Design and test of online flatness measuring system for large-scale chassis of combine harvester

Abstract: The large-scale harvester chassis is usually made up of 2 long stringers and several crossbeams by welding, and most important parts of harvester are mounted on its upper surface, such as engine, cab and gearbox. Because of the welding thermal stress, the upper surface of chassis cannot be a perfect plane, and the important parts would not be in the pre-set position. If the error is too big, the harvester would not run smoothly as hoped. So it is important to get the flatness error before chassis goes toward the production line. Three-coordinate measuring machine, laser measuring machine and 3D (three-dimensional) camera are traditional flatness error measuring devices, and their precision is high, but they cannot measure such a big chassis with fast enough speed; in addition, their price is very high. In this paper, a test system equipped with laser ranging finders is proposed to measure the flatness error of chassis. The system includes a hydraulic lifting platform and a reference platform. Hydraulic lifting platform is made up of positioning devices, clamping devices and shearing mechanism, and it can lift chassis to a certain height and keep still. The reference platform is a 0 stage precision granite platform, 2 parallel linear guide rails are mounted on its surface, and 2 groups of laser ranging finders (3 finders for each group) will move along the guide rails to get the distance between reference platform and measuring point on chassis. In this system, hydraulic lifting platform is just used to fix chassis, and the position error between hydraulic lifting platform and reference platform can only change the coordinate value of measuring point, and has nothing to do with the flatness error result. The measurement process is carried out in 3 steps. First, hydraulic lifting platform lifts the chassis to the position where stringers are under the laser ranging finders. Second, laser ranging sensors scan the chassis to get the coordinate values of measured points on chassis. At last, computer calculates the flatness error result, and the chassis is put down and took away at the same time. After measuring, the chassis plane is transformed into a great number of spatial coordinates with the same rectangle distribution, and then software will call the MATLAB to calculate the error result. The measurement and control software above-mentioned based on LabWindows/CVI (C for Virtual Instrumentation) is developed to control the system, which can make the test system autonomous or semi-autonomous working, and all the data are stored in an excel file. The system is tested by 9 times on one chassis of a certain type corn combine harvester. The length is 4 000 mm, the width is 1 110 mm, and 66 measuring points are measured along the 2 stringers. The flatness error of 10.27 mm is obtained with the least square method, which takes all the measuring points into account, and the uncertainty degree of this result is just ±0.05 mm. All these show that the result is authentic and credible, and the system and error evaluation method can meet the accuracy and speed requirements of the test, so there is a great significance to ensure the assembly quality of large-scale harvesters and improve the competition of the products.