Abstract:
An inclinometer utilizing synchronous demodulation for high resolution and electronic offset adjustment provides a wide dynamic range without any moving components. A device encompassing a tiltmeter and accompanying electronic circuitry provides quasi-leveled tilt sensors that detect highly resolved tilt change without signal saturation.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to tilt sensing devices, and more particularly to metrological inclinometers. 
     2. Description of Related Art 
     Inclinometers for metrology have resolution that is about three orders of magnitude less than those employed for geophysical detection. Inclinometers or tiltmeters for geophysical studies have more resolution than required for metrology in an application such as slope stability monitoring in road cuts and road beds in slide-prone areas. 
     A wide range of metrology inclinometers are presently available having resolutions of 0.00005 degrees (1 microRadian). However, a need exists for an instrument that can increase the resolution of metrology inclinometers using some of the techniques employed in geophysical inclinometers, but without such complexity and are also cost effective. An object of the present invention is to provide an inclinometer for metrology having improved resolution compared to present metrology inclinometers. 
     SUMMARY OF THE INVENTION 
     The invention includes an electrical and/or electronic self-adjusting inclinometer having essentially no moving parts during the leveling of the inclinometer&#39;s tilt sensors. The inclinometer or tilt sensing system includes (1) at least one tilt sensor providing an AC analog signal obtained from exterior movement, (2) a synchronous demodulator to provide high resolution to the analog signal received from the sensor, and (3) means for providing offset to the analog signal to provide the system with a quasi-initial leveling status and also signal amplification, without saturating the signal. Some offset can be sent to the synchronous demodulator to allow greater AC gain, thus improving the signal-to-noise ratio. 
     The system works effectively in inclinometers having tilt sensors that were initially not level with the horizontal. Prior to detection of exterior object movement, e.g., tilt or inclination changes in earth formations, the tilt sensors are electronically quasi-leveled by employing the offset. After tilt detection by the sensor(s), the more highly resolved analog signal can be converted to digital, sent to microcomputer means where it can be stored, filtered, further sent to central collection, and/or the data monitored and/or returned to the system. If tilt changes reach a critical level, an alarm can be triggered from the microcomputer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic diagram of a self-adjusting tilt sensing system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention includes a tilt-sensing device having at least one tilt sensor connected to electrical and/or electronic circuitry. For the device, method and use of the invention, a tilt sensor having a single longitudinal axis is described, although a 2-axis tilt sensor or others can readily be adapted to the invention by the skilled artisan. A preferred sensor is a electrolytic sensor, i.e., containing an electrolytic fluid, typically a liquid having a gas bubble, although non-bubbled fluid sensors can be utilized. 
     FIG. 1 illustrates a schematic diagram of a tilt sensing device of the invention having at least one tilt sensor circuit  2  connected to an amplification system such as synchronous demodulator  4  connected to offset means  16  for providing an electrical or electronic offset that can be transmitted to the amplification system (i.e., synchronous demodulator  4 ). The tilt sensing device encompasses one or more tilt sensors  7  (e.g., a single 1-axis electrolytic tilt sensor circuit shown) having analog output electrodes  8  and  10  adapted to detect a tilt angle (or angles) of inclination with the horizontal and to transmit an analog signal (or signals) therefrom. The tilt sensor circuit  2  can include at least one excitation electrode  12 , at least one ground  14 , and at least a pair of fixed resistors  9 . 
     The device includes synchronous demodulator  4  adapted to receive the analog signal(s) from the tilt sensor(s) via electrodes  8  and  10  and to increase resolution of the analog signal(s). Synchronous demodulator  4  receives analog output signals from the sensors as well as offset from a digital-to-analog conversion means such as digital-to-analog converter  16 . Oscillator  18  includes the drive circuitry for sensor(s) circuit  2  (e.g., a 10 volt peak-to-peak 2 KHz sine wave.) Oscillator  18  output signals typically range from 3 to 20 volts, peak-to-peak, at about 100 Hz to about 10 KHz. The sensor outputs are transmitted to a differential amplifier 20 (via sensor output electrodes  8  and  10 , respectively). Resistors  9  complete a Wheatstone bridge with tilt sensor  7 . A first amplifier  22  is capable of generating signals of equal amplitude and phase from the output of differential amplifier  20 . Second amplifier  24  is capable of generating a signal of equal amplitude, but inverted with respect to the output of differential amplifier  20 . Accordingly, first and second amplifiers  22  and  24  have equal gain, but opposite polarities with respect to each other. For example, first amplifier  22  can have a gain of  10  and second amplifier  24  would then have a gain of negative  10 . Usually such amplifiers provide a gain from about 1 to about 1000. The drive voltage from oscillator  18  controls an analog multiplexor  15 , which gates the synchronized output signals of first and second amplifiers  22  and  24  thereby demodulating and providing a synchronous rectified signal which can be filtered through low pass filter  26  to obtain a smooth output voltage proportional to the sensor tilt angle, and of positive or negative polarity, depending on the direction of tilt. The filtered output can then be coupled to analog-to-digital signal conversion means. Usually analog multiplexor  15  is controlled by the output of a comparator  25  which detects the zero crossing of oscillator  18 . Thus, the comparator  25  output is logic high when the output of oscillator  18  is positive and a logic low when oscillator  18  is negative. 
     A feature of the device of the invention involves a means for providing an offset to the analog signal obtained from the tilt sensors. Such means for providing an offset to such an analog signal are adapted to return a portion of the offset to the synchronous demodulator and are adapted to amplify the analog signal and simultaneously prevent signal saturation. The offset is typically in the form of a voltage increase or reduction to the output analog signal from the tilt sensor, although other forms of offset adjustments should be apparent to the skilled artisan. Such means for providing the offset can include analog-to-conversion means such as a digital-to-analog signal converter  16 . A computer  30  (e.g., microcomputer) is adapted to receive and/or monitor the analog signal or to receive and/or monitor a digital signal converted from the analog signal. The computer can include (or be connected to) signal conversion means such as an analog-to-digital converter  28  for converting the analog signal to an digital signal and/or a digital-toanalog converter for converting the digital signal to an analog signal. Normally the offset is transmitted from digital-to-analog converter  16  via computer  30  wherein an operator makes adjustments, or a program code is activated, to control the offset to the demodulator via analog-to-digital signal converter  16 . Such an offset can also be provided by resistors (not shown) to demodulator  20 . 
     The invention includes a method for leveling the sensing device or tiltmeter wherein a digital-to-analog converter is set to a given offset, an initial, non-level tilt angle from horizontal is measured by receiving and recording an analog output signal from the tilt sensor in the tiltmeter, and the offset is adjusted to null the analog output signal from the tilt sensor. The tiltmeter is initially positioned to measure an object of measure at the initial tilt angle other than level or horizontal, and the digital-to-analog converter is set to a relatively small or minimum offset. The amount of offset is adjusted to essentially null the analog output signal from the tilt sensor(s) by adjusting a converted analog output signal from the digital-toanalog converter to essentially null the output of the synchronous demodulator. Such an amount can be up to (plus or minus) the full scale of the amplifier or synchronous demodulator. Thus, in effect, the adjusted offset electronically produces a quasi-leveling of the initially non-level sensor(s) without any mechanical movement of components of the inventive device. 
     The method includes measuring a tilt change by receiving and recording a second analog output signal from the leveled tilt sensor, the second analog output signal being amplified and digitized. The digitized output signal from the second analog output signal is transmitted to the computer for monitoring, recordation, storage, and/or further amplification or demodulation. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited by the scope of the appended claims.