Robotic mower simplified PLZ!!

Thula,

          you have posed an interesting question to which there is no simple answer but to start the ball rolling I'll throw in a few pointers.

With regard to safety, that is paramount and so therefore a fundamental requirement is that the cutting blade is properly guarded against accidental access.  You will never guard against some moron who deliberately sticks their fingers into moving machinery.  Apart from physically covering the rotating blade you could add something like a tilt switch to cut the power if for example the machine was tilted out of its normal plane.

As with many projects it depends what you want the machine to do and you noted some items.  In my opinion you should tackle each function one at a time, once you have successful control of each function then you look at integrating all the functions into the whole machine.

For example you have a motor to power the blade, no big deal if it were a plastic fan on a small DC motor.  However it isn't likely to be a small motor if it is turning a metal blade to cut grass.  For the sake of these notes I am envisaging that the 'lawn mover'  will be a rotary blade similar to a hover mover but on a wheeled chassis, it could, of course, be a cylinder mower as well.  Again if we make the assumption the power source will be a 12 volt battery then there are plenty of DC motors to be found on the net.  Therefore you have to investigate the option to control this motor from a basic relay, which is probably adequate for this particular function, to more sophisticated electronic control.  You could easily add current sensing to detect if the motor is overloaded or beginning to stall.

Next you would need to address the drive to the wheels and steering.  Do you have individual control of each wheel?  Do you have a steering system?  or do you use a tracked machine?

Then have to consider how you control it, radio control like a model aircraft, some form of system using GPS,  and do you build in some intelligence like collision detection?

All of these options are widely covered on robotic forums, Arduino forums etc.  Generally the biggest challenge is up sizing the electronics to cope with the more powerful motors necessary for such a project.

Such a project is easily doable and all the concepts could be proven using light weight models before adding them to the overall full sized project.

I hope that helps but there are so many possibilities it could almost become a thread on its own.

Hi bobcroft

Nice answer, thanks...

ok, so to take up what you've said there, we can start to list what's necessary.  I agree tilt sensor so that's an IMU something.  There's a freeIMU board (EUR49.00, eek!) a whole board with IMU on it http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=39478. 

not sure which board to use so difficult to say just yet whether the IMU comes with it, or whether it's a separate shield. Any suggestions...?
Requirement 1 (shortened to R0001): IMU for protection against accidental access

Pretty much everything I've read has a motor for each wheel (individually controlled) and another motor for the blade

Requirement 2 (R0002): 2 x wheel motors (DC, ...)

R0003: 1 x blade motor (DC, ...)

Definitely need current sensing (found this article: Arduino Current Sensor and this one (slightly easier to understand): Arduino Playground - CurrentSensing ). How does the basic relay you mentioned work, and can current sensing be included into that?

R0004: current sensing for blade motor overload/stall (and/or sensor for RPM of blade speed)

For control: NO control, NO RC, NO GPS - they just take up time and money. First up, i'd rig a perimeter wire to set the boundary, mower would be manually placed within the boundary and turned on by a big red ON/OFF button. it would then drive in as straight a line as it can manage until it senses the perimeter wire and changes direction. It would then just bounce backwards and forwards until it either runs out of juice (then it can give a strangled sigh and stop) or the big red ON/OFF button is pressed by its owner.  Simples. So it can't go too fast because I wouldn't want out-of-breath owners slagging off the design after being dragged around their yard for half an afternoon because the mower was going so fast they had to dive on it to stop it.

R0005: guided by perimeter wire only

R0006: manual start and stop

R0007: 4mph max

It would need a bump sensor as well so it can change direction if it hits something

R0008: bump sensor

Probably makes sense to have a drop off sensor so that it can tell if it's about to fall into an unmarked grave/flower bed

R0009: drop off sensor

probably worth having a few LEDs somewhere to give an indication of charge in the battery, although a (cheap!) LCD screen might also be possible although that's probably more programming than is really necessary.

R0010: battery charge levels (if charge <=20%, light up 1 LED, if charge <=40% light up 2 LEDs etc...)

cheers,

thula

Thula,

               you have made some useful progress in beginning a design specification.  Here are some further thoughts.

R0001 an IMU seems to be an over kill, all you need is a basic tilt switch or perhaps two set a at 90° to each other.  some of these switches used to be a mercury filled glass vial with two contacts, if tilted to a certain angle the mercury 'closed' the contacts and made a circuit.  You have to allow for the fact that the grass being cut may be uneven so some natural angular movement of the mower is likely in normal use.  There are probably much better alternative devices these days since mercury is very toxic.

R0002/3.  This is where you need to sort out the mechanical size, shape, capacity, weight  etc. of the machine.  You need to decide on wheel type and diameter, I would advise a least 150 mm diameter with rubber tyres.  Then you need to decide how to power the wheels via a motor with a gearbox.  Since you will only need a very low RPM from the gearbox you will get a good torque increase and so the motor itself may not need to be huge.  At this stage you need to design the mechanical bits to get an assessment of the weight of the machine including batteries.  This will give you an idea of the likely power / torque needed to move the machine along.  You could look at motor drives from golf buggies or disabled peoples scooters since these are both sources of low voltage battery powered devices designed to carry considerable weight.

R0004  The current sensing modules you mentioned, the ACS range, are perfect for your application and easy to interface to an Arduino as per the articles you listed and lots of others on the net.

The relay drive is easy to implement using, preferably an opto coupled relay to the Arduino.  You need to select the relay so that its contacts can handle the current taken by the motor.  Peter Oakes has also published a very good article on high current drives on E14 so have a look at that.

That's all for now, I need to go to work!

Bob

You are forgetting something, before the mower is tilted it will have already hurt or damaged something.  A robot can not tell the difference between a foot and a ball so a kid playing around the mower can easily get hurt, or a ball laying in the yard could shut off the mower.  Tilt sensors would also only work if your yard is flat enough as well so I doubt many robotic mowers will even have them.

I would start with wireless remote control so you can sit on the porch and mow the yard and work up from that point.  And from someone in the industry I would say the hardest part is getting the job to look good while not hurting anyone and without spending a lot of money.  Remember the lawnmower is one of the most dangerous items that most people use, thousands of kids are hurt each year and hundreds of people are killed around them.

Hi Robert,

thanks for the comments... keep 'em coming. You're right there are loads of injuries and deaths as well. From the fair bit of reading on mowing injuries I've done, most of the extreme ones are from ride on mowers, so not to minimise the injuries from a rotary mower but they seem to be more hands and feet going under the cutting deck, and items flicked out by the mower.  Blade construction could solve the first one - by having the blades retractable on impact i.e. they'd just be loose and pushed out by centrifugal force, but if they hit a solid object they would just flip back - it wouldn't stop injuries completely but it would minimise them. likewise a mechanical solution could prevent the flicking injuries, perhaps a skirt around the cutting area...

Cheers,

thula

Hi Robert,

thanks for the comments... keep 'em coming. You're right there are loads of injuries and deaths as well. From the fair bit of reading on mowing injuries I've done, most of the extreme ones are from ride on mowers, so not to minimise the injuries from a rotary mower but they seem to be more hands and feet going under the cutting deck, and items flicked out by the mower.  Blade construction could solve the first one - by having the blades retractable on impact i.e. they'd just be loose and pushed out by centrifugal force, but if they hit a solid object they would just flip back - it wouldn't stop injuries completely but it would minimise them. likewise a mechanical solution could prevent the flicking injuries, perhaps a skirt around the cutting area...

Cheers,

thula