DATA ACQUISITION SYSTEM DAS Data Acquisition System DAS

DATA ACQUISITION SYSTEM (DAS)

Data Acquisition System (DAS) • The system of measurement and recording data is called data acquisition system. • The two major uses of data acquisition system are: 1. Instrumentation or measurement system. 2. Recording system. • There are two types of DAS: 1. Analog data acquisition system. 2. Digital data acquisition system.

Data Acquisition System (DAS) DAS consists of individual sensors with the necessary signal conditioning, data conversion, data processing, multiplexing, data handling and associated transmission, storage and also display systems. To optimize the characteristic of the system in term of performance, handling capacity and cost, the relevant sub system can be combined together. Analog data is generally acquired and converted into digital form for the purpose of processing, transmission, display and storage.

Data Acquisition System (DAS) Data may be transmitted over long distances (from one point to another) or short distances (from test centre to a nearby PC). It may be displayed on a digital panel or on a Cathode Ray Tube (CRT). To match the input requirements with the output of the sensor, some form of scaling and offsetting is necessary, and can be done by using amplifier/attenuators. For converting analog information from more than one source, either additional transducers or multiplexers are employed.

Data Acquisition System (DAS) Transducer 1 Transducer 2 Signal conditioner 1 Signal conditioner 2 Transducer 3 Signal conditioner 3 Transducer 4 Signal conditioner 4 Printer Analog M data U L T I P L E X E R -Recorders -Display -Meter A/D Converter Digital Display Magnetic Tape Transmission Computer Processing Schematic block diagram of a General Data Acquisition System (DAS)

Data Acquisition System (DAS) The characteristics of the DAS depend on both the properties of the analog data and on the processing carried out. Based on the environment, a board classification divides the DAS into 2 categories. I. Those suitable for favorable environments -minimum RF interference and electromagnetic induction- (laboratory instrument applications, test systems for collecting long term drift information, high calibration test instruments, etc) II. Those intended for hostile environments (aircraft control systems, turbovisous in electrical power systems, industrial process control systems)

Data Acquisition System (DAS) The important factors that decide the configuration and sub systems of the DAS: 1. Accuracy and resolution 2. Number of channels to be monitored 3. Analog or digital signal 4. Single channel or multichannel 5. Sampling rate per channel 6. Signal conditioning requirements of each channel 7. Cost

Data Acquisition System (DAS) The various general configuration include: 1. i. iii. iv. Single channel possibilities Direct conversion Pre-amplification and direct conversion Sample and hold, and conversion Pre- amplification, signal conditioning and any of the above 2. i. iii. iv. Multi channel possibilities Multiplexing the output of single channel converters Multiplexing the output of sample-hold circuits. Multiplexing the input of sample-hold circuits. Multiplexing low level data

Data Acquisition System (DAS) Signal Conditioning of the Inputs It is primarily utilized for DAS, in which sensor signals must be normalized and filtered to levels suitable for analog-to-digital conversion so they can be read by computerized devices Two methods of signal conditioning which are particularly applicable with advantage to DAS are: i. Ratio metric conversion ii. Logarithm conversion

Single Channel DAS Consists of a signal conditioner followed by an analog to digital (A/D) converter. The outputs are in digital code words including over range indication, polarity information and a status output to indicate when the output digits are valid. A/D Converter Convert Command Buffer To Computer Print Out/ Storage

Analog to Digital Converters used for DAS Usually designed to receive external command to convert and hold. For DC or low frequency signals, such as from thermocouple (especially in the presence of noise), a dual slope type converter is often used. The advantage is that it has a linear averaging capability and has a null response for frequencies harmonically related to the integrating period. The most popular type of converter for data system applications is the successive approximation type, since it is capable of high resolution and high speed at moderate cost

Pre-amplification and Filtering For signal levels which are low compared to input requirements, amplification may be used in order to bring up the level of the input to match converter input requirements, so that optimum use can be made in terms of accuracy and resolution. If the signal levels are below a 10 of an m. V, or when resolution of 14 bit or 16 bits is needed, the use of differential amplifiers can become a necessity. If the input signals are to be physically isolated from the system, the conductive paths are broken by using transformer coupled or an optocoupled isolation amplifier (usually using for high voltages sources, transmission towers, biomedical applications, etc).

Multi-Channel DAS 1. Multi-Channel Analog Multiplexed system The individual analog signal are applied directly or after amplification and/ or signal conditioning, whenever necessary, to the multiplexer. For the most efficient utilization of time, the multiplexer is made to seek the next channel to be converted while the previous data stored in the sample/hold is converted to digital form. When the conversion is complete, the status line from the converter causes the sample/hold to return to the sample mode and acquires the signal of the next channel. On completion , the S/H is switched to the hold mode, and conversion begin again for the next channel.

Analog Signal 1 Scaling, Amplification, Signal Conditioning 1 Analog Signal 2 Scaling, Amplification, Signal Conditioning 2 Analog Signal 3 Scaling, Amplification, Signal Conditioning 3 * Multi-Channel Analog Multiplexed system Multiplexer S/H Analog Signal 4 Scaling, Amplification, Signal Conditioning 4 A/D Logic Buffer To Computer or Data Transmission

2. Multiplexing the Outputs of Sample/Hold When a large number of channel are to be monitored at the same time (synchronously) but at moderate speeds, the technique of multiplexing the outputs of the S/H is particularly attractive. The S/H outputs are connected to an A/D converter through a multiplexer, resulting in a sequential readout of the outputs. Applications that might require this approach include wind tunnel measurements, seismographic experimentation, radar and fire control systems.

Analog Inputs After Signal Conditioning Sample/ Hold (S/H) * Multiplexing the Outputs of Sample/Hold (S/H) Multiplexer A/D (S/H) Timing Interface To Transmission or Processing system

3. Multiplexing After A/D conversion Since each A/D converter is assigned to an individual channel, the conversion rate of the A/D need only be as fast as is needed for that channel, compared to the higher rate that would be needed if it were used as in a multi channel analog multiplexed system. The parallel conversion provides additional advantages in industrial DAS where many strain gauges, thermocouple, and LVDT are distributed over large plant areas. The data converted to digital form is used to perform logic operation and decisions.

Multiplexer Sub-channel Analog Data for Signal Conditioning S/H A/D Processor Buffer Timing Digital Multiplexer * Multiplexing After A/D conversion

4. Multiplexing Low Level Data It enables the use of a single high quality data amplifier for handling multichannel low level input. Low level multiplexing can be attractive when a large number of channel (25), all having low level outputs, need to be used at moderate speeds. Several factors have to be considered to accomplish low level multiplexing. Guarding may have to be employed for every channel, and each individual guard may have to be switched, so that guard is driven by the common mode pertaining to that channel.

* Multiplexing Low Level Data Differential Input Guard Multiple Switches To other Channel Instrumentation Amplifier Guard S/H A/D Buffer

Computer Based DAS aids operate in the following manner. 1. Display information instantly in condensed, understandable and legible manner so that it can be easily assimilated. 2. Display spatial as well as time variation. 3. Display vital parameters grouped together logically and concisely, eliminating the need of looking at many scattered instruments. 4. Display CRT graphic displays of plant sub-systems. 5. Display short trends on a long and short term basis, as required.

Computer Based DAS 6. Analyze the data and present the highest priority problem first, and display operator guidance message. 7. Analyze the data and present the derived data. 8. Display alarms, indicating abnormal plant operating conditions on the CRT. 9. Provide trending of analog variable on strip chart recorders, in the form of a histogram on the CRT, and provide dynamic updating of parameters. 10. Produce a hard copy record of all plant operating events and various plant logs. 11. Provide a recording of the sequence of events, whenever an emergency occurs.

Digital to Analog(D/A) and Analog to Digital (A/D) Converters DAC involves translating digital information into equivalent analog information. v E. g. the output of a digital system might be changed to analog form to drive a pen recorder. DAC also be considered as a decoding device. DAC is usually an integral part of any ADC used for the reverse process of changing analog signals to equivalent binary signal. ADC is often referred to as an encoding device.

Variable Resistor Network The basic problem in converting a digital signal into an equivalent analog signal (D/A) is to change the n digital voltage level into one equivalent analog voltage. A resistive divider having three digital inputs and an analog output is shown in figure 20 21 Resistance Divider Analog Output 22 In this case, assume digital levels 0=0 V and 1=+7 V

Variable Resistor Network For an input of 001, the output is +1 V; similarly for 010, the output is +2 V and for 100, the output is +4 V. Now digital input of 011 is seen to be a combination of signals 001 and 010. If +1 V from the 20 bit is added to +2 V from the 21 bit, the output is +3 V for an input of 011. Hence, the resistive ladder must do two things in order to change the digital input into an equivalent analog output. 1. 20 bit must be changed to +1 V, 21 bit to +2 V and 22 bit to +4 V 2. The three voltages representing the digital bits must be summed together to form the analog output voltage

Con’t (Modified Millman’s Theorem) Where: V 0, V 1, V 2, …. . , Vn-1 are digital input voltage levels (0 and +V) and n is number of input bits.

Example 5: For a 5 bit resistive divider, determine the following 1. The weights assigned to the LSB. 2. The weights assigned to the 2 nd and 3 rd LSB. 3. The change in output voltage due to the change in the LSB, 2 nd LSB and 3 rd LSB. 4. The output voltage for a digital input of 11011 and 10110. (Assuming 0=0 V and 1=+10 V)

Binary Ladder Binary ladder is constructed of resistors having only two values and thus overcome the disadvantages of weighted resistors. The left end of the ladder is terminated in 2 R, as shown in figure

Binary Ladder Assume all inputs are at ground, beginning at point A, the total resistance looking into terminating resistor is 2 R (total resistance looking out towards the 20 input is 2 R). These two resistors combine to form a value of R.

Binary Ladder At node B or at 21, the input is still 2 R It is also the same for resistance looking back towards node C is 2 R (resistance looking at 23 input)

Binary Ladder From the previous circuit, we can conclude that the resistance looking back from any node towards the terminating resistance or out toward the digital input is 2 R. We can use this to determine the various digital inputs. Assume digital input 1000

Binary Ladder After simplification, we got : Therefore :

Binary Ladder To determine the output voltage due to the 2 nd MSB, assume an input of 0100, as shown in figure.

Binary Ladder After simplification, we got : Therefore: This process can be continued, and it can be shown that the 3 rd MSB gives an output voltage of +V/8, the 4 th MSB gives an output voltage of +V/16 and so on.

Example 6: For a 5 bit binary ladder, if the input levels are 0=0 V and 1=+10 V. What are the output voltage for each bit?

Thank you QUESTIONS?

RECORDERS

Recorders The output device which gives output for permanent record is called a recorder Generally provide a graphic record of variation in the quantity being measured, as well as an easily visible scale on which the indication is displayed. Recording usually provides instantaneous indication for monitoring at the same time as it makes a graphic record. Electronic recording instruments may be divided into three groups.

Types of Recorders 1. The easiest type is simply a meter having an indicating needle and a writing pen attached to the needle. This type is called a galvanometer recorder. 2. Null or potentiometric recorder, operating on a selfbalancing comparison basis by servomotor action. This recorder is basically a voltage responsive positional servo system using a motor to move a writing device back and forth across a piece of paper. 3. Magnetic recorder is a thin magnetic tape or wire, is magnetized in accordance with a varying signal as the tape passes rapidly across a magnetic recording head.

Recorders Two types of recording devices: i. Analog ii. Digital ü Analog recorders can be classify into two groups: i. Graphic recorders ii. Magnetic recorders ü There are three main types of graphic recorders: i. Strip chart recorders ii. Circular chart recorders iii. X-Y recorders

Strip Chart Recorder Strip chart recorders are those in which the data is recorder on a continuous roll of chart paper moving at a constant speed The recorder records the variation of one or more variables with respect to time The basic element of a strip chart recorder consists of : i. A pen (stylus) used for making marks on a movable paper ii. A pen (stylus) driving system iii. A vertically moving long roll of chart paper iv. Chart paper drive mechanism v. Chart speed selector switch

Applications of Strip Chart Recorder 1. Temperature Recording – A strip chart recorder may be used to provide a graphical record of temperature as a function of time. There are two primary methods used for recording temperature, thermocouple method and the resistance method. 2. Sound Level Recording – It is frequently desirable to obtain a record of the sound level over a period of time, near highways, airports, hospitals, schools or residence. 3. Recording Amplifier Drift – Transistor amplifier are sensitive to temperature changes. Temperature changes causes the bias voltage of the transistor to change, thereby changing the operating or quiescent point, generally by a small amount. This change of the point is called Drift.

X-Y Recorder In most research field, it is often convenient to plot the instantaneous relationship between two variables [y=f(x)], rather than to plot each variable separately as a function of time In such cases, the X-Y recorder is used, in which one variable is plotted against another variable In an analog X-Y recorder, the writing head is deflected in either the x-direction or the y-direction on a fixed graph chart paper The writing head consists of one or two pens, depending on the application

Magnetic Recorders The major advantage of using a magnetic tape recorder is that once the data is recorded, it can be replayed an almost indefinite number of times The recording period may vary from a few minutes to several days The recorders described earlier have a poor high frequency response Magnetic tape recorders, on the other hand, have a good response to high frequency, i. e. they can be used to record high frequency signals. Hence, magnetic tape recorders are widely used in instrumentation systems

Magnetic Tape Recorder A magnetic tape recorder consists of the following basic components 1. Recording head 2. Magnetic head 3. Reproducing head 4. Tape transport mechanism 5. Conditioning devices There are three methods of magnetic tape recording which are used for instrumentation purposes 1. Direct recording 2. FM recording 3. Pulse duration modulation recording (PDM)

Advantage of Direct Recording Has a wide frequency response, ranging from 50 Hz-2 MHz for a tape speed of 3. 05 m/s. It provides the greatest bandwidth obtainable for a given recorder Requires simple electronic circuits Has a good dynamic response and takes overloads without increase in distortion. In general, instrumentation recorders have a signal to noise ratio of 22 -30 db at 1% total harmonic distortion (THD) It is used to record signal where information is contained in the relation between frequency and amplitude, such as the spectrum analysis of noise It can be used for recording voice signals

Advantages of FM Recording Useful primarily when the dc component of the input signal is to be preserved Has a wide frequency range and can record from dc voltages to several k. Hz No drop-out effect due to inhomogenetics of the tape material Independent of amplitude variations, and accurately reproduces the waveform of the input signal Used extensively for recording voltages derived from nonelectrical quantities, such as force, acceleration and pressure Extremely useful for multiplexing in an instrumentation system

Digital Data Recording Advantages • High accuracy • Insensitivity to tape speed • Use of simple conditioning equipment • The information is fed directly to a digital computer for processing and control Disadvantages • Poor tape economy • The information from transducers is in analog form, hence an A/D converter is required • A high quality tape and tape transport mechanism are required