Build a system to produce smooth sinusoidal oscillations with a variable stroke of up to 8" and variable frequency up to 30hz. The driver required will be around 3 hp.
since this is supposed to be running by the end of august, and there isn't time to machine all the necessary parts, my basic idea was to cannibalize an existing single cylinder engine and connect the drive shaft to a D/C or variable speed A/C . The arrangement will require a connecting rod as the thing that I am driving cannot sustain the side force. The stroke will be varied by allowing for different crank arm lengths.
My question is. Where in tarnation do I find a single speed engine that might be a candidate? Any other ideas for doing this? Any help you can provide would be greatly appreciated.
Jim, That seems like a great idea. I bought the Basic Stamp Homework board and kit and am learning about how these devices work. Right now I'm learning how to move a hobby servo. I can see how an electric valve could be actuated by a signal from a microcontroller. But how easy would it be to write a program for a microcontroller so that Andrew could change both the stroke and frequency with a pot? That sure would make his life easy. ERS
Keep in mind that the motion from a slider crank mechanism (which I think is what you're proposing) is not truly sinusoidal unless the connecting rod is infinitely long. A Scotch yoke is the easiest way to get a sinusoidal acceleration and velocity. I've built walking beam mechanisms for parts feeding with strokes up to about 10", with a linear ball slide taking the place of the slot and follower of a traditional Scotch yoke. Changing the stroke is a matter of moving a pin that couples the motor crank to the linear bearing car.
A picture would be helpful here, but I'm afraid I don't have a CAD model easily available. I can supply more detail if you're interested. The construction is pretty simple.
OTOH, my particular favorite for variable stroke is the twin crank variable phase single using the offset W linkage. It has perfect balance, but is not all that easy to make. Drop me an email if you want a PS emulator for one.
I suspect not very hard. Microcontrollers with built-in ADCs are cheap and widely available, and sticking a scaling factor into some code isn't that hard.
Yeah, except then he'll need a 30 HP motor to drive the hydraulic pump. Also, you'll need a $5000 Moog proportional servo valve, and an encoder or LVDT and conditioner to regulate the acceleration, position and velocity of the cylinder. Of course, this is the way the pros do this sort of work. They use something like a MTS hydraulic test stand which is already set up for tests of this type.
30 Hz is at the upper end of hydraulic testing systems, and the HP requirement goes through the roof as the frequency goes up.
If the energy output was not in the 3 Hp range, electromagnetic shaker tables would do a much better job at the 30 Hz level. But, if you demand force in the 500 - 1000 Lb range, the electromagnetic shakers get to be the size of truck engines, and the drive electronics gets to be several relay racks.
The small engine starts to look good, if this is not on a NASA budget. But, I don't know of any small engines with an 8" stroke! Not even a small truck engine uses a stroke like that, although the over the road Diesels get close.
Turning an engine into a variable stroke mechanism is not going to be real easy. The counterweight needs to be adjusted to properly balance a load WAY heavier than the piston, and unless you put giant blocks of Tungsten (or Plutonium, but you won't be able to get that, and you don't want too much of that in one place, anyway) in the crankcase, you won't be able to fit enough counterweight to the crank.
At 30 Hz, the crank needs to be spinning at 1800 RPM, so the whole rig really does need to be properly balanced, or it will shake itself apart. What the HELL do you need to shake 8" P-P at 30 Hz, anyway? I haven't figured out the G force, but it has to be a whopper! Making electronics to be shot out of guns?
Oh yeah, an MTS hydraulic test stand can do this. I think the one we have at the university can only do modest strokes of 1/2" at 20 Hz, but it is a valve and pump limitation. Ours only has 1" hoses and a 15 Hp pump, I think.
The aerospace guys do this kind of torture testing all the time. Of course, an 8" stroke at 30 Hz is really heavy stuff, and would require a MASSIVE power source, and massive cooling.
I'm sure, for the appropriate quantity of $, MTS or one of the other outfits who make hydraulic test stands could configure and deliver such a machine.
If the oscillations have to be truly sinusoidal, a crank won't do it. It depends on how accurately sinusoidal the motion has to be.
A Scotch-yoke drive would do it. At such low speeds, they can be made with a standard roller bearing on the crankpin, which makes them quite efficient.
However, if it doesn't have to be *that* perfectly sinusoidal, a conventional crankshaft probably is better.
Jim, Why the antipathy toward Basic Stamps? As a novice I'd like to know. I already know that if I start to use these things a lot I'll be buying the microcontrollers themselves because of the price. I've seen circuits that use the bare devices that I can build. And circuits to make the programmers to plug them into. But learning with the Basic Stamp Homework Board with it's breadboard area, battery clip, etc. sure is an easy way for me to learn. Eric
I tried several versions of a ride in Las Vegas where the riders sat on a stage that was sitting on several hydraulic cylinders. You watched a movie and the stage moved in time to the action. It moved very fast. I'm sure that some cylinders cycled faster than 30 HZ. But if it was done for a long time there might be a problem getting rid of the heat. Back to the ride though, one was a race in outer space. There was a point where the spacecraft you are supposed to be driving turns upside down and goes through a hole in a huge plate. It really felt as if you turned upside down. During another part of the ride the spacecraft is struck by an object that cracks the "windshield". When the impact occurs the timing was perfect. It felt just like I was driving and struck something. The ride had several themes. One was a train going down a steep mountain and almost flying off the tracks. Even though the ride lacked the fancy trappings of Disneyland it was the most realistic ride I've ever been on. While my wife and her mom gambled I stood in line with a bunch of kids to go on the ride several times. ERS
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