Automonous Bot: Arduino, Ardumoto board, and the Dagu Magician Chassis
Late am I with this posting, but life gets in the way and takes over. The Little Weed wanted to delve into robotics, so we bought an Arduino Uno and the Dagu Magician Chassis. This chassis is a robot platform for a three wheeled vehicle, complete with two wheels and two motors, a single ball caster, and all the nuts and bolts to put it together. Programming of the robot is done with the Arduino IDE or in Eclipse. This posting steps you through our experience putting together this robot platform.
The Little Weed wanted to progress from building Lego, Kinex and RC cars and devices, to something more autonomous. Robotics and physical computing is a natural progression but a little daunting for a youngster. There are issues with attention span when it comes to learning the Arduino, programming, the Arduino IDE, not to mention the new world of physical computing that includes battery supplies, motors, electronics, soldering, etc. An adult is certainly needed to get the kid on the right track. It was easy to pique his interest with the subject of an autonomous robot platform.
The Dagu Magician chassis DG007 comes with a single page of instructions, sufficient to put the platform together. We paid $25CAD from a local bot shop. There is a bottom layer, which holds the motors, central caster and battery packs, and there is the top layer, which will hold your Arduino, power switches and any sensors you need to guide your ‘bot.
Dagu Magician Chassis kit, underside view of motors: Note the missing motor power wires, the thin plastic straps.
Dagu does a pretty good job of cutting the acrylic parts for the platform. There were a couple of the motor stays that fit a little bit too tight and needed some sanding to fit, but overall the pieces fit together pretty well. Getting the motor stays and the motors bolted together took some fiddling. This fiddling is definitely an adult task, as if you force the parts together you risk cracking the acrylic. The acrylic is strong enough but would break with rough use, so be warned.
Dagu Magician Chassis kit, underside view of motors: Note the missing motor power wires, the caster wheel and cushy tires
The two DG01D motors are adequate for a beginner bot, pull well enough and are simple to install. Note that the DG01G motors have a preferred direction and both should be installed in this preferred direction. There are arrows on the diagram showing the direction, and there is an additional plastic knob on one side of the motor. Ensure that the plastic knob on both motors point to the same side of the bot. Initially I inadvertently installed the motors in opposite directions, because the motors run both forwards and backwards. What happens is that the motors are stronger in their preferred direction, so when you run the bot one wheel pulls harder than the other, resulting in a turn rather than a straight line. I had to completely take apart the chassis and reinstall the motors, after which pulled more iniformly.
Autonomous Bot: Arduino, Ardumoto board, and the Dagu Magician Chassis. Used light sensors in this iteration.
The DG01D motor and gearbox are not particularly strong and require a battery voltage of 4.5 – 7v. We used a 6v RC battery with good results. There are some pretty cheap wires running from the motors, two that eventually dropped off. Soldering them back on was required. The motors are held together in its individual plastic case with a very thin plastic strap. Be careful when you solder to very carefully remove the plastic strap because they are very close to the motor and would probably melt with the heat of soldering. Once soldering is complete, carefully reinstall the plastic straps. Run the battery wires through the top chassis layer so they do not get caught in the wheels. There is no protection for the motor wires under the chassis so running over rough terrain might rip the motor wires off, disabling your bot. You could fabricate some protection, or run the bot with the wheels in front. The battery wires come with pin end connectors which promptly broke off. I cut some wire back and tinned it with solder, and it works fine. I could not find replacement pin end connectors from my local electronics supplier.
The motors come with speed board holders, bit I have not figured out their purpose. These are the flower looking wheels that attach to the motors on the inner axles. As they were binding on the lower chassis, I removed them. Maybe they are used with a sensor to count the number of revolutions of the motor, though you would also need a sensor there. Maybe we can do this in the future.
In researching the Arduino and the Dagu Magician chassis there were numerous negative comments that the unevenness of the motors. I also found this to be the case. Even in the right orientation, the motors are are unevenly matched, resulting in the bot consistently pulling in one preferred direction. This makes running in a straight line near impossible. One motor is more than 30% stronger than the other. I needed to add more code into my programs to throttle back the strong motor, which overly complicates development for the Little Weed. This mismatch of motor power is the one strong criticizm of the Magician Chassis kit.
Autonomous Bot: Arduino, Ardumoto board, and the Dagu Magician Chassis. Used light sensors in this iteration.
The wheels are 65mm in diameter and 30mm in width. They have plastic rims with solid rubber tires, but the tires are a little squishy. These tires are the best part of the chassis kit. They roll very well over hardwood and carpeted surfaces. I have not tried them outdoor or in water. They look good and seem strong. The rims are a 5 spoked plastic design. The wheels look cool.
The metal caster attaches to the lower chassis. We chose to run the bot with the caster in front. We could have run the wheels in front and the caster dragging behind. This is up to you, but will affect your programming of the bot. The caster does add drag to the bot. I thought of oiling the caster, but it would have picked up lots of dirt and lint, which might accumulate inside the sealed unit, and I would not be able to remove the dirt. You could replace the caster with a wheel that could rotate, but then when the bot radically turns direction, you would need extra time and energy for the wheel to correctly orient itself. I opted to leave the caster alone. Radical changes of direction are felt immediately with the caster, and programming would be easier.
The 4AA battery pack fits very well over the casters. The battery pack comes with a 5mm mono plug that conveniently plugs right into our Arduino unit. There is no on/off switch. We opted to add velcro with an adhesive backing to attach the battery pack to the lower chassis, otherwise the battery pack moves around too much. Velcro is not included in the kit. There is just enough room between the lower and upper layer to separate the velcro on the battery pack in order to remove it, with very little wasted space. We did not want to permanently bolt the battery pack to the chassis to allow us to change the batteries. Note this battery pack is used to run the Arduino CPU and does not power the motors. A separate motor battery pack is required.
The motor battery pack required some thought. I used an RC car 6v battery pack, 5.5 x 7.3 x 1.6cm. It is flat and compact, and we had a couple of them with chargers. The battery pack fit nicely between the wheels and under the top chassis layer. We cut a block of wood the exact dimensions of the battery pack. I had some broken red acrylic in the garage, which I cut down to size, and then removed the corners. Then with the block of wood clamped against the plastic I used a heat gun to soften the plastic and bend the acrylic sides to the shape of the wood form. On cooling the acrylic bent back slightly from the form. This aids in removing the battery. The fit of the batter into the battery holder is close. The positive and negative power wires use simple bolts through the battery holder, which contact the battery. I had to heat up the area where the contacts touch the battery and flare them back slightly in order to get the bolt heads to fit. I ground down the battery case slightly so that it can slide between the chassis layers, with a tight, wedged fit. The black dot, done with Sharpie, shows the proper orientation of the battery.
My motor battery pack and holder for the Dagu Magician Chassis, in red acrylic and 6v RC battery.
My motor battery pack holder for the Dagu Magician Chassis, in red acrylic. Note the discolouration of the acrylic due to heat from the heat gun.
Metal spacers are used to separate the upper and lower layers of the chassis. There are lots of different holes and slots in the layers to be able to customize and find a nice fit for your bot CPU and sensors. We tried to buy a few more of these metal spacers and found them to be quite expensive. Maybe we should shop around, as we don’t need too many. There are short 1.0 cm ones and longer 2.5cm ones.
There is a lot of space on the top platform. We bolted down our Arduino with the 1.0 cm spacers. Thankfully there are lots of slots in the platform, because fitting the slots to the mounting holes in the Arduino proved challenging. There was lots of space left over for an additional breadboard for sensors and connecting wires. We initially used an Ardumoto board, with piggybacks the Arduino unit, thus saving space. We then created our own H-bridge board using an L293D IC. This additional board will need a readjustment of the positioning of the Arduino, but there is still enough space.
The Magician Chassis kit did not take very long to assemble, maybe one hour. Tweaking it to work better, adding the battery holder and programming the Arduino took much longer. Overall I think the Magician chassis, while it has its flaws, worked very well for a beginner bot platform. It is very kid friendly. We still have it, and it has not broken, yet. As an entry level platform it did its job admirably, and inexpensively. The next bot we build will probably have better motors and I will fab my own platform, but will cost much more than the Magician Chassis. We have purchased more light sensors, pillaged light sensors from nightlights, purchased a proximity sensor as well as other sensors.
Where robotics and physical computing will lead us is unknown. I do like the direction towards physicality because so much of computers is virtual, nice but inadequate to work in the real world. The Dagu Magician Chassis has taken us one large step forward to who knows where.
