View Full Version : My dumb droid

03-17-2014, 09:13 AM
This idea, like many, I am sure, has evolved from a simple idea - maybe I have gotten carried away? I am shopping for 8-10 acres of land now and was trying to decide what kind of equipment to use to manage it. 3 acres of it will be plowed and sown with some kind of grain. I will have a large green house to grow veggies, fruits and flowers. The terrain will mostly be flat with some hills. My original idea is that I wanted an electric tractor to pull a plow, a disc, tiller, etc as well as a vehicle that I can use to transport materials on it (up to 500 pounds) or pull a small trailer with about 2,000 pounds. Then I realized that if it had a crane/arm it would prove to be even more useful - I am not as young as I used to be - so I added the requirement that it have a dexterous arm that can lift 200 pounds and life and manipulate about 60 pounds. Later, I realized that if it would fit into my greenhouse, it would be doubly useful, so I decided it has to be 36" wide, that if it had a big, healthy battery pack it would serve as a mobile power station for other appliances (water pumps). Some of the properties that I am considering get snow, so being able to plow snow was put on the list. I decided that I would like the vehicle to have the format shows in this picture, though possibly with 6 wheels - 6 wheel drive, though maybe 6 wheel drive is not necessary - and not tracks, with an additional electric motor to power a pto (other option is to go with tractor accessories that have their own gas or electric motors. Then...I thought, gosh, if this were remote control, or had a cpu, I could have it plow the fields and sit under an umbrella sipping on mint tea, or even go inside and read a book...so that is the present status of my plan, or just go inside and do something else. I want the top speed to be at least 10 km/hr. 1.5 to 2 mph for the plowing, etc would be more than fast enough. The land will not be "virgin" land, meaning that it will already have been contoured and prepared. Hub motors might save some complexity and could deliver the power needed to do the farm work, but I am not sure how much hp/torque I would need. I have looked at geared hub motors for electric vehicles rated at 1 and 2kw at 350 rpm. I am talking to the mfg about an option to reduce the top rpm. Most robots use stepper motors, I know, but it seems to me that they would not produce the brute strength that I need. 5439It is certainly still on the drawing board. I am hoping to learn about the power requirements - motors & controllers - now, and then will tackle the cost/feasibility of the arm/crane. The rest of the specs of my droid will be determined by these two aspects.

03-17-2014, 11:03 AM
Most robots use stepper motors

I don't think that's generally true -- certainly not for locomotion. As you say, stepper motors don't have a good torque/weight ratio.

Are you looking at pre-existing software and control circuitry? If you start from scratch, just a simple thing like "drive in a straight line" is surprisingly difficult!

03-17-2014, 05:44 PM
Stepper motors are used primarily in industrial spaces for arms and CNC control. AC motors, and increasingly BLDC motors, are used for mostly constant velocity drive situations (fans/blowers, conveyors, elevators, etc.). Most bots use DC (brushed or brushless) gearmotors for powering their wheels because they are light and compact for their power output and very easy to control.

Adding an arm would probably best be solved with hydraulics. Industrial, electric robotic arms capable of 200 lbs payloads tend to be really expensive and usually have a large cabinet of electronics to control everything. A hydraulic arm would use parts common to most other construction and farm equipment, which are easily acquired, repaired, and replaced. It would still be pretty expensive, but a lot easier to maintain and control than an industrial robotic arm (a motor to drive the pump, then a couple valves per piston).

Tommy_T has a very relevant project thread (http://forums.trossenrobotics.com/showthread.php?4314-More-Mule-then-Rover).

03-18-2014, 08:44 AM
Thanks! Yes, I read Tommy_t's thread and will follow it. Starting from scratch? I want to do this in as simple a way as possible. I am not even sure if this will stay as one machine or will split off into two. One will certainly save $$ but will increase the complexity. Can robotic arms use hydraulics instead of stepper motors? I am thankful that I have about a year and a half to do the research and testing on this, before I will start building. I am collecting data now on robotics, to see if it will pay off in time savings and convenience for me, as well as saving my back from throwing disks. For right now, the most useful piece of detailed information that I need is the specifications and characteristics for hub motors. Most power curves for electric motors that I have seen are relatively flat - rpm vs. power. If I can get a hub motor that turns at 150 rpms, using 13" wheels I will get 11 mph out of it - an adequate speed, especially if torque is high at the lower speeds needed during plowing, etc. My first phase is the design for the tractor/farming purposes, second to add the accessories to the droid and then look at how to connect the necessary electronics to the machine to make it all sing :)

03-18-2014, 12:52 PM
What's your budget? There exists various mini-tractors or bomb-disposal robotics chassis you can start from.

03-18-2014, 05:45 PM
I am hoping to finish it off with $3,000.

03-18-2014, 10:58 PM
$3000 USD gets you a gasoline powered riding lawnmower (either entry-level zero-turn-radius version, or deluxe ackermann-steering version). It will not buy you very much of what you describe from any manufacturer, especially if it is to be a purely electric vehicle. The battery pack and charger(s) together will probably be at least $1000 USD, even if you go with crap Lead-Acid batteries that will last only a couple years before requiring replacement.

If you have experience building/welding metal structures, then you might be able to build something close to what you want. Batteries will still take a large portion of the cost. Possible bright-side of a massive, high-current battery pack is that there have been people using them for welding (cannot remember where I saw it, but they basically tapped into their golf-cart's pack after charging it).

03-19-2014, 04:41 AM
Thanks! I can have the frame/platform made for about $500 (I live in China so can take advantage of cheaper labor), buy the motors for about $600 and the battery pack and charger, as you said, for about $1,000....probably. I say "probably" because I have yet to determine the size of motors needed. The size of motors needed and amount of use time required on a charge will determine the size of the battery pack, and therefore the cost of both the motors and the battery pack.

03-20-2014, 03:46 PM
My guess is you'd be better off with a small Diesel or Gasoline engine.
If I were to build that kind of tool, I'd look for a small Bobcat mini-backhoe, and then adapt the steering to be remote/autonomously actuated.
But Bobcats cost more than $3,000...

Working the soil is heavy duty with lots of things that can go wrong, and farm equipment, even being designed for that heavy duty, often breaks.