Kurumbalapitiya, D, Hoole, SRH. " Data Acquisition The Electrical Engineering Handbook Ed. Richard C. Dorf Boca raton crc Press llc. 2000
Kurumbalapitiya, D., Hoole, S.R.H. “Data Acquisition” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
84 Data acquisition 84.1 Introduction 84.2 The Analog and Digital Signal Interface 84.3 Analog Signal Conditioning Dhammika 84.4 Sample-and-Hold and A/D Techniques in Data urumbalapitiva Acquisition Harvey Mudd College 4.5 The Communication Interface of a Data Acquisition System S. Ratnajeevan H. Hoole 84.6 Data Recording Harvey Mudd College 84.7 Software Aspects 84.1 Introduction Data acquisition includes everything from gathering data, to transporting it, to storing it. The term data cquisition is described as the " phase of data handling that begins with sensing of variables and ends with a magnetic recording of raw data, may include a complete telemetering link(McGraw-Hill, Dictionary of scientifi and Technical Terms, Second Edition, 1978). Here, the term variables refers to those physical quantities that are associated with a natural or artificial process. a data acquisition phase involves a real-time computing envi- ronment where the computer must be keyed to the time scale of the process. Figure 84. 1 gives a simplified block diagram of a data acquisition system current in the early 1990s 9. The path the data travels through the system is called the data acquisition channel. Data are first captured subsequently translated into usable signals using transducers. In this discussion, usable signals are assumed to be electrical voltages, either unipolar(that is, single ended, with a common ground so that we need just one lead wire to carry the signal)or bipolar(that is, common mode, with the signal carried by a wire pair, so that ne reference of the rest of the system is not part of the output). These voltages can be either analog or digital, depending on the nature of the measurand (the quantity being captured). When there is more than one analog input, they are subsequently sent to an analog multiplexer(MUX). Both the analog and the digital signals are then conditioned using signal conditioners. There are two additional steps for those conditioned analog signal First they must be sampled(see Chapter 73.4)and next converted to digital data. This conversion is done by analog-to-digital converters(ADC)(see Chapter 32) Once the analog-to-digital conversion is done, the rest of the steps have to deal with digital data only. The calendar/clock block shown in Fig 84. 1 is used to add the time-of-date information, an important parameter of a real-time processing environment, into the half-processed data. The digital processor performs the overall system control tasks using a software program, which is usually called system software. These control tasks also include display, printer, data recorder, and communication interface management supply unit(PSU)and a stable clock are essential components in many data acquisition systems. There are systems where massive amounts of data points are produced within a very short period of time, and they are equipped with on-board memory so that a considerable amount of data points can be stored locally. Data are transmitted to the host computer once the local storage has reached its full capacity. Historically, data acquisition evolved in modular form, until monolithic silicon came along and reduced the size of the modules c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 84 Data Acquisition 84.1 Introduction 84.2 The Analog and Digital Signal Interface 84.3 Analog Signal Conditioning 84.4 Sample-and-Hold and A/D Techniques in Data Acquisition 84.5 The Communication Interface of a Data Acquisition System 84.6 Data Recording 84.7 Software Aspects 84.1 Introduction Data acquisition includes everything from gathering data, to transporting it, to storing it. The term data acquisition is described as the “phase of data handling that begins with sensing of variables and ends with a magnetic recording of raw data, may include a complete telemetering link” (McGraw-Hill, Dictionary of Scientific and Technical Terms, Second Edition, 1978). Here, the term variables refers to those physical quantities that are associated with a natural or artificial process. A data acquisition phase involves a real-time computing environment where the computer must be keyed to the time scale of the process. Figure 84.1 gives a simplified block diagram of a data acquisition system current in the early 1990s. The path the data travels through the system is called the data acquisition channel. Data are first captured and subsequently translated into usable signals using transducers. In this discussion, usable signals are assumed to be electrical voltages, either unipolar (that is, single ended, with a common ground so that we need just one lead wire to carry the signal) or bipolar (that is, common mode, with the signal carried by a wire pair, so that the reference of the rest of the system is not part of the output). These voltages can be either analog or digital, depending on the nature of the measurand (the quantity being captured). When there is more than one analog input, they are subsequently sent to an analog multiplexer (MUX). Both the analog and the digital signals are then conditioned using signal conditioners. There are two additional steps for those conditioned analog signals. First they must be sampled (see Chapter 73.4) and next converted to digital data. This conversion is done by analog-to-digital converters (ADC) (see Chapter 32). Once the analog-to-digital conversion is done, the rest of the steps have to deal with digital data only. The calendar/clock block shown in Fig 84.1 is used to add the time-of-date information, an important parameter of a real-time processing environment, into the half-processed data. The digital processor performs the overall system control tasks using a software program, which is usually called system software. These control tasks also include display, printer, data recorder, and communication interface management. A well-regulated power supply unit (PSU) and a stable clock are essential components in many data acquisition systems. There are systems where massive amounts of data points are produced within a very short period of time, and they are equipped with on-board memory so that a considerable amount of data points can be stored locally. Data are transmitted to the host computer once the local storage has reached its full capacity. Historically, data acquisition evolved in modular form, until monolithic silicon came along and reduced the size of the modules. Dhammika Kurumbalapitiya Harvey Mudd College S. Ratnajeevan H. Hoole Harvey Mudd College
ONTRO IGGER PROGRAMMABLE SAMPLE zz≮ PROGRAMS DIGITAL MUX COMMUNICATION INTERFACE MPU TIME CALENDA DDR BUS FIGURE 84.1 The block diagram of a data acquisition system The analysis and design of data acquisition systems is a discipline that has roots in the following subject areas: signal theory, transducers, analog signal processing, noise, sampling theory, quantizing and encoding theory, analog to-digital conversion theory, analog and digital electronics, data communication, and systems engineering Cost, accuracy, bit resolution, speed of operation, on-board memory, power consumption, stability of operation under arious operating conditions, number of input channels and their ranges, on-board space, supply voltage require- ments, compatibility with existing bus interfaces, and the types of data recording instruments involved are some of the prime factors that must be considered when designing or buying a data acquisition system. Data acquisition ms are involved in a wide range of applications, such as machine control, robot control, medical and analytical instrumentation,vibration analysis, spectral analysis, correlation analysis, transient analysis, digital audio and video, seismic analysis, test equipment, machine monitoring, and environmental monitoring 84.2 The Analog and Digital Signal Interface The data acquisition system must be designed to match the process being measured as well as the end-user requirements. The nature of the process is mainly characterized by its speed and number of measuring point whereas the end-user requirement is mainly the flexibility in control. Certain processes require data acquisition with no interruption where computers are used in controlling On the other hand, there are cases where the acquisition starts at a certain instance and continues for a definite period. In this case the acquisition cycle is repeated in a periodic manner, and it can be controlled manually or by software. Controllers access the process via the analog and digital interface submodules, which are sometimes called analog and digital front ends. Many applications require information capturing from more than one channel. The use of the analog MUX in Fig. 84. 1 is to cater to multiple analog inputs. a detailed diagram of this input circuitry is shown in Fig. 84.2 e 2000 by CRC Press LLC
© 2000 by CRC Press LLC The analysis and design of data acquisition systems is a discipline that has roots in the following subject areas: signal theory, transducers, analog signal processing, noise, sampling theory, quantizing and encoding theory, analogto-digital conversion theory, analog and digital electronics, data communication, and systems engineering. Cost, accuracy, bit resolution, speed of operation, on-board memory, power consumption, stability of operation under various operating conditions, number of input channels and their ranges, on-board space, supply voltage requirements, compatibility with existing bus interfaces, and the types of data recording instruments involved are some of the prime factors that must be considered when designing or buying a data acquisition system. Data acquisition systems are involved in a wide range of applications, such as machine control,robot control, medical and analytical instrumentation, vibration analysis, spectral analysis, correlation analysis, transient analysis, digital audio and video, seismic analysis, test equipment, machine monitoring, and environmental monitoring. 84.2 The Analog and Digital Signal Interface The data acquisition system must be designed to match the process being measured as well as the end-user requirements. The nature of the process is mainly characterized by its speed and number of measuring points, whereas the end-user requirement is mainly the flexibility in control. Certain processes require data acquisition with no interruption where computers are used in controlling. On the other hand, there are cases where the acquisition starts at a certain instance and continues for a definite period. In this case the acquisition cycle is repeated in a periodic manner, and it can be controlled manually or by software. Controllers access the process via the analog and digital interface submodules, which are sometimes called analog and digital front ends. Many applications require information capturing from more than one channel. The use of the analog MUX in Fig. 84.1 is to cater to multiple analog inputs. A detailed diagram of this input circuitry is shown in Fig. 84.2 FIGURE 84.1 The block diagram of a data acquisition system
MODE CONTROL DECODER CHANNEL GAIN LIST REGISTER FIGURE 84.2 Analog input circuitry-the analog front end. and the functional description is as follows. When the mUx is addressed to select an input, say, x n), the same address will be decoded by the decoding logic to generate another address, which is used in addressing the programmable register. The programmable register contains further information regarding how to handle x(o) The outcome of the register is then used in subsequent tuning of the signal conditioner. Complex programmabl control tasks might include automatic gain selection for each channel, and hence the contents of this register are known as the channel gain list. The MUX address generator could be programmed in many ways, and one imple way is to scan the input channels in a cyclic fashion where the address can be generated by means of a binary counter. Microprocessors are also used in addressing MUXs in applications where complex channel selection tasks are involved. Multiplexers are available in integrated circuit form, though relay MUXs are widely sed because they minimize errors due to cross talk and bias currents. Relay MUX modules are usually designed as plugged-in units and can be connected according to the requirements There are applications where the data acquisition cycle is triggered by the process itself. In this case an analog or digital trigger signal is sent to the unit by the process, and a separate external trigger interface circuitry is supplied. The internal controller assumes its duties once it has been triggered. It takes a finite time to settle the ignal x, (n) through the MUX up to the signal conditioner once it is addressed. Therefore, it is possible process x_(n) during the selection time of x (t) for greater speeds. This function is known as pipelining and will be illustrated in Section 84.3 In some data acquisition applications the data acquisition module is a plugged-in card in a computer, which is installed far away from the process. In such cases, transducers-the process sensing elementsare connected to the data acquisition module using transmission lines or a radio link. In the latter case a complete demodu lating unit is required at the input. When transmission lines are used in the interconnection, care must be taken to minimize electromagnetic interference since transmission lines pick up noise easily. In the case of a single ended transducer output configuration, a single wire is adequate for the signal transmission, but a common ground must be established between the two ends as given in Fig. 84.3(a). For the transducers that have common mode outputs, a shielded twisted pair of wires will carry the signal. In this case, the shield, the transducer's encasing chassis, and the data acquisition module's reference may be connected to the same ground as shown Fig. 84.3(c). In high-speed applications the transmission line impedance should be matched with the output pedance of the transducer in order to prevent reflected traveling waves. If the transducer output is not strong enough to transmit for a long distance, then it is best to amplify it before transmission. Transducers that produce digital outputs may be first connected to Schmitt trigger circuits for pulse shaping purposes, and this can be considered as a form of digital signal conditioning. This becomes an essential requirement when such inputs are connected through long transmission lines where the line capacitance significantly affects the rising and falling edges of the incoming wave Opto-isolators are sometimes used in coupling when the voltage levels of the two sides of the transducer and the input circuit of the data acquisition unit do not match each other. Special kinds of connectors are designed and widely used in interconnecting e 2000 by CRC Press LLC
© 2000 by CRC Press LLC and the functional description is as follows. When the MUX is addressed to select an input, say, xi (t), the same address will be decoded by the decoding logic to generate another address, which is used in addressing the programmable register. The programmable register contains further information regarding how to handle xi (t). The outcome of the register is then used in subsequent tuning of the signal conditioner. Complex programmable control tasks might include automatic gain selection for each channel, and hence the contents of this register are known as the channel gain list. The MUX address generator could be programmed in many ways, and one simple way is to scan the input channels in a cyclic fashion where the address can be generated by means of a binary counter. Microprocessors are also used in addressing MUXs in applications where complex channel selection tasks are involved. Multiplexers are available in integrated circuit form, though relay MUXs are widely used because they minimize errors due to cross talk and bias currents. Relay MUX modules are usually designed as plugged-in units and can be connected according to the requirements. There are applications where the data acquisition cycle is triggered by the process itself. In this case an analog or digital trigger signal is sent to the unit by the process, and a separate external trigger interface circuitry is supplied. The internal controller assumes its duties once it has been triggered. It takes a finite time to settle the signal xi (t) through the MUX up to the signal conditioner once it is addressed. Therefore, it is possible to process xi–1(t) during the selection time of xi (t) for greater speeds. This function is known as pipelining and will be illustrated in Section 84.3. In some data acquisition applications the data acquisition module is a plugged-in card in a computer, which is installed far away from the process. In such cases, transducers—the process sensing elements—are connected to the data acquisition module using transmission lines or a radio link. In the latter case a complete demodulating unit is required at the input. When transmission lines are used in the interconnection, care must be taken to minimize electromagnetic interference since transmission lines pick up noise easily. In the case of a singleended transducer output configuration, a single wire is adequate for the signal transmission, but a common ground must be established between the two ends as given in Fig. 84.3(a). For the transducers that have common mode outputs, a shielded twisted pair of wires will carry the signal. In this case, the shield, the transducer’s encasing chassis, and the data acquisition module’s reference may be connected to the same ground as shown in Fig. 84.3(c). In high-speed applications the transmission line impedance should be matched with the output impedance of the transducer in order to prevent reflected traveling waves. If the transducer output is not strong enough to transmit for a long distance, then it is best to amplify it before transmission. Transducers that produce digital outputs may be first connected to Schmitt trigger circuits for pulse shaping purposes, and this can be considered as a form of digital signal conditioning. This becomes an essential requirement when such inputs are connected through long transmission lines where the line capacitance significantly affects the rising and falling edges of the incoming wave. Opto-isolators are sometimes used in coupling when the voltage levels of the two sides of the transducer and the input circuit of the data acquisition unit do not match each other. Special kinds of connectors are designed and widely used in interconnecting FIGURE 84.2 Analog input circuitry—the analog front end
ACQUISITION TRANSDUCER OR SENSO MODULE OR SENSOR Vs2VR2 DATA IsE上己usmN MODULE FIGURE 84.3(a) Connecting transducers to the data acquisition unit, (b)single-ended(unipolar)output, and (c)com- - mode(bipolar) output I/P FIGURE 84.4 Programmable gain instrumentation amplifie transmission lines and data acquisition equipment in order to screen the signals from noise. Analog and digital ignal grounds should be kept separate where possible to prevent digital signals from flowing in the analog ground circuit and including spurious analog signal noise. 84.3 Analog Signal Conditioning The objective of an analog signal conditioner is to increase the quality of the transducer output to level before analog-to-digital conversion. a signal conditioner mainly consist of a preamplifier, whic an instrumentation amplifier or an operational amplifier and/or a filter. Coupling more and more the data acquisition channel has to be done taking great care that these signal conditioning circuits do not add more noise or unstable behavior to the data acquisition channel. General purpose signal conditioner modules are commercially available for applications. Some details were given in the previous section about programi signal conditioners and the discussion is continued here. Figure 84. 4 shows an instrumentation amplifier with programmable gain where the programs are stored in the channel-gain list. The reason for having such sophistication is to match transducer outputs with the maximum allowable input range of the ADC. This is very important in improving accuracy in cases where transducer output voltage ranges are much smaller than the full-scale input range of an ADC, as is usually the case. Indeed, this is equally true for signals that are larger than the full-scale range, and in such cases the amplifier functions as an attenuator. Furthermore, the instrumentation amplifier converts a bipolar voltage ignal into a unipolar voltage with respect to the system ground. This action will reduce a major control task far as the ADC is concerned; that is, the aDC is always sent unipolar voltages, and hence it is possible to maintain unchanged the mode control input which toggles the adC between the unipolar and bipolar modes of an ADc e 2000 by CRC Press LLC
© 2000 by CRC Press LLC transmission lines and data acquisition equipment in order to screen the signals from noise. Analog and digital signal grounds should be kept separate where possible to prevent digital signals from flowing in the analog ground circuit and including spurious analog signal noise. 84.3 Analog Signal Conditioning The objective of an analog signal conditioner is to increase the quality of the transducer output to a desired level before analog-to-digital conversion. A signal conditioner mainly consist of a preamplifier, which is either an instrumentation amplifier or an operational amplifier and/or a filter. Coupling more and more circuits to the data acquisition channel has to be done taking great care that these signal conditioning circuits do not add more noise or unstable behavior to the data acquisition channel. General purpose signal conditioner modules are commercially available for applications. Some details were given in the previous section about programmable signal conditioners and the discussion is continued here. Figure 84.4 shows an instrumentation amplifier with programmable gain where the programs are stored in the channel-gain list. The reason for having such sophistication is to match transducer outputs with the maximum allowable input range of the ADC. This is very important in improving accuracy in cases where transducer output voltage ranges are much smaller than the full-scale input range of an ADC, as is usually the case. Indeed, this is equally true for signals that are larger than the full-scale range, and in such cases the amplifier functions as an attenuator. Furthermore, the instrumentation amplifier converts a bipolar voltage signal into a unipolar voltage with respect to the system ground. This action will reduce a major control task as far as the ADC is concerned; that is, the ADC is always sent unipolar voltages, and hence it is possible to maintain unchanged the mode control input which toggles the ADC between the unipolar and bipolar modes of an ADC. FIGURE 84.3 (a) Connecting transducers to the data acquisition unit, (b) single-ended (unipolar) output, and (c) common-mode (bipolar) output. FIGURE 84.4 Programmable gain instrumentation amplifier