selecting dc motor for baby papisan

Status
Not open for further replies.
S

starcrossed

Member
I am trying to automate an old baby papisan at home. I am not sure which motor can be used? I want to use a motor that runs on battries...say 4 aa or d size batteries making a total of 6 volts..

The motor has to be capable enough to swing a baby of 10 kgs to 15 kgs?

can any one suggest how to select the motor?
 
starcrossed said:
can any one suggest how to select the motor?
Click to expand...
You'll want a gear-motor in order to get the RPM's down low enough, and an output shaft you can attach a crank of suitable length for the amount of travel you'll need.

Electronic hobbyist magazines often have advertisements from companies that have an assortment of motors available. Check out a local book and magazine store.
 
SEO said:
What is a baby papisan?
Click to expand...

It swings a baby.

100_0033.jpg
 
A baby papasan is a baby swing or an infant cradle.

I have been looking at advertizements from dc micro motor manufactures, but how do i decide which motor to get..i mean how do i decide the torque rating, the current rating, and the speed?

I definately require high torque, but what torque and current? any idea?
 
081116-1117 EST

starcrossed:

You indicate that you are an electrical engineer. Consider this:

You have a device that is resonant. The average energy to keep this circuit oscillating is that to overcome the losses. This is way below 1 HP. Probably like 1/100 HP. Note: an adult can for a short time produce about 1 HP. For example running up a stairway, one of our high school physics experiments. At the Henry Ford Museum there is a hand crank generator that you can check your arm capability and for most people it is somewhat below 1 HP. I have seen one person produce 1 HP. My peak arm output is between 3/4 and 1 HP.

How can you get the energy into the swing to overcome the friction. Instead of a continuous motor drive see if you can create a synchronized pulse of energy to the swing. Also note the swing velocity is approximately sinusoidal. Maximum velocity is at the bottom of the swing and zero at each peak. Pulsing at the bottom of the swing might be best because the location is fixed. You might use a permanent magnet on the swing and a fixed coil on the frame.

.
 
gar said:
081116-1117 EST

starcrossed:

You indicate that you are an electrical engineer. Consider this:

You have a device that is resonant. The average energy to keep this circuit oscillating is that to overcome the losses.
Click to expand...
Interesting application.
Gar, I understand your point and I don't disagree with it.
If the baby swing operates at its natural resonant frequency then a little impulse of energy at zero speed or top of the swing would do the job.
If it can operate at two speeds, see image posted, then at least one of them won't be the natural resonance. The motor would still have to make up losses but it would have to work over the entire cycle either in motoring or generating mode.
 
i believe the best point to give a push is at the ends and not middle. if you see the children playing on swings in a garden,lwho swing using their own body weight....ike almost all of us have done at some time in our life,...they apply change in center of gravity or push the swing at the ends for max effect.....also i want to use a motor and not magnet, hence fix point is not much of consideration...

Also the initial torque required to set the swing in motion will be very high compared to force required to keep in motion as gar rightly said.

any thaughts on the right motor config for this? i want to use a micro dc motor...

i require a force of (weight)tan(tetha) to move a weight in horizantal direciton by angle tetha..which comes to 3.6 kg initial push to set the swing in motion...which motor can give it?

gar said:
081116-1117 EST

starcrossed:

You indicate that you are an electrical engineer. Consider this:

You have a device that is resonant. The average energy to keep this circuit oscillating is that to overcome the losses. This is way below 1 HP. Probably like 1/100 HP. Note: an adult can for a short time produce about 1 HP. For example running up a stairway, one of our high school physics experiments. At the Henry Ford Museum there is a hand crank generator that you can check your arm capability and for most people it is somewhat below 1 HP. I have seen one person produce 1 HP. My peak arm output is between 3/4 and 1 HP.

How can you get the energy into the swing to overcome the friction. Instead of a continuous motor drive see if you can create a synchronized pulse of energy to the swing. Also note the swing velocity is approximately sinusoidal. Maximum velocity is at the bottom of the swing and zero at each peak. Pulsing at the bottom of the swing might be best because the location is fixed. You might use a permanent magnet on the swing and a fixed coil on the frame.

.
Click to expand...
 
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.
 
Last edited:
starcrossed said:
i believe the best point to give a push is at the ends and not middle.
Click to expand...
Indeed. See post #9. But it's not critical where you do it.
starcrossed said:
Also the initial torque required to set the swing in motion will be very high compared to force required to keep in motion as gar rightly said.
Click to expand...
That's the crunch. You have no kinetic energy for the first quarter of a cycle.
To do that you'd need a motor capable of delivering around 200W.
Thats based on a +/- 45 deg swing, a distance from fulcrum to COG of about 600mm (2 ft) and a sing from left to right of around 1.5 seconds.
 
081117-0752 EST

starcrossed:

Using a different calculation and my dimensions:
To raise 30 # to the 45 deg angle at the end of a 3 ft pivot arm is a rise of 3 - 0.707*3 ft = 0.879 ft. This is 30*0.879 ft-# of work. If this is done in 1.5 seconds, then the rate is 30*0.879/1.5 ft-#/sec = 26.37/1.5 = 17.6 ft-#/sec .

Since 1 HP is equivalent to 550 ft-#/sec we should need 17.6/550 HP = 0.032 HP or 24 W.

My guess at the oscillation frequency is wrong. For a simple pendulum the period is T = 2*Pi*(sq-root of (L/g) ). A simple pendulum is defined for small angles. Thus, the approximate period of the natural frequency is 6.28*sq-rt(3/32) = 6.28*0.306 = 1.92 seconds. This is about 1/3 the time I had assumed. Thus, my power would increase by 3 times.

But if you operate at the natural frequency, then make use of the stored energy.

If a DC motor with no gearing is used and pulsed with energy for maybe 1/2 of the period and we assume the swing is 90% efficient, then we need 30*0.879*0.1 = 2.64 ft-# of energy every 0.75 sec or 3.52 ft-#/sec. Apply energy for 1/2 the time, then during that applied time we need 7.04 ft-#/sec. Or 0.013 HP while energized.

This does not mean you can use a 1/78 HP motor. You need to look at motor torque capability. If we use the required torque to raise the 30 # to 0.879 ft and take 10% (losses) the result is 30*3*0.707*0.1 = 6.4 #-ft. Assume the motor is on 50% of the time, then it can be overloaded by 100%. A motor with a torque rating of 3.2 #-ft could be used. If direct drive is used and base speed is 2000 RPM, then HP rating is 1.21 HP.

As soon as you add gearing the motor drive losses increase rapidly and you will require a more elegant motor control vs on-off with current limiting or you won't get the normal pendulum motion.

There may be mistakes above, but it should guide you with some ideas.

.
 
one more question...how much currrent the motor will draw? I mean i want the motor to run on 4 aa size batteries, with total 6 volts. i dont want the current drain to be more than 200 to 300 mAmps. Is it possible to get this torque from this current?
 
081118-0820 EST

starcrossed:

What is 6*0.3? It is 1.8 W.

What is 6*3*0.1 (3 A at a duty cycle of 10%)? It is 1.8 W average.

Based on a resonant circuit, adding lost energy to the pendulum, this is more than enough average power assuming 100% transfer of the energy. Thus, the means and efficiency of the means to supply the energy to the swing is the important subject.

You need to study how you are going to transfer energy from the actuator to the swing and make use of the fact that the swing is a resonant circuit. To force the swing to operate off resonance will require much more energy than at resonance.

The task you have chosen is interesting from the point of view of how to minimize the input energy required.

Go back to your engineering education and consider various ways to do this. Think outside the box.

What country are you from? Are you where various components are easy to obtain?

.
 
All of these baby swings (my terminology) that I've seen, the adult gives the swing a push to get it started.
This eliminates the need for a large motor to give it it's "breakaway" force.

If I was going to do something like this, I would probably go to www.harborfreight.com (or some other like) and buy a cheap 12 or 18V battery powered drill and disassemble it to remove the motor.
I would then incorporate the motor/planetary gear from the drill into my drive system.

This would give you a DC motor with good output torque, a rechargeable battery, and a charger....all for very little money.

Just a idea

steve
 
gar said:
081117-0752 EST

starcrossed:

Using a different calculation and my dimensions:
To raise 30 # to the 45 deg angle at the end of a 3 ft pivot arm is a rise of 3 - 0.707*3 ft = 0.879 ft. This is 30*0.879 ft-# of work. If this is done in 1.5 seconds, then the rate is 30*0.879/1.5 ft-#/sec = 26.37/1.5 = 17.6 ft-#/sec .

Since 1 HP is equivalent to 550 ft-#/sec we should need 17.6/550 HP = 0.032 HP or 24 W.
Click to expand...
I did the calculations mentally and was obviously suffering from brain fade at the time.
Taking your figures, 3ft is about 1 metre, and 15kg exerts a force due to gravity of 150 Newtons (using the approximation that g is about 10)
At an angle 45deg, you get the 15kg displaced horizontally by 0.7m.
That would give a torque of 150*0.7 Nm
If the swing gets from zero to 45deg in 1.5 seconds, that equates to 1 revolution in 12 seconds, or 5 rpm. Or about 0.5 radians per second using the approximation of dividing by 10 for the conversion.
Power is thus 150*0.7*0.5 W
Or about 50W

Of course it isn't that simple.
The power required from standstill to 45deg varies depending on the angle - that's where the difference in your 24W and my 50W comes in.
And, if you went from 5RPM to zero instantly at 45deg, the baby just might not consider it a soothing experience.......

Levity aside, once moving I guess the swing would take power in a roughly sinusoidal manner, positive for the upwards swing and negative for the downward swing. Average power might not be much but the motor still has to provide peak torque and maximum RPM even if not simultaneously.
 
081118-1206 EST

Besoeker:

If we can somehow couple the motor or some other device to the swing such as to synchronously add energy to the swing, then we do not need the peak torque capability required to lift the weight X distance.

.
 
Status
Not open for further replies.
Top