FIELD

The present patent document relates to piezoelectric (PE) sensor circuits and generally a way to provide output signal and power to a circuit over the same two wire connection. The present patent document describes two-wire output remote charge amplifier circuits (CAC) designed for a high-temperature, piezoelectric (HTPE) transducers that can operate at temperatures up to +815° C. (+1500° F.).

BACKGROUND

HTPE sensors are used in extremely high temperature environments such as those experienced on aircraft gas turbines. Although many HTPE sensors are used on aircraft, they can be used in the industrial, environmental, military, aviation, and space vehicle fields. A HTPE transducer, like any typical PE transducer, transforms an input signal, like acceleration for example, into an output charge electrical signal. A typical HTPE transducer, similarly to other PE transducers, has a high resistance (typically greater than 100 MΩ) at room temperature, but, in contrast to other PE transducers, exhibits a much lower resistance, down to 10 kΩ at high temperatures.

A remote charge amplifier is used with HTPE transducers to boost the signal. The remote charge amplifier is connected to the HTPE transducer with a high temperature cable. These high temperature cables are often coaxial but other cable types may be used. The charge amplifier converts the charge signal coming from the transducer into a voltage output signal. At the same time, the charge amplifier transforms the transducer's high impedance into the amplifier's low output impedance allowing transmission of the voltage signal over long cable lengths.

In some embodiments, charge amplifiers can be integrated with the transducers such as with IEPE sensors.

The basic advantage of a charge amplifier over a voltage amplifier is that a charge amplifier features independence of a charge gain on the transducer's capacitance. This feature is critical for HTPE transducers because the HTPE transducer is located in a hot zone and connected to the electronic circuit with a high temperature cable having a large capacitance value. Therefore, only a charge amplifier (not a voltage amplifier) can be used as an electronic circuit for HTPE transducers.

Typical charge amplifiers are only able to operate with high impedance capacitive sources. However, when resistance of the signal source decreases due to environmental conditions, a typical charge amplifier generates large spikes in its frequency response. These large spikes in the frequency response occur when the HTPE transducer is exposed to high temperatures. The large spikes are caused because the resistance of the HTPE transducer, which normally has values of hundreds MΩ at room temperature, can decrease to 10 kΩ at temperature of +815° C.

In many embodiments, the charge amplifier is designed to have a two-wire output. The first wire is the actual output and carries the output signal and voltage supply at the same time. The second wire is the ground. The source of power for the circuit may be a constant current source.

Various different charge amplifier circuits designed for PE sensors are known in the art. A remote charge amplifier circuit used with a PE sensor is shown inFIG. 1. This charge amplifier is based on an Operational Amplifier (“Op Amp”). In the embodiment shown inFIG. 1, the charge amplifier connects to a PE transducer with a coaxial cable and has a two-wire output. The configuration shown inFIG. 1may be found at the URL https://www.springer.com/us/book/9783319080772.

Another known configuration for a charge amplifier, and one that is more often used, is illustrated inFIG. 2. The charge amplifier shown inFIG. 2is two stages and based on a field effect transistor (“FET”) and a bipolar junction transistor (“BJT”). The embodiment shown inFIG. 2may also be found at the URL above. The charge amplifier shown inFIG. 2also has a two-wire output. However, the charge amplifier inFIG. 2is part of an IEPE sensor.

In both of the known charge amplifiers shown inFIG. 1andFIG. 2, only a single 1-pole low-pass filter (“LPF”) was used. Moreover, neither of these charge amplifiers can be effectively used with HTPE transducers due to the large spikes in their frequency response that will occur at high temperatures.

To these ends, one object of the present invention is to provide a PE sensor charge amplifier circuit design, and in particular a sensor circuit for an HTPE sensor, that has better performance and can work at high temperatures. Even more preferably, the circuit design would have a two-wire output so that it is compatible with current setups, cabling and sensors designs. Such a design would allow for compatibility with current high-temperature PE sensors hardware and systems while providing for improved performance of these sensors. In particular, it is desirable that the new circuit design could be used as a replacement for current circuits used with HTPE sensors.

SUMMARY OF THE EMBODIMENTS

In view of the foregoing, an object according to one aspect of the present patent document is to provide an improved circuit for sensors and in particular HTPE sensors. In preferred embodiments, the circuit provides extended frequency range for transducers and in particular HTPE transducers. Embodiments described herein preferably have the capability to operate with HTPE transducers even when they have a low impedance due to exposure to high temperatures or other reasons. Preferably, the circuits herein provide resonance suppression to the HTPE transducer's output.

In preferred embodiments, a charge amplifier circuit is provided. In some embodiments, the circuit comprises: an input and a two-wire output; a charge amplifier comprising an operational amplifier and a feedback capacitor; a first low-pass filter directly coupled to an output of the charge amplifier; a second low-pass filter coupled to an output of the first low-pass filter by a decoupling capacitor; a constant current source located between a positive power supply input of the operational amplifier and a first output of the two-wire output wherein the first output of the two-wire output carries the signal and power of the charge amplifier circuit.

In some embodiments, the charge amplifier circuit further comprises an emitter follower directly coupled to an output of the second low-pass filter.

In yet other embodiments, the first low-pass filter is a passive 1-pole low-pass filter. In embodiments with a passive 1-pole low-pass filter, the low-pass filter may be comprised of a resistor and a capacitor.

In preferred embodiments, the second low-pass filter is an active 2-pole low-pass filter. In embodiments that include an active 2-pole low-pass filter, the active 2-pole low-pass filter may comprise two junction field effect transistors (JFET) connected in series with each other, three resistors, and two capacitors.

In preferred embodiments, the charge amplifier circuit may have one or more feedback circuits. In some embodiments, a first feedback circuit provides direct current bias for a non-inverting input of the operational amplifier. In some embodiments, a second feedback circuit provides direct current bias for an inverting input of the operational amplifier. In some embodiments, a third feedback circuit is provided.

Although the charge amplifier circuits taught herein can be used for any purpose, they are ideally suited for use with a piezoelectric transducer and in particular an HTPE transducer. In preferred embodiments, and HTPE is in electrical communication with the input of the charge amplifier circuit.

Embodiments described herein preferably consist of an input and a two-wire output; a first stage including a charge amplifier comprising an operational amplifier (Op Amp), feedback capacitor, and decoupling capacitor; wherein feedback capacitor is connected between the Op Amp output and the circuit input and the decoupling capacitor is connected between the Op Amp inverting input and the circuit input. A second stage including a 1-pole low-pass filter, wherein the output of the first stage is directly coupled to the input of a second stage. A third stage that includes a 2-pole low-pass filter comprising two junction field effect transistors (JFET), three resistors, and two capacitors; wherein the first JFET's drain is connected to the circuit output and the first JFET's source is connected to the second JFET's drain and the input of the emitter follower, the second JFET's gate is connected to the second output wire, and wherein the output of the second stage is connected to the third stage input with a decoupling capacitor in between. A fourth stage including an emitter follower comprising two bipolar junction transistors (BJT) connected to each other as a Darlington transistor. A constant current source (CCS) connected between the first wire of the two-wire output and a positive power supply input of the Op Amp. Three feedback circuits, where the first feedback circuit comprising bipolar junction transistor and two resistors is connected between constant current source and non-inverting input of the Op Amp. A second feedback circuit comprising three resistors is connected between the constant current source and inverting input of the Op Amp. A third feedback circuit comprising three resistors connected between the first wire of the two-wire output and a gate of the first JFET, wherein the collectors of both BJT transistors is electrically connected to a second wire of the two-wire output.

In preferred embodiments, the circuit is used for HTPE transducer and connected to the HTPE transducer with a coaxial cable.

In some embodiments, the fourth stage emitter follower is comprised of two bipolar junction transistors (BJT) connected to each other as a Darlington transistor.

In some embodiments, the third stage 2-pole LPF is comprised of two JFETs connected to each other in series.

In some embodiments, the first feedback circuit is comprised of one BJT and two resistors; each of the second and third feedback circuits is comprised of at least three resistors.

As described more fully below, the charge amplifier circuit of the present patent document addresses and at least ameliorates some of the shortcomings of known charge amplifier circuits used with HTPE transducers. Further aspects, objects, desirable features, and advantages of the apparatus and methods disclosed herein will be better understood from the detailed description and drawings that follow in which various embodiments are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the claimed invention.

