JPH04225166A - Semiconductor electrostatic servo type force sensor and actuator using it, and acceleration sensor - Google Patents

Abstract

PURPOSE:To enable a posture of a movable electrode to be controlled stably and obtain a stable output characteristic by specifying so that the dimensions, shape, etc., of the movable electrode and a cantilever satisfy certain conditions. CONSTITUTION:A cantilever 2 is deformed when a servo system does not operate and fixed electrodes 3 and 4 can be displaced up and down. A difference C=C1-C2 between electrostatic capacities C1 and C2 between the electrodes 3 and 4 and a movable electrode 1 is measured by a C detection circuit 5, thus enabling a position of the electrodes 3 and 4 for the electrode 1 to be detected. Also, a pulse width modulation circuit 7 controls a duty ratio D of a pulse voltage which is applied to the electrodes 3 and 4 so that C=0 can be obtained and an inverter 8 applies an inverted pulse voltage of the pulse voltage to be applied to the electrode 3 to the electrode 4. Then, shape, dimensions, and application voltage of the electrodes 1, 3, and 4 and the lever 2 are set so that a posture where a sum of a change in electrostatic energy and change in elastic energy due to deformation of the lever 2 always becomes positive for an arbitrary fine change of the posture of the electrode 1.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optimal structure and driving method for a semiconductor electrostatic servo acceleration sensor.

0002

[Prior Art] A semiconductor electrostatic servo acceleration sensor basically consists of a movable electrode with a certain mass provided at the tip of a cantilever, and a fixed electrode formed opposite to the electrode surface with a gap of a certain size in between. Consisting of electrodes, a voltage is applied between the movable electrode and the fixed electrode to balance the inertial force exerted on the movable electrode due to acceleration, and electrostatic force is applied to the movable electrode, thereby controlling the posture of the movable electrode to remain constant regardless of acceleration. are doing.

0003

SUMMARY OF THE INVENTION In order for such a sensor to operate stably, the attitude of the movable electrode must always be stably controlled to a constant position. In the prior art, it cannot be said that sufficient consideration has been given to this point.

An object of the present invention is to provide a detection section and a driving method for a semiconductor electrostatic servo acceleration sensor that can stably control the posture of a movable electrode and provide stable output characteristics.

[0005]

[Means for solving the problem] In order to achieve the above object, the dimensions of the movable electrode and the cantilever supporting it,
It is necessary to impose conditions such that the movable electrode can maintain a stable state on the shape, the mutual positional relationship between the movable electrode and the fixed electrode, and the voltage waveform for controlling the posture of the movable electrode.

[0006]

[Operation] In the operating state of the electrostatic servo acceleration sensor, the external forces acting on the movable electrode are the inertial force due to acceleration, the electrostatic force between it and the fixed electrode, and the restoring force of the cantilever. In order for the movable electrode to maintain a stable posture, these forces must be well balanced with each other. The inertial force is determined by the mass of the movable electrode, the electrostatic force is determined by the shape of the movable electrode and the fixed electrode, the positional relationship between them, and the waveform of the applied voltage, and the restoring force of the cantilever is determined by the shape.

[0007]

Embodiments Hereinafter, embodiments of the present invention will be explained based on FIGS. 1 to 5.

FIG. 1 shows a basic configuration of a detection section and a block diagram of a signal processing circuit in a pulse width modulation (PWM) type acceleration sensor, which is a type of electrostatic servo type acceleration sensor. The detection section is composed of a movable electrode 1 formed at the tip of a cantilever 2 and upper and lower fixed electrodes 3 and 4, and is formed by microfabrication of a substrate such as a silicon semiconductor or glass. When the servo system is not operating, the cantilever deforms and the movable electrode can move up and down.

5 is a ΔC detection circuit which measures the difference ΔC=C1-C2 between the capacitances C1 and C2 between the movable electrode and the upper and lower fixed electrodes, and detects the position of the movable electrode with respect to the fixed electrode. 6
is an amplifier, 7 is a pulse width modulation circuit, and the duty ratio D of the pulse voltage applied to the fixed electrode is set so that ΔC=0, that is, the capacitance between the movable electrode and the upper fixed electrode and between the movable electrode and the lower fixed electrode are equal. Control. Reference numeral 9 denotes an inverter which applies a pulse voltage that is inverted from the pulse voltage applied to the upper fixed electrode 3 to the lower fixed electrode 4. 9 is a low-pass filter that converts the pulse voltage into DC and outputs it.

Now, consider a flat plate-shaped movable electrode that is symmetrical with respect to a certain point O, and a pair of fixed electrodes that are parallel to each other above and below the movable electrode, separated from the movable electrode by a gap. FIG. 2 is a side view of such a detection section. The size of the gap is sufficiently smaller than the width and length of the electrode, and the area of the fixed electrode is larger than that of the movable electrode. As mentioned above, when the servo system is in operation, the system is controlled so that ΔC=0. However, even if the condition ΔC=0 is imposed, the posture of the movable electrode with respect to the fixed electrode is not uniquely determined, and there remains a degree of freedom for rotation around any axis that passes through point O and is parallel to the fixed electrode surface. has been done. That is, even if the movable electrode rotates around this axis, the condition that ΔC=0 is always satisfied.

Now, consider the case where the movable electrode rotates around such an axis. For example, assuming that the surface of the movable electrode facing the fixed electrode is rectangular, if both ends of the movable electrode are rotated by Δd (Δd《d) as shown by the broken line in Figure 2, the movable electrode and either the upper or lower fixed electrode The electrostatic capacitance C (d, Δd) between the two is expressed by the following equation.

0012

[Math 1]

S: Opposing area of the electrodes l: Length of the opposing surfaces θ: Rotation angle εr: Relative dielectric constant of the medium between the electrodes ε0: Permittivity of vacuum The amplitudes V between the movable electrode and the upper and lower fixed electrodes are mutually inverted. When a pulse voltage of

[0014]

[Math 2]

[0015] On the other hand, since the movable electrode is constrained by the cantilever, the rotation of the movable electrode causes the cantilever to deform, and elastic energy is stored therein. The spring constant kθ of the cantilever when the movable electrode rotates around the axis shown in FIG. 2 is defined by the following equation.

[0016]

[Math 3]

T: Torque at which the cantilever attempts to restore its original state against rotation.At this time, the elastic energy Uc of the cantilever is expressed by the following equation.

Therefore, the energy U of the entire system is

001
9]

[Math 4]

[0020]

[Math 5]

It is expressed as follows. here,

[0022]

[Math 6]

[0023]

The system attempts to take a rotation angle θ such that the energy U becomes minimum. In Equation 4, θ of energy U
The shape of the dependence is completely different depending on the sign of the coefficient Q of θ2. That is, when Q>0, as shown in the graph on the left side of FIG. 3, U becomes a minimum at θ=0, and the system attempts to occupy a state where θ=0, that is, the movable electrode and the fixed electrode are parallel. This state is stable, and the movable electrode is always controlled to a constant posture even if disturbances such as vibrations or shocks are applied to the sensor.

On the other hand, when Q≦0, θ=0 is in a metastable state as shown in the graph on the right side of FIG. The value becomes larger and larger. That is, the movable electrode remains rotated around the axis and does not return to its original position. Therefore, the above P
In order for the WM electrostatic servo acceleration sensor to operate stably, it is necessary to design the shape of the detection section and the waveform of the applied voltage so that Q>0. That is, from Equation 6, the rotational spring constant kθ, the opposing area S of both electrodes, the size d of the gap between the movable electrode and the fixed electrode, and the amplitude V of the pulse voltage must satisfy the following relational expression.

[0026]

[Math 7]

By the way, the spring constant kθ with respect to the rotation of the movable electrode differs depending on the rotation axis selected.

For example, in the single beam type shown in FIG.
The mode of deformation of the cantilever due to rotation of the movable electrode about -A' is "deflection", and the mode of deformation due to rotation about axis B-B' is "twisting". Also, axis C
-C' is a mixture of "deflection" and "twisting". The spring constants kθ of rotation about these axes are different. Further, in the two-beam type shown in FIG. 5, the rotation around the axis AA' is accompanied by "deflection", and the rotation about the axis B-B' is accompanied by both "deflection" and "twisting".

In order for the state where the movable electrode and the fixed electrode are parallel, that is, the state where θ=0, to be stable, the left side in FIG. The shapes, dimensions, and applied voltages of the movable electrode, fixed electrode, and cantilever must be determined so that the potential diagram looks like the graph shown below.

According to this embodiment, since the posture of the movable electrode in a PWM electrostatic servo type acceleration sensor with good linearity of output characteristics can be stably controlled, acceleration can be stably measured with high precision and high sensitivity. There is.

By the way, in the electrostatic servo type acceleration sensor, since the inertial force acting on the movable electrode and the electrostatic force between the movable electrode and the fixed electrode are balanced, the acceleration measurement range can be expanded, that is, the maximum measurable acceleration αmax can be increased. In order to do this, it is necessary to increase the electrostatic force that acts on the movable electrode. Therefore, it is necessary to increase the voltage V applied between both electrodes.

On the other hand, from number 7

[0033]

[Math. 8]

[0034] Then, when V<Vth, the movable electrode and the fixed electrode are parallel to each other in a stable state, and when V≧Vth, the state is in a quasi-stable state. Therefore, in order to stably control the movable electrode, the voltage V must be lower than Vth,
Vth will determine αmax. Furthermore αma
In order to increase x, it is necessary to increase Vth. For this purpose, it is necessary to design a sensor structure with a large rotational spring constant kθ according to Equation 8. As aforementioned,
The magnitude of kθ varies depending on the rotation axis selected. In reality, the stability of the movable electrode is determined by the minimum value of kθ for various rotational axes, so in order to increase αmax, the detector structure must have a large minimum value of kθ. For example, 1 in Figure 4
For this beam type and the two-beam type shown in Fig. 5, if kθ about axis B-B' is minimum in each case, and the total width of the beam (in the case of two beams, the sum of each beam width) is equal. , that of two beams is larger than that of one beam. Therefore, the two-beam type has a larger acceleration measurement range. however,
If the spring constant k for displacement in the direction perpendicular to the movable electrode surface is too large, it will have various adverse effects on the sensor characteristics, so the spring constant kθ for rotation must be increased by a method that does not increase k too much. Therefore, as shown in FIGS. 6 and 7, for example, if the length of the cantilever is made as long as possible, preferably at least 50% of the diameter of the movable electrode, and the movable electrode is supported from the surroundings, the spring for displacement in the direction perpendicular to the surface of the movable electrode can be prevented. The minimum value of the rotational spring constant kθ can be increased without increasing the constant k too much, and the acceleration measurement range can be expanded without impairing sensor characteristics.

[0035] For an electrostatic servo type acceleration sensor that has a movable electrode, a fixed electrode, and a cantilever in a general shape, the attitude of the movable electrode can be stably controlled and stable output characteristics can be obtained. The movable electrode is arranged so that for any minute change in the posture of the movable electrode, there is a posture in which the sum of the change in electrostatic energy and the change in elastic energy due to the deformation of the cantilever is always positive.
It is necessary to determine the shape, dimensions, and applied voltage of the fixed electrode and cantilever.

The present invention is not particularly limited to the pulse width modulation method, but can also be applied to electrostatic servo type acceleration sensors of other methods. At that time, the electrostatic energy between the movable electrode and the fixed electrode must be calculated according to the applied voltage waveform.

Furthermore, the present invention can be applied to a sensor that measures not only inertial force due to acceleration but also other external forces acting on the movable electrode. For example, if a part of the flat plate constituting the movable electrode is made of a magnetic material, the magnetic force received from the magnetic field can be measured, and the movable electrode can be used as a magnetic sensor.

Furthermore, if other parts are formed on a part of the flat plate constituting the movable electrode, an electrostatic servo type actuator can be created in which the position of the part is controlled by the voltage applied between both electrodes. The invention is also applicable to such actuators.

[0039]

According to the present invention, since the posture of the movable electrode can be stably controlled to always be in a constant position, there is an effect that acceleration and other external forces acting on the movable electrode can be stably measured. Furthermore, there is an effect that stable position control is possible as an actuator for electrostatic positioning.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of a pulse width modulation electrostatic servo type acceleration sensor.

FIG. 2 is a diagram for explaining changes in the posture of a movable electrode.

FIG. 3 is a diagram showing changes in energy in the movable electrode-fixed electrode-cantilever system with respect to changes in the posture of the movable electrode.

FIG. 4 is a plan view of the movable electrode-cantilever portion for explaining the rotation axis of the movable electrode when there is only one cantilever.

FIG. 5 is a plan view of the movable electrode-cantilever portion for explaining the rotation axis of the movable electrode when there are two cantilevers.

FIG. 6 is a diagram (part 1) showing a movable electrode supported by four cantilevers.

FIG. 7 is a diagram (part 2) showing a movable electrode supported by four cantilevers.

[Explanation of symbols]

1... Movable electrode, 2... Cantilever, 3... Fixed electrode, 4...
Fixed electrode, 5... ΔC detection circuit, 6... amplifier, 7... pulse width modulation circuit, 8... inverter, 9... low pass filter.

Claims (9)

[Claims] Claim 1: A movable electrode having a constant mass and whose entire surface or at least a portion of the surface is made of a conductor, one or more cantilevers supporting the movable electrode, which faces the movable electrode surface and has a gap formed therein. It consists of at least one fixed electrode placed apart from each other, and a voltage is applied between the movable electrode and the fixed electrode, so that the electrostatic force acting on the movable electrode, the inertial force due to acceleration, and the restoring force due to the deformation of the cantilever are balanced, independent of acceleration. In a sensor that controls electrostatic force so that the mutual positional relationship between a movable electrode and a fixed electrode is always constant, and calculates acceleration from the controlled amount, the movable electrode - The shape and dimensions of the movable electrode, fixed electrode, and cantilever, and the size of the gap between the two electrodes so that the sum of the change in electrostatic energy between the fixed electrodes and the change in the elastic energy of the cantilever is always positive. 1. A semiconductor electrostatic servo acceleration sensor, which is characterized by setting the speed and applied voltage. 2. A movable electrode having a constant mass and whose entire surface or at least a portion of the surface is made of a conductor, and one or more cantilevers supporting the movable electrode, which face the movable electrode surface and are separated by a gap. A voltage is applied between the movable electrode and the fixed electrode, and the electrostatic force acting on the movable electrode and the external force and the restoring force due to the deformation of the cantilever are balanced, and the movable electrode is fixed without depending on the external force. In a sensor that controls the electrostatic force so that the mutual positional relationship between the electrodes is always constant, and determines the external force from the controlled amount, the change between the movable electrode and the fixed electrode due to any minute change in the posture of the movable electrode. The shapes and dimensions of the movable electrode, fixed electrode, and cantilever, the size of the gap between the two electrodes, and the applied voltage are adjusted so that the sum of the change in the electrostatic energy of the cantilever and the change in the elastic energy of the cantilever is always positive. A semiconductor electrostatic servo type force sensor characterized by the following settings. 3. A movable electrode having a constant mass and having at least a portion of its surface made of a conductor, and one or more cantilevers supporting the movable electrode, which face the movable electrode surface and are separated by a gap. A voltage is applied between the movable electrode and the fixed electrode, and the electrostatic force acting on the movable electrode is balanced by some external force and the restoring force due to the deformation of the cantilever, so that the movable electrode is independent of acceleration. In an actuator that controls electrostatic force so that the mutual positional relationship between fixed electrodes is always constant, and controls the position of a movable electrode by the applied voltage, the movement corresponding to any minute change in the posture of the movable electrode is controlled. The shapes and dimensions of the movable electrode, fixed electrode, and cantilever, and the gap between the two electrodes are carefully selected so that the sum of the change in electrostatic energy between the electrode and the fixed electrode and the change in the elastic energy of the cantilever is always positive. A semiconductor electrostatic servo actuator characterized by setting the size and applied voltage. 4. A semiconductor electrostatic servo type acceleration sensor according to claim 1, characterized in that a pulse width modulation method is used as the electrostatic servo type acceleration sensor. 5. The semiconductor electrostatic servo force sensor according to claim 2, characterized in that the electrostatic servo force sensor uses a pulse width modulation method. 6. The semiconductor electrostatic servo actuator according to claim 3, characterized in that a pulse width modulation method is used as the electrostatic servo actuator. 7. In claims 1 and 4, the movable electrode has a flat plate shape and is supported from all sides by cantilevers parallel to the plate surface, and the length of the cantilever is 50% of the diameter of the movable electrode. % or more. 8. In claims 2 and 5, the movable electrode has a flat plate shape and is supported from all sides by cantilevers parallel to the plate surface, and the length of the cantilever is 50% of the diameter of the movable electrode. % or more. 9. In claims 3 and 6, the movable electrode has a flat plate shape and is supported by cantilevers from all sides parallel to the plate surface, and the length of the cantilever is 50% of the diameter of the movable electrode. % or more.