Multi-frequency eddy-current system and method
FIELD OF THE INVENTION 
     This invention relates generally to object examination and pertains more particularly to nondestructive testing by eddy current practice. 
     BACKGROUND OF THE INVENTION 
     In various known practices in nondestructive object evaluation, a standard object is passed into, through, or by a coil which is suitably excited to induce eddy currents in the standard object. The coil provides output signals indicative of the response of the standard object to the induced eddy currents. Associated detector circuitry effects display of the amplitude and phase of the coil output signals, indicative of standard object parameters such as electrical conductivity. Test objects are then passed into or through the coil and display of test object characteristics is effected to enable comparison of same with the standard object. 
     Fundamental to the described practice is a setup procedure termed &#34;balancing&#34;, i.e., adjusting the testing system such that it operates in use in a preselected subrange of its dynamic operating capability. In this practice, one adjusts the system such that none of its circuits will operate in a saturation range and thereby introduce gross nonlinearities in signal handling. In a manual setup procedure using, for example, Varimac eddy current test apparatus manufactured by Magnetic Analysis Corporation, one observes an oscilloscope display while moving an object through the test coil and manually adjusts a balance potentiometer until an in range condition is observed on the scope. Such manual procedure is cumbersome and can be time consuming. 
     While the balancing procedure contributes to system accuracy, other considerations bear on performance, one of which is the presence of regions of nominal nonlinear gain in the operative dynamic subrange of the system. Such nominal aberrations result from variations in phase angle between in-phase and quadrature channels from the perfect ninty-degree angle. In this area of what may be termed &#34;calibration&#34;, the above-referenced and other known systems either do not have the several required capabilities or implement calibration to a degree which has less than the accuracy desired in certain applications. 
     From applicant&#39;s viewpoint, the present state of the art of object examination through eddy current practices lacks method and system correlative with current microprocessor capability in the area of balancing and calibration and further in ascertaining the validity of generated data. As respects the last item, applicant notes that the art heretofore has not had a validity checking capability in accordance with what might be derived from theoretically determinable parameters, such as electrical conductivity of specimens, given their dimensional and other known character. 
     SUMMARY OF THE INVENTION 
     The present invention has as its primary object the provision of improved non-destructive object testing methods, apparatus and systems. 
     A more specific object of the invention is to provide for expeditious balancing and calibration in a multi-frequency eddy current object testing system. 
     More general objects of the invention are the provision of balance and calibration systems and software therefor and the provision of a data validity check according with theoretical parameter determination. 
     In achieving the above and other objects, the invention provides a system including oscillator means for exciting an eddy current coil at one or more frequencies, detector circuitry for processing coil output signals and balance and control calibration means connected with the oscillator and the detector circuitry for so operating upon coil output signals in the detector circuitry as to force operation of the detector circuitry into a prescribed subrange of its overall operation range, and to effect correction of above-noted nominal nonlinearity parameters bearing on system accuracy. The balancing offsets are retained in memory so that off-scale comparisons can be made by shifting balance a previously known amount. 
     The system is preferably multi-frequency in makeup, the coil being excited at plural frequencies and the detector circuitry including separate detector channels corresponding respectively with the coil excitation frequencies. The coil output signals can be monitored on a continuous basis or monitored in the detector channels for the occurrence in one or more of the channels of a preselected condition, e.g., the peaking of the coil output signal to a given magnitude above a threshold level. On this event, samples of the coil output signals are taken in each detector and the system includes means to convey the sampled signals or the continuous detector output signals to microprocessor means for particularized gathering of intelligence from the sampled signals. Such microprocessor means also may operate the system at large. 
     In realizing the list of the more general objects above noted, the invention effects balancing on a computer-assisted basis and by implementing a quite simplified balancing approach running through a two-step rough or coarse phase into a fine phase. As for its attainment of the second general object, the invention provides apparatus and method for correcting such nominal nonlinearities throughout the dynamic operating subrange of the system. With respect to the third general object, applicant provides a method for determining information providing a validity check on system generated data. 
     The foregoing and other features of the invention will be further understood from the following detailed description of the preferred embodiment thereof and from the drawings, wherein like reference numerals identify like components throughout. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the system of the invention. 
     FIG. 2 is schematic diagram of the input network of the FIG. 1 system. 
     FIGS. 3 and 3(a) are schematic diagrams of the detector-oscillators of the FIG. 1 system. 
     FIG. 4 is a block diagram of the filter units of the FIG. 1 system. 
     FIG. 5 is a schematic diagram of the flaw trap of the FIG. 1 system. 
     FIGS. 6, 6(a), 6(b) are schematic diagrams of the data collector and controller of the FIG. 1 system. 
     FIGS. 7(a) through 7(d) are drawings illustrative of the balancing practice of the invention. 
     FIG. 8 is a flow chart of a program for coarse balancing. 
     FIG. 9 is a flow chart of a program for fine balancing. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS AND PRACTICES GENERAL SYSTEM 
     Referring to FIG. 1, system 10 includes coil 12, which may be of single winding or plural winding type. In the latter type, the primary winding is excited over lines 14 and 16 and the secondary winding applies its output signals to lines 18 and 20. Where a single winding coil is used, the winding is connected across lines 18 and 20 and lines 14 and 16 are not used. 
     Either of the foregoing types of coils may be constructed, for improved measurement accuracy, in accordance with copending commonly-assigned U.S. patent application Ser. No. 631,484 of applicant herein, entitled &#34;Compensated Transducer and System&#34;, filed concurrently herewith. In this referenced application, a transducer for magnetic testing has coil structure employing a first winding of given length and a second winding which has the same length as the first winding but wherein one-half of the second winding is in reverse winding direction to the other half, both such winding parts being of equal turns. Two such coil structures are arranged in a bridge circuit, the first winding of one coil structure being series-connected with the second winding of the other structure, and vice versa. Thermal resistivity change is thus compensated for, and inductive drift may be greatly reduced by deposition of the coil structures upon a substrate selected to have a substantially lower thermal expansion coefficient than the metal of the coil structures and sufficiently greater in thickness than the metal to effectively give rise to lessened coil structure dimensional change due to temperature. 
     Input network 22 serves to coact with coil 12 and further to provide selective filtering and attenuation of coil output signals. In addition to its connection to lines 14-20, network 22 receives input signals on lines 24, 26 and 28 and 30 from detector-oscillator unit 32, which includes individual channels having detector-oscillators 34, 36, 38 and 40. The output signals of network 22 are furnished over lines 42, 44, 46 and 48 to unit 32. An input control signal for a calibration mode, discussed in detail below, is provided on line 50 to network 22. 
     Detector-oscillators 34-40 receive input control signals for the calibrate mode and for a balance mode over lines 52, 54, 56, 58 and 60. The output signals of detectors-oscillator 34-40 are applied in zero-degree or in-phase (sine) to lines 62, 64, 66 and 68 and in ninety-degree or quadrature phrase (cosine) to lines 70,72, 74 and 76. These lines provide inputs to filter 78, which receives input control signals over lines 80. 
     The output signals of filter 78 are provided on lines 82, 84, 86, 88, 90, 92, 94 and 96 to flaw trap 98, which is operative, in accordance with the states of control input lines 116, to furnish on output lines 100-114 either all or selected parts of the input signals on lines 82-96. 
     Data collector and controller unit 118 processes the signals on lines 100-114 and communicates with microprocessor MP through interface I-F. Unit 118 furnishes the various filter selection, flaw trap control and balance and calibration control signals on its output lines 119. 
     INPUT NETWORK 
     Turning to FIG. 2, input network 22 includes power driver 120, input to which is provided on lines 24-30. Output is on lines 14 and 16 where coil 12 has two windings, in which case, switches 122 and 124 are in open state. Where the system is operating with a single winding coil, switches 122 and 124 are closed and the power driver 120 output is applied to lines 18 and 20. Instrumentation amplifier 126 receives inputs on lines 128 and 130 and provides an output signal on line 132 indicative of the difference between the inputs on lines 128 and 130. Depending upon the states of switches 134, 136, 138, 140, 142, and 144, one can apply the signals on lines 18 and 20 in full or attenuated amplitude to lines 128 and 130. In full amplitude setting, only switches 136 and 138 are closed. In attenuated setting, switches 136 and 138 are open and switches 134, 140, 142, and 144 are closed. 
     The line 132 signal may be filtered or unfiltered as applied to operational amplifier 146, depending upon the states of switches 148 and 150. Band reject filter 152 is in series with switch 148. When switch 148 is closed and switch 150 is open, the line 132 signal is filtered in filter 152. When switch 150 is closed and switch 148 is open, the line 152 signal is applied unfiltered directly to amplifier 146. The output of amplifier 146 is applied to output lines of 42, 44, 46 and 48 of network 22. 
     Band reject filter 152 is of type tunable by potentiometer input over a range of twenty to twenty thousand hertz. It is also adjustable by potentimeter input to provide for equal gain above and below the band reject range. 
     Line 50 is set to ground potential in the calibrate mode. Driver 120 accordingly, is rendered inactive to allow calibration, discussed in detail below. Briefly, here, it can be said that calibration establishes the minus ninety degree resistive phase response axis for each of the separate frequencies of the respective channels and allows a direct phase response reference to be made. 
     DETECTOR-OSCILLATOR UNIT 
     Each of detectors-oscillator units 34-40 of system 10 of FIG. 1 may be realized by the circuit embodiment of FIGS. 3 and 3(a). Considering this as detector-oscillator 34, the line 42 signal from input network 22 is applied to tuned filter 158, the output of which is applied over line 160 to attenuator 162, and over line 164 to multiplexer 166. 
     Each of lines 168, preferably six in number, extends from a distinct junction of the resistive ladder in attenuator 162 to multiplexer 166. Counter 170 is cleared by control input signal on line 60-2 and toggled (advanced one step) by control input signal on line 60-1. The state of counter 170 is applied to multiplexer 166 over lines 172, 174 and 176. The line 164 and 168 signals, representing different amplitudes of the output of filter 158, are thus selectable individually through multiplexer 166 in accordance with toggling of counter 170. 
     The output of multiplexer 166 is amplified in operational amplifier 178 and filtered in tuned filter 180. Zero-degree (in-phase) content of the output of filter 180 is stored, after passage through multiplexer 181, in sample and hold circuit 182 per control input signal thereto on line 184. Ninety-degree (quadrature) content of the filter 180 output is stored in sample and hold circuit 186 per control input signal thereto on line 188. The respective in-phase and quadrature sampled contents on lines 190 and 192 are low pass filtered in filters 194 and 196, which drive unit 34 output lines 62 and 70. 
     Oscillator 198 provides a zero-degree output on line 200 and a ninety-degree output on line 202. The line 200 output is applied to line 24, which provides coil excitation through input network 22 (FIG. 2) 
     The line 200 and 202 signals are clipped by diodes 208 and 210 to furnish inputs to monostable flip-flops 212 and 214 which control the states of sampling lines 184 and 188 above discussed. Based upon the ninety degree phase difference between the line 202 and line 204 signals, the line 184 and line 186 sample control signals will occur at corresponding time difference, thus providing for sampling of the output signal of filter 180 for in-phase and quadrature content, respectively. 
     Oscillator 198 output signals are applied also over lines 216 and 218 to multipliers 220 and 222. Direct current (d.c.) voltages are applied to the multipliers over lines 224 and 226 from variable d.c. controller 228, an embodiment of which is shown in FIG. 3(a) and discussed below. In response to the states of input control lines 60-3, 60-4, 60-5 and 60-6 (up and down phase toggle signals), controller 228 furnishes the line 224 and 226 signals independently of one another. 
     The outputs of multipliers 220 and 222 are applied over lines 230 and 232 through resistors 234 and 235 to line 236. The line 236 signal accordingly effects an offset injection of d.c. voltage by a determined amount, along with in-phase and quadrature components. The line 236 signal is injected into the line 164 input to multiplexer 166. 
     Turning to FIG. 3(a), injection controller 228 includes identical channels 228-1 and 228-2, respectively for in-phase and quadrative injection of d.c. voltage. The ensuing discussion of channel 228-1 will be seen as applying also to channel 228-2. 
     Lines 60-3 and 60-4 provide in-phase up and in-phase down toggle signals to counter stage 238-1. Lines 240-1 and 242-1, connect stage 238-1 to counter stage 244-1. Lines 246-1 and 248-1 connect counter stage 244-1 to counter stage 250-1. Stage 238-1 applies its state to lines 252-1. Stage 244-1 applies its state to lines 254-1. Stage 250-1 applies its state to lines 256-1. Digital-to-analog convertor (DAC) 258-1 receives the signals on lines 252-1, 254-1 and 256-1 and generates an analog output signal over lines 260-1 and 262-1 to operational amplifier 264-1, which drives line 224. 
     In the case of channel 228-2, lines 60-5 and 60-6 provide quadrature phase up and quadrature phase down toggle signals. 
     FILTER UNIT 78 
     The in-phase and quadrature output signals for each of the four detector-oscillators, i.e., the signals respectively on lines 62 and 70, lines 64 and 72, lines 66 and 74 and lines 68 and 76 are applied to filter 78 (FIG. 4), which receives control input signals for directing filter operation on lines 80-1, 80-2 and 80-3. The filter is structured to function either as a low pass or band pass unit, responsively to the state of line 80-1. Four frequencies (fifteen, forty, one-hundred and fifty and four hundred hertz) may be selected from as either the upper cutoff frequency or the center band pass frequency, in accordance with the states of lines 80-2 and 80-3. Thus, with lines 80-2 and 80-3 both LO (at ground potential) and line 80-1 HI (at +5 volts d.c.), filter 78 is called on to function as a low pass filter with an upper cutoff frequency of fifteen hertz. With lines 80-2 and 80-3 both HI and line 80-1, LO, the filter is band pass with center frequency at four hundred hertz. Filter 78 provides its output signals, in channel pairs of in-phase and quadrature, on lines 82-96 to flaw trap 98. 
     FLAW TRAP 98 
     An embodiment of flaw trap 98 is shown in FIG. 5. Lines 82-96 from filter 78 are applied to sample and hold circuits 266-280. At least one pair of the in-phase and quadrature filter output signals is selected to be monitored, namely, the pair on lines 82 and 84, and these lines are applied to dual operational amplifier 282. The amplified signals are squared in multipliers 284 and 286. The squared signals are now applied to a high pass filter 288-290-292 and summed. Comparator 294 receives the summed signals and a threshold level from potentiometer 296. Output line 298 changes state when the summed signal amplitude exceeds the threshold level. 
     Operational amplifier 300 offsets the attenuation of the summed signal in filter 288-292 and provides a full amplitude version of the summed signal to differentiator 302 and zero-crossing detector 304. Line 306 changes state on each zero crossing and is applied with line 298 to gate 308. Gate 308 applies its output to monostable flip-flop 310 whose output is applied through gates 312-1 and 312-2, to switch 314, the state of which is controlled by line 116-1. This line is HI, when the flaw trap circuitry above discussed is to be used in providing for output information corresponding to threshold and peak levels in the line 82 and 84 signals. In the illustrated position for switch 314, line 116-1 is HI and the pulse from flip-flop 310 is conducted through switch 314 to line 316 and causes circuits 266-280 to sample and hold signals then on lines 82-96. Output signals from these circuits are applied to lines 100-114. In the alternate state of switch 314, line 316 is connected to plus 5 volts d.c. and the sample and hold circuits conduct signals continuously from input to output lines. 
     An output control signal of flaw trap 98 is provided on line 116-2, this being a trigger to activate analog-to-digital conversion in data collector and controller unit 118 (FIG. 1). Control inputs on lines 116-3 and 116-4 permit direct triggering of line 316 from software or external sources. 
     DATA COLLECTOR AND CONTROLLER 118 
     In FIGS. 6, 6(a),6(b) and 6(c), an embodiment of data collector and controller 118 is shown. Unit 318 is a Data Aquisition Unit, such as a Burr-Brown SDM856JG. In function, unit 318 multiplexes and digitizes analog input signals, i.e., the signals applied on lines 100-114 upon control input from line 116-1 of flaw trap 98, above discussed. Control input signals on lines 320, 322 and 324 are DS (Data Strobe), BUSY and BUSY ENABLE signals. These signals and other control input signals on line 326 (BUSY) and on lines 328, 330 and 332 are generated by circuitry of FIG. 6(a) discussed below. The digital output of unit 318 is furnished on lines 334 and 336 to FIFO (first in, first out) memories 338 and 340. 
     The bits on lines 334 and 336 are fifteen individual bits, twelve data bits and three channel address bits. Nine lines, 334, apply nine bits to memory 338 and six lines 336 convey six bits to memory 340. As successive fifteen-bit words issue from acquisition unit 318 to the FIFO units, each prior word is shifted through and ultimately arrives at the output stages of the FIFO units. Here, portions of the partial word in FIFO 338 are applied to multiplexers 344 and 348 over lines 342 and 346, respectively. Likewise, portions of the partial word in FIFO 340 are applied respectively over lines 350 and 352 to multiplexers 344 and 348. One state of line 353 gates lines 342 and 346 to lines 354 and 356 and hence to universal asynchronous receiver/transmitter 358, which may be a UAR/T A4-3-1015D. The other state of line 353 gates lines 350 and 352 to lines 354 and 356 and thus to UAR/T 358. 
     UAR/T 358 is operative to transmit or receive binary data with appended control and error detecting bits. Typically, manual switches (not shown) are connected to ports of the UAR/T and are set as desired to select number of stop bits, baud rate, bits per word, parity mode, etc. 
     To accommodate the particulars of the system under discussion, latches 360 and 362 receive inputs over lines 364 and 366 from UAR/T 358. The latch outputs, the meaning and use of which is covered hereinafter, are on lines 370 (channel C address), 372 (channel B address), 374 (channel A address), 376 (toggle down on auto balance), 378 (toggle up on auto balance), 380 (toggle up attenuator), 382 (clear attenuator), 384 (calibrate toggle), 386 (flaw trap enable), 388 (filter select A2), 390 (filter select A1), and 392 (filter mode select). The states of lines 370-392 are desirably set by suitable software control as discussed below. 
     UAR/T 358 is operative to apply output data serially to line 394 or to receive input data serially on line 396 in accordance with the state of line 398. In control of the state of line 398, a communication modem, (interface I-F in FIG. 1), such as the customary RS-232 unit, informs as to request to send or to receive by input to gate 400. Gate 402 receives the line 404 output of gate 400 and a second input from line 406. Line 406 is controlled by gate 408, the inputs to which are furnished by FIFO units 338 and 340 over lines 410 and 412. In operation, when both FIFO units have a word resident in their output stages and when there is a modem request concurrent therewith, gate 402 places its output line 414 in first logic state and such state is furnished as one input to gate 416. 
     UAR/T issues a readiness strobe over line 418 to monostable flip-flop 420, the output of which is applied over line 422 to gate 424. Gate 424 has a second input on line 426, which is the gate 408 output (FIFO units ready for output). The gate 424 output on line 428 is applied to gate 416. When both inputs, on lines 414 and 428, are in selected common state, gate 416 pulses output line 430 and monostable flip-flop 432 strobes line 434, rendering UAR/T 358 operative to send data on line 394. 
     The state of line 353 is required to change state during each full word transmission by UAR/T 358 to effect the multiplexing of signals on lines 342, 346, 350, and 352, above discussed. Counter 436 and monostable flip-flop 438 so control line 353, as follows. Each output pulse of monostable flip-flop 420 is applied over line 440 and stops counter 436. The first counter stage output is applied to line 353. The second counter stage output is applied over line 442 to monostable flip-flop 438 to pulse its output line 444 and reset counter 436 to zero. Counter 436 accordingly changes the state of line 353 once per each two strobes on line 418. 
     Referring now to FIG. 6(a), the generation of signals on control lines 326, 328, 330 and 332 of FIG. 6 to data acquisition unit 318 will now be discussed. Counter 446 is of four-stage type, the first, second, third and fourth stage states being present on lines 448, 450, 452 and 454, which are separate inputs to switch 456. The output of switch 456 is applied over line 458 to monostable flip-flop 460. Output line 462 of flip-flop 460 is connected to the clearing input of counter 446. Line 332-1, which is the BUSY output line of acquisition unit 318, drives the input of the counter first stage. 
     If switch 456 is set to line 448, the first pulse on line 332-1 will change the state of line 448, will trigger flip-flop 460 and reset the counter to zero. The channel address lines 326, 328 and 330 will not experience a change of state, and acquisition unit 318 will be informed of first channel selection (the channel having oscillator-detector 34 of FIG. 1). Conversely, if switch S1 is set to line 454, line 458 will not be pulsed until state change of the fourth counter stage. At that time, all of address lines will have changed state, and the channel address indication will be the fourth channel, (the channel having oscillator-detector 40 of FIG. 1). 
     In a flaw trap enable mode, acquisition unit 318 will have processed through to FIFO units 338 and 340, under line 454 setting of switch 456, sampled information at in-phase and quadrature of all four diverse frequency channels, i.e., eight distinct pieces of information. 
     The line 332-1 BUSY signal is applied also over line 464 to monostable flip-flop 466 whose output pulse is conveyed over line 468 to FIFO units 338 and 340 to shift into same the next word available from data acquisition unit 318. 
     As monostable flip-flop 460 clears counter 446, it also clears flip-flop 470 over line 472. Line 332-2 (BUSY ENABLE) changes state to advise data acquisition unit 318 of completion of the cycle. Flip-flop 470 is preset at the outset of each cycle by an external or internal trigger on lines 474 and 476, in which cases gate 478 pulses monostable flip-flop 480 which is connected to the preset input of flip-flop 470 by line 482. The pulse on line 482 is applied over line 484 to gate 486, to which is also applied the BUSY signal on line 332-3. Gate 486 controls the state of line 332-4 (DS). 
     FIG. 6(b) depicts the attenuation control portion of data collector and controller unit 118. Decoder 488 receives addresses C and B on input lines 370 and 372. Output lines 506, 508, 510 and 512 provide inputs to gating unit 514, the further input to which is the attenuation toggle up signal on line 380. One of the four output lines of gating unit 514 will have a state change, in accordance with the channel being set for attenuation. The uppermost output line is noted as 60-1, corresponding to the same line in FIG. 3 i.e., the toggle attenuator line for the channel having detector-oscillator 34. 
     Output lines 506, 508, 510 and 512 also provide inputs to gating unit 516, the further input to which is the clear attenuator signal on line 382. One of the four output lines of gating unit 516 will have a state change, in accordance with the channel being set for attenuation. The uppermost output line is noted as 60-2, corresponding to the same line in FIG. 3, i.e., the clear attenuator line for the channel having oscillator-detector 34. 
     FIG. 6(c) depicts the balance control portion of data collector and controller unit 118. Decoder 518 has inputs from lines 370, 372, and 374, i.e., addresses A, B and C. Output lines 526, 528, 530 and 532 of decoder 518 extend to both of gating units 534 and 536. These gating units respectively receive control inputs of in-phase toggle up and toggle down on lines 376-1 and 378-1. The outputs of gating 534 and 536 are the in-phase control signals for the four channels, the upper output lines being lines 60-3 and 60-4, corresponding to these same lines in FIGS. 3 and 3(a). 
     Turning to the lower half of the balance control portion, which generates the quadrature toggle up and toggle down signals, lines 538, 540, 542 and 544 extend to gating units 546 and 548. These gating units respectively receive control inputs of quadrature toggle up and a toggle down on lines 378-2 and 376-2. The output lines are the quadrature control signals for the four channels, the upper output lines being lines 60-5 and 60-6 of FIGS. 3 and 3(a). 
     BALANCE MODE 
     The first (MF) of the two programs listed at the end of this text includes subsections for what are termed &#34;rough&#34; balance, &#34;single-sided rough&#34; balance and &#34;fine&#34; balance. An understanding of same, and of applicant&#39;s method embodied therein, will be facilitated by reference to FIGS. 7(a), 7(b), 7(c), 7(d), 8 and 9. FIG. 7(a) is a plot of the operating range of the system. The vertical and horizontal extents of regions 0 from origin OR defines the desired dynamic subrange in which the system should be operated and may be selected to be a given number of units, i.e., a given voltage level VL. As will be seen from FIG. 7(a), and the enlarged view of the dynamic subrange in FIG. 7(b) in areas labelled 1, one of the horizontal and vertical components would be in-range , i.e., less than level VL. In areas labelled 2, neither component would be in-range. 
     Considering the typical balance problem of FIG. 7(c), the starting point SP is in an area 2 and both components are out-of-range. Per the subject method, one proceeds initially along a path P1 to intersect the in-range area of one of the components. In upward movement from SP, one goes off screen without having entered area 1. However, upon continuing movement along P1 upon reentering the screen, one component becomes in-range. Travel is now along a second path, P2, until the second component becomes in-range. This is seen in FIG. 7(c) as the end of path P2 is in the lower right-hand quadrant of the dynamic subrange. This practice is termed &#34;rough&#34; balance, the flow chart of which is shown in FIG. 8. From this juncture, one proceeds in the single-sided rough balance and then to &#34;fine&#34; balance practice, shown in FIG. 7(d), the flow chart of which is shown in FIG. 9. 
     Refering to FIG. 8, and to program MF below, program step 530 calls out a key entry for balance (BAL) and an advance to step 2000. At step 2070 (550 of FIG. 8), inquiry is made as to whether either or both of the absolute values of B8 (in-phase or zero degree data) or B9 (quadrature or ninety-degree data) are less than nine hundred units (VL), i.e., whether rough balance exists. If yes for either, the program (552 of FIG. 8) advances to single-sided rough balance and then to fine balance. If no, step 2080 brings on the rough balance subsection starting at step 4000. 
     Step 4006 asks whether one of the data is in range (554). If no, M1 is set to one hundred and twenty-eight times the 1/ (attenuator setting+1) (556) and an up zero increment occurs (558). In-range is checked again (560) and if no and if counter M1 is not exceeded in count (562), another up zero increment occurs (558). If in-range is not reached at full count, the counter is reset (564) and a ninety degree increment is made (566). In-range and count are checked through further ninety degree incrementing (568, 570). In the event that an in-range condition does not occur, output indication is provided (572). 
     If any one of the in-range inquiries is answered yes, (554, 560, 568), the practice proceeds to the inquiry of whether both data are in-range (574). The MF program steps are 4010-4200. Step 4210 (576) will proceed to fine balance if both data are in-range. If no, the one data in-range is determined (578), the counter is set (580) and both zero and ninety degree are incremented (582). Counter settings are determined which keep both data in-range during incrementing (584), are set (586) and scale if determined (588) and the test is executed (590). Balance (both data in-range) is now tested (592). If yes as to balance, then or after reversal (594), the practice proceeds to fine balance (596). The corresponding program steps are 4220-4610. 
     On return, the program is at MF step 2090 and fine balance proceeds, if a recheck of rough balance (598) does not call for a repeat (600) of rough balance. Current attenuator setting is recorded, and the attenuator is set, read and reset (602). Count is now set (604), an up zero increment is made (606) and a reading is made (608) In-range inquiry is made (610). If no, a reverse, twice-count movement, down zero increment (612) is made. In-range is checked again (614). If no, rough balance is repeated (618). If yes, an up ninety increment is made (616) and a reading is made (620). In-range inquiry is made (622). If no, a reverse, twice count movement (down) ninety increment, 624 is made. In-range is checked and, if no, rough balance is repeated. If yes, scale is determined for zero and ninety (628) and executed (630). Balance is checked (632) and if needed, a reverse is made (634), thus reaching the end (636) of the routine. 
     THE MF PROGRAM 
     The MF program has various capabilities beyond the balance subroutines above discussed. In the order of their occurrence, these subroutines are as follows. The key entry of step 500 calls for the subroutine extending through steps 800-830, which permits module (channel) selection to establish conditions therein. The key entry of step 510 calls for the subroutine extending from step 7500 to 7550, which steps the attenuator of the current channel. The key entry of step 520 calls for the subroutine of steps 7400-7440, which clears the attenuator of the current channel. The key entry of step 530 calls up the balance subroutine, covered above, fine balance in steps 2000-2880 and rough in steps 4000-4630. The key entry of step 540 calls up the subroutine of steps 1000-1180 which provides for display of system settings of filters, attenuators and current data generated by the system. The key entry of step 554 calls up the filter subroutine of steps 650-780, which permits selection of filter mode and frequency and closes with the display subroutine. 
     Steps 900-950 provide for display of the current channel settings and data. Steps 7000-7050 effect an up zero phase step. Steps 7100-7150 effect a down zero phase step. Steps 7200-7250 perform an up ninety degree step and steps 7300-7350 a down ninety degree step. Steps 8000-8300 read system data. Steps 8500-8610 read system data for one channel. Steps 9000-9090 set up the RS-232 interface for an eight bit word, two stop bits, ninety six hundred baud and enable receiver and transmitter. Steps 9800-9850 comprise an error trap subroutine. Steps 7600-7630 and 8700-8920 set forth the CAL or calibrate subroutine of MF, this practice being next discussed in connection with the calibrate mode. 
     CALIBRATE MODE 
     In its calibrate mode, the system is operated with a given injection voltage applied to lines 18 and 20 of FIG. 1. This voltage is trimmed by toggle commands to a voltage of plus or minus one multivolt for a full scale signal change at the lowest setting of the attenuator 162 of FIG. 3. The gain of filter 180 of FIG. 3 is trimmed to provide equal values on the in-phase and quadrature output lines 62 and 70 (FIG. 3). 
     The quadrature error is now obtained by determining the angle between the signals on lines 62 and 70. The calibrate circuitry of FIG. 3 includes multiplexer 181, above noted, and counter 640. Counter 640 is stepped by the calibrate toggle signal applied thereto over line 642 and is cleared by a clear signal applied thereto over line 644. Four inputs are applied to multiplexer 181, sine and cosine from oscillator 198 over lines 646 and 648, ground over line 650 and a signal over line 652. The state of counter 640 is conveyed to multiplexer 181 over lines 654, 656 and 658. As the counter is stepped from a cleared condition, its state changes and multiplexer is stepped through its several inputs to provide measured parameters for use in implementing calibration through the following. ##EQU1## Data is Z 0 , Z 90 . The following steps apply. 
     (1) (Z 0 , Z 90 )=) (Z 0 ,Z 90  * C) (raw data) 
     (2) (Z 0 , 0) M 1  =) (Z&#39; 0  Z&#39; 90 ) (transforms X; matrix multiplication) 
     (3) (0, Z 90  * C) M 2  =) (Z&#34; 0  Z&#34; 90 ) (transforms Y; matrix multiplication) 
     (4) (Z 0   C , Z 90   C )=(Z&#39; 0  +Z&#34; 0 , Z&#39; 90  +Z&#34; 90 ) (corrected data) 
     Matrix multiplications are as follows: ##STR1## Signals indicative of these correction factors are suitably stored for use by the microprocessor which receives data from the system. 
     SC PROGRAM 
     The program SC, second listed below, was developed by applicant as an exact solution of the Maxwell field equations for a one-dimensional cylindrical geometry. The program is driven by operator prompts to provide data about the cylindrical system disposed within the field of a solenoidal eddy current encircling coil or external to a probe coil. The program generates the response of the eddy current system in a phase and amplitude format that is characteristic of the coil size, frequency, sample size and structure, and electrical conductivity centered within the coil. Units used are MKS units, except for magnetic permeability which is in relative terms with vacuum being unity. Of its many uses, SC allows a check to be performed as whether system data is valid and whether the selected frequencies of test are the optimum frequencies for inspection purposes. 
     Referring to the SC program below, steps 180-230 thereof permit an operator to select a measurement frequency, to enter the coil radius and to identify the number of layers in the test object. Step 330 permits the operator to set up the boundary layer problem as to the extent of the Bessel expansion. Layer radius, electrical conductivity and relative permeability are entered respectively in steps 370, 390 and 410. Steps 430-1641 are computational, solving the Maxwell field equations for the one-dimensional geometry. Readout of calculated object parameters and modification of input setup conditions are provided by the remaining steps of the SC program. 
     The SC program will be seen to have a wide variety of applications. In its primary use in connection with the subject invention, the SC program provides a basis for validation of data provided by the system of the invention and to assist in selection of operating frequencies to optimize the accuracy of system output data to measure electrical conductivity, magnetic permeability, specimen dimensions and flaw identification. ##SPC1## 
     SYSTEM SUMMARY 
     In one of its system aspects, the invention looks to expedited balancing of an eddy-current object examination, i.e., a system having drive means for generating an object excitation signal of preselected frequency, a coil for applying the excitation signal to the object and generating an output signal indicative of interaction of the signal with the object, and detection circuitry for generating output signals separately indicative of different phase components of the coil output signal. To this end, the invention provides means connected to the detection circuitry for injecting first and second phase component correction signals into the detection circuitry to conform operation of same to within a preselected operating subrange thereof. Further means are introduced into the detection circuitry for combining the injection signals with the coil output signals. 
     In its specifically depicted and described embodiment, the injection means includes a microcomputer implementing the balance routines of the MF program, the microcomputer having inputs comprising the detection circuitry output signals and applying up and down outputs controlling unit 228 of FIG. 3. Fundamental to this approach is that there occurs an initial activity wherein injection takes place successively by input to one of the two channels, to get one parameter in desired range. When one channel parameter is in range, injection then takes place jointly in both channels. 
     In another system aspect, the invention provides calibration in apparatus of the type discussed above. Here, means are provided for connection to the detection circuitry for introducing linearity compensation, in effect signals for compensating for non-linearities which would otherwise be present at various locations in the dynamic operating subrange of the system, as above discussed in the Calibrate Mode. Here, multiplexer 181 cycles through its signal, ground, sine and cosine inputs to provide raw data for the calibration calculations which provide correction factors applicable to running measured data to compensate for nonlinearity in the operating subrange. This practice is implemented by the calibrate subroutine of the MF program. 
     Method aspects attend both of the discussed system aspects and further method of the invention is made available by the procedure for validating sensed data by the practice of the SC program. Here, one identifies preselected externally determinable characteristics of the coil and the object, e.g., coil size, object radius, etc., and generates electrical signals indicative thereof. This is done by key entries in the SC program as called for by operator prompts. Validation signals indicative of a latent characteristic of the object are now generated by the SC program, independently of the coil excitation and output signals and are available for comparison with the system output signals. 
     Various changes to the system described and variations in the methods discussed can be made without departing from the invention. The particularly illustrated and explained preferred embodiments and practices are thus intended in an illustrative and not in a limiting sense. The true spirit and scope of the invention are set forth in the following claims.