# What sensor to inspect for object motion detection in an open but limited space?

I'm playing around with motion detection for fun and came across the problem of detecting the motion of uniform objects (e.g. cubes, balls, etc) from a fixed position. Concise: electronically capture the path of such a small object that enters an open empty space (field, flat ground, no other objects).

QUESTION: What sensors do you employ to fulfill the following requirements (within the given enrionment).

Environment and scenario:

• empty, cubic space (air only),
• no side longer than 40 feet / 12 meters.
• different but natural outdoor or indoor air pressure.
• no temperature difference between object and sensor.
• temperature between 5° and 55° celcius
• objects enter space from one side of the cube only.
• sensor must be opposite of the object entry side.

Requirement:

• location of object must be captured highly frequenty (min 10 times) within 1-3 seconds.
• each measurement event must deliver x, y and z (full relative position to the sensor).
• the sensing angel must be 80° (from what I read, that is really high. However, I think that can be resolved by installing multiple sensors to reach the 80°.)

So, dear e-engineers. What sensor is to solve that problem best under the given conditions? ultrasonic sensors?

Thanks for hints (that go beyond my concrete question).

• Well, I think you'll need at least 3 sensors to get a x,y,z coordination system Dec 4, 2013 at 12:52
• What kind of precision is required? If the lighting and background is controllable I'd go straight for a computer vision solution. Dec 4, 2013 at 12:53
• @user1410908 yep, that makes sense. I thought that too, and is absolutely doable on the host system. Dec 4, 2013 at 13:01
• @pjc50 The required precison is given by the need to compute the speed of the object. The speed of the object is not linear. Might go up and down. Because of that I need to measure multiple times. At the end I certeinly want a speed that is accurate within +-3kmh, +-1.5mph. Think of a baseball scenario, hm.. tennis outdoor, a speedy tractor on a field, etc. background is not something I could influence. And since there is no other object in this cubic space, I thought ultrasonic might make sense (though, I have no clue on their professional capabilities). Dec 4, 2013 at 13:08