Build a Simple Robotic Metal Detector (part one)

One of the things that I have been wanting to experiment with on the robot is adding a metal detecting sensor. I wrote this page to explain about metal detectors and how to build one. Although I will be building this metal detector to put on a robot there is not too much that you would have to do to modify this project into just a regular metal detector that anyone can use. Another thing is that since I already have the robot I only have to add the metal detector onto it so that will save a lot of time for me. If you are looking to build the entire robot as well I suggest you see some other posts.

About Metal Detectors
Metal detectors can detect nearly all metallic objects. That means anything that contains elements like gold, silver, iron, nickel, copper , aluminum, tin and lead or mixtures and combinations like bronze and brass. Metal detectors cannot detect nonmetal items such as wood, plastic, stones and bone. Some metal detectors are able to discriminate which means that they can differentiate between various types of metal.

Like everything there is more than one way to detect metal and thus more than one way to make a metal detector. There are three main types of metal detectors that are available. They are VLF, PI and BFO types.
  1. VLF (Very Low Frequency) - These are usually high-end metal detectors but they are the most widely used. These metal detectors are very good at discriminating.
  2. PI (Pulse Induction) - Metal detectors that use alternating electromagnetic fields to detect metal. This type is not very effective in differentiating between metals.
  3. BFO (Beat Frequency Oscillation) - These metal detectors use two oscillating coils that produce magnetic fields that are altered when it gets near a metal object.
BFO metal detectors are the easiest metal detector to make but they are not as effective as PI or VLF types. For this project I will be demonstrating how to build a BFO metal detector from scratch. I plan on attaching the coil to an Arduino Uno Programmable controller so that the robot can interact with the metal detector easier.

Before we begin it is a good idea to understand how BFO metal detectors work.

One of the two oscillating wire coils is called the search loop. The other coil is smaller and is usually inside the control box or at least it is located somewhere where the search loop is not. The smaller coil is called the reference oscillator. The two oscillating coils will have their frequencies adjusted so that they are as close to being identical as possible. When the search loop goes near a metallic object the frequency at which it oscillates is altered. This creates a difference between the oscillation of the search loop and the reference oscillator. This difference becomes audible and is called a beat note. The beat note will change as it gets closer or further from the piece of metal. The beat note stops being audible or 'nulls' when the two oscillators are about equal.