INTRODUCTION TO SENSORS, TRANSDUCERS & ACTUATORS

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INTRODUCTION Transducers play a major role in mechatronics engineering & technology. These are the basic elements that convert or transform one form of energy to another form.

1 INTRODUCTION Transducers play a major role in mechatronics engineering & technology. These are the basic elements that convert or transform one form of energy to another form. Let us change the word energy into the word signal which is more convenient to use. Thus transducers convert one form of signal to another form. Transducers are mainly used to quantify the physical, electrical, fluidic and mechanical variables such as temperature, pressure, magnetic field, voltage, flow and vibration. Such variables or parameters are also called as measurands. Q. What is the difference between transducer and sensor? Transducers are the physical element which is an active part of a sensor. In another word, a sensor is a sophisticated transducer which contains signal conditioning circuits. This signal conditioning circuit is used for amplifying and refining the weak signal that is available at the output of the transducer. Some of the commonly used signal conditioning circuits are amplifiers, filters and Analog to Digital Converters (ADC). The input signal is referred to as measurands. The output of the transducer is referred to as equivalence. Q. List the sensors or transducers. 1. Limit switches 2. Proximity switches 3. Electric sensors 4. Light sensors- optical encoders Q. Write the working and applications of limit switches. Limit switches are normally not operator accessible. Generally they are activated by moving parts on the machine. They are mechanically or magnetically operated. They are sometimes called cam switches because many are operated by a camming action when a moving part pushes the switch. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 1

2 Functions of limit switches: Applications: Detecting presence/ absence Counting Detecting range of movement Detecting positioning and travel limit Breaking a live circuit when unsafe conditions arise. Material handling Food packaging Manufacturing- Automotive/heavy equipments, machining, marine/aviation Metal forming Q. What do you mean by proximity switches? List and give their applications. A proximity sensor consist of an element that changes either its state or an analog signal when it is close to, but often not actually touching, an object. Magnetic, electrical capacitance, inductance and eddy current methods are particularly suited to the design of a proximity sensor. Type: Functions: Applications: Inductive type Capacitive and Optical sensors Reed switches Detecting presence/ absence Counting Material handling Food packaging In AGVs (Automated Guided vehicles) Q. Write the working of inductive sensor. Inductive sensors use currents induced by magnetic fields to detect nearby metal objects. The inductive sensor uses a coil (an inductor) to generate a high frequency magnetic field. If there is a metal object near the changing magnetic field, current will flow in the object. This resulting current flow sets up a new magnetic field that Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 2

3 opposes the original magnetic field. The net effect is that it changes the inductance of the coil in the inductive sensor. By measuring the inductance the sensor can determine when a metal have been brought nearby. Q. Write the working of capacitive sensor. Inside the sensor is a circuit that uses the supplied DC power to generate AC, to measure the current in the internal AC circuit, and to switch the output circuit when the amount of AC current changes. Remember that capacitors can hold a charge because, when one plate is charged positively, negative charges are attracted into the other plate, thus allowing even more positive charges to be introduced into the first plate. Unless both plates are present and close to each other, it is very difficult to cause either plate to take on very much charge. Only one of the required two capacitor plates is actually built into the capacitive sensor! The AC can move current into and out of this plate only if there is another plate nearby that can hold the opposite charge. The target being sensed acts as the other plate. If this object is near enough to the face of the capacitive sensor to be affected by the charge in the Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 3

sensor's internal capacitor plate, it will respond by becoming oppositely charged near the sensor, and the sensor will then be able to move significant current into and out of its internal plate. Q.

4 sensor's internal capacitor plate, it will respond by becoming oppositely charged near the sensor, and the sensor will then be able to move significant current into and out of its internal plate. Q. Write the working of Optical sensor. These sensors are more commonly known as light beam sensors of the thru-beam type or of the retro reflective type. Both sensor types are shown below. The light beam is generated on the left, focused through a lens. At the detector side the beam is focused on the detector with a second lens. If the beam is broken the detector will indicate an object is present. The oscillating light wave is used so that the sensor can filter out normal light in the room. The light from the emitter is turned on or off at a set frequency. When the detector receives the light it checks to make sure that it is at the same frequency. If light is being received at the right frequency then the beam is not broken. Retro-reflective type light sensors have the transmitter and receiver in the same package. They detect targets that reflect light back to the sensor. Retro-reflective sensors that are focused to recognize targets within only a limited distance range are also available. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 4

5 Q. Write the working of Reed switch sensor. It consists of two magnetic switch contacts sealed in a glass tube. When a magnet is brought close to the switch, the magnetic reeds are attracted to each other and close the switch contacts. It is widely used for checking the closure of doors. Q. Write the construction & applications of thumb wheel switch sensor. Thumb wheel adjustment with 10 positions is available. This switch consists of a glass epoxy stator with tin plates copper tracks for ways. The switches are sealed to protect it from dust. The pole is spring loaded brass contact. The operating lever and assembly is plastic coated. Applications: computers and measuring instruments Q. Write the working and applications of optical encoders. Optical encoders are used to detect the position, velocity, acceleration and direction of movement of rotors, shafts, pistons of rotary machines and translational systems. The optical encoder provides encoded pulsed signals in response to the movement. Continuous optical signals are coded by the use of a specially designed rotating disk containing code patterns called track. The pattern or track on the disk consists of alternate appearance of opaque and transparent segment and the pattern is circular. Basically, two types of encoders, such as incremental encoder and absolute encoder exist. Incremental encoder: this device consists of a circular disk that contains alternative evenly spaced opaque and transparent segments over a circle. A light source is located on one side of the disk and a photo detecting device (photo detector) is placed on the other side of the disk. Light emitting diode (LED) is used as the light source that provides continuous light signal. A photo diode or photo transistor is commonly used as the light detector. Light signals can be received if the transparent segment of the pattern is in between the light source and photo detector. In the real application the disc is rigidly fixed with the rotor or shaft of the rotating element. The evenly spaced transparent radial lines on its surface, rotates past the light source. The output is taken from the photo detector. The number of Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 5

They provide a unique output for every position. The coded disk consists of a number of concentric patterns of opaque and transparent segments. The concentric patterns are called tracks.

6 pulses determines the position of the disk and the number of pulses per second measures the velocity of the disk. Absolute encoder: It also provides angular position and velocity value, which is derived from the pattern of the coded disc. They are more capable than incremental encoders. They provide a unique output for every position. The coded disk consists of a number of concentric patterns of opaque and transparent segments. The concentric patterns are called tracks. The number of tracks determines the resolution of the encoder. Each track has its own photo detector. The length of the transparent segments of the track towards the center of the disk increases in a specific order to satisfy the binary coding technique. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 6

Q. What is actuator? List them. Actuation is the process of conversion of energy to mechanical form. Actuator is a device that accomplishes this conversion.

7 Q. What is actuator? List them. Actuation is the process of conversion of energy to mechanical form. Actuator is a device that accomplishes this conversion. The microcontroller provides command signal to the actuator for actuation. They are responsible for transmission of power and motion from one place to another by the use of mechanical components. Solenoids Relays DC and AC motors. Q. Write the working and applications of solenoid. Solenoids are the most common actuator components. There is a moving ferrous core (piston) that will move inside wire coil. Normally the piston is held outside the coil by a spring. When a voltage is applied to the coil and current flows, the coil builds up a magnetic field that attracts the piston and pulls it into the center of the coil. The piston can be used to supply a linear force. Applications: pneumatic valves and car door openers. Q. Write short note on relays. Relays are electrically operated actuators or switches in which changing a current in one electrical circuit switches a current on or off in another circuit. Relays are like remote control switches and are used in many applications because of their relative simplicity, long life, and proven high reliability. Relays are used in a wide variety of applications throughout industry, such as in telephone exchanges, digital computers and automation systems. Highly sophisticated relays are utilized to protect electric power systems against trouble and power blackouts as well as to regulate and control the Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 7

8 generation and distribution of power. In the home, relays are used in refrigerators, washing machines and dishwashers, and heating and air-conditioning controls. Q. what are ON-OFF and latching applications in relays? ON-OFF Applications: In figure (a) the relay is off. The metal arm is at its rest position and so there is contact between the Normally Closed (N.C.) switch contact and the 'common' switch contact. If a current is passed through the coil, the resulting magnetic field attracts the metal arm and there is now contact between the Normally Open (N.O.) switch contact and the common switch contact, as shown in figure (b). Latching relay circuit: If a relay is connected as shown in figure, it will become 'latched' on when the coil is energized by pressing the Trigger button. The only way to turn the relay off will then be to cut the power supply by pressing the Reset button (which must be a push-to-break type). Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 8

9 Q. Write short on solid state relays. These active semiconductor devices use light instead of magnetism to actuate a switch. The light comes from an LED, or light emitting diode. When control power is applied to the device s output, the light General Purpose Relay is turned on and shines across an open space. On the load side of this space, a part of the device senses the presence of the light, and triggers a solid state switch that either opens or closes the circuit under control. Often, solid state relays are used where the circuit under control must be protected from the introduction of electrical noises. Advantages of Solid State Relays include long life, no moving parts, no contact bounce, and fast response. The drawback to using a solid state relay is that it can only accomplish single pole switching. Q. write short note on motors. Motors or electromechanical actuators convert electrical energy into mechanical energy. The fundamental principle of operation of such actuators comes from the fact that when an electric current is passed through a group of wire loops placed in a magnetic field, the loop rotates, and the rotating motion is transmitted to a shaft, providing useful mechanical work. They are used in robotics, machine tools, petrochemicals, paper mills. Besides industrial control applications, the electromagnetic actuators are used in many consumer products such as, fans, vacuum cleaner, mixer, hair dryer and CD player. There are three types of motors such as AC, DC and stepper motors. Q. How DC motor works? And explain their types. DC motor is the most versatile actuator and sometimes called rotating machine. The DC motor has two parts, stator and rotor. The stator is the outer part of the motor which contains evenly spaced magnetic poles. The stator is fixed and rotor rotates inside the stator. Rotor consists of an electromagnet with poles facing toward the stator poles. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 9

The electromagnet is formed by current carrying conductors (copper wires), which are wound around the iron core (rotor). The wound wire is called armature coil or armature winding.

10 The electromagnet is formed by current carrying conductors (copper wires), which are wound around the iron core (rotor). The wound wire is called armature coil or armature winding. When a DC current is applied to the armature windings, the rotor is immediately turned in to a group of electromagnets. The current that passes through the rotor winding is called armature current. When this current carrying rotor is placed in magnetic field which is established by use of magnet of stator, a force is exerted that makes it possible to rotate a rotor. The magnetic field, armature current and force are mutually perpendicular to each other. The direction of current, the direction of magnetic field and the stator-rotor assembly are such, based on the law that the force exerted on the coil makes it possible the rotor to rotate. The force exerted is given by, F= I L B Where, I is armature current, B is magnetic field and L is the length of the conductor coil. DC motors are categorized as brushed, brushless and coreless type. Q. Write the construction and working of brushed DC motor. Brush and commutator are two important parts of the DC motors. When current is passed through the armature winding, it develops a group of electromagnets whose poles are attracted by the opposite poles of stator poles. Among the group, consider a single electromagnet. Assume that only one field magnet (two poles) is present. The poles of this electromagnet will be attracted by the opposite poles of the stator poles (Fig a) and by virtue of which the rotor electromagnet would make a motion and then stop in the position as shown in fig b. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 10

That is, once the magnetic lines of forces of the two electromagnets (stator and rotor) become parallel and opposite shown in fig b, no further motion of the rotor would occur.

11 That is, once the magnetic lines of forces of the two electromagnets (stator and rotor) become parallel and opposite shown in fig b, no further motion of the rotor would occur. However, key to a DC motor is that the moment the rotor electromagnet aligns, its field flips, i.e., the direction of flow of current through the coil is deliberately altered (Fig c). The alteration of current causes the poles of the rotor electromagnet to alter. Notice that prior to the flipping the pole of the rotor electromagnet and the pole of stator electromagnet were not similar. In this particular example they were North Pole and South Pole respectively, and just after flipping the poles of the two electromagnets become similar. The alteration allows the rotor electromagnet to continue its motion since the same pole will now repel each other. If the field of the rotor electromagnet flipped at just the right moment at the end of each alignment, the rotor would rotate freely. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 11

12 Q. Write working of brushless and coreless DC motors. Brushless motors A brushless motor operates much in the same way as a brushed motor. In a brushed motor, the current is supplied to the rotating coil by a mechanical commutator (splitring) and brush arrangement. In a brushless motor, on the other hand the commutation is achieved electronically. The electronic commutator can reverse the external connections at a high rate. The commutator and brushes are in contact and during rotation both the elements rub against each other. The brushless motor is a better construction because the commutation is achieved using electronic circuit that overcomes the problem of wearing arising out of physical contact. A brushless motor is often used when high reliability, long life and high speeds are required. Coreless motors Iron core construction has several major disadvantages. The core has relatively high inertia that limits acceleration. They also own electrical inductances and core losses, which stay even with laminated structure. To overcome these drawbacks many motors are designed without a core. These types of motors are called coreless motors. Coreless motors depend upon the winding itself to provide structural integrity for the armature. Q. Write the working of AC motors. AC motors AC motors consume alternating electrical power to produce mechanical actuation in terms of angular movement. The principle of operation of all AC motors relies on the interaction of a revolving magnetic field created in the stator by AC current, with an opposing magnetic field at the rotor. The opposing magnetic field is originated by virtue of induction or by supplying an armature current by a separate the DC current source. The principle of operation of AC motor in relation to the origin of opposing field differs. Accordingly, AC motors are of two types induction motor synchronous motor AC motors are either single phase or multiphase, depending upon the input signal requirement and internal construction. Induction motors The interaction of magnetic fields of the rotor and the stator makes the induction motor to rotate. The stator windings are connected to the power supply, which could Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 12

be a one or multiphase type. For example, a single phase induction motor is connected to the single phase power line and three-phase induction motors to the three-phase power line, respectively.

13 be a one or multiphase type. For example, a single phase induction motor is connected to the single phase power line and three-phase induction motors to the three-phase power line, respectively. When an alternating voltage across the stator windings is applied, a radial rotating magnetic field is produced. The rotor has conductive loop along its periphery. The rotating magnetic fields produced by the stator induce a current into the conductive loops of the rotor. Once that occurs, the magnetic field causes forces to act on the current carrying conductors, which results in a torque on the rotor. Synchronous motors The synchronous motors are a 3-phase system in which the magnets are mounted on the rotor and are excited by the DC current. The stator winding is divided into three parts, which are fed with three-phase AC current respectively. Because of the nature of the connection, the magnetic field rotates at a constant speed that is determined by the frequency of the current in the AC signal. The variation of the three waves of current input to the stator winding causes a varying magnetic interaction with the poles of the magnets of the rotor. This, in turn, causes the rotor to rotate. Typical characteristics of synchronous motors can be attributable to as follows. They: run at constant speed fixed by frequency of the input powers signal require DC current for excitation have low starting torque Q. Write the working of stepper motor. Many applications require precise positioning control. Traditionally this has been achieved by the use of small DC motors. However, small DC motors have very gradual acceleration and deceleration providing low stabilisation property. Gearing mechanism can help to reduce this problem to some extent. But overshoot still persists deviating the reachability, which means that desired position control cannot be achieved. The only way to effectively use a DC motor for precise positioning is to Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 13

14 apply servo mechanism, a control realisation principle based on feedback rule furthermore; DC motors are not efficient for producing high torque at relatively low speeds. The stepper motor also consists of a rotor and stator. As the name suggests, the stepper motor steps a bit at a time. The motor can be controlled using a microcontroller as it can responds to digital pulse trains. The rotor of the motor rotates a specified number of degrees by each pulse the motor receives from its controller. The motion caused by one pulse is called one step. There are two forms of stepper motor Variable reluctance motor Figure shows the basic form of the variable reluctance stepper motor. With this form the rotor is made of soft steel (having less reluctance than air) and is cylindrical with four poles, i.e. fewer poles than on the stator. When an opposite pair of windings has current switched to them, a magnetic field is produced with lines of force which pass from the stator poles through the nearest set of poles on the rotor. Since lines of force can be considered to be rather like elastic thread and always trying to shorten themselves, the rotor will move until the rotor and state poles line up. This is termed the position of minimum reluctance. This form of stepper motor generally gives step angles of 7.5 or 15. Permanent magnet stepper Figure shows the basic form of the permanent magnet motor. The motor shown has a stator with four poles. Each pole is wound with a field winding, the coils on opposite pairs of poles being in series. Current is supplied from a DC source to the windings through switches. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 14

15 The rotor is a permanent magnet and thus when a pair of stator poles has a current switched to it, the rotor will move to line up with it. Thus for the currents giving the situation shown in the figure the rotor moves to the 45 position. If the current is then switched so that the polarities are reversed, the rotor will move further 40 in order to line up again. Thus by switching the currents through the coils the rotor rotates in 45 steps. With this type of motor, step angles are commonly 1.8, 7.5, 15, 30, 34 or 90. Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 15

Note 8. Electric Actuators

Note 8. Electric Actuators Note 8 Electric Actuators Department of Mechanical Engineering, University Of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada 1 1. Introduction In a typical closed-loop, or feedback, control