Help please with DC motor control

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

 

[Jim Thompson]

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

Just use a current source, or a current-limited power supply. If
rotation is slow enough torque is constant for a fixed current value.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Maynard A. Philbrook Jr.]

In article <5mhft91agr5phdpac3sohkah28v7pnsd6n@4ax.com>,
etpm@whidbey.com says...

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

A prony brake with a catenary arm will serve you with no need to use a
drive motor. The string would pick the arm up as you need it and release
the brake. This type of brake can have a spring on one side so the load
will not spin backwards to easy but will allow it to have tension on the
forward movement. The tension most likely would be more than you need so
the catenary arm, as it gets picked up via the string, will release the
strap around the friction wheel and remove the brake tension.

But if the weight of the spool is still heavy for you when the prony
brake has released you can assist it with a motor by putting a
pot on the catenary arm to drive the spool forward. some people have
used Rheostats to push the motor forward as the arm get's picked up.

Also, you can use a dancer regulator system, look here and study the
image.

http://www.carotron.com/systeminterfacecomponents/pidanapp/

Jamie

 

[Tim Wescott]

On Tue, 29 Jul 2014 09:34:01 -0700, etpm wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on a
nylon cord. The cord comes is pulled off of a spool a certain amount and
cut. I have drag on the spool because I don't want it keep spinning when
I'm not pulling. But pulling against this drag is tiresome. So I was
thinking that I could use a motor to help pull the cord. The motor would
not have quite enough torque to pull the cord off of the spool by
itself. But when I pull on the cord it will be easy because the motor
would be helping me. I'm wondering how to control the torque of the DC
motor. I think that if I keep the current limited to a certain value the
torque the motor can develop will also be limited. If the voltage to the
motor is set at a certain value then it will only be able to reach a
certain speed. If I set the voltage so that the motor will spin fast
enough to keep up with the maximum rate at which I pull the cord will I
be able to pull the cord slower without having the motor try to feed the
cord too fast? I'm hoping I can just use a current and voltage limited
power supply to do what I want. One drawback I see is that when the cord
is not being pulled the motor will be stationary but will be consuming
power because it is pulling against the drag on the spool. So I would
need to use a motor that could be stalled forever without overheating.
If a brushed motor is used will this be really hard on the brushes? And
could a brushless motor be used instead?
Thank You,
Eric

I vote for the Prony brake.

But if you absolutely must use a motor, yes, a permanent-magnet DC motor
will generate a torque that is proportional to current, and yes, you can
use it as an aid as you describe.

Get a motor that will give you your desired torque at a current that's at
or below its rated current, and you should be fine from a heating and
brush-wear standpoint.

Note that some motors do not put out consistent torque: their torque
constant differs depending on where the armature is in relation to the
magnets, and they exhibit "cogging torque" where the armature seeks to
maximize the amount of iron that "sees" the magnets. Both of these can
muck you up -- the best solution that doesn't involve picking through
data sheets or tons of experimentation is to use a servo motor.

A brushless motor will also work, but you'd need to get a drive for it
that can be commanded to a specific current level. These can be had, for
$$$, but finding one surplus will require a lot of diligence.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Phil Allison]

<etpm@whidbey.com>

I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes?

** Can't you just reduce the drag on the spool a bit ?

If you must use a DC motor to assist, go for a 12V automobile blower motor.

http://www.50plymouth.com/07-cha/blowermotor08.jpg

These have lots of poles ( 11 is typical ) so torque is very smooth.

Current regulation / limiting can be done with a couple of 12V auto brake
light bulbs, with wattage picked to suit the torque you want. Start with a
single 21W as see how that goes.



..... Phil

 

[Jasen Betts]

On 2014-07-29, etpm@whidbey.com <etpm@whidbey.com> wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut.I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome.

Move the drag to rub against the circumference of the line wound on
the spool. That gets you constant force.

If the required drag to stop the spool is too great run the cord through
a block and tackle arrangement to lift the drag when pulled.


--
umop apisdn


--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

[Helmut Wabnig]

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

I have read several answers to this post already,
all of them are off topic.

A DC motor can be regulated to deliver constant torque
by using a series resistor which measures the motor current,
then feeds the signal back to a servo amplifier with gain adjustment.
But then you will not have constant (rotational) speed.

This is not the same problem as constant speed control.
For DC speed control you would use a tacho-generator
or when pulsing the motor you can use the motor itself
as tacho-generator during the pulse pause.


This is not a simple task, but there are some hand drill machines
on the market which have adjustable speed and adjustable torque.
To be precise: adjustable maximum torque.

One cannot have constant speed and constant torque in one
when the mechanical load changes permanently.
You cannot have constant torque if the load changes.
Only a maximum torque regulator can be achieved
but at the cost of changing speed.

w.

 

[amdx]

On 7/29/2014 1:58 PM, Tim Wescott wrote:

On Tue, 29 Jul 2014 09:34:01 -0700, etpm wrote:


A brushless motor will also work, but you'd need to get a drive for it
that can be commanded to a specific current level. These can be had, for
$$$, but finding one surplus will require a lot of diligence.

I got an electric motor here, https://www.surpluscenter.com/
Just click on electric motors on the left side.
Decent supply at good prices.
Mikek



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

 

[John Larkin]

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

Suppose you used a constant-speed motor, turning at some low RPM. It could turn
a shaft or a drum.

The feed cord would make one or more loops around the drum. If you don't pull on
the cord, it slips on the rotating drum. If you pull, it tightens up and the
motor assists. It's self-regulating.

I've seen this done on ships to let a crew guy apply an easily regulated amount
of force to a big line or cable. He loops it around a slowly rotating thingie
(nautical term) and pulls as needed. The mechanical force multiplication is
huge.

Vaguely like this:

http://www.davitmaster.com/products/images/BR1000.WEG.LG.jpg



--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation

 

[John Larkin]

On Wed, 30 Jul 2014 08:39:54 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

Suppose you used a constant-speed motor, turning at some low RPM. It could turn
a shaft or a drum.

The feed cord would make one or more loops around the drum. If you don't pull on
the cord, it slips on the rotating drum. If you pull, it tightens up and the
motor assists. It's self-regulating.

I've seen this done on ships to let a crew guy apply an easily regulated amount
of force to a big line or cable. He loops it around a slowly rotating thingie
(nautical term) and pulls as needed. The mechanical force multiplication is
huge.

Vaguely like this:

http://www.davitmaster.com/products/images/BR1000.WEG.LG.jpg

google electric capstain

http://www.nabrico-marine.com/pages/electriccapstan.html



--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation

 

<SNIP>

A DC motor can be regulated to deliver constant torque
by using a series resistor which measures the motor current,
then feeds the signal back to a servo amplifier with gain adjustment.
But then you will not have constant (rotational) speed.

This is not the same problem as constant speed control.
For DC speed control you would use a tacho-generator
or when pulsing the motor you can use the motor itself
as tacho-generator during the pulse pause.


This is not a simple task, but there are some hand drill machines
on the market which have adjustable speed and adjustable torque.
To be precise: adjustable maximum torque.

One cannot have constant speed and constant torque in one
when the mechanical load changes permanently.
You cannot have constant torque if the load changes.
Only a maximum torque regulator can be achieved
but at the cost of changing speed.

w.

Thanks for the replies everyone. After reading the replies I am a
little confused. If I use a current and voltage limited power supply
and there is not quite enough power for the motor to pull against the
drag the motor will be stalled. When I pull on the cord the motor will
of course turn, but only when I pull on the cord. If I pull the cord
the motor will then just draw less current, right? So the torque it
develops plus the pulling force will be just enough to overcome the
drag, won't it? If not, then what am I missing?
Thanks,
Eric

 

[amdx]

On 7/30/2014 10:50 AM, John Larkin wrote:

On Wed, 30 Jul 2014 08:39:54 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

Suppose you used a constant-speed motor, turning at some low RPM. It could turn
a shaft or a drum.

The feed cord would make one or more loops around the drum. If you don't pull on
the cord, it slips on the rotating drum. If you pull, it tightens up and the
motor assists. It's self-regulating.

I've seen this done on ships to let a crew guy apply an easily regulated amount
of force to a big line or cable. He loops it around a slowly rotating thingie
(nautical term) and pulls as needed. The mechanical force multiplication is
huge.

Vaguely like this:

http://www.davitmaster.com/products/images/BR1000.WEG.LG.jpg


google electric capstain

http://www.nabrico-marine.com/pages/electriccapstan.html

Yes, you have it, I was looking for the term to do an image search.
The word used by deckhands is cathead or catshead.
This url shows several, in pictures 3, 4, 5, and 6.
http://alariobros.com/winches-hydraulicandelectric.aspx
When lifting a load out of the hold on the boats, the rope goes from the
load up over a pulley and down to the cathead, wrapped around once, just
a little tension on the loose end of the rope causes enough friction to
start lifting the load. On the boats they usually have
hydraulic motors.
I like the idea.
Mikek




---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

 

[John Larkin]

On Wed, 30 Jul 2014 11:03:24 -0500, amdx <nojunk@knology.net> wrote:

On 7/30/2014 10:50 AM, John Larkin wrote:
On Wed, 30 Jul 2014 08:39:54 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

Suppose you used a constant-speed motor, turning at some low RPM. It could turn
a shaft or a drum.

The feed cord would make one or more loops around the drum. If you don't pull on
the cord, it slips on the rotating drum. If you pull, it tightens up and the
motor assists. It's self-regulating.

I've seen this done on ships to let a crew guy apply an easily regulated amount
of force to a big line or cable. He loops it around a slowly rotating thingie
(nautical term) and pulls as needed. The mechanical force multiplication is
huge.

Vaguely like this:

http://www.davitmaster.com/products/images/BR1000.WEG.LG.jpg


google electric capstain

http://www.nabrico-marine.com/pages/electriccapstan.html


Yes, you have it, I was looking for the term to do an image search.
The word used by deckhands is cathead or catshead.
This url shows several, in pictures 3, 4, 5, and 6.
http://alariobros.com/winches-hydraulicandelectric.aspx
When lifting a load out of the hold on the boats, the rope goes from the
load up over a pulley and down to the cathead, wrapped around once, just
a little tension on the loose end of the rope causes enough friction to
start lifting the load. On the boats they usually have
hydraulic motors.
I like the idea.
Mikek

Yeah, I had a hard time remembering the search terms. I used to have a sailboat,
and design marine automation systems, and go out on ships and stuff, but it's
been a long time. Warm and dry has its own appeal.

I've seen steam-powered capstains on older ships.

The sailboat capstain winch things cost kilobucks. "Why have a sailboat when you
can throw your money directly in the water?"


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation

 

[amdx]

On 7/30/2014 11:37 AM, John Larkin wrote:

On Wed, 30 Jul 2014 11:03:24 -0500, amdx <nojunk@knology.net> wrote:

On 7/30/2014 10:50 AM, John Larkin wrote:
On Wed, 30 Jul 2014 08:39:54 -0700, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension on
a nylon cord. The cord comes is pulled off of a spool a certain amount
and cut. I have drag on the spool because I don't want it keep
spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull the
cord. The motor would not have quite enough torque to pull the cord
off of the spool by itself. But when I pull on the cord it will be
easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the current
limited to a certain value the torque the motor can develop will also
be limited. If the voltage to the motor is set at a certain value then
it will only be able to reach a certain speed. If I set the voltage so
that the motor will spin fast enough to keep up with the maximum rate
at which I pull the cord will I be able to pull the cord slower
without having the motor try to feed the cord too fast? I'm hoping I
can just use a current and voltage limited power supply to do what I
want. One drawback I see is that when the cord is not being pulled the
motor will be stationary but will be consuming power because it is
pulling against the drag on the spool. So I would need to use a motor
that could be stalled forever without overheating. If a brushed motor
is used will this be really hard on the brushes? And could a brushless
motor be used instead?
Thank You,
Eric

Suppose you used a constant-speed motor, turning at some low RPM. It could turn
a shaft or a drum.

The feed cord would make one or more loops around the drum. If you don't pull on
the cord, it slips on the rotating drum. If you pull, it tightens up and the
motor assists. It's self-regulating.

I've seen this done on ships to let a crew guy apply an easily regulated amount
of force to a big line or cable. He loops it around a slowly rotating thingie
(nautical term) and pulls as needed. The mechanical force multiplication is
huge.

Vaguely like this:

http://www.davitmaster.com/products/images/BR1000.WEG.LG.jpg


google electric capstain

http://www.nabrico-marine.com/pages/electriccapstan.html


Yes, you have it, I was looking for the term to do an image search.
The word used by deckhands is cathead or catshead.
This url shows several, in pictures 3, 4, 5, and 6.
http://alariobros.com/winches-hydraulicandelectric.aspx
When lifting a load out of the hold on the boats, the rope goes from the
load up over a pulley and down to the cathead, wrapped around once, just
a little tension on the loose end of the rope causes enough friction to
start lifting the load. On the boats they usually have
hydraulic motors.
I like the idea.
Mikek



Yeah, I had a hard time remembering the search terms. I used to have a sailboat,
and design marine automation systems, and go out on ships and stuff, but it's
been a long time. Warm and dry has its own appeal.

I've seen steam-powered capstains on older ships.

The sailboat capstain winch things cost kilobucks. "Why have a sailboat when you
can throw your money directly in the water?"

LOL!
When I talk to anyone thinking about getting a boat, I always tell
them the same thing. "Get a friend with a boat, every time you go
fishing on the boat, give them some money for fuel.
If you really feel energetic, help clean the boat, change the oil,
clean out the bilge, fix the bilge pump, pay the insurance,wash and wax
it, help them fix the water pump, replace the broken pin in the
propeller, Winterize, clean the battery connections, replace the
battery,lube the bearings on the trailer, replace the wiring on the
trailer lights.
There's much more, but you get the point. I don't really say the
second paragraph, but that's some of the reason I say,
"Get a friend with a boat, every time you go fishing on the boat, give
them some money for fuel.
Mikek


---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

 

[Baron]

John Larkin prodded the keyboard

On Tue, 29 Jul 2014 09:34:01 -0700, etpm@whidbey.com wrote:

Greetings All,
I am working on a project in my shop that requires constant tension
on a nylon cord. The cord comes is pulled off of a spool a certain
amount and cut. I have drag on the spool because I don't want it
keep spinning when I'm not pulling. But pulling against this drag is
tiresome. So I was thinking that I could use a motor to help pull
the cord. The motor would not have quite enough torque to pull the
cord off of the spool by itself. But when I pull on the cord it will
be easy because the motor would be helping me. I'm wondering how to
control the torque of the DC motor. I think that if I keep the
current limited to a certain value the torque the motor can develop
will also be limited. If the voltage to the motor is set at a
certain value then it will only be able to reach a certain speed. If
I set the voltage so that the motor will spin fast enough to keep up
with the maximum rate at which I pull the cord will I be able to
pull the cord slower without having the motor try to feed the cord
too fast? I'm hoping I can just use a current and voltage limited
power supply to do what I want. One drawback I see is that when the
cord is not being pulled the motor will be stationary but will be
consuming power because it is pulling against the drag on the spool.
So I would need to use a motor that could be stalled forever without
overheating. If a brushed motor is used will this be really hard on
the brushes? And could a brushless motor be used instead?
Thank You,
Eric

Suppose you used a constant-speed motor, turning at some low RPM. It
could turn a shaft or a drum.

The feed cord would make one or more loops around the drum. If you
don't pull on the cord, it slips on the rotating drum. If you pull,
it tightens up and the motor assists. It's self-regulating.

I've seen this done on ships to let a crew guy apply an easily
regulated amount of force to a big line or cable. He loops it around
a slowly rotating thingie

Its called a "Capstan"

(nautical term) and pulls as needed. The
mechanical force multiplication is huge.

Vaguely like this:

http://www.davitmaster.com/products/images/BR1000.WEG.LG.jpg



--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation

--
Best Regards:
Baron.

 

[Tim Wescott]

On Wed, 30 Jul 2014 08:56:25 -0700, etpm wrote:

SNIP

A DC motor can be regulated to deliver constant torque by using a series
resistor which measures the motor current,
then feeds the signal back to a servo amplifier with gain adjustment.
But then you will not have constant (rotational) speed.

This is not the same problem as constant speed control.
For DC speed control you would use a tacho-generator or when pulsing
the motor you can use the motor itself as tacho-generator during the
pulse pause.


This is not a simple task, but there are some hand drill machines on the
market which have adjustable speed and adjustable torque.
To be precise: adjustable maximum torque.

One cannot have constant speed and constant torque in one when the
mechanical load changes permanently.
You cannot have constant torque if the load changes.
Only a maximum torque regulator can be achieved but at the cost of
changing speed.

w.
Thanks for the replies everyone. After reading the replies I am a little
confused. If I use a current and voltage limited power supply and there
is not quite enough power for the motor to pull against the drag the
motor will be stalled.

You mean current, not power. Torque in a permanent-magnet DC motor is
proportional to current, not power.

When I pull on the cord the motor will of course
turn, but only when I pull on the cord. If I pull the cord the motor
will then just draw less current, right?

That depends on where the voltage limit on the power supply kicks in. If
you have it set high enough then the current in the motor will stay
constant, and the torque will, too.

So the torque it develops plus
the pulling force will be just enough to overcome the drag, won't it?

Probably.

> If not, then what am I missing?

Nothing that I can think of off the top of my head, but given the other
suggestions so far it does seem to be a wasteful way to go about the job.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

On Wed, 30 Jul 2014 16:07:28 -0500, Tim Wescott
<tim@seemywebsite.really> wrote:

On Wed, 30 Jul 2014 08:56:25 -0700, etpm wrote:

SNIP

A DC motor can be regulated to deliver constant torque by using a series
resistor which measures the motor current,
then feeds the signal back to a servo amplifier with gain adjustment.
But then you will not have constant (rotational) speed.

This is not the same problem as constant speed control.
For DC speed control you would use a tacho-generator or when pulsing
the motor you can use the motor itself as tacho-generator during the
pulse pause.


This is not a simple task, but there are some hand drill machines on the
market which have adjustable speed and adjustable torque.
To be precise: adjustable maximum torque.

One cannot have constant speed and constant torque in one when the
mechanical load changes permanently.
You cannot have constant torque if the load changes.
Only a maximum torque regulator can be achieved but at the cost of
changing speed.

w.
Thanks for the replies everyone. After reading the replies I am a little
confused. If I use a current and voltage limited power supply and there
is not quite enough power for the motor to pull against the drag the
motor will be stalled.

You mean current, not power. Torque in a permanent-magnet DC motor is
proportional to current, not power.

When I pull on the cord the motor will of course
turn, but only when I pull on the cord. If I pull the cord the motor
will then just draw less current, right?

That depends on where the voltage limit on the power supply kicks in. If
you have it set high enough then the current in the motor will stay
constant, and the torque will, too.

So the torque it develops plus
the pulling force will be just enough to overcome the drag, won't it?

Probably.

If not, then what am I missing?

Nothing that I can think of off the top of my head, but given the other
suggestions so far it does seem to be a wasteful way to go about the job.

Greetings Tim,
I understand that torque is proportional to current. When I used the
word power I was thinking about watts, voltage times current. But I
see where I was wrong. Low current and high volts would be the same
wattage as high current and low volts. In this situation current is
what matters for torque. Though the other solutions have their merits
I think the convenience of having the same pulling force no matter
what speed I pull the cord, within reason, will be best for me. So
another question. I see lots of chips for driving low current BLDC
motors. If I was to use one of these and the motor was stalled would
the motor driver still try to make the motor move? Do any work like
that? They need no commutation but instead supply a constantly
rotating field and the motor tries to spin at the correct RPM? I need
to look on the web to see if what I want is available.
Eric

 

[Tim Wescott]

On Wed, 30 Jul 2014 16:36:15 -0700, etpm wrote:

On Wed, 30 Jul 2014 16:07:28 -0500, Tim Wescott
tim@seemywebsite.really> wrote:

On Wed, 30 Jul 2014 08:56:25 -0700, etpm wrote:

SNIP

A DC motor can be regulated to deliver constant torque by using a
series resistor which measures the motor current,
then feeds the signal back to a servo amplifier with gain adjustment.
But then you will not have constant (rotational) speed.

This is not the same problem as constant speed control.
For DC speed control you would use a tacho-generator or when pulsing
the motor you can use the motor itself as tacho-generator during the
pulse pause.


This is not a simple task, but there are some hand drill machines on
the market which have adjustable speed and adjustable torque.
To be precise: adjustable maximum torque.

One cannot have constant speed and constant torque in one when the
mechanical load changes permanently.
You cannot have constant torque if the load changes.
Only a maximum torque regulator can be achieved but at the cost of
changing speed.

w.
Thanks for the replies everyone. After reading the replies I am a
little confused. If I use a current and voltage limited power supply
and there is not quite enough power for the motor to pull against the
drag the motor will be stalled.

You mean current, not power. Torque in a permanent-magnet DC motor is
proportional to current, not power.

When I pull on the cord the motor will of course turn, but only when I
pull on the cord. If I pull the cord the motor will then just draw
less current, right?

That depends on where the voltage limit on the power supply kicks in.
If you have it set high enough then the current in the motor will stay
constant, and the torque will, too.

So the torque it develops plus the pulling force will be just enough
to overcome the drag, won't it?

Probably.

If not, then what am I missing?

Nothing that I can think of off the top of my head, but given the other
suggestions so far it does seem to be a wasteful way to go about the
job.
Greetings Tim,
I understand that torque is proportional to current. When I used the
word power I was thinking about watts, voltage times current. But I see
where I was wrong. Low current and high volts would be the same wattage
as high current and low volts. In this situation current is what matters
for torque. Though the other solutions have their merits I think the
convenience of having the same pulling force no matter what speed I
pull the cord, within reason, will be best for me. So another question.
I see lots of chips for driving low current BLDC motors. If I was to use
one of these and the motor was stalled would the motor driver still try
to make the motor move? Do any work like that? They need no commutation
but instead supply a constantly rotating field and the motor tries to
spin at the correct RPM? I need to look on the web to see if what I want
is available.
Eric

The "needs no commutation" part will kill you in this application. AFAIK
those chips _do_ need commutation -- they generate it by sensing the back-
EMF of the spinning motor, which you can't do with a motor that's
stationary.

You'd need a BLDC motor with commutation outputs (usually three Hall
sensors), and a driver that knows what to do with those outputs. And if
you're moving it slowly, expect to feel a perceptible "bump" every time a
Hall detector switches.

For all that I'm usually in the business of providing fancy solutions to
simple problems -- I think you're over-complicating things here. If
you're going to do it with a DC motor, just get a surplus brushed motor
and a cheap constant-current supply, and don't mess with BLDC stuff
unless you're in it for the journey and not the destination.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

On Thu, 31 Jul 2014 12:33:34 -0500, Tim Wescott
<tim@seemywebsite.really> wrote:

On Wed, 30 Jul 2014 16:36:15 -0700, etpm wrote:

On Wed, 30 Jul 2014 16:07:28 -0500, Tim Wescott
tim@seemywebsite.really> wrote:

On Wed, 30 Jul 2014 08:56:25 -0700, etpm wrote:

SNIP

A DC motor can be regulated to deliver constant torque by using a
series resistor which measures the motor current,
then feeds the signal back to a servo amplifier with gain adjustment.
But then you will not have constant (rotational) speed.

This is not the same problem as constant speed control.
For DC speed control you would use a tacho-generator or when pulsing
the motor you can use the motor itself as tacho-generator during the
pulse pause.


This is not a simple task, but there are some hand drill machines on
the market which have adjustable speed and adjustable torque.
To be precise: adjustable maximum torque.

One cannot have constant speed and constant torque in one when the
mechanical load changes permanently.
You cannot have constant torque if the load changes.
Only a maximum torque regulator can be achieved but at the cost of
changing speed.

w.
Thanks for the replies everyone. After reading the replies I am a
little confused. If I use a current and voltage limited power supply
and there is not quite enough power for the motor to pull against the
drag the motor will be stalled.

You mean current, not power. Torque in a permanent-magnet DC motor is
proportional to current, not power.

When I pull on the cord the motor will of course turn, but only when I
pull on the cord. If I pull the cord the motor will then just draw
less current, right?

That depends on where the voltage limit on the power supply kicks in.
If you have it set high enough then the current in the motor will stay
constant, and the torque will, too.

So the torque it develops plus the pulling force will be just enough
to overcome the drag, won't it?

Probably.

If not, then what am I missing?

Nothing that I can think of off the top of my head, but given the other
suggestions so far it does seem to be a wasteful way to go about the
job.
Greetings Tim,
I understand that torque is proportional to current. When I used the
word power I was thinking about watts, voltage times current. But I see
where I was wrong. Low current and high volts would be the same wattage
as high current and low volts. In this situation current is what matters
for torque. Though the other solutions have their merits I think the
convenience of having the same pulling force no matter what speed I
pull the cord, within reason, will be best for me. So another question.
I see lots of chips for driving low current BLDC motors. If I was to use
one of these and the motor was stalled would the motor driver still try
to make the motor move? Do any work like that? They need no commutation
but instead supply a constantly rotating field and the motor tries to
spin at the correct RPM? I need to look on the web to see if what I want
is available.
Eric

The "needs no commutation" part will kill you in this application. AFAIK
those chips _do_ need commutation -- they generate it by sensing the back-
EMF of the spinning motor, which you can't do with a motor that's
stationary.

You'd need a BLDC motor with commutation outputs (usually three Hall
sensors), and a driver that knows what to do with those outputs. And if
you're moving it slowly, expect to feel a perceptible "bump" every time a
Hall detector switches.

For all that I'm usually in the business of providing fancy solutions to
simple problems -- I think you're over-complicating things here. If
you're going to do it with a DC motor, just get a surplus brushed motor
and a cheap constant-current supply, and don't mess with BLDC stuff
unless you're in it for the journey and not the destination.

Greetings Tim,
I am in it partly for the journey. To begin with I am just going to
use one of the DC motors I have sitting around waiting to be used.
However, after thinking about BLDC motors and commutation I realized
that the motor drivers must initially energize the motor coils to make
a rotating magnetic field so the motor will spin up to some minimum
RPM so the the driver can detect the back EMF or hall effect switches
for commutation. Which makes me wonder if I can find a driver which
doesn't need commutation but instead just energizes the coils at a
certain frequency, like the way an induction motor works. Then the
motor can spin at any RPM from stall up to the rotating field RPM. In
fact, I bet someone has written the code for an Arduino to do this. I
should look for it.
Thanks,
Eric

 

[Tim Wescott]

On Thu, 31 Jul 2014 17:13:33 -0700, etpm wrote:

On Thu, 31 Jul 2014 12:33:34 -0500, Tim Wescott
tim@seemywebsite.really> wrote:

On Wed, 30 Jul 2014 16:36:15 -0700, etpm wrote:

On Wed, 30 Jul 2014 16:07:28 -0500, Tim Wescott
tim@seemywebsite.really> wrote:

On Wed, 30 Jul 2014 08:56:25 -0700, etpm wrote:

SNIP

A DC motor can be regulated to deliver constant torque by using a
series resistor which measures the motor current,
then feeds the signal back to a servo amplifier with gain
adjustment.
But then you will not have constant (rotational) speed.

This is not the same problem as constant speed control.
For DC speed control you would use a tacho-generator or when
pulsing the motor you can use the motor itself as tacho-generator
during the pulse pause.


This is not a simple task, but there are some hand drill machines on
the market which have adjustable speed and adjustable torque.
To be precise: adjustable maximum torque.

One cannot have constant speed and constant torque in one when the
mechanical load changes permanently.
You cannot have constant torque if the load changes.
Only a maximum torque regulator can be achieved but at the cost of
changing speed.

w.
Thanks for the replies everyone. After reading the replies I am a
little confused. If I use a current and voltage limited power supply
and there is not quite enough power for the motor to pull against
the drag the motor will be stalled.

You mean current, not power. Torque in a permanent-magnet DC motor is
proportional to current, not power.

When I pull on the cord the motor will of course turn, but only when
I pull on the cord. If I pull the cord the motor will then just draw
less current, right?

That depends on where the voltage limit on the power supply kicks in.
If you have it set high enough then the current in the motor will stay
constant, and the torque will, too.

So the torque it develops plus the pulling force will be just enough
to overcome the drag, won't it?

Probably.

If not, then what am I missing?

Nothing that I can think of off the top of my head, but given the
other suggestions so far it does seem to be a wasteful way to go about
the job.
Greetings Tim,
I understand that torque is proportional to current. When I used the
word power I was thinking about watts, voltage times current. But I
see where I was wrong. Low current and high volts would be the same
wattage as high current and low volts. In this situation current is
what matters for torque. Though the other solutions have their merits
I think the convenience of having the same pulling force no matter
what speed I pull the cord, within reason, will be best for me. So
another question. I see lots of chips for driving low current BLDC
motors. If I was to use one of these and the motor was stalled would
the motor driver still try to make the motor move? Do any work like
that? They need no commutation but instead supply a constantly
rotating field and the motor tries to spin at the correct RPM? I need
to look on the web to see if what I want is available.
Eric

The "needs no commutation" part will kill you in this application.
AFAIK those chips _do_ need commutation -- they generate it by sensing
the back-
EMF of the spinning motor, which you can't do with a motor that's
stationary.

You'd need a BLDC motor with commutation outputs (usually three Hall
sensors), and a driver that knows what to do with those outputs. And if
you're moving it slowly, expect to feel a perceptible "bump" every time
a Hall detector switches.

For all that I'm usually in the business of providing fancy solutions to
simple problems -- I think you're over-complicating things here. If
you're going to do it with a DC motor, just get a surplus brushed motor
and a cheap constant-current supply, and don't mess with BLDC stuff
unless you're in it for the journey and not the destination.
Greetings Tim,
I am in it partly for the journey. To begin with I am just going to use
one of the DC motors I have sitting around waiting to be used. However,
after thinking about BLDC motors and commutation I realized that the
motor drivers must initially energize the motor coils to make a rotating
magnetic field so the motor will spin up to some minimum RPM so the the
driver can detect the back EMF or hall effect switches for commutation.
Which makes me wonder if I can find a driver which doesn't need
commutation but instead just energizes the coils at a certain frequency,
like the way an induction motor works. Then the motor can spin at any
RPM from stall up to the rotating field RPM. In fact, I bet someone has
written the code for an Arduino to do this. I should look for it.
Thanks,
Eric

The hall effect switches are static; they let the controller know what
quadrant the armature is in whenever they have power applied.

There are "sensorless" brushless motor controllers, but they are really
only useful for driving loads that present little or no back-torque at
low speeds, and that tend to run at 10% of top speed or higher - like
fans, or RC airplane propellers.

Induction motors are qualitatively different from BLDC motors. Induction
motors work because the armature gets dragged along with the rotating
magnetic field. As such they are "self timed". BLDC motors are
synchronous machines: the rotor must be in step with the rotating
magnetic field. You can run a BLDC motor by rotating the magnetic field
open-loop, but then its working like a stepper motor. You really can't
generate much torque reliably that way, and if the motor slips out of
synchronization the _average_ torque will go to zero, no matter how much
vibration is added by the coils.

For all of the above reasons, whenever a BLDC motor must generate
significant torque at low speeds, it's got hall-effect sensors.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com