Sensor for motion detection on small scale

Hi,

I need a sensor to measure motion of small particles to a close distance. Think of it like stones of diameter between 1 and 10mm falling down a pipe (diameter about 15mm), and I want to be able to measure the event when one passed the sensor (the flow is not continuous, so only one passes at a time).

So the simplest setup would be a sensor that just sits there and gives a HIGH output if it detected motion… My ideas so far where as follows:

  1. Infrared Sensor: Problem: sensitive to light conditions, I guess not high enough resolution?
  2. Ultrasonic: insensitive to light conditions, but also not precise enough (?), as I need measurement accuracy to 1mm (the sensor should detect such small particles)
  3. Photoresistor and light source: should be sensitive enough, but would that work?

I am happy for all suggestions and, hopefully, some proven concepts, if you have any

julius_a: I need a sensor to measure motion of small particles

So the simplest setup would be a sensor that just sits there and gives a HIGH output if it detected motion

Let's first clarify that you actually mean motion, or do you mean presence?

If a sensor like ultrasonic detects no object present, then a moment later does detect an object in range, it detected its presence not the actual motion. The motion is implied since one moment the object wasn't there, and the next moment it was, so it must have moved. But the sensor didn't see the motion: it looked and there was nothing; looked again and there was something.

To detect actual motion with one sensor is different from sensing presence.....

sayHovis: Let's first clarify that you actually mean motion, or do you mean presence?

If a sensor like ultrasonic detects no object present, then a moment later does detect an object in range, it detected its presence not the actual motion. The motion is implied since one moment the object wasn't there, and the next moment it was, so it must have moved. But the sensor didn't see the motion: it looked and there was nothing; looked again and there was something.

To detect actual motion with one sensor is different from sensing presence.....

Ok, I am sorry for the confusion, as I described I meant the presence of an object. I just want the information "no object / object" but for objects of 1-10mm and a distance from 0 to 15mm to the sensor... would that work with an ultrasonic? Afaik the resolution is more in the area of 3-5mm, at most.

How fast do the objects move? If they take more than 1/10th of a second to pass the visual field, then the obviouse answer is a raspberrypi + camera (you can actually get up to ~ 400 Hz framerate from the rpi camera, at least for thw first ~ 100 lines)

zwieblum: How fast do the objects move? If they take more than 1/10th of a second to pass the visual field, then the obviouse answer is a raspberrypi + camera (you can actually get up to ~ 400 Hz framerate from the rpi camera, at least for thw first ~ 100 lines)

The objects are more or less in free fall, and are sensed shortly after they started falling. However, I would like to use an Arduino since so far all other Control mechanisms are based on it.

Hm .. try a light barrier. But you'll need to take a modulated IR carrier (the usual 38kHz will do) and you will need the receiver to be quite sensitive, as the light variance for the 1mm ball will be 1/100th of the 10 mm ball. Giving different distances, you might end up with a variance of 1/1000th. Might be that you need an amplifier or you can get away with taking multipole samples. Or you get a i2c 16-bit adc

OK, an object falling in some sort of environment and you want to measure there not there and you want fast accurate measurements.

A TFMiniPlus, doing its thing getting distance return measurements might do the deed.

... or not. Please take a look at the datasheet.

julius_a: I need a sensor to measure motion of small particles to a close distance.

What are the particles made of?

Come to that what is the pipe made of?

I’m also thinking at a simple light gate / break beam sensor. As long as you can make sure the particles drop through one at a time, and can be guaranteed to fall through the beam.

In your case, that can not be done as 1 mm is less than half the diameter of the pipe. That requires a number of sensors to covert the thing.

Another option may be a bounce plate (piezo sensor), that the particles fall on. Then the diameter doesn’t matter any more as long as they hit hard enough.

If they’re made of iron or other magnetisable material, an inductive sensor may work: a coil around the pipe, look for changes in inductance.