Data Acquisition System

Syllabus

• Design of Data Acquisition System (DAS)

Introduction to Data Acquisition System

Data acquisition systems are widely used in renewable energy source applications in order to collect data regarding the installed system performance, for evaluation purposes. 

Data Acquisition system consists of:

Block Diagram

DAS Design

Example:-Design a DAS system to measure Temperature, Humidity, Pressure and display these parameters on LCD for weather monitoring application

Selection of Sensors

Temperature sensing

Temperature sensing is very straight-forward with the LM335. The output of this device is equal to the absolute temperature in degrees Kelvin divided by 100, so:
Vout = Temperature (°K)/100
To determine the temperature in o C, we use the equation:
ºC = 100 *Vout – 273

Humidity sensing

For humidity sensing, we opted for the Humirel HS1101 capacitive sensor. This device, when combined with a CMOS 555 timer operating as an astable multivibrator, produces a signal with a humidity dependent frequency. To minimize temperature effects, it is important to use the Texas Instruments TLC555 device in this design. Also note that care must be taken at the node of the HS1101 and the 555 timer. Stray capacitance values will lead to erroneous and unpredictable measurements.
A first order equation that relates relative humidity hu to frequency f is: hu = 565.1 – 0.0767 * f

Pressure sensing

Pressure sensing is provided by a Motorola MPX5100A, which operates from 0 to 16 PSI. However, we are interested in only a very small part of that range. Barometric pressure readings fall between 28 and 32 in. of mercury. This translates to 13.75–15.72 PSI. To increase the dynamic range of the output, we added an amplifier circuit (IC), which subtracts about 3.7 V from the sensor output and then multiplies the difference by 4.

Signal Conditioning

The output of sensor is not always in the form of change in voltage which is required for microcontroller, it may be in the form of change in current, change in Resistance, Change in Capacitance, so Signal Conditioning block  is required in order to convert the these parameters into appropriate change in voltage.

Temperature sensor:- As we are using LM35 temperature sensor it provides output in the form millivoltage, Hence there is no need of sinal conditioning
Humidity sensor:-Change in capacitance is converted into change in frquency by Astable multivibrator which works as signal conditioning
Pressure Sensor:-To increase the dynamic range of the output, we added an amplifier circuit (IC), which subtracts about 3.7 V from the sensor output and then multiplies the difference by 4.

Data Conversion

Interface three sensors to different channels of ADC, Select appropriate channel
Analog to digital converter will convert the Analog data into digital which is then fed to 8051 microcontroller

Data Processing

8051 microcontroller will process the data and will send to display or to storage device or to printer

Display

16x2 LCD Display will display real time temperature , Humidity and Pressure of the environmentwith their units

Interfacing Diagram

Assembly Language Program for Interfacing

References

• Intrfacing diagram by Jayesh Gopal ,Student,SKNCOE
• Created and Edited by Prof. S.M.Wagh, SKNCOE, Pune
• WikiNote Foundation