1. Field of the Invention
The present invention relates to a drive unit for a measuring device, in particular a drive circuit for a capacitance type transducer used for an ultrasound probe of an ultrasonograph, and a drive method therefor.
2. Description of the Related Art
There is a diagnostic principle using a photoacoustic system as one of diagnostic principles using an ultrasonic wave. The photoacoustic system uses a photoacoustic vibration such as an ultrasonic vibration generated by instantaneous thermal expansion of a tissue which has absorbed a laser beam when the inside of a biological body is irradiated with a pulsed laser beam. The photoacoustic vibration is received on the body surface to acquire information on the inside of the biological body. For the reception of the ultrasonic vibration, an ultrasonic transducer is used. As one type of ultrasonic transducers, there is a capacitance type ultrasonic transducer. The capacitance type ultrasonic transducer is composed of, for example, a space maintained in a substantially vacuum condition, which is called “cavity”, and two electrodes provided with the cavity therebetween. One of the electrodes is provided on a thin film called “membrane” and supported so as to vibrate. The other of the electrodes is fixed onto a substrate of the capacitance type ultrasonic transducer. Hereinafter, the electrode supported so as to vibrate is also referred to as “vibrating electrode” and the electrode fixed onto the substrate is referred to as “fixed electrode”.
When the capacitance type ultrasonic transducer (hereinafter, also referred to simply as “ultrasonic transducer”) receives an ultrasonic wave, the membrane vibrates and a distance between the two electrodes changes. The change in the distance between the electrodes changes a capacitance between the two electrodes. While a voltage is applied between the two electrodes, the change in capacitance is converted into a current signal. In this configuration, a structural unit of the ultrasonic transducer composed of one cavity and two opposed electrodes is referred to as “cell”. In addition, a structural unit composed of a plurality of cells electrically connected to each other is referred to as “element”. In an ultrasonic transducer used for an ultrasonograph, normally a plurality of elements are arranged in a 1D (one-dimensional) or 2D (two-dimensional) array. A drive circuit is provided for each element, and each drive circuit is referred to as “channel”. Since a change in the distance between the electrodes changes the capacitance therebetween, the element is able to be considered to be a variable capacitor in an electric circuit.
Regarding the drive technology of the foregoing ultrasonic transducer, there is a suggestion of decreasing the number of drive circuits of the ultrasonic transducer by connecting electrodes between elements (see Japanese Patent Application Laid-Open No. 2008-022887).
In the technique described in Japanese Patent Application Laid-Open No. 2008-022887, however, a distance between a plurality of elements increases in some cases though the number of channels of the ultrasonic transducer decreases. Therefore, it is an object of the present invention to provide a technique capable of decreasing the number of drive circuits without increasing the distance between variable capacitors such as elements.
In view of the above problem, the present invention provides a drive unit for a measuring device comprising a plurality of variable capacitors as sensor elements each having first and second electrodes opposed to each other, the plurality of variable capacitors being provided with a drive circuit for each pair, wherein the first electrodes of the two variable capacitors in each pair are electrically connected to each other, and wherein the drive circuit for each pair includes a bias supply which applies two AC bias voltages to the second electrodes respectively of the two variable capacitors to produce an output signal at the first electrodes connected to each other of the two variable capacitors in each pair, the two AC bias voltages being relatively 90° out of phase with respect to each other, a multiplier which multiplies the output signal by two AC signals to produce two multiplication signals, the two AC signals being relatively 90° out of phase with respect to each other, and an integrator which integrates the two multiplication signals for each cycle of the corresponding AC bias voltages to acquire two integration signals for the two variable capacitors in each pair. The variable capacitors are, for example, elements of a capacitance type ultrasonic transducer.
Moreover, in view of the above problem, the present invention provides a drive method for a measuring device comprising a plurality of variable capacitors as sensor elements each having first and second electrodes opposed to each other, the method including the steps of electrically connecting the first electrodes of the two variable capacitors in each pair of the plurality of variable capacitors, applying two AC bias voltages to the second electrodes respectively of the two variable capacitors to produce an output signal at the first electrodes connected to each other of the two variable capacitors in the pair, the two AC voltages being relatively 90° out of phase with respect to each other, multiplying the output signal by two AC signals to produce two multiplication signals in the pair, the two AC signals being relatively 90° out of phase with respect to each other, and integrating the two multiplication signals for each cycle of the corresponding AC bias voltages to acquire two integration signals for the two variable capacitors in the pair.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
The present invention is characterized by electrically connecting the first electrodes of the two variable capacitors to each other in each pair of a plurality of variable capacitors as sensor elements of a measuring device, applying two AC bias voltages which are relatively 90° out of phase with respect to each other to the second electrodes respectively of the two variable capacitors to produce an output signal at the first electrodes connected to each other in the pair, multiplying the output signal by two AC signals which are relatively 90° out of phase with respect to each other to produce two multiplication signals in the pair, and integrating the two multiplication signals for each cycle of the corresponding AC bias voltages to acquire two integration signals for the two variable capacitors in the pair. The present invention is not limited to a drive unit and method for a capacitance type ultrasonic transducer set forth in the embodiments described later, but is also applicable to a drive unit and a drive method for any measuring device as long as the measuring device has a plurality of variable capacitors as sensor elements.
Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.
A first embodiment relates to a drive unit and method for a capacitance type ultrasonic transducer. First, description is made on an element constituting a variable capacitor of an ultrasonic transducer of this embodiment. As illustrated in
Subsequently, the operations of the bias supply 21, the bias supply 22, the element 31, and the element 32 will be described by using
Vb
1(t)=A11*sin(ω*t) (1)
In the above expression, t is time, ω is an angular velocity [rad/s], and A11 is amplitude [V].
The angular velocity ω is set to a frequency higher than a frequency at which the membrane is able to vibrate mechanically, thereby preventing the vibration of the membrane from being caused by the bias supply 21. Specifically, the frequency of the AC bias voltage is set to a frequency higher than a mechanical vibration band of the element. In this condition, the current i1(t) flowing through the wiring p21 is represented by the following expression (2):
i
1(t)=(A11/(ω*C11))*cos(ω*t) (2)
In the above expression, C11 is the capacitance of the element 31.
Vb
2(t)=A12*cos(ω*t) (3)
In the above expression, t is time, ω is an angular velocity [rad/s], and A12 is amplitude [V].
Also in this state, the angular velocity ω is set to a frequency higher than a frequency at which the membrane is able to vibrate mechanically, thereby preventing the vibration of the membrane from being caused by the bias supply 22. In this condition, the current i2(t) flowing through the wiring p22 is represented by the following expression (4):
i
2(t)=−(A12/(ω*C12))*sin(ω*t) (4)
In the above expression, C12 is the capacitance of the element 32.
Vb
1(t)=A11*sin(ω*t) (1)
The angular velocity ω is set to a frequency higher than a frequency at which the membrane is able to vibrate mechanically. In this condition, the current i1(t) flowing through the wiring p21 is represented by the following expression (5):
i
1(t)=(A11/(ω*C11(t)))*cos(ω*t) (5)
In the above expression, C11(t) is a time-varying capacitance of the element 31.
Vb
2(t)=A12*cos(ω*t) (3)
Also in this state, the angular velocity ω is set to a frequency higher than a frequency at which the membrane is able to vibrate mechanically. In this condition, the current i2(t) flowing through the wiring p22 is represented by the following expression (6):
i
2(t)=−(A12/(ω*C12(t)))*sin(ω*t) (6)
In the above expression, C12(t) is a time-varying capacitance of the element 32.
In the above expressions (5) and (6), the terms are defined as follows:
I
41(t)=(A11/(ω*C11(t)) (7)
I
42(t)=(A12/(ω*C12(t)) (8)
Then, the expressions (5) and (6) are evaluated as follows:
i
1(t)=I41(t)*cos(ω*t) (9)
i
2(t)=−I42(t)*sin(ω*t) (10)
The current i3(t) flowing through the wiring p3 is represented based on Kirchhoff's laws by the following expression:
i
3(t)=i1(t)+i2(t) (11)
The current i3(t) is defined using the expressions (9), (10), and (11) as follows:
i
3(t)=I41(t)*cos(ω*t)−I42(t)*sin(ω*t) (12)
The current-to-voltage converter 4, in which two currents flow, converts the current i1(t) and the current i2(t) to voltage V31(t) and voltage V32(t), respectively, as follows:
V
31(t)=E3*I41(t) (13)
V
32(t)−E3*I42(t) (14)
In the above expression, E3 is a current-to-voltage conversion constant [V/I].
This type of circuit is feasible using a transimpedance circuit. The value of the current-to-voltage conversion constant E3 is an arbitrary real number. The values of the transimpedance circuit and the current-to-voltage conversion constant E3 are not the essence of the present invention and therefore the description of the values is omitted here. The voltage V4(t) output from the current-to-voltage converter 4 is represented using the above expressions (12), (13), and (14) by the following expression (15):
V
4(t)=V31(t)*cos(ω*t)−V32(t)*sin(ω*t) (15)
Subsequently, the operation of the signal separator 5 will be described by using
A voltage signal represented by the expression (15) is input to the wiring p4 in
V
4(t)=V31(t)*cos(ω*t)−V32(t)*sin(ω*t) (15)
The reference signal S71 output from the reference signal generator 71 is the following voltage signal of a cosine wave:
S71=V5*cos(ω*t) (16)
In the above expression, t is time, V51 is arbitrary amplitude [V], and ω is an angular velocity [rad/s]. The value of the angular velocity ω in the expression (16) is the same as the angular velocity ω in the expression (1).
The multiplier 81 multiplies the voltage V4(t) by the reference signal S71 to generate the following signal S81:
S81=(V31(t)*cos(ω*t)−V32(t)*sin(ω*t))*(V51*cos(ω*t)) (17)
The integrator 91 integrates the signal S81 and outputs the following signal S91:
S91=(1/(V51*n))*F1 (18)
In the above expression, F1 represents a formula for integrating the signal S81 with respect to time t in the interval from time 0 to time (2 π/ω).
The value of the signal S91 in the expression (18) is an average value of the voltage V31 from time 0 to time (2 π/ω).
On the other hand, the reference signal S72 output from the reference signal generator 72 is a voltage signal of a negative sine wave. This is an AC signal which is relatively 90° out of phase with respect to the reference signal S71.
S72=−V52*sin(ω*t) (19)
In the above expression, V52 is arbitrary amplitude [V].
The multiplier 82 multiplies the voltage V4(t) by the reference signal S72 to generate the following signal S82:
S82=(V31(t)*cos(ω*t)−V32(t)*sin(ω*t))*(−V52*sin(ω*t)) (20)
The integrator 92 integrates the signal S82 and outputs the following signal S92:
S92=(1/(V52*π))*F2 (21)
In the above expression, F2 represents a formula for integrating the signal S82 with respect to time t in the interval from time 0 to time (2 π/ω).
The value of the signal S92 in the expression (21) is an average value of the voltage V32 from time 0 to time (2 π/ω).
The voltage V31 is represented by the expression (13) as follows:
V
31(t)=E3*I41(t) (13)
Then, when I41(t)=(A11/(ω*C11(t)) in the expression (7) is substituted into the expression (13), the following expression is obtained:
V
31(t)=E3*(A11/(ω*C11(t)) (22)
The current-to-voltage conversion constant E3, the amplitude A11 of the bias voltage, and the angular velocity ω of the bias voltage are known constants. When a constant F11 is defined as in the following expression (23), the expression (22) is expressed by using the expression (23) as in the following expression (24):
F
11=(E3*A11)/ω (23)
V
31(t)=F11/C11(t) (24)
Similarly, when a constant F12 is defined as in the following expression (25), an expression (26) is obtained:
F
12=(E3*Al2)/ω (25)
V
32(t)=F12/C12(t) (26)
Since the capacitance C11(t) and the capacitance C12(t) are the capacitances of the element 31 and the element 32 of
Referring to
In this device, irradiation light 24 projected from the light source 50 toward a subject 17 hits against a light absorber 51 inside the subject, thereby generating an acoustic wave 52, which is called “photoacoustic wave”, due to a photoacoustic effect. While the frequency of the acoustic wave 52 depends on the size of material and/or individual pieces constituting the light absorber 51, the frequency is on the order of 300 kHz to 10 MHz. The acoustic wave 52 passes through acoustic impedance matching material 25 with good propagation properties and is detected by a capacitance type ultrasonic transducer 53 having a drive unit of the present invention. A signal amplified in current and voltage is sent to a signal processing section 55 via a signal line 54. The detected signal is processed by the signal processing section 55 and physical information on the subject is extracted. Although the signal processing section 55 is mainly a calculator, a part of the signal processing section 55 may be an integrated circuit and it is possible to reconstruct the two- or three-dimensional image thereof. The use of the ultrasonic transducer 53 having the drive unit of the present invention enables the achievement of signals in a compact configuration. Naturally, the measuring device of the present invention is also able to be used in a subject diagnostic device which detects an acoustic wave from a subject to which an acoustic wave such as an ultrasonic wave is applied. Also in this case, the acoustic wave from the subject is detected and the converted signal is processed by a signal processing section, thereby enabling the acquisition of information on the inside of the subject.
According to the present invention, two variable capacitors, which are elements or the like of a capacitance type ultrasonic transducer, are able to be driven as one pair by one drive circuit and two signals (ultrasonic signals or the like) corresponding to the two variable capacitors of each pair, are able to be detected separately. Accordingly, the number of drive circuits, namely channels is able to be decreased without increasing the distance between variable capacitors such as elements.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2012-126095, filed Jun. 1, 2012, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
---|---|---|---|
2012-126095 | Jun 2012 | JP | national |