Roomba repair - common problems

It's quite a while I play with Roomba.
Here a list of the common problems for these little robots (3rd and 4th generation) placed in the most probable happening order.

Battery

THE Roomba common problem is of course the battery.
On average, at least once per year, the battery dies.
It depends about the use of course, and contrary to the common sense, more a Roomba is used, more the battery last.
But as said, on average the battery last about a year.

NiCd batteries are lasting even less, mostly due to the "memory effect".
NiHi batteries are better.

Deck/motor

The second typical  problem that can happens to your Roomba, is in the deck motor or in the deck gearbox.
The deck motor is the electric motor that drives the brushes.
Typically the problems in these areas are happening when the Roomba entangle in wires or thick carpets.
When trying to remove itself,  more trust is placed on the brushes and if this happens often, after a while the mechanic can be damaged.
Actually there are different kind of problems that can happens to that area  :

1. broken motor gears
   The motor has a gearbox used to increase the torque.
   There are 3 gears inside the motor and they are made in teflon or plastic.
   The typical problem is that one or two of these gears, broken up, losing one or more "teeth".
   The final result is to have the motor spinning and  the main shaft not moving.
   It's enough to replace the gears to fix the problem.  Unfortunately  a new set of gears can cost more than the motor ... so often is more cheap to buy a new motor with a new gearbox (they come as single piece).
   
2. broken motor
   More rare but it happens that some internal connections broke.
   In this case the only thing is to replace the motor.
   
3. broken electronic
   The motor is driven by some electronic. Specifically a Mosfet is feeding the motor.
   Sometime, even more rare than the broken motor, is the electronics that fails.
   In this case "in theory" is enough to change the mosfet to fix the unit.
   Unfortunately it is extremely difficult because you need to take TOTALLY apart the Roomba in order to extract the electronic.
   Then you change the component and in order to test if the repair is correct, you need to put back totally the unit !
   It's about 2 hours work and if something goes wrong, you need to do it again !
   Unless to have a Roomba simulator , in order to test the electronic without the need to put it in a Roomba, is one of the worse case scenario for repairing a Roomba.

Also the deck gearbox can have problems :

1. broken gears
   Like the gears in the motor, the gears in the deck gearbox can loose teeth or broke down.
   Having other broken decks is possible to substitute the broken ones.
   
2. broken gearbox cage
   A couple of times I had to repair a gearbox deck cage broken.
   The gearbox cage is screwed to the deck and if broken can not keep the gears in the right place.
   Change the gearbox deck cage or the entire deck.

Wheels

The third typical problem with Roomba are the wheels.
The Roomba wheel is quite complex object. It is composed by four parts :

1. electric motor
2. planetary gearbox
3. sensor
4. tire

Each one of these elements can develop problems, but the most recurring problem is on the sensor.

The sensor is used to determine the speed  of the wheel and is very simple.

A special wheel with some openings is mounted on the main shaft and the light from an infrared LED can reach a photodiode only when an opening in the wheel happens.

The problem is that dust can, and does on time, cover the photodiode, so that become insensitive to the infrared light.

The result is that Roomba loses feedback from the wheel, starting to act crazy.

The phenomenon is well known as circle dance and usually cleaning the sensor solve the problem.
With some luck can be cleaned with compressed air, otherwise the wheel needs to be taken apart.

In all these years I own many Roomba, I never saw a planetary gearbox broken.
It is rare, but is possible to have tires used so much that the rubber is incredibly thin or the electric motor defective.