APPARATUS FOR USE WITH A DENSITOMETER

Abstract: Apparatus for use with a densitometer having a probe (4) with an output and a probe driver (18) for driving the probe includes a loop circuit coupled between the probe output and the probe driver. An amplifying circuit (12) is coupled in the loop circuit to amplify the signal provided by the probe (4). A phase/gain control device (16) is also coupled in the loop circuit between the amplifying device (12) and the probe driver (18). The phase/gain control device (16) controls the loop circuit to provide a loop gain of at least one and a substantially zero degree phase shift between the probe driver (18) and the probe output signal. The amplifying device includes an op-amp which is preferably coupled to a shielded cable (14) carrying the probe output signal. The shield conductor and one input of the op-amp are held at ground potential, and the op-amp acts to hold the centre conductor at ground potential also. With the centre conductor held at ground potential, the output signal from the probe varies with current, not voltage. A resistance in the feedback path of the op-amp converts current changes to voltage changes proportional to the output signal of the probe. Since the centre conductor of the shielded cable varies in current only, the output signal from the probe is insensitive to cable length, cable loading, and cable capacitance.

Description: The invention relates to apparatus for use with a densitometer having a probe with an output, probe driver means coupled to the probe, a connection between the probe output and the probe driver means so as to form a loop, and amplifier and phase/gain control means in said connection for amplifying the probe output signal. Such a form of apparatus is known from US-A-4 215 566. More specifically, the apparatus described here is a vibration densitometer having a magnetostrictive drive with a coil and a crystal pick-up. The loop circuit includes a driver amplifier providing the coil with a voltage which is twice that ordinarily provided. The amplifier is configured as a differential amplifier coupled to the crystal output through a transformer. The circuit is thus sensitive to capacitive loading from the connection lead. This precludes the use of protective shielding cable and the sensitivity to cable capacitance requires an interface unit which has to be located close to the probe, this interface unit conditioning the probe output signal for subsequent processing. This requirement for an interface unit close to the probe causes practical disadvantages. Densitometers of this type may be used, for example, to determine the density of a fluid in a tank such as an aircraft fuel tank. A large aircraft may have many fuel tanks, each thus requiring a separate interface unit; this increases cost and weight and decreases reliability. Corresponding disadvantages can arise in other applications. The invention aims to overcome these problems. Accordingly, the apparatus first set forth above is characterised in that the connection includes a cable having a grounded shield conductor and a signal conductor, in that the amplifier means has an input connected to the signal conductor and is operative to hold the signal conductor at ground potential so that the probe output is a current-varying signal whereby the amplifier means amplifies the probe output signal with a phase shift which is independent of the length and capacitive loading of the cable. Apparatus according to the invention for use with a vibrating spool densitometer, and a vibrating spool densitometer embodying the invention, will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a block diagram of the densitometer; and Figure 2 is a detailed schematic of the densitometer. The densitometer to be described allows a densitometer sensor to be located in a fuel tank with no active electronics associated with the densitometer in the tank hardware. The required signal conditioning for the densitometer sensor is located in an electronics package which does not have to be located near the fuel tank. The densitometer is designed with a view toward keeping the probe output signal immune to cabling effects. The densitometer requires no problem wiring coaxial or shielded cable to interconnect the density sensor with the appropriate signal conditioning electronics package. The design of the electronics also limits tank energy to less than 0.1 microjoules, and the current to less than 10 milliamperes. These current and energy levels are intrinsically safe. The electronics package for the densitometer may be supplied with appropriate driving voltage (typically 28 volts DC for an aircraft) from the appropriate fuel quantity channel. Internal circuitry sustains densitometer spool vibration (which relates to fuel density) and the sine wave output of the electronics package is sent to a processor unit. Calibration constants for the densitometer may be carried as resistors in a potted area behing the densitometer sensor. The resistor values are read by a processing unit, so that a densitometer frequency output can be converted directly into fuel density. The calibration values are carried on each density sensor; therefore, the densitometer may be replaced as required without affecting system calibration and without any calibration data entry required by aircraft maintenance personnel. There is no need to do anything to the electronics package if a densitometer is replaced, and no need to do anything to the densitometer if the electronics package is replaced. The design of the densitometer also allows its signal conditioning circuitry to be placed on the same processor electronics package that measures the fuel tank quantity, thus eliminating the additional interface electronics package required by known vibration densitometers. Since the densitometer provides for signal conditioning which is not sensitive to cabling effects, a number of advantages accrue, such as increased system reliability, easier installation, lower weight, and lower cost. In Figure 1, fuel tank 2 includes a probe 4 which may comprise a vibrating spool 6 given by an excitation device, for example an excitation coil 8. Sensor coil 10 picks up spool vibrations and provides a probe output signal which is output from fuel tank 2. A variety of alternative sensors may be provided, such as a crystal. The probe output signal from sensor coil 10 is provided to a densitometer preamplifier cable compensation circuit 12 via cabling 14. It is particularly preferred that cable 14 comprise a coaxial cable, or any other known shielded cabling suitable for such use. The shielding conductor of cable 14 is coupled to ground, for reasons described later herein. The shielding conductor is also coupled to the densitometer preamplifier circuit 12, as is the signal conductor of cable 14. Densitometer preamplifier 12 is an important part of the densitometer. It allows the signal from the sensor coil 10 to be amplified with a phase shift independent of the length of capacitance loading of the cable connecting the densitometer to the densitometer preamplifier 12. In order for the sensor to operate properly, the phase of the signal from the sensor coil 10 must be carefully controlled. An uncontrolled phase shift will cause the sensor to stop vibrating, resulting in a loss of signal. Previous methods of signal conditioning required the sensor to be located close to the electronics signal conditioning package to avoid such phase shifts. Densitometer preamplifier 12 makes the system relatively insensitive to cabling effects, thus allowing the densitometer preamplifier 12 to be located in any convenient location. The amplifier signal from densitometer preamplifier 12 is applied to a loop gain and control circuit 16. Loop gain and control circuit 16 provides an input to coil driver 18 and also samples the output signals from coil driver 18. Loop gain and control circuit 16 is configured to ensure a substantially zero degree phase shift between the sensor coil output signal and the coil driver output signal. The densitometer operates with a resonant element sensor. At the frequencies of interest, the sensor has a gain peak and a phase of zero degrees between the input (coil driver 18) and the output (sensor coil 10). In order to sustain oscillation of the vane at the resonant frequency, loop gain and control circuit 16 must provide a loop of at least one, and a phase shift of zero degrees. The conditions required to sustain oscillation are well known to anyone versed in the art. Loop gain and control section 16 monitors the outputs of coil driver 18 and adjusts the loop gain such that spool oscillations are just sustained. Any phase errors in the loop circuit are detected and corrected beforehand by making part substitutions in the loop gain and control circuit, as described later herein. The loop gain and control circuit 16 provides an output signal which corresponds to the density of the fuel in fuel tank 2. This output signal may be provided to a processor 20 which may provide a fuel density indication on indicator 22. As above indicated, the output signal from loop gain and control circuit 16 is also applied to coil driver 18 which, in turn, drives excitation coil 8 to provide appropriate vibration to vane 6. Coil driver 18 provides enough power gain for the output of the loop gain and control circuit 16 to drive the excitation coil 8. Coil driver 18 also provides a fixed gain of approximately 11 to help make up for losses in probe 4. Furthermore, coil driver 18 provides a balanced output to minimize interference with other systems and also provides a low pass filter characteristic which prevents the excitation of resonant frequencies outside the range of interest. The output impedance of coil driver 18 is low so that cable loading effects will not affect the system operation. Both the densitometer preamplifier cable compensation circuit 12 and coil driver 18 may be located remotely from the fuel tank 2. This is because the output signal of sensor coil 10 is relatively immune to degradation due to cabling effects. Figure 2 illustrates more details of the presently preferred embodiment. The output signal from sensor coil 10 is provided to densitometer preamplifier 12 through cable 14. The centre conductor of cable 14 is preferably connected to the inverting input of an operational amplifier U1 through a summing junction 17. The shielding conductor of cable 14 is coupled to both the noninverting input of U1 and to ground. Since the operation of op-amp U1 is to keep summing junction 17 at the same potential as the noninverting input, and since the noninverting input is kept at ground potential, the net result is that the centre conductor of cable 14 carrying the probe output signal is held at ground potential. With the centre conductor at ground potential, there is no voltage across any stray resistance or capacitance to ground on the return wire from sensor coil 10. Since there is no voltage across these stray elements, no current flows through them. With no current flow, the cable strays cannot affect the system phase or gain. Thus, the system is independent of cabling effects, and the cable can be made as long as desired. With the centre conductor of cable 14 held at ground potential, the output signal from sensor coil 10 is a current. Thus, the probe output signal carried on cable 14 is current-varying and not voltage-varying. The feedback action of the op-amp U1 causes the input current to flow through feedback resistor R1. Current in resistor R1 results in an output voltage porportional to the input current provided by the sensor coil 10. Thus, the voltage invariant probe output signal carried on cable 14 is converted to a voltage-varying signal at the output of preamplifier 12. Therefore, an effective measure of the density of fuel in fuel tank 2 may be provided without regard to the effects or length of cabling 14. Op-amp U1 amplifies the output signal from probe 4 with a phase shift relatively independent of a length or capacitance loading of the probe output cable 14. This is because op-amp U1 has a low input impedance. Therefore the capacitance load on the probe output cable 14 does not change the phase. Thus, the length and capacitance loading of the cable output 14 does not affect the phase of the signal provided at the output of preamplifier 12. The output of op-amp U1 is provided to loop gain and control circuit 16. Preferably, loop gain and control circuit 16 comprises two variable gain stages composed of amplifiers U2 and U3 for controlling the loop gain. Both stages are identical and comprise a feedback loop in the form of a T attenuator which controls gain. The ground leg of the T is partially formed by the drain-source leg of a JFET transistor (Q1 and Q2). By changing the gate voltage of the JFET transistor, the drain source resistance is modulated, thus forming a voltage-controlled T attenuator which controls gain. The gate voltage for each JFET transistor Q1 and Q2 is derived by amplifier U7, which samples the drive signals provided by coil driver 18 to the excitation coil 8. These signals are rectified by diode CR 1 and low passed filtered by capacitor C11 and resistor R29 to give a DC signal proportional to the sense coil drive. If there is no signal present at the output, the DC voltage is zero, which modulates the drain-source resistance of transistors Q1 and Q2 to a minimum value, and sets the loop gain to a maximum. As oscillations build up in the closed resonant system, the DC voltage on the gates of transistors 01 and 02 goes negative, causing an increase in drain-source resistance, and a decrease in the gain of amplifiers U2 and U3. The gain stabilizes at a value such that the drive to the sense coil is approximately 1 volt peak-to-peak. The phase of the circuit is preadjusted by component selection at installation of the densitometer equipment. Briefly, the circuit is tuned depending upon the particular amplifiers used in preamplifier 12, loop gain and control circuit 16 and coil driver 18. In Figure 2, capacitor C2 may be specially preselected in order to control the phase of the circuit. Capacitor C2 can be chosen by taking the loop circuit output and plotting phase versus frequency. Then, capacitor C2 is selected to ensure a substantially zero degree phase shift in the loop circuit. Changes in capacitors C3 and C7 in coil driver 18 may also affect the phase of the circuit. Again, empirically tuning the circuit may require substitutions in capacitors C3 and C7. Thus, the densitometer can be pre-tuned by part substitution with the amplifiers or capacitors of the loop gain and control circuit 16 and coil driver 18. All such part substitutions are to be included within the scope of the appended claims. The output of loop gain and control circuit 16 is provided in sensor coil driver 18 to a balanced output driver U5 which provides a positive drive output on line 24 to excitation coil 8 and amplifier 25 U7. The output signal of loop gain and control circuit 16 is also applied in parallel to a unity gain inverter stage U4 which provides an inverted signal output to standard active filter U6. Driver U6 develops on line 26 the negative phase of the balanced drive output to excitation coil 8 and amplifier U7. Both balanced output drivers U5 and U6 are standard active filters configured as 3dB Chebyshev low pass filters. The filtered output signals from filters U5 and U6 are returned to excitation coil 8 which then properly drives vane 6 at resonance. While the circuit of Figure 2 is a detailed schematic of a preferred embodiment, those of skill in this field will readily appreciate that many alternative circuits may be designed depending upon the particular use to which the sensor probe is put. All such alternative circuits are to be included within the scope of protection afforded to the present invention. Thus, what has been described is densitometer apparatus for making the probe output signal immune to cabling effects, thereby allowing signal conditioning circuitry to be located remotely from the sensor probe. Where a plurality of probes are utilized in a single system, appropriate signal conditioning circuitry may be unified in one package, for example, co-located with processor and indicator devices. The densitometer described overcomes advantages of known vibration densitometers by eliminating the need for interface units located at each fuel tank. The densitometer provides a means of signal conditioning a vibrating spool densitometer signal in such a fashion that the vibration frequency of the densitometer is dependent only upon the fluid in which the sensor is immersed, and is not dependent upon the length, shielding or capacitive loading of the interconnection cable. Therefore, the interconnection cable may now be shielded. This further protects the system from influences of high energy fields, for example, RF interference. Since the output signal of the vibration densitometer is independent of the length of cabling, the signal condition circuitry which transmits the probe output signal to the appropriate processor may now be located at any desired location in an aircraft, thus eliminating extra electronic packages. In fact, the signal conditioning circuitry may now be located at the processor location itself. While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment. On the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: Apparatus for use with a densitometer having a probe (4) with an output, probe driver means (18) coupled to the probe (4), a connection (14,12,16) between the probe output and the probe driver means (18) so as to form a loop, and amplifier and phase/gain control means (12,16) in said connection for amplifying the probe output signal, characterised in that the connection includes a cable (14) having a grounded shield conductor and a signal conductor, in that the amplifier means (12) has an input connected to the signal conductor and is operative to hold the signal conductor at ground potential so that the probe output is a current-varying signal whereby the amplifier means (12) amplifies the probe output signal with a phase shift which is independent of the length and capacitive loading of the cable (14). Apparatus according to claim 1, characterised in that the phase/gain control means (16) controls the loop to provide a loop gain of at least one and a substantially zero degree phase shift between the probe driver means (18) and the probe output signal. Apparatus according to claim 1 or 2, characterised in that the amplifier means (12) has a second input connected to ground and to the shield conductor. Apparatus according to claim 3, characterised in that the amplifying means (12) comprises an operational amplifier (U1). Apparatus according to claim 4, characterised by a circuit path connected from the output of the operational amplifier (U1) to the input connected to the signal conductor. Apparatus according to claim 5, characterised in that the said circuit path comprises a feedback path including a resistance (R1) to convert the voltage-invariant signal into a voltage varying output. Apparatus according to claim 6, characterised in that the feedback path includes capacitance (C1) connected in parallel with the resistance (R1). Apparatus according to any preceding claim, characterised in that the phase-gain control means (12) includes first (U2) and second (U3) variable gain stages coupled in series between the amplifying means (12) and the probe driver means (18) for providing the loop gain. Apparatus according to claim 8, characterised in that each of the first and second variable gain stages comprises a responsive T-attenuator feedback loop (R3 to R5; R6 to R8) having a grounded leg including a transistor (Q1;Q2) for adjusting the loop gain. Apparatus according to claim 9, characterised in that the first and second variable gain stages respectively comprise first and second amplifiers (U2;U3) having said T-attenuator feedback loops respectively, said first amplifier (U2) having an input coupled to said amplifying means (12) and an output, said second amplifier (U3) having an input coupled to a said first amplifier output (U2) and an output coupled to said probe driver means (18). Apparatus according to claim 9 or 10, characterised in that said phase/gain control means (16) includes an amplifier (U7) coupled to said probe driver means (18) for controlling a gate voltage of both said T-attenuator feedback loop transistors (Q1;Q2) to control said loop gain. Apparatus according to any preceding claim, characterised in that said probe driver means (18) has first and second outputs (24,26) connected to drive the probe (4), the probe driver means (18) including unity gain inverter means (U4) for inverting the output of the amplifier and phase/gain control means (12,16), first low pass filter means (U5) coupled to the output of the amplifier and phase/gain control means for providing a positive drive output to the first output (24) of the probe driver (18), and second low pass filter means (U6) coupled to said unity gain inverter means (U4) for providing a negative drive output to the second output (26) of the probe driver (18).

Inventors: MAIER, LAWRENCE CARL
Applicants: SIMMONDS PRECISION PRODUCTS INC.
Application year: 1988
Main classification: G01N9
IPC codes: G01N9