Motor Control: Start and Stop at the same point

[Buqpa]

Hello and thanks in advance for your answers!
Please, excuse my English as it is not my mother tongue and I may make some mistakes.

Let me ask you for some help: I am looking for a system like a fan that starts and stops at the very same points. Image a stopped fan that starts spinning. When I swicht off the motor of the fan I want it to stop in the very same place that it started, no matter how many turns it makes. It is ok if the motor still spins (but slower each time) before it "finds" where it has to stopped, this means, once the motor is switched off it can continue spinning as long as it needs until it stops in the desired position.

I think the best way to do it is using stepped motors, but I have no clue. I think that a digital control (PIC, Arduino for example) may do the task.

The arms of the fan will be like 2kg heavy.

What do you think?

Thanks a lot for your interest!

 

[karesz]

Hi,
I think you need an proximity detector & a controlled bracke, maybe trough eddycurrents...
How exact do you means "stops at the same point"?
K.

 

[Buqpa]

Dear Karesz,
Kind regards for your reply.
Let me explain you want I mean. Imagine there is an arrow attached to the motor of the fan. Before it starts spinning it points to a direction, for example, 12 o'clock.
Then it starts spinning and spins for sometime. Then you stop it. The arrow will keep spinning, but slower each time until it stops. Then, once it is stopped the arrow has to point to the very same position again, 12 o'clock.

It does not matter if the arrow just stops at 12 o'clock or once it is stopped it takes some steps to reach the initial position again.

I think that using stepped mottors and counting how many pulses has received, just using a PIC (Arduino for shake of simplicity) would be enough.

What do you think?

Kind regards,
Diego

 

[elmuhamady]

Dear I think you should use Stepper Motor as it will be very optimized for your purpose and you can also know how many steps to reach your desired position. I think you wouldn't find more accuracy than that. Also stepper motor work great with microcontrollers.Other wise your circuit will be more complicated as you will use encoder detector and brake mechanism.

Best Wishes,

Amr

 

[Buqpa]

Dear Elmuhamady,

Thanks a lot for your reply. Yes, I think that will work properly. Now I have a doubt. I will be movin an "arrow" that will weight around 2kg, and I would like to reach sucha spin speed I could not distinguish the arrow. Do you know any powerful step motor not very expensive?

Thanks a lot!
Diego Mellado

 

[bosparra]

The important thing to look for is the "holding torque" of the stepper motor. With such a large weight, you will have to work out the torque that's required to move the weight, as well as to keep it stationary. The required torque will be determined by the distance of the weight from the center shaft of the motor. Make sure that the motor you select can deliver more than what you require.

 

[Buqpa]

Dear bosparra,

Thanks a lot for your information! You all are being really helpful!
Any recomendation on what a motor to buy or where to look for it?

Kind regards,
Dieg

 

[ScOe]

I would rather go with ac motor than stepper and use absolute encoder to determine it position in any time. So if you cut off the power supply on motor, on next start you will know your absolute position (if that is important to you).

Take the motor from some old washer or buy some equivalent. It has around 100W power. How long is that arm ? Lets say it is 1m long. It will need 20Nm torque to hold it still. So motor with 100W can drive it with around 1revolution per sec.

 

[karesz]

Hi,
In my opinion & basically it dont need encoder if (I guessed AC motor too) should not rotate with high speed. After cut off the motor supply; you must have logically coupled rest supply voltage and (i.e. an inductive/light) position sensing of rotor, but such angle before its "home position"(to have enough time to brake it /take the moving energy of rotor), then apply a controlled brake force (i.e. timed/pwm) & you can stop it every time on the same position.
K.

 

[Buqpa]

Dear ScOe, thanks a lot for your reply. And thanks to everybody, I din not expected so much atention!
Let me explain you all the project. Instead of an arrow, there will be a capital letter attached to each motor. Letters must sping fast enough so they can not be distinguish. There will be several letters, all them conforming a word. Before any of the letters start spinning, the word should be readable, this means all letters has to be "up side up". Letters start spinning, but they do not have to do it all the same time. Second letter may spin for 2 minutes, third for 1 minute, and first and fourth remain stopped. So, after spinning each letter has to reach its initial position. And this initial position will not change ever.

That is why I thought about step motors. Using an absolute encoders seems too far complicated for the application.
Letters will be 1 meter length, and the motor would be attached at the center.
Then, taking a capital I, 2kg weight, after my own calculation (they may be wrong) minimum torque would be 4Nm
(twice the integer between 0 and 0.5m of 9.8*2*s ds) so 5Nm would be enough.
In addition, it is not mandatory for the letter to stop suddenly. It may keep spinning swloly until it stops. Also we it starts spinning it has not to do it at the maximum speed since the beginning. One or two revolutions before maximum speed are allowed.

Hope this explanation make things easier to understand.

Added after 14 minutes:

Hi,
In my opinion & basically it dont need encoder if (I guessed AC motor too) should not rotate with high speed. After cut off the motor supply; you must have logically coupled rest supply voltage and (i.e. an inductive/light) position sensing of rotor, but such angle before its "home position"(to have enough time to brake it /take the moving energy of rotor), then apply a controlled brake force (i.e. timed/pwm) & you can stop it every time on the same position.
K.

Dear Karesz,
Thanks for your suggestion.
I think it is a good idea, but it is too "tight". I mean you have to make a very precise calculation on the "anlge before home position". I prefer using some digital control, I think. That angle is related with the weight of each letter, where they are installed... I think it is too complicated to calculate :S

Anyway, kind regards for yor idea, I think it is a very good guess

 

[elmuhamady]

Dear I think you can get a very suitable stepper motor for your project from an old copier machine (we have done that before) then you can improve holding torque using Stepper Chopping Driving.
For more details see:

**broken link removed**


Best Wishes,
Amr

 

[Buqpa]

Thanks a lot Elmuhamady!
I will have a look at your link (no I can not) Any idea about the model of the stepper motor? I need 6 motors and I have no six copier machine to break close to me

Kind regards!

 

[elmuhamady]

Each copier has 2 or 3 motors, so you will need 3 machines only

Try Digikey to find suitable one :

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=983126&k=stepper motor


Best Wishes,
Amr

 

[Buqpa]

LoL!
Ok, I will have a look asap!

You all are being really helpful!
Thanks a lot!

 

[karesz]

Dear Buqpa,
Tnx a lot for your words!
You can sens a rotor position not only on the rotors self, but in the center axle too.
You can design on the rotor axle a (adjustable to rotor angle) rotatable disc(or pin)-marker of metal for inductance or plastic disc for optical indexing/sensing.
This index signal you has in all rotation one time, but if you do coupling it with "motor out" switch-signal = you has the sensing in right moment & adjustable to rotor angle in right moment to have enough time & energy/way to stopping ervery time on same position ..
K.

 

[Buqpa]

Dear Karesz,
thanks for all your ideas. I think I will take the optical indexing into account also.
It is not very easy to do, but it lets me use DC motors, which are more powerful and cheaper!
What do you think? Why AC better than DC motors?

Kind regards,
Diego