Schematic Microcontroller Applications and Analogue Interfacing for input and output | Controller Circuit

Thursday, June 7, 2012

Schematic Microcontroller Applications and Analogue Interfacing for input and output

Microcontroller Applications and Analogue Interfacing for input and output

In this post we will learn how to Interface 8051 with the Input and Output Devices like Transducer/Sensors ,Analogue-to-Digital Conversion (ADC) and Digital-to-Analogue Conversion (DAC).These three types of Interfaces will be discussed in this post and some coming posts.

1. The interface of Transducer or sensors with Microcontroller 8051:-

The 8051 microcontroller is a general purpose microcontroller and widely used for different types of applications including industrial applications. In the process or even anyindustry there are many process parameters which are desired to be measured and process by microcontrollers for some suitable decisions. These process variables are sensed or measured by using appropriate sensors or transducer.As the digital computer and microcontrollers use binary values but in the physical world most things (like process variables in industry i.e temperature, pressure, humidity, etc) is in analog nature (continuous).
Thus the data to be measured of these parameters namely for example temperature, pressure, humidity, velocity, voltage are analog data. Most of the physical parameters are not in the form of electrical signals.A device called transducer is used to convert the physical quantity to electrical signals (i.e. voltage, current). One notable things is that the transducer are also referred to as sensors.
Sensors for temperature, velocity, pressure, light, and many other natural quantities can produce an output voltage (or current) which the value is proportional to the quantity being measured.
Then an analog-to-digital converter is used to translate the analog voltage to digital numbers so that microcontroller can read and process them. Lets take only the temperature for better understanding.

Temperature sensors :-

There are a wide variety of temperature sensors on the market today, including Thermocouples, Resistance Temperature Detectors (RTDs), Thermistors, Infrared, and Semiconductor Sensors.Most electronic temperature sensors used in engineering fall into one of two classes: resistive or thermoelectric. But this is not a boundary line as well i think.A more general classification of temperature sensors thermocouples, resistive sensors, bi-metallic sensors, infrared sensors, pyrometry based sensors, semiconductor sensors and thermographic liquid crystals.

Resistance Temperature Detector (RTD)

The RTD is a temperature sensing device whose resistance changes with temperature. Typically built from platinum, though devices made from nickel or copper are not uncommon, RTDs can take many different shapes.To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. We then use a resistance vs. temperature plot to determine the temperature of the surrounding medium.). RTDs exhibit fairly linear resistance to temperature curves over their operating regions, and any non-linearities are highly predictable and repeatable.

Thermistor

Thermistors are semiconductor based transducers, manufactured in the shape of flat disc, beads, or rods. They are manufactured by combining two or more metal oxides. Similar to the RTD, the thermistor is a temperature sensing device whose resistance changes with temperature. Thermistors, however, are made from semiconductor materials.). Resistance is determined in the same manner as the RTD, but thermistors exhibit a highly nonlinear resistance vs. temperature curve.Like the RTD, thermistors require external current excitation and significant signal conditioning. Chips do exist that will convert a thermistor value directly to digital data (MAX6682, Maxim / Dallas Semiconductor.

IC Temperature Sensors

RTDs and thermistors may be simple devices, but they are likely not suited to any mechatronics application. We need to buy the sensor, purchase a chip or create our own circuitry to apply a constant current and measure the resulting voltage, and run this output through and ADC. All these components need to be matched, for example the ADC needs to have high enough resolution to take advantage of the 5mv/°C change from the ADT70. This can end up being quite complicated and costly.

Thermocouples

When two dissimilar metals are in contact with each other, a thermal load existing over the junction of the metallic couple induces a measurable electrical potential. A thermocouple is a self-generating transducer made up of two or more junctions between dissimilar metals. It will be noted that one junction (the cold junction) has to be maintained at a known reference temperature, for instance by surrounding it with melting ice. The other junction is attached to the object to be measured. . The junctions between the two wires and the voltmeter do not cause any error signal to appear as long as they are at the same temperature. Since there is no proper reference junction with this approach, the system is liable to give an erroneous output if the temperature of the surrounding environment changes. This small error may be avoided by using a cold junction compensation system in which the characteristics of the signal conditioning circuitry are modified by including another temperature sensor such as a thermistor in the circuitry.

Semiconductor diode temperature sensors

PN junctions in silicon have become popular as temperature sensors due to their very low cost.The forward bias characteristic of a silicon diode is used here for temperature sensing. It is well-known that a voltage Vf has to be applied across the junction before a current will flow. For silicon Vf (which is often termed the diode voltage drop) is of the order of 600-700 mV. Vf is temperature dependent, and is very nearly linear over the temperature range from – 50 to + 150 °C. The voltage Vf has a temperature characteristic which is essentially the same for all silicon devices of about - 2 mV/°C.

Liquid crystal temperature sensors

A number of liquids (mainly organic) than be made to exhibit an orderly structure, in which most or a1l of the molecules are aligned in a common direction. The structure can be altered by electric or magnetic fields. Most people are familiar with the liquid crystal displays used in watches and calculators which use compounds sensitive to electric fiends. Less well- known, however, is the fact that some liquid crystal materials are temperature sensitive.

Temperature sensors (LM34 and LM35)

LM34 series are precision integrated-circuit temperature sensors whose output voltage is linearly proportional to Fahrenheit temperature. LM35 series are precision integrated-circuit temperature sensors whose output voltage is linearly proportional to Celsius temperature. They require no external calibration since it is internally calibrated. Their output voltage changes 10mV for each degree of temperature change.
Item        Temperature Range             Accuracy          Output
LM34      -50 F to +300 F                 +3.0 F             10 mV/F
LM34C    -40 F to +230 F                +3.0 F             10 mV/F
LM35      -55 °C to +150 °C             +1.5 °C           10 mV/°C
LM35C    -40 °C to +110 °C            +1.5 °C           10 mV/°C
LM35D     0°C to +100°C                +2.0°C            10 mV/°C
The LM34 an LM35 series are precision integrated-circuit temperature sensors.

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