Highly productive assembly of guide sleeves for headrests
04. April 2018
Ros, a family-owned Franconian company, employs a workforce of about 550 at five locations within the Ros Group. It is a renowned manufacturer of custom-made plastic parts for the automotive, medical and electrical engineering industries. The company produces about 250 million parts per year and places great emphasis on an above-average vertical integration level.
This also applies to the guide sleeves of the headrests that are manufactured completely by injection moulding process and assembled in Ummerstadt. These guide sleeves differ in essential details according to vehicle model and their use in the driver’s, front passenger or rear seat. The newly commissioned assembly plant had to be capable of handling a total of eight different variants produced by Ros. Two Yaskawa type MOTOMAN MH5LS and MOTOMAN MH12 robots, a rotary indexing table with eight stations, diverse feed systems and multiple sensors form the basis for the highly flexible rotary transfer machine.
The adjusting sleeve essentially consists of three plastic parts: a guide sleeve, button with a cross bar, spring and cap.
Christopher Lamprecht, production planner at Ros and project manager for this plant, describes the real challenge in the assembly of these individual parts: “Depending on the version, in order to meet the high level of demand we must achieve an output of several hundred parts per hour. That’s why we chose a concept that guarantees overall availability at the highest level with a rotary indexing table and two dynamic robots.”
The assembly stages in detail
The assembly process starts with the feeding of the guide sleeves, which reach the transfer position individually by means of a vibratory bowl in a linear system. Here a camera system identifies the position of the sleeve and transmits this data to the Yaskawa robot, which then picks up the part and conveys it to a second vision system that reads the part number and checks it for plausibility. Then the compact type MH5LS six-axis robot deposits the sleeve quickly and accurately into the workpiece carrier of the rotary indexing table.
The rotary indexing table immediately moves to Station 2. The next step is pneumatic ‘insertion” of the return springs, before a further vision system at Station 3 checks the correct location and positioning of the springs. The task of Station 4 is to deliver the button and assemble the cross bar. After passing empty Station 5 a second integrated QS control takes place at Station 6, by which a number of features are checked.
A particularly intricate step is performed at Station 7, where the second Yaskawa six-axis robot performs the final task of fitting the cap. The MH12 picks up a cap with its vacuum gripper and mounts it on the sleeve in an ingenious movement pattern. In this process, in which the cap must be positioned at a predetermined angle in order to carefully click it into place, the robot must operate with extreme precision.
Similarly stringent demands exist at the following station, at which the robot takes hold of the complete assembled locking sleeves and ejects them via a chute. Here again, the robot takes up the challenge in terms of precision and path accuracy, as Lamprecht stresses: “The removal of the finished part from the workpiece carrier is rather difficult because the sleeve is located at an angle to the horizontal head. The robot cannot simply pull the part vertically out of the workpiece carrier; it must adjust its trajectory to the sloping position of the sleeve, which may vary by up to 13 degrees depending on the component version. Thanks to excellent motion control, the MH12 performs this task with speed and precision, enabling us to adhere to the cycle times.”
State-of-the-art control technology
Lamprecht explains the reasons why the decision makers at Ros opted for Yaskawa as robot supplier: “Firstly, the company’s experience with the Japanese robots was positive and it saw no reason to change its single sourcing strategy. Secondly, the decision in favour of Yaskawa was clinched by the good repeat accuracy of the robots, and their speed and easy connection to image processing systems via Profinet.”
When it comes to control and communications technology, the plant reflects the state-of-the-art. Conventional wiring was renounced in favour of communication via Profinet, thus significantly reducing costs. All workpiece carriers are equipped with an RFID chip that, among other things, greatly facilitates their identification. This is an important consideration, as several different workpiece carrier designs are required for assembly of the eight sleeve versions. Via RFID it is easy to verify that the suitable attachment for the selected component version is located on the rotary indexing table. One positive spin-off is that the data of each assembly stage is also stored on the chip.
Furthermore, the operating team was able to operate the plant with the greatest of ease. While loading and unloading tasks are performed to a large part manually, all assembly stages run fully automatically. The operation of the system is extremely simple: the operator selects the appropriate variant on the control unit and starts the assembly process. The system would immediately recognise a faulty part and stop the assembly process.
All in all, the system that was designed and built in-house satisfies all specifications. Lamprecht sums up: “The plant achieves the specified cycle times, has the required flexibility for the assembly of all versions. It has an extremely compact design, so that we can manage with a small footprint, and it consists of first-class components, including the two Yaskawa robots that guarantee high performance and availability. All that without any costs for external engineering – what more could you wish for?”
Text & Pictures: Ralf Högel
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