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Re: Cheap Motors

I agree that it would be better to use multi-pole motors - if money were no
object. In the UK, cost is of primary importance when you are trying to
persuade school children, school teachers and even universities to take an
interest in robotics and micromouse. I can only recount my own experiences
of using small 3-pole DC motors. My own micromouse uses Swallow Systems'
standard chassis with 8 pole tacho and 16 to 1 reduction gearbox. Using
8-bit PWM the mouse will start rolling (overcoming 'stiction') at about 30%
PWM. This gives me at least 180 steps of speed control which I consider to
be more than adequate. If the mouse is not allowed to stop then it will keep
rolling at less than 10% PWM increasing the range still further. These are 6
volt motors, but I only use 4 x AA NiCds = 4.8 volts for the supply. The
H-bridge drivers lose more so they are probably only seeing 2/3 of their
normal voltage! The key is probably the 16 to 1 gearbox providing torque
multiplication. Of course all this limits speed, but to be honest, that is
not a factor. I personally am more concerned that students learn about
practical control systems, maze solving algorithms, etc, working with
hardware they can afford. As a university lecturer I saw a lot of money
wasted on fancy motors and equipment: classic cases of trying to run before
you can walk.

Bill Marshall

----- Original Message -----
From: <jclarke@qbe-europe.com>
To: <micromouse@cs.rhul.ac.uk>
Sent: Wednesday, April 17, 2002 5:02 PM
Subject: Re: Cheap Motors


> In my experience, the cheaper the motor, the more difficult they are to
> control.
>
> There are two main components of a motor.  Fixed magnets, and the number
of
> "poles" or "coils" that make up the comutator of the motor.  The more
fixed
> magnets there are in the motor, the "smoother" the motor will rotate.  The
> more "poles" you have on top of that, the motor will rotate even smoother.
> As an experiment, take any cheap motor and slowly rotate the shaft.  Count
> the times you feel resistance as you rotate.  This "resistance" is the
> motor pole entering the magnetic field around the fixed magnet in the
motor
> casing.  Most 3 pole motors have two fixed magnets.  Therefore, the poles
> are 120 degrees apart moving through two seperate magnetic fields.  You
> should feel five areas of resistance as you rotate the shaft through 360
> degrees.
>
> As you know, there are three components to electro motive motion:
> Movement, Magnetic Field, and Current.  By manually rotating the rotor
> through the magnetic field, you produce an output current (e.g dynamo
light
> on a bicycle wheel).  This current in turns sets up a reverse magnetic
> field in the rotor coil which is the resistance you can feel when you turn
> the shaft.
>
> Anyway the end result is , the more poles (or fixed magnets) you have in
> the motor, the smoother the shaft rotation will be.
>
> This also has an effect when you are driving the motors from a PWM source
> (as this is the most effective way to control the average voltage applied
> to a motor in these types of conditions).  The smaller the number of poles
> means the voltage is being applied to the motors less (3 pole motors 3
> times a revolution, compared to 7 pole motors at 7 times a revolution).
> Depending on the construction of the motor (and we are looking at cheap
> motors aren't we), you could be in a situation where the voltage being
> applied to the motors is only happening for about 10% of a revolution.
> This means that the motor is accelerating for 10% of the time and
> develerating for 90% of the time.
>
> With high pole motors, the voltage is being applied for more, maybe up to
> 60% to 70% of a revolution.  As a result, the motor spends more time in a
> driven state than an idle state.  This results in less power required to
> maintain a given output.  The downside is that motors with this amount of
> efficiency tend to be more expensive to buy and source than your typical
> slot car motor.
>
> I investigated this when I was doing my masters degree in the early
> nineties.  The end result was that a "three pole motor" only started
moving
> after a 60% PWM cycle was introduced compared to 15% PWM cycle for a high
> efficient 9 pole motor.  (Both driving the same load, and at the same
> voltage)
>
> I ask a simple question, whats the point of using a PWM signal against the
> motor if the effective range is only going to be less than 40% for cheap
> motors compared to around a 70% - 80% for more efficient motors.  You
loose
> the benefit of PWM, and you should look at other means of motor control.
>
> Threads over the last year or so have been based on software for maze
> solvers etc.  Whats the point of having a fast maze solver if you can't
> drive to the center of the maze quickly and safely.
>
> The end result is, that you have to build a gear box either way (and your
> more than likely to be able to buy a gearbox that mounts directly onto the
> more expensive motor than the cheap one), so why not spend the extra money
> on what is probably the most important part of a mouse!
>
> Just my 10cents worth
>
> James
>
>
>
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