Data Acquisition Systems
Applications of data acquisition system
Data acquisition systems are required to test products prior being used in service or by public. Many products are required to be subjected to extreme conditions to ensure the product would still be operational and not detrimental to environment. For instance, air craft motor drive must be tested under same atmospheric condition that it would be exposed to during flights from part of the land to other. Different temperature, pressure and humidity might mean motor drives fail to operate as expected and we really do not want to risk an aircraft motor drive failure. Examples where products required testing are numerious: from prosthetic lower limb to blood pressure measurement, an actuator in Eurofighter Typhoon to piece of furniture in house. Products must be tested to prevent disastrous accidents.
Why data acquisition systems:
In the above applications, we are required to take measurements very fast. Air craft when drives motors at high speed, moment of ‘high-torque’, unexpected load profile might only occur for less than micro-second duration. If we are taking measurements 1000 times a second, we would never capture faults as we require to take over 1,000,000 times a second to capture ‘spikes’ in readings that are happening for micro-second durations. Data Acquisition Systems are deployed in test industry to capture such fast measurement and display in forms of scientific waveforms, graphs and charts so engineers and design scientists could analyse data while testing of units prior shipping units out to airlines and aircraft manufacturers.
Hardware Architecture of data acquisition system
Usually, a PC based solution is very user-friendly and cost-effective. Scientists and engineer need mouse, keyboard and display to work on large amount of data. We also need storage so high-capacity hard drive is also essential. As well as PC, we need Sensors, Signal Conditioning, Signal connectivity, USB or PCI DAQ cards as well as drivers associated with these DAQ cards. Software apps then could be written to acquire data, calibrate into engineering format, log and display for analysis.
Sensor Analogue data acquisition
In most aircraft and automotive testing, sensors that would provide millivolts, milliamps, volts, and current data are used for data acquisition. Such measurements change linearly or non-linearly with change in physical quantity. For example, pressure sensor might be used to achieve 0 to 10 Volts when pressure change occur between 0 to 7500 psi. Using right sensor for application is also essential as there might be restrictions based on industry. Food industry might impose restrictions to only install IP69K sensors so sensor body is immune to steam as well as high water jet sprays. Chemical industry might ask to install only Safety Instrumented System SIS to maintain Safety Integrity Level SIL for process plant.
“Data Acquisition System is designed to offer abstraction layer to ensure it would work same without major modifications irrespective of industry, Khurram Waris said”
Signal Conditioning, Amplification and Isolation
When sensor data mismatches format relevant to DAQ cards – USB/PCI hardware that plugs into PC- then we might need to provide amplification by means of additional electronics prior signal is connected to DAQ cards. For instance, a force sensor might only provide measurement data in format of mV where as DAQ cards could only measure Volts signal. Signal amplifier could then be used to amplify mV to Volts by multiplying measurements value with 1000 and output amplified signals to connect to DAQ cards.
Some industries request signal isolation so measurement and control systems are isolated and kept ‘safe‘ from harsh pieces of process and machineries that could potentially damage measurement and control stations.
Signal Connectivity terminal blocks are available to wire sensors cable so only a single cable then runs to the measurement PC plugged with DAQ card – USB or PCI based.
Variety of PC bus based DAQ cards are available from National Instruments. Daq cards are available for AI,AO, DI, DO as well as those with built in signal conditioning that measure physical phenomenon directly e.g force, temperature etc without requiring conditioning and amplification to measurement data. It is always advisable to look up on NI website for hardware available.
Data Acquisition System Specification
Following are some of the specifications that DAQ system might need to deliver:
- High Frequency Sampling and Logging Rates.
- Signal Type: +-10V,+-5V,4-20mA – Options for selecting singled ended or differential.
- Ability to terminate sensor cables in to terminal block
- Data must be time sampled as it is critical in certain application to analyse against time of the day.
- Representation of sensor data in various readable and scientific formats such as historical trends, bar charts, graphs, digital read-outs.
- Configuration Mode to reconfigure channel engineering minimum,maximum, offset data and save configuration file permanently.
- Calibration Mode to prompt user when sensor calibration has run out.
- Analysis Mode for live plotting of data with timestamps. Upto four different Y-scales
available with separate plot colours and scale colours.
- Ability to Log test file and replay.
- Ability to add cursors, scroll cursors, zoom in/out and automatic update of signal minimum, maximum, average values between cursors.
- Log File Management mode to automatically sort log files according to the day of the month of the year the file was created. Using search mode, Log file could be searched by name.
- XY mode to plot channel data on X and Y axis.
- Function Generator mode to generate signal-sine,cosine, sawtooth, triangular- at
sampling rate upto 3MHz.
- Support mode to provide remote desktop assistance for selected duration.
- Advanced Error management system to report system errors and diagnostics.
- It has to be also platform independent so it could run on windows, Mac as well as linux OS.
My recent data acquisition project
Recently I developed data acquisition system which met above requirements. I also provided access restrictions so only engineers could modify calibration data. System also provided reporting capability so results would automatically populate in Microsoft Word document for email communication to team members. Software provided configurability to hook up any card from NI – USB or PCI irrespective of channel count.
System developer must ensure during all stages of software and hardware development that system would offer modularity, scalability and configurability. In this manner, tried and tested system and software could be re-used over and over in newer projects without modification. Engineers save time which is vital to provide productive services.
In this article, I presented a brief overview of the main elements required for the design and manufacturing of data acquisition systems. Should you require assistance with your next DAQ solution, then get in touch..