JPH0711534B2 - Piezoelectric acceleration sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acceleration sensor that detects vibration of an object, and more particularly to a piezoelectric acceleration sensor that applies a piezo effect.

2. Description of the Related Art Acceleration sensors of devices that detect the inertial velocity of an object are widely used to detect impacts acting on an object, measure vibrations, and the like.

As the acceleration sensor, various types such as an electrostatic type, a piezoelectric type, a photoelectric type, and a servo type have been proposed and put into practical use depending on the difference in operation principle and form.

Among them, the piezoelectric acceleration sensor has a simple structure,
It is one of the most widely used because of its small size, light weight and robustness.

Piezoelectric accelerometers utilize the properties of ferroelectric ceramics such as barium titanate and zirconate, or quartz, which generate an electrical signal when subjected to mechanical deformation-the piezoelectric effect. The inertial force applied to the weight is applied to the piezo element, the resulting electrical output is detected, and the inertial acceleration is obtained.

At this time, there are a method of taking out an open circuit voltage and a method of taking out a short-circuit charge as a method of taking out an electric signal from the piezo element, and one of the methods is used in a conventional piezoelectric acceleration sensor.

Problems to be Solved by the Invention The sensitivity of the piezoelectric acceleration sensor configured as described above is proportional to the open-circuit voltage generated per unit working force or the short-circuit charge. Sex matters. This is mainly due to the temperature dependence of the material constant of the piezo element, but this problem will be described below.

The following relationship is established between the force acting on the piezo element and the generated electric output, paying attention to an appropriate directional component.

Q = A · V + B · F F: Working force Q: Generated charge V: Generated voltage Here, A and B are proportional coefficients determined by the shape of the piezo element, material constants, and the like.

The ratio of F and V (i.e., open circuit voltage) at the time of the Q = 0 in the above equation: K _V, Q (i.e., short-circuit charge) when the V = 0 and F ratio: K _Q is proportional to the sensor sensitivity, respectively Will be done.
When K _V and K _Q are represented by A and B, K _V = −B / AK _Q = B Therefore, when A and B change with temperature, the sensor sensitivity also changes. At this time, among the determinants of the proportional constants A and B, the temperature dependence of the material constant that can be ignored for the shape is a problem.

Generally, the dominant material constants for the proportional constants A and B are the dielectric constant: E ₃₃ and the piezoelectric constant D ₃₁ , which are approximately connected by the following proportional relationship.

AαE ₃₃ BαD ₃₁ therefore well a typical example of the temperature dependence of _{K V αD 31 / E 33 =} -G 31 K Q αD 31 The piezoelectric constant G _31, D ₃₁ to the piezoelectric type acceleration sensor The piezoelectric ceramics used are shown in FIG.

Therefore, when an acceleration sensor is constructed using this piezoelectric material, its sensitivity is as shown in FIG. 3 (a) when a method for extracting an open circuit voltage is used, or a method for extracting a short circuit charge is used. When used, it has temperature dependence as shown in FIG. 3 (b).

And such temperature dependence-that is, D ₃₁ and G ₃₁ have temperature dependences, respectively, and their inclinations are opposite to each other, is not peculiar to the example given here, and the difference in degree is not. That is true for piezoelectric materials in general.

It is an object of the present invention to provide a highly accurate acceleration sensor with such sensitivity having little temperature dependency.

Means for Solving the Problems In order to solve the above problems, the present invention has a bimorph structure in which two piezoelectric elements are bonded together so that one electrode surface is mechanically coupled to each other and electrically connected. Using elements,
The above-mentioned object is achieved by taking out an electric signal proportional to the open circuit voltage from one piezo element, taking out an electric signal proportional to the short circuit charge from the other piezo element, and adding them.

Operation According to the above means, it is possible to cancel the temperature dependence of the material constant and realize a highly accurate acceleration sensor with a small change in sensitivity with respect to a temperature change.

Embodiment One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the piezoelectric type acceleration sensor of the present invention.

In FIG. 1, 1a and 1b are a pair of piezoelectric elements having the same shape and material with electrodes formed on both sides, and 2 is mechanically coupled to one surface of each of the two piezoelectric elements and electrically connected. An electrically conductive metal plate forms a bimorph structural element as a whole, and the bimorph structural element is supported by a fixing member 3 to form a cantilever structure.

Further, 4 is a voltage amplifier that generates an output proportional to the open-circuit voltage of the piezo element 1a, 5 is a charge amplifier that generates an output proportional to the short circuit charge of the piezo element 1b, and the voltage amplifier 4 The outputs of the charge amplifiers are added by weighting of Ka and Kb by the adder of 6.

When an inertial acceleration occurs in the fixing member 3 with such a configuration, an inertial force as a reaction force due to the inertial acceleration acts on the bimorph element to cause bending, and the piezo elements 1a and 1b have an electric power corresponding thereto. A signal is generated, and its sensitivity has temperature dependence associated with the temperature characteristics of the piezoelectric constants G ₃₁ and D ₃₁ when taken out as an open circuit voltage and taken out as a short circuit charge.

Since this temperature dependence becomes as shown in FIG. 3, the ratio of the gain of the voltage amplifier, the gain of the charge amplifier, and the weighting of the adder is set so as to cancel out the change, so that the temperature change is suppressed. The change in sensitivity
It can be much smaller than if the voltage or charge amplifier were used alone.

For example, when the piezoelectric ceramics given as an example in the description of the conventional technique is used, the absolute value of the slope is about 1.5 times larger in D ₃₁ at around room temperature, so charge amplification was performed at around room temperature. If the sensitivity ratio is set to 1.5: 1, the temperature dependence of the combined sensitivity is improved as shown in Fig. 4, and it becomes almost flat near room temperature.

At this time, since the piezo elements 1a and 1b both form a part of the bimorph structure, the response from the acting force to the flexure is completely the same, and the same shape and material are used. There is no inconvenience caused by the mismatch of the responses of both, which is a problem when they are independently provided and each output is subjected to voltage detection, charge detection and addition.

In addition, when compared with a system in which a special temperature detecting means is provided and the gain is corrected based on this information, not only the structure is much simpler, but the two adjacent metal plates having a very high thermal conductivity are provided. Since the two piezo elements complementarily correct the temperature dependence, a correction error caused by a temperature error between the acceleration response unit and the temperature detection unit does not occur.

The above is a description of the configuration and operation of one embodiment of the present invention. A specific example of the circuit portion will be described in detail with reference to the circuit diagram of FIG.

In FIG. 2, 1a, 1b, and 4-6 are as described in FIG. 1, and a specific example of 4-6 is shown as a circuit diagram.

Reference numeral 4 is a voltage amplifier for extracting the electric output generated by the piezo element 1a as an open circuit voltage. An impedance for extracting voltage information from the piezo element having a capacitive high impedance output characteristic, converting it to low impedance, and making it easy to process. It works as a converter.

Here, R ₁ is a bias resistor for biasing the positive input of the operational amplifier A ₁ and is indispensable for operation, but since it becomes a factor that hinders the low-frequency characteristics of the voltage amplifier 4, there is no problem with output offset and noise. It is desirable to use as large a value as possible within the range, and in order to do so, consideration must be given to using a low input bias current (for example, FET input type) for the operational amplifier A ₁ .

Reference numeral 5 is a charge amplifier for taking out the electric output generated by the piezo element 1b as a short-circuit charge, and it functions as a charge-voltage converter that generates a voltage signal proportional to the amount of charge generated by the piezo element 1b.

More specifically, the negative feedback action of the operational amplifier A ₂ charges (integrates) a current equal to the short-circuit current (differentiation of the short-circuit charge) into the capacitor C and extracts the terminal voltage thereof → restores the short-circuit charge by the integration operation. Then, a voltage output proportional to that is obtained.

Here, R ₂ is for biasing the negative input of the operational amplifier A _{2 in} the same way as R ₁ in the voltage amplifier 4, and it is also a factor that hinders the low-frequency characteristic, so the above consideration is required. .

Reference numeral 6 is an adder for adding the outputs of the voltage amplifier 4 and the charge amplifier 5 with predetermined weighting, and amplifies the outputs with gains R ₃ / Ra and R ₃ / Ra, respectively, and then linearly synthesizes the output. (Therefore, the weighting ratio is 1 / Ra: 1 / Rb).

In this embodiment, the bimorph has a structure in which a metal plate is sandwiched between piezo elements, but this is merely for the purpose of mechanical reinforcement and electrical terminal drawing, and if there is no problem with these two, There is no problem even if the structure is such that the piezo element is directly attached.

Further, although the cantilever structure is used in this embodiment, the present invention is not limited to this and can be applied to all structures generally used as a piezoelectric acceleration sensor.

EFFECTS OF THE INVENTION As will be apparent from the detailed description above, the piezoelectric acceleration sensor of the present invention uses a pair of piezo elements having a bimorph structure, and extracts an electrical signal proportional to the open circuit voltage from one piezo element, Since the electric signal proportional to the short-circuit charge is taken out from the other piezo element and this is added, the temperature change of the sensitivity is offset by canceling the temperature change of the material characteristic peculiar to the piezo element. It is possible to realize a highly accurate characteristic with a small value.

[Brief description of drawings]

FIG. 1 is a perspective view showing a bimorph element of an embodiment of the present invention, FIG. 2 is a circuit diagram showing a circuit configuration of the embodiment, and FIG. 3 is a typical example of temperature dependence of material constant of piezoelectric ceramics. The graph shown in FIG. 4 is a graph showing the temperature dependence of sensitivity in one example of the present invention. 1a, 1b ... Piezo element, 2 ... Metal plate, 3 ... Fixing member, 4 ... Voltage amplifier, 5 ... Charge amplifier, 6 ... Adder.

Claims (3)

[Claims] 1. A bimorph structure element in which two piezo elements each having electrodes formed on both surfaces thereof are bonded to each other so that one electrode surface is mechanically coupled and electrically connected to each other; A mass element for applying an inertial force according to the acceleration to be detected to cause bending, a voltage amplifier that produces an output proportional to the open circuit voltage of one piezo element, and an output proportional to the short-circuit charge of the other piezo element. A piezoelectric acceleration sensor, comprising: a charge amplifier that generates a charge; and an addition unit that adds the output of the voltage amplifier and the output of the charge amplifier. 2. The bimorph element has a structure in which two piezo elements are bonded together via a metal plate,
The piezoelectric acceleration sensor according to claim (1). 3. The mass element for applying an inertial force according to the acceleration to be detected to the bimorph element to cause the bimorph element to bend is the bimorph element itself, according to claim (1). Piezoelectric acceleration sensor according to the item.