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The Mindstorm NXT kit comes with three servos and four sensors. The sensors include a touch/bump, an audio input sensor, an optical sensor (Fig. 6) and an ultrasonic range finder (Fig. 7). This collection essentially uses all of the control module's IO, but it will be possible to plug in a hub in the future that increases the number of devices, allowing more complex systems to be created. The standard collection is sufficient for some rather sophisticated projects, and will likely be all that the majority of NXT users will ever utilize.
The three servo motors are identical. They can operate in full rotation mode so they can drive wheels or gears. There are a number of ways to mount or link Lego components or shafts, allowing a servo to be used in a number of different rotational applications. The interface is more sophisticated than many robotic servos in that a program can specify rotation based on degrees, revolutions or seconds. It is possible to specify whether the stopping process will be immediate (brake) or coasting. Each servo has its own tachometer.
The bump sensor has been slightly improved over the older Mindstorm product by allowing a shaft connection to the sensor input. The audio sensor provides a threshold intensity support, and is omnidirectional. It is not suitable as a recording device.
The optical sensor can detect colors, and it includes a white LED to generate its own light. This is handy for line following because it provides a consistent light level and eliminates external lighting from the equation. The LED is less useful as the distance between the sensor and a detectable object increases.
The ultrasonic range sensor is probably the most sophisticated of the bunch. It looks like a pair of robotic eyes, but in fact one side is the transmitter and the other is the receiver. The sensor sensitivity varies a little depending upon the environment, but it should handle distances up to 100 in. with pretty good accuracy.
Making Movement
Lego kits have included gears and wheels since their inception. The Mindstorm products take advantage of these by allowing designers to employ the servo motors in a host of applications. Many of these designs tend to be fragile because of the kinds of gears and wheels provided (Fig. 8). The approach is to make the parts durable since there is plenty of play between gears.
Shafts tend to be a bit more robust. Most of the beams in the kit have holes, allowing linkages and pivots to be easily incorporated into design while remaining quite solid in terms of construction. All the designs in the book use press fit parts found in the kit, but it is also possible to add nuts and bolts to a system to make it more rugged.
Over all, the Mindstorm NXT is an excellent learning and basic research platform. It is obviously in a different class from the iRobot Create or White Box Robotics' PC-BOT, but they can be complementary.
Future Projects
As a standalone system, the Mindstorm NXT is great for learning about robotics. Still, more sophisticated designs can be done by combining kits or using the kit with other platforms. I plan on using the NXT on the iRobot Create. I also happen to have some older Lego platforms that are compatible with the NXT. A little use of the drill and a Dremel tool will allow some based units to be bolted to the iRobot Create's mounting holes. Linking the Mindstorm NXT control module to the iRobot Create Command Module will take a little electronic expertise, but it should lead to some interesting interactions.
For more information, check out iRobot, Lego (and Lego Engineering), Microsoft, National Instruments, and White Box Robotics.