The environment for sensors in automatic gearboxes is harsh: large temperature fluctuations and oil which aggressively attacks surfaces. Bosch now uses ovens from Vötsch Industrietechnik to cross-link the thermosetting plastic housings of speed sensors and then subject them to an extensive stress test. The ovens are integrated into fully automated production lines. The rotation sensors are integrated into the 9G-TRONIC automatic gearboxes of a leading German car manufacturer.
The oven
The vötschtechnik® heating and activation oven consists of two chambers. In one of the chambers, there is a constant temperature during the process. The second chamber has two temperature zones, with greatly differing temperatures. The sensors are passed from one temperature zone to
the other. The oven is equipped with a separate controller and is integrated into a fully automated production line. The production line uses industrial robots to place the sensors in the oven. The oven doors, four for each chamber, are pneumatically powered, and opening and closing is requested with an input/output signal. Wolfgang Paasch, Production Project Manager at Robert Bosch Fahrzeugelektrik Eisenach GmbH summarises the integration into the production line: “vötschtechnik® ovens are ideal for processing our small sensors in large packaging units and therefore only require a small amount of space. The ovens are notable for their robustness, reliability, and once-a-year maintenance. Cooperation during the various projects was good and constructive and deadlines were always met. A second oven is planned to increase capacity.”
The process
The robot places the sensors, which are plugged onto carrier plates, directly into the hot left-hand chamber of the oven. The thermosetting plastic, the packaging of the sensors, if you will, is cross-linked at high temperatures. Cross-linking creates the finished structure of the plastic.
This process takes several hours, during which the temperature of the oven is held stable. Sensors which have already gone through this process are moved into the cooler of the two temperature zones in the right-hand chamber. The repeated transfer from one temperature zone to the other is triggered by the production line control system and is carried out automatically by the oven. The rapid change in temperature, from hot to cold or from cold to hot, simulates the thermal stresses on the sensor in the gearbox. The sensors are therefore being subjected to a stress test under extreme conditions. A function test is carried out once the heat treatment is complete. Only sensors which have passed this test are installed in the gearboxes.
The sensors
Three speed sensors are installed in automatic gearboxes: one measures the engine speed, a second measures the speed of a ring gear in the gearbox, and a third measures the speed at the output of the gearbox. The vehicle control unit receives the data from the sensors to decide when
to shift gears.
