Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 15
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The practical maximum lengthof linear encoder scales is about 10 ft (3 m), but commercial catalogmodels are typically limited to about 6 ft (2 m). If longer distances are tobe measured, the encoder scale is made of steel tape with reflective graduations that are sensed by an appropriate photoelectric scanning unit.Linear encoders can make direct measurements that overcome theinaccuracies inherent in mechanical stages due to backlash, hysteresis,and leadscrew error. However, the scale’s susceptibility to damage frommetallic chips, grit oil, and other contaminants, together with its relatively large space requirements, limits applications for these encoders.Commercial linear encoders are available as standard catalog models,or they can be custom made for specific applications or extreme environmental conditions.
There are both fully enclosed and open linearencoders with travel distances from 2 in. to 6 ft (50 mm to 1.8 m). Somecommercial models are available with resolutions down to 0.07 µm, andothers can operate at speeds of up to 16.7 ft/s (5 m/s).Magnetic EncodersMagnetic encoders can be made by placing a transversely polarized permanent magnet in close proximity to a Hall-effect device sensor. Figure 1-39shows a magnet mounted on a motor shaft in close proximity to a twochannel HED array which detects changes in magnetic flux density asthe magnet rotates. The output signals from the sensors are transmitted tothe motion controller. The encoder output, either a square wave or aFigure 1-39 Basic parts of amagnetic encoder.Chapter 1Motor and Motion Control Systems49quasi sine wave (depending on the type of magnetic sensing device) canbe used to count revolutions per minute (rpm) or determine motor shaftaccurately.
The phase shift between channels A and B permits them to becompared by the motion controller to determine the direction of motorshaft rotation.ResolversA resolver is essentially a rotary transformer that can provide positionfeedback in a servosystem as an alternative to an encoder. Resolversresemble small AC motors, as shown in Figure 1-40, and generate anelectrical signal for each revolution of their shaft.
Resolvers that senseposition in closed-loop motion control applications have one winding onthe rotor and a pair of windings on the stator, oriented at 90º. The statoris made by winding copper wire in a stack of iron laminations fastened tothe housing, and the rotor is made by winding copper wire in a stack oflaminations mounted on the resolver’s shaft.Figure 1-40 Exploded view of abrushless resolver frame (a), androtor and bearings (b). The coilon the rotor couples speed datainductively to the frame forprocessing.50Chapter 1Motor and Motion Control SystemsFigure 1-41 Schematic for aresolver shows how rotor positionis transformed into sine andcosine outputs that measure rotorposition.Figure 1-41 is an electrical schematic for a brushless resolver showingthe single rotor winding and the two stator windings 90º apart. In a servosystem, the resolver’s rotor is mechanically coupled to the drive motorand load.
When a rotor winding is excited by an AC reference signal, itproduces an AC voltage output that varies in amplitude according to thesine and cosine of shaft position. If the phase shift between the appliedsignal to the rotor and the induced signal appearing on the stator coil ismeasured, that angle is an analog of rotor position. The absolute positionof the load being driven can be determined by the ratio of the sine outputamplitude to the cosine output amplitude as the resolver shaft turnsthrough one revolution.
(A single-speed resolver produces one sine andone cosine wave as the output for each revolution.)Connections to the rotor of some resolvers can be made by brushesand slip rings, but resolvers for motion control applications are typicallybrushless.
A rotating transformer on the rotor couples the signal to therotor inductively. Because brushless resolvers have no slip rings orbrushes, they are more rugged than encoders and have operating livesthat are up to ten times those of brush-type resolvers. Bearing failure isthe most likely cause of resolver failure. The absence of brushes in theseresolvers makes them insensitive to vibration and contaminants.
Typicalbrushless resolvers have diameters from 0.8 to 3.7 in. Rotor shafts aretypically threaded and splined.Most brushless resolvers can operate over a 2- to 40-volt range, andtheir winding are excited by an AC reference voltage at frequencies from400 to 10,000 Hz. The magnitude of the voltage induced in any statorwinding is proportional to the cosine of the angle, q, between the rotorcoil axis and the stator coil axis.
The voltage induced across any pair ofChapter 1Motor and Motion Control Systems51stator terminals will be the vector sum of the voltages across the twoconnected coils. Accuracies of ±1 arc-minute can be achieved.In feedback loop applications, the stator’s sinusoidal output signalsare transmitted to a resolver-to-digital converter (RDC), a specializedanalog-to-digital converter (ADC) that converts the signals to a digitalrepresentation of the actual angle required as an input to the motioncontroller.TachometersA tachometer is a DC generator that can provide velocity feedback for aservosystem.
The tachometer’s output voltage is directly proportional tothe rotational speed of the armature shaft that drives it. In a typical servosystem application, it is mechanically coupled to the DC motor andfeeds its output voltage back to the controller and amplifier to controldrive motor and load speed. A cross-sectional drawing of a tachometerbuilt into the same housing as the DC motor and a resolver is shown inFigure 1-42. Encoders or resolvers are part of separate loops that provideposition feedback.As the tachometer’s armature coils rotate through the stator’s magnetic field, lines of force are cut so that an electromotive force is inducedin each of its coils.
This emf is directly proportional to the rate at whichFigure 1-42 Section view of aresolver and tachometer in thesame frame as the servomotor.52Chapter 1Motor and Motion Control SystemsFigure 1-43 The rotors of theDC motor and tachometer sharea common shaft.Figure 1-44 This coil-type DCmotor obtains velocity feedbackfrom a tachometer whose rotorcoil is mounted on a commonshaft and position feedback froma two-channel photoelectricencoder whose code disk is alsomounted on the same shaft.the magnetic lines of force are cut as well as being directly proportionalto the velocity of the motor’s drive shaft. The direction of the emf isdetermined by Fleming’s generator rule.The AC generated by the armature coil is converted to DC by thetachometer’s commutator, and its value is directly proportional to shaftrotation speed while its polarity depends on the direction of shaft rotation.There are two basic types of DC tachometer: shunt wound and permanent magnet (PM), but PM tachometers are more widely used in servosystems today.
There are also moving-coil tachometers which, likemotors, have no iron in their armatures. The armature windings arewound from fine copper wire and bonded with glass fibers and polyesterresins into a rigid cup, which is bonded to its coaxial shaft. Because thisarmature contains no iron, it has lower inertia than conventional copperand iron armatures, and it exhibits low inductance. As a result, the moving-coil tachometer is more responsive to speed changes and provides aDC output with very low ripple amplitudes.Chapter 1Motor and Motion Control Systems53Tachometers are available as standalone machines.
They can berigidly mounted to the servomotor housings, and their shafts can bemechanically coupled to the servomotor’s shafts. If the DC servomotor iseither a brushless or moving-coil motor, the standalone tachometer willtypically be brushless and, although they are housed separately, a common armature shaft will be shared.A brush-type DC motor with feedback furnished by a brush-typetachometer is shown in Figure 1-43. Both tachometer and motor rotorcoils are mounted on a common shaft.
This arrangement provides a highresonance frequency. Moreover, the need for separate tachometer bearings is eliminated.In applications where precise positioning is required in addition tospeed regulation, an incremental encoder can be added on the sameshaft, as shown in Figure 1-44.Linear Variable Differential Transformers (LVDTs)A linear variable differential transformer (LVDT) is a sensing transformer consisting of a primary winding, two adjacent secondary windings, and a ferromagnetic core that can be moved axially within thewindings, as shown in the cutaway view Figure 1-45. LVDTs are capableof measuring position, acceleration, force, or pressure, depending onhow they are installed. In motion control systems, LVDTs provide position feedback by measuring the variation in mutual inductance betweenFigure 1-45 Cutaway view of alinear variable displacementtransformer (LVDT).54Chapter 1Motor and Motion Control SystemsFigure 1-46 Schematic for a linear variable differential transformer (LVDT) showing how themovable core interacts with theprimary and secondary windings.their primary and secondary windings caused by the linear movement ofthe ferromagnetic core.The core is attached to a spring-loaded sensing shaft.
Whendepressed, the shaft moves the core axially within the windings, coupling the excitation voltage in the primary (middle) winding P1 to thetwo adjacent secondary windings S1 and S2.Figure 1-46 is a schematic diagram of an LVDT. When the core is centered between S1 and S2, the voltages induced in S1 and S2 have equalamplitudes and are 180º out of phase. With a series-opposed connection,as shown, the net voltage across the secondaries is zero because bothvoltages cancel.
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