Data Acquisition and Data Logging

1. Data Acquisition and Data Logging Some basics

2. What is data acquisition and data logging? Data acquisition is simply the �gathering of information about a system or process� Information may be gathered for various reasons knowledge research and development systems or process monitoring systems or process control Data acquisition requires one or more transducers (sensors) to sense the system or process, and a signal measuring instrument

3. What is data acquisition and data logging? Real Time Data Acquisition relates to data being acquired and used in the same time frame, such as when monitoring or controlling a system or process Data Logging is when acquired data is stored or logged for later use or analysis. Involves some form of memory.

4. What is data acquisition and data logging? Data acquisition and data logging is not a complex or mysterious science as viewed by some Data acquisition in its simplest form could be to measure the length of a piece of string with a ruler Data logging in its simplest form could be to write that measured length down on a sheet of paper

5. A data acquisition or data logging system A data acquisition or data logging system generally includes the following components: sensors or transducers which provide the fundamental information of the parameters to be measured a device to convert the primary signal from the sensors into a form compatible with information processing systems a computer or other controller which supervises the overall system, and manages the generated data

6. Plan your task Before you start: What am I trying to measure? How am I going to measure? How accurately do I want measure? How often do I need to measure? During: Calculations, Data reduction, Alarms After: How am I going to recover my data? What am I going to do with my data? Post processing

7. Parameters to measure There are many different parameters for which sensors or transducers are commonly available: temperature pressure, force, mass, weight velocity, acceleration, vibration strain, stress, distortion, fatigue flow, volume, level length, width, depth, thickness, displacement state, pulse, counter composition, concentration

8. Parameters to measure Selecting suitable sensors is often the most difficult task

9. Types of sensors Sensors produce an electrical output that is proportional to the quantity of the parameter being sensed Electrical output from sensors can be Voltage, direct or alternating Current, current loop Resistance, conductance Frequency Binary states or pulses (counts) Serial data - RS232, RS422, RS485, SDI-12 Parallel data - BCD, Gray Code, quadrature encoded

10. Characteristics of sensors General characteristics to consider Accuracy Degree of agreement between the measured value and the true value Linearity How well the measured data points fit a straight line Repeatability The degree of variance of successive readings of a measured value. Match the sensor characteristics to the parameter being measured

11. Support for sensors Sensors often require support that must be provided by the data acquisition system or data logger sensor excitation signal conditioning compensation for cable effects management of references, zeros, etc scaling of the electrical output to parameter units Ability of data acquisition system or data logger to support sensors can determine the effectiveness of a project

12. Reading information from sensors Many sensors produce an analog signal, such as voltage, current, resistance, frequency, etc. Analog signals are measured by a process of Analog to Digital Conversion or ADC Important characteristics of ADC are accuracy resolution linearity repeatability speed common mode range electrical noise rejection

13. Resolution, accuracy, linearity, repeatability These terms are often confused, however they have very different and important meaning Apply to both the ADC and to sensors Important to consider when matching sensors and data acquisition systems to requirements

14. Resolution, accuracy, linearity, repeatability Resolution is the degree to which the ADC can determine that two readings are different. usually expressed as a binary proportion of the measuring range for example a 16 bit ADC resolves to 1:65535 of range, for a 1 Volt range this is 15.26 microvolt small data loggers commonly have 12 bit resolution, larger systems have 16 bit resolution or greater resolution is usually fixed, but may vary with sampling speed

15. Resolution, accuracy, linearity, repeatability Accuracy is the degree to which the ADC can measure the true value of a signal. usually expressed as % full scale of measuring range for example an accuracy of 0.1% for a 1 Volt measuring range is �1 millivolt accuracy will often vary with operating conditions such as ambient temperature, signal type, environmental noise, etc.

16. Resolution, accuracy, linearity, repeatability Linearity is the degree to which the relationship between the true value of the signal and the measured value of the signal remain constant over the measuring range. usually expressed as % full scale of measuring range for example a linearity of 0.01% for a 1 Volt measuring range is �0.1 millivolt linearity will often vary with operating conditions such as ambient temperature, signal type, environmental noise, etc.

17. Resolution, accuracy, linearity, repeatability Repeatability is the degree to which the ADC will measure the same value for the same signal level on successive occasions. usually expressed as % full scale of measuring range for example a repeatability of 0.01% for a 1 Volt measuring range is �0.1 millivolt repeatability will often vary with operating conditions such as ambient temperature, signal type, environmental noise, etc.

18. Common mode range Common mode range is a measure of the ability of the ADC to measure a signal separate from local ground

19. Electrical noise rejection Noise rejection is the ability of the ADC to eliminate local environmental electrical noise from the measurements

20. Sampling Speed Sampling speed is the number of readings per unit time of a single channel, or of a group of channels Expressed as samples/second (Hz, KHz) Varies from less than 1 Hz to 100�s KHz for different data acquisition systems May be variable within the same system

21. Sampling Speed Appropriate sampling speed is dependent on many factors what type of information is to be captured - short term events, long term trends, both? when is information to be captured? what is the level of importance of various information? Aim to set a sampling speed to maximize sensitivity of data. minimize redundancy of data. optimize generation of reports. simplify archiving of data.

22. Number of channels Most data acquisition systems have a single analog to digital converter, and many input channels Input channels are selected for measurement by a switch or multiplexer (mux), which may be sequential or random. Switching may be by relay or solid state. Only one channel can be measured at a time Consideration must be given to �sensor bounce� when a sensor is selected for measurement

23. Data recovery and transfer Data can be recovered from data acquisition systems and data loggers in various ways. Serial comms interface via direct connect, modem, cell phone, radio, satellite PCMCIA - modem, cell phone, LAN, memory card, USB memory device Network port - Ethernet, field bus, proprietary, etc Universal Serial Bus (USB) Data can be transferred and published using : Local Ethernet network Intranet - email, web pages Internet - email, world wide web pages

24. Data processing and reporting Collected data is analyzed and reported using any of generic packages such as ASCII text editors Spreadsheets Databases general purpose data processing packages general purpose SCADA packages proprietary host software packages �To summarise the function of a data acquisition device�[next slide]�To summarise the function of a data acquisition device�[next slide]

25. Other functionality In addition to the core functionality of data acquisition, modern data acquisition and data logging systems have other functionality including real-time calculations, statistics, FFT, etc. alarms testing for out of range conditions. control feedback to the measured system or process. a range of communications and memory options etc.

26. Form factor for dataacquisition systems Data acquisition systems can be of a number of forms: Pocket loggers - small, battery powered, stand alone devices with simple functionality Intelligent data loggers - stand alone devices with own intelligence, sophisticated data manipulation, alarms, backed up power supply, displays, etc. Accessed continuously or periodically by a PC. Plug in cards - plugged into the ISA bus or USB port of a PC to provide basic data acquisition functionality. The PC provides power, control, data storage, etc.

27. Exercise Write down the following Types of sensors you will use. The accuracy / repeatability required. Sensor power requirements. Sensor scaling. Rate of measurements that need to be made. The method of powering the logger, sensors and communications systems. The memory requirements of the application. The method of communicating with the logger. The method of displaying, distributing and analysing the data collected.