Newsgroups: comp.robotics
Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!news2.near.net!news.mathworks.com!uunet!portal.austin.ibm.com!awdprime.austin.ibm.com!awdprime.austin.ibm.com.!nahshon
From: nahshon@vnet.ibm.com (Itai Nahshon)
Subject: Re: Motor selection for micromouse?
Sender: news@austin.ibm.com (News id)
Message-ID: <NAHSHON.95May12112258@rs3bt-01.haifa.ibm.com>
In-Reply-To: cmcmanis@Sun.COM's message of 11 May 1995 19:54:18 GMT
Date: Fri, 12 May 1995 08:22:58 GMT
References: <D8E9vt.96z@ucc.su.OZ.AU> <3otq1b$b24@handler.Eng.Sun.COM>
Organization: IBM Israel Science and Technology, Haifa, Israel
Lines: 77

yIn article <3otq1b$b24@handler.Eng.Sun.COM> cmcmanis@Sun.COM (Chuck McManis) writes:

>Path: awdprime.austin.ibm.com!portal.austin.ibm.com!uunet!europa.chnt.gtegsc.com!gatech!swrinde!cs.utexas.edu!venus.sun.com!news2me.EBay.Sun.COM!handler.Eng.Sun.COM!pepper!cmcmanis
>From: cmcmanis@Sun.COM (Chuck McManis)
>Newsgroups: comp.robotics
>Date: 11 May 1995 19:54:18 GMT
>Organization: FirstPerson Inc., Palo Alto, CA
>Lines: 33
>References: <D8E9vt.96z@ucc.su.OZ.AU>
>NNTP-Posting-Host: pepper.eng.sun.com
>X-Newsreader: TIN [version 1.2 PL2]

>Mark Hedley (hedley@ee.su.oz.au) wrote:
>: I am building a micromouse (small mobile robot for finding way through maze),
>: and need help selecting a suitable motor, and a supplier.

>: I presume that a DC motor is best (the microcontroller I'm using is 68HC11K1
>: which has PWM outputs).

>Could be a false presumption. A stepper motor is more controllable, especially
>at low speeds.

Stepper motors need current to hold in place. DC motors are far more
energy efficient. That's important when you design a battery operated
application.

>: Is it possible to get motor with good torque at very low speeds (say 100 rpm)
>: or is a gearbox mandatory?

>A gearbox is pretty much required. Basically the speed of the motor is 
>ultimately controlled by the physical properties of the motor. The windings
>exert a force on the armature, the armature accellerates (f = ma) until
>the friction of the bearings and the wind resistance of the armature and
>and the inertia of the load all balance out. Unloaded, a small DC motor
>can easily run at 9000 RPM. There are good decscriptions of this in
>most elecromechanics textbooks.

No No No... The friction of the bearing is (we hope) negligible
and the resistance is low...
Every DC motor (with permanent magnets) also works as a generator.
Suppose the supply voltage is Vbat. The voltage generated by the
motor is Vgen(RPM) (It is a function of the motor speed RPM), and
the resistance is R (that's the sum of the resistances of the wires,
battery, switches, motor windings, motor brushes...).
The current through the motor is R.

Summing the voltager over the circuit:
  Vbat = Vgen(RPM) + I*R.

If the motor is free running, or lightly loaded Vgen(RPM) is
the dominant part in the right hand side of the equation.
Try to stop the motor and the current will jump HIGH...

The free running speed of a DC motor at a given voltage is
about the same speed where Vgen(RPM) = Vbat.

Note 1: Add more windings to the motor armature and it will turn slower.
        reduce the number of windings and it will run faster. (assuming
        nothing else changes).
Note 2: Just as motor speed is a function of voltage, the maximum torque
        is a function of the current through the motor windings.

>A couple of suggestions:
>   1) Use neutered Radio Control Servos (keep the motor and the
>      gearbox.)
>   2) Build a custom gearbox using gears from any one of the suppliers
>      in the FAQ or near you in Australia.
>   3) Use stepper motors.

>--
>--Chuck McManis			     All opinions in this message/article are
>Sun Microsystems Inc.                those of the author, who may or may not
>Internet: cmcmanis@Eng.sun.COM       be who you think it is.
>Crypto-puzzle: *0U0JPFPrWRN9PkWRKeP5WRmIR9wP5QAWuIQP9Pu9tnIZ7AD1SIS

Itai Nahshon
Haifa, Israel
