# When Solenoids Fight, Which one Wins?

Discussion in 'General Electronics Chat' started by squatchy, Dec 10, 2012.

1. ### squatchy Thread Starter Member

Nov 21, 2012
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If two identical solenoids are fixed to a common based, and provided the same power, I assume they will be at a stalemate provided that the red Christmassy twine connecting their pistons does not break.

But what is the relationship between the two solenoids when their provided voltages are not equal? If one is given 9v and one 8v, will their pistons adjust so that the central point is somewhere between, but not at the center? Or will the one with more power completely overtake the other?

Why I'm asking: I'm exploring new ways to build a linear motor which can be controlled for robotic/animatronic purposes. If a reliable algorithm for two battling solenoids could be developed to position point anywhere between them, that would be very helpful. I'm open to better ideas.

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2. ### kubeek AAC Fanatic!

Sep 20, 2005
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There is so many variables in this that you can´t really know until you try it, and each solenoid model will be different. Solenoids are generally designed to be either fully on or nothing, so I guess the linearity will be pretty bad, but there may be some solenoids designed for a more linear action.

3. ### tshuck Well-Known Member

Oct 18, 2012
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Theoretically, they will reach a stalemate, though in practice, no two solenoids will have the same characteristics. Each will have slight imperfections as to how the magnetic field is generated, impurities in the slug(affecting inductance), differences in windings, especially if hand-wound(but they probably wouldn't be), and much more... These imperfections will leave you with less than accurate positioning along the axis of pull. That being said, it should already disqualify this from actual positioning in a robotic application.

If you were to power them unequally, the one that gets more current, in theory, will be stronger and have more pull than the other solenoid.

A rack and pinion driven with a stepper motor gives you much better accuracy, but without knowing the application, a good solution is only a stab in the dark...

4. ### squatchy Thread Starter Member

Nov 21, 2012
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So if it has more pull, will it win entirely (100% pull), or will the amount of pull resemble a ratio of the current each is getting? In other words, would a 12v versus 9v solenoid pull the center point all the way toward it, or only partially?

I realize this must be dependent on the type of solenoid, but I'm curious about the general way these magnetic fields will battle against each other.

5. ### tshuck Well-Known Member

Oct 18, 2012
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As you said, each solenoid will be different. It depends on a myriad of things, but generally, as the slug is pulled into the solenoid, it will be drawn more into the center, meaning the one that wins a little will win a lot...

6. ### squatchy Thread Starter Member

Nov 21, 2012
43
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So then for a solenoid, generally, the energy required to "pull" its plunger decreases with the distance the plunger is already pulled?

A fully extended solenoid that takes 12v to "pop" would take less than 12v if the plunger were placed halfway before activation?

7. ### kubeek AAC Fanatic!

Sep 20, 2005
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First, unless you mean a spring-loaded solenoid, there is no energy required to pull the solenoid apart from friction, no energy gets stored in the plunger.
Second, the main factor that decides the force is current as with any onther electromagnet, and then the depth of the plunger.
The relationship between plunger depth, current and force is very strongly dependant on the magnetic circuit - lenghts of the coil and plunger and materials used. So you could have one solenoid that goes almost only on-off, and there could be solenoids that can do more of linear motion. But I would agree that you should need less current to exert the same force with a plunger half way in.

8. ### tshuck Well-Known Member

Oct 18, 2012
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hmmmm... there are, again, a number of things that would go into this as the more a slug is in there, the more friction is has to overcome, but I can't imagine that that should be an issue.

Again, the current is what dictates your solenoid actuation, not the voltage. It is the current moving through the coil windings that generates a magnetic field which attracts,in this case, the slug toward the center of the solenoid. As this happens, the inductance increases, causing an increase in the inductance of the solenoid, which, in turn, causes impedance to go up, meaning current decreases.

The [warning-PDF] figure on page 2 relates current,position, and velocity of a solenoid.

Last edited: Dec 10, 2012
9. ### squatchy Thread Starter Member

Nov 21, 2012
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Can anyone think of any existing commercially-available solenoids that could be purchased and pitted against each other in this way to arrive at a controllable linear motion solenoid pair?

10. ### tshuck Well-Known Member

Oct 18, 2012
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Not anything with any reliability.... temperature does murder to theory

11. ### kubeek AAC Fanatic!

Sep 20, 2005
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I think you could get slightly better controlability if you pull against a spring, but still it won´t be very accurate.
But then again, a voice coil from a speaker is basically a solenoid pulling against a light spring, which is made for pretty linear response, but only within a few millimeters from the center point for large (think 10" - 15") speakers.

12. ### John P AAC Fanatic!

Oct 14, 2008
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Solenoids are so highly non-linear in the amount of force they produce versus the plunger position, that any contest of this kind is going to be won by the unit that has its plunger nearest to home at the start. If you think of "balance" between them, it's a case of "balanced on a knife edge" where it's immediately going to fall one way or the other. The only way you can make this work is with position feedback of some sort, and a clever control system.

strantor and squatchy like this.
13. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
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so what some hydraulic systems use is fluid dampened solenoids. Either solenoid has a fluid volume it must displace through an orifice to gain movement. Adjusting the orifice adjusts speed.