Build a Robotic Lawn Mower: Is It Worth Pursuing?

[kevindeschamp]

I have been Thinking about building a robotic lawn mower. Right now, I am thinking of powering it with a 3000 watt generator. The mower would be driven by four ac electric drill motors and the mower would cut grass with one 12 amp motor. The drill motors would draw about 3 amps each. I need about 140 ft lbs to drive the estimate 200 lbs up a 30 degree incline.
Is this a concept worth persuing?
Thank you

 

[Mech_Engineer]

Picking out the motors and power source is relatively easy compared to deciding how you will control the beast. How will it know where it is? How will it cover the yard? What happens if it gets stuck? How will it avoid bumping into things? Will it be safe to be around with children and/or animals in the yard? There is a lot of control system hardware and software development that will have to happen.

Using a 12 amp motor for the blade sounds a little small to me, but it will depend on the cutting width (blade diameter). Most lawnmowers have 5+ horsepower driving the blade for a 22" width, so I might start there for a spec.

Also, drill motors by themselves will be useless as drive motors because they will have a speed of multiple thousand rpms. This means you will need gearboxes for them.

 

[BenchTop]

You really need DC motors to be able to control speed.

You can also regulate the load on the blades by enclosing them in a disk so only the ends do the cutting. The amount of exposed blade will determine the power required.

Your main problem after that is coverage of area & boundaries.
That's going to be your fun part. You can work that as simple as feedback circuits or as complex as microprocessors plus.

 

[Mech_Engineer]

You can also regulate the load on the blades by enclosing them in a disk so only the ends do the cutting. The amount of exposed blade will determine the power required.

The power required will be determined by how fast the mower is moving forward. The power requirement will be different for the same mower going 1mi/hr or 3mi/hr. Slower is probably better anyway since it will have to sense its environment and at the same time reduces power required for the drive wheels.

 

[kevindeschamp]

Mech,
I use a 12 amp black decker mower with a 19 inch blade now on my lawn. Works great even with leaves and tall grass.
I only want to use it in the backyard because it is good size.
I plan to use some of the component from the RoboCut lawn mower. Here is the link:http://www.robotshop.se/micro/wwwrc_us/indext.htm

BenchTop,
I have been looking at corded AC dewalt 1/2 drills with a torque increase of 3. I think that the motors inside are DC motors. The dewalt website says that the motors are good from 0-600 rpm. Can I reduce the motors easily to make them crawl?

 

[Mech_Engineer]

It sounds like you've got a handle on the blade motor power, just make sure the speed is in an acceptable range as well.

For the drive motors, you might look for a motor/gearbox combo here: http://motion-controls.globalspec.com/SpecSearch/Suppliers/Motion_Controls/Motors/DC_Motors . Depending on the diamter tires you plan to use, you can specify a desired shaft output speed and see what you'll need to achieve it.

 

[kevindeschamp]

Mech,
Thank you for the constructive criticism and that link is awesome.

 

[Mech_Engineer]

I have to say I am attracted to the idea of having a little robot slave to mow my lawn. But I'm lazy enough as it is, mowing the lawn every other week at least gets me off my butt

 

[BenchTop]

Mech,
I have been looking at corded AC dewalt 1/2 drills with a torque increase of 3. I think that the motors inside are DC motors. The dewalt website says that the motors are good from 0-600 rpm. Can I reduce the motors easily to make them crawl?

Yes. DC motors can be speed controlled by pulse width modulation or by voltage, with feedback or without.
You'll need to know more about the motors ratings to proceed with design.
Higher voltages means more expensive.

 

[BenchTop]

The power required will be determined by how fast the mower is moving forward. The power requirement will be different for the same mower going 1mi/hr or 3mi/hr. Slower is probably better anyway since it will have to sense its environment and at the same time reduces power required for the drive wheels.

Quite right. The typical set of variables, as in milling, is feed, speed, materials and number of teeth.
In point of fact, however, the blades do virtually all their cutting on the outermost edge and the rest of the blade mostly provides unneccesary friction and a little shaft to wind crap up on.
Weed whackers employ the trick of a disk to solve that

 

[kevindeschamp]

Can I control the speed of a ac motor cheap and easily? Is there an ac motor with awesome starting torque?

 

[Mech_Engineer]

Can I control the speed of a ac motor cheap and easily?

Cheap and easy are relative terms, but it can be done. Overall I think it is probably "easiest" to control the speed of a DC motor, but I'm not an electrical engineer. You can find off the shelf speed controllers for most any motor, and their prices will vary widely.

You might read about the different kinds of motors to gain a better understanding of their pros and cons:

http://en.wikipedia.org/wiki/Electric_motor
http://en.wikipedia.org/wiki/AC_motor
http://en.wikipedia.org/wiki/Brushed_DC_electric_motor
http://en.wikipedia.org/wiki/Brushless_DC_electric_motor

Is there an ac motor with awesome starting torque?

All you need to do is decide what an "awesome" starting torque is, and then do a search for a motor fitting your requirements on GlobalSpec.

 

[Topher925]

If you have no experience working with electromechanics of any kind, then you should definitely only focus on DC motors. When choosing a DC motor for an application you don't really care so much about torque, you care about the power band. What ever motor you choose it will have to be geared and that gearing can be changed depending on what kind of torq and speed you want. Also keep in mind that unlike AC motors, DC motors often have a bell shaped efficiency curve so its very adventagous for the motor to operate at an RPM which as at the top of the curve.

 

[dr dodge]

make it a "z-rider" style (zero turn radius) and then use the weed eater style head. that way it does the trimming, too. electric RC cars have some pretty good solid state DC motor controllers. if you set it up as an RC mower, the good radios have a usb connector, so you could control it with a laptop/pc remotely. remember one other thing, the conventional mower is made to get the yard done ASAP. being as this thing could just cruise around everyday, I'd think light small cut and 3 days to get the yard done.

dr