Photo Credit By Vibration Research
In order to be able to collect data from actual service environment, we will need at least two components: sensor/transducer and data acquisition/recording/logger system.
Sensor will be responsible to convert the physical measurand into electrical signal (voltage, charge, capacitance, resistance) and that can be transferred and read by data acquisition system.
The characteristic of the sensor (linearity, frequency response, thermal effect, etc) will influence the accuracy of data.
But other than sensor characteristic, the data acquisition system itself also plays important role on providing accurate and meaningful result, especially when the data acquisition system converts the analogue electrical signal into digital form (to be recorded and stored) and also during data post processing.
The capabilities and features offered by the data acquisition system for data post processing will be key factor that we need to consider when we want to select and purchase a data acquisition system.
Sensor/Transducer Compatibility
It is obvious that we have to ensure that the data acquisition system can smoothly be connected to sensor and receive the electrical signal.
We may want to list up all the type of sensor/transducer we plan to use during data acquisition.
Data acquisition manufacturer may have different module for different type of sensor. The module difference may not only be due to type of signal but may also be due to type of connector.
Below are examples of module of data acquisition system:
Voltage Sensor Module
IEPE Sensor Module
Charge Sensor Module
Microphone Module
Thermocouple Module
Strain Gage Module
Variable Capacitance Sensor Module
Photo credit: Vibration Research
Module for voltage, charge, IEPE and microphone may look similar, but they may be different internally since IEPE sensor needs bias voltage to power up its built-in amplifier, while charge sensor needs additional signal amplifier.
Sampling Rate
Sampling rate is considered as number of data recorded within one second period of time, describe in Hertz (Hz) unit.
For some physical measurand like temperature, static air pressure or strain measurement during static tensile test, sampling rate may not be important.
For these type pd measurements, 5 Hz to 10 Hz sampling may be good enough. But if we are running dynamic measurement, such as vibration or acoustic data recording, then sampling rate will be significantly important.
The data acquisition system should be able to provide minimum sampling rate as 2.5 times of maximum frequency of interest.
For vibration measurement, many data acquisition system manufacturer offer measurement module that can go up to 50 kHz sampling rate.
For acoustic measurement, since the frequency of interest can go op to 20 kHz, then many manufacturers will offer higher than 100 kHz sampling rate.
Photo credit: Vibration Research
Synchronized Data Sampling
Looking at how the data acquisition system is managing the data recording between channels, data acquisition system manufacturer may have two alternative products, one with synchronized channels and another one with non-synchronized channels. Synchronized channels mean all data at each channel will be collected at the same time.
At data acquisition system with non-synchronized, data at each channel may not be collected at the same time, creating time lag between channels.
For dynamic measurement and/or for dynamic control, using data acquisition system with synchronized channels should be more appropriate.
Meanwhile, when we are running measurement, which channel are relatively independent to each other, using non-synchronized channel may be allowed.
High Number of Channel, Online Analysis
In the case of no budget constraint, and we are running laboratory measurement, where we need to deploy high number of sensors, no constraint in terms of size and weight, then most of data acquisition system manufacturer will offer high scalable and modular data acquisition product, where we can plan the investment, change or add in measurement module in more flexible way.
We may want to have also online analysis feature, where we will be able to connect the data acquisition system to our PC, monitoring the process and perform real time dynamic analysis.
Photo credit: Vibration Research
Compact Size, Offline Recording, Wireless Monitoring
In another case, when we need to run measurement in the moving vehicle (automobile, motorcycle, train, airplane, vessel, etc), where the size and weight can be carried are limited, then we should initially determine the number of channels needed, and then look at the compact size of data acquisition system offered by manufacturer.
This type of data acquisition system may be running without the need of PC connected to it. The measurement setup and monitoring should be done wirelessly from mobile phone or tablet. Since no PC connected to data acquisition system during data collection, then we may want to ensure that it has enough capacity of data storage. Another aspect that we may need to check is the battery capacity. We want to ensure that data acquisition system has enough power for long duration data collection.
