Micropower apparatus for low impedance measurements

A micropower apparatus and method for milliohm resistance detection includes a drive circuit and a monitoring circuit. The drive circuit includes a step up current transformer that is driven by a square wave oscillator via a low pass filter and voltage-driven current source. The drive circuit drives a series arranged test coupon and reference coupon, the latter being exposed to the elements. The coupons are Kelvin connected to the monitoring circuit, which includes a pair of low noise, low offset pre-amplifiers, a pair of post amplifiers connected to outputs of the pair of pre-amplifiers, a pair of full wave rectifiers connected to outputs of the pair of post amplifiers, and a pair of low pass filters connected to the outputs of the pair of full-wave rectifiers. Resistance values of the test and reference coupons can accordingly be monitored ratiometrically to determine a state of a selected environment by, for example, detecting changes in electrical resistance due to corrosion of the test coupon.

BACKGROUND

1. Field of the Invention

The present invention relates to a micropower instrument of high accuracy to perform ratiometric measurement of milli-ohm level resistances using milliamp-level current. The invention can be used for general environmental monitoring including, but not limited to, corrosion measurement, strain measurement and other monitoring uses that rely on ratiometric comparisons of a milli-ohm sense resistance to a milli-ohm reference resistance.

2. Background of the Invention

Corrosion can lead to failures in infrastructure, machines, and mission critical systems. Such failures are expensive to repair, can lead to lost or contaminated products, can cause environmental damage, and ultimately, can even cause unsafe environments or situations for humans. Decisions regarding the future integrity of a structure or its components depend substantially upon an accurate assessment of the conditions affecting its corrosion and rate of deterioration. Only with accurate information in hand, can an owner or operator make an informed decision as to the type, cost, and urgency of repair or replacement.

Corrosion monitoring is particularly important in areas that cannot be readily inspected visually or are difficult to inspect due to the inherent structural arrangement of a particular device, machine or structure. For example, there may be cavities within vehicles that are generally not accessible because of equipment or other structures that block an opening to the cavity. Nevertheless, corrosion monitoring of such spaces is desirable, and perhaps critical.

One well-known method of monitoring corrosion is the electrical resistance technique. This technique effectively measures material loss, i.e., corrosion, by measuring a change in electrical resistance of a metallic element, which is exposed to a selected environment, with respect to a reference element that is arranged to be immune from that environment's corrosive effects. While this technique is very popular and has found wide acceptance, the technique requires the availability of electric power. In some cases, power for the electric resistance technique is obtained from a battery. However, since it is often desirable to monitor environments for relatively long periods of time, battery life becomes an issue for these instruments. Accordingly, there is need for a micropower instrument with greatly-improved battery life and accuracy.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a micropower instrument of high accuracy to perform ratiometric measurement of milli-ohm level resistances using milliamp-level current. The invention can be used for general environmental monitoring including, but not limited to, corrosion measurement, strain measurement and other monitoring uses that rely on ratiometric comparisons of a milli-ohm sense resistance to a milli-ohm reference resistance. The apparatus in accordance with the present invention is an analog measurement instrument that consumes an extremely low amount of power (thereby providing longer battery life), has high precision and accuracy, and, in a preferred embodiment, operates on and measures the resistance of a sacrificial electrical resistance coupon. The instrument is able to detect and monitor resistance in the mili ohm range.

In accordance with a corrosion detection implementation of the present invention, corrosion is measured by comparing the resistance of a corroding test coupon to a protected reference coupon that was identical or nearly so at the time of manufacture. The resistances of these coupons are very low, typically on the order of a few milliohms. Because low average power consumption is a desirable feature, high-current excitation of the coupons is not an option. On the other hand, low-current excitation results in signals of microvolt magnitude. The present invention was developed in view of the fact that available commercial instruments fail, by a wide margin, to meet low-power requirements for an environmental analysis detection system that is intended to be located or positioned in places that may generally be inaccessible, or that need to be on-station for long periods of time.

Features of the present invention include, but are not limited to:

the use of AC current to excite the coupons to avoid errors due to DC offset in amplifiers and thermoelectric potentials at various connection points;

the use of a 10:1 current step-up transformer in a drive circuit to gain a tenfold increase in power efficiency in driving the very low-impedance load (the coupons);

the use of very low-noise, low-offset, high-gain instrumentation operational amplifiers in a first signal-processing stage; and

the use of ratiometric measurement by current driving the reference and sensor coupons in series, sensing and signal-processing their responsive voltages, and taking the ratio of these voltages in subsequent digital signal processing.

The present invention can also be employed as a resistive straingage. More specifically, the present invention provides an instrument that can be used to monitor straingages or other resistances that are proportional to strain. The term straingage commonly refers to a resistive element that changes resistance with strain. Resistive straingages are routinely used for measurements of strain in structural elements or members. Such straingages have relatively high resistance to aid in ease of instrumentation, but they are relatively delicate, fragile, and require considerable care and skill to affix. They are often configured or arranged as bridges to facilitate ratiometric measurement. The ability to measure very low resistances with a micropower instrument in accordance with the present invention makes it possible to observe strain in a metal structural member (as in a sheetmetal skin) by directly measuring changes in resistance between various points in the strained member itself, obviating the need for straingages.

The features and attendant advantages of the present invention will be more fully appreciated upon a reading of the following detailed description in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to FIGS. 1 and 2 , which illustrate, respectively, a block diagram and schematic of a corrosion monitoring implementation of the present invention.

An oscillator 120 , preferably a 100 Hz symmetrical hysteretic oscillator using a RRIO (rail-to-rail input and output) opamp, produces a symmetrical square wave oscillating between ground and Vcc. This type of oscillator has the advantage over sine wave oscillators, such as Wein bridge, state-variable, phase shift or twin-tee oscillators, of starting immediately at full magnitude. Thus, an oscillator of the kind depicted minimizes the total time required from application of power to availability of stable measurement data. The present inventors have determined that post-filtering such a square wave signal provides much faster response than was possible with alternatives while providing acceptable spectral purity. Oscillator 120 is comprised of U 1 a and associated circuitry. The rationale for selection of the 100 Hz frequency is described below. It is noted that other oscillating frequencies may be selected depending on the particular application.

The resulting square wave is filtered by, in this particular exemplary implementation, a 4 th -order Butterworth-response low pass filter 125 comprised of U 1 b , U 1 c and associated circuitry. The corner frequency of this filter is 125 Hz. The filtered output is a sine wave with no second harmonic component because it is symmetrical, and it has less than 1% third-harmonic component. The output sinusoid is stable to within less than 0.1% of steady state magnitude within fewer than 200 milliseconds. Those skilled in the art will appreciate that other types of filters may be employed instead of the Butterworth type and that the present invention should not, therefore, be construed to be limited to this type of filter. The choice of response and order of the filter is governed by desired spectral purity versus rapidity of response and freedom from overshoot upon startup.

The sinusoidal voltage thus produced is presented to a voltage-driven current source 130 comprised of U 2 a and associated circuitry. It drives its load with a sinusoidal current of 5 mA peak regardless of load impedance or voltage required to produce the intended current. The current source is referenced to Vcc so it provides AC current as the drive voltage varies sinusoidally and symmetrically above and below Vcc.

Inverting amplifier 140 (U 2 b ) inverts the sinusoidal voltage with unity gain. This provides differential drive for the primary of transformer 135 . The current drive's output voltage can only vary from near Vcc to near ground, but since the other end of the primary of transformer 135 is connected to a voltage source varying out of phase with the drive current, drive voltage can approach Vcc in magnitude if necessary to reach peak values of 5 mA. The actual voltage appearing on the primary of transformer 135 will depend on the resistance of the leads to coupons 300 a (the test coupon) and 300 b (the reference coupon), also referred to herein collectively as a device under test.

Since coupons 300 a and 300 b are current driven in series, they are excited with identical current. Transformer isolation prevents undesirable ground loop current in the common return.

The test and reference coupons 300 a , 300 b (also shown in FIG. 3 ) are Kelvin connected as shown in FIG. 2 . As shown, sense voltage preferably returns on wires separate from those conducting the drive current. The drive frequency of 100 Hz was chosen as high enough in frequency to result in a transformer of acceptable size and weight (about 2 cm 3 and about 4 grams), but low enough to minimize the effects of coupling between drive wires and sense wires in the wiring between the instrument and the coupons.

The sense voltages from the coupons are supplied to instrumentation pre-amplifiers U 3 a and U 3 b , which correspond generally with 150 a , 150 b . These pre-amplifiers are preferably selected for very low noise and very low DC offset. Low DC offset is desirable to keep the amplifiers out of saturation, given the high gain and the low supply voltage available. They are preferably set to a gain of 1000 (60 dB). Pre-amplifiers 150 a , 150 b are also preferably arranged such that the phase-opposite voltages from coupons 300 a , 300 b are amplified in phase to minimize crosstalk, although crosstalk isolation between pre-amplifiers 150 a , 150 b is believed to exceed 120 dB.

The outputs of pre-amplifiers 150 a , 150 b are AC-coupled to post amplifiers U 4 a and U 5 a , which correspond to elements 155 a , 155 b . AC coupling is used to remove any amplified DC offset error from the pre-amplifiers.

The post-amplified signals are presented to precision full-wave rectifiers 160 a , 160 b , comprised of U 4 b and U 5 b and associated circuitry. The theory of operation of this block is straightforward and well-documented in the literature. See, e.g., LB-9 (Linear Brief 9) from National Semiconductor, Santa Clara, Calif.

The resulting full-wave rectified signals are low pass filtered by, for example, 2d order Bessel response filters 170 a , 170 b , comprised of U 4 d for one channel and U 5 d for the other channel. In the implementation shown, these are Sallen-Key filters with unity gain regardless of tolerance in resistor values. These filters have a 3 dB corner frequency of 5 Hz. This form of filtering is preferred over the usual R-C post-detection filtering to achieve fast response with good rejection of post-rectification ripple. The DC output of filters 170 a , 170 b is proportional to the average value of the rectified AC input. The output is settled to within 0.2% within 200 milliseconds of oscillator startup, including delay in the oscillator filter. At 1 volt output, ripple is about 1 mV RMS.

Filters 170 a , 170 b limit system bandwidth to 5 Hz, which gives the system very good immunity to EMI and to Johnson noise in the low-level stages.

As added protection, the inputs to pre-amplifiers 150 a , 150 b are preferably protected against transients by resistors followed by diode clamps to ground and Vcc. DC offset from leakage current of the diodes used is negligible over the full MIL temperature range due to the low impedances involved. In addition, in a preferred implementation, the instrumentation opamps used are internally protected for overvoltage up 40 volts.

U 6 ( FIG. 2 ) is a charge-pump voltage inverter operating at about 35 KHz to produce a negative bias voltage for the instrumentation opamps. The input signals are typically within less than a millivolt of ground potential so the first stage amplifiers require negative bias. Power supply rejection of these amplifiers exceeds 120 dB at the frequencies of interest and the 5 Hz lowpass filters eliminates any noise significantly above that frequency, so the negative bias is not regulated.

FIG. 3 depicts an exemplary coupon 300 that may be used in conjunction with the apparatus of the present invention. Coupon 300 comprises two portions, a test coupon portion 300 a and a reference coupon portion 300 b . Reference coupon portion 300 b and its associated leads are preferably coated with a water tight and air tight coating (the border of which is identified by dashed lines 310 ) to seal the reference coupon from the ambient environment. Accordingly, only test coupon portion 300 a is exposed to the elements and experiences corrosion and corresponding change in resistance that is detected by the apparatus in accordance with the present invention.

As mentioned, the present invention may also be used for straingage applications, or other applications in which milli-ohm range sensing might be desirable.