Newsgroups: comp.robotics
Path: brunix!sgiblab!news.cs.indiana.edu!lynx.unm.edu!Mr-Hyde.aoc.nrao.edu!news
From: browen@aoc.nrao.edu (Bruce Rowen)
Subject: Re: PWM DC motor control
Message-ID: <1994Mar21.171526.28204@Mr-Hyde.aoc.nrao.edu>
Sender: news@Mr-Hyde.aoc.nrao.edu
Reply-To: browen@aoc.nrao.edu
Organization: National Radio Astronomy Observatory, Socorro NM
References: <PJG.94Mar18132910@parint.esl.com>
Date: Mon, 21 Mar 94 17:15:26 GMT
Lines: 65


>In article <Mar18.181352.31507@acs.ucalgary.ca> morrow@cns8.cns.ucalgary.ca (Bill Morrow) writes:
>   Is anyone trying to actually vary the driving voltage to the motor,
>   using some feedback from shaft encoders? You know, just like they 
>   taught you in that control systems course. It seems to me that this
>   type of control system would be more efficient, responsive and cover
>   a wider range of motor speed.
>

Paul Gyugyi writes (I think!):

>From what I understand, while the ratio of voltage to speed is fairly
>linear, the tourqe from a motor increases greatly with higher voltage.
>That is, for a given speed, you get more torque by using PWM at the
>max voltage than you do by having a constant, lower voltage.

(the following is assuming a standard fixed field DC brushed motor)

Assuming you are avoiding the non-linear realms of field
saturation, motor TORQUE is proportional to motor CURRENT.

To control the SPEED of a motor, some form of feedback is required to control
the motor current. If you don't believe it try this test:

Connect a small motor to a voltage regulated power supply and set the voltage
so that the motor spins at a fair velocity. Now grab the shaft and load the
motor down. 

If the shaft speed (velocity) was linear with respect to voltage, the motor 
would have maintained velocity independent of load. This didn't happen did
it?

There are several important benefits to using PWM control vs. linear control
of a motor.

1. Efficiency. PWM allows "switches" to control current flow, The "switches"
   operate with very low I^2 * R loss and can be quite compact. The mentioned
   switches can be ordinary transistors, SCRs, or MOSFETS, among others

2. Better torque modulation. You can drive the motor with high voltages (up to
   the insulation limits) which allow faster coil saturation times.  This
   allows better low speed torque control. 

A motor usally is rated at a maximum continious current. This is almost always 
a thermal limit. You can get far greater torque from a motor than its nominal
rating by dumping more current in. You only have to observe the motors thermal
limits and thermal conductivity. (of course you have to consider any mechanical
limits such as gear trains etc.) Motors that are rated as a voltage 
(i.e. 12 VDC) may be interpeted as saying you can continiously drive the motor
with the current that flows when the motor is connected to a 12 VDC source.

It is common to drive 12 VDC servos with 48 VDC PWM sources. You get fast rise
times, quick response, and loads of torque. Just observe that you do not want
to use a 100% PWM signal (full on) that exceeds the motors thermal limit.


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Bruce Rowen 				National Radio Astronomy Observatory
Scientific Programmer				Array Operations Center
browen@aoc.nrao.edu			  P.O. Box O  Socorro, NM   87801
(505)385-7329					     (505)385-7000
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