Data Acquisition with Comedi

AEI maintains a number of testing chambers for diesel engines, known as test cells. In a cell, an engine is equipped with a number of temperature and pressure measurement devices. A frequency measurement device also is used to measure the rotational speed of the engine. Finally, the engine is connected to a dynamometer, which simulates actual driving conditions by varying the resistance against the spinning engine. This results in generated torque, which is measured as well.

Figure 3. An Engine Being Instrumented

The actual scan rate of the engine data is slow, only 20 times per second. If the measurement of this data were the only required job, the overall setup would be straightforward. However, a number of variable parameters must be tuned and controlled with the newest acquisition of each set of numbers. The engine throttle position and dynamometer load amounts must be varied slightly to maintain the engine speed at a specific condition. Valves in the cell controlling cooling water flow must be adjusted to keep engine coolant temperatures at constant levels. Safety measures must be checked to determine that no catastrophic problem has occurred.

Figure 4. Overview of Engine Measurement and Control with Comedi

All of these checks and new control values must be taken care of before the kernel can return to handling the rest of its scheduling. If the Linux kernel were to handle this scheduling on its own, it is quite possible that everything would work properly. However, it's impossible to determine beforehand when each stage of the process will be executed. With real-time extensions, however, the problem becomes trivial.

A real-time kernel is not without its downsides. While the real-time scheduler is executing some process at a fixed interval, the Linux kernel basically is put on hold. This means that a real-time process must be fast and efficient, and it must relinquish control back to the kernel as quickly as possible. Failure to do so results in sluggishness in the non-real-time portion of the system. If something goes wrong in the real-time process and control never goes back to the kernel, a complete system lockup can occur as well.

Laboratory aside, sometimes it's interesting and fun to put Comedi to work at home. Low-end multipurpose data acquisition cards can be purchased for $99–$299 US, depending on brand, complexity and acquisition rate. Some examples of home projects include monitoring temperature in various parts of the house or scanning a magnetic sensor on a garage door to remind you that it's still open.

One interesting aspect of the personal computer is that parallel port lines can be controlled individually. Using Comedi, it's trivial to turn on and off these digital lines. When used with some form of relay, these digital lines can turn off and on anything imaginable.

Although parallel ports toggle between 0 and 5 volts, they typically do not have the capacity to source much electrical current. That said, it's a bad idea to connect the parallel port line directly to a device to turn it on or off without adding some kind of buffer circuitry. Many Web sites exist that explain how to create these circuits.

I use Comedi, an old 486 and two parallel ports to create an annual holiday light show. Lights are hung on the house in normal fashion, and a pair of wires for each set of lights is run back into the control room (a spare bedroom, in this instance). These power wires are connected to a custom-built circuit board that houses mechanical relays that send the power to the lights when they receive a 5-volt signal from the parallel port. A simple C program uses Comedi function calls to control the parallel port lines digitally, that is, to turn on and off the lights. Simple text files tell the program when to turn various lights on and off. And, the neighborhood receives a treat.

Figure 5. Interface Board for Parallel Port Light Show Display

Data acquisition is extremely valuable in the laboratory. The generic interface that Comedi provides allows great ease of use in Linux for a large number of available DAQ cards. As the popularity of Linux grows, the importance of having an interface such as Comedi's becomes vital.

Furthermore, as the low-end DAQ cards become even less expensive, Linux-based data acquisition becomes more and more appealing to hobbyists and do-it-yourselfers. What used to be an expensive set of software and hardware now is a viable method of implementation for a multitude of applications.

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