Natick-based Cognex Corp., which makes sensors and monitors used in manufacturing, lost ground on Wall Street after Piper Jaffray cut its rating to “underweight” on concern about a slowdown in manufacturing in China and uncertainty about conditions in the European Union, Dow Jones reported. Cognex has invested heavily in China, but revenue in the region could decline 15 to 20 percent in calendar 2012 from a year before, Piper said. Cognex’s US revenue, the firm said, could fall 10 to 15 percent.