The Robotic Toothbrush

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The objective of this experiment is to design a very simple robotic car using easily available materials.

Robot Toothbrush

Introduction

You can begin the class by introducing the following topics:
Electricity: Current flows from the positive electrode (+) to the negative electrode (–). Current is the movement of invisible particles inside a conductor. It is helpful to think of current flow like the flow of water. For example, when you think of a waterfall, the water at the top has more potential (energy) than the water at the bottom of the falls. This difference in potential is the reason that water will flow downwards (you don’t see water flowing upwards!). In electrical terms, the potential difference is called voltage, and causes electrical current to flow. Voltage is measured in a unit called volts (V), and current is measured in a unit called amperes (A). A multi-meter allows you to measure the voltage and the current.

Permanent magnets: Permanent magnets create a magnetic field without requiring any energy input. Take a moment to examine the interaction of a magnet placed under a sheet covered with iron filings. Now, place two permanent magnets next to each other and determine how many configurations create an attraction and repulsion. How many poles does a permanent magnet have? Poles are usually named North and South. Poles with the same name repel each other and those with different names attract each other.

Motor: A motor converts electrical energy into mechanical energy. The motor’s operation relies on the fact that electricity traveling in a conductor creates a magnetic field. This is Ampere’s rule. By coiling the conductor, the magnetic field becomes shaped into 2 poles. Our conductor has thus become an electromagnet (a magnet created by an electric current). We will use the electromagnet’s poles to oppose the poles of a permanent magnet placed nearby. The arrangement of the electromagnet is in such a way that when the coil is repelled by the permanent magnet, it rotates and the current is turned off in the coil because it is no longer connected to the power supply (notice that there is a gap between the two pads of the rotor). When the coil reaches the 180-degree (horizontal) position again, contact is established and poles that oppose the magnetic field of the permanent magnet are again formed (see Figure below). Since the power supply is not rotating, the North and South poles of the electromagnet always occur in the same position.
Robotic Toothbrush Step Schematic of Motor
Torque: as shown in the figure below, the force exerted by the motor is separated from the point where the force is applied by a distance equal to the radius of the wheel. The torque of a motor is the product of the radius and the force perpendicular to the radius.
Robotic Toothbrush Step Torque Schematic

Materials

  • 1 tooth brush
  • 1 vibration motor
  • 1 piece cardboard (2 cm x 3 cm)
  • 3V power supply (3V disc battery or, 2 AAA batteries or,2 1.5V wristwatch batteries)
  • aluminum foil strip (1 cm x 6 cm)
  • 10 cm electrical tape
  • 3 cm double-sided tape
  • sponge (optional)
  • Scissors
  • Cutter/ utility knife

Procedure

  1. Cut off the handle of the toothbrush as close to the bristles as possible.Robotic Toothbrush Step 1
  2. Shorten or cut off inner bristles of the toothbrush head with scissors.Robotic Toothbrush Step 2
  3. Using double-sided tape, attach a piece of cardboard on the back of the toothbrush head.Robotic Toothbrush Step 3
  4. Tape the vibration motor on one end of the cardboard (this will be the front side of the robot).
    Robotic Toothbrush Step 4
  5. Make the 3V power supply by either:
      • Connecting two AAA batteries in series using aluminum foil and electrical tape.
      • Stacking two 1.5V watch batteries on top of each other to make a 3V power supply (positive to negative).
      • Taping down one 3V disk battery.

    Robotic Toothbrush Step 5

  6. Attach the motor leads to the power supply. Make sure that one of the leads or the power supply can be easily disconnected; this will conserve the battery when you do not want the robot to run.
    Robotic Toothbrush Step 6
  7. Secure the power supply onto the cardboard using tape.
    Robotic Toothbrush Step 7
  8. Attach a piece of trimmed sponge using tape. Place weights as necessary (pebble or clay).
    Robotic Toothbrush Step 8

Your robot is now ready to clean your desk!

Science Behind the Scenes

Short Circuits

What do you think will happen if the two leads of a battery are attached together directly? The battery will begin to dangerously heat up, and if connected for a long enough period, the battery could leak, and a fire could start.

This is an example of a short circuit. A short circuit occurs when a low resistance path is created between the two leads of the battery. If it was unintentional, then the low resistance path bypasses the real circuit (causing the real circuit to stop working). A large amount of current begins to flow through that low resistance path and as a result, the wires will heat up and start a fire. Therefore, always make sure there is a load connected to the battery such as an LED or resistor.

Vibration Motors

Vibration motors are usually small motors that are designed to move a mass either up and down or around a shaft, and in the process, to create a vibration. While rotating shafts of motors don’t normally create a vibration, it is the fact that the mass is placed off-centered and the rotated at high speed that creates the vibration.

It is common to characterize motors in terms of their speed with no load (or mass attached) at a given voltage. That speed is given in RPM, which is an abbreviation or ‘rotations per minute’.

Vibrations motors usually have a high RPM (more than 5000 RPM) but very low torque, so they would not be very useful to power a car (unless it is an extremely light one!) because the motor would not have enough force to move that car.
Modern vibration motors are usually utilized in cell phones or tablets to notify the user of an event. Usually a stronger vibration is more useful. The vibration is the result of the force generated by the moving mass of the motor on, for example, a cell phone. To compare the vibration force, it is common to normalize the weight of a reference phone to 100 grams and divide the force by this reference mass. This new value has the same units as acceleration — Newtons per gram (N/g) or meters per second squared (m/s2). Rather then reporting directly this acceleration, it is reported as a function of the gravity G, which has a standard value of 9.8 m/s2. So if you find a vibration motor rated at 1G, that means that when the vibration force is exerted upwards, a 100 gram phone may appear to almost float off a table (essentially cancelling gravity for a brief instant like when you jump up). However, the vibration is very rapid so the phone falls back immediately. Likewise, attaching an object lighter than 100 grams to the motor would cause it to bounce (the force exerted by the motor is greater than the one exerted by gravity).

Safety

Avoid electrically shorting batteries. Shorted batteries will heat up and can burn the user. Or, the metal casing of the battery can melt and expose the user to harmful chemicals. When using utility knives (cutters), exercise caution and wear protective equipment to avoid cutting and puncturing your skin, or inflicting other injuries.

Authors

We thank Calvin Cheng, Dr. Mayrose Salvador, Danilo Joksimovic and Dr. Martin Labrecque for creating this write-up and developing the experiment