Threshold compensating detector for magnetostrictive transducer

A magnetostrictive linear position transducer including a position indicating magnet. Magnet position information is fed back to the detector to automatically control the detector threshold as a function of magnet position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to magnetostrictive transducers or gauges. 
The magnetostrictive position transducer gauge described herein, 
incorporates a novel signal reception subsystem, which improves noise 
performance. This permits the construction of longer gauges, or gauges for 
use in noisier environments. 
The inventive structure includes a threshold detector system which 
automatically compensates for the position of a location indicating 
magnet. 
The invention is disclosed in the context of a "4-20 ma transmitter 
standard", magnetostrictive linear position transducer. 
2. Description of the Prior Art 
In general, magnetostrictive position sensors incorporate a ferromagnetic 
delay line, or "waveguide". A pulse generator supplies a current pulse to 
the delay line which generates a magnetic field which surrounds the delay 
line. A remote and movable, position indicating magnet is positioned along 
the delay line. The magnetic field of the position magnet disturbs the 
magnetic field generated by the current pulse. 
The interaction between the permanent magnetic field of the position magnet 
and the magnetic field induced by the current pulse causes a strain or 
mechanical reaction within the delay line. This strain induced reaction 
force within the delay line, is propagated along the length of the delay 
line as a delayed acoustic torsional wave. 
A device, called a mode converter, is typically attached to one end of the 
waveguide. This element responds to the passage of the torsional acoustic 
wave and converts it into a representative electrical signal. 
The time delay period from the excitation of the waveguide to the reception 
of the corresponding acoustic wave at the mode converter indicates the 
position or location of the position magnet along the length of the delay 
line. 
A variety of time measurement, or intervalometer techniques have been used 
to convert the time period information into a position indicating signal. 
For example, U.S. Pat. No. 3,898,555 to J. Tellerman, uses a fixed 
frequency oscillator to excite the delay line. The returned acoustic 
signal, in conjunction with the fixed frequency oscillator, develops a 
signal which is "pulse width modulated" by the position of the magnet 
along the delay line. An integrator converts the pulse width modulated 
waveform to a dc voltage level which forms the transducer output. 
U.S. Pat. No. 4,721,902 to J. Tellerman et al. teaches inter alia, a method 
to convert the "pulse width modulated signal" into a digital value. The 
patent teaches the use of a conversion counter to collect "counts" from a 
conversion oscillator during the "on" time of the pulse width modulated 
signal. 
This patent also teaches a method to enhance the detection of the delayed 
acoustic signal through the use of a time domain filter which sets the 
duration of an inhibit timer based upon the historical output of the 
transducer. This time domain filtering technique eliminates the 
contribution of noise to the output signal, however it limits the rate at 
which the position indicating magnet can move along the gauge. 
Magnetostrictive position sensor devices of this type are used in the 
measurement and control industry. They find use in machine tools; in 
robotics; as liquid level indicators, as well as other applications. 
To facilitate the use of various types of transducers, produced by a 
variety of manufacturers, industry has adopted a current mode transducer 
standard referred to as the "4-20 milliamp transmitter" standard. Under 
this standard, transducers are supplied as a two terminal device. In use, 
the two terminal transducer device is coupled to a power supply (24 volts, 
D.C.) and the amount of current drawn by the transducer from the power 
supply indicates the measured value of the transduced signal. For example, 
a pressure sensor may draw 4 ma of current from the remote power supply at 
the minimum pressure, and 20 ma at the maximum pressure, while 
intermediate pressures would correspond to intermediate current draws. 
The magnetostrictive measurement technique requires the reliable detection 
of the delayed acoustic pulse. These acoustic pulses are attenuated during 
the course of transmission in the waveguide. In general, the amplitude of 
the acoustic pulses are the greatest when position indicating magnet is 
closest to the mode converter; the acoustic pulses are faintest when the 
magnet is remote from the mode converter. 
In the prior art, the maximum length of a magnetostrictive gauge was 
limited by the detectabilty, in the presence of noise, of the delayed 
acoustic pulse. 
SUMMARY OF THE INVENTION 
In contrast to prior art magnetostrictive measurement systems, the present 
invention includes an automatic threshold circuit which sets the detection 
threshold for the delayed acoustic pulse as a function of location of the 
position indicating magnet. 
An understanding of the invention and the best mode for practicing it, 
requires some familiarity with the overall architecture of the 
illustrative transducer described herein. 
The elements of the transducer, include a low frequency, sampling clock 
which excites the delay line at a relatively low, fixed rate. This pulse 
generator circuitry generates high current interrogation pulses, but has a 
low average current draw. 
The elements also include, a time measurement system which converts the 
acoustic delay time to a digital value through the operation of a 
conversion clock and a conversion counter. A fixed time interval blanking 
timer initiates the time interval measurement. The reception of a delayed 
acoustic pulse aids the time interval measurement. The digital value of 
the measurement is then reconverted to an analog signal. 
The elements also include circuitry to convert the averaged measurement to 
a current draw for the transducer which encodes the position signal onto 
the power supply leads. 
The elements also include a resolution enhancement system where the 
resolution of time measurement value is enhanced by averaging the analog 
signal over a time interval which reflects several position measurements. 
The enhanced analog position signal as developed by the low pass averaging 
filter corresponds to the location of the position magnet. This signal is 
used to control the magnitude of mode converter output required to trip a 
comparator which indicates the reception of a returned acoustic pulse. In 
this fashion, the position of the magnet, is used to set the detection 
threshold of the system. Automatic feedback control of the threshold 
requirements of a returned signal compensate for the position of the 
magnet and improve detectability of the acoustic pulses on the waveguide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the following description, reference is made to an illustrative 
embodiment for carrying out the invention. It is understood that other 
embodiments may be utilized without departing from the scope of the 
invention. 
OVERVIEW 
As shown in FIG. 1 the magnetostrictive transducer 10 is connected to a 
remote power supply 12 through a two conductor current loop 14. The 
position of the magnet 16 along the gauge length of the delay line 18 is 
reflected by the current which the transducer 10 draws from the power 
supply 12. In general, the magnitude of this draw will vary between 4 and 
20 milliamps depending on the position of the moveable position indicating 
magnet 16. 
The delay line may comprise a ferromagnetic tube 18 with a coaxial return 
conductor as shown, or the delay line may be a solid ferromagnetic rod, 
either round or rectangular in cross-section with a parallel return 
conductor. 
The electronic module 24 contains the control logic and signal processing 
circuitry. A low frequency sampling clock is provided which triggers a 
short 1 microsecond, 20 volt pulse which is supplied to the delay line. 
The induced magnetic field associated with this pulse interacts with the 
magnetic field of the position magnet 16 which results in a mechanical 
reaction within the tube 18 at the location of the magnet 16. This 
interaction generates an acoustic pulse which propagates along the tube 
and is detected by a mole converter 22. 
The mode converter may take many forms, however a common configuration 
involves a tape armature coupled to the periphery of tube 18, which moves 
within a coil of wire when the sonic pulse passes the location of the 
converter. The translational motion imparted to the armature by impulse 
rotation of the tube gives rise to an electrical signal within the coil 
which is supplied to the electronic module 24. 
Since the speed of propagation in the tube is fixed, one can determine the 
location of the acoustic wave source by measuring the time required to 
receive the delayed acoustic pulse. In operation, the time interval 
between the excitation pulse, initiated by the sampling clock, and the 
reception of the delayed acoustic pulse returned from the permanent 
magnet, indicates the distance from the mode converter 22 to the magnet 
16. 
This propagation time interval is measured, averaged and converted into a 
current for transmission along the power supply lines. 
The digital time measurement process is accomplished by accumulating counts 
in a conversion counter during a time period or conversion window defined 
by control logic. 
The control logic includes a "blanking" timer converter 22, which occurs as 
a direct result of excitation which operates to exclude the spurious 
output of the mode the delay line 18. In the preferred and illustrative 
circuit shown, the conclusion of the blanking interval, defines the 
starting point of the conversion window. 
As previously mentioned, the delayed signal, returned by the waveguide is 
used to end the time interval measurement process. This returned acoustic 
signal has an amplitude dependant upon the distance that the pulse has 
travelled in the waveguide. 
In the present invention, the "effective gain" of the acoustic pulse 
reception circuitry is varied as a function of the position of the magnet 
on the gauge, since the amplitude of the acoustic pulse is dependent on 
the position of the magnet. This result is achieved by altering the amount 
of mode converter signal required to trip a comparator which generates the 
signal which corresponds to the detection of an acoustic pulse. In this 
fashion, the location of the magnet along the length of the gauge sets the 
threshold for detection of the delayed acoustic pulse. 
SYSTEM TIMING 
FIG. 2 is a timing diagram depicting the relationship between control logic 
waveforms and signal conversion waveforms generated during the course of a 
position measurement. 
The waveform 26 reflects the output of the low frequency sampling clock as 
taken from the output pin 60 of the comparator forming a portion of the 
sampling clock 52. In a preferred embodiment, position measurements are 
taken at a rate of 32 samples per second. The rising edge 28 of the 
sampling clock defines time, t0 and initiates the delivery of an 
excitation pulse to the delay line waveguide 18 as indicated by pulse 30 
on waveform 32. 
Waveform 32 is taken from the output pin of the pulse forming one shot 54. 
This signal is the triggering pulse for the power amplifier 56. 
The waveform 40 presents the output of the mode converter 22. In general, 
the mode converter output is amplified by a high gain amplifier 60. The 
amplified output, is compared to a threshold value in a comparator 62. The 
logic level output of comparator 62 is depicted on waveform 43. 
Waveform 40 corresponds to the mode converter 22 output. Signal complex 38 
and signal complex 39, are generated by the delivery of the excitation or 
interrogation pulses 30 and 31. These events are excluded by the blanking 
timer 58. Coincident with the generation of the excitation pulse is the 
initiation of a blanking pulse to exclude the mode convverter output pulse 
which results directly from the excitation of the delay line. This 
blanking pulse 34 shown on waveform 36 is fixed and extends for 
approximately 20 microseconds from t0 to t2 on FIG. 2. This blanking pulse 
34 is used to blank out mode converter pulse 38 shown on waveform 40, by 
gating out (through NOR 171) the mode converter comparator output. Also, 
the blanking time resets the conversion counter 66 after the pulse 38 has 
occurred, enabling the counting process, at the conclusion of the blanking 
time. 
In this fashion, the system is configured to receive the delayed, position 
indicating acoustic pulse 42, shown on waveform 40. The duration of the 
measured time period (t2 to t3) corresponds to the travel time for the 
acoustic pulse along the wave guide 18. During this time period (t2 to 
t3), a high frequency conversion oscillator 64 supplies "counts" to the 
conversion counter 66. The counts collected are shown as 44 on wave form 
46. This time interval is referred to as the conversion window. 
The end of the conversion window corresponds to the latch output pulse to 
the digital to analog converter 68. 
It should be appreciated that the conversion clock toggles at a relatively 
high 1.8MHz rate and draws substantial current from the supply. It is 
preferred to turn the clock off with the acoustic return pulse 42 which 
activates the digital to analog conversion latch as shown at 47 on 
waveform 45. It is preferred to turn the conversion clock on with the RC 
timer formed by resistor 71 and capacitor 72. The time constant of this 
network is short enough to permit conversion clock turn on well in advance 
of the measurement cycle insuring frequency stability. Operating the 
oscillator in this fashion eliminates the need for a separate high current 
draw gate to control the oscillator. 
FIG. 3 is a block diagram partitioning the electrical schematic to 
facilitate discussion of the functional relationships between elements and 
structures of the magnetostrictive transducer system. 
The current loop 14 is interfaced with the transducer system 10 through 
protection circuitry 48. This circuitry serves to shunt over voltages and 
to protect against over current conditions due to polarity reversal. A 
current source and several voltage regulators 50 supply the operating 
voltages to the remaining circuitry. 
The sampling clock 52 output serves to initiate the excitation pulse 
delivered to the waveguide 18 through the power amplifier 56 and 
triggering one shot 54. The sampling clock 52 also initiates the blanking 
one shot 58. 
The acoustic pulse transduced by the mode converter 22 is amplified in 
amplifier 60. After suitable amplification the amplitude of the returned 
signal is compared with a threshold value in comparator 62. 
In general, the output of the mode converter is amplified by a high gain 
amplifier stage 60 which generates an output signal which varies in 
amplitude (in the absence of noise) as the magnet is moved along the 
gauge. The output of the is amplifier is AC coupled to the comparator. If 
the AC excursion of the mode converter signal exceeds the threshold of the 
comparator, a logic detect signal is generated. This signal indicates the 
reception of a returned pulse, and turns off the conversion oscillator 64, 
ending the digital time conversion process. 
Once the digital value of the time interval is latched in the conversion 
counter 66, the digital value is converted to an analog voltage level in 
digital to analog converter 68. Several sequential time interval 
measurements are converted to an average current value through the 
operation of the filter and the voltage to current converter 70. The 
output of the voltage to current conversion is used to modulate the 
current drawn by the transducer system 10 from the supply 14 through the 
current sense resistor 49. 
FIG. 4 is a schematic diagram of an illustrative circuit for carrying out 
the invention. Component types and values are set forth as follows: 
______________________________________ 
RESISTORS CAITORS 
______________________________________ 
20.OMEGA. 
49,134 47 pf 72 
1M.OMEGA. 
71 10 .mu.f 
82,83,153 
2K.OMEGA. 
84 560 pf 135 
2.2.OMEGA. 
86,87,104,105,106 
1 .mu.f 136,139,140,152 
150k.OMEGA. 
90 100 pf 137,144,147,151 
10M.OMEGA. 
91 .01 .mu.f 
138 
10.OMEGA. 
92,108,116,129,132,133 
0.1 .mu.f 
141,142,143, 
100k.OMEGA. 
93,99,101,110 145,146,148, 
10M.OMEGA. 
94 150,156 
80.6K.OMEGA. 
95 .033 .mu.f 
154 
121k.OMEGA. 
102 3.3 .mu.f 
155 
732.OMEGA. 
96 
31.6k.OMEGA. 
97 
470k.OMEGA. 
100,111 
162k.OMEGA. 
98 
15k.OMEGA. 
103 
1k.OMEGA. 
107,113,114 
10k.OMEGA. 
117,123,126,131 
49.9k.OMEGA. 
112,120,121,122,128 
24.9k.OMEGA. 
115,125 
787.OMEGA. 
118 
60.4k.OMEGA. 
119 
100k.OMEGA. 
127 
4.99k.OMEGA. 
124,130 
______________________________________ 
Turning to FIG. 4A, the transducer circuitry is connected to the remote 
power supply through terminals 61 and 63. Protection circuitry includes 
diode 157 and the spark gap 177. The capacitor 142 prevents noise from 
entering the system. The current source 176 and the voltage regulators 
159,160,161, and 162 form the internal power supply for the remaining 
circuitry. 
The sampling clock 52, on FIG. 4b may be implemented with an RC oscillator 
formed about comparator 80. The sampling clock 52 triggers the edge 
triggered one shot 54. The oneshot generates the narrow trigger pulse used 
to excite the waveguide or delay line 18. It is preferred to use a step up 
transformer 81 in conjunction with capacitor discharge circuitry to 
generate the relatively high energy pulse required to excite the delay 
line. In operation, the capacitors 82 and 83 are charged slowly through 
resistor 84, to minimize instantaneous current draw. Upon receipt of a 
triggering pulse from the one shot 54, the FET 169 conducts the charge to 
common energizing the primary winding of transformer 81. Pulse shaping 
circuitry including resistors 105 and 106 cooperate with the Schottky 
diode 167 to form a rapid rise time pulse to drive the magnetostrictive 
delay line 18. 
The sampling clock 52 also initiates the blanking circuitry implemented as 
a 20 microsecond oneshot 58. The blanking circuit has two related 
functions. The output from the one shot, operates through logic gate 171 
to blank out the output while the caparator comparator out oneshot is 
high, thus preventing false detection of the comparator output resulting 
directly from excitation of the waveguide 18. Another function of the 
oneshot 58 is to initiate the count conversion process by removing the 
reset on the counter to permit counting, as depicted in FIG. 2 on waveform 
46. 
On FIG. 4A, the mode converter coils 22 are coupled to the input of a high 
gain amplifier shown on the figure as 60 and associated components. The 
gain of this amplification stage generates an approximately 75 millivolt 
signal. The amplifier output is compared with a reference voltage in 
comparator 62. For a typical gauge it is preferred to allow the comparator 
to toggle on signals, which exceed a fixed 50 millivolts amplitude 
threshold set by the resistive voltage divider formed by resistor 115 and 
114. 
In operation, the amplitude of the AC component of the amplified mode 
converter signal is supplied to the signal input of the comparator 62. If 
the magnitude of this applied signal exceeds the reference level set by 
the resistive divider then the comparator will toggle generating a logic 
level output indicating the reception of an acoustic pulse. 
However, when the switch 178 is closed an additional controlled reference 
DC voltage is summed at the signal input of the comparator. In this 
instance, the AC component required to trip the comparator is reduced by 
the amount of DC bias supplied through the switch 178 to the signal node 
of the comparator. 
As described in more detail elsewhere, the voltage at the switch 178 
represents the time averaged position measurement developed from the 
output of the filter 70. Consequently the value of DC supplied to the node 
of the comparator reflects the measured position of the magnet. In this 
preferred mode the voltage at the switch is time averaged, however the 
principle motivation for the time averaging is resolution enhancement and 
the use of an instantaneous value for magnet position is both operable and 
desirable in some applications. 
The logic level output of the comparator 62 turns off the conversion clock 
64 formed by the NOR gate 89 and associated crystal oscillator components. 
Once valid counts are collected by the counter 66, the digital to analog 
converter 68 converts the number to a and used to current corresponding 
analog value. This analog valve is averaged, system 10. The resolution of 
the transducer is enhanced by averaging many analog voltage readings. In 
general, a digital counter cannot resolve beyond the least significant 
digit because of the quantizing error of plus or minus one bit. 
Consequently, the resolution of a twelve bit counter is normally limited 
to 1 out of 4096 bits or 0.024%. However, if the quantizing error is 
random then a large number of sequential measurements would statistically 
favor one bit state for the least significant bit of the counter. 
Therefore by averaging the analog output of the digital to analog 
converter the resolution is improved beyond the normal capacity of the 
counter. In the preferred embodiment a sampling rate of 32 Hertz is 
preferred and a one half second time constant, low pass, three pole, 
active filter 70 is provided to average the output voltage over sixteen 
measurements. This procedure generates a square root of 16 or 4 times 
improvement of the underlying resolution, resulting in an effective 
resolution of 0.006%. 
Op amp 164 and the associated switches allow the gauge starting point to be 
adjusted. In some applications it is desirable to have the "4ma" draw at 
one end of the gage. This circuitry permits selecting the end of the 
desirable to permit the effective gauge to length to be adjusted. The 
variable resistor 123 sets this span value while the variable resistor 126 
is used to zero the gauge. 
The op amp 165 forms part of the "current draw" circuitry. In operation, 
the op amp .and the transistor 178 sink current from the remote power 
supply to indicate the magnitude of the Vsig signal. Feed back from the 
actual current required to operate the circuitry of the transducer is 
measured across the current sense resistor 49. 
In summary, the sampling clock 52 triggers the interrogation pulse supplied 
to the waveguide and also starts the blanking timer 58. The blanking timer 
removes the reset condition on the conversion counter 68 at the conclusion 
of a fixed blanking time. The mode converter output which results directly 
from the excitation of the waveguide occurs during this blanking interval 
and is effectively ignored since the conversion counter is reset during 
the blanking interval. The next, mode converter output pulse, is the 
delayed output pulse and this signal is used to turn the conversion clock 
off. The digital number developed in the counter during this counting 
window is converted to an analog value and low pass filtered to enhance 
the resolution of the gauge. The averaged signal is then used to control 
the current draw of the gauge from a remote supply, and is used to control 
the amplitude of mode converter signal required to terminate the 
measurement time interval window.