I'm doing a project which using ultrasonic sensors to measure the distances to objects.
As far as I know the sensor can only measure the distance of the closest object to it. Is it right?
And in case there are multiple objects, can the ultrasonic sensor be able to detect distances to those?
Typically ultrasonic sensors are designed to only return the distance to the first object. They send a pulse out, then time until that pulse comes back from a reflection, and that is the result. They ignore any remaining returns from other reflections.
It may be possible to modify or design your own ultrasonic sensor that pays attention to the other reflections as well, giving multiple outputs from multiple objects, but unless you control the environment carefully it can be very complicated to do well.
But the answer to your question is no, unmodified ultrasonic sensors do not perform multiple object detection.
Unltrasonic sensors have a cone in which they measure. The aperture is usually specified in the data sheet and quite wide. Within this cone, the closest object will give the "sensor reading", although objects on the bounds of the cone might not be recognized correctly.
So for getting a good idea of e.g. a robots surroundings, laser sensors mounted on a servo might be better. You can sweep the surroundings in well-defined steps with this thing.
There are some sensors with an analog envelope output. Which enables you to detect all the reflections detected by the receiver. And of course you can allways just use a transmitter and receiver without additional electronics, so that you deal with the raw signals, and then extract from it whatever you want.
