I have a steel ball of 8 mm diameter passing through a plastic tube of 10 mm diameter. The wall of the tube is 1 mm thick plastic. Is it possible to identify the movement of the ball through one or multiple ultrasound devices which is/are allowed to be placed anywhere but not less than 3 cm away from the tube. If we assume that ultrasound is not a viable option, can there be another option?
Ultrasonic signals will be reflected from the tube, regardless of whether the ball is present.
A magnet and Hall effect sensor might work, or try a light source and phototransistor or LDR.
Hall Effect Sensor could be a perfect idea but I cannot find a hall sensor that can sense a magnetic field of a 8 mm cylindrical diametrically magnetized neodymium magnet from 3 cm away. I tried with TI DRV5055 Hall Sensor and Honeywell SS 49 but could not get a good results. Thank you for your suggestion.
Is the plastic transparent?
laser barrier?
Are you stuck with that magnet or can you use another? An axially magnetized magnet would narrow the magnetic field and perhaps work with one of your sensors. I am getting >3cm range with a 1/4" x 1/2" N52 magnet with whatever hall sensor is in the Eve door/window switch.
Try some others. I'm quite sure that the basic idea can work. You could also use a magnetometer (digital compass) designed to measure the Earth's magnetic field. Those can detect the distortions in the Earth's field caused by passing vehicles, several meters distant.
Never mind, the magnet might be too strong and nor allow the steel ball to pass, now that I think about it.
Take a look at a capacitance sensor.
Active metal detector should do the trick. The cheap diy kit ones should be effective enough to blip.
As the tube is not metal, you can also use coils wrapped around parts of the tube.
They will be part of an RF oscillator.
When the ball passes "inside" the coil, it will change the frequency of the oscillator.
It is similar to metal detectors used to search for buried metal.
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