My pet peeve about my house? The white kitchen floor. It’ll be the first thing to go if we ever remodel, but in the meantime it needs to be cleaned constantly. Unfortunately, it typically gets cleaned once a week; the other six days it looks horrible. So I jumped at the chance to check out the US $250 Mint floor-cleaning robot.
Mint, from Evolution Robotics, isn’t the first floor-washing robot—Scooba, a sibling of the popular Roomba vacuum robot, also mops floors. But there’s a lot of prep to running a Scooba—pick up the room, fill the tank beforehand, empty and clean the tank after, hope it doesn’t run onto rugs or get the floor too wet (a problem for my wood floors elsewhere in the house, which also don’t get cleaned often enough). That’s a lot of work to save some work.
Mint operates more the way I do. It has two modes, making long passes in sweep mode and short back-and-forth zigzags in mopping mode. It can use any reusable or disposable cloths that fit the Swiffer style of manual mop and any type of floor cleaner—even a vinegar-and-water mix worked fine. I also already had Swiffer-type mop pads, though the microfiber reusable ones packaged with Mint worked better.
Mint’s first pass was awesome, although it didn’t save me any time because I followed it from room to room, marveling. I purposely left the cotton throw rug in front of the sink; Mint scrubbed neatly around it, lifting the edge just a tiny bit to get underneath. It buzzed along the walls, got into the corners, painstakingly worked around table legs, and scooted under furniture.
Upstairs, in sweeping mode, it slammed on the brakes to avoid zooming off the top of the staircase, pirouetted, and headed back to push an embarrassingly large clump of dust bunnies out from under the bed. When Mint’s done, it chirrups cheerfully and then goes back to wherever it was when it started. It’s otherwise very quiet; we typically didn’t hear Mint running in another room, except for the occasional thump when it found a wall.
After butting up against the same obstacle a few times, it would pause for a moment and ponder its situation before taking another approach. It got into trouble in two places—on a heat intake floor grating (the wheels seemed to catch in the grates), and between the rockers of a child’s rocking chair: When trying to back out, the Mint pushed the rockers to a new position and seemed baffled. When it does get in trouble, it tries to extricate itself and then beeps. If no help comes, it shuts down. This was reassuring—it meant I could leave the house when Mint was operating.
All told, it’s a pretty smart little robot. It doesn’t just wander around randomly; it maps out the area it’s cleaning and covers it efficiently. To do this, it needs a little help, in the form of a tiny cube that you set on a counter and point to the center of the room. The cube shines two infrared spots on the ceiling, and a processor on board tracks those spots. Another processor, an ARM7, does the bulk of the computation, creating the map and understanding Mint’s location on it.
Remarkably, the Mint doesn’t need those infrared spots to be in the same room it’s cleaning. The cube, which the company calls the NorthStar navigation system, typically sat on the kitchen counter while Mint was cleaning my office, after passing through the living room and turning left to go through the open office door. The robot is a little less efficient in these situations, because it pokes out of the distant room occasionally to check its map calibration, then goes back in to continue its cleaning. It manages with the help of a gyroscope, along with optical encoders on the wheels that track wheel rotations. Its front bumper has two switches that let it know when it’s stuck—or when someone steps on it, in which case it stops in place, rather than scooting ahead and causing a fall. Two other mechanical sensors detect drops—that’s why it didn’t zoom off my staircase. It sends an infrared signal ahead of itself, so it slows down before touching a wall, instead of just crashing into it. This system got tripped up sometimes when an obstacle didn’t go all the way to the ground; Mint would crash into it at full speed. Even so, it didn’t damage anything. (It did push a bedroom scale under a dresser, however.)
Life with Mint was working out pretty well, particularly for marital harmony—instead of ignoring the increasingly filthy kitchen floor, my husband or I would say, ”Want to start that robot thingy?”
Then Mint had a bad day. A series of bad days, actually, getting stuck in places it hadn’t before—at the edge of the kitchen where there is a slight drop, in the fireplace, or under the dishwasher. I figured my successful Mint runs had just been luck. But the light dawned when Evolution CEO Paolo Pirjanian explained in an interview that one design challenge had been figuring out what to do when the wedge-shaped robot jammed itself under low furniture. Mint’s usual response is to increase the motor power beyond its normal operating parameters and try to yank itself out. However, if the floor is wet and slippery, that yank doesn’t work, and it gets stuck, in spite of traction control mechanisms on the wheels. Uh oh, I realized, those bad days were my fault: Those areas get particularly dirty, so besides wetting the cleaning pad attached to Mint, I splashed some extra cleaner on those floors, figuring Mint would then get them that much cleaner (it seemed like a good idea at the time).
I also had some concerns about power consumption, because Mint runs for 3 hours or so before it needs a recharge but takes at least twice that long to charge. Pirjanian explained that Mint charges its nickel metal hydride batteries very slowly: Its designers found they could build a much safer trickle-charging system at a lower cost than a fast-charging system, and they also believed that most of their customers would typically charge overnight.
Could I sweep and mop a lot faster than Mint does? Sure. But is my house a whole lot cleaner after a month of living with Mint? Absolutely. By the way, it rained this afternoon; did you see the footprints the kids made? Could someone start up that robot thingy in the kitchen?