LEGO[R] moves up-market.

Having reported on the LEGO ROBOLAB Kit in April 2005, it seemed natural to write a sequel to that article based on the MINDSTORMS[R] NXT Robotics Toolset. The newer kit definitely has gone up-market in its programmability and sensor and motor capabilities. There is much less emphasis on low-level gearing and mechanical engineering in contrast to the earlier ROBOLAB that had basic motors.

[ILLUSTRATION OMITTED]

In the MINDSTORMS kit, the motors include gear reduction as well as angular feedback sensors. Accordingly, you can program a motor to turn clockwise for 180 degrees at a certain speed. That would have been difficult if not impossible in the earlier product.

A tremendous amount of effort has gone into the step-by-step on-screen instructions for the various models. For example, there are 60 illustrated steps to build a robotic arm and an additional 16 steps for a hand. Given the complexity of some of the models, this effort is necessary to ensure a satisfying user experience, as they say. Nevertheless, some of the illustrations show only one piece being added. Perhaps an exploded view with explanatory text would be an alternative approach worth considering.

Programming also is very different from ROBOLAB, reflecting some of the changes introduced in recent versions of LabVIEW. Although wires can be used to interconnect ports on icons, you seldom need to. Instead, most of an icon's behavior is determined by parameter values entered on a form.

When programming, you start with the common palette, which is useful, but the options offered by the complete palette are more interesting. At least initially, I felt I had to work hard to overcome what seemed like excessive graphic gimmicks. There are self-locating blocks, lots of different colors and shapes, and very large symbols.

Icons are positioned on a sequence beam, which greatly limits the types of networks you can build, especially compared with ROBOLAB. A positive result of constraining the arrangement of icons is very predictable program execution. As I found, the real power of a program is in the configuration panels.

Without reading any instructions, you soon learn that clicking here and there opens up connector blocks on the icons. It's helpful that the wires are different colors for different kinds of data, and if you connect two things that shouldn't be connected, the wire goes dotty and grey. However, in my experience, it wasn't necessary to wire the icons together at all because the control-panel information was very comprehensive. Nevertheless, I'm sure that advanced MINDSTORMS builders find wiring useful.

Installing the software on my PC wasn't completely straightforward, and after installing it, you are exposed to LEGO and other advertising. You need to restart the computer for the short-cut icon to appear. After doing that, I could get the MINDSTORMS software to start up.

Example

A general comment on LEGO construction: It helps to have the parts spread out in a large flat box. There may be some kids that are so organized that they actually keep similar parts in separate containers; however, I suspect that most of the time all the parts end up mixed together. Finding a specific component among the 571 that make up the kit is helped by color and having only a layer or two to work through.

Initially, it's easy to find a part because there are several of each type. But as the project nears completion, you are trying to locate one particular part, and it can be difficult if you can't spread out all the remaining components.

Rather than dive into a pre-engineered example, I wanted to become familiar with the kit by building something original. My idea was to make a mechanism that had balls drop down slides and eventually get picked up and recirculated over and over.

[FIGURE 1 OMITTED]

Two motor drives were required: one to close a pair of jaws around the ball and the other to swing the ball and jaws up and over to a track and then release the ball and return to the original position. I didn't build the track but did complete the necessary programming for the LEGO mechanism. Figure 1 shows the operation of the model as a progression of separate positions.

Because I didn't know where the main arm would be positioned at the start of the program execution, I programmed the drive motor to traverse the arm relatively slowly to the right until a stationary photodetector triggered on a sufficiently high light level reflected from the white beam on the side of the arm. That action stopped the main motor and started a second motor that activated the jaws.

I used a worm gear as a rack driven by a pair of pinions geared together and turning in opposite directions on either side of the rack. This linear motion activated a symmetrical bell-crank arrangement that opened and closed the jaws, actually more like a pair of claws interleaved to grab and hold one of the 2" balls.

After the photodetector triggered and the main motor stopped, the claw actuator motor was started and ran a short amount of time until the touch actuator was pressed. The mechanism was designed so that the jaw opening would be more than sufficient to drop the ball but not so wide that the bell cranks would jam. Also, the jaw opening could not be too narrow to restrict tolerance in the positioning of the next ball to pick up.

Having dropped the ball, the machine was in a known state set by two reliable sensors. A short 3-s wait icon held the machine in that state. Because the main motor drives a fixed gear ratio, it was simple to compute that the arm would reach the desired position at the other side of the machine after 1,120 degrees of rotation. As a result, a main motor icon was inserted and its panel programmed for 1,120 degrees with braking after the rotation.

Also, the claws then needed to be closed by running the second motor. Because of the design of the mechanism, a rubber-band drive was the easiest to implement and acted as an overriding clutch, providing holding power for the jaws after the motor had stopped. I deliberately programmed the motor to run a little longer than required so the rubber bands would be stretched.

By using a sensor to determine one end point in each of the mechanical loops, both the claws and the traversing mechanism could operate repeatedly without tolerance build-up.

Programming

I simply couldn't understand why I was having very basic programming problems. In desperation, I e-mailed MINDSTORMS but never received a reply or acknowledgement. Finally, I contacted an acquaintance at National Instruments, and soon the problem was solved. In effect, each icon in the program describes a state. For example, you may insert a motor icon and in its associated control panel set a number of conditions, such as run clockwise forever with servo feedback to hold a 50% speed.

I had assumed that if I put a similarly configured motor inside a while loop that when the loop terminated the motor would stop. No, the motor's local variables set on its control panel take precedence. When I tried to run that kind of program, the motor did indeed stop, but only because the program had stopped.

To gain control of the motor, you must insert another motor icon on the sequence beam but with different conditions programmed into the icon's setup panel. You could have a motor icon set to run forever, then a wait icon set to 10 s, and finally another motor icon set to stop. That kind of process underlies operation of any MINDSTORMS program.

However, because the programming panels work at a high level of abstraction, you can directly program a single motor icon to run for 10 s, avoiding the need for three separate steps. In addition, you can specify whether the motor should coast to a stop or brake. And, you can allow further program steps to execute while the motor runs or inhibit the program from moving on until the motor motion completes.

Obviously, there is a lot of powerful low-level software running in the background to support the actions programmed on each panel. All of this motion control required for the machine was possible using only six icons, giving some idea of the high-level nature of MINDSTORMS. Additional icons that I did not use have even more functionality. The move icon allows simultaneous control and synchronization of at least two motors necessary to simplify programming Rex the Robot to walk.

Also, the basic while loop could be linked to the state of a particular sensor. For example, if the loop were to run until the touch sensor was pressed, the loop icon would include the touch-sensor symbol, and the control panel would present appropriate choices such as actuate when pressed, when released, or when depressed 50%.

A lot of detail is handled on the individual panels associated with the elements, which makes the overall diagram higher level and cleaner. On the other hand, there's no way that a MINDSTORMS diagram is self-documenting to the degree a ROBOLAB diagram is. There is so much detail hidden in the panels that you have very little information on the diagram with which to work.

Construction

This kit uses the TECHNIC[R] beam-and-pin type of construction found in many LEGO mechanical kits. In common with ROBOLAB, the controller block can be built into the mechanism. Similarly, there are plenty of attachment points for the motors and sensors.