GB2488139A - Rotary torque sensor with optical signal output - Google Patents

Abstract

A rotary torque sensor comprising a strain gauge 8 (e.g. of resistive type) bonded to a rotating shaft 6, wherein the gauge is powered by a slip ring and brush assembly 1,9 and its output signal is transmitted optically via optical emitting and receiving diodes (e.g. LED 7 and photo-diode 5 respectively). The slip rings may be printed circuits etched onto the stator PCB, plated with a metallic (e.g. Nickel and Gold) layer 2 and the brushes may be beryllium copper wire brushes. The emitting diodes may be Light Emitting Diodes (LED) mounted in an optical non-contacting slip ring light guide 4, the light guide comprising optically transmitting material with a reflective element applied to one side such that light is emitted in a direction parallel to the shaft. Multiple strain gauges may be arranged in a Wheatstone bridge arrangement (e.g. for temperature compensation).

Description

BACKGROUND:

14 In general, there is known prior art on torque sensors which is listed below: 17 Torque: 18 DE102004056049 discloses a torque sensor which has stator unit with coil 19 circuit having flexible substrate running along sensor region, and evaluation circuit to evaluate signal received from one coil to determine value for rotary 21 position of rotor units.

22 DEl 0200734 discloses a torque sensor which has a source attached to 23 the shaft and a cooperating stationary sensor with a pair of sensor 24 elements and an integrated evaluation unit. The sensor elements are provided by Hall elements spaced apart by a distance of between 1 and 26 4mm.

27 DE 4323960 discloses a torque sensor which includes a stationary 28 sensor for detecting axial displacement e.g. of an axial force, and/or a 29 stationary dynamic torque control mechanism coupled to the axially displaceable centre section of a hollow torsion casing.

31 US 6601462 discloses a capacitive torque sensor and method of 32 detecting torque comprising of a dielectric layer variable in dielectric 33 constant in dependence upon the strain is formed on at least a part of 34 an outer peripheral surface of a bar having a circular cross section.

Inter digital electrodes are faced to the dielectric layer to thereby form a 36 single capacitor. The inter digital electrodes have a plurality of linear 37 electrodes.

39 Torque measurements by magnetism: Torque sensors utilising magnetism as mode of measurement has been used and is listed 41 below: 42 US2004035222 discloses the sensing of magnetic fields in contactless 43 torque sensor system for shafts and other bodies subject to torque. The 44 invention is based on the concept that a hollow part, such as a hollow shaft, has a wall section of ferromagnetic material internally bounded by 46 a bore. The torque or force dependent field is detected by a magnetic 47 field sensor or sensor arrangement positioned within the bore. The 48 magnetically active region may also be established by other means, e.

49 g. an active source by which the region is magnetised to provide the desired force or torque-dependent field within the hollow.

511 US472471 0 discloses an electromagnetic sensor for measuring the 52 twist of a rotary shaft in response to the transmission of torque. Rotary 53 magnetic sleeve means coupled to longitudinally displaced points on 54 the shaft rotate relative to each other in proportion to the shaft deformation. Permanent magnets provide a source of magnetic flux, 56 and a magnetic flux path is defined by a combination of the rotary 57 sleeve and a segmented stationary sleeve. Relative rotation of the 58 rotary sleeve with deformation of the shaft progressively alters the 59 density of the magnetic flux in the segmented stationary sleeve, and a magnetic flux transducer positioned in an air gap between the 61 stationary sleeve segments detects the flux density to provide an 62 indication of the amount of torque transmitted through the shaft.

63 JP 58009034 describes a method to make the non-contact detection of 64 torque possible by fixing a thin amorphous magnetic strip having a large magnetostriction constant by winding and fixing it to a revolving 66 shaft.

68 DE4231 646 discloses a measuring arrangement to determine the 69 torsion as well as a torque of a shaft. On the shaft annular bodies from soft magnetic material are mounted at axial positions located in the 71 distance, which rotate against each other with torsion. The bodies 72 exhibit a rotationally symmetric arrangement with projections and gaps.

73 In a stator member several coils are provided, which become generated 74 with an electromagnetic alternating field. The torsion displacement of the projections results in measurable change of the inductance of the 76 magnetic circuit, from which a measurement value for the torsion is 77 obtained.

78 US 6260422 describe a torque sensor and rotation restrictor for stator.

79 An improved magnetostrictive torque sensor for sensing torque applied to a shaft that is rotatably supported in housing. A magnetostrictive 81 cylinder is fixed to the shaft. A stator is supported on the shaft by 82 bearings to surround the magnetostrictive cylinder and is 83 accommodated in the housing. The stator incorporates exciting coils 84 and detecting coils such that the coils are located about the shaft.

JP 1209773 describes a method to employ the surface layer of a rotary 86 shaft itself as a torque sensor by heating and quickly cooling the 87 surface layer along a circumferential direction by applying a high 88 energy density beam to the local width region of the shaft formed of 89 normal temperature normal magnetic steel. A carbon dioxide gas laser beam is, for example, applied to the local width region of a rotary shaft 91 formed of a SUS3O4 material, and a whole surface layer is heated and 92 quickly cooled over the whole outer periphery, thereby forming a 93 ferromagnetic layer on the outer periphery of the shaft.

Imve4 rotation anqje: Torque measurement combined with rotation 96 angle is also well known and is listed below: 97 DEl 9745823 describes an apparatus for measuring torque and rotation 98 angle of a shaft. Two axial from remote locations of the shaft disc 99 members are mounted to measure torque.

W098/48244 shows a turning sensor and a torque sensor for detection 101 of the angular position by means of a Hall Effect sensor. At a shaft 102 magnets are arranged, while a stator member is between river 103 guidance pieces where air gaps are formed. The magnetic flux in the air 104 gaps builds and from it the rotational position of the magnetic member is determined.

106 DE4232993 discloses an apparatus to the measure the torsion and/or 107 relative angular movement of a wave configuration. On the wave 108 configuration which can be measured from two coil assemblies with an 109 air gap located between them are mounted as rotor. A coil ring arrangement provided is stator. The coil assemblies are wrapped with 111 ring segments from ferrite-filled material with magnet wire. The torsion 112 of the wave configuration expresses itself in a displacement of the coil 113 assemblies to each other, which leads with current flow to the increase 114 or decrease of an electric field in the coil assemblies. These electrical signals into the coil ring arrangement of the stator are transmitted and 116 the resultant measuring curves evaluated.

118 Rotary toçQue sensor Rotary torque sensors in general are known and 119 listed below: CN 101629861 discloses a torque rotary speed sensor, which 121 comprises a torque sensor, a rotary speed sensor and a signal 122 processor, wherein the torque sensor is used for measuring the torque 123 and outputting a torque frequency signal having a mathematic relation 124 with the torque; the rotary speed sensor is used for measuring the rotary speed and outputting a rotary speed frequency signal having a 126 mathematic relation with the rotary speed.

127 CN 201277894 discloses a rotary torque sensor which comprises a 128 sensor main body connected with a probe; one end of a drive rod at the 129 centre of the probe is symmetrically connected with a rotary blade by a cross shaft, and the other end is connected with an input shaft of the 131 sensor main body by a connector; the sensor main body comprises a 132 cylindrical shell, a torque generating mechanism in the middle of the 133 shell, a torque magnifying mechanism for connecting the input shaft 134 and the output shaft, and a data acquisition mechanism.

CN 2741124 discloses a rotary torque sensor from bearing power 136 supply.

137 DE10057468 discloses a rotary shaft torque sensor system for 138 automatic brake system in motor vehicle, includes roller bearing 139 arranged in flux path between shaft and excitation coil so that air gap is absent.

141 JP 2000055751 discloses a sensor with rotary angle limitation 142 equipment for detecting rotary torque.

S

143 JP 61155930 describes a method to make it possible to control torque 144 with high accuracy, by simple constitution such that a bush energized by a return spring is connected between an input side rotary shaft and 146 an output side rotary shaft through a cam mechanism and proximity 147 switch is provided.

149 Strain _qauqs_as torque sensor: strain gauges have been used in torque and rotary torque sensors and are listed below: 151 DE4216234 discloses a electrical machine for sensing torque and 152 speed for testing active and passive rotary machines -has strain gauge 153 torque sensor coupled to rotor of measuring machine so that shaft of 154 measuring machine remains free of rotational friction of bearing.

CN 101762349 discloses a high-precision intelligent standard torque 156 sensor, aiming at mainly solving the technical problem of improving the 157 linearity and the stability. The elastic body of the high-precision 158 intelligent standard torque sensor is made of maraging steel containing 159 alloy component; two groups of strain gauges for measuring bending moment and twisting are adhered to the elastic body; and a circuit 161 board for a processing is arranged in the shell.

162 WO 2010088922 discloses a multiaxial force/torque sensor assembly 163 comprising a set of at least two sensors each being made of strain 164 gauges each being arranged at a definite angle and distance relatively to each other and each being fixed to a transducer body, preferably a 166 metal body, which is direct or indirect mechanically in contact with a 167 printed circuit board (PCB) provided with clearances for each strain 168 gauge and provided with associated electronic components and wiring 169 located on the remaining area of the printed circuit board, which will monitor compressive and tensile stresses in the measurement 171 directions of the sensors.

172 DE 10247972 discloses a device which has at least one multi- 173 component sensor consisting of a deformation element extending along 174 the sensor's longitudinal axis between two force applying flanges with strain gauges on its outer surfaces.

176 GB238791 1 describes a torque transducer for measuring torque in a 177 circular shaft which is formed by a saddle mounted to the shaft, the 178 saddle having a seat which is profiled to complement the curvature of 179 the outer periphery of the shaft. A plate on which a strain measuring device is located is rigidly attached in the saddle body so it is flat when 181 no torque is applied to the transducer. The transducer provides a 182 mounting member allowing attachment of a flat sensor to a circular 183 shaft. The saddle may be of cylindrical or rectangular shape and may 184 include a cover to form an enclosed housing for the strain sensor i.e. strain gauges. Connection between the strain sensor and associated 186 circuitry is wired or wireless.

187 DE 10161183 describes a sensor for measuring the torque between 188 two machine assemblies with contact-free energy. Torque is measured 189 using force sensors positioned to measure the deformation of a component caused by an applied torque. The measured torque value is 191 decoupled from interference forces. The strain sensors on which the 192 strain gauges are mounted are H or U shaped.

193 DE 10142480 discloses a torque-transfer sensor which has two rotating 194 connection pads that rotate relative to each other and are connected to each other via a deformation body on which strain gauges are 196 mounted, the signals of which are connected to a bridge measurement 197 circuit. The deformation body is a linking spiral spring whose bending 198 axis coincides with the rotation axis. Strain gauges are mounted on 199 either side of the spring axis with their measurement direction perpendicular to the rotation axis.

201 JP 10206252 describes a method and apparatus for correcting linearity 202 of torque sensor using strain gauge. The output signal of a strain gauge 203 is connected through an amplifier to an analogue-digital converter. An 204 output terminal of the analogue-digital converter and an output terminal 205 of an output circuit of a corrected value are connected to the input 206 terminal of an adding signal of an adder, and the output terminal of the 207 digital-analogue converter is connected to an output terminal of a 208 measuring signal. An analogue signal of the output terminal becomes 209 the signal of a value proportional to torque in the correction of linearity 210 even if the torque exceeding the linearity of a torque sensor is applied, 211 so that the correct measurement can be performed irrespective of the 212 size of the torque.

213 DE 10321210 describes a method which involves adjusting the 214 amplification of output signals of several strain sensors attached to a 215 flexible outer gear wheel and combining sensor output signals 216 according to the amplification adjustment. The output signal 217 amplification of each sensor is adjusted so that the rotary harmonics in 218 their output signals caused by distortion of the flexible outer gear wheel 219 and not related to the transmitted torque is eliminated or controlled.

221 Photo Dkdes: Torque signals using photo diodes are known and listed 222 below: 223 CN 1369695 discloses a torque transducer in photoelectric type for 224 vehicle belonging to special electronic equipment of transport vehicle is 225 a photoelectric torque transducer in non-contacting type, which mainly 226 composes of two photoelectric cells, a light-transparent arm with light- 227 transparent hole and a light barrier arm with light-guiding hole, luminous 228 diode, torsion bar and pencil of conducting line. The light transparent 229 arm and light barrier arm are fixed at two ends of torsion bar separately, 230 the photoelectric cell and luminous diode will be placed at outer side of 231 light-transparent arm and light barrier arm separately and the 232 conducting wires of photoelectric cell and luminous diode will be 233 connected with outside through winding box.

234 JP 2002071480 discloses an optical torque and rotation sensor, and 235 electric power steering device. A rough part comprising groove parts 236 and protruding parts is formed on the outside perimeter of a torsion bar.

237 The light emitted from a light-emitting diode is reflected on the rough 238 part, and the reflected light is received with photodiodes. A control 239 device uses a difference between voltages outputted from the 2 240 photodiodes to acquire a bridge output, and detects a torque applied on 241 the torsion bar from the bridge output.

242 US56061 37 discloses an optical torque sensor for incorporation in a 243 vehicle steering system comprises a light-emitting diode (LED), a light 244 sensor which receives a light signal, a signal processor for receiving 245 output signals from the light sensor, and a light-transmitting medium 246 which alters the direction of propagation of light propagating through 247 the medium to an extent which is dependent on the torque applied to 248 the medium. The medium is defined by a generally cylindrical body 249 which is attached to input and output ends of a torsion bar.

250 JP 6086757 describes an oscillation circuit which applies an oscillation 251 voltage to both a torque detecting coil and a temperature compensating 252 coil, a difference amplifying circuit which applies the voltage, depending 253 on the AC voltage of the torque detecting coil and that of the 254 temperature compensating coil, to a positive input terminal (+) and a 255 negative input terminal (-), respectively, and a diode which applies the 256 negative input terminal (-) of the difference amplifying circuit with the 257 reference voltage, obtained by dividing the voltage of a power supply E 258 with voltage-dividing resistors.

259 DE 3804767 describes a sensor for the measurement of moments 260 occurring on test objects having rotational bearings has a measurement 261 pick-off from a light-emitting diode (LED) connected to a sensor 262 housing, a two-part differential photodiode likewise fastened to the 263 housing and a mirror connected to a magnetic holder. The mirror 264 reflects the light emitted by the LED and partially illuminates the 265 differential photodiode. In the case of a deflection of the magnetic 266 holder provided with a coupling for the test object, the current in one 267 area of the differential photodiodes increases and in the other area 268 decreases.

269 CN 2741127 discloses a rotary torque sensor of photoelectric coupling 270 sending out signals, comprising a shell and a gauging spindle. A first 271 round hole and a second round hole are arranged on the shell; the 272 gauging spindle travel through the shell; the shell is provided with a 273 processing circuit and a power supply device; an infrared luminous tube 274 is fixedly installed on the surface of the gauging spindle; an infrared 275 receiving tube is arranged on the shell. The sensor adopts non-contact 276 photoelectrical coupling connection to cause the export of the 277 measuring signal, and the measuring signal has no interference from 278 outside.

280 Laer torque measurement by laser is listed below: 281 US 2008156972 discloses a non-contact torque sensing apparatus and 282 method for measuring the instantaneous torque, or torsional 283 stress/strain, transmitted through an elongated power transmission 284 member such as a rotatable shaft. Polarized light is directed along a 285 measurement light path in a cavity of a shaft where it intercepts a 286 polarizing filter. The polarizing filter is operable to alter the polarization 287 angle of the light according to torsional twisting of the shaft. A 288 measurement device measures the change in the polarization angle of 289 the light to obtain the shaft twist angle. Shaft torque is then calculated 290 from the twist angle.

292 Data: data accusation for torque measurement is listed below: 293 DE 10040356 describes a system which includes memory for 294 selectively storing measurement data output by true sensor, and battery 295 or capacitor to supply stored energy to sensors. Control logic regulates 296 data storage and power supply operations based on output of a signal 297 detector.

299 This invention is based on rotary torque measurement methodldevice 300 which utilises combination of known elements and novel elements. The 301 device uses strain gauges as sensing elements which are bonded to a 302 calibrated rotating shaft. The strain gauges are used in multiple 303 numbers arranged in such a fashion as to cancel out all extraneous 304 forces other than torque and provide temperature compensation. The 305 strain gauges are powered via a slip ring and brush assembly. The 306 torque readings from the rotating shaft are obtained by photodiodes 307 utilising optical ring.

308 Key novelty aspect of this invention is the method of transmission of 309 power to the sensing elements and the transmission of the signals from 310 sensing elements.

Summary of Invention

311 An embodiment of this invention is a rotary torque sensor device in 312 which the electrical power transmission is provided via slip ring and 313 brush assembly and comprises of: 314 a) slip rings produced by etching printing circuits onto rotor PCB and 315 metallic plating, 316 b) connection to slip rings via metal brushes, 317 c) brush assemblies with variable tension capability to negate any 318 resonant effects on power transmission, caused by external 319 vibrational forces, 320 d) power is processed onto rotor PCB to amplify and to process 321 electronics which powers strain gauges whose signal is amplified 322 and digitised, 323 rotary torque is sensed by number of strain gauges, bonded to an 324 elastic element which distorts under external forces (in this case 325 torsional force but can be other types of force) which are arranged to 326 negate vibratory noise and extraneous forces, and to provide 327 temperature compensation, 328 and the method for signal transmission comprises of: 329 a) digitised signals power Light Emitting Diodes or diodes, 330 b) output of transmission is received by optical diode, 331 and the device allows digital signal transmission to be made of the 332 torque signal from the rotating rotor to the stator of the transducer.

334 Another embodiment of this invention is that slip rings are made of 335 metal plating, preferably with hard Gold and Nickel.

337 Another embodiment of this invention is that connection to slip rings is 338 via multiple metallic brushes, preferably made of copper beryllium.

340 Another embodiment of this invention is combination of Gold and Nickel 341 slip ring plating and copper beryllium brushes produce high conducting 342 and low wearing contacts, i.e. wear rates are lower than 1 xlO-09 m/m, 343 preferably lxlO-1O m/m 345 Another embodiment of this invention is that strain gauges of resistive 346 type or of any similar sensing type are used in multiple numbers which 347 are arranged in an electrical whetstone bridge configuration for full 348 signal torsion performance and cancel out vibratory noise and reduce to 349 a minimum all extraneous forces, and to provide temperature 350 compensation.

352 Another embodiment of this invention is that the method for signal 353 transmission comprises of digitised signals power LED's or diodes 354 which are mounted on optical transmissive material slip ring, which is a 355 circular ring of transmissive material with reflective element applied to 356 one side, enabling light to travel around the ring and be emitted at 357 perpendicular angle along its perimeter and the output of transmission 358 is received by fixed optical diode positioned at the front of optical ring.

360 Another embodiment of the invention is that the device allows digital 361 signal transmission to be made of the torque signal from the rotating 362 rotor to the stator of the transducer and then the digital signal can 363 further be electronically processed further to comply with data 364 transmission protocols i.e. RS232, R5488, and the many field bus 365 systems.

367 Another embodiment of the invention is that signals can be wirelessly 368 transmitted.

DESCRIPTION

Detailed Description:

370 Figure 1: An expanded side view of rotary torque device 371 Figure 2: a side view of rotary torque device 372 Figure 3: Pin on Disc configuration for measuring wear coefficient 374 There are numerous ways of putting electrical power into a rotating 375 shaft/component.

376 For transducer use where considerable effort has been made to 377 ensure. Brushes and slip rings for transmitting electrical power to a 378 circuit are well known and currently in use. The key requirements for 379 this inventive device are that the power to be transmitted is only a few 380 volts, typically up to 18 volts direct current and at current levels of up to 381 50 milli-amps. The sliding contacts to transmit the power (brushes and 382 slip rings) should be in contact with each other at all times during the 383 shaft rotation (which has the slip rings mounted on them) whilst 384 subjected to vibrationary forces in all planes. There should be very low 385 friction so that they do not interfere or there is minimal effect on the 386 torque being measured by the transducer otherwise frictional forces will 387 introduce a frictional drag torque affecting the measurement accuracy.

388 They should also have long life in design for minimum maintenance, 389 and the sliding surfaces should not deteriorate due to environmental 390 conditions.

392 This narrows down the choice of material to either coin silver slip rings 393 with silver graphite brushes or to beryllium copper multiple wire brushes 394 with hard Gold slip rings. Both of these methods have been used in the 395 past for signal and power transmissions in such critical systems such as 396 radar aerials/antenna, cranes, and gyroscopes as examples. In most 397 cases there are provided two brushes per slip ring to maintain contact 398 between the brush and the slip ring. Commercially these combinations 399 are supplied as units within their own right. However they are physically 400 large and expensive. In this invention the brushes and slip rings are a 401 composite assembly within the transducer allowing an improved, more 402 compact, lower cost transducer product to be produced.

403 This invention is based on number of elements; firstly, the sensing 404 elements are strain gauges, which are bonded to a calibrated rotating 405 shaft to measure torque/torsion forces. These strain gauges are of the 406 standard resistive type or of any similar sensing type. They are used in 407 multiple numbers arranged in such a fashion (multiples which are 408 arranged in an electrical whetstone bridge configuration for full signal 409 torsion performance) and to cancel out all extraneous forces. The 410 cancellation of the unwanted monitored/measured forces are achieved 411 by mounting the gauges in positional balanced pairs on the torsion 412 shear axis of the sensor material and electrically connected such that 413 bending, longitudinally compressive and tension forces are electrically 414 cancelled within the whetstone bridge configuration. Therefore there will 415 be no differential signal from the whetstone bridge. This arrangement 416 also provides temperature compensation as all gauges are effected 417 similarly and therefore do not provide a differential signal from the 418 whetstone bridge. However, these devices need to have electrical 419 power for them to work and secondly a novel way to power them via a 420 slip ring and brush assembly. Finally, a novel way to collect the torque 421 reading from the rotating shaft is demonstrated.

422 This assembly as shown in Figures 1 & 2 comprises not the usual slip 423 ring types but ones produced by etching printed circuits onto the stator 424 P.C.B. {printed circuit board}(1) and plating them with metallic layer, 425 preferably specific combinations of Nickel and hard Gold (2). The 426 devices to connect power to these slip rings are beryllium copper wire 427 brushes (9) of specific diameter (0.05 inches) hardness (500-700 MPa) 428 and in 5-1000 multiples, preferably 100. There are two or more of these 429 brush block assemblies with different bending forces presented by the 430 bushes to the slip rings to ensure resonant effects do not effect contact 431 transmission of the power. The power is then processed on the rotor 432 P.C.B. (3) to be regulated, electrically noise filtered to power the 433 whetstone bridge sensors, signal amplifiers and process electronics. it 434 powers the sensing gauges (8) to rotating shaft (6) whose signals are 435 very low in value. An onboard amplifier raises the signal level to a 436 useful value for the signal to be digitised.

437 This digital signal then powers a Light emitting diode or diodes (7). This 438 L.E.D or L.E.Ds are mounted in an optical non-contacting slip ring light 439 guide (4). This unique optical slip ring is a circular ring of optically 440 transmissive material, with a reflective element applied/ or abraded to 441 one side. The diodes are positioned into the material such that their 442 optical output is into the optical ring. The properties of the optical ring 443 enable the light to travel around the ring and be emitted at a 444 perpendicular angle along its perimeter. The output of this optical 445 transmission is detected by a fixed optical receiving diode(s) (5) 446 positioned in front of the optical ring. This allows digital signal 447 transmission to be made of the torque signal from the rotating rotor to 448 the stator of the transducer This digital signal can then be electronically 449 processed further to comply with data transmission protocols i.e. 450 RS232, RS488, and the many Field bus systems. Additionally signals 451 can be wirelessly transmitted.

452 This method on rotating parts reduces the need to have batteries 453 mounted on the shaft that adds errors due to inertia effects. Similarly it 454 reduces electrical noise on the torque electrical signal by using slip 455 rings and brushes only for powering the sensors, not for transmission of 456 low or high power signals. For the same reason friction errors, 457 dampening the torque measured, or affecting the torque to be 458 measured, especially with very low torques values are reduced greatly.

459 It also reduces inertia affects from bulky and expensive slip rings, it 460 allows high speed torque sensing and data transmission to take place 461 at high orders of accuracy and low uncertainty of measurement.

462 It allows for intelligent transducers, which can stand-alone. To have 463 digital signals which can be easily processed/read from any commonly 464 compatible data logger, computer, and display? 465 This method can also be used with many other types of measurement 466 and types of forces other than torque as a means of isolating electrical 467 signals in electrically noisy areas, Radio intensive field transmissions, 468 and for intrinsic safety applications.

469 This device demonstrated power transmission of only a few volts, 470 typically up to 18 volts direct current and current levels of up to 50 milli-471 amps.

472 Calculation of wear rate: A common used equation to compute the wear 473 rate is (Archard,1953).

474 W1=kFs 475 where F is the normal load, s the sliding distance, W1 the wear volume 476 and k1 the specific wear rate coefficient. Index i identifies the surface 477 considered. The k-value is given in m3/Nm or m2/N, sometimes in 478 mm3/Nm. However, the wear displacement h is more convenient than 479 W. With h1 =V /A, the contact pressure p=F/A where A is the area 480 subjected to wear then: 481 h1=k1ps 482 The sliding distance s can be replaced by s=v.t where v is the mean 483 value for the slide rate and t the running time. Because the k-value 484 depends just like the friction coefficient on a lot of parameters this factor 485 is to be found experimentally.

486 The starting point for any discussion of wear on the macro scale is the 487 Archard Equation, which states that: 488 WK*s*P 489 where W is the worn volume, s is the sliding distance, P is the applied 490 load and K is the wear per unit load per sliding distance.

492 The Archard equation assumes that the wear rate is independent of 493 apparent area of contact. However, it makes no assumptions about the 494 surface topography (surface roughness effects are encompassed by 495 the experimental wear coefficient) and it also makes no assumptions 496 about variations with time. It must also be stated that although it is 497 widely used, the Archard equation only provides for an order of 498 magnitude estimate and is a true calculation of wear. One of the more 499 common methods for determining the value for K is to press a 500 stationary pin using a preload of P into the surface of a rotating disk.

501 The load P is known and the sliding distance S can be determined from 502 the rotational speed of the disk and time that the disk has rotated. The 503 amount of wear on the pin is determined by change in mass (weight) of 504 the pin and the constant K calculated as shown in Figure 3. The disc 505 (10) has surface layer made of Gold plated Nickel (11) and pin made of 506 Copper Beryllium (12).

507 According to some researcher, there is no correlation either between 508 the coefficients of friction and overall hardness. Although hardness is 509 undoubtedly an important factor in wear performance, its role is more 510 complex than was once thought and, is closely linked to the structure of 511 the materials involved. It is evident that the combination of one hard 512 and one less-hard material is an important feature of a successful 513 matching pair. The hard surface controls the interaction and the softer 514 surface conforms. The softer material is able to embed hard abrasive 515 particles thereby minimising damage to the surfaces. Its lower shear 516 strength means that, should contact occur, seizure is less likely to 517 happen. The softer material, being the one that experiences most wear, 518 can be designed to be the cheaper and more easily replaced 519 component.

520 For slip rings 521 Gold hardness = 437.5 MPa 522 Nickel hardness = 700 MPa 523 For brushes 524 Copper hardness = 535 MPa 525 Copper Beryllium hardness = 600-700 MPa 526 Nickel-gold plating copper PCB traces: Components are normally 527 connected to microwave circuits by soldering, or by using wire bonds 528 onto the PCB trace. Other plating finishes are used, most commonly 529 gold. However, copper and gold tend to undergo solid state diffusion 530 into each other (with copper doing so at a faster rate); the process is 531 accelerated by increased temperature. Copper on a trace surface 532 oxidizes, resulting in increased contact resistance (copper migrating 533 into the gold can cause the gold to tarnish and corrode). This can be 534 minimized by plating a barrier layer between the copper and gold.

535 Nickel is commonly used as a barrier layer to prevent the gold migrating 536 into the copper on the tracks. (The nickel barrier helps to reduce both 537 the number and the effect of pores compared with plating gold directly 538 over the copper base). The nickel protective coating provides several 539 benefits. It serves as a backing to the gold for extra hardness as well as 540 providing an effective diffusion barrier layer between gold and copper.

541 The nickel/gold provides a finish that is heat and corrosion resistant, 542 environmentally stable, wire solderable and durable (the nickel under 543 plate enhances the wear characteristics of gold) albeit at a higher cost 544 than simple solder finishes. Traditionally, nickel/gold plating has been 545 applied over copper tracks used for keyboard contacts or edge fingers 546 to provide the conductive, corrosion resistant coating. This approach 547 provides benefits for soldering, but at higher frequencies the presence 548 of the nickel layer can produce additional loss due to skin effect. Most 549 current at high frequencies flows in the outside of the conductor (in a 550 very narrow skin on the conductor), which in the case of nickel plated 551 copper concentrates most of the current in the nickel.

552 Nickel is a ferromagnetic material and its magnetic permeability results 553 in a higher skin effect resistance than that of copper. In other words the 554 skin-effect resistance on the upper (nickel plated) surface of the trace 555 will be much higher than that of the (copper) core side. Additionally, 556 copper's resistivity (at room temperature) is 1.7x1 08 ohm metre, with 557 that of nickel over four times greater at 7x1 08 ohm metre.

558 Selective nickel plating: The cost of nickel plating is a reduction in 559 usable PCB trace length. The effective usable length of a P.C.B. trace 560 at high frequencies is roughly inversely proportional to the resistive 561 trace loss. At or around 1GHz nickel plating a P.C.B. microstrip trace 562 reduces the useful length of the trace by a factor of three.

564 Copper Beryllium supplied by Brush Wellman and of type 017500 or 565 ASTM B534 annealed and tempered at standard conditions and heated 566 treated at 900-925F for 2 hrs has Tensile strength of 100-1 30 ksi (155- 567 200 MPa) and Tensile Strength is 1/3 of Hardness (H = 465 -600 568 MPa), which matches with Gold plated Nickel. In addition, 017500 has 569 highest conductivity of 45-60%. The wire diameter of 0.05-0.08 inches 570 was used and the brush was made with bunch of wires.

572 Experiments were conducted on Pin-on-Disc machine on Copper- 573 beryllium metal fibre brushes in high current density sliding electrical 574 contacts with gold plated Nickel surface shoed low wear properties.

575 Brush linear wear rates of 4x10-11 m/m was achieved at sliding 576 speeds and current densities as high as 7.5 mIs and 215 A/cm2 577 (2MA/m2).

579 Table 1

580 Test conditions.

581 Variable Value 582 Sliding speed 2.5 mIs and 7 mIs 583 Nominal current density 0, 125, 175, 220 A/cm2 584 Brush normalforce 0.IN 585 Fibres per brush 1000 586 Fibre packing fraction 0.5 587 Slip-ring diameter 25mm 588 Relative humidity >90% 589 Dew point temperature 23°C 590 Slip-ring temperature 20-25°C

592 Table 2

593 Sliding Speed Current density Wear rate Friction Coefficient Contact resistance 594 (mIs) (A/crn2) (rn/rn) (mU) 596 2.5 175 4x10-11 0.29 7-11 597 7.0 220 4x10-11 0.14 5-7 599 Radius of Gold plated ring = 25mm 600 Speed at 100 rpm 601 Distance travelled in 1 year = 8.25 x 106 m 603 Wear rate (k) = 4 x 10h1 604 Load (F)=lOg 0.IN 606 W(wear)=ksF 608 W = 8.25 x 106 x 4 x 10h1 x 0.1 = 3.3pm for 1000 brushes 609 0.0033pm for I brush 611 At speed of 10000 rpm, W 0.33 pm for each brush.

613 Linear brush wear rate (brush displacement divided by sliding 614 distance), friction coefficient, and contact resistance data were 615 recorded. Table 2 summarizes measured values at each of the 616 operating conditions. Linear brush wear rates were on the order of 617 4x10-1 I mlm at high current density during 2.5 and 7 rn/s sliding 618 speed operation. Friction coefficient was sensitive to a change in sliding 619 speed, with a steady-state value of approximately 0.3 at 2.5 rn/s 620 followed by a lengthy transition to 0.14 upon changing speed to 7 rn/s.

621 Contact resistance was sensitive and varied proportional to friction 622 behaviour, though steady-state values on the order of I OmC) were 623 attained at both sliding speeds.

625 The copper-beryllium (UNS CI 7200) is demonstrated as a robust 626 material for the construction of metal fibre brushes for operation in high 627 speed, high current density sliding electrical contacts. A wear rate of 628 4*10-11 m/m was achieved for the case of a positively biased brush at 629 sliding speeds up to 7.0 mIs and average current densities as high as 630 220 A/cm2. Friction coefficients in the range of 0.14-0.29 and contact 631 resistances on the order of lOmC] were observed.