In The Name of Allah Instrumentation Dr. Ali Karimpour Associate Professor Ferdowsi University of Mashhad
Position Sensors Topics to be covered include: v v v v v v Introduction Resistive Displacement Sensors (Potentiometer type) Capacitive Sensor - Displacement Inductive Displacement Sensors Optical Encoder Displacement Sensors Synchros & Resolvers 2
Introduction v Need for position detection? u u u u u... v Types u u Robotics CNC systems Production line Measuring other quantities by displacement. Continuous Discontinuous 3
Introduction u Measuring other quantities by displacement. 4
Resistive Displacement Sensors (Potentiometer type) v Type of motion u u Linear motion potentiometer Rotary motion potentiometer Linear motion (Wire-wound) Linear motion (Layer based) Different Resolution 5
Resistive Displacement Sensors (Potentiometer type) v Type of motion u u Linear motion potentiometer Rotary motion potentiometer Rotary motion (Wire-wound) Rotary motion (Layer based) 6
Resistive Displacement Sensors (Potentiometer type) Rotary Type Movement Limitations Fraction of turn (Near) One Turn Multi Turn 7
Resistive Displacement Sensors (Potentiometer type) Multi resistance displacement sensor Four resistance displacement sensor 8
Resistive Displacement Sensors (Potentiometer type) v Output of displacement sensor Output of displacement sensor in no load R M E p R s E i R p x l R p Load effect?? E p?? 9
Resistive Displacement Sensors (Potentiometer type) Example 5-1: Suppose E i =10 V, R s =0, R p =1000 Ω and Rm=5 KΩ. Derive FSO% of system in different positions. 10
Resistive Displacement Sensors (Potentiometer type) v Output of displacement sensor with load compensation 11
Resistive Displacement Sensors (Potentiometer type) v Technology u u u u Wire-wound Metal layer Carbon layer Polymer layer Characteristics of Conductive Plastic, Wirewound, and Hybrid 1000 rev/min 300 rev/min 12
Resistive Displacement Sensors (Potentiometer type) Belts and pulleys: Gears scale the mapping between input and pot shaft motions according to gear ratio. Rack-and-pinion mechanisms convert between linear and rotary motion. Lead-screws convert rotary motion to linear motion via the screw principle. 13
Resistive Displacement Sensors (Potentiometer type) Cabled drums convert between linear and rotary motion according to the drum circumference. Cams map rotary motion into linear motion according to the function programmed into the cam profile. Flexible shafts transmit rotary motion between two non-parallel axes with a 1:1 mapping. 14
Resistive Displacement Sensors (Potentiometer type) Implementation Protect the shaft from thrust, side, and bending loads (i.e., not use the pot as a bearing) Provide hard limit stops within the pot s travel range (i.e., not use the pot s limit stops) Protect the pot from contaminants. Advantages and drawbacks Pots are easy to use (2.5 to 500 mm) Relatively economical Almost no support electronics Poor repeatability Large hysteresis Subject to wear, particularly when exposed to vibrations. 15
Resistive Displacement Sensors (Potentiometer type) Typical resistance function for nonlinear potentiometers 16
Resistive Displacement Sensors (Potentiometer type) Example 5-2: It is necessary to measure the position of a panel. It moves 0.8 m. Does a rotary motion potentiometer can help? Example 5-3: Suppose in example 5-2 we need a 0.1 cm accuracy. We have a shaft that rotates 250 when the panel moved from one extreme to other. A rotary motion potentiometer has been found which is rotated 300 full-scale movement. It has 1000 turns of wire. Can this be used? It is ok. 17
Resistive Displacement Sensors (Potentiometer type) Example 5-4: A control potentiometer is rated as: 150 Ω 1 W (derate at 10 mw/ C abov 65 C) 30 C/W thermal resistance Can it be used with 10-V supply at 80 C ambient temperature? Answer: The power dissipated by the potentiometer is:c The actual temperature of the potentiometer is: Allowed power dissipation is: It is not ok. 18
Capacitive Sensors-Displacement It is Ok for small movements (μm-mm) 19
Inductive Displacement Sensors Hall effect sensor Relatively poor temperature performance Effectively used for short-range sensing Cost effective It is Ok when cost is most important and temperature is not an issue. Hall sensors work best when movements are less than (25 mm). 20
Inductive Displacement Sensors Horizontal or vertical movement One pole, bipolar, one pole and piece. One application as counter. 21
Inductive Displacement Sensors Eddy current sensors 22
Inductive Displacement Sensors LVDT (Linear variable differential transformer) LVDT is a friction less device nothing to wear out. LVDT has essentially infinite mechanical life. LVDT has no need to isolation and very good resolution. 23
Inductive Displacement Sensors 24
Inductive Displacement Sensors 25
Inductive Displacement Sensors Instrumentation used in LVDT Fortunately there are several commercial LVDT instrumentation. 26
Inductive Displacement Sensors Example 5-5: An LVDT can move ±15mm and its linearity is ±0.3%. Suppose sensitivity coefficient be 23.8 mv/mm. It is use in a CNC and measure between -12mm till +14mm. What is the output range and output error. Answer: Range of LVDT is: 12mm 14mm 12*23.8mv 14*23.8mv 285.6mv 333. 2mv Maximum error is: output _ error V meas displacement 1 23.8*0.997 Vmeas sensitivity 1 23.8*1.003 2.53 10 4 V meas ( mm) 27
Inductive Displacement Sensors RVDT (Rotational variable differential transformer) 28
Inductive Displacement Sensors RVDT (Rotational variable differential transformer) Features Brushless, non-contacting technology Repeatable position sensing with infinite resolution Housed and frameless versions available Geared configurations available Benefits Long life High reliability High accuracy Repeatable performance Robust, compact construction Custom electrical and mechanical designs available contact factory with requirements 29
Optical Encoder Displacement Sensors This part presented by: Mr. Ali Rahmanzadeh. Optical encoder A rotary encoder, also called a shaft encoder, is an electromechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. 30
Optical Encoder Displacement Sensors This part presented by: Mr. Ali Rahmanzadeh. Main parts of encoders - Light source - Condenser Lens - Graduated Disk - Photo detector (Photovoltaic Cells ) - Electronics board 31
Optical Encoder Displacement Sensors This part presented by: Mr. Ali Rahmanzadeh. Different types of optical encoders: Incremental encoders: It provides a pulse each time the shaft has rotated a defined distance. absolute encoders: It provides a output with a unique code pattern representing each position. 32
Optical Encoder Displacement Sensors This part presented by: Mr. Ali Rahmanzadeh. Incremental encoders: An incremental rotary encoder provides cyclical outputs (only) when the encoder is rotated, that can be easily interpreted to provide motion related information such as velocity. reference or "index", which happens once every turn. This is used when there is the need of an absolute reference, such as positioning systems. The index output is usually labeled Z. 33
Optical Encoder Displacement Sensors This part presented by: Mr. Ali Rahmanzadeh. absolute encoders: An "absolute" encoder maintains position information when power is removed from the system. [ The position of the encoder is available immediately on applying power. The relationship between the encoder value and the physical position of the controlled machinery is set at assembly; the system does not need to return to a calibration point to maintain position accuracy. 34
Optical Encoder Displacement Sensors Output techniques: In commercial absolute encoders there are several techniques for transmission of absolute encoder data, including parallel binary, analog current, analog voltage, PWM, Profibus, Profinet, EtherNet TCP/IP, CANopen, depending on the manufacturer of the device. This part presented by: Mr. Ali Rahmanzadeh. 35
Optical Encoder Displacement Sensors This part presented by: Mr. Ali Rahmanzadeh. Benefits High resolution : until 26 bit single turn Excellent angular accuracy Operating in extreme environments High reliability Miniaturized encoders Large hollow shaft encoders Very light weight Easy adaptation to the application 36
Synchros & Resolvers Synchros and resolvers are angular position transducers. v Synchro Primary on rotor 3 Secondaries on stator @ 120 physical angle v Resolver Primary on rotor 2 Secondaries on stator @ 90 physical angle 37
Synchros & Resolvers Synchros and resolvers advantages over other position transducers. It is absolute position encoder. Synchro converter resolution is typically 12 to 14 bits. Its input is an ac source(220/110 V, 50/60 Hz) so, no need to separate dc excitation. Its output is a high level ac. Any noise induced into signals is small, since, one must divide the stator voltage by the rotor voltage. How do one convert these sinusoidal signals into an indication of position? The answer is a receiver synchro. 38
Synchros & Resolvers Receiver synchro. 39
Synchros & Resolvers Receiver synchro. The output from the receiver synchro is not linear. There are two angles, as the shaft is rotated through a complete circle. Synchro to digital (S/D) or resolve to digital is a convenient solution. It uses the S1, S2, S3, and R1 and R2 signals from transmitter synchro to produce a binary word. 40
Synchros & Resolvers Angular value for the binary bits of a S/D converter. 41
Synchros & Resolvers Functional Diagram of S/D 42
Synchros & Resolvers A position control system by synchro. 43
References v Industrial Control Electronics J.M. Jacob, Prentice-Hall, 1989 مبانی اندازه گیری در سیستمهای ابزار دقیق حمید رضا تقی راد و سید v علی سالمتی انتشارات دانشگاه صنعتی خواجه نصیرالدین طوسی 1392 v Principles and Practice of Automatic Process Control, Third edition, C. A. Smith, A. Corripio, Wiley 2006 v Instrument Control Engineers' Handbook Fourth Edition, Béla G. Lipták, editor-in-chief. CRC Press 2003 44
Some Useful websites for the course v http://saba.kntu.ac.ir/eecd/ecourses/instrumentation.htm v http://profsite.um.ac.ir/~shoraka/instrumentation.htm v http://karimpor.profcms.um.ac.ir/index.php/courses/10328 45