How to maintain a 12V 150 RPM geared DC motor's torque in all speeds?

[royz7]

Hi everybody,

This is my first message here and it looks like an awesome forum!

I have a 12V geared DC motor and i am having a hard time figuring out how to
maintain it's torque in from speed 0 to 150 RPM..

I know about PWM and figured it's not going to do it..
I am looking for a very simply designed solution.

from what i understand:
voltage correlates to speed.
current correlates to torque.

and i know it's not 100% exact but i understand that it's mostly true..

so if I'll take my motor and use a potentiometer as a rheostat and cut the voltage
by half to 6V i will have around half the speed, but i will also have half the current
and half the torque.. which that means that i could stall the motor with my fingers,
so it's not good.

is there any simple way to maintain the current in all speeds
and just changing the voltage for the speed?

or the voltage and current go together and there is no way to change that?

the answers i got from people by now were very confusing and didn't help me
to solve the problem, please try to use simple explanations since i am just a hobbyist..
if you are about to advise me of any electronic components, please please,
be as specific as you can since sometimes i find some terms that i google and don't get nothing..

what i want in the end is something that i can buy or build myself which will be common
enough for me to go to the electronics shop and get it (or the components) that will be simple,
cheap and a well working solution.

I'll really appreciate any help cause this problem is driving me nuts already..

Thanks a lot!

Roy.

 

[russ_watters]

You're neglecting the most important part of the issue: the load. The characteristics of the load are what determine the required torque and the motor will deliver whatever torque is necessary at a certain rpm to maintain the load or fail trying. This means that if you're at a certain rpm and the load (torque requirement) decreases, you have to decrease the voltage to maintain constant rpm.

 

[royz7]

Hi Russ,

this is my second problem actually,

1. I am trying to run the motor at around 30 RPM with high enough torque
for me not to be able to stall it with my fingers.

2. second problem is after I'll have the high torque i will have to maintain the speed
due to the changes in load..

I've heard there are some drivers that i can use to solve these..

please tell me what can i do..

Thanks a lot!

Roy.

 

[Calimero]

Servo motor? I don't know your power requirements, but it seems like easiest way to go, and probably cheapest if you want good results.

 

[royz7]

Servo motor? I don't know your power requirements, but it seems like easiest way to go, and probably cheapest if you want good results.

How will a servo motor function different or better? please tell me..

roughly,
my power requirement is pushing less than 1 kilogram (2 pounds) for 15cm (6 inch)

 

[Calimero]

Servo motor will function exactly as you need it.

As for power requirements, it is not just what mass you are pushing, but also rate at what you want it to accelerate. Of course this goes for horizontal movements, where gravity is not a factor. Also you have to add for friction in the system.

I think that small bruch DC servo with gecko drive will do it. Anyway how will you transfer power from shaft to moving mass? At what rate you want it to accelerate? Is it only horizontal movement?

 

[Calimero]

You should hook up http://www.geckodrive.com/product.aspx?c=2&i=14474" . Then you need simplest pulse generator (usually built around NE 555 circuit, very cheap, available as kits) and with potentiometer you can adjust frequency, thus RPM, anyway it suits you.

 

[skeptic2]

I have a 12V geared DC motor and i am having a hard time figuring out how to
maintain it's torque in from speed 0 to 150 RPM..

I know about PWM and figured it's not going to do it..
I am looking for a very simply designed solution.

from what i understand:
voltage correlates to speed.
current correlates to torque.

and i know it's not 100% exact but i understand that it's mostly true..

so if I'll take my motor and use a potentiometer as a rheostat and cut the voltage
by half to 6V i will have around half the speed, but i will also have half the current
and half the torque.. which that means that i could stall the motor with my fingers,
so it's not good.

is there any simple way to maintain the current in all speeds
and just changing the voltage for the speed?
Roy.

You are correct that voltage correlates to speed and current to torque, however one thing you left out is the internal resistance of the motor. Depending on the current through the motor there is a voltage across the internal resistance that does not contribute to the speed. You need to compensate for that voltage by determining what that resistance is and multiply that resistance by the instantaneous current and add that value to your speed voltage.

This is not as hard as it sounds. First you must monitor the current through the motor. For small motors it is usually done by monitoring the voltage across a small resistor in series with the motor. With a digital solution that voltage is multiplied by the internal resistance and added to the speed set point of the motor. For an analog solution, the voltage representing the current is amplified by an opamp whose gain represents the internal resistance and that is added to the set point voltage. Once you have this circuit working you can find the internal resistance by trial and error by varying the load of the motor and adjusting the internal resistance until the speed doesn't change. Note, if you are operating your motor at 12 V, your motor voltage may have to go higher than 12 V to compensate for the internal resistance.

If you adjust your voltage from 12 V to 6 V, your required torque may or may not also drop in half. If for instance your required torque is constant regardless of speed then you will see, as you reduce your speed, the apparent resistance of the motor will drop, requiring more current to maintain the same speed. Your power source must be capable of supplying sufficient current for all conditions. Note however, if your motor stalls and your power supply continues to supply max current, your motor can burn up. Many motor controllers have a max current limiter or trip point to prevent that.

 

[royz7]

Thanks everybody!

Servo motor will function exactly as you need it.

As for power requirements, it is not just what mass you are pushing, but also rate at what you want it to accelerate. Of course this goes for horizontal movements, where gravity is not a factor. Also you have to add for friction in the system.

I think that small brush DC servo with gecko drive will do it. Anyway how will you transfer power from shaft to moving mass? At what rate you want it to accelerate? Is it only horizontal movement?

I don't really care about the acceleration, i don't have to do it too fast.

I am translating the rotary motion to linear motion using a 5" control arm:

to this arm i will be connecting a shaft.
so yes, it's only for horizontal movement..

lets say i have a servo motor in a size of about "nema 23" motor.
and it ranges from 0-150 rpm (if there is such a thing),
will it have the same torque at 30 rpm as it will have at 150 rpm?
will i still need a gear motor?
does a servo motor have a low speed- high torque characteristic?

The motors that you have suggested are going really high on rpm and i don't need it
and i really want to stay in the 12V zone, i am not a pro, and would like to avoid high voltage in this project..

as for the gecko drive, it looks good but it's very expensive for me for this project..

You are correct that voltage correlates to speed and current to torque, however one thing you left out is the internal resistance of the motor. Depending on the current through the motor there is a voltage across the internal resistance that does not contribute to the speed. You need to compensate for that voltage by determining what that resistance is and multiply that resistance by the instantaneous current and add that value to your speed voltage.

This is not as hard as it sounds. First you must monitor the current through the motor. For small motors it is usually done by monitoring the voltage across a small resistor in series with the motor. With a digital solution that voltage is multiplied by the internal resistance and added to the speed set point of the motor. For an analog solution, the voltage representing the current is amplified by an opamp whose gain represents the internal resistance and that is added to the set point voltage. Once you have this circuit working you can find the internal resistance by trial and error by varying the load of the motor and adjusting the internal resistance until the speed doesn't change. Note, if you are operating your motor at 12 V, your motor voltage may have to go higher than 12 V to compensate for the internal resistance.

If you adjust your voltage from 12 V to 6 V, your required torque may or may not also drop in half. If for instance your required torque is constant regardless of speed then you will see, as you reduce your speed, the apparent resistance of the motor will drop, requiring more current to maintain the same speed. Your power source must be capable of supplying sufficient current for all conditions. Note however, if your motor stalls and your power supply continues to supply max current, your motor can burn up. Many motor controllers have a max current limiter or trip point to prevent that.

Thanks for all that information,
I would like to have the same torque in all speeds and i think i would like to go the analog way, so I'll build that circuit to compensate for the current the motor lacks at load and at low speeds but how will i control the motor's speed? by a potentiometer? wouldn't that cut the current too? so where does this compensation gets in the picture?
if i understand right everything is connected in a series and it should work?

For an analog solution, the voltage representing the current is amplified by an opamp whose gain represents the internal resistance and that is added to the set point voltage.

if you can please advise me of which small resistor i should get and if you can redirect me to
any other information that i can look for on this circuit, I'll appreciate it..

Thanks a lot everybody!

Roy.

 

[royz7]

Hey guys,

PLEASE HELP ME OUT HERE..

I was looking at an assambled circuit that is ready to work,
they claim it does exactly what i need it to do..

can you please take a look?

[PLAIN]http://www.electronickits.com/kit/complete/motor/ck1400smallr.jpg

http://www.electronickits.com/kit/complete/motor/ck1400.htm"

and they have these too:

[PLAIN]http://www.electronickits.com/kit/complete/motor/CANUK1102.jpg
http://www.electronickits.com/kit/complete/motor/CANUK1102.htm"

and:

[PLAIN]http://www.electronickits.com/kit/complete/motor/CKMX033PIC.jpg
http://www.electronickits.com/kit/complete/motor/CKMX033.htm"

my last questions:
do you think any of there will do the job for me? i can't figure out if besides PWM they have any sort of way to compensate in voltage and current when the there is a load on the motor at low speeds?

Please Please just tell me if it's good for me..

Thanks a million.

Roy Z.

 

[royz7]

anybody? please? thanks..

 

[kubing]

i have project relate to your need i think. control 12v motor
http://simf1.blogspot.com/