Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 16
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This is called the null position of the core.However, if the core is moved to the left, secondary winding S1 ismore strongly coupled to primary winding P1 than secondary windingS2, and an output sine wave in phase with the primary voltage isinduced. Similarly, if the core is moved to the right and winding S2 ismore strongly coupled to primary winding P1, an output sine wave thatis 180º out-of-phase with the primary voltage is induced. The amplitudesof the output sine waves of the LVDT vary symmetrically with core displacement, either to the left or right of the null position.Linear variable differential transformers require signal conditioningcircuitry that includes a stable sine wave oscillator to excite the primarywinding P1, a demodulator to convert secondary AC voltage signals toDC, a low-pass filter, and an amplifier to buffer the DC output signal.The amplitude of the resulting DC voltage output is proportional to themagnitude of core displacement, either to the left or right of the nullposition.
The phase of the DC voltage indicates the position of the corerelative to the null (left or right). An LVDT containing an integral oscillator/demodulator is a DC-to-DC LVDT, also known as a DCDT.Linear variable differential transformers can make linear displacement (position) measurements as precise as 0.005 in. (0.127 mm).Chapter 1Motor and Motion Control SystemsOutput voltage linearity is an important LVDT characteristic, and it canbe plotted as a straight line within a specified range. Linearity is thecharacteristic that largely determines the LVDT’s absolute accuracy.Linear Velocity Transducers (LVTs)A linear velocity transducer (LVT) consists of a magnet positioned axially within a two wire coils.
When the magnet is moved through thecoils, it induces a voltage within the coils in accordance with the Faradayand Lenz laws. The output voltage from the coils is directly proportionalto the magnet’s field strength and axial velocity over its working range.When the magnet is functioning as a transducer, both of its ends arewithin the two adjacent coils, and when it is moved axially, its north polewill induce a voltage in one coil and its south pole will induce a voltagein the other coil. The two coils can be connected in series or parallel,depending on the application.
In both configurations the DC output voltage from the coils is proportional to magnet velocity. (A single coilwould only produce zero voltage because the voltage generated by thenorth pole would be canceled by the voltage generated by the southpole.)The characteristics of the LVT depend on how the two coils are connected. If they are connected in series opposition, the output is added andmaximum sensitivity is obtained. Also, noise generated in one coil willbe canceled by the noise generated in the other coil.
However, if the coilsare connected in parallel, both sensitivity and source impedance arereduced. Reduced sensitivity improves high-frequency response formeasuring high velocities, and the lower output impedance improves theLVT’s compatibility with its signal-conditioning electronics.Angular Displacement Transducers (ATDs)An angular displacement transducer is an air-core variable differentialcapacitor that can sense angular displacement. As shown in explodedview Figure 1-47 it has a movable metal rotor sandwiched between asingle stator plate and segmented stator plates.
When a high-frequencyAC signal from an oscillator is placed across the plates, it is modulated by the change in capacitance value due to the position of therotor with respect to the segmented stator plates. The angular displacement of the rotor can then be determined accurately from thedemodulated AC signal.5556Chapter 1Motor and Motion Control SystemsFigure 1-47 Exploded view ofan angular displacement transducer (ADT) based on a differential variable capacitor.The base is the mounting platform for the transducer assembly. It contains the axial ball bearing that supports the shaft to which the rotor isfastened.
The base also supports the transmitting board, which contains ametal surface that forms the lower plate of the differential capacitor. Thesemicircular metal rotor mounted on the shaft is the variable plate orrotor of the capacitor. Positioned above the rotor is the receiving boardcontaining two separate semicircular metal sectors on its lower surface.The board acts as the receiver for the AC signal that has been modulatedby the capacitance difference between the plates caused by rotor rotation.An electronics circuit board mounted on top of the assembly containsthe oscillator, demodulator, and filtering circuitry. The ADT is poweredby DC, and its output is a DC signal that is proportional to angular displacement. The cup-shaped housing encloses the entire assembly, andthe base forms a secure cap.DC voltage is applied to the input terminals of the ADT to power theoscillator, which generates a 400- to 500-kHz voltage that is appliedacross the transmitting and receiving stator plates.
The receiving platesare at virtual ground, and the rotor is at true ground. The capacitancevalue between the transmitting and receiving plates remains constant,Chapter 1Motor and Motion Control Systemsbut the capacitance between the separate receiving plates varies withrotor position.A null point is obtained when the rotor is positioned under equal areasof the receiving stator plates. In that position, the capacitance betweenthe transmitting stator plate and the receiving stator plates will be equal,and there will be no output voltage. However, as the rotor moves clockwise or counterclockwise, the capacitance between the transmitting plateand one of the receiving plates will be greater than it is between the otherreceiving plate.
As a result, after demodulation, the differential outputDC voltage will be proportional to the angular distance the rotor movedfrom the null point.InductosynsThe Inductosyn is a proprietary AC sensor that generates position feedback signals that are similar to those from a resolver. There are rotaryand linear Inductosyns. Much smaller than a resolver, a rotaryInductosyn is an assembly of a scale and slider on insulating substratesin a loop. When the scale is energized with AC, the voltage couples intothe two slider windings and induces voltages proportional to the sine andcosine of the slider spacing within a cyclic pitch.An Inductosyn-to-digital (I/D) converter, similar to a resolver-todigital (R/D) converter, is needed to convert these signals into a digitalformat.
A typical rotary Inductosyn with 360 cyclic pitches per rotationcan resolve a total of 1,474,560 sectors for each resolution. This corresponds to an angular rotation of less than 0.9 arc-s. This angular information in a digital format is sent to the motion controller.Laser InterferometersLaser interferometers provide the most accurate position feedback forservosystems. They offer very high resolution (to 1.24 nm), noncontactmeasurement, a high update rate, and intrinsic accuracies of up to 0.02ppm.
They can be used in servosystems either as passive position readouts or as active feedback sensors in a position servo loop. The laserbeam path can be precisely aligned to coincide with the load or a specificpoint being measured, eliminating or greatly reducing Abbe error.A single-axis system based on the Michaelson interferometer is illustrated in Figure 1-48. It consists of a helium–neon laser, a polarizingbeam splitter with a stationary retroreflector, a moving retroreflector that5758Chapter 1Motor and Motion Control SystemsFigure 1-48 Diagram of a laserinterferometer for position feedback that combines high resolution with noncontact sensing,high update rates, and accuraciesof 0.02 ppm.can be mounted on the object whose position is to be measured, and aphotodetector, typically a photodiode.Light from the laser is directed toward the polarizing beam splitter,which contains a partially reflecting mirror.
Part of the laser beam goesstraight through the polarizing beam splitter, and part of the laser beam isreflected. The part that goes straight through the beam splitter reachesthe moving reflectometer, which reflects it back to the beam splitter, thatpasses it on to the photodetector. The part of the beam that is reflected bythe beam splitter reaches the stationary retroreflector, a fixed distanceaway. The retroreflector reflects it back to the beam splitter before it isalso reflected into the photodetector.As a result, the two reflected laser beams strike the photodetector,which converts the combination of the two light beams into an electricalsignal. Because of the way laser light beams interact, the output of thedetector depends on a difference in the distances traveled by the two laserbeams.
Because both light beams travel the same distance from the laserto the beam splitter and from the beam splitter to the photodetector, thesedistances are not involved in position measurement. The laser interferometer measurement depends only on the difference in distance betweenthe round trip laser beam travel from the beam splitter to the movingretroreflector and the fixed round trip distance of laser beam travel fromthe beam splitter to the stationary retroreflector.If these two distances are exactly the same, the two light beams willrecombine in phase at the photodetector, which will produce a high electrical output. This event can be viewed on a video display as a brightlight fringe.
However, if the difference between the distances is as shortas one-quarter of the laser’s wavelength, the light beams will combineout-of-phase, interfering with each other so that there will be no electrical output from the photodetector and no video output on the display, acondition called a dark fringe.Chapter 1Motor and Motion Control Systems59As the moving retroreflector mounted on the load moves farther awayfrom the beam splitter, the laser beam path length will increase and a pattern of light and dark fringes will repeat uniformly. This will result inelectrical signals that can be counted and converted to a distance measurement to provide an accurate position of the load.
The spacing betweenthe light and dark fringes and the resulting electrical pulse rate is determined by the wavelength of the light from the laser. For example, thewavelength of the light beam emitted by a helium–neon (He–Ne) laser,widely used in laser interferometers, is 0.63 µm, or about 0.000025 in.Thus the accuracy of load position measurement depends primarily onthe known stabilized wavelength of the laser beam. However, that accuracy can be degraded by changes in humidity and temperature as well asairborne contaminants such as smoke or dust in the air between the beamsplitter and the moving retroreflector.Precision Multiturn PotentiometersThe rotary precision multiturn potentiometer shown in the cutaway inFigure 1-49 is a simple, low-cost feedback instrument.
Originally developed for use in analog computers, precision potentiometers can provideabsolute position data in analog form as a resistance value or voltage.Precise and resettable voltages correspond to each setting of the rotarycontrol shaft. If a potentiometer is used in a servosystem, the analog dataFigure 1-49 A precision potentiometer is a low-cost, reliablefeedback sensor for servosystems.60Chapter 1Motor and Motion Control Systemswill usually be converted to digital data by an integrated circuit analogto-digital converter (ADC).
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