1、CHAPTER 6POSITION AND MOTION SENSORS,Modern linear and digital integrated circuit technology is used throughout the field of position and motion sensing. Fully integrated solutions which combine linear and digital functions have resulted in cost effective solutions to problems which in the past have
2、 been solved using expensive electro-mechanical techniques. These systems are used in many applications including robotics, computer-aided manufacturing, and factory automation.,POSITION AND MOTION SENSORS Linear Position: Linear Variable Differential Transformers (LVDT) Hall Effect Sensors: Proximi
3、ty Detectors Linear Output (Magnetic Field Strength) Acceleration : Accelerometers,1. LINEAR VARIABLE DIFFERENTIAL TRANSFORMERS (LVDTs)The linear variable differential transformer (LVDT) is an accurate and reliable method for measuring linear distance. LVDTs find uses in modern machine-tool, robotic
4、s, avionics, and computerized manufacturing. By the end of World War II, the LVDT had gained acceptance as a sensor element in the process control industry largely as a result of its use in aircraft, torpedo, and weapons systems. The publication of The Linear Variable Differential Transformer by Her
5、man Schaevitz in1946 made the user community at large aware of the applications and features of the LVDT.,The LVDT (see Figure below) is a position-to-electrical sensor whose output is proportional to the position of a movable magnetic core. The core moves linearly inside a transformer consisting of
6、 a center primary coil and two outer secondary coils wound on a cylindrical form. The primary winding is excited with an AC voltage source (typically several kHz), inducing secondary voltages which vary with the position of the magnetic core within the assembly. The core is usually threaded in order
7、 to facilitate attachment to a nonferromagnetic rod which in turn in attached to the object whose movement or displacement is being measured.,The secondary windings are wound out of phase with each other, and when the core is centered the voltages in the two secondary windings oppose each other, and
8、 the net output voltage is zero. When the core is moved off center, the voltage in the secondary toward which the core is moved increases, while the opposite voltage decreases. The result is a differential voltage output which varies linearly with the cores position. Linearity is excellent over the
9、design range of movement, typically 0.5% or better. The LVDT offers good accuracy, linearity, sensitivity, infinite resolution.,A wide variety of measurement ranges are available in different LVDTs, typically from 100m to 25cm. Typical excitation voltages range from 1V to 24V RMS, with frequencies f
10、rom 50Hz to 20kHz. Key specifications for the Schaevitz El00 LVDT are given in Figure below. Nominal Linear Range: 0.1 inches ( 2.54mm) Input Voltage: 3V RMS Operating Frequency: 50Hz to 10kHz (2.5kHz nominal) Linearity: 0.5% Fullscale Sensitivity: 2.4mV Output / 0.001in / Volt Excitation Primary Im
11、pedance: 660 Secondary Impedance: 960,Note that a true null does not occur when the core is in center position because of mismatches between the two secondary windings and leakage inductance. Also, simply measuring the output voltage VOUT will not tell on which side of the null position the core res
12、ides.A signal conditioning circuit which removes these difficulties is shown in Figure below where the absolute values of the two output voltages are subtracted. Using this technique, both positive and negative variations about the center position can be measured.,AC SOURCE,T,While a diode/capacitor
13、-type rectifier could be used as the absolute value circuit, the precision rectifier shown in Figure below is more accurate and linear.,INPUT,The input is applied to a V/I converter which in turn drives an analog multiplier. The sign of the differential input is detected by the comparator whose outp
14、ut switches the sign of the V/I output via the analog multiplier. The final output is a precision replica of the absolute value of the input. These circuits are well understood by IC designers and are easy to implement on modern bipolar processes.The industry-standard AD598 LVDT signal conditioner s
15、hown in Figure below (simplified form) performs all required LVDT signal processing. The on-chip excitation frequency oscillator can be set from 20Hz to 20kHz with a single external capacitor.,Two absolute value circuits followed by two filters are used to detect the amplitude of the A and B channel
16、 inputs. Analog circuits are then used to generate the ratiometric function A-B/A+B. Note that this function is independent of the amplitude of the primary winding excitation voltage, assuming the sum of the LVDT output voltage amplitudes remains constant over the operating range. This is usually th
17、e case for most LVDTs, but the user should always check with the manufacturer if it is not specified on the LVDT data sheet.,A single external resistor sets the AD598 excitation voltage from approximately 1V RMS to 24V RMS. Drive capability is 30mA RMS. The AD598 can drive an LVDT at the end of 300
18、feet of cable, since the circuit is not affected by phase shifts or absolute signal magnitudes. The position output range of VOUT is 11V for a 6mA load and it can drive up to 1000 feet of cable. The VA and VB inputs can be as low as 100mV RMS.,2. HALL EFFECT MAGNETIC SENSORSIf a current flows in a c
19、onductor (or semiconductor) and there is a magnetic field present which is perpendicular to the current flow, then the combination of current and magnetic field will generate a voltage perpendicular to both (see Figure below).This phenomenon is called the Hall Effect, was discovered by E. H. Hall in
20、 1879.The voltage, VH, is known as the Hall Voltage. VH is a function of the current density, the magnetic field, and the charge density and carrier mobility of the conductor.,=MAGNETIC FIELD T=THICKNESS VH=HALL VOLTAGE,The Hall effect may be used to measure magnetic fields (and hence in contact-fre
21、e current measurement), but its commonest application is in motion sensors where a fixed Hall sensor and a small magnet attached to a moving part can replace a cam and contacts with a great improvement in reliability. Since VH is proportional to magnetic field and not to rate of change of magnetic f
22、ield like an inductive sensor, the Hall Effect provides a more reliable low speed sensor than an inductive pickup.,Although several materials can be used for Hall effect sensors, silicon has the advantage that signal conditioning circuits can be integrated on the same chip as the sensor. CMOS proces
23、ses are common for this application. A simple rotational speed detector can be made with a Hall sensor, a gain stage, and a comparator as shown in Figure below. The circuit is designed to detect rotation speed as in automotive applications. It responds to small changes in field, and the comparator h
24、as built-in hysteresis to prevent oscillation. Several companies manufacture such Hall switches, and their usage is widespread.,VTHRESHOLD,ROTATION,MAGNETS,There are many other applications, particularly in automotive throttle, pedal, and valve position sensing, where a linear representation of the
25、magnetic field is desired. The AD22151 is a linear magnetic field sensor whose output voltage is proportional to a magnetic field applied perpendicularly to the package top surface (see Figure below). The AD22151 combines Hall cell technology and conditioning circuitry to minimize temperature relate
26、d drifts associated with silicon Hall cell characteristics. The architecture maximizes the advantages of a monolithic implementation while allowing sufficient versatility to meet varied application requirements with a minimum number of external components.,Principal features include dynamic offset d
27、rift cancellation using a chopper-type op amp and a built-in temperature sensor.Designed for single +5V supply operation, low offset and gain drift allows operation over a 400C to +1500C range. Temperature compensation (set externally with a resistor R1) can accommodate a number of magnetic material
28、s commonly utilized in position sensors. Output voltage range and gain can be easily set with external resistors. Typical gain range is usually set from 2mV/Gauss to 6mV/Gauss.,3. OPTICAL ENCODERSOptical encoder is the popular position measuring sensors. An incremental optical encoder (left-hand dia
29、gram in Figure below) is a disc divided into sectors that are alternately transparent and opaque. A light source is positioned on one side of the disc, and a light sensor on the other side.,As the disc rotates, the output from the detector switches alternately on and off, depending on whether the se
30、ctor appearing between the light source and the detector is transparent or opaque. Thus, the encoder produces a stream of square wave pulses which, when counted, indicate the angular position of the shaft. Available encoder resolutions (the number of opaque and transparent sectors per disc) range fr
31、om 100 to 65,000.Most incremental encoders feature a second light source and sensor at an angle to the main source and sensor, to indicate the direction of rotation.,Without some form of revolution marker, absolute angles are difficult to determine. A potentially serious disadvantage is that increme
32、ntal encoders require external counters to determine absolute angles within a given rotation. If the power is momentarily shut off, or if the encoder misses a pulse due to noise or a dirty disc, the resulting angular information will be in error.,The absolute optical encoder (right-hand diagram in F
33、igure up) overcomes these disadvantages but is more expensive. An absolute optical encoders disc is divided up into N sectors (N = 5 for example shown), and each sector is further divided radially along its length into opaque and transparent sections, forming a unique N-bit digital word with a maxim
34、um count of 2N - 1. The digital word formed radially by each sector increments in value from one sector to the next, usually employing Gray code. Binary coding could be used, but can produce large errors if a single bit is incorrectly interpreted by the sensors.,Gray code overcomes this defect: the
35、maximum error produced by an error in any single bit of the Gray code is only 1 LSB after the Gray code is converted into binary code. A set of N light sensors responds to the N-bit digital word which corresponds to the discs absolute angular position. Industrial optical encoders achieve up to 16-bi
36、t resolution, with absolute accuracies that approach the resolution (20 arc seconds). Both absolute and incremental optical encoders, however, may suffer damage in harsh industrial environments.,4. RESOLVERS AND SYNCHROSMachine-tool and robotics manufacturers have increasingly turned to resolvers an
37、d synchros to provide accurate angular and rotational information. These devices excel in demanding factory applications requiring small size, long-term reliability, absolute position measurement, high accuracy, and low-noise operation. A diagram of a typical synchro and resolver is shown in Figure
38、below. Both sycnchros and resolvers employ single-winding rotors that revolve inside fixed stators. In the case of a simple synchro, the stator has three windings oriented 120o apart and electrically connected in a Y-connection. Resolvers differ from synchros in that their stators have only two wind
39、ings oriented at 90o,ROTOR,STAROR,R1,R2,S2,S4,S2,S1 TO S3=V sin t sin S3 TO S2=V sintsin(+120o) S2 TO S1=V sint sin(+240o),RESOLVER,S1 TO S3=V sint sin S4 TO S2=V sint sin(+90o)=V sintcos,SYNCHRO,Because synchros have three stator coils in a 120o orientation, they are more difficult than resolvers t
40、o manufacture and are therefore more costly. Today, synchros find decreasing use, except in certain military applications.Modern resolvers, in contrast, are available in a brushless form that employ a transformer to couple the rotor signals from the stator to the rotor. The primary winding of this t
41、ransformer resides on the stator, and the secondary on the rotor.Other resolvers use more traditional brushes to couple the signal into the rotor winding.,Brushless resolvers are more rugged than synchros because there are no brushes to break or dislodge, and the life of a brushless resolver is limi
42、ted only by its bearings. Most resolvers are specified to work over 2V to 40V RMS and at frequencies from 400Hz to 10kHz. Angular accuracies range from 5 arc-minutes to 0.5 arc-minutes. (There are 60 arc-minutes in one degree, and 60 arc-seconds in one arc-minute. Hence, one arc-minute is equal to 0
43、.0167 degrees).In operation, synchros and resolvers resemble rotating transformers. The rotor winding is excited by an AC reference voltage, at frequencies up to a few kHz. The magnitude of the voltage induced in any stator winding is proportional to the sine of the angle, , between the rotor coil a
44、xis and the stator coil axis.,In the case of a synchro, the voltage induced across any pair of stator terminals will be the vector sum of the voltages across the two connected coils.For example, if the rotor of a synchro is excited with a reference voltage, Vsint, across its terminals R1 and R2, the
45、n the stators terminal will see voltages in the form: S1 to S3 = V sint sin S3 to S2 = V sint sin (+ 120o) S2 to S1 = V sint sin (+ 240o), where is the shaft angle.,In the case of a resolver, with a rotor AC reference voltage of Vsint, the stators terminal voltages will be: S 1 to S3 = V sint sin S4
46、 to S2= V sint sin(+ 90o) = V sint cosIt should be noted that the 3-wire synchro output can be easily converted into the resolver-equivalent format using a Scott-T transformer. Therefore, the following signal processing example describes only the resolver configuration.,A typical resolver-to-digital
47、 converter (RDC) is shown functionally in Figure below. The two outputs of the resolver are applied to cosine and sine multipliers. These multipliers incorporate sine and cosine lookup tables and function as multiplying digital-to-analog converters. Begin by assuming that the current state of the up
48、/down counter is a digital number representing a trial angle, . The converter seeks to adjust the digital angle, , continuously to become equal to, and to track , the analog angle being measured.The resolvers stator output voltages are written as:,Vsint,V1 = V sint sin V2 = V sint cos where is the a
49、ngle of the resolvers rotor. The digital angle is applied to the cosine multiplier, and its cosine is multiplied by V1 to produce the term: V sint sin cos The digital angle is also applied to the sine multiplier and multiplied by V2 to product the term: V sint cos sin These two signals are subtracte
50、d from each other by the error amplifier to yield an AC error signal of the form: V sint sin cos) - cos sin),Using a simple trigonometric identity, this reduces to: V sint sin ( - )The detector synchronously demodulates this AC error signal, using the resolvers rotor voltage as a reference. This results in a DC error signal proportional to sin( - ).The DC error signal feeds an integrator, the output of which drives a voltage- controlled-oscillator (VCO). The VCO, in turn, causes the up/down counter to count in the proper direction to cause:sin( - ) 0 When this is achieved, - 0,
