coil, solenoid force.....

[bob g]

i have a question for you electromagnetic guru's

for the sake of discussion and to illustrate the question assume the following

1.  an electromagnetic solenoid,, say with a 12vdc coil
   
2. the core is extended and is held in the outward position with a spring.. and assume a linear tension over the travel of the solenoid.
   
3. when 12vdc is applied the core fully retracts against the spring pressure..
   
4. at voltages under 12 volts the core retracts at a rate proportional to the voltage.
   
   

now the question..

1. is this rate linear?
   
2. such as if the total travel is 1" at 12vdc, would it be approximately 1/2 inch at 6 volts?
   
3. i guess is the magnetic field proportional to the voltage applied to the coil?
   
4. if not is there a formulae to illustrate the non linear function?
   
   

thanks for the input

bob g

[Flux]

bob g

If you use an air cored solenoid that is fairly long with an iron core that operates from just in to about 1/3 in the solenoid you can get some approximation to linearity.

It is inefficient and not suited to sustaining a powerful pull, but as you mention control you may not be worried about that.

As normally constructed with a steel yoke round the solenoid, the pull is very non linear, increasing drastically as the steel poles come together.

Often if a long travel with a  reasonable pull at the start is needed it is necessary to wind the coil for a lower voltage and arrange contacts to insert a resistor at the end of the travel to prevent burn out.

If you want reasonable linearity over half an inch, over a 2/1 volt range and significant force, you have a challenge.

Others may have more experience on this one.

Flux

[tecker]

  Right I Think you can discount Linearity In most cases you get all or nothing .

Solenoids are usually configured for retract or eject . Poles develop that causes the

armature to seek magnetic stability .

[Electric Ed]

The increase in pulling force is proportional to the reduction in the airgap squared.

In other words, if the remained constant, and you had 1 lb of pulling force with a 1 inch airgap, the force would be 4 lbs at 1/2 inch, and 16 lbs at 1/4 inch gap.

Electric Ed

[Electric Ed]

[edit] That should read - "In other words, if the CURRENT remained constant"

(Wish we could edit our own posts, like most other boards)

Electric Ed

[Norm]

   The voltage to a small PM motor is fairly linear in relation to the speed so if you want a simple homebrewed control like what you might be thinking about.....this might work...small PM motor with the right gear ratio hooked to a speedometer (mechanical type) the more voltage the farther the needle travels....now on a little larger scale instead of the needle shaft it could operate the shaft of a dimmer switch.....( Aha! A  Home-Brewed Rube Goldberg voltage regulator for 'Look what happened to my snowblower'

     Just some wacky thoughts and ideas

                 ( :>) Norm  

[finnsawyer]

The magnetic field strength is actually proportional to the current not voltage.  The internal resistance can become a factor as battery voltage is reduced.  As the iron core is pulled into the core the field distribution and strength changes.  Systems such as this that exert external forces are trying to get to a state of lowest (field) energy.  The iron core has a much larger permeability than air and concentrates the magnetic field resulting in a decrease in system energy.  Lowest energy occurs with the core centered in the coil.  Why don't you get a spring scale and measure the force needed to move the core to various displacements from center?  Then you'll have your answer.  I doubt the relationship is linear.

[JW]

Im willing to second what Finsawyer said.

 To determine the potential "pull", current should be what your mainly concerned with. In some of the most modern trends in auto development 42vdc systems seem to be the direction. It has been observed that low impeadence low voltage coils overheat, with continious cycling schedules. When a coil overheats it gains resistance lowering circuit current, weakening the solenoid pull. However assuming the resistance of an overheated low voltage coil, you could compensate with more input voltage, as ohm's law proves. I guess it all boils down to what your projected coil operating temp is.

 I think what Electric Ed said about air gaps is pretty much right on square.

 Just remember, if you have the perseverence to get thru reluctor and coil designs, you still have to contend with solidstate driving of the switch gear. Which really is not that tuff.

 Have fun

 -JW    

   

[jacquesm]

hm, if you need some linear movement due to a varying current why not loop a few coils on a hinge above a magnet so that the coil can rotate parallel to the magnet.

Increase current->coil will move. And even if it's not perfectly linear it will work a whole lot better than any system based on attraction (inverse 3rd power of the distance to your magnet, if you are 'inside' the magnet it gets better but still pretty nonlinear).

Another way is to emulate the voice coil of a loudspeaker, but that needs some pretty tricky alignment to get it linear.