dc micro motor design

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starcrossed

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I have been designing this project of automating a baby cradle. i have done some calculations of the motor torque i require at a given speed. i tried talking with few dc micro motor manufacturers but they are unable to help me. can you please help.

I want the motor to run on 6 volts dc through 4 D-Size batteries. The current drawn shall not be more than 1 Amp at stall. the stalling torque i require is at various possible speeds is

Sr. Speed Stall Torque
1 22000 0.63
2 20000 0.70
3 18000 0.77
4 16000 0.87
5 14000 0.99
6 15000 0.93
7 12000 1.16
8 10000 1.39
9 9000 1.55
10 8000 1.74
11 7000 1.99
12 6000 2.32
13 5000 2.78


Is it possible to design a dc micro motor with any one of the above possible speed torque options at 6 V and 1 amp stall current?
 
It might help if you specify units. I guessed that you were using RPM for speed and oz-inches for torque, in which case I believe that with a stiff 6V supply you will just barely meet your current limits (linear back of the envelope).

Additionally, I believe that you have made a serious error in your assumptions: A supply consisting of 4 'D' cells will not be able to maintain a 6V output under 1A load for very long, perhaps not at all. The voltage drop at the battery will keep you in your current limits, but drop your torque severely.

-Jon
 
The speed is in RPM and torque is in Kg-Cm. I am assuming a maximum current of 1 amp. I believe the ideal current should be around 0.5 to 0.7 amps. duty cycle of the motor is going to be aound 50% that is in every second the motor is going to run for around 0.5 seconds only.
 
I agree with winnie, you're proposing a design that simply doesn't have enough power to operate for anywhere near a reasonable amount of time. And the cost of D cells, and silly performance of most rechargeable NiCad D cells.

I'd be looking at a small lead-acid, sorry, baby thing, Gel-Cell battery.
 
I took the average of all the currents you've listed and get 1.37 amps.

If you want the thing to work for four hours you'd multiply 1.37 by 4 and get about 5.48 amp hours.

Then I would only expect satisfactory performance for 50 to 75% of that time.

If it were me I might be considering a 6v. 7.5 Amp hour Gel-cell battery.
 
winnie said:
It might help if you specify units. I guessed that you were using RPM for speed and oz-inches for torque, in which case I believe that with a stiff 6V supply you will just barely meet your current limits (linear back of the envelope).
Click to expand...

I like you winnie. :smile:
 
Besoeker said:
Not sure that a range of surrents was given?
Click to expand...

I'll repost it:

The current drawn shall not be more than 1 Amp at stall. the stalling torque i require is at various possible speeds is

Sr. Speed Stall Torque
1 22000 0.63
2 20000 0.70
3 18000 0.77
4 16000 0.87
5 14000 0.99
6 15000 0.93
7 12000 1.16
8 10000 1.39
9 9000 1.55
10 8000 1.74
11 7000 1.99
12 6000 2.32
13 5000 2.78
Click to expand...

Edit: Those are torques huh? Oops. :grin:

Then 5 Amp hours'll do.
 
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kg-cm huh? IMHO that is a horrible unit to use; the kg is not really a unit of force...but that is a discussion for another time. My thoughts are:

1) I will assume that you want to use some sort of permanent magnet motor, given the very small size of these machines. I assume that the speed that you list is the 'no load' speed, meaning the speed with zero torque output. Finally I will assume that the torque increases linearly from 0 at the 'no-load' speed to the stall torque at zero speed.

2) Given the above assumptions, maximum _power_ is achieved at half of the 'no-load' speed. Power is the product of torque and speed, and is zero at both the 'no load' speed (speed is max but torque is zero) and zero at the stall speed (torque is max but speed is zero), and peaks half way down.

3) From this we can calculate the maximum power of each of your motor selections. Say picking motor 8, the peak speed is 10000 RPM and the stall torque is 1.39 kg-cm. Peak power is thus at 5000 RPM at 0.695 kg-cm. Power is torque times speed, but you also need a conversion factor to get to reasonable units. I always convert speed to radians per second and torque to newton meters, because in those units torque * speed gives watts. 5000 RPM = 523.6 rad/sec. 0.695 kg-cm = 0.06811 Nm Peak power of motor 8 is thus 36 watts. It looks to me like you will get a similar answer for each of the motors listed.

4) Assuming 100% efficiency, and assuming 6V supply, you are talking 6A at peak power delivered, and _12A_ at stall,

Thus if the motor is actually being operated at stall, you will not even come close to your current limits.

-Jon
 
Yet, they do make baby cradles that work on a few D cells.

I don't know where you are getting your RPM's and torques from, but I wonder if the numbers are right.

We have discussed a similar post before, and I think we came to the conclusion that your motor only needs to overcome any losses (friction, wind resistance, etc.) This is assuming someone gives the cradle a good push to start it.

IMO, since losses are somewhat hard to predict, I would work this in reverse via trial and error. Pick a generous power supply to start with (maybe 8 or 10 D cells), and pick a motor with enough torque to give you a good run time. (Maybe 3 or 4 hours).

Then try it. If the crib stops after 10 min, you need more torque. If it keeps running like the energizer bunny until the batteries are dead, then back off the torque some and try to mazimize the battery life. Then maybe you will find you can back off on the number of batteries a little.
 
Don't cut yourself short on torque.

If you do, when the little bugger rolls over, moves, squirms, etc, it will disrupt the movement and the motor needs enough torque to maintain the oscillation of the cradle.

10,000 rpm what are you doing? Even with a worm gear that's pretty fast... isn't it?
( Am I in left field here? But even loaded and running at 5k, this sound nuts for the app. ??? )

Having had a couple of the battery swing deals they sell commercially they are way under torqued to save battery power, not to mention the mechanics are pretty cheap. (I returned the first two after only a few weeks.) AND after dealing with batteries for a while I simply rigged up a 2a 6v power supply.

Thankfully the little ones are now 4 and 6, so they can swing on their own...


Have fun,
Doug S.
 
starcrossed said:
I have been designing this project of automating a baby cradle. i have done some calculations of the motor torque i require at a given speed. i tried talking with few dc micro motor manufacturers but they are unable to help me. can you please help.

I want the motor to run on 6 volts dc through 4 D-Size batteries. The current drawn shall not be more than 1 Amp at stall. the stalling torque i require is at various possible speeds is


Is it possible to design a dc micro motor with any one of the above possible speed torque options at 6 V and 1 amp stall current?
Click to expand...

From the speeds and torques gives, it looks aiming for the same final output torque to move the cradle just with different motor/gearbox options.
I suppose it would be possible to design a motor with the stall torque you require. But, based or rough calculations, I don't you will get that using 4 D type cells.
 
Maybe a clutch would help with the stalled condition. It seems like this is where a lot of the trouble will be.

Edit: Still, how many batteries do you want go through in a day?
 
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While you are at it, design an automatic feeder and diaper changer.

Seriously, I would scavenge one of those motor set ups for the fake reindeer that are so popular around Christmas and try that first. Just make sure you disconnect the flashing lights as they may keep the young'un awake.

Keep the reindeer around in case Junior catches on to the rouse and won't stop crying until a real person rocks his crib.

When I was an infant the only thing that would put me to sleep at night was a ride in my Dad's 1957 Chevy. You know what's weird...that was 50 years ago and I still remember parts of it.

What were we talking about???
 
thanks a lot for your responces..one thing i like about mike holt's forum is the enthusiasm and the responce i get here...:smile:
 
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