Positional Feedback of solenoid actuator

[brashear]

Is it possible to get a rough idea of position or velocity without some type of position sensor from a solenoid actuator? I am thinking that solenoid actuators are usually designed so that the inductance of the solenoid increases as the armature moves when the solenoid is turned on. Could this effect be used to roughly detect the position of the solenoid? Maybe it can't be used to detect precisely the position, but can be used to detect when the armature is stuck. For example, perhaps we can monitor the rate of increase of the voltage across the solenoid. If the rate of voltage is faster than normal, it might mean that its inductance is lower and that the armature is stuck somewhere along its travel.

The second question is it possible to detect back EMF (Lenz's Law) during the bounce back period when the solenoid is turned off and the spring pulls the armature back? If cycled fast enough, the moving armature will have some remaining magnetic field due to hysteresis which will cut across the solenoid coil and should generate a small voltage in it. If the armature is stuck, then this voltage would not be seen.

Any thoughts?

 

[waross]

For an AC solenoid, you may detect a stuck solenoid by monitoring the current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brashear]

Could you explain a bit more in detail? How would this be done and what would we be looking for? Thanks

 

[waross]

The inductive reactance increases as the armature or core is drawn towards the center of the core. As the reactance increases, the impedance increases. As the impedance increases the current decreases.
Beware, an AC solenoid that does not complete the stroke may easily overheat and burn out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[brashear]

Wouldn't the AC solenoid only overheat if it was stuck in the position where the armature is far away from the center of the core?

 

[itsmoked]

They typically fry in about one second if they don't completely seat. Your position sensor could simply be a smoke detector.

The inductance changes with extension but not much until the armature is seated.

If you are going to run the solenoid in various extensions you need to use a DC solenoid.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Or pneumatic...

You can buy pneumatic actuators that have a magnet embedded in the plunger so that you can mount a magnetostrictive sensor on the outside as a position feedback device. They are very accurate. Electric solenoids are not intended to accomplish partial strokes, they are On-Off devices. If you have no access to air or hydraulics, you can use a Linear Actuator, they automatically come with position feedback.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[OperaHouse]

Way back I was designing some high speed solenoid drivers. I don't remember any proportional distance info that junped out but you could definitely see when the solenoid closed as a difference in the coil current slope on DC solenoids. This is very useful in determining the speed of relays as you don't have to depend on contacts and associated bounce. Personally, I think the idea doesn't stand a chance.