sensors and goods detection

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  aj_onduty

  You can use laser transmitter-receiver, or infrared transmitter-receiver, as your budget may permit.
  The containers are usually higher than other vehicles, even they are taller than the vehicles which bring them, when mounted. Use three laser sensors, in a horizontal line, at a height which is higher than other vehicles but lower than the container height mounted on the vehicle. The purpose of three sensors, well, let me put it straight.
  They are spaced such that the distance between two sensors is less than the length of the smallest container. So, when the container gets in, it would activate two sensors at a time. then you can make sure that it is a container. Now, let us name the sensors 1,2 and 3.
  When an object obstructing the transmitter-receiver gets in, it first intersects sensor 1. Then our processor checks if it obstructs sensor 2 without changing the status of sensor 1. If it does, then the processor verifies that it is a container. And as it obstructs the sensor 1 first, it verifies that it is getting in. So, your computer tells you that one container has got in. Similar for getting on, it would be vice-versa.
  This is a fully automatic version which won't satisfy that tricky part of the question.
  Now, For the other consignment (tricky) problem. If you know the details of the drivers who drive the first containers(those of your concern), call them up and tell them that there is a button provided at the gate, which they need to hit when they get in and they clear out the containers. This way, you can count the number of containers, do all the things required without the need of sensors. But this is a semi automatic type, it won't work without human intervention.