In the previous DAQ series, What is a Data Acquisition (DAQ), and How DAQs Work at a High Level, terminology that is analogous with DAQs was mentioned. To delve further, this series will discuss the some of the methodology associated with data acquisition.
Transducers, Sensors, Actuators:
As we know, data acquisition begins with a physical phenomenon or physical property and electrical signals needing measurement. No matter the type of property to be measured, it must be transformed into a medium that can be sampled. Transducers sense physical phenomena and provide electrical signals that the DAQ system can measure. To convert these physical properties into electrical signals, the device used is a sensor. A sensor or actuator (a type of transducer) is responsible for converting physical property into an electrical signal (voltage or current) or electrical characteristics (resistance or capacitance) that can be converted into electrical signals. Sensors can also convert temperature into analog signals that an analog-to-digital converter (ADC) can measure. 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.
Transducers need signals to communicate within the DAQ. Signals may be digital (also called logic signals sometimes) or analog depending on the transducer used. If the transducer is not suitable for the DAQ hardware, the signal may need to be amplified, filtered or demodulated. This process is called signal conditioning. There are other reasons for signal conditioning, which depends on the needs of the sensor.
Recoders and Dataloggers:
Products used with sensors to document information relating to a process are recorders and dataloggers. Recorders usually have a pen that deflects as a percentage of an input span, while paper moves beneath it at a defined speed in relation to time. The recorder output is an easy-to-read, continuous trend line. Dataloggers typically print the actual value of the input with a time stamp. The advantages of the datalogger include less paper usage, higher resolution of the reading, and less chance of misinterpretation of the data. Hybrid recorders are instruments that have both trend recording and datalogging ability. Some of these units also include features such as math calculations and communication ability to transfer data to a host computer for further analysis.
Your personal computer may be used for a data acquisition system. The computer provides a processor, a system clock, a bus to transfer data, and memory and disk space to store data. Depending upon the computer, it can drastically affect the maximum speeds at which you are able to continuously acquire data. Today’s technology boasts IC processors coupled with the higher performance PCI bus architecture as well as the traditional ISA bus and USB. The processor controls how fast the converter accepts data. The system clock provides time information about the acquired data. Knowing when measurement occurred is vital; therefore, a recorded sensor reading is generally not enough. Historically, data was transferred from the hardware to system memory through dynamic memory access (DMA) or interrupts. DMA is hardware controlled and therefore extremely fast. Interrupts might be slow because of the latency time between when a board requests interrupt servicing and when the computer responds.
Today, an alternative to DMA is the Programmed Input/Output (PIO) interface in which all data transmitted between devices goes through the processor. A newer protocol for the ATA/IDE interface is Ultra DMA, which provides a burst data transfer rate up to 33 MB (megabytes) per second. Hard drives that come with Ultra DMA/33 also support PIO modes 1, 3, and 4, and multiword DMA mode 2 (at 16.6 megabytes per second).Data transfer capabilities of the computer you use can significantly affect the performance of your DAQ system. Applications requiring real-time processing of high-frequency signals need a high-speed, 32-bit processor with its accompanying coprocessor, or a dedicated plug-in processor such as a digital signal processing (DSP) board. If the application only acquires and scales a reading once or twice a second, however, a low-end PC can be satisfactory. Traditional platforms include Mac OS, which is known for its simple graphical user interface, Windows 7 (32- and 64-bit),Windows XP (32-bit), Windows Vista (32- and 64-bit), Windows 2000, Linux (SUSE, Redhat, Mandriva).
The interface between the signal and a PC is usually the DAQ hardware. It could be in the form of modules that connects 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 motherboard. 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 through 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. The beauty of DAQ hardware is that it does not have to run permanently to a PC. Stand-alone loggers and oscilloscopes, which can be operated from a PC, can operate completely independent of the PC.
For the DAQ hardware to work with a PC, you need DAQ software. 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. It is common for an application that runs on Windows will also run on Linux and BSD.