selecting dc motor for baby papisan

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i hail from india. The components are very easy to obtain. also i am considering using "mechanical advantage" to drive the cradle. this will futher reduce the power needed. if i use a lever, with ma of around 5, ie. the arm is pivoted and the longer arm receives the push from the motor...i will be able to do with smaller dc motor.

i have contacted one local supplier of dc motor and he is sending me a motor in couple of days...so will keep my self posted...
 
081118-1408 EST

starcrossed:

You are half way around the world. Do you sleep during the day?

Here is an idea. The point on the locus of a circle moving at constant velocity and projected to an axis follows a sine curve.

Create a connecting rod from a point on a disk driven by your motor and connect the connecting rod to the pendulum. Now your driving function will approxmate the free motion of the pendulum.

I suggest you measure the resonant frequency of the swing with intended weight. Pick your motor RPM to approximate this frequency. But design for the torque for maximum elevation assuming a non-moving condition. So without going back and looking at the calculations about 20 to 50 W or 6/100 HP. So maybe a 1/10 HP motor with which to start playing.

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Gar, i am using a work geared dc motor. but as you said, the problem is that it cant be back driven. now my triggering circuits depend on free rotation. is there any way a dc gear motor with worm gear mechanism be back driven? The speed reduction by use of worm gearl will be from 3000 rpm to say around 40-60 rpm?

gar said:
081116-1435 EST

starcrossed:

It does not matter where in the swing you add the energy it is still only the amount of energy to overcome losses.

To get the swing moving you have to add more energy than the losses. This excess energy is added to the mass of the system. But again it does not matter where the energy is added, but it has to be synchronized properly.

If you use a worm gear type of drive, then it is typically less than 50% efficient and can not be back driven. This means you will need a motor control with a program to control the shape of the acceleration curve. You can calculate the torque to raise the mass to some angle. Suppose the angle of the peak swing is 45 deg, the weight is 30 #, and the lever arm is 3 ft, then the torque is 30*0.707*3 = 63.6 #-ft. If the period of 1 full swing (Up-down-up and down) is 6 seconds, then RPM is 10/2 if we assume constant velocity in both directions. HP = 63.6*5/5252 = 6.05/100 HP. Check my math.

I just did and it was wrong. Now maybe it is correct.
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081119-1158 EST USA

starcrossed:

The approximate general rule is that backdriveable vs non-backdriveable occurs at about 50 % efficiency.

Sliding friction worm gear reducers are less than 50% efficient for almost anything except very low ratios. But do not use a worm gear reducer for a crain and not also supply a brake because vibration to the gear reducer will allow backdriving.

Spur gear reducers are more efficient, but in high ratios are not very efficient.

A 100 to 1 ratio in anything has a lot of loss.

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man I hate to deflate this whole idea but this does not sound very cost effective to me..I just bought on at the second hand store for grand baby and it was like new and got it for 50 dollar..you are not going to be able to make this thing work that cheaply..
 
may be not cheap...but its a lot of fun making it...i dont intend to sell it ne ways...

btw, if you have bought a 2nd hand swing, can you please tell me the rating of the motor it uses...is possible....show a picture...

cschmid said:
man I hate to deflate this whole idea but this does not sound very cost effective to me..I just bought on at the second hand store for grand baby and it was like new and got it for 50 dollar..you are not going to be able to make this thing work that cheaply..
Click to expand...
 
I will give it a shot for you..I will be cutting up deer tonight so be patient..I will get you pics of the whole setup..


I like to create things as well..
 
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i had one more thaught on the mechanism..how about using a solenoid..???


i believe a solenoid is small tunnel of coils that produce magnetic field when energised. A plunger type push-pull solenoid has an iron core inside the tunnel..by using an iron core instead of air, the power will increase. when energised this iron core is thrown out or pulled in creating a stroke...now if i connect a strong permenant magnet to the iron core such the magnet will experience repelling force from the solenoid when energised, then the electric power requirement will reduce further.

Can i get the required power stroke from soloneid...i mean about 3 kg forece with stroke lenght 1 to 2 inch...can make it little bit bigger if required...

I mean power effeciency is an electric motor better then solenoid for same movement..??

what is the best stroke that i can get with 6 Volts battery and 200 to 300 mAmps current. i dont mind making it little big bigger. is there any website that can guide me on the design parameters of solenoids.
 
081124-1349 EST

starossed:

With all the information we have given you it should be possible with your general background, based on your profile, for you to start figuring this out yourself.

One of my professors from long ago would have told you to figure it out on your own almost from the start. This professor would also ask what seem to be dumb questions in program review meetings in order to force the speaker to more clearly present their idea. This professor was not dumb and at one time was head of the department. He also wrote our textbook on Fundamentals of Engineering Electronics which was a book on electron tube theory.

In one of my earlier posts I mentioned a magnet and a coil instead of a motor.

If you have a continuous supply of 6 V and 0.3 A that is an average power of 1.8 W. If the average power dissipation in your swing system is less than 1.8 W, then your criteria will work. You could use pulses of 25 W with a duty cycle of 7% and be within 1.8 W average. Or pulses of 250 W at 0.7% duty cycle.

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gar said:
081124-1349 EST

starossed:

With all the information we have given you it should be possible with your general background, based on your profile, for you to start figuring this out yourself.

One of my professors from long ago would have told you to figure it out on your own almost from the start. This professor would also ask what seem to be dumb questions in program review meetings in order to force the speaker to more clearly present their idea. This professor was not dumb and at one time was head of the department. He also wrote our textbook on Fundamentals of Engineering Electronics which was a book on electron tube theory.

In one of my earlier posts I mentioned a magnet and a coil instead of a motor.

If you have a continuous supply of 6 V and 0.3 A that is an average power of 1.8 W. If the average power dissipation in your swing system is less than 1.8 W, then your criteria will work. You could use pulses of 25 W with a duty cycle of 7% and be within 1.8 W average. Or pulses of 250 W at 0.7% duty cycle.

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Click to expand...

Yet if I read correctly the item you quoted would only have muscle in one direction which would entail a timing mechanism to synchronize the movement..I will get the info tonight on the one I have but I believe it entails the use of a circular movement on a cam to maintain the mechanical momentum..you do need to start the movement and the motor maintains the movement..so all it replaces is you having to walk over and reestart the swing..
 
081124-1819 EST

cschmid:

Yes you need timing information. A magnetic and coil can be made to be self starting. You just start with small motions. If you supply more energy per pulse than is dissipated, then the swing magnitude will increase. As swing magnitude increases more energy is lost per cycle. Stability of the magnitude, steady state, occurs when the input energy equals the dissipated energy. With a magnetic and a coil you can pulse energy into the swing each half cycle.

A long time ago I invented a very simple LC oscillator with minimum components that was quite stable in amplitude relative to input supply voltage that was based upon removing the excess energy each cycle to maintain a stable output amplitude. The circuit was too simple and thus not patentable. Simple equivalent circuits read on it. There was no damped oscillation of the amplitude with step changes in supply voltage.

All oscillators are based on a feedback circuit that can supply more energy per cycle than is dissipated and then either adjusting the input energy, such as an HP oscillattor does (automatic gain control via a small incndescent lamp), or extracting excess energy each cycle.

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