Forums

Help please with DC motor control

Started by Unknown July 29, 2014
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
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 | | | Voice:(480)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.
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
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
<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
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 ---
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.
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
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
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