Thanks to optical identification systems and identification using radio waves (RFID), the industrial identification of tools, materials and products is state of the art. However, it only allows the selective detection of individual objects that move past the respective reader, but not the comprehensive detection of all objects in the plant. This is where real-time locating systems (RTLS for short) come in, which provide constantly available links between the type of object (what?), the location (where?) and the time at which the location was determined (when?). Typical applications in industry include the following:

• In large manufacturing plants, there can be several thousand objects in circulation - from pallets and containers with delivered material to partial and finished products. The constantly updated mapping and tracking of these material flows can be used for the analysis and optimization of the production process as well as for the actual control of production processes. Set-up processes can be carried out as soon as a specific material has been delivered to a machine. Resources that are rarely available, such as overhead cranes or temporary storage areas, can be used more efficiently if the goods currently in circulation are known and expensive search and allocation processes can be eliminated.
• Driverless transport systems require location information of the vehicle and the objects to be transported for free navigation as well as for loading and unloading processes.
• Support processes such as system, vehicle or container maintenance can benefit from location information by shortening search processes and avoiding incorrect assignments between systems and tools.
• Production processes can be designed to be error-free if the use of a tool is only released once it has been correctly positioned, for example in the case of a large number of manual screwdriving processes with torque specifications. This also enables automated documentation of tool use.

Components of local radiolocation systems

A local radio location system such as 'Simatic RTLS' from Siemens usually consists of an infrastructure that is installed in a fixed location and mobile components that are to be located by the infrastructure. Terms such as anchor, node, base station or sensor have become established for infrastructure components. In many systems, gateways are anchors with additional IT interfaces such as Ethernet or WLAN. Mobile components are usually called transponders or tags. Transponders are available in a wide range of designs - from very small designs to devices with input and output options such as buttons, LEDs and ePaper displays.

With the Time Difference of Arrival (TDOA) measurement method, only the transponder transmits, which means that several thousand transponders can be localized with a measurement rate in the seconds range.

© Siemens

The measurement accuracy that can be achieved in industrial production environments is largely determined by the signals used for localization. UWB (Ultra Wide Band) signals, for example, achieve a maximum accuracy of 30 cm. Locating with narrow-band communication signals such as Bluetooth and WLAN is significantly worse, as errors in the meter range can occur when these are used. UWB technology is significantly more robust, especially in halls with many metallic surfaces.

The data communication between transponders and anchors, which is also required for localization, can take place in different ways. A rough distinction can be made between a classic wired or wireless network infrastructure and communication via the wireless link used for measurement. Before deciding on a system, the advantages and disadvantages - costs, susceptibility to interference and real-time behavior - must be weighed against each other; especially in applications with large transponder populations, care must be taken to ensure that the channel capacity available for transit time measurement is not unnecessarily used up by communication.

Usually, central locating servers - for example 'Simatic Locating Manager' - calculate the positions of all transponders. With several thousand objects and a high measurement rate, this is no easy task and the hardware should be correspondingly powerful. A redundant hardware design and replication of the measured location information in the software ensure uninterrupted operation.

Radiolocation measurement method

With the Time Difference of Arrival (TDOA) measurement method, only the transponder transmits, which means that several thousand transponders can be localized with a measurement rate in the seconds range.

© Siemens

Two methods are commonly used today for radiolocation, which have different advantages and disadvantages depending on the application:
For large transponder populations, high measurement rates and the requirement for long battery life, the measurement of runtime differences is particularly advantageous. Here, the mobile transponders transmit signals that are received by the positioning infrastructure. This process is known as 'Time Difference of Arrival' (TDOA). Due to the simultaneous reception at many anchors, such a system scales very well as the number of transponders increases. And as the transponders only emit short signals - also known as 'flashing' - this method results in low energy consumption at the transponder. This enables battery operation for several years. With the TDOA measurement method, several thousand transponders can be measured at a rate of seconds.

'Two Way Ranging' (TWR), also known as 'Roundtrip Time of Flight' (RTOF), enables direct distance measurement between an anchor and a tag. This measurement method has a detrimental effect on the number of possible transponders on a site, as a large number of communication links are required, which cost time, channel capacity and energy. Battery life is also severely limited as a result. By measuring the distance to several anchors, the 2D or 3D position of the transponder can be calculated using tri- and multi-lateration methods. This measurement method is preferably used for applications with high accuracy requirements and only a few transponders, such as tool location.

Real-time radiolocation

The real-time term in RTLS refers to the constantly available position information of all objects that are equipped with a transponder. The timeliness of the measurement depends largely on the set measurement rate - usually in the seconds or minutes range. This is important, for example, when controlling production processes based on location events.

Excerpt from the 'Simatic RTLS' portfolio with the RTLS4030G gateway and the RTLS4030T transponders for small load carriers and RTLS4084T with ePaper display for displaying information directly on the object.

© Siemens

The large number of conceivable target systems requires interfaces that the server must offer in order to make location information available in a targeted manner. Filtering and distribution to different communication channels can be based on the type of object - container, vehicle, tool - for example. Deriving events from position data - 'material reaches machine' instead of the x/y position of the material - enables a qualified response by the target system.

A setup process can be triggered via a manufacturing execution system when a material delivery is expected and the position of the tool to be set up in the tool store can be displayed to the worker at the same time. In Enterprise Resource Planning, resources are managed efficiently thanks to constantly updated location information, as search processes are no longer necessary and local bottlenecks or oversupply can be quickly identified. Position information can also be used at the control and process control level, for example to open and close barriers and to set points.

With 'Simatic RTLS', Siemens offers a real-time radio location system with UWB signals and the TDOA and TWR measurement methods. In addition to gateways and anchors, which are used to set up an RTLS infrastructure, a wide selection of transponders is available for various applications in industry and local public transport. A server (Simatic Locating Manager) with multiple redundancies on request calculates the location information and makes it available to higher-level systems. The 'Scalance' family can be used as the basis for the Industrial Ethernet and Industrial WLAN network infrastructure in order to network gateways with each other and with the server.

Author:
Dr. Stefan Schwarzer is Product Manager for Industrial Identification and Localization Systems atSiemens in Nuremberg.