Data Acquisition Systems - Review

Data Acquisition Systems Review

When defining a Data Acquisition System, this can be can be accurately described by explaining what a Data Acquisition System "does" not what it is. A Data Acquisition System catches or captures data about an actual system and stores that information in a format that can be easily retrievable for purposes of engineering or scientific review and analysis. Another requirement of a Data Acquisition System should be that it captures information programmatically or automatically in other words, without any hands-on human intervention or guidance.

Generally speaking, there are seven elements or functions in a Data Acquisition System. The seven elements/functions are (in no particular order) data collection, measurement, timing and triggering, a real-time clock, system control, data communication and data archiving. All seven elements must be in place for a structure to be considered a Data Acquisition System. If only a number of these elements are part of the system, the module could be defined as a component of a Data Acquisition System. If a system has all seven elements along with additional features, it is probably a larger system with a Data Acquisition System being part of the larger structure.

The actual components or elements of a Data Acquisition System to perform the seven essential functions are critical to the efficiency of the system. There must be a series of sensors as inputs to a Data Acquisition Board; in addition, there must be a trigger to synchronize the sensor inputs (the data stream), as well as a control for the Data Acquisition Board. Between the Data Acquisition Board and the processor of the system and system clock, a data communications bus (I/O) is also required. While the data is being stored real-time, the analysis and review of the information is performed after data is gathered. By definition, information cannot be analyzed in real-time, otherwise data events will be missed or overlooked. The Data Acquisition System must collect, sort, catalog and store data to be reviewed and analyzed in a meticulous (read: offline) manner.

The following is the methology associated with Data Acquisition

Source: Data acquisition begins with the physical phenomenon or physical property to be measured. Examples of this include temperature, light intensity, gas pressure, fluid flow, and force. Regardless of the type of physical property to be measured, the physical state that is to be measured must first be transformed into a unified form that can be sampled by a data acquisition system. The task of performing such transformations falls on devices called sensors.

A sensor, which is a type of transducer, is a device that converts a physical property into a corresponding electrical signal (e.g., a voltage or current) or, in many cases, into a corresponding electrical characteristic (e.g., resistance or capacitance) that can easily be converted to electrical signal.

The ability of a data acquisition system to measure differing properties depends on having sensors that are suited to detect the various properties to be measured. There are specific sensors for many different applications. DAQ systems also employ various signal conditioning techniques to adequately modify various different electrical signals into voltage that can then be digitized using an Analog-to-digital converter (ADC).

Signal: Signals may be digital (also called logic signals sometimes) or analog depending on the transducer used.

Signal conditioning may be necessary if the signal from the transducer is not suitable for the DAQ hardware being used. The signal may need to be amplified, filtered or demodulated. Various other examples of signal conditioning might be bridge completion, providing current or voltage excitation to the sensor, isolation, linearization. For transmission purposes, single ended analog signals, which are more susceptible to noise can be converted to differential signals. Once digitized, the signal can be encoded to reduce and correct transmission errors

DAQ hardware: DAQ hardware is what usually interfaces between the signal and a PC. It could be in the form of modules that can be connected to the computer's ports (parallel, serial, USB, etc.) or cards connected to slots (S-100 bus, AppleBus, ISA, MCA, PCI, PCI-E, etc.) in the mother board. Usually the space on the back of a PCI card is too small for all the connections needed, so an external breakout box is required. The cable between this box and the PC can be expensive due to the many wires, and the required shielding.

DAQ cards often contain multiple components (multiplexer, ADC, DAC, TTL-IO, high speed timers, RAM). These are accessible via a bus by a microcontroller, which can run small programs. A controller is more flexible than a hard wired logic, yet cheaper than a CPU so that it is permissible to block it with simple polling loops. For example: Waiting for a trigger, starting the ADC, looking up the time, waiting for the ADC to finish, move value to RAM, switch multiplexer, get TTL input, let DAC proceed with voltage ramp. Many times reconfigurable logic is used to achieve high speed for specific tasks and digital signal processors are used after the data has been acquired to obtain some results. The fixed connection with the PC allows for comfortable compilation and debugging. Using an external housing a modular design with slots in a bus can grow with the needs of the user.

Not all DAQ hardware has to run permanently connected to a PC, for example intelligent stand-alone loggers and oscilloscopes, which can be operated from a PC, yet they can operate completely independent of the PC.

DAQ software: DAQ software is needed in order for the DAQ hardware to work with a PC. The device driver performs low-level register writes and reads on the hardware, while exposing a standard API for developing user applications. A standard API such as COMEDI allows the same user applications to run on different operating systems, e.g. a user application that runs on Windows will also run on Linux and BSD.

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