www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS UNIT 8 (a) Interfacing resistive transducers to electronic circuits. Introduction to data acquisition systems. 2 Hours (b) Display Devices and Signal Generators: X-Y recorders. Nixie tubes. LCD and LED display. Signal generators and function generators. 4 Hours UNIT 8a: Interfacing resistive transducers to electronic circuits. Introduction to data acquisition systems. Interfacing resistive transducers to electronic circuits The major problem with resistive transducers, which includes strain gages, temperature transducers is that the resistance change is very small. As an example, consider measuring the current through a resistance transducer such as an RTD. A simple panel meter is used as an Indicator to provide a remote reading of temperature. The change in the meter Indication is very small for small temperature changes. As an example, the change in resistance of a platinum resistance thermometer is 0.385 per cent per degree Celsius. In this case, a 1- degree change in temperature will produce a 0.385 Per cent change in the indicating meter, which will be hardly visible. A solution to this Problem is to connect the resistance transducer in a bridge circuit as shown in Fig.1 www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS Fig. 1 Resistance Transducer in a Bridge Circuit First, the zero output voltage Point can be set for a convenient point such as 0 degrees Celsius or 0 degrees Fahrenheit instead of an absolute zero, which would be the situation if only the transducer current were measured. The setting of the zero output point can be achieved by adjusting the values of R1, R2 , and R3 to provide bridge balance at the desired temperature. The change of output voltage in Fig 1. is Eqn.1 The output voltage is not a linear function of the change in resistance because of the ∆R term in the denominator of Eqn. 1. An improvement may be made by providing a constant current source for the bridge as shown in Fig.2. www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS Fig. 2 Wheatstone bridge powered from a constant current source The equation for the output voltage as a function of resistance is There is an improvement in the linearity of the output voltage as a function of the change in resistance, as the ∆R term in the denominator is divided by a factor of 4R rather than 2R and thus the effects of the ∆R term in the denominator are reduced (as compared to Eqn. 1) If the change in resistance is small, which is often the case with a resistance thermal device, the error due to the lack of linearity is small. Absolute linearity can be achieved by using two transducers as shown in Fig. 3 www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS Fig. 3 Wheatstone bridge employing two RTDs The output voltage as a function of R is This technique requires the use of two matched transducers in the environment to be measured. Most resistive transducers are not expensive items, and providing two matched transducers on a common header is not a difficult task. Often when a transducer is eventually connected to a digital system, a microprocessor can be employed to "linearize" the bridge output voltage. INTERFACING TRANSDUCERS TO ELECTRONIC CONTROL AND MEASURING SYSTEMS The output voltages and currents from many transducers are very small signals. In addition to the low levels, it is often necessary to transmit the transducer output some distance to the data collection or control equipment. Also in an industrial environment, where there is large electrical machinery, the electrical noise present can cause serious difficulties in low-level circuits. These www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS noises can be either radiated as an electromagnetic field or induce in the wiring of the plant as ground loops, and induced spikes on the ac power supply. Regardless of the source of noise, low-level signals must be transmitted from place to place with care. One effective method of combating noise is to increase the strength of low level signals before transmission through wires. This is often done with an amplifier called an instrumentation amplifier. There are several characteristics of an instrumentation amplifier that set them apart from operational amplifiers. The features of a Three Op-Amp Differential topology of an instrumentation amplifier are: Combination of inverting and non-inverting topologies; The output signal is an amplified version of the difference between the two input signals; AVD = R2/R1; V0 = AVD(V2 – V1); Fig. Instrumentation Amplifier www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS Data acquisition systems Data acquisition systems are used to measure and record signals obtained in basically two ways: 1. Signals originating from direct measurement of electrical quantities; these may include dc and ac voltages, frequency , or resistance, and are typically found in such areas as electronic component testing, environmental studies, and quality analysis work. 2. Signals originating from transducers, such as strain gages and thermocouples Instrumentation systems can be categorized into two major classes: analog systems and digital systems. Analog systems deal with measurement information in analog form. An analog signal may be defined as a continuous function, such as a plot of voltage versus time, or displacement versus pressure. Digital systems handle information in digital form. A digital quantity may consist of a number of discrete and discontinuous pulses whose time relationship contains information about the magnitude or the nature of the quantity. Data acquisition systems are used in a large and ever-increasing number of applications in a variety of industrial and scientific areas, such as the biomedical, aerospace, and telemetry industries. The type of data acquisition system, whether analog or digital, depends largely on the intended use of the recorded input data. In general, analog data systems are used when wide bandwidth is required or when lower accuracy can be tolerated. Digital systems are used when the physical process being monitored is slowly varying (narrow bandwidth) and when high accuracy and low per-channel cost is required. www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS Digital systems range in complexity from single-channel dc voltage measuring and recording systems to sophisticated automatic multichannel systems that measure a large number of input parameters, compare against preset limits or conditions, and perform computations and decisions on the input signal. Digital data acquisition systems are in general more complex than analog systems, both in terms of the instrumentation involved and the volume and complexity of input data they can handle. Digital systems require converters to change analog voltages into discrete digital quantities or numbers. Conversely, digital information may have to be converted back into analog form, such as a voltage or a current, which can then be used as a feedback quantity controlling an industrial process. Analog Data Acquisition System An analog data acquisition system typically consists of some or all of the following elements: 1. Transducers for translating physical parameters into electrical signals. 2. Signal conditioners for amplifying, modifying, or selecting certain portions of these signals. 3. Visual display devices for continuous monitoring of the input signals. These devices may include single- or multichannel oscilloscopes, storage oscilloscopes, panel meters, numerical displays, and so on. 4. Graphic recording instruments for obtaining permanent records of the input data. These instruments include stylus-and-ink recorders to provide continuous records on paper charts, optical recording systems such as mirror galvanometer recorders, and ultraviolet recorders. 5. Magnetic tape instrumentation for acquiring input data, preserving their original electrical form, and reproducing them at a later date for more detailed analysis. www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS www.bookspar.com | VTU NOTES | QUESTION PAPERS | NEWS | RESULTS | FORUMS Digital Data Acquisition System A digital data acquisition system may include some or all of the elements shown in Fig. 4 The essential functional operations within a digital system include handling analog signals, making the measurement, converting and handling digital data, and internal programming and control. Fig. 4 Elements of a digital data-acquisition system 1. Transducer Translates physical parameters to electrical signals acceptable by the acquisition system. Some typical parameters include temperature, pressure, acceleration, weight displacement, and velocity. Electrical quantities, such as voltage, resistance, or frequency, also may be measured directly. 2. Signal conditioner Generally includes the supporting circuitry for the transducer. This circuitry may provide excitation power, balancing circuits and calibration elements. An example of