I think I've figured out how to make the 220 lb robot drive train, please corre
latigerlilly
Posts: 114
Hi guys,
I didn't know that you could run a 24 VDC motor with just 12 VDC. Armed with this knowledge, this is what I think I'll choose for a drive train for my planned robot wheelchair with robot arm. Estimated max weight of passenger/payload and bot should be 220 lbs or less (hopefully much less).
I plan to use two NPC T-64 motors, each with 0.7 HP at 24 VDC. I estimate that if I run it at 12 VDC with the Parallax HB-25 motor controller, it will probably output 0.35 HP. This should not overload the max 0.5 HP rating of the HB-25. At 24 VDC, the maximum (just before stall) HP is 0.95 HP at 43.1 amps and 181 rpm. Therefore, the max amps should be under the max 25 amps rating of the HB-25 controller if I run it at 12 VDC. The amps should be 1/2 that at 24 VDC or 21.55 amps. Stall amps is 110 at 24 VDC, so as long as I ramp the motor and not strain it by doing quick reversals without ramping I should be fine.
I am choosing the NPC motor for direct drive because NPC sells hubs and wheels that bolt directly on the gearmotor. It is the easiest way to put together a drive train. It is not cheap, but I think it should do the job with a minimum of fuss. Here is a link to the motor: robotcombat.com/marketplace_gearmotors.html . I think that the motor with direct drive might grind the gears if it gets caught where it cannot move, but power is applied. I will caution my friend to not apply additional power if he is stuck. I can also attach a counter that will count how many partial revolutions the motor has moved. If it expects movement where there is none detected, then power to the motor will be automatically cut off. Also speed will be limited to 1 to 2 mph to limit possibility of damage to the motors. The HB-25 has a fuse, overamperage, and overvoltage protection, so I will not attach any fuses. I will build a set of brakes with an electromagnet attached to a set of bicycle brakes: scientificsonline.com/product.asp?pn=3060435&sid=merbizrate&mr:referralID=89a555ce-b113-4a16-8900-86765b91a57f .
What do you think guys?
Thanks,
Lilly.
I didn't know that you could run a 24 VDC motor with just 12 VDC. Armed with this knowledge, this is what I think I'll choose for a drive train for my planned robot wheelchair with robot arm. Estimated max weight of passenger/payload and bot should be 220 lbs or less (hopefully much less).
I plan to use two NPC T-64 motors, each with 0.7 HP at 24 VDC. I estimate that if I run it at 12 VDC with the Parallax HB-25 motor controller, it will probably output 0.35 HP. This should not overload the max 0.5 HP rating of the HB-25. At 24 VDC, the maximum (just before stall) HP is 0.95 HP at 43.1 amps and 181 rpm. Therefore, the max amps should be under the max 25 amps rating of the HB-25 controller if I run it at 12 VDC. The amps should be 1/2 that at 24 VDC or 21.55 amps. Stall amps is 110 at 24 VDC, so as long as I ramp the motor and not strain it by doing quick reversals without ramping I should be fine.
I am choosing the NPC motor for direct drive because NPC sells hubs and wheels that bolt directly on the gearmotor. It is the easiest way to put together a drive train. It is not cheap, but I think it should do the job with a minimum of fuss. Here is a link to the motor: robotcombat.com/marketplace_gearmotors.html . I think that the motor with direct drive might grind the gears if it gets caught where it cannot move, but power is applied. I will caution my friend to not apply additional power if he is stuck. I can also attach a counter that will count how many partial revolutions the motor has moved. If it expects movement where there is none detected, then power to the motor will be automatically cut off. Also speed will be limited to 1 to 2 mph to limit possibility of damage to the motors. The HB-25 has a fuse, overamperage, and overvoltage protection, so I will not attach any fuses. I will build a set of brakes with an electromagnet attached to a set of bicycle brakes: scientificsonline.com/product.asp?pn=3060435&sid=merbizrate&mr:referralID=89a555ce-b113-4a16-8900-86765b91a57f .
What do you think guys?
Thanks,
Lilly.
Comments
In stead, I'd look at the NPC 2212 or 41250 on the same page. These are "worm gear" drives, and would be "self locking" in that the way the gears work, I don't think you'd be able to "drive" the motor from the wheel end.
Also, you can/should NOT, under ANY circumstances, depend on telling your friend "don't apply additional power if he's stuck". While this may be good advice, in some cases, there may not be alternatives, and you'll need to have other mechansims (like the sensors you describe) to prevent problems. Temperature sensors on the HB-25s and/or motors may also be a viable alternative, and may be a good idea anyway.
Also keep in mind that fuses only offer protection for things that are "down stream" of them. The fuses in the HB-25s won't protect the circuits between the battery(ies) and the HB-25s. This may, or may not be an issue, depending on what's in between said batteries and HB-25s, and what the liklihood of problems such as vibration or rubbing causing a short are.
Again, I would SRONGLY urge you to try your setup on some smaller type device, for example a "hacked" toy. You've got to learn about things like differential steering, how PWM works with motors, steering with low radius turns, etc. Doing this one full scale out of the gate, and with no/minimal electrical/electronics experience is asking for trouble. You might also want to look at some of the advanced Lego sets for ways to prototype this. I don't know that the "robotics" kits are what I'd go for, but the "Technics" sets might be a good source of parts to use for experimentation. It would be important for the "prototype" to use the same methods for steering (and braking) that your full scale version would use.
Remember, you are talking about building a device that a human will be dependant on. We're not saying "don't do this". What we are saying is that there are a gazillion ways something like this can fail, and "the consequences of failure could be extreme." The various ways that this can (will) fail will not be readily visible until after the fact. I can almost certainly promise, after you build it, and test it, and your friend is using it, he will find a way for it to fail, and it will fail at the worst possible time, possibly in a truely spectacular way. That's just the way life works. Been there. Done that. Survived. Did it some more, and will keep doing it.
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John R.
8 + 8 = 10
I'm really upset. I just found out that iRobot (the company that makes the Roomba robotic vacuum) makes killer robots for use in Iraq and Afghanistan: www.brown.edu/Students/INDY/archives/2005-03-10/articles/spread-smith_robot-war.htm . I own a Roomba! Did I help finance this?! All the technology for a robotic wheelchair with a robot arm exists, but there is no commercially available unit you can buy. A 17 year old girl has to build one for her friend. Why? Because outside of the defense industry, there are precious few professional roboticists. That's because all the money is in killerbots. "The root of all evil is money," it says in the Good Book. How true. How sad.
Boys think it's fun to shoot guns and blow stuff up. Girls think it's fun to play with dolls and pretend to cook. If more girls build robots, we'd have robot butlers instead of robot killers. Did you know that the dishwasher machine was invented by a woman? Her name was Josephine Garis Cochrane: www.ecolitgy.com/it/josephine.html
www.uh.edu/engines/epi1476.htm.
I really enjoyed the Bicentennial Man movie (I cried)....
By the way, thanks a bunch to all the guys who answered my questions. I will be posting pics when I'm done!
Much appreciation,
Lilly.
Post Edited (latigerlilly) : 1/31/2007 2:15:06 AM GMT
I would echo most of John R's sentiments about carefully proceeding on this project -- but I also know you've heard that already from a number of members of the forum (including myself)
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When the going gets weird, the weird turn pro. -- HST
kelvin
You don't halve the power output by halving the voltage...
Remember:
W= V * A
A= V / R
A = Ampere
R = Resistance(In the motor)
V = Voltage
W = Watts
The resistance of the motor is pretty much a constant, so half the voltage means half the Amperage which means a QUARTER power output...
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Don't visit my new website...
As far as "real" comment, I agree with those before me who admonished you to remember that this device is intended to be a functioning part of a living being's life and so, engenders all of the responsibilities pertaining thereunto (can you tell that my wife works for a lawyer?). In short, every effort should be made NOT to "rely" on the human to be the "safety device" (i.e. don't drive too fast or apply power while you are up against the wall), but rather implement automatic safety devices/indicators with any over-ride capabilities necessary to deal with "out of bounds" conditions. I admire our drive and desire to make the world a better place, but don't let idealism cloud reality.
Good Luck!!
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Truly Understand the Fundamentals and the Path will be so much easier...
But that sounds cold and heartless to even say that, so I'll just emphasize how big of an issue safety is here (I know you know that already). It really is a matter of life and death. Just last weekend a man was burned alive and killed in his electric wheelchair because it caught fire and he couldn't get out of it. Remember Murphy's Law when you're building this.
Don't know if you live near any companies that build robots, but it would be great if you could get a mentor to help you or a company to sponsor you. Any colleges near you with an engineering department? It would really be great if you had a professional to run ideas by.
Best of luck to you. I can't even believe a 17 year old is taking on a project like this - I am extremely impressed with you. I wouldn't even know where to start!
Good luck, and be safe.
Update -- robotcombat.com looks like a great source, substantial stuff
Post Edited (PJ Allen) : 2/3/2007 5:32:56 PM GMT
Oh and from experience, make sure there is a way to disconnect the power/motors FAST for whoever is trying to ride this... trust me, something unexpected will happen, and deep-cycle marine batteries are not light.
Good luck!
A·kill switch wired as close to the battery as possable is a good idea.· I wouldn't recommend using the chassis as a ground wire.· Cars do this to save money, but it makes short-circuits much easier and can introduce noise issues.· If you really feel paranoid, put a fuse on both the ground and power wire from the battery.··That should even·help protect from some shorts·in the battery box.·
I look forward to pictures of this thing,
Marty
P.S. I think you'll be fine!· just don't make·any 0-awg wires glow orange [noparse]:D[/noparse]·
I should also mention that the Basic Stamp2 can talk to these speed controllers and many other speed controllers that except a PWM signal normally from a radio receiver, that we use in large robots.
Parallax products Rock!
When combined with the technology that has been deveolped for large combat robots anything could be possible.
Robotic combat stuff.....
http://www.teamkiss.com/
http://www.teamkiss.com/newkiss1220.html
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Post Edited (slamer) : 2/26/2007 11:37:20 PM GMT