Sensors And Actuators

SENSORS AND ACTUATORS Introduction An electronic control unit is in many ways similar to human brain. It received messages from various sources and after processing the information, it either instruct actuators to perform some physical action or it stores the data away in its memory for use at some time in the future. Electronic sensors perform the information gathering role in this system. Each sensor feeds the electronic control module (ECU) with information that relates to some particular mechanical action or thermal effect. A sensor or transducer as it is called when it gives an output signal proportional to the physical quantity it is measuring converts physical actions it notices into either an analogue or digital electrical signal. Sensors can be separated into two main classes, viz., active or self gathering and passive or modulating. The passive type requires an external energy source to drive it and the sensor acts only as an energy controller. Pressure sensors Different types of pressure sensors commonly available are a) Variable resistor potentiometer b) Variable inductance type c) Variable differential transformer d) Strain gauge and piezo resistive e) Capacitor capsule Variable resistor potentiometer type: When a wiper contact blade is rubbed over a resistance material, it acts as a divider of the voltage that is applied to the potentiometer resistor. In the circuit a drop in air pressure causes the wiper blade to move towards the earthed end of the resistor thus reduces the output voltage signal. In order to obtain reasonable sensitivity, a comparatively large movement of the wiper blade must be provided. Position and level sensors This group of proximity sensors is designed to signal either a given position of mechanical component or the level of a liquid in a reservoir. The many applications of these position sensors are a) Crankshaft angular position for timing of the ignition and injection systems b) Crankshaft movement for computation of the engine speed for engine management and tachometer operation c) Throttle position for fuel injection and automatic transmission systems d) Gearbox output shaft movement for speedometer, odometer and trip computer operation e) Axle to body position for indication of axle loading f) Maximum wear of brake friction material g) Door position for door hazard indication h) Road wheel movement for anti skid braking (ABS) systems The different types of position sensors used in automobiles are 1. Magnetic-variable reluctance 2. Magnetic-d.c-excited inductive 3. Magnetic-Hall effect 4. Optical and fiber optics type

Optical opto-electronic sensors can be used signal a shaft position when used in conjunction with fiber optics. The numerous advantages suggest that this system will be used extensively in modern vehicles. A LED light source is positioned opposite a phototransistor and a chopper plate in the form of a disc is attached to the moving shaft. A slot or hole in the disc allows the light to pass to the phototransistor at the instant when the slot is aligned with the LED. At this point it signals the position of the shaft. Although it suggests a visual light radiation, the system often uses light frequencies outside the vision of the human eye. The LED fitted generally has a frequency which falls within the range infra red to ultra violet. Voltage supplied to the LED is set to give a phototransistor output which is sufficient after amplification to suit the ECU. A square wave pulse is given by this type of sensor, this makes is compatible with the requirement of a digital system. One drawback of this system is there is need to maintain the lens of the emitter and receiver in a clean condition. Dirt on the surfaces reduces the sensor output. So periodic attention to this task is necessary. Being the sensor is passive; it has the capability of detecting zero motion. Air flow sensors

Generally hot wire air flow sensors are used in automobiles. A hot wire air mass meter relies on the cooling effect of the air as it passes over the heated wire. If this wire is heated by passing a constant current through it then the temperature of the wire will fall as the air flow is increased. Similarly if a hot wire is kept at a constant temperature will be governed by the air flow, the large air flow requires the greater current. Both the constant current and constant temperature methods use electronic mean to measure the temperature. Generally this is achieved by utilizing the change in resistance which occurs where the temperature is changed. Hot wire systems take into account changes in air density. This is particularly important in cases where the vehicle is operated at different attitudes. Atmospheric pressure decreases with attitude. So in an area situated well above sea level the air mass supplied for a given throttle opening is reduced considerably unless this feature is taken into account. The richer mixture received by the engine would cause high exhaust pollution. A thin platinum hot wire of diameter 0.070 mm is exposed to air which passes through a tube situated where the galvanometer is placed in a bridge circuit, controls the current supplied to the four arms of the bridge. When a signal shows that the bridge is unbalanced the amplifier adjusts the heating current to restore the bridge to a balanced state. The armature of the cold wire compensating resistor acts as a standard. Throttle position sensor Generally this type of sensor is a potentiometer, which is meant for altering electrical potential. One of the simplest arrangements is a variable resistor. In this case the throttle is connected to a contact blade which wipes across the resistor coil. As the throttle is opened the number of resistor coils in the circuit is reduced. This alters the voltage and potential of the output in relation to earth. An improved accuracy and a longer life are obtained by using alternative resistor materials to wire. A typical sensor contains two pairs of contact blades one pair acts as a micro switch to signal the throttle closed position. A constant voltage of 5v is applied to the sensors. As the contact blades slide along the resistor in accordance with the opening of the throttle valve. The output voltage increases proportionally. This linear signal is transmitted to the ECU. Temperature sensor A number of sensors are used on a vehicle to measure temperature. These include: a) Engine temperature for ignition, fuel metering and instrumentation b) Air intake temperature for fuel metering and vaporization control c) Ambient conditions for driving safety d) Exhaust temperature for fuel metering The majority of the temperature uses a thermistor along with the thermocouples where it is needed. Thermistor sensor:

This type normally consists of a brass bulb, which is in contact with the substance it is sensing. The bulb contains a capsule called a thermistor. Resistance of common metals increases with temperature and thermistors which have a sensing capsule that responds in this way are said to have a positive temperature coefficient (p.t.c). A capsule made of a semiconductor material has a resistance which decreases with temperature. These materials of which silicon is most common have a negative temperature coefficient (n.t.c). Such thermistors are used in engine block to measure the temperature of coolant. Thermocouple: The thermistor is excellent for measuring temperature upto about 2000 C. But above this temperature a thermocouple is normally used. It consists of two wires of dissimilar materials joined together and connected to a galvanometer. When the hot junction is heated, an e.m.f is generated which is registered by the galvanometer. Depending on the materials upto a given temperature the current increases with an increase in the temperature difference between the hot and cold ends of the wires. Indication of the temperature is achieved by scaling of the galvanometer accordingly. A thermocouple can be made by using two metals from antimony, iron, zinc, lead, copper and platinum. Current will flow from the higher to the lower in this list across the cold junction. Nowadays other metals and alloys are used for a thermocouple like nickelchromonium/ nickel aluminium alloys. This type is suitable for temperature range of about 0-11000C, which is most common in exhaust system of the engine. Knock detecting sensors The main purpose of a knock sensor is to detect the combustion knock in an engine combustion chamber. A knock sensor has to detect vibrations from combustion knock in the frequency range 1-10 kHz. When severe knock occurs in the combustion chamber, the transference of pressure waves through the cylinder block causes the metal particles to be accelerated to and fro. The accelerometer type knock sensor detects this oscillatory motion by using a piezo-ceramic semiconductor. Pressure on the semiconductor generates a small electrical charge and this is used to provide the signal current. The body of the knock sensor is screwed into the side of the cylinder block and piezo electric crystal is

clamped by a seismic mass which tunes the sensor to the required frequency range. When an oscillation is applied to the body of the sensor, the sound waves vary the compression of the crystal. This causes a small e.m.f of the order of 20 mV/g to be generator. Te small analogue signal current produced by the sensor when the engine is knocking is transmitted to the ECU. After filtering to remove the unwanted waves, the signal is then averaged and converted to a digital form to represent the knock/noknock conditions. Whenever a digital pulse is sensed by a logic circuit, the system responds by reducing the spark timing advance or changing the A.F strength. Oxygen sensor/EGO sensor/Lambda sensor

The detection of oxygen in the exhaust gas from an engine provides useful means for controlling the air fuel mixture. Excess oxygen in the exhaust indicates that the air fuel mixture is weak, whereas the deficiency of oxygen shows that the mixture is rich and exhaust gas is polluting the atmosphere. The gas detector used to provide this signal is called lambda sensor. This type of exhaust gas oxygen sensor normally uses zirconium oxide (ZrO2) for its active material. It consists of a thimble shaped portion of ZrO2 covered with two thin and porous platinum electrodes. The internal electrode is in contact with air in the center of the dome and the outer electrode is placed so that it is in contact with exhaust gas. Extra protection again gas erosion is given by covering the outer electrode with a porous alumina ceramic coating through which gas can penetrate. The operation of the sensor is similar to a galvanic battery cell, in the case of the sensor the ZrO2 acts as the electrolyte. At high temperatures, this electrolyte becomes conductive so if the two plates are in contact with different amount of oxygen, then a small voltage will be generated across the two plates. This action is produced because oxygen atoms carry two electrons. So this means that the atom carries a negative charge. The ZrO2 attracts oxygen ions with the result that negative charges build upon the surface of ZrO2 adjacent to the platinum electrode. When the sensor is exposed to exhaust gas a greater concentration of oxygen on the air side of the Z rO2 cause this side to have a greater number of negative charges. In consequence a potential difference is built up across the plate which will depend on the difference in oxygen level. (Air factor for proper air fuel ratio is 1, Air factor is referred as lambda. The value of lambda is given by λ=Actual volume of air drawn/theoretical volume of air The value of λ>1, it is referred as lean mixture The value of λ<1, it is referred as rich mixture)