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'DC motor speed regulation'
1998\06\22@071057 by de.vlieger

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Hi all,

As a newcomer to the list, I am interested in speed regulation of a
small DC brush motor (e.g. as used in model trains) by using a PIC. Most
information that is available refers to  applying a PWM output signal to
a mosfet H-bridge. The H-bridge is used for direction control. The PIC
produces a PWM signal and the speed is controlled by the duty cycle of
the PWM signal.
However, does anyone have a PIC solution to the feedback control
problem: i.e. a regulated constant speed by measuring the actual motor
speed and comparing it to the desired speed.

An additional limitation is that the hardware should be very small (SMD
components) in order to fit in models.

Here is my first idea:
A 'simple' way is to measure the current through the motor since it has
a 'linear' relation to motor speed (via the EMF: Electro Magnetic
Force). This can be done by a small resistor in serie of the H-bridge.
The resistor's voltage can be the input of a AD convertor. However the
resitor will produce to much heat. A mosfet in serie with a current
sense output instead of a resitor could probably be a solution.

Another solution may be to count the number of rotations within a
certain time, e.g. with a hall sensor or a tacho. IMHO this seems not
the most appropriate way because of the available space in the model
(but suggestions are welcome).

My question is:

Does anyone knows a PIC16F84 or PIC16C72/73 kind of solution to this
problem. The PIC16C72/73 has a AD converter that could be used to
measure the motor current. I'm also very interrested in code or parts of
code.

Any information is very welcome.
Thanks in advance for any suggestions,


Joost

spam_OUTde.VliegerTakeThisOuTspamnet.hcc.nl

1998\06\22@072520 by org Hager

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On Mon, 22 Jun 1998, de.vlieger wrote:

> ...
> However, does anyone have a PIC solution to the feedback control
> problem: i.e. a regulated constant speed by measuring the actual motor
> speed and comparing it to the desired speed.
>
> An additional limitation is that the hardware should be very small (SMD
> components) in order to fit in models.
>
> ...
> Another solution may be to count the number of rotations within a
> certain time, e.g. with a hall sensor or a tacho. IMHO this seems not
> the most appropriate way because of the available space in the model
> (but suggestions are welcome).

I haven't got any part numbers handy, but hall sensors can be made very
small (I've once used one made by Siemens that was so tiny that I lost it
by dropping it onto the carpet; it was never seen again...). Using another
resistor you could feed a variable voltage to a PIC A/D converter. By
attaching tha sensor to an appropriate point at the motor housing, rpm
measurement is possible.

Just my 2c.

Georg.

1998\06\22@074137 by Ron Fial

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Another (perhaps old fashiond) way to measure DC permanent magnet motor speed is
to measure the back EMF from the motor when the voltage to the motor is momenta
rally off.  If you are doing PWM, then when the current is off, if you read the
back EMF voltage from the motor, it is directly related to speed.  I have used t
his and the speed control is quite accurate -- You are using the motor as a DC g
enerator for an instant.

This method takes more parts than measururing ON current, usually needs an op-am
p circuit, but way cheaper than a tach.  This is probably to complex a method fo
r an H bridge driver, but its quite easy for a PWM single direction of rotation
application.
 Regards,
    Ron Fial

At 12:56 PM 6/22/98 +0200, you wrote:
>Hi all,
>
>As a newcomer to the list, I am interested in speed regulation of a
>small DC brush motor (e.g. as used in model trains) by using a PIC. Most


Regards,
 Ron Fial

1998\06\22@084313 by Timothy D. Gray

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I dont think a tach would be so expensive, 1 led, 1 phototransistor, 1
tiny piece of plastic on the motor shaft. total cost 98 cents and no
special circutry to translate to PIC compatable signals, just measure the
frequency.


On Mon, 22 Jun 1998, Ron Fial wrote:

> Another (perhaps old fashiond) way to measure DC permanent magnet motor speed
is to measure the back EMF from the motor when the voltage to the motor is momen
tarally off.  If you are doing PWM, then when the current is off, if you read th
e back EMF voltage from the motor, it is directly related to speed.  I have used
this and the speed control is quite accurate -- You are using the motor as a DC
generator for an instant.
>
> This method takes more parts than measururing ON current, usually needs an op-
amp circuit, but way cheaper than a tach.  This is probably to complex a method
for an H bridge driver, but its quite easy for a PWM single direction of rotatio
n application.
{Quote hidden}

1998\06\22@093102 by paulb

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Ron Fial wrote:

> ... measure the back EMF from the motor when the voltage to the motor
> is momentarally off.

> This is probably to complex a method for an H bridge driver, but its
> quite easy for a PWM single direction of rotation application.

 Not too hard.  If the "H-bridge" uses FETs which are essentially
symmetrical switches when turned on, you simply leave the "lower" FET
turned on after the pulse and measure the voltage at the other end of
the motor.  It'll probably work with bipolar transistors too.

 This latter could be by diodes from each end of the motor, an extra
pair of FET switches from the two ends, or just a resistive divider
giving you half of the back EMF notwithstanding which direction the
motor is spinning.

 I believe the back EMF needs to be well integrated though!

 Cheers,
       Paul B.

1998\06\22@110906 by Harijs Melders

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At 08:02 98.06.22.§Zª GxŸÞZª€šx -0400, Timothy D. Gray wrote:
>I dont think a tach would be so expensive, 1 led, 1 phototransistor, 1
>tiny piece of plastic on the motor shaft. total cost 98 cents and no
>special circutry to translate to PIC compatable signals, just measure the
>frequency.

Cheaper way- measure frequency of collector switching spikes on supply.

>
>
>On Mon, 22 Jun 1998, Ron Fial wrote:
>
>> Another (perhaps old fashiond) way to measure DC permanent magnet motor
speed is to measure the back

1998\06\22@155552 by Michael S. Hagberg

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there are pic controlled trains for as small as 'N' scale that I have seen
on the web. The pic receives the digital signal (Train id, and speed) from a
signal on the track. there is a site http://www.tttrains.com that has the specs and
if you do an http://www.altavista.com search for +pic +microchip +train +hobby
there are many links to poke around.

michael

1998\06\22@155557 by Calvin

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You can also check out page 134 of the May 13, 1998 issue of Electronic
Design, where is a simple design of a Rotation Counter, using no moving
parts, taking advantage of the momentary short created by the brush of a
permanent magnet motor.

Calvin
TGO Electronics


{Original Message removed}

1998\06\22@172824 by ORCUTT, KEVIN

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> -----Original Message-----
> From: .....owner-piclistKILLspamspam@spam@MITVMA.MIT.EDU
> Sent: Monday, June 22, 1998 3:55 PM
> To: PICLISTspamKILLspamMITVMA.MIT.EDU
> Subject: Re: DC motor speed regulation
>
>
> there are pic controlled trains for as small as 'N' scale
> that I have seen
> on the web. The pic receives the digital signal (Train id,
> and speed) from a
> signal on the track. there is a site http://www.tttrains.com that
> has the specs and
> if you do an http://www.altavista.com search for +pic +microchip
> +train +hobby
> there are many links to poke around.
>
> michael
>
>
Howdy All,
       The common standard that is used today in the Model railroading world
is the DCC standard from the National Model Railroad Association (NMRA).
It stands for Digital Command Control.  There are many (6-8)
manufacturers in the DCC field (Digitrax, Lenz, Wangrow.. and others).
There is quite a lot of information out there on DCC.  You might want to
start at  http://www.nmra.org.  There is a section for DCC and that are links
to most of the manufacturers own web pages from there...  The standard
only states what the signal is on the rails.  How each manufacturer gets
the signal there is up to them.
A brief synopsis of command control:
       Command control of model trains has been around a long time.  General
Electric developed a system back in the fifties called ASTRAC,  It
wasn't too popular and never really took off in the hobby.  In the
seventies a series of articles in Model Railroader was published
covering the CTC-16 system.  This was the first major system to gain a
wide acceptance in the hobby.  It was an analog system.  A few years
later the CTC-16 system was improved upon to create the CTC-16e.  again
is was digital.  The creator of the CTC-16 system (Keith Gutierrez)
developed the system as part of his master thesis for his EE degree.
You can get more info at his company's web site: http://www.cvpusa.com.  Other
systems came onboard with some of them compatible with the CTC-16
systems and other that were not.  There are/were two-three other systems
out there that were not compatible with every one else.  In the eighties
a German company (Lenz) developed a digital system with more
capabilities that the other analog systems out.  As more companies
became interested in the digital system, the NMRA pushed for a standard
that everyone can live with and be compatible at the track level.  With
the standard in place many companies decided to develop systems that
adhered to this new way of communication with the locomotive.  Back in
the seventies the pc boards in each locomotive was quite large compared
with today's decoders.  With the advent of Surface mount and the
Microcontroller these boards can be quite small.  The smallest today is
about the size of a dime. Like I said earlier the only standard is the
signal on the track.  As for the way the different systems communicate
between each device is different.  Some manufacturers use a RS-485
polled network and others use a slowed down version of a Ethernet.
Different stroke for different folks...

       I am in the process of trying to add some devices to one of these
networks. As I am new to the PIC world I would certainly welcome any
help alone these lines.  The system I am working with uses the Ethernet
approach.  It's slowed down to about 16K baud which is well in the range
of the PICs out there to day.

Thanks for the bandwidth,

Kevin Orcutt
Software Engineer
Vickers E.S.D., Inc.
1151 W. Mason-Morrow Road
Lebanon, OH 45036-9699
(513) 494-5752 - Phone
(513) 494-5400 - Fax
.....kevin.orcuttKILLspamspam.....vickers-systems.com (Work)
EraseMEkntorcuttspam_OUTspamTakeThisOuTfuse.net (Home)
#include <stddisclaimers.h> // "It just doesn't Matter!"

1998\06\22@175242 by ORCUTT, KEVIN

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Hello again,
       I did forget to mention in my previous post about DCC
That some of the companies out there have developed a back EMF decoder
that lets the user set the speed of the train and no matter what the
hill is it tries to maintain that same speed, up hill, down hill and on
the flat.

> {Original Message removed}

1998\06\23@064638 by Marco DI LEO

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You can also find the article at the web site of Electronic Design:

http://www.penton.com/ed/Pages/magpages/may1398/ifd/0513ifd.htm

Ciao
 Marco


Calvin wrote:
>
> You can also check out page 134 of the May 13, 1998 issue of Electronic
> Design, where is a simple design of a Rotation Counter, using no moving
> parts, taking advantage of the momentary short created by the brush of a
> permanent magnet motor.
>
> Calvin
> TGO Electronics

1998\06\24@123033 by Claudio Rachiele IW0DZG

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                  Status Distribution June 23, 1998 14:42:06

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1998\06\26@073552 by Joost de Vlieger

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Marco DI LEO wrote:

> You can also find the article at the web site of Electronic Design:
>
> http://www.penton.com/ed/Pages/magpages/may1398/ifd/0513ifd.htm
>
> Ciao
>   Marco
>
> Calvin wrote:
> >
> > You can also check out page 134 of the May 13, 1998 issue of Electronic
> > Design, where is a simple design of a Rotation Counter, using no moving
> > parts, taking advantage of the momentary short created by the brush of a
> > permanent magnet motor.
> >
> > Calvin
> > TGO Electronics


Thanks to all who have replied on this topic so far.
Especially counting pulses as proposed in the Electronic Design article
seems promissing.

New suggestions are still welcome. Therefore, see below a part of my original
message:

I am interested in speed regulation of a small DC brush motor (e.g. as used in
model trains)
by using a PIC. Most information that is available refers to  applying a PWM
output signal to
a mosfet H-bridge. The H-bridge is used for direction control. The PIC
produces a PWM signal and the speed is controlled by the duty cycle of
the PWM signal.
However, does anyone have a PIC solution to the feedback control
problem: i.e. a regulated constant speed by measuring the actual motor
speed and comparing it to the desired speed.
An additional limitation is that the hardware should be very small (SMD
components) in order to fit in models.


Joost de Vlieger
KILLspamde.VliegerKILLspamspamnet.hcc.nl

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