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farmer
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Posts: 13461
Joined: December 16th, 2002, 7:09 am

gas powered robot?

August 23rd, 2017, 1:22 am

My neighbor stopped mowing lawns so I decided to build a robot. For many tasks including cleaning and painting. My requirements included

1) be sturdy enough to carry and guide a weed whacker
2) be able to move quickly on smooth terrain
3) be able to go up the various types of stairs and uneven land necessary to cut all my grass

Through suspension prototyping, I moved from aluminum for an indoor-outdoor robot, to 5/16 steel. to 3/8 steel for basically an outdoor robot. So this is a pretty heavy robot that needs some pretty strong motors. I had assumed I would use multiple brushless electric motors. Because that seems to be the standard for large-scale RC cars and such. Gas engines are a very sloppy power source. So far as I know, they are adjustable by throttle and gearing. Whereas electric motors offer much more precise control. For a nimble and precise robot.

But I would not even use a battery-powered weed whacker, that would be ridiculous and would suck miserably. I am worried that the motors I am going to purchase will require too much battery and will not last very long. And the necessary batteries will add so much weight. that the suspension will need to be sturdier, the engines stronger, and the batteries even larger, so that it might suck no matter what the combination.

Maybe electric power is only practical for the extremely easy scenario of a vehicle which goes on a flat, smooth road, or on rails. When I see those MIT guys testing that four-legged dog robot, for example, it is usually hooked up to power cables. How long can that beast really go on batteries? I saw a poster in the hardware store illustrating how many logs you can cut with a cordless electric chain saw. But I have thrown all my rechargeable tools in the trash, the cordless ones are a source of endless disappointment.

What do you think? Can a device like that robot dog, or anything where the weight is not babied by a smooth road, be very useful running on batteries?
 
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Traden4Alpha
Posts: 23951
Joined: September 20th, 2002, 8:30 pm

Re: gas powered robot?

August 23rd, 2017, 2:10 pm

Gasoline has about 100X energy density than even the best Lithium batteries which is why cordless tools suck, drones only last 15 minutes, and electric cars either need huge batteries or can't go very far.  If you want your robot to run all day without swapping battery packs every 15 minutes, a gas engine-generator is probably the best solution.  It could be hacked together by putting a chain saw or lawnmower engine on an alternator from a motorcycle or small car (with suitable gearing to match RPMs).  You might need to add a small battery or power management logic to mediate between the spiky power requirements of the motors and the smoother power output changes in gas engine.  Also, a fly wheel on the engine (with the engine mounted so the shaft is vertical) would also buffer energy and provide some gyroscopic stability for the robot.

You'll want to run through bit of math on the power requirements or run the robot tethered to wall power and watch the pattern of wattage so you can pick the right size alternator and small gas engine. And obviously you'll have a bit if design iteration or math when you add, for example, the 10 pounds of gas genset to the 50 pounds frame, need to boost the frame a bit, need another 20% power so have to go to a 12 pound genset which adds 2 pounds etc etc. ending up with maybe a 13-15 pound power plant.  But it will work out.  Obviously engineers can successfully build motor cycles and ATVs that go extremely fast on relatively small gas engines.

Finally, the total power requirements for flat-land operation will be a strong function of the efficiency of your suspension and motion system.  If you look at animal's legged motion, it's all based on pendulums and spring-loaded linkages that require little energy to start movement and keep moving.  The legs and legged motion are designed around natural resonant frequencies with most of the kinetic energy in the legs and body being recovered internally by swinging, springing, and bouncing motions.

Have fun!
 
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farmer
Topic Author
Posts: 13461
Joined: December 16th, 2002, 7:09 am

Re: gas powered robot?

September 1st, 2017, 9:11 pm

You have any thoughts on this?
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