Necessity of data acquisition system in instrumentation: -
- All engineering activities require collection of data, in development, production, quality control, management, process control etc.
- We have been using thermometer, voltmeters, scales, oscilloscopes and such variety of measuring devices and instruments to read the physical data.
- The emphasis is therefore on getting the machines to meet most of our data acquisition and control needs.
- Data acquisition is collecting data or information that describes the given situation.
- Modern data acquisition system translet the real word signals into a format that digital computer can accept, and can also regenerate analog and other control signal from computer instruction.
- These features provide extensive capabilities for mathematical analysis, storage, display, report generation, control and communication.
- Most of real world data signal like temperature, pressure, flow, speed, intensity, position etc. cannot be read directly into electrical signal.
- Transducers are used for this purpose, and the data in form of continuous analog output levels has to be translated into digital from to suit the computer.
Various configuration of data acquisition system: -
These are two types of data acquisition system:
1. Single channel DAS
2. Multi channel DAS
- The various sub-system of data acquisition system can be timed shared by two or more input sources. Depending on the desired properties of multiplexed system, a number of techniques are employed for such time shared measurement.
- The multi-channel analog multiplexed data acquisition system is shown in figure. It has a single analog to digital convertor. This analog to digital convertor has an input which is selected from multiplexer.
- The individual analog signals are applied directly or after amplification and or signal conditioning, whenever necessary to multiplexer. These are further converted to digital signal by using analog to digital convertor.
- For most efficient utilization of time, multiplexer is made to seek the next channel to be converted while the previous data stored in sample is converted to digital form.
- When conversion is complete the status line from convertor causes the sample to return to sample mode and acquires the signal of next channel. On completion of acquisition either immediately. The S/H is switched to hold mode a conversion begin’s and multiplexer selected the channel.
- This method is relatively slower than system, where S/H outputs on even A/D convertor O/P are multiplexed, but it has the obvious advantage of low cost due to sharing of a majority of sub-systems.
- Sufficient accuracy in measurement can be achieved without the S/H, in case where signal amplification is extremely slow.
Signal conditioning in DAS:
- Since all the data have to be acquired, originate generally from different sources so signal conditioning in some cases.
- A simple attenuator is used to down the input gains.
- Analog differentiation, precision rectification and averaging, phase detection, logarithmic conversion, ratio computating using dividers and many other types of processors are used before DAS.
There are two methods of signal conditioning are as follows:
A. Radiometric conversion
B. Logarithmic conversion
A. Radiometric conversion:
- If the bridge output is conditioned in such a way that output of signal amplifier is a voltage proportional to strain only and independent of the excitation voltage, the system accuracy improves since variation in the excitation voltage do not affect the sensitivity of the system.
- This method incorporates an analog divider. The output of divider and excitation is fed to amplifier. So that the output of amplifier is the ratio of the amplifier output voltage to the excitation voltage.
B. Logarithmic conversion:
- This circuit enables the measurement of a fractional change in the input as a percentage of the input magnitude.
- Consider that a change of 1% i.e. the input changes from 100mV to 101mV. The output of the logarithmic amplifier would change by,
Application of DAS:
The applications of data acquisition system are as follow:
1. It is used for collecting information.
2. It converts data into useful form.
3. It is used to generate information for display.
4. It is used in aircraft control system
5. It is used in industrial process control system.