solenoid vs pnumatic actuator

[solson16]

Okay, here's the deal
I'm looking to find a small linear actuator, either a solenoid or a pneumatic actuator triggered by a pic microcontroller and whatever else ends up being necessary.

I need about an inch of stroke, and the force requred is in the ounce range (essentially just needs to move itself plus 4 or 5 grams) but I'm trying to get an actuator that can do this somewhere near 100hz ideally that way when a delay during hold open is taken into account, it can still cycle around 30hz.

If a solenoid can reasonably do this, what kind of information is available to me to design it? And if not, can a pneumatic actuator and solenoid valve reasonable accomplish this?

Could a permanent magnet (rare earth type) be used for the plunger of the solenoid, this way it can be dual action and not need a spring for return (thus reducing force requirements?)

Or is this just not feasible in a cost effective manner?

Thanks in advance for the advice,
Shane Olson

 

[israelkk]

Working at 100hz meaning that the cycle time is 10msec. Therefore, the 1 inch stroke has to be accomplished in 5 msec. Just to move the 5 grams 1 inch at 5 msec requires a 10 N force. But to build a solenoid that can give 10 N force from a distance of 1 inch you will need a very large solenoid with a massive plunger which in turm will weight 10 times the 5 grams rasing the force needs to as much as 100 N, and so on.

On the other hand a pneumatic actuator contolled by a solenoid valve can develop high forces but the combination of the 1 inch travel and 100 hz seems to be a real challenge. I had the opportunity to develop and design fast ON-OFF electro-pneumatic valves with 1 msec up to 4 msec ON and OFF time delays (electrical delay plus mechanical delay) and it was not easy. Therefore, to achieve the 5 msec you need the pneumatic acuator to be extreemly fast and don't forget the pneumatic delay time to fill the actuator inside volume. So the actuator volume should be minimal, on the otherhand it has to be sized to provide the force to complete the 1 inch travel in 1-2 msec.

 

[solson16]

.025m/.005s=5m/s ave, 10 m/s end
10m/s/.005s =2000m/s^2
2000m/s^2*.005kg=10N

huh. indeed. maybe I should have done that before..

well, pneumatic it is. difficult but acheivable at least?
thanks again.
any tips?

 

[GregLocock]

Look at a long stroke subwoofer, as used in doof-doof cars.

You should be able to find one with a 1" stroke, and 100 Hz is obviously within their capabilities. Then cut some windows in the loudspeaker cone and you are all set.

At the very least the coil and magnet size would give you a good idea of the required design.

However 1" stroke at 100 Hz is, frankly, an awful lot for an electrodynamic design. I think you'll end up with molten coils. To get that reliably we'd use an electromagnetic shaker that would cost $50000, and have watercooling, and a power amp the size of a fridge.



Cheers

Greg Locock

 

[MintJulep]

100 Hz will be a serious challange for pneumatics.

I'd suggest hydraulic, or even pure mechanical. Cam or crank and linkage.

 

[MikeHalloran]

As Greg didn't quite fully explain, a solenoid with a permananet magnet plunger is called a voice coil.

The rotary voice coils used to move the heads in hard disk drives are capable of 100Hz response; note that they use relatively long lever arms to move milligram loads about an inch, in an arc.

Just as in a disk drive head actuator, your eventual solution will probably involve a mechanical displacement amplifier (e.g. lever arm) designed for minimum mass and high acceleration, and the actual force driver will work over a much shorter stroke.

Along the way, you will learn to design for stiffness, not just for strength. Points of reference; airplanes resonate around 7Hz, buildings a bit lower. Hydraulic fluid compressibility has significant effects well below 100Hz. Air at ordinary pressures is just not going to get you there, and at high pressures it becomes dangerous in at least two ways.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[MintJulep]

Mike,

There is no shortage of hydrauliclly actuated shaker tables that appear to operate just fine from about 5 Hz to well over 100 Hz. What compressibility effects are you refering to?

 

[MikeHalloran]

The fluid's bulk modulus, acting over the actuator area, behaves like a flexible structural member at particular frequencies. Lots of fluid motion in, no mechanical motion out. In our helicopter flight simulators, we shifted the command signal frequency a little to either side of the anti- resonance. I don't recall the details from 30 years ago, but I remember the notch was at a surprisingly low frequency, and surprisingly narrow.

In a shaker table given pseudo- random excitation, you'd never notice such a thing. I wonder if you'd detect it in a slow sweep.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[MintJulep]

Meitherike,

Interesting. But it seems tunable by either selecting a fluid with a different density, or changing the volume of the actuator.

 

[Barry1961]

A stepper motor with a crank.

Barry1961