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3.2 Hardware and Software Analysis

Both the Cricket hardware from the MIT Media Lab and the different types of programming software will be analyzed and discussed thoroughly in separate subsections (3.2.2 and 3.2.3, respectively). But first, a summary is given on the analysis results of the various surrounding systems, namely Festo Pneumatics, Märklin Metall, Lego Technic, various motors, and available options for power supply. 

3.2.1 Surrounding Systems

 - Festo Pneumatics
From the great variety of products Festo (see [c]) offers in the catalogue for industrial users, only a few items can be used in an environment like the Computer Clubhouse Esslingen. Obviously, there is a limitation to the mechanical dimensions of the parts. Although the pneumatic devices should be fully functional and provide a certain power to drive working models, their size should be small enough to be handled by kids. 
  
To provide the CCE with a flexible selection of pneumatic devices, a variety of Festo products has been chosen, which seemed to offer all the parts needed to offer a basic understanding of how a pneumatic system works. 
  
Cylinders – as the acting part in pneumatics – are provided in two different types. On the one hand, there are very small single-acting cylinders. Single-acting means, they extend powerfully when supplied with air; when the air supply is shut, a spring inside the cylinder pulls back the piston. 
 
As the spring is rather weak, this backward movement cannot be used drive a load. The cylinders chosen for the CCE have the following specifications: 
  • Piston diameter: 6mm
  • Outside diameter: 10mm
  • Overall length: 86mm
  • Stroke:  25mm
  • Pressure handling: up to 7bar
  • Drive attachment: M3 outside thread on piston rod
  • Connecting part: axial M8 outside thread on front end
  • Air connector: 3mm fitting for plastic tube, non-removable

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On the other hand, there are also cylinders of a slightly larger type. These are double-acting cylinders, which means they can be used to drive a load in both directions. Thus, they have no spring inside but use a second air supply for the backward movement. 
 
As discussed later on, this demands for another and more complicated type of valves. Their characteristics are as follows: 
  • Piston diameter: 12mm
  • Overall size:  30mm (square), length 83mm
  • Stroke:  40mm
  • Pressure handling: up to 10bar
  • Drive attachment: M3 inside thread on piston rod
  • Connecting parts: angle sections on both ends with two 4mm holes each
  • Air connectors: M5 inside threads to attach throttle valves or tubes

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For each type of cylinders, there is a suitable valve type, namely 3/2-way valves and 5/2-way valves. While both valves have two positions (idle and active), they differ in the internal structure and in the number of connectors, as indicated in the name. 
 
For an easy understanding, the two types can be compared to simple electrical switches: The first one acts like an ordinary switch – the air flow is either enabled or disabled. The second type acts like a change-over switch – the air is guided either this way or that way, which is needed for double-acting cylinders connected with two air tubes. 
 
Both types are designed as solenoid valves, requiring a 24V DC voltage for electrical control. In active mode, each valve consumes about two watts of electrical power. Besides, they can also be operated manually by using a tiny button. As very little air throughput is needed for the selected cylinders, the smallest and lightest version of valves has been chosen, which is only 10mm wide. Under the socket cover, the valve has a built-in electronic circuit, consisting of two diodes, a resistor and a LED. 
 
While the LED just indicates if the valve is active, the two diodes are essential for proper and save operation: one protects the circuit and the coil from wrong polarity, the other one is connected in parallel to the coil in order to conduct the negative voltage peaks induced in the coil in the moment of switching off. 
 
A flow control valve can be used to regulate a cylinder’s speed. This device is inserted into the air tube near the cylinder and throttles the air coming out of the cylinder in forward movement. For air flowing in the opposite direction (i.e., filling the cylinder for the backward movement), this flow control valve is no obstacle. 
 
Thus, you can regulate the forward and reverse speeds of a cylinder separately, using one flow control valve on each side. Obviously, this is only possible with double-acting cylinders; in order to provide the same functionality for single-acting cylinders, a special device has been created (see "Mechanical Preparations"). 
 
For an efficient, yet removable air connection between valves, cylinders, and the air supply (compressor), Festo offers plug-in systems, which are very easy to use: 
 
You just plug the end of a tube into a socket – and by pressing a small ring on the socket, you can remove it as often as you wish. This plug-and-socket system includes connectors for valves and cylinders as well as different kinds of couplings and dividers. 
  
The air compressor that Festo provided the CCE with is a rather powerful machine with an air tank and variable pressure. During normal operation of pneumatic-driven models, the compressor needs to work only once in a while, when the pressure in its tank has fallen significantly. 
 - Märklin Metall
Since the second half of the 19th century, the Märklin company (see [d]) has been a manufacturer of metal toys and construction kits. The popular Märklin Metall system is marketed as several basic and supplemental kits that may still be expanded with so-called theme kits. While theme kits provide special parts dedicated to certain applications, the basic and supplemental kits available at the CCE contain only very universal elements like beams, angle sections, screws, gear wheels, and axles. 
 
Most parts are fully zinc-plated and painted, while the screws and nuts are made from brass. From an educational point of view, this construction system has several advantages over other toys:
As the time you need to assemble a model is comparatively high due to the screw connections, it encourages a detailed advance planing of the construction, demanding for an abstract and analytic way of thinking. The results mostly honor the efforts that have been summoned up for building the model, as the structures can be extremely strong and durable. This may create a true sense of achievement for kids. 
  
The basic parts feature an array of 4mm diameter holes in a half-inch grid, which allows the elements to be connected at various angles and positions. As tools for its construction kits, Märklin includes a screwdriver, two 8mm wrenches, as well as a small allenwrench needed to fix gear wheels on their axles. 
 - Lego Technic
The basic idea of the well-known Lego (see [e]) bricks – the easy-to-use snapping connection – have been taken over into the compatible Technic system. It is based on an array of round 5mm buttons on the top of a brick that are pressed into their counterparts on the bottom of another one. The main differences to the plain Lego bricks are additional holes as well as a great variety of new parts that are useful for technical applications. 
 
While standard Lego bricks could only be assembled by putting one brick onto another, a bar of Lego Technic can also be attached to the long side of another one with the help of plug bolts that snap into the holes in the bars. This is just one example of how much more flexible this system is, compared to the plain Lego bricks. 
 
However, the fact that makes Lego Technic interesting for the use in Computer Clubhouses is the system’s ability to allow even younger and inexperienced kids to build up their own working models that may then be powered by motors. The reasons for this are the simplicity of the connection principle as well as the availability of pre-built elements of complex structure, e.g. a differential gearbox or steering shafts for an automobile. 
  
On the other hand, Lego has two severe shortcomings, when compared to the Märklin system. They have their roots in the plastic material and the snap joints: The models built with Lego are much less durable and weaker in the application. This means e.g., a Lego crane is not capable of carrying heavy loads because it will either bend too much, or even break apart at the snap joints. 
  
Nevertheless, Lego Technic is the kids’ favorite system for the quick assembly of a model they have in mind. 
 - Motors
To power the Märklin or Lego models, three different motors are available at the CCE. There are two types of Lego motors – micro motors and standard motors – and an inexpensive type of larger standard motors which can be found at electronics stores. The latter are in fact of the same size as those marketed by Märklin, which is why the original Märklin motors will not be described here. 
 
The Lego micro motor is the only one that can be used with the MIT Cricket modules. It is less than one cubic inch in size and has a built-in gearbox. At 9V, the shaft of this tiny motor reaches about 30 turns per minute when idling. Even when stalled at 9V (which is its nominal voltage), the current consumption stays below 80mA, which equals about 0.7W of input power. 
 
This motor is very useful where slow motion is  needed and energy is valuable, e.g. as a driving unit in small battery-powered Lego vehicles. Despite the gearbox’s high transmission ratio, the motor’s torque is very low. It cannot be used to drive larger models, so its applications are quite limited. 
 
The standard Lego motor, however, is much more powerful and is also designed for 9V operation. Just like the micro motor, it has a Lego axle shaft, but there is no built-in gearbox. The shaft turns at high speed, and this motor’s idle current equals the stalled current of the micro motor! 
 
Because of its maximum current of 680mA when stalled at 9V, the Lego standard motor cannot be driven by the Cricket’s motor output, which allows only 150mA. 
  
The overall advantages of this motor – high power and speed, combined with its compact brick-shaped design, and the Lego snap joint compatibility – make it the most desirable drive for larger Lego models. If the Crickets could handle this type of motor, it would mean a great improvement in respect to the Crickets’ universal usability within the CCE’s environment. 
 
The largest motor available at the CCE is a high-power motor of the Johnson brand, which has a nominal voltage range of 3..15V. Designed as a slow motor with a five-part rotor, it has an idle speed of 5500rpm at 500mA (at 6V). At the point of highest efficiency, it uses about 2.8A (at 6V). The ratings at 9V will be significantly higher. 
 
An angle section can be attached to the front plate of the motor’s housing. This can in turn be combined with Märklin Metall elements to integrate the motor in functional models. Gear wheels of various sizes that fit on the motor’s 3.12mm shaft are also available from Märklin. 
 - Power supplies
The standard 9V batteries for which the Crickets have been designed provide an electrical capacity of about 120mAh. This applies to the popular NiCd batteries, while the latest NiMH types can store up to 20% more energy. When attached to a Cricket running two of the tiny Lego micro motors, a battery of this kind will last less than two hours, where the power that sensor devices and the Cricket itself will consume has not yet been taken into consideration. 
  
As standard batteries have a recharging time of about 14 hours, a large amount of batteries and chargers would be needed in order to supply a couple of Crickets with electricity for several hours. 
  
While a stabilized power supply which can deliver 0..30V DC and up to 3A is available at the CCE, this is not suitable to be operated by the kids: the variable voltage involves the risk that the Crickets may easily be destroyed. In addition to this, most Cricket applications will probably require a mobile power supply. 
 
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