Fixing one of Rovio's wheel servos

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antec
antec's picture
Fixing one of Rovio's wheel servos

The other day I was having fun chasing my dog around the house, when suddenly Rovio began deviating to the right side. Then I lost the right wheel altogether. Thinking of all the posts on this forum about defective wheel sensors, I figured that now after months of almost daily use, my bot now had the same problem. So I took Rovio apart, and to my surprise, it was not at all what I thought. If anyone is experiencing a problem like this, the first and easiest thing to do is to swap the sensors, from left to right. Still have the same problem with the same wheel not wanting to turn with a known good sensor? Then the problem isn't the sensors.

I then figured either the problem was inside the MCU unit (beyond my capabilities to fix) or a problem with the servo itself (possibly repairable depending on if it can be replaced). As it turned out, plugging in the opposite servo to the problem servo's MCU board socket revealed that the MCU was still functioning normally. Looked like a bad servo...

I then disassembled the servo itself. As it turns out, the electric motor itself had developed a problem. When a movement command was given, the motor would stall for a while and then spin a little weakly.

So, to make a long story short, a little cleaning of the armature's commutator and the brushes inside the electric motor, and Rovio is back to prowling the house again. Might be helpful if folks are aware that a dead wheel may in fact not be the fault of the sensors. At any rate, if the servo itself cannot be replaced, the motor itself looks like an extremely common electric motor that could be either purchased or found in many smaller electric RC cars.

GWJax
GWJax's picture

Nice find and great troubleshooting. If you have any photos of this procedure please post it for the rest of the members.

Jax

WeJa
WeJa's picture

Hi antec,
thank you for this great post.
I have the same problem, but unfortunately I can't figure out how to disassemble the servo without destroying something. Could you give me a hint how you separated the wheel from the servo?

antec
antec's picture

Well, now I wish I had taken pictures of the whole process! But here's how you would go about servicing one of the wheel servos if you are sure that is the problem:

The first thing you need to do is remove the wheel on the side that is broken. To do this, take a really tiny screwdriver or a knife, and pop the center cap in the center of the wheel off. Then, if you look straight down the center, there is a screw that goes into the center of the servo's axle. The wheel should pull right off revealing a keyed plastic shaft.

Then you obviously need to remove the six screws that hold Rovio's upper half of the body together. Now, before taking the two halves apart, turn Rovio over. You'll notice that there are more screws in the bottom of the black lower half of the body. You have to take all of these screws out, not just by the defective wheel. What is confusing is that the lower bucket that makes up Rovio's body is actually two halves in of itself, in addition to the upper part with the lights and neck on it.

Now, inside of Rovio, you'll notice there are screws holding plastic brackets around the servo in question. Remove these. You also have to remove the clips that hold the two vertically oriented circuit boards. Now, you'll see that there are a bunch of screws down in the bottom of the bucket that hold the two halves of the black plastic bucket together. Remove all of them. This really is a pain, but you have to separate the two halves of Rovio's lower plastic bucket to be able to get any one of the servos loose. With the halves able to be separated, you can withdraw the servo in question.

Removing the motor from the servo is easy with a tiny screwdriver. Only take the screws out of the servo that hold the cap over the back of the motor. As far as finding an exact replacement for the motor, I found many that were the same physical size, but had different winding counts in the armature and different fixed magnets. (meaning different torque and speed than my original motor) So, if your motor has gone bad in the servo, your best bet to fix your problem is to just find a motor of the same similar type, take it apart, and harvest the rear plastic portion with its brushes intact. Save your original motor's metal casing and armature, and slide the rear plastic portion of the new motor with its new brushes already in place onto your original motor's casing and armature. Do this and you will be rewarded with a replacement motor that spins with the exact same speed and torque as the other side. Unfortunately the exact make and model of Rovio's stock motors is all but an obscurity. But luckily the stock motor is standard in size enough that you can use parts out of any other similar motor, and it will work.

Hope this helps

doctpd
doctpd's picture

Thanks.. very helpful advice. Mine did the same. Lost power and wouldn't reverse. I dismantled the motor on the duff wheel to find one brush had worn through completely and fallen off and the commutator was worn with a central groove. They ain't high quality motors ! As suggested I easily found exactly the same dimension motors on eBay for 1ukp each advertised with the spec..

"Miniature DC Motor, 3Vdc Voltage Rating, 1.046W Power Rating, 16000rpm No Load Speed, 12407rpm Nominal Speed, 1.001A Load Current, 8.2g-cm Maximum Continuous Torque, 36.6g-cm Stall Torque, 19.9mm Diameter, 2mm Shaft Diameter, 25mm Long, 17g Weight, 0.29A No Load Current, 34.81% Efficiency, 8.1mm Shaft Length, Zinc Plated Steel Casing, Two Flat Sides Make the Motor Ideal for Mounting on a PCB, Rotates Counter-Clockwise when Viewed from Shaft End, Solder Tag Termination
Operating voltage 1.5 to 4.5V DC
Two flat sides make the motor ideal for mounting on a PCB"

Given that there may be differences in the windings/torque/etc I bought 3 and replaced all wheels so that they were all the same. Before desoldering the old motors check the polarity compared to the new ones and make sure you get the rotation direction and polarity right. Rovi is now reassembled and bombing around on his new motors.

antec
antec's picture

It's good to hear when folks are able to solve problems like this with a little good old fashion tinkering! I think of Rovio like a really high-tech R/C vehicle. I never expected any RC car that I ever bought to not eventually require repair. But it seems some undue cost cutting really made some stuff break awful fast! My Rovio has been driving around my house used regularly for about a year and a half now, and in that time has had two motors replaced and is running on two of my homemade IR wheel encoders. Also ditched the original power supply for a 3A 9V transformer and now have excellent battery life. With all those problems ironed out, its been nice using Rovio to its fullest potential.

Waynosan
Waynosan's picture

Do you guys know of a good source for Rovio replacement motors?  One went back on my unit and I want to replace all three since I'm already in there.  Thanks.