# Material Property Effects On Magnetic Circuit

1. Aug 21, 2014

### carpekd

I'm building a linear actuator and I don't have much experience with magnetic circuits...

A rough sketch of what I'm building is attached. I'm trying to determine if the materials I'm using for the shaft and shaft collar will have a detrimental effect on overall force output. Right now, my shaft is made of 1566 steel and the shaft collars are zinc plated steel (don't know the material number, but it is magnetic). I've noticed that when the shaft, magnet, and shaft collars are assembled that the attractive force of the magnet is reduced vs. when the magnet is by itself. (I'm just putting a piece of metal close to it and felt the attractive force). I'm assuming that the shaft has a low magnetic reluctance and is directing a good bit of the flux through the shaft, but i'm not sure. I'm also assuming that this would reduce the performance of the actuator (overall force). My plan is to replace the shaft with a 303 stainless steel shaft and nylon or aluminum shaft collars. Would this be a sound plan? Could someone explain what is happening to the magnetic field in this application?

Thanks.

#### Attached Files:

• ###### actuator.gif
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2. Aug 22, 2014

### Baluncore

I do not understand how the proposed linear actuator will work.
What is the orientation of the coils?

You need to draw in the lines of magnetic field that you require.
You must work out where to place the magnetic conductors = iron? to make the field follow that path.

If you want it to operate fast with AC signals you will need to use iron powder based magnetic conductors.
The permanent magnets may not work well at AC because of skin depth in their conductive coating.

3. Aug 22, 2014

### carpekd

The coil will be a cylindrical coil with back iron on the outside (not shown). The coil will be energized with dc current and will operate on the voice coil principle. A similar commercial product is described here: http://www.designworldonline.com/moving-magnet-voice-coil-actuators/#_

My concern is the detrimental effects on overall force production due to magnetic field interactions with the shaft/shaft collar materials...

4. Aug 22, 2014

### Baluncore

The shaft should be non-magnetic or it will short circuit the permanent magnet path.
The collars on the shaft should be magnetic as they are part of the magnetic circuit.
There needs to be a magnetic outer shell to carry the external field from one end to the other.
That shell should include magnetic end plates that guide the shaft.
The cross section area of the shell and endplates should be similar to PM section area.

The shaft will exert an axial force determined by DC current.
If solid iron is used for the shell and end plates there will be a slow response to changes in coil current.

#### Attached Files:

• ###### actuator2 .gif
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5. Aug 23, 2014

### Baluncore

A voice-coil has a moving coil in a permanent magnet field. The permanent magnet field can use thick iron components because the PM field does not need to change. The moving coil has a low mass and a high frequency response, maybe 10 kHz.

You are building a transducer that uses thick iron components about a solenoid coil with significant inductance. The mass of iron and the inductance of the coil will reduce the frequency response of the changes in field, probably to less than 1Hz.

Your armature has permanent magnets that have more mass than a voice-coil in an air-gap. But that does not really matter here since your stator field will be so stable.

Because the bandwidth of the control loop is so low, your actuator will be very hard to position accurately. It will need to be used in a way that applies a force, but does not move anything.