Velodyne LiDar Inc., a Morgan Hill company that makes one of the key sensors for self-driving cars, has opened a factory in San Jose designed to expand along with the fledgling industry, the firm reported Tuesday. The new plant on Hellyer Avenue will build Velodyne’s lidar, a sensor that works on the same principle as radar but uses laser light instead of radio waves. With a few notable exceptions, most of the companies developing self-driving vehicles use lidar to scan the roadway environment with high precision. And while truly autonomous cars capable of driving themselves in all situations remain several years away, Velodyne is banking on rapid growth in the number of test vehicles on the road.