How Is Cybot Driven?

The Motors

Cybot has two motors - one for each wheel, of course.  These can be controlled independently so that Cybot can be steered.  These motors are powered by four 1.5V AA batteries, which give a total voltage of 6V.

The magazine (issue 2 ) gave a brief description of how a motor works.  But in case you haven't got the issue, I will explain the operation here as well.

There are two basic physical laws behind a motor.  The first is that if you pass a current through a piece of wire, a magnetic field will be produced around the wire.  Secondly, if two magnetic forces of the same polarity (i.e. North or South) are brought together, they will repel each other.

That's the physics out of the way, let's look at these laws a little more closely.  First, let's look at the first statement that says we will produce a magnetic field around a piece of wire if a current passes through it.  The diagram below shows this graphically.

The diagram above shows how, if we could see it, the magnetism surrounds the wire while current is flowing, and bear in mind that in the real world, the magnetic field is all along the wire.  If we switch off the current, the magnetic fields disappear.  Notice that with the current flowing as if it were coming out of the screen, the magnetic fields are travelling clockwise.  If we reverse the direction of the current, then the direction of the magnetic fields will change to go anti-clockwise.

Now, using just a single piece of wire won't give us much magnetism.  To increase the magnetism, we do two things.  First, the more pieces of wire we have carrying a current, the stronger the magnetic fields.  Rather than using many different wires, we coil a very long single piece of single wire.  If we wrap this long piece of wire round a piece of iron, we not only have something to put the wire on, but the iron gives the magnetic field a bit of a boost - hence giving us a much stronger magnetic field.  Pass a spindle through the middle of the iron, and we have the start of our motor, called the rotor, as shown below:

Now, let us look at the second statement, where I said that two similar magnetic poles will repel each other.  You have probably already experienced this yourself if you have ever played with two magnets.  Turn one of them one way, and the two magnets will snap together.  Turn round the other way, and it will be difficult to bring them together.  This brings us to the second component of our motor - the magnets.

If we place two magnets either side of our rotor, one with the North pole facing the rotor and the other with the South pole facing the rotor, we get the arrangement shown below:

If we were to hold the spindle at both ends, and pass a current through the coil of wire, because the magnets can't move the whole rotor will rotate until the magnetic field around the coil matches the corresponding opposite pole of each magnet (i.e. North to South and South to North).  If we now reverse the direction of the current (i.e. reverse the voltage) then the magnetic field on the coil changes direction.  This will cause the magnets to repel the magnetic field around the coil as two North poles and two South poles will be facing each other.  The rotor will continue to rotate until the North pole of the coil matches with the South pole of one magnet, and vice versa.  If we continue to do this, we can create a continuous rotation of the rotor.

In order to reverse the current using a DC voltage, the coil is connected to the supply via a commutator.  This is essentially a metal ring cut in half, with the positive side of the battery on one half and the negative side of the battery on the other.  Now, as the rotor turns, it will have its supply voltage revered on each half turn.  The diagram shows our complete motor.

The motor we have just looked at won't be very good if we were to make it for real.  The problem is, the supply is reversed on every half turn of the rotor.  It is probable that the rotor will have slowed down quite a bit due to friction by the time we have reversed the voltage.  This will result in a motor with jerky movements.  To overcome this, we reverse the supply voltage every quarter of a turn.  In order to do this we now need four magnets, instead of two, our commutator has to be split into four, and when we wind the coil round the iron core, we must ensure that we reverse the direction of the winding a quarter of the way round.  This is how most motors are configured, and certainly the one inside Cybot.

The Gears

What follows is an explanation of the gear box in Cybot.  It does get a little heavy, even though I have tried to make it simple.  Don't worry if you can't follow this - there isn't an exam afterwards!

So, we have made our motors.  All we need to do now is put some wheels on them and we're off, right?  Well, no - not really.  You see, our motors are so good, the spindles rotate extremely fast.  This is a problem if you are trying to drive something like Cybot.  For example, if the motors rotated at 5600 times a minute (rpm), then Cybot will travel at around 28 miles per hour!!!  Great if you want a race, but not if you want to stop Cybot crashing in a wall.

The solution is to use a series of gears that connect the motor to the wheels.

Let us have some gearbox theory.  Consider the gear diagram below:

The diagram shows a small gear (which we shall call Gear 1) with radius R1 meshed with a larger gear (which we shall call Gear 2) with radius R2. The speed at the point where the two gears meet, called the mesh point, equals the speed of rotation of Gear 1 times R1, which must be equal to the speed of rotation of Gear 2 times R2 (if this wasn't true then one gear would travel faster than the other and the teeth wouldn't mesh together).  In other words, the speed of the mesh point is unique; we can express it in terms of Gear 1 or Gear 2. This is all based on the physics of speed equals rotation velocity times radius for a point on a rotating object. In mathematical terms, this equals:

Mesh Point Speed = Rotation Velocity of Gear 1 x R1 = Rotation Velocity of Gear 2 x R2
or
Rotation Velocity of Gear 2 ÷ Rotation Velocity of Gear 1 = R1 ÷ R2

So, based on the above equation, you can see that by meshing a small gear to a larger gear we can reduce the rotation speed of the shaft attached to the larger gear. This is the principle behind using gear trains to obtain a reasonable wheel speed. The ratio of the output gear radius to the input gear radius is called, appropriately enough, the gear ratio.

There is a second reason why we need a gear box.  It is to increase wheel torque. For those who haven't come across the term torque before, it is the amount of turning power.  Most common hobby or toy electric motors are high speed (several thousands of rpm) and low torque (about 20 gcm). In order to provide the torque necessary to drive a robot's wheels, we need to increase the torque. 

A gear box will not only reduce the wheel speed, but also increase the torque at the same time!

Let's take our gear diagram again.

If we ignore energy losses such as friction and heat, then the power input to the little gear has to equal the power output to the big gear. This is called the conservation of energy. Energy simply can't be pulled in from the Fourth Dimension; it has to be accounted for and conserved. What is the power of a gear? From basic physics, it's the torque on its shaft times its rotation velocity. Thus, we have:

Power In = Power Out

Torque 1 x Rotation Velocity 1 = Torque 2 x Rotation Velocity 2
or
Torque 2 ÷ Torque 1 = Rotation Velocity 1 ÷ Rotation Velocity 2

But, from velocity reduction we know that:

Rotation Velocity 1 ÷ Rotation Velocity 2 = R2 ÷ R1, which means Torque 2 ÷ Torque 1 = R2 ÷ R1

And so, a gear reduction increases the output torque as well as decreases the output speed.

When Cybot is complete, I will show you how to calculate the amount of power consumed by the motors, and how to predict when you will need to replace the 4 AA batteries!