Chap. 5 Electronic Components and Sensing Devices

Transcription

1 1 Chap. 5 Electronic Components and Sensing Devices Today, practically all mechanical devices contain some sort of electronic components. The function of a product often relies on the integration of mechanical and electronic components coordinated by a control structure ructure. This chapter provides a fundamental introduction on the following electronic components and sensors: esistor Switch elay Diode Transistor DC motor Step motor Force sensors Location sensors Temperature sensors Capacitor Operational amplifier

2 2 esistors (I) A resistor is a dissipative element that converts electrical energy into heat. If a resistor s material is homogeneous and has a constant cross-sectional area A and length L, then the resistance is given by = ρl A where ρ is the resistivity of the material. For example, metals with low resistivity are regarded as conductor whereas plastics and ceramics are regarded as insulator.

3 3 esistors (II) The most common type is the carbon-film resistor. The metal film resistor has accurate resistance value, and is usually used on sophisticated circuits. A wire wound resistor can sustain high power. If packaged with a cement block or a ceramic block, it is also called cement resistor or ceramic resistor.

4 Color Codes of esistors 4 color brown A 1 B 1 C 10 1 D ±1% For example: (4 color bands) red ±2% orange yellow green blue violet gray white black golden ±5% In addition, 5 or 6 color bands can be found in some precision resistors silver ±10%

5 5 Specifications of esistors esistance and Power The unit of resistance is Ω (Ohm), or KΩ, MΩ. The relation between resistance, voltage, and current can be expressed by Ohm s Law : V I = A resistor has a limited power dissipation capability designated in watts, and it may fail after this limit is reached. When selecting a resistor, decide the required resistance first, and then calculate the power according to the flow of current: P = I 2 For example, the most common wire-lead resistors used in ordinary electronic circuitry are 1/4 watt 5% tolerance carbon or metal-film resistors. esistor values of this type range between 1Ω to 24MΩ.

6 6 Variable esistor, Light Dependent esistor, and Thermistor A variable resistor (V) can be used to adjust the flow of current in a circuit. When the resistor s spindle is rotated, a sliding contact puts more or less resistance material in series with the circuit. A light dependent resistor (LD) is a component whose resistance depends upon the amount of light falling on it. A thermistor is a component whose resistance varies with temperature.

7 Switch 7 A switch is used for making and breaking an electric circuit. A switch has two parts: a pole and a contact. There are four types of switch: single pole single throw (SPST), single pole double throw (SPDT), double pole single throw (DPST), and double pole double throw, (DPDT). The pole on a switch is often labeled COM. The contacts on a switch are often labeled N/C or N/O. N/C stands for normally closed. This is the contact which is connected to the pole when the switch is not activated. The N/O, or normally open contact, connects to the pole when the switch is activated.

8 Limit Switch, otary Switch, and Membrane Switch 8 Limit switch (or micro switch) is commonly used in sensing mechanical motion. Switching is activated by touching the arm or roller. Membrane switch otary switch

9 elay 9 A relay is a switch which is turned on and off by an electromagnet. As shown in Figure, when a small current flows through the coil, it produces a magnetic field that magnetizes an iron core. This attracts the armature that forces the switch contacts to touch. The important specifications of a relay include the maximum voltage and current loads (e.g., 5A 240VAC) and the activated-voltage of the coil (e.g., 5V, 9V or 12V, etc) SPST relay DPDT relay

10 Example 1. elay eversing Circuit 10 Coil

11 Example 2. elay Latch 11 A

12 Project 1. The Automatic Door (I) 12

13 Project 1. The Automatic Door (II) 13

14 Project 1. The Automatic Door (III) 14

15 Project 1. The Automatic Door (IV) 15

16 Project 1. The Automatic Door (V) 16

17 Project 1. The Automatic Door (VI) 17

18 Project 1. The Automatic Door (VII) 18

19 Project 1. The Automatic Door (VIII) 19

20 Project 1. The Automatic Door (VIIII) 20

21 Inductance 21 The wire coil in the relay is the simplest form of inductor, which is a passive energy storage element that stores energy in the form of a magnetic field. An inductor s voltage-current relationship can be expressed as di 1 V ( t) = L I( t) = V ( τ d )τ dt L t 0 The current through an inductor cannot change instantaneously because it is the integral of the voltage. It takes time to increase or decrease the current flowing through an inductor, such as a relay or motor. Sparks induced by the high induced voltage may occur when suddenly switching off the relay. The switching design of the solid-state relay (SS) uses no moving parts or contacts that can wear out. The solid-state relay has better performance than the electromechanical relay (EM), and suited to applications that demand long life and high reliability where fast response and high switching rates are required. The cost of SS is much higher than EM.

22 Diode 22 A diode is a component which allows current to flow in one direction only. The most common diode consists of a junction of P-type and N-type silicon semiconductor material. A real diode requires about 0.7V of forward bias to enable significant current flow. It can withstand a reverse voltage up to the breakdown voltage (50V to kilotvolts). Like ordinary diodes, light emitting diodes (LED) also allow current to flow in one direction only. The LEDs emit light when current passes through them. LEDs are used mainly as visual indicators that a circuit is working or an appliance is on. Now LEDs have seen applications in traffic lights and other lighting applications. breakdown voltage current 0.7V voltage failure

23 Example 3. Half-Wave ectifier 23

24 Example 3. Full-Wave ectifier 24

25 Transistor 25 The transistor is another semiconductor device, which was invented in 1947 by Bell Lab engineers. The transistor is made of three layers of N- and P- type semiconductor material. The three layers are called emitter, base, and collector. There are two basic types of transistors, NPN and PNP. NPN type is more commonly used. A small current between base and emitter will turn on the large current between collector and emitter. This is known as a current amplification. The current gain is defined as h fe = I I c b = collect current base current

26 Example 4. Calculate the Current Gain 26 The current gain the transistor in the figure is: h fe = = 200 Darlinton pair: In order to obtain higher current gain, two transistors can be combined by connecting the emitter of one to the base of the other. With the Darlington pair, the current gain will be =40,000.

27 Example 5. Light Activated Switch 27 As shown in the figure below, the current flowing through the base of the transistor depends on the resistance variation of the LD. The V is added to adjust the sensitivity. With the same principle, the LD can be replaced by a thermistor and therefore this circuit becomes a temperature activated switch. A photo interrupter is a switch composed of an LED and a phototransistor. The ON/OFF switching is determined by whether the light of LED is interrupted or not.

28 Capacitor 28 A capacitor is a component that stores energy in the form of an electrostatic field. In its simplest form, a capacitor consists of two conducting plates separated by an insulating material. The capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separation between the plates. Capacitance is measured in Farads. A capacitance of 1 F produces 1 V of Q potential difference for an electric charge of one coulomb (1 C), V =. The C smaller values microfarads (μf) or picofarads (pf) are more convenient. electrolytic capacitor ceramic capacitor plastic capacitor

29 Capacitor Code 29 The capacitance and sustainable voltage are usually printed on the capacitors. For small capacitors such as plastic capacitor, their capacitance are expressed as codes. For example, Tolerance code Letter symbol B C D E F G H Tolerance ±0.10% ±0.25% ±0.5% ±0.5% ±1% ±2% ±3% Letter symbol J K M N P Z Tolerance ±5% ±10% ±20% ±0.05% +100%, -0% +80%, -20%

30 Example 6. Time Delay Circuit 30 When S1 is closed, the relay is activated and the light bulb is on while the capacitor starts to store electrical energy. If S1 is open, the capacitor releases electricity to hold the relay activated till the capacitor is empty. PS1 is used to reset the capacitor. I S1 + - PS1 C 燈 time

31 Example 7. Automatic Transportation Vehicle (I) 31

32 Example 7. Automatic Transportation Vehicle (II) 32

33 Operational Amplifier (I) 33 The circuits discussed so far has been made up from separate components such as resistors, transistors etc. Integrated circuits (ICs), however, are complete circuits in themselves Intel 386 CPU contains 275,000 transistors Intel 586 CPU contains 12,000,000 transistors Intel Pentium III contains 28,100,000 transistors, and Intel P4 CPU contains over 40,000,000 transistors. ICs contain very small chips of silicon, into which numerous components have been formed. Each silicon chip is mounted in a plastic case and is connected to pins set in the side of the case. The 741 operational amplifier (op amp) as shown in the figure, is a very useful IC.

34 Operational Amplifier (II) 34 The op-amp is a low-cost and versatile IC consisting of many internal transistors, resistors, and capacitors. It is commonly used to build amplifiers. The op-amp has an open-loop gain: V output ( V V ) = A non inverting inverting-input 10,000, μv Inv input 2 7 V μv Non-inv input V output An ideal op amp has an infinite open-loop gain, the voltage output is the saturation voltage, and the op amp becomes a comparator.

35 Example 8. Application of OP-amp (I): Inverting Amplifier 35 An op amp circuit usually includes feedback from the output to the inverting input to form a close loop configuration, which results in stabilization of the amplifier and control of the gain. An ideal OP-amp : Infinite impedance at both inputs, short between the two inputs. Infinite gain, zero output impedance.

36 Example 8. Application of OP-amp (I): Inverting Amplifier 36 By Kirchhoff s Current Law, i = at node C. i in 0 Vin Vout 0 = = iout = F in i out V V F = This ratio is called closed-loop gain, the input and output voltage is proportional to the their resistance. out in

37 Example 8. Application of OP-amp (II): Non -Inverting Amplifier 37 Input voltage at + i in = V in 0 = i out = V out V F in V V out in = 1+ F

38 Example 8. Application of OP-amp (II): Summer 38 Kirchhoff s Current Law: I f = I I 2 I 3 Ohm s Law: Vout 0 = f 0 V V V 3 3 If 1 = 2 = 3 = in V ( V + V ) f out = V3 in The output voltage of the OP-amp is equal to sum of the input voltage multiply by the ratio of f / in. The minus sign indicates that input and output are in opposite phase.

39 Example 9. High-Sensitivity Light Activated Switch 39 The left part of this circuit is a Wheatstone bridge, in which small resistance variation will generate high voltage output. ( When light emits on the LD, V 1 <V 2, and the output of the OP-amp is negative. The transistor and the relay is not activated. When in dark, V 1 >V 2, and the output of OP-amp is positive. The transistor and the relay is activated. The V is used to adjust the sensitivity to light. The diode is used to reduce current arcs between switch contacts when the switch is opened.

40 Electric Motors 40 Almost every mechanical movement that you see in our daily life is caused by an AC (alternating current) or DC (direct current) electric motor. An electric motor contains two magnets: the armature (or rotor) is an electromagnet, while the field magnet is a permanent magnet. An electric motor uses the attracting and repelling forces between the armature and the field magnet is to create rotational motion and converts electrical energy into kinetic energy. Direct Current Electrical Motor Generator

41 Characteristic Curves of a Permanent Magnet (PM) Motor 41 Important specifications of motors: working voltage, torque output (g cm or k gm), and working rotational speed (rpm)

42 DC Motor Control Pulse Width Modulation (PWM) 42 高電位脈波寬度較窄 Decrease duty cycle to decrease the average current through the motor. Duty Cycle Period( 週期 ) Increase duty cycle to increase the average 高電位脈波寬度較寬 current through the motor.

43 Stepping Motor 43 Stepping motors are widely used in digital control systems. A stepping motor receives pulse signal input and is capable of movement in discrete steps. Therefore, stepping motors are well-suited for use with open-loop control in positioning control applications. In a stepping motor, stators are energized in turn to attract the rotor to rotate a step angle. Usually step angle is 30, 15, 5, 2.5, and 1.8 degrees (12, 24, 72, 144, 180, and 200 pulses per revolution).

44 Example 10. Control of a Stepping Motor 44 In order to control a stepping motor with 15 step angle to keep at 150 rpm (2.5 rev/sec), the required pulse rate is: (360/15) 2.5=60 pulse/sec. If this motor drives a worm whose pitch is 2.4mm, per input pulse will contribute 2.4/24=0.1mm in forward displacement of the worm. There is holding torque in the output shaft of a stepping motor when the stators are not energized to create a self-lock effect. Note that stalling might occur under large loading, which results in positioning error in open-loop control.

45 Sensors 45 A sensor is a device that responds to or detects a physical quantity and transmits the resulting signal to a controller. Sensors are often used in close-loop control to feed back the output of the system in order to generate proper control actions. A transducer is a sensor that converts (transduces) one form of energy to another form, usually electrical signals.

46 Force Sensors 46 Strain gage, load cell, piezoelectric transducer and accelerometer are common force sensors used to measure force, stress and acceleration. When a conductor is stretched under force, the length and area of the conductor will vary slightly. The resulting resistance variation can be calculated as: = ρl A A strain gage is used to detect the surface deformation by measuring the change of resistance.

47 47 47 Load Cell A load cell consists of two strain gages and a Whetstone bridge circuit. The active gage ( 1 ) lies in the same direction as the force applied and the compensated gage ( 4 ) perpendicular to the force. The voltage output of the bridge: in in in B A out V - V V -V V V + + = + + = = ( )( ) in V = ( ) ( ) in in out V V V Δ + Δ + Δ =

48 Other Force Sensors 48 Piezoresistive effect: resistance of the material varies due to external force loading. Piezoelectric effect: piezoelectric current is induced by external force loading. If a voltage is applied on the material, deformation of the material will occur. Accelerometer is a typical application of the piezoelectric materials. Converting analog signals to digital signals:

49 Potentiometer 49 Potentiometers measures linear or rotary mechanical positions by means of a variable resistor. wiper B A resistor A B C rotary potentiometer A linear potentiometer C B C

50 Optical otary Encoder 50 發光二極體 LED photo 光電晶體 transistor 1 0 output 輸出訊號 signal t

51 Incremental Encoder and Absolute Encoder 51 The gray code is designed so that only one bit changes state for each count transition.

52 Temperature Sensors 52 Thermocouple is commonly used in sensing temperature. Seedbeck effect: as two distinct metals are connected together, the temperature difference between the hot joint and the cold joint (thermocouple) will induce a voltage. Temperature difference can be measured by measuring this voltage. The thermistor is also used for temperature measurement. But the relation of voltage vs. temperature is often non-linear. There are temperature sensing ICs too.