Study of Automatic Test System of Surface Flatness in No-Till Field Based on the PLC Technology - Yanbo Su; Hongwen Li; Yarong Mi; Jin He; Qingjie Wang; Hui Li; Rabi G. Rasaily (Computer and Computing Technologies in Agriculture V - 5th IFIP TC 5/SIG 5.1 Conference, CCTA 2011, Beijing, China, October 29-31, 2011, Proceedings, Part I, 2011)