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Design of a Virtual Instrument (Lab Report Sample)


A conditioning circuit for a temperature measurement using PT100 senor is given as
shown A diagram WITH A BRIDGE. The analogue signal IS THEN digitalised via a NI data
acquisition card before the DSP.


(School of Computing, Engineering and Digital Technologies, Teesside University)
Table of Contents
 TOC \o "1-3" \u \h  HYPERLINK "#_Toc59520369" Abstract 1
 HYPERLINK "#_Toc59520370" 1 Introduction 2
 HYPERLINK "#_Toc59520371" 2 Software 2
 HYPERLINK "#_Toc59520372" 3 Block Diagram 3
 HYPERLINK "#_Toc59520373" 3.1 Temperature sensor 4
 HYPERLINK "#_Toc59520374" 3.2 Signal conditioning circuit 5
 HYPERLINK "#_Toc59520375" 3.3 Amplification 6
 HYPERLINK "#_Toc59520376" 3.4 Anti-aliasing filter. 7
 HYPERLINK "#_Toc59520377" 3.5 ADC circuit unit (My DAQ-Data Acquisition) 8
 HYPERLINK "#_Toc59520378" 3.6 Digital signal processing unit 10
 HYPERLINK "#_Toc59520379" 4 Conclusions 14
 HYPERLINK "#_Toc59520380" 5 References 15

Data acquisition is the automation of a system for data collection and analysis. It entails making measurement of physical parameters and logically representing them. This paper will discuss a Laboratory Virtual Instrument Engineering Workbench (LabVIEW) platform to measure the temperature using Resistance Temperature Detectors (RDT). The measurement was achieved by a virtual instrument (VI) connected to a data acquisition device. A VI uses customisable software and modular measurement hardware to create user defined measurement system (Sumanthi and Srekha, 2007) LabVIEW software based VI not only combines software and instrumentation technology but also makes full use of computing capacity of computers available to us to break the limitation of traditional measurement in data processing. The LabVIEW program is called VI because it emulates the operation and appearance of a physical instrument such as a multi-meter (Gary and Richard,2011). Compared to traditional temperature measurement devices, this system has the advantage of simple structure, low cost, easy operation and high stability.
Day to day manufacturing activities demand for measurement of temperature because equipment and process work best within optimal temperature ranges. With an advancement in technology, virtual measurement technology is taking position in modern measurement and control area.
Data Acquisition (DAQ) cards and software can be used to create devices called virtual instruments (VI). Software is just a basic element of a data acquisition system. A typical industrial PC-based DAQ system may consist of components such as transducers, signal conditioning, plug-in DAQ boards and application software to create virtual instrument. The parameter to be measured is converted to a signal that can be easily detected and transmitted by means of device known as a sensor. Such a device converts the physical signal into an electrical signal, such as voltage or current. In a data acquisition system, one cannot connect signals directly to a plugin DAQ board. Typically, the signal must be conditioned and optimised before the plug-in DAQ board converts them to a digital signal; Finally, the software controls the data acquisition system by capturing the data, analysing, and displaying the results
The VI framework converts temperature measured into analog signals through a transducer at the front end of the temperature device. The analog signal is then treated by signal conditioning circuit through amplification, filtering and AD conversion. Signal conditioning aims at standardisation of the voltage signal. The signal is then collected by a data acquisition card and then to the computer bus under the direction of data collection. The collected data is then treated by VI software and changes in temperature can be analysed more accurately.
LabVIEW is based on graphical programming language to describe actions (Gary and Richard,2007). Memory allocation/deallocation is automatically managed by LabVIEW. The development of a virtual instrument (VI) with LabVIEW consists of a front panel, a block diagram and an icon and controller panel (Venkatesh, 2012). The front panel is a graphical user interface (GUI) of the VI containing switches, meters and other type of devices. The VI is built by drag and drop selection of controls and indicators. From the front panel, the user can interact and change the values of switches, gauges, and controls etc. The front panel also displays data in the forms desired by the user. The block diagram on the other hand contains the source code for the virtual instrument. The user can select functions such as file I/O, instrument I/O, or data acquisition, and place them on the block diagram of your VI. The user can connect these functions together with wires like a schematic or a flow chart to define execution of the VI.
Virtual instruments are structured as follows:
The control or user interface of virtual instruments is known as the front panel. The front panel simulates the panel of a physical instrument.
The instructions given to the virtual instrument are in the form of a block diagram wired together. A block diagram node executes when all its inputs are available. LabVIEW executes the block code as it is created.
The design in this paper is based on Pt100 sensor incorporated into a signal conditioning circuit. The signal VO is then digitised using Data Acquisition Card (DAQ) followed by digital signal processing. The measured signal (from sensor) is weak and must be pre-processed before entering the DAQ card. The bridge is made up of a temperature sensor Pt100 and three impedances (fixed) sends voltage levels corresponding to changes in temperature to the op amp for amplification

Figure 1 Signal conditioning circuit
Block Diagram
The circuit can be simplified into a block diagram of its functional units.

Figure 2 Block diagram of the system.
Temperature sensor
The sensor used in the design is a Resistance temperature detector (RTD). The sensor simply measures the temperature by correlating the resistance of the RTD element with temperature. RTDs are used for accurate temperature measurement. Their non-linearity is however a serious drawback in temperature monitoring where precise measurement and control are crucial. Linearisation of RTD’s output can be achieved by use of feedback compensation, by using constant current sources to excite the bridge and the use of software.
A RTD is a passive resistive circuit component and when current is made to flow through it, the RTD start to heat a phenomenon called self-heating. Self-heating is typically specified as the amount of power that will raise the RTD temperature by 1° C, or 1 mW/°C. If the resistance to be considered in measurement is small, then the resistance change due to self-heating will influence the output least it is ignored (Wu, 2018).
The law of variation of the resistance with the temperature for the RTD sensors is given by the Calendars-Van Dusen equation (Debnath, 2019)
 QUOTE   Eq 1
The coefficients in the Eq. 1 defined by the IEC-60751 standard are as follows
Ro is the resistance of the RTD at 0°C. For a PT100 RTD, Ro is 100 О©:
• A = 3.9083x10-3 • B = –5.775x10-7 • C = 0 for temperature greater than 0°C.
For the Pt100, at the bridge is balanced because Rrtd=Ro
At the bridge is unbalanced since Rrtd is given by
The resolution of the sensor is determined as the smallest change in temperature that can cause a change in resistance.
Signal conditioning circuit
A change in environmental temperature conditions causes the resistance of Pt100 sensor to vary. It therefore follows that change in resistance value causes the bridge to be unbalanced and an output will be observed from the bridge. The bridge is designed according to Pt100 indexing table, that is, when the environment temperature in 00c, PT100 resistance of 100 О©.
When the RTD is excited by a voltage source, the resulting bridge output voltage is proportional to the resistance of the bridge with inherent nonlinearity. However, with the use of a resistor divider, a Wheatstone bridge or a feedback compensation, the non-linearity can be significantly reduced. These linearization techniques can be applied to other resistive sensors, mainly with second-order non-linearity.
The above-mentioned techniques though do not solve non-linearity completely and there is need to use software to linearize the relationship between the temperature being measured and the corresponding voltage. This is done by the Digital signal processor.
An equivalent circuit for the bridge used for signal conditioning Wheatstone bridge is as follows

Figure 3 Bridge equivalent circuit

 QUOTE   Eq 2
Substituting for the values of resistances in equation Eq. 2

The expression relating the bridge’s output to the change in RTD resistance is
 QUOTE   Eq3

The voltage range because of change in temperature is 0volts to 0.3999volts at the output of the bridge.
The operational amplifier is configured as a differential amplifier that amplifies the output of the bridge. This circuit requires precise resistor matching to achieve high common-mode rejection ratios (CMRR).

Figure 4 Differential amplifier.

The differential gain is given by

Output voltage measured at the output is therefore

Anti-aliasing filter.
The filter formed after amplification is called the ant...

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