Get the distance to non perpendicular surface So I'm using the HC-SR04 modules to get the distance to objects, the problem is that if it's not a perpendicular surface I dont get any reading, and I assume that the reason is that the ultrasounds get reflected in another direction, so they don't come back to the sensor. What's the way to get over this issue?

Thanks in advance
You mean to say that you get proper reading when the surfaces are FLAT?
Ultrasonic sensors have this property that the waves emitted from then are only 15 degree from the line of sight, but this means that you can surely calculate distance of objects having a curved shape. The only condition is the object should be directly in a straight line