Reciprocating engine for ornithopter

[rjenkinsgb]

Wikipedia even has an image of such a motor

That's the same principle as the hard drive actuator system, extended. It is not a solenoid actuator in the conventional sense.
You could use a halbach array to avoid the need for the heavy external iron backplates, which link the magnets in that diagram.

There are numerous forms of linear motor. Many use three phase drive, rather than quadrature.

The type Buk suggested would be better with a coil either side of the permanent magnet, that gives more torque; a single side version suffers from inverse square strength reduction as the gap increases.
Adding a "C" frame to connect the outer ends of the coils with a direct magnetic path would give another major improvement.

 

[Buk]

The type Buk suggested would be better with a coil either side of the permanent magnet, that gives more torque; a single side version suffers from inverse square strength reduction as the gap increases.
Adding a "C" frame to connect the outer ends of the coils with a direct magnetic path would give another major improvement.

Funny you should mention that, but its gonna take a couple more hours to process:

 

[spammmer]

I apologize. It's hard for me to understand what you mean.

 

[Buk]

The type Buk suggested would be better with a coil either side of the permanent magnet, that gives more torque; a single side version suffers from inverse square strength reduction as the gap increases.
Adding a "C" frame to connect the outer ends of the coils with a direct magnetic path would give another major improvement.

Turns out that adding a second coil and a c-frame costs so much in weight that it negates any gains. Better to use a bigger magnet and stronger spring on the single sided version.

 

[Pommie]

I think you'll find that modern brushless motors will give the best power to weight ratio.The gearing could be tricky as they spin very fast but will still be far superior to solenoids.

Mike.

 

[Buk]

I think you'll find that modern brushless motors will give the best power to weight ratio.The gearing could be tricky as they spin very fast but will still be far superior to solenoids.

I thought the same thing until I started looking into it.

I looked at various minature bldc motor manufacturers product lines (eg.) at their weight & power ratings and they looked pretty good, until you start considering that you need at least a 2, if not 3 stage reduction gearing and then a mechanism to turn rotation into reciprocation. You can look at 3-5 times the weight of a 6.3mm dia. motor for the entire gears train.

So then I started thinking about the "voice coil" actuator motors they used to use in harddrives:

They could provide the required reciprocating motion directly, using minimal space and lightweight components.
It was just a case of could they provide enough force.

So I ran a simple simulation and found that I could get 20N force using two 2mm x 2mm x 1mm neos and a few hundred turns of 0.1mm motor wire. Still playing with the simulation, and working out what kind of circuit would be required to drive it -- probably a simple variable speed sine wave generator (555) and the ability to reverse the polarity at the appropriate point, which probably means a hall sensor -- but I think it would work very well for this.

 

[Buk]

The smallest free flying, self power robot uses peizo actuators to "supercede the thrust efficiency of real insects":