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one on the right is a geared DC brushed motor. You may
be surprised to learn that the stepper motor shown is
significantly less powerful than the smaller DC motor next
to it. At six volts, the stepper motor probably cannot reliably
run faster than about 50 RPM, whereas the DC motor runs
happily at 250 RPM.
Because of the nature of a stepper motor, it will
actually get weaker as you increase its speed — much
weaker unless you employ carefully designed power circuits
and use PWM (known as a chopper in stepper jargon) to
power the motor. These circuits are commonly used with
stepper motors that run CNC routers and similar tools.
As you have surmised by now, I don't recommend
stepper motors for your balancing robot. They will be
heavy, power hungry, and possibly not fast enough without
a lot of engineering dedicated to their drivers. You will get
much better performance (and probably faster success)
with the DC motor and encoders.
Q. I have had my Roomba for maybe three years now, and have noticed that it has gradually gotten less and less running time on a charge. I leave my
Roomba on its charger when I am not sending it out
cleaning my rooms.
I have also noticed that my battery pack gets very
warm after it has been on the charger for a while. What is
wrong and how can I fix it to get my vacuum cleaner back
on the job?
A. You don't say what type of Roomba you have. At 3+ years old, you could have either a NiCd battery pack or a newer higher capacity NiMH pack. Regardless,
the symptoms that you describe point clearly to a damaged
battery pack that has probably been losing cells. This power
loss is very typical of a battery pack that has been abused
by a cheap charging circuit. Leaving the Roomba plugged in
allows the charger to continually trickle charge your pack.
As more current is absorbed initially, the battery gains
charge. After it has become charged, if the charge voltage
is still there you begin over-charging the cells and they get
warm. Then, they get hot and start taking on damage. I
would have thought that iRobot would put a more
sophisticated charger in their robots, but, maybe not. You
have a few options to fix your problem:
1) Buy a replacement battery pack. This will fix your
problem, but it is likely the most expensive solution.
2) Find and replace the damaged cells in your pack.
There may be more than one bad cell. This will
mean that some cells will be "fresher" than others,
and it may speed the demise of more cells in the
3) Build a new pack using some other technology that
doesn't use the Roomba's internal battery charger.
Solution #1 will be the easiest way to get your Roomba
up and running again. As stated, it will also be the most
certain and the most expensive solution. Everything in the
real world revolves around three related forces: time,
money, and effort. If you don't have the time or a way to
expend the effort, then money will solve your problem.
Solution #2 works well if you have the time (but not
the money) to crack open the pack and find the bad cells.
All you need is a voltmeter.
Measure across each cell. I am willing to bet that most
of the cells will be 1.4V (fully charged), but one or more
will measure at a very low voltage — perhaps even 0V.
These are the damaged cells you will need to replace.
Find cells with a similar rating (the pack will have its
mAh — milliamp hour — rating printed on it). This is the size
of the cell you are using. My guess is that you have a NiCd
(Nickel Cadmium) pack. Look for battery cell retail sellers.
The real hassle will be soldering them to their mates in
the cell. I just use needle-nosed pliers to pop the thin metal
tabs from the bad cells and solder these tabs onto the new
cells. I recommend using a fine file to scrape off the
coating on the poles of your new cells. They won't hold
solder well until you do that.
Solution #3 may or may not be cost-effective and is
certainly the most time-consuming way to fix your problem.
So, of course, that is what I did when my Roomba did the
same thing as yours! Here is how I replaced my Rooba
I always scour the Internet looking for interesting deals,
and this year I found someone selling off a bunch of 2,400
mAh Lithium-Ion rechargeable cells for cheap. I bought a
couple dozen thinking that I could make some powerful
robot packs with them. These were the big cylindrical cells,
not LiPo (Lithium-Polymer) flat packs. They looked like they
would fit, so, a project was born!
First, I had to remove the old pack from the case. The
screws in this pack required a triangular bit to get them
out. This is no barrier to a dedicated hacker! I found a hex
bit for one of my old multi-tools that I didn't care about, so
I went to the bench and ground it into a triangle shape.
Eight screws were now sitting in a cup, but ... surprise, the
lid was also glued on!
Okay, this is another challenge to my determination. I
scored the glue with a hobby knife and used a flat-blade
screwdriver to "pop" a corner, then worked around the
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