Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

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54 Chapter 1 Motor and Motion Control Systems Figure 1-46 Schematic for a linear variable differential transformer (LVDT) showing how the movable core interacts with the primary and secondary windings. their primary and secondary windings caused by the linear movement of the ferromagnetic core. The core is attached to a spring-loaded sensing shaft. When depressed, the shaft moves the core axially within the windings, coupling the excitation voltage in the primary (middle) winding P1 to the two 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 equal amplitudes and are 180º out of phase. With a series-opposed connection, as shown, the net voltage across the secondaries is zero because both voltages cancel. This is called the null position of the core. However, if the core is moved to the left, secondary winding S1 is more strongly coupled to primary winding P1 than secondary winding S2, and an output sine wave in phase with the primary voltage is induced. Similarly, if the core is moved to the right and winding S2 is more strongly coupled to primary winding P1, an output sine wave that is 180º out-of-phase with the primary voltage is induced. The amplitudes of 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 conditioning circuitry that includes a stable sine wave oscillator to excite the primary winding P1, a demodulator to convert secondary AC voltage signals to DC, a low-pass filter, and an amplifier to buffer the DC output signal. The amplitude of the resulting DC voltage output is proportional to the magnitude of core displacement, either to the left or right of the null position. The phase of the DC voltage indicates the position of the core relative 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 1 Motor and Motion Control Systems 55 Output voltage linearity is an important LVDT characteristic, and it can be plotted as a straight line within a specified range. Linearity is the characteristic 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 the coils, it induces a voltage within the coils in accordance with the Faraday and Lenz laws. The output voltage from the coils is directly proportional to the magnet’s field strength and axial velocity over its working range. When the magnet is functioning as a transducer, both of its ends are within the two adjacent coils, and when it is moved axially, its north pole will induce a voltage in one coil and its south pole will induce a voltage in 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 coil would only produce zero voltage because the voltage generated by the north pole would be canceled by the voltage generated by the south pole.) The characteristics of the LVT depend on how the two coils are connected. If they are connected in series opposition, the output is added and maximum sensitivity is obtained. Also, noise generated in one coil will be canceled by the noise generated in the other coil. However, if the coils are connected in parallel, both sensitivity and source impedance are reduced. Reduced sensitivity improves high-frequency response for measuring high velocities, and the lower output impedance improves the LVT’s compatibility with its signal-conditioning electronics. Angular Displacement Transducers (ATDs) An angular displacement transducer is an air-core variable differential capacitor that can sense angular displacement. As shown in exploded view Figure 1-47 it has a movable metal rotor sandwiched between a single stator plate and segmented stator plates. When a high-frequency AC signal from an oscillator is placed across the plates, it is modulated by the change in capacitance value due to the position of the rotor with respect to the segmented stator plates. The angular displacement of the rotor can then be determined accurately from the demodulated AC signal.
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vi Contents Commutation 34 Installa
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viii Contents Chapter 4 Wheeled Veh
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x Contents Industrial Robots 258 In
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xii Introduction a unique robot. Wh
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xiv Introduction outdoor and indoor
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xvi Introduction Rapid prototyping
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xviii Introduction Because the phot
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xx Introduction Meanwhile, the opaq
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xxii Introduction Laminated-Object
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104 Chapter 2 Indirect Power Transf
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110 Chapter 3 Direct Power Transfer
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164 Chapter 5 Tracked Vehicle Suspe
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202 Chapter 7 Walkers when crossing
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204 Chapter 7 Walkers Figure 7-1 On
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206 Chapter 7 Walkers Figure 7-4 Tw
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208 Chapter 7 Walkers and the relat
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210 Chapter 7 Walkers Figure 7-10 E
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212 Chapter 7 Walkers Figure 7-12 T
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214 Chapter 7 Walkers ROLLER-WALKER
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216 Chapter 7 Walkers Walkers have
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220 Chapter 8 Pipe Crawlers and Oth
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230 Chapter 9 Comparing Locomotion
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242 Chapter 10 Manipulator Geometri
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292 Index CAM-LEM, Inc., xxiii camm
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294 Index (ground pressure cont.) a
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296 Index (mobility systems cont.)
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298 Index shear pin torque limiters
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About the Author Paul E. Sandin is
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Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

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