Quick review: Zumo Robot Kit for Arduino

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For today I have a quick (and unexpected) review: the Zumo Robot Kit for Arduino (v 1.2) from Pololu. This weekend a friend of mine came asking for help to assemble one Zumo kit he bought for his kid. He opened the kit content and felt intimidated by the content, and came to me as he knows I’m good at soldering. He told me he ordered the kit by mistake, he should have ordered the assembled version. Anyway, a good opportunity for a review…

So, what’s so intimidating:

Pololu Zumo robot kit for Arduino: unboxing

Pololu Zumo robot kit for Arduino: unboxing

Oh, I see. Lots of small parts there… It takes a bit of elbow grease to put this thing to work. Just kidding! The amount of soldering needed is not much bigger than soldering the connectors onto an Arduino Shield, and the mechanical assembly is well described in the manual. Note that the user manual is available only in pdf format and has to be downloaded from Pololu’s website.

Assembling the Zumo Robot kit

First, some mechanical assembly. The idler sprockets are fixed with the appropriate shoulder bolts and washers. Care should be taken not to overtighten the bolts, as the chassis is quite fragile.

Then the motors have to be installed. The kit came with no motors, so I’ve added two 6V HP 100:1 motors from my collection. The recommended motors are 75:1, so my robot will be a bit slower.

Pololu motors, with the polarity sign highlighted

Pololu motors: mind the polarity

A little surprise here: the motors have polarity, and the marking is quite hard to see. If you install the motors with the wrong polarity – don’t worry, it can be fixed in software, but some minor changes in the code will be needed. So, I have first soldered the wires, then I have placed the motors inside the chassis. Note that after placing the top shield the motors won’t move anymore.

Pololu Zumo: assembly detail

Pololu Zumo: assembly detail

Now it’s time to assemble the electronics. The Zumo shield comes with only the SMD components installed, and one has to solder the buzzer, buttons, and the Arduino connectors. So, first I have soldered the buzzer, then the buttons and finally the connectors for the Arduino Uno. I decided to skip the jumpers for the buzzer and the I2C connections. The height of those jumpers can later interfere with the board on top, and I prefer to have as much free space as possible available. I have placed two solder bridges instead, one to enable pin A1 to read the battery voltage, the other to connect the buzzer to Arduino pin 3 – I will use an Uno to drive the robot.

With the shield configures, I placed the two spacers on top of the chassis, then the shield. Notice the wire from the motors coming out of their holes. After the shield was securely fixed with the screws, I soldered those wires and trimmed them short.

Pololu Zumo: assembling the shield

Pololu Zumo: assembling the shield

Once this assembly stage is done, it’s quite hard to disassemble the robot, but not impossible. Given the choice, I would replace the motor wires with some longer, flexible wire. There’s a bit of space between the motors, so the cable can be fit in there.

This leads to another issue: one needs a careful planning before assembling the robot. I find quite difficult to modify the robot to add extra sensors and such one the assembling is completed. With this being said, here’s the assembled robot, ready to go:

Pololu Zumo Robot kit for Arduino

Pololu Zumo Robot kit for Arduino

Some conclusions

First of all, I would like to emphasize that this little robot meets size requirements to participate in Mini Sumo competitions. Even without motors, sensors and Arduino boards, the Zumo Robot kit for Arduino is one of the cheapest and simplest ways to enter the competitions realm. Or it might be a good starting point to build your final year project.

No motors!!! One has to add it’s own motors. Here comes the fun – there are plenty options regarding gear ratio. Lower gear ratios will mean a faster robot, but with less pushing force. Higher gear ratios will bring you a more powerful, albeit slower robot. I think it will be fun to see the same chassis but with different motors in direct competition.

No sensors!!! The kit comes without sensors. At the very minimum one has to add the Zumo Reflectance Sensor Array. The line sensing array is needed both for line follower configurations, and for sumo competitions as well. In sumo tournaments you need to know when the robot is being pushed out of the dohyō. Then some distance sensors have to be added to “see” where the opponent is – add as many sensors as you can afford and can fit onto the chassis.

In many sumo competitions, for safety reasons, the robot must be equipped with an IR kill switch to be operated by the judge. The role of the start/kill switch enables the judge to give the start of the competition, and to cut the power to the motors in case something goes wrong. I have seen many robots using the kill switch module from http://www.startmodule.com/. It would have been nice to have a dedicated connector for this on the Zumo shield. One must use the small prototyping area and wires to add the kill switch.

The Arduino Uno sits upside down – this creates a nasty set of problems. First, some Arduino clones will not fit due to restricted space. Second, there’s little room for expansion. Only a handful of my Arduino shields will be able to fit between the Zumo shield and the Arduino board. Second, you might wish to add some (plastic) protection on top of the Arduino board. When in sumo competition, the adversary can come on top of the Zumo robot – short circuits can happen, disabling your robot.

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