In May, BMW’s assembly plant in Munich celebrated its 100th anniversary. Spanning nearly 5.4 million square feet and employing some 7,800 people, the factory assembles 1,000 cars and 2,000 engines daily. The factory’s current lineup includes the 3 series, 4 series and the i4 electric vehicle. In 2016, the plant produced its 10 millionth vehicle.

On the engine assembly line, bearing shells of different thicknesses are used to provide optimum support for crankshaft bearings, but the shells are difficult to differentiate visually. To prevent assembly errors, BMW employs the Local Positioning System (LPS) from Sarissa GmbH of Weingarten, Germany.

The LPS uses ultrasonic sensors to determine the location of a tool or a worker’s hands with millimeter precision. When linked to a manufacturing execution system or work instruction software, the system error-proofs assembly operations by guiding workers to the correct part and showing them where to install it. If assemblers attempt to install a part in the wrong location, the software issues an alarm or can deactivate the tool.

The LPS consists of one or more ultrasonic receivers and transmitters that are integrated into the tool or attached to the worker’s hand. The transmitters emit ultrasonic signals at millisecond intervals that are silent and physiologically harmless. In addition, the system unaffected by ambient light sources or audible sounds.

The spatial coordinates of the transmitter are identified and localized in real time by the receivers, which are located above the work area. If the transmitter and receiver are 2 meters apart, the system can locate an object with a precision of 0.21 millimeter at an angle of 0 degree. With an inclination of 40 degrees between transmitter and receiver, the system can locate an object with a precision 0.33 millimeter.

The LPS communicates with a PC or PLC in real time via Profinet, EtherCAT or TCP/IP interfaces.

BMW has been using the LPS system in Munich since 2017. Engines are assembled on mobile workpiece carriers. To ensure perfect bearing arrangements for the six- and twelve-cylinder crankshafts, the right bearing shells must be installed on each bearing bracket. This requires an exact measurement of the respective bearing bracket before the bearing shells are installed. Based on this data, the appropriate bearing shells with the correct thickness can be determined for the respective location in the bearing bracket.

Although the bearing shells are different, they can be almost indistinguishable visually from one another and are easily confused. The LPS ensures that the right shell is installed on the right bracket.

In this application, the LPS software does not determine the coordinates of a tool tip, as is usually the case, but rather the coordinates of the bearing shell.

A PLC tells the LPS what motor is in in the assembly station as well as the bearing classifications to be installed for each bearing bracket. Assemblers then see on a screen which bearing shells are required for that specific engine. Assemblers place the shells, which are marked with a Data Matrix code, in the designated positions in the bearing bracket. Assemblers scan the code with a special code reader equipped with an ultrasonic transmitter. At the same time that the reader is scanning the code, the transmitter is sending high-frequency ultrasonic signals into the room, which are received by a Sarissa ultrasonic receiver. Based on the QR code and after communicating with the PLC, the LPS determines in real time whether the correct bearing shell is in the prescribed mounting position.

Although the transmitter is located on the code reader, Sarissa’s software always determines the XYZ position of the QR code and thus the bearing shell, and not the position of the code reader. It makes no difference whether the code reader is held at different angles during the reading process or by a right- or left-handed person.

If a bearing shell has been accidentally mounted in the wrong position, the engine block is prevented from leaving the workstation. The error is visualized on the screen, and assemblers can correct the mistake. After the correction, the software confirms on the screen that all bearing shells have been mounted correctly.

For more information on error-proofing sensors, call 49-751-50915900or visit