Apparatus for measuring a mechanical quantity

Abstract

A mechanical quantity measuring apparatus is provided which can make highly precise measurements and is not easily affected by noise even when it is supplied an electricity through electromagnetic induction or microwaves. At least a strain sensor and an amplifier, an analog / digital converter, a rectification / detection / modulation-demodulation circuit, and a communication control circuit are formed in one and the same silicon substrate. Or, the silicon substrate is also formed at its surface with a dummy resistor which has its longitudinal direction set in a particular crystal orientation and which, together with the strain sensor, forms a Wheatstone bridge. With this arrangement, even when a current flowing through the sensor is reduced, measured data is prevented from being buried in noise, allowing the sensor to operate on a small power and to measure a mechanical quantity with high precision even when it is supplied electricity through electromagnetic induction or microwaves.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to an apparatus capable of measuring a mechanical quantity and outputting a measured value wirelessly. [0002] What is generally called an RF tag has been developed which uses an electricity supplied through electromagnetic induction to active a circuit and thereby transmit a preset ID number wirelessly and which is beginning to be applied to a goods distribution management and a management of admission tickets. Attempts are currently under way to connect a physical quantity sensor to such an ID tag. For example, as disclosed in JP-A-2001-187611, a temperature sensor is connected to an RF tag circuit on a printed circuit board, and the temperature sensor mounted on the printed circuit board is then entirely molded with plastic to form an ID tag with a sensor. BRIEF SUMMARY OF THE INVENTION [0003] When a strain sensor and a mechanical quantity sensor applying the strain sensor, such as pressure sensor, vibration sensor a...

AI Technical Summary

Benefits of technology

[0005] A second problem is that the mechanical quantity sensor using a semiconductor has a larger temperature dependency of the measured value than other physical quantity sensors, so that unlike other sensors, the mechanical quantity sensor is required to perform a temperature correction. Normally, the strain sensor is combined with a dummy resistor having the same temperature dependency as the strain sensor to form a Wheatstone bridge circuit to perform the temperature correction. At this time, considerations must be taken to ensure that the dummy resistor and the sensor have the same temperatures. It is also necessary to keep the dummy circuit in the bridge circuit in a non-strained condition. For this purpose, the dummy resistor and the sensor need to be arranged separately and connected together. However, the lead wires for connection easily pick up noise, making the measurement practically impossible without special considerations as in the case with the first problem. It is therefore an object of this invention to provide a mechanical quantity measuring apparatus which is not susceptible to noise and influences of temperature and can make highly precise measurements even when an electric power is supplied to the circuit through electromagnetic induction or microwaves.

[0006] To make the apparatus resistant to influences of noise even when a power consumption by the apparatus is lowered, a strain sensor taking advantage of a piezoresistive effect and a circuit operating on an electricity supplied by electromagnetic induction or microwaves are formed in the same silicon substrate.

[0011] This invention offers an advantage that, even when a mechanical quantity sensor is operated by using a small electricity supplied through electromagnetic induction or microwaves, noise picked up by the sensor can be made very small. Since all the circuits are formed in a small area on the same silicon substrate, a current that would otherwise be induced by a phase difference of radio waves when RF feeding (supply of electricity in the form of radio wave energy) is performed can be prevented from being generated in the sensor, making it possible for a sensor to perform its intended sensing operation even when a small electric power is used. That is, when activating the circuits by using an induced current as a power source, it is essential to reduce the power consumption of the sensor. In that case, data from the sensor is not buried in noise, allowing for correct measurement.

[0012] Further, since the Wheatstone bridge circuit of the above arrangement and construction is provided in the same single crystal silicon substrate, a small change in the resistance of the mechanical quantity sensor can be compensated for. Also, a good thermal conductivity of the silicon substrate assures an accurate temperature correction and thereby improves a measurement precision. Further, since a small strain sensor and a dummy resistor are formed in the same single crystal silicon substrate, noise does not easily enter the Wheatstone bridge circuit, preventing the measured data from being buried in noise even when the current flowing in the sensor is reduced. This in turn makes for a reduction in the power consumption of the sensor.

[0013] Furthermore, since the sensor's power consumption can be reduced, this invention enables the sensor to be operated with a small energy. This means that the sensor can operate on an electric power supplied through electromagnetic induction or microwaves and also on electricity locally generated by vibrations and solar cells.

Problems solved by technology

When a strain sensor and a mechanical quantity sensor applying the strain sensor, such as pressure sensor, vibration sensor and acceleration sensor, are connected to a circuit that uses an electricity supplied through electromagnetic induction or microwaves to transmit the result of measurement, however, the following problems characteristic of the mechanical quantity sensor arise.

First, the strain sensor has a very small output for a measured strain and is very vulnerable to noise as compared with other sensors such as temperature sensor.

At this time, if any noise, even at a small level, enters the circuit, it can cause large measuring errors.

Further, when the electricity supplied by electromagnetic induction or microwaves is used, an amount of electricity that can be supplied to the strain sensor is very limited and is required to be set two or more orders of magnitude smaller than when a commonly marketed strain gauge and an amplifier are used.

Thus, if the current flowing through the strain sensor is set at a level of the order of μA, the apparatus becomes susceptible to noise, rendering the measurements practically impossible without special considerations.

Considering this condition of use, it is difficult to cover the sensor and its lead wires with a conductive material for perfect electromagnetic shield.

A second problem is that the mechanical quantity sensor using a semiconductor has a larger temperature dependency of the measured value than other physical quantity sensors, so that unlike other sensors, the mechanical quantity sensor is required to perform a temperature correction.

However, the lead wires for connection easily pick up noise, making the measurement practically impossible without special considerations as in the case with the first problem.

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