Science Buddies: "Ask an Expert"

My 7th grader is working on the Mass Driver project currently and we do not happen to have any hall effect sensors at the moment. Would IR sensors work in their place?

I had an additional question, could he reverse the polarity of the electromagnet using an L298N motor driver? I thought it might work but I don't want to frustrate him with my possibly bad idea. He wanted to try to push the projectile after it passed the sensor by reversing it and then chart the difference with and without the reversal process.

Thank you for the help!

Moderator note: I've combined your posts so the experts see them together. Thanks!

Project: https://www.sciencebuddies.org/science- ... ass-driver

Hi, so two answers:

1) No, an IR sensor is not a good replacement for the Hall effect sensor in this project. In theory, you could use an optical sensor (either infrared or visible light) looking through a clear tube just after the coil to detect when light is blocked by the projectile passing in front of the sensor. However, at that point the projectile has already passed through the center of the coil and has started to slow down (which we'll explain more in the next point). The Hall effect sensor is going to detect a changing magnetic field sooner (before the projectile has even exited the coil), shutting the coil off so you aren't going to lose as much speed.

2) Yes, you can reverse the polarity of an electromagnet using an H-bridge. BUT, if you are using a ferromagnetic projectile (not a permanent magnet), that will NOT "push" the projectile out the other side of the coil - it will still pull it back in! The projectile is a temporary magnet that is magnetized by, and attracted to, whatever external magnetic field is present. This is why, per the previous point, you want to shut the coil off the moment the projectile has passed through the center - otherwise the coil will start to pull it back in. Perhaps a more intuitive way to explain this is just to demonstrate picking paper clips up with a permanent magnet. You can always pick the paper clips up with either the north or south ends of the magnet, but you can never push them away. You CAN push away another permanent magnet though. So in theory, you could try using a cylindrical permanent magnet for the projectile in this project, and reverse the polarity of the electromagnet at exactly the right time so you pull the magnet into the coil and then push it out. That would be a cool addition to the project, but getting the timing right might be difficult and I haven't tried it myself, so I can't guarantee that it would work. I'd recommend starting out with the project instructions and if you get that working first, you could then move on to trying a permanent magnet projectile and switching the electromagnet polarity.

This is a fun project - let us know how it goes!

Hi!

Thank you for your reply. We will use hall effect sensors for the electromagnets as you suggested in your comment. He is setting up a separate program on a separate arduino uno to measure the speed at the end of the tube with two IR sensors outputting to a LCD display. The IR sensors should work for that correct?

Your polarity reversal advice makes absolute sense! I should have realized that. The good thing is we have both a magnetized projectile and ferromagnetic projectile so it will be very interesting to fiddle with the differences. Hopefully we can get the timing down for the magnetic projectile.

We have a 20 inch acrylic tube that I thought we would set up with 4 magnets 2 inches wide and 2 inches apart. Then at the end of the tube would be the speed measurement device. I have eight 6v lantern batteries. I thought he could compare magnetic vs ferromagnetic projectile speeds, measure speeds of 1 vs 2 vs 3 vs 4 magnets, measure the speeds based on the number of batteries hooked up in the series, and measure the change in speed of the magnetic projectile with polarity reversal if we can get it working. I figured he could do some research on scientific/industrial/military applications for mass drivers and get some nice pictures we could print on poster boards along with his code and test result graphs.

I will definitely let you know how it goes. Looks like we are having a snow day tomorrow so we should be working on this all day. Thank you so much again for your help! It is very much appreciated.

Sounds like you have a good plan! For a basic school science project I think testing that many different variables is going well above and beyond the basic requirements where students many times just need to test one independent variable, but for a more advanced project or an engineering design project where he is trying to optimize the performance of the device, testing all those different things makes sense.

We don't have our own tutorial for the IR sensor setup you're describing, but I believe you're correct and you should be able to find other tutorials for it online. Measure the distance between the sensors, use an Arduino to measure the time difference between when the first sensor is triggered and when the second sensor is triggered, and then you can calculate the velocity.