Metal Oxide Semiconductor Co Sensor

Published Date: 02 Nov 2017

Chapter 3

In this project used three different sensor, carbon monoxide sensor, temperature sensor and humidity sensor. Each sensor has different type of sensor and different characteristics. Therefore, we must know all the sensor principle operation for each sensor. Select the most suitable sensor for our project because various sensors will have different sensitivity characteristics.

3.1.1 Metal-Oxide Semiconductor CO Sensor

Sensing element, sensor base and sensor cap are the main three things for gas sensor. The sensing element contains sensing material and heater. Heater is needs to heat up sensing element like 400°C. The sensing element will utilize different materials depending on the target gas. When a metal oxide crystal such as SnO2 is heated at a certain high temperature in air, oxygen is adsorbed on the crystal surface with a negative charge. Then donor electrons in the crystal surface are transferred to the adsorbed oxygen, resulting in leaving positive charges in a space charge layer.

Thus, surface potential is formed to serve as a potential barrier against electron flow. Electric current flows through the gain boundary or conjunction parts of SnO2 micro crystals inside the sensor. Adsorbed oxygen forms a potential barrier at grain

boundaries, which prevents carriers from moving freely. The electrical resistance of the sensor is attributed to this potential barrier. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, so the barrier height in the grain boundary is reduced .The reduced barrier height decreases sensor resistance. [7]

3.1.2 Thermistor

Thermistors are semiconductor devices that are used to measure temperature. The name comes from a combination of the words "resistor" and "thermal". Thermistors have an electrical resistance that is proportional to temperature. The resistance decreases as temperature increases which is just opposite for RTD’s. Thermistors are an inexpensive alternative to RTD’s when temperature ranges are below 150°C. Thermistors can be used from temperatures of –80°C to 300°C. Thermistors are made by sintering various metal oxides together, attaching leads and packaging them in a small epoxy coated body. These style thermistors are normally limited to a maximum temperature of 150°C. Some of the higher temperature thermistors, which are glass coated, can be used up to 300°C. [8]

3.1.3 Capacitance Humidity Sensor

They consist of a substrate on which a thin film of polymer or metal oxide is deposited between two conductive electrodes. The sensing surface is coated with a porous metal electrode to protect it from contamination and exposure to condensation. The substrate is typically glass, ceramic, or silicon. The incremental change in the dielectric constant of a capacitive humidity sensor is nearly directly proportional to the relative humidity of the surrounding environment. The change in capacitance is typically 0.2–0.5 pF for a 1% RH change, while the bulk capacitance is between 100 and 500 pF at 50% RH at 25°C. Capacitive sensors are characterized by low temperature coefficient, ability to function at high temperatures (up to 200°C), full recovery from condensation, and reasonable resistance to chemical vapors. The response time ranges from 30 to 60 s for a 63% RH step change. [9]

3.2 Data Acquisition System

A data acquisition system is comprised to three main part: input and output system (sensor/transducer), analog to digital signal (data acquisition hardware) and computer (software). Signal conditioning may be required to adjust the signal type and range of the output signal to align with the requirements of the data acquisition system.

3.2.1 Data Acquisition hardware

Data acquisition devices are designed to provide a communication bridge between a laboratory instrument or sensor and a computer system or acts as the interface between a computer and signals from the outside world. It primarily functions as a device that digitizes incoming analog signals so that a computer can interpret them. The three key components of a DAQ device used for measuring a signal are the signal conditioning circuitry, analog-to-digital converter (ADC), and computer bus. [10]

3.2.1.1 Signal conditioning

The process of modifying the output of a sensor is called signal conditioning. Signal conditioning is often needed to make the output from a sensor compatible with data acquisition systems.[11] In the past, signal conditioning was routinely needed and often a source of problems when collecting data using computers. Today, many sensors output a signal that is compatible with common data acquisition systems. Reason to used signal conditioning is aligning a sensor output to a data acquisition system input constraint (amplification and offset) and filtering the noise signal.

3.2.1.1.1 Voltage Divider

The process of converting a resistance to a voltage involves the use of a circuit known as a voltage divider. Voltage divider circuit is a very important type of electronic circuit. This type of circuit, in its simplest form, consists of two or more resistors connected allows us to obtain several different voltage of resistance. The power supply system of a transistor radio is a good example of a system that uses voltage divider circuit to obtain various voltage to operate the radio circuitry.

Formula for dividing a voltage

Figure 1: Basic Voltage Divider Circuit

If one of the resistors in the circuit is replaced with a variable resistance, then the output voltage is proportional to the change in resistance of the variable resistor.[12] As a resistive sensor operates as a variable resistor, the resistive sensor can be used to replace one of the resistors, giving a voltage output which is proportional to the resistance of the sensor.

The most suitable value for the other resistor in the circuit can be determined using the voltage divider equation, the minimum and maximum resistance values for the sensor and the input voltage

3.2.1.1.2Amplifier

Sometimes a sensor will produce a small output voltage, perhaps with only a few mV of range. In this case it is necessary to amplify the voltage so that it covers as much of the range of the analog-to-digital converter as possible. The easiest way to achieve this amplification is to use an Opamp (Operational Amplifier). [13] Defining the operation of amplifier:

Gain: The primary function of an amplifier is to amplify, so the more gain the better. It can always be reduced with external circuitry, so we assume gain to be infinite.

Input Impedance: Input impedance is assumed to be infinite. This is so the driving source won’t be affected by power being drawn by the ideal operational amplifier.

Output Impedance: The output impedance of the ideal operational amplifier is assumed to be zero. It then can supply as much current as necessary to the load being driven.

Response Time: The output must occur at the same time as the inverting input so the response time is assumed to be zero. Phase shift will be 180°. Frequency response will be flat and bandwidth infinite because AC will be simply a rapidly varying DC level to the ideal amplifier.

Offset: The amplifier output will be zero when a zero signal appears between the inverting and non-inverting inputs.

3.2.1.2 Analog-to-digital converter (ADC)

Electronic circuits that process such signals are known as analog circuits. An alternative form of signal representation is that of a sequence of numbers, each number representing the signal magnitude at an instant of time. The resulting signal is called a digital signal. As indicated,the analog to digital signal converter (also called an ADC) accepts an analog sample vA and produces an TV-bit digital word. An ADC has an analog reference voltage or current against which the analog input is compared. The digital output word tells us what fraction of the reference voltage or current is the input voltage or current. So, basically, the ADC is a divider. [13]

http://iamechatronics.com/images/CommonImgs/GenEng/Digital_Data_Acquisition_and_Networks/Digital_Data_Acquisition_and_Networks_Fig_001.JPG

3.2.1.3 USB

Universal Serial Bus (USB) is defines the cables, connectors and communications protocols used in a bus for connection, communication and power supply between computers and electronic devices. In this project USB is a communication which is get digital signal from ADC to computer.

The USB is a polled bus. The Host Controller initiates all data transfers. [14] Most bus transactions involve the transmission of up to three packets. Each transaction begins when the Host Controller, on a scheduled basis, sends a USB packet describing the type and direction of transaction, the USB device address, and endpoint number. The USB device that is addressed selects itself by decoding the appropriate address fields.

In a given transaction, data is transferred either from the host to a device or from a device to the host. The source of the transaction then sends a data packet or indicates it has no data to transfer. The destination, in general, responds with a handshake packet indicating whether the transfer was successful. Some bus transactions between host controllers and hubs involve the transmission of four packets. These types of transactions are used to manage the data transfers between the host and full-/low- speed devices. The USB data transfer model between a source or destination on the host and an endpoint on a device is referred to as a pipe.