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Author Topic: Where/What parts for remote heavy duty platform?  (Read 2115 times)
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Hi,

I'm looking for RC 4 wheel car type platform that can carry at least 10-20lbs. load.

It can run slow but needs to work on carpet.  I would like to be able to put a 1/2" thick maybe 12" x 12" box on it to carry soup cans, books,etc... across rooms on carpeted floor.

There are a ton of RC cars, parts out their but I think they are built for speed not carry heavy loads.

Is there such a platform out there, what names would I search form?

Anything on ebay that might do the job?

Hopefully pre built modules, really don't have the mechanical background to build from scratch.

Thanks
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Phoenix, Arizona USA
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10-20 lbs of payload on carpeted floor for a robot chassis isn't going to be cheap to find. Do you have -any- mechanical aptitude - or are you willing to develop some (hey, you are developing robots - surely you know that this is something you need to know, right)?

If so - then I would suggest looking into using a modified 6 volt DC PowerWheels (or similar) ride-on toy. They will easily work on carpets, they are larger than an R/C car, but still small enough to not be too much of a nuisance inside a house, and they are meant to carry small children (easily 10-20 lbs). The drawback is that you have to build your own interface to the motor that drives them, and you have to come up with a solution to control the steering of the vehicle (a gearmotor, or a powerful R/C servo, or a small linear actuator all could do the trick - but there will be some mechanical modifications to be done - hope you are handy with tools!).

Beyond that, unless you are willing to spend a fair amount of cash ($200.00 or more - typically much more), building your own platform may be your next best option. A piece of plywood, some angle brackets, a couple of nice surplus gearmotors (from All Electronics or Electronic Goldmine, or similar online surplus outlets - or repurpose a couple of cheap cordless drill gearmotors from Harbor Freight), and a couple of wheels mounted on the motors, a battery, etc - you can easily make a simple differential drive two-wheel (plus casters) robot platform.

Take a look into this book (if you can find and purchase the older editions - get all of them; it is well worth it, believe me!):

http://www.amazon.com/Robot-Builders-Bonanza-Gordon-McComb/dp/0071750363

It details how to build platforms like I've described (and has done so for around 20 years now); you might also find this book enlightening, too:

http://www.amazon.com/Building-Robot-Drive-Trains-Robotics/dp/0071408509

Both of these books will teach you enough to give you the skills needed to start building your own platforms; from scratch or otherwise.
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Thanks for the links and info.

Another expensive hobby!   $1K-2K for just motors,wheels and some metal. smiley-eek

I did some more digging and decided try the surplus car seat motors, at 9.50 each + SH.
Will try just as you mentioned, differential for steering. Thought of stepped motor for steering but don't want
to complicate it further, keeping simple as possible, simple aluminum tube frame.

The motor details: Draws 1.3A no load, stalls at approx 9.5A and spins 2,500 rpms.

Does that mean it needs 9.5A to stop the motor?!?!?

I'm planning to use the L298 arduino shield. It says it's rated 2A per side or parallel for higher power.

Some quick questions maybe you can answer before I get the book.

1. motor says 1.3A no load, so how much current could it increase with load, is only way to know by building it up?
   my concern is if it draws like 5A or more, then that would blow out the L298.

2. any info on how to current limit the L298 for safety? the datasheet mentions adding current sense resistors but does
not show any equation on how to calculate the resistor value.

3. I should use Low on Enable pin to stop motor, since Fast motor stop limit is 2A?

4. Lead acid battery best for this high current application?

Thanks
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Thanks for the links and info.

Another expensive hobby!   $1K-2K for just motors,wheels and some metal. smiley-eek

I did some more digging and decided try the surplus car seat motors, at 9.50 each + SH.
Will try just as you mentioned, differential for steering. Thought of stepped motor for steering but don't want
to complicate it further, keeping simple as possible, simple aluminum tube frame.

The motor details: Draws 1.3A no load, stalls at approx 9.5A and spins 2,500 rpms.

Does that mean it needs 9.5A to stop the motor?!?!?


No, that means if you were to mechanically lock the motor shaft preventing any rotation and applied full voltage the motor would draw 9.5 amps.

I'm planning to use the L298 arduino shield. It says it's rated 2A per side or parallel for higher power.

Some quick questions maybe you can answer before I get the book.

1. motor says 1.3A no load, so how much current could it increase with load, is only way to know by building it up?

Well there are math solutions if you know all the variables involved, max motor torque, RPMs under load, amount of load, etc.
   my concern is if it draws like 5A or more, then that would blow out the L298.

Most likely. Some motor controllers have self protection circuitry added to simply shutdown if too much current is attempted to be drawn by the motor.

2. any info on how to current limit the L298 for safety? the datasheet mentions adding current sense resistors but does
not show any equation on how to calculate the resistor value.

Maybe check schematics of various vendors motor controller boards/shield that use that chip to see how they implemented over current protection.

3. I should use Low on Enable pin to stop motor, since Fast motor stop limit is 2A?

4. Lead acid battery best for this high current application?

They are usually the most cost effective rechargeable battery type for higher current applications. Li-Po packs properly sized give the best energy density values, but at higher costs

Lefty
.

Thanks

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I did some more digging and decided try the surplus car seat motors, at 9.50 each + SH.

Something to keep in mind: Since you are planning on carrying a load, it is likely that these motors do -not- have the bearings necessary (radial bearings) to carry the load, so you can't just put wheels on them and expect them to last for any length of time. Instead, mount your wheels on shafts, and mount the shafts to your platform using some kind of bearing - pillow blocks with ball bearings or bushings will be fine. Then attach the motor's shaft to the shaft of the wheel (use a piece of rubber hose and a couple of hose clamps or something). That way, the bearings (instead of the motor) will carry the load of the robot.


Will try just as you mentioned, differential for steering. Thought of stepped motor for steering but don't want
to complicate it further, keeping simple as possible, simple aluminum tube frame.

Differential steering is definitely easier to work with, vs Ackermann.

The motor details: Draws 1.3A no load, stalls at approx 9.5A and spins 2,500 rpms.

Does that mean it needs 9.5A to stop the motor?!?!?

Stall of a motor occurs when:

1) The motor is under a complete and full load such that the shaft no longer turns. Generally gears in a gear motor break long before this, though.
2) It can also occur (very briefly) upon startup of a motor - the current will "shoot up" to the stall current before settling back down.
3) If can also occur if you switch the motor rapidly from "forward" to "reverse" (which isn't good for a motor or the mechanics of a gear box - so don't do it).

The result of #2 (and 3) is that you need to get an h-bridge that can handle these conditions (plus 15-25% extra - never design to the margin, always allow for some overage). So - you would want an h-bridge that can handle about 12 amps per motor. That's not a huge h-bridge, but it isn't small, either.

I'm planning to use the L298 arduino shield. It says it's rated 2A per side or parallel for higher power.

Won't work for that motor - so forget about it.

Some quick questions maybe you can answer before I get the book.

1. motor says 1.3A no load, so how much current could it increase with load, is only way to know by building it up?
   my concern is if it draws like 5A or more, then that would blow out the L298.

I've already answered this: At startup, the motor will draw about 9.5 amps (rated stall condition).

2. any info on how to current limit the L298 for safety? the datasheet mentions adding current sense resistors but does
not show any equation on how to calculate the resistor value.

The L298 already has built-in current limiting (thermal overload protection). But you likely can't use the L298; I wouldn't recommend it for this motor, anyhow.

4. Lead acid battery best for this high current application?

As noted, cost for amperage is nice for this type of battery, but they are heavy. The next best (they'll cost more, but weigh less) would be NiMH packs (commonly used in the R/C hobby world). Next would be LiPo packs (again, RC world - very light, but very expensive, and must be watched while charging; they have a nasty habit of exploding/bursting into flames when overcharged).

Interestingly, there are companies that make "drop in replacement" LiFePo4 batteries for many of the common SLA/Gel-Cell lead-acid batteries commonly used for alarms and UPS systems. They are very lightweight, but very expensive. Something to consider, though!
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