Linear Actuator (magnetic field repellant force)

[Korichnevijgigant]

I am working on a home made electromechanical actuator to replace the vacuum actuator on the front axle of my Truck.

The vacuum actuator isnt the greatest and I have replaced it and the system lines few times.

SO I figure, I know what force the vacuum actuator is putting out, I know the stroke length, I have stuff to make this, why not?

BUT I want to get the physics right in my head first.

SO, Lets say I have a spring that will push a cylindrical magnet away from the shift fork, and a solenoid made from wiring that push the magnet into the shift fork. Which engages the hubs.

I already figured out the spring portion and drawn out the magnetic field interaction.

Does anybody have an equation to get an accurate (or conservative) repelling force for the solenoid and the magnet?

I havent really found a good source, wiki has some, but I dont trust wikipedia.

Thanks

 

[dgallup]

First off, before getting into force and the fact that repelling is a lot more difficult than attracting, how much travel do you need? Magnetic actuators are NOT good at acting over the sort of long strokes that hydraulic or pneumatic actuators typically work over.

Second, you can do some back of envelope calcs but they are not too accurate because they neglect a lot of effects. There is a free 2D magnetic FEA solver called FEMM that will do a much better job.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Korichnevijgigant]

its not much stroke, if I remember right, its just between .25 to .5 inches

all the shift fork does is push a splined coupler over to connect some splined shafts.

I will take a look at that FEMM.

I appreciate it

 

[IRstuff]

That's probably too much already. The field at a pole drops off faster than one might expect.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[dgallup]

Most of our magnetic actuators stroke less than 100 microns. A quarter of a mm (250 microns) is considered a huge stroke. However, ours have to move very fast, not a consideration fro you.

Power dissipation may be an issue for you, I assume the actuator has to stay in either position for long periods of time. You may want to look into a latching solenoid design.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Korichnevijgigant]

Thanks guys. I was hoping it would be easier than that.

It would have been an elegant solution. The transfer case as is has a hole that is threaded for the vacuum switch which supplies vacuum to the front axle. When you engage the 4wd it pushes it in. So I found that this hole and the switch design was almost identical to the reverse light switch. So I thought I could just stick another reverse light switch in there to power the coil for the solenoid. No other components needed.

I had initially though of a regular electric motor that would turn a worm gear and engage the shift fork, but then I would need a way to control the travel of the fork and then reverse the polarity to disengage it. Got pretty complex pretty quick.