Data Acquisition Hardware and Software PDF

# Data Acquisition Hardware and Software This lesson describes DAQ hardware and DAQ software. ## Topics - DAQ Hardware Overview - Components of a DAQ Device - Choosing Appropriate DAQ Hardware - DAQ Software Overview - Overview of NI-DAQmx VIs ## DAQ Hardware Overview A typical DAQ system has t...

# Data Acquisition Hardware and Software This lesson describes DAQ hardware and DAQ software. ## Topics - DAQ Hardware Overview - Components of a DAQ Device - Choosing Appropriate DAQ Hardware - DAQ Software Overview - Overview of NI-DAQmx VIs ## DAQ Hardware Overview A typical DAQ system has three basic types of hardware: a terminal block, a cable, and a DAQ device. ### Components of a DAQ Device - **Signal:** Converted to a measurable electrical signal. - **Terminal Block:** A physical connector for signals, consists of screw terminals, spring terminals, or BNC connectors. - **Cable:** Transports the signal from the terminal block to the DAQ device. - **DAQ Device:** Acquires the signal and transforms it into a format the computer can understand. - **Computer:** Processes the signal and displays the data. ### Terminal Block and Cable - Terminal block provides a place to connect signals. - The type of terminal block depends on: - The measuring device, - The number of signals being measured. - A cable transports the signal from the terminal block to the DAQ device. ### BNC-2120 Signal Accessory The NI BNC-2120 is a connector block with signal labeled BNC connectors. - Can be used with NI X Series, M Series, E Series, and S Series multifunction DAQ devices. - Simplifies the connection of analog signals to the DAQ device. - Includes a function generator with a switch to select the frequency range of the signal. ### DAQ Devices - **Typical NI multifunction DAQ devices have four standard elements:** - Analog input - Analog output - Digital I/O - Counters. - **National Instruments also makes high-speed digital devices that offer:** - Timed digital I/O - High-speed analog output devices for advanced waveform generation - Dynamic signal acquisition (DSA) devices for analyzing rapidly changing signals. - **You can transfer the signal you measure with the DAQ device to the computer through a variety of different bus structures.** - A DAQ device that plugs into the PCI bus of a computer - A DAQ device connected to the PCMCIA socket of a laptop - A DAQ device connected to the USB port of a computer. - **If you do not have a DAQ device, you can simulate one in Measurement and Automation Explorer to complete your software testing.** - **You can also use PXI/CompactPCI to create a portable, versatile, and rugged measurement system.** ## Components of a DAQ Device - **Computer I/O Interface Circuitry:** Transfers information between the DAQ device and the computer. - **I/O Connector:** Connects the terminal block to the DAQ device. - **Real-Time System Integration (RTSI) Bus:** Shares and synchronizes signals between multiple DAQ devices in the same computer. ## Analog Input Circuitry - **Multiplexer:** Connects only one of several input channels to the instrumentation amplifier at a time. - **Instrumentation Amplifier:** Amplifies or attenuates the signal. ## Analog-to-Digital Converter (ADC) - **The ADC is an electronic device that converts an analog voltage into a digital number that you can send to the computer for interpretation.** - **Analog input circuitry combines with the ADC to acquire an analog signal so you can measure the level, shape, or frequency of that signal.** ## Analog Output Circuitry - **A DAC performs the opposite task of an ADC. It takes a digital number that was sent from the computer and converts it into an analog signal that is output through the I/O connector.** - **A DAC is useful for generating DC signals (level), specific tones (frequencies), and waveforms (shapes).** ## Digital I/O Circuitry - **The digital I/O circuitry can perform both input and output functions. A typical DAQ device has multiple digital lines that can acquire or generate digital signals using software timing or hardware timing.** - **You can use the digital I/O functionality of the DAQ device in applications ranging from monitoring a switch to see if it has changed states to controlling a relay.** ## Counter Circuitry - **Counters acquire and generate digital signals. Their built-in timing signals called timebases make them ideal for measuring the rate of a digital signal. You use the counter functionality of a DAQ device in applications ranging from measuring the frequency of a motor shaft to controlling stepper motors by generating a specific frequency pulse train.** ## Choosing Appropriate DAQ Hardware When choosing DAQ hardware, you must determine application needs related to: - Bus of DAQ device - Signals to measure - Accuracy of measurements - **How much data will I be streaming across this bus?** - All PC buses have a limit to the amount of data that can be transferred in a certain period of time. - This is known as the bus bandwidth and is often specified in megabytes per second (MB/s). - **What are my single-point I/O requirements?** - Bus latency is the responsiveness of I/O. It is the time delay between when a driver software function is called and the actual hardware value of the I/O is updated. - **Do I need to synchronize multiple devices?** - Some DAQ devices have access to programmable function input (PFI) lines that can be used to route clocks and triggers between difference devices. - **How portable should this system be?** - External buses, like USB and Ethernet, are particularly good for portable data acquisition systems. - Bus-powered USB devices provide additional convenience because they do not require a separate power supply. - **How far will my measurements be from my computer?** - One solution to this problem is to use a portable computing platform to move the entire system closer to the signal source. ## Signal Considerations - **Code width:** the smallest change in the signal that the DAQ device can detect. - **Resolution:** the number of bits used to represent an analog signal determines the resolution of the ADC. ## Amplification and Device Input Range Amplification or attenuation of a signal might occur before the signal digitizes to improve the representation of the signal. ## Code Width Code width is the smallest change in a signal that a system can detect. ## Accuracy Considerations The result of any measurement is only an approximate estimate of the actual value being measured.