Overall, the new Lego Mindstorm NXT is a promising platform for anyone interested in hobby robotics, and it is especially nice for Mac lovers. The brick comes with four input ports one doubles as an expansion slot for a still-hypothetical sensor hub and three output ports. The nice-but-small LCD can display black and white text and graphics, while the brick's loudspeaker plays a variety of NXT sound files. Unfortunately, the brick can only play.
However, you can find Mac-friendly utilities that convert. You connect your computer to the brick using either a USB cable or a Bluetooth wireless connection. The wireless connection is very cool. The brick can link with a computer, with other bricks or even with Bluetooth phones or PDAs.
The Mindstorm community has already produced a number of PDA and cell phone remote controls. I suspect we will see interesting Bluetooth sensors in the future. The kit comes with three motors. This is the NXT's biggest limitation, since unlike sensors , with only three ports, this looks like a hard-and-fast limit.
However, including a third motor in the basic kit feels like big improvement over the RIS 2. The new servomotors are bigger than the old motors, but they include a built-in rotation sensor. This gives you a lot of fine control over the robot's drive train. The kit also comes with four sensors: a touch sensor, a light sensor, a sound sensor and an ultrasonic range finder. The touch and light sensor are bigger than, but otherwise similar to, their RIS 2.
The sound sensor can detect the volume of the ambient sound in either adjusted or standard decibels. The dBA setting focuses on sounds within human hearing, while the dB setting includes sounds too high or too low for our ears.
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In either case, it can distinguish volumes up to about 90 dB, which is roughly equal to a typical lawnmower. The Ultrasonic Sensor uses sonar to determine the range to objects. It can detect items from 0 to cm a little over 8 feet 4 inches , with an accuracy of 3 cm. However, this accuracy depends a lot on the object you are sensing. Ultrasonic sensors work best when approaching hard, flat objects straight on. If you approach the wall at too steep an angle, a lot of the sound may reflect away from you.
For similar reasons, the sensor struggles with round, soft or thin objects. Around my apartment, the robot keeps running into my beanbag probably because of the roundish shape and the soft cover , and it cannot see the screen door leading to our balcony. Other than that, it does a reasonably good job. Finally, no discussion of hardware would be complete without mentioning the third party expansions.
Several companies offer new, high-quality sensors: everything from magnetic compasses to DYI prototype boards. Even Lego's getting in on the hardware-hacking game. You can buy legacy cables on their online store, letting you connect old RIS 2. Overall the NXT seems well designed with an eye towards future expansion. Sure, it still has its limits, but it is much more promising than any of the earlier Mindstorm products.
Lego's Mindstorm software lets you program and manage the files on your brick. Programs are written using a drag-and-drop graphical programming language, referred to as NXT-G. Great for simple projects or quick prototypes, it still becomes frustrating when you try to create larger or more complicated programs. They offer a free, academic version to Mindstorm owners--but the free version only runs on Windows. I don't. So let's talk about something else.
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If graphical programming is not your cup of tea, you can find a growing number of third-party tools scattered across the web. I will only mention the Mac-friendly ones in this article. These third-party projects can be divided into three main camps: software that runs on a computer and sends commands to the brick over Bluetooth iCommand and ruby-nxt , languages that compile to the same bytecode used by the existing NXT firmware NBC and NXC , and systems that completely replace the brick's firmware LeJOS NXJ. I will briefly examine each of these options, but be warned: these projects are still green, and most of them are undergoing frantic development.
Anything I say now will undoubtedly be outdated by the time this article prints. Of course, there has to be a catch somewhere. As I am writing this, the Lego software is not available as a universal binary. It only runs under Rosetta, and this creates some problems. So, I can only communicate with my brick using the USB cable. While that definitely knocks the software down a few pegs on the coolness-O-meter, it's not by itself a deal breaker. Unfortunately, the Lego software also seems slow and buggy. Dropping in a series of simple commands works fine, but don't even think about using data wires.
Don't get me wrong; the software works great on my wife's PowerMac.
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The Windows version also runs smooth-as-silk under Parallels. But if you have to use the Lego software on an Intel Mac, save often and don't try anything too complicated. Still, all hope's not lost. This won't help you with Lego's software, but it does let you use iCommand and ruby-nxt. I also recommend checking out the NXTBrowser for uploading and managing files over a Bluetooth connection. The rest of this article focuses on programming the tribot robot. If you followed the instructions when you opened the NXT box, it is the first robot you built.
It's a differentially steered, three-wheeled robot with snapping mandibles--you can't miss it. If, like me, you've lost the original instructions, you can find a digital version in the Mindstorm software's Robo Center. We will only use the ultrasonic sensor in these samples, but go ahead and make the whole contraption--after all, snapping mandibles are just too cool! Launch the Mindstorm NXT application. In the Start New Program text box, type wanderer and click Go. This gives you a blank grid. You can create your program by dragging blocks from the toolbar.
Let's start by dragging a loop block and dropping it onto the sequence beam as shown.
This is our program's main loop. We want it to run forever, which should be the default. To double-check, select the loop block, then look at the control settings below the workspace. Select Forever from the drop-down options. Now drop a switch block inside the loop block. This time we will want to change the settings. Control should say Sensor. You might want to play around with the distance setting, but 25 inches seemed good for running around my apartment. This is the skeleton of the world's simplest behavior-based robot.
Behavior-based robotics also called reactive robotics builds a robot from the bottom up. Instead of worrying about complicated data processing and planning, behavior based robotics focuses on immediate reactions to the robot's environment. These robots tend to be simple but very robust.
They effortlessly handle unexpected changes in their environment. And, often, surprisingly complex behavior can emerge from the interaction of otherwise simple rules. Given the relatively limited sensors and on-board processing power, behavior based approaches seem the perfect fit for Mindstorm projects. Here, we are building a simple rules-based architecture. Each rule has a prerequisite, if that prerequisite is met, the rule fires.
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In our case, we have two behaviors: avoid obstacles and explore. Since we only have two options, we can express them in a single if If we are closer than 25 inches, avoid the obstacle, else explore. To actually program this, drop a move block into the top and bottom of the switch. If our robot senses an object closer than 25 inches, it will execute the top beam indicated by the flower icon. If it is further than 25 inches, it will execute the bottom note the mountain icon--get it? Flower for close. Mountain for far away. Select the top move block. Set the Power to 25, the Duration to Unlimited and slide the Steering control all the way to the right, as shown.
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Normally, a move instruction will cause the program to block until the movement finishes. When you set the block's duration to unlimited, it starts the motors, and then continues onto the next command on the sequence beam. In our case, it will just keep looping around. On each iteration it checks the ultrasonic sensor, then executes the appropriate block.
If something's in our way, we turn to the right. Otherwise, we drive straight ahead. So far this is a very simple example. NXT-G programs can become a lot more complicated. Most blocks have a pullout drawer on the bottom-left corner. Opening this drawer exposes a series of input-output ports.
You can connect blocks using data wires, allowing you to modify the block's settings at runtime. While the exact details are beyond the scope of this article, I will include a slightly more complicated example with the online source code.
NXT-G's biggest limitation is that it quickly becomes unwieldy. I find scrolling around the desktop quite awkward, especially when the project no longer fits on a single screen. An update is expected to be made available later this year. Kind of like BricxCC-lite. This is an early release with a number of rough edges and defects.
Please report any problems you have using the editor and integrated compiler. You can configure the refresh rate and enable or disable polling. You can also set the scale from 1x to 4x. It supports a wide variety of actions for such things as downloading the NXT firmware, downloading and uploading files to and from the NXT, checking the battery level, playing sound and melody files, playing tones, listing all files on the NXT, and reading input and output values.