in practice the distinction between the two different applications is not so precise and the resulting application somewhere in-between. Machine builders will also work with the VNCK and the NX Virtual Machine to explore new technological functions and their impact on the mechanical design, and to create functioning models of the machines to highlight their functional selling points.

Machine tool operators, on the other hand, are often involved in the engineering phase of their new machines, therefore coming into contact with the digital twin for machine tool engineering and commissioning. For customized versions of series machines, for example, the special functions are often worked out in detail using digital twins. With special machines or large machines, it is essential for machine builders to discuss possible solutions with the future user as early as the design phase.

All in all, digital twins are not only powerful tools for development, production, and servicing, but also represent a highly efficient means of communication when it comes to conveying ideas, features, and requirements.