Resin level detection in additive manufacturing

An apparatus useful for making a three-dimensional object from a polymerizable resin, comprising:

• (a) a build platform on which a three-dimensional object can be made;
• (b) a light transmissive window having a build surface operatively associated with said build platform, said build platform and said build surface defining a build region therebetween, said window configured to support a resin pool thereon;
• (c) an elevator assembly operatively associated with said build platform and/or said window, said elevator assembly configured for advancing said build platform and said window member away from one another to draw said polymerizable liquid into said build region;
• (d) a light engine operatively associated with said window and positioned to irradiate said build region with light to form a growing three-dimensional object from said resin;
• (e) a force sensor operatively associated with said platform and/or said window and configured to detect an impact of said build platform with a top surface portion of said resin pool;
• and
• (f) controller operatively associated with said carrier platform, said light engine, and said force sensor, said controller configured to determine a resin pool fill level from a detected impact of said carrier platform with a resin pool top surface portion.