JPH0477860B2 - - Google Patents

Description

[Detailed description of the invention] (b) Industrial application fields The present invention relates to an infrared detection device.

(b) Prior Art An infrared detection device is often used as a means for converting the amount of infrared rays emitted from a measured object into an amount of electricity in order to measure the temperature of the object.
As is well known, infrared detection devices require an optical chopper. That is, the optical chopper intermittently injects the infrared rays to be measured into an infrared detecting element made of a crystal or the like to give a heat pulse corresponding to the intensity of the infrared rays.

This type of optical chopper according to the prior art, as seen in Japanese Patent Application Laid-Open No. 55-97612, places a disk having a cutout such as a hole in front of an infrared detecting element, and rotates the disk at a constant speed with a motor. It is something. Therefore, this structure is not suitable for downsizing the device.

JP-A-58-32131 discloses a new type of optical chopper aimed at downsizing infrared detection devices. The structure of this chopper uses a bimorph that vibrates in response to an electric signal to vibrate the shutter and interrupt the incident infrared rays.

In order to perform accurate infrared detection in such a device, the shutter must be mounted with extremely high precision, but this is extremely difficult.

(C) Purpose of the Invention The present invention provides an optical chopper consisting of a bimorph and a shutter driven by the bimorph to reduce the size of an infrared detection device without making the mounting accuracy of the shutter extremely high. The present invention provides a device that can perform infrared detection with high precision.

(d) Structure of the invention The infrared detection device of the present invention includes an infrared detection element,
An optical chopper for intermittently injecting the infrared rays to be measured into the element, and a drive section for driving the chopper, the chopper comprising a bimorph and a slit attached to the bimorph to perform chopping action by displacement of the bimorph. and a shutter section having a shutter section, the drive section applies a drive voltage for displacing the bimorph to one electrode terminal of the bimorph, and when the drive voltage is applied to the other electrode terminal, the infrared rays to be measured are emitted. The present invention is characterized in that a reference voltage is applied to adjust the position of the shutter portion to a position where the amount of passage becomes a predetermined amount.

(E) Embodiment FIG. 1 shows an infrared detector used in the device of this embodiment. The exterior of this detector is composed of a substrate 1 and an external shield case 2 fitted to the substrate, and the infrared rays to be measured enter through a circular hole window 3 provided on the top surface of the external shield case 2. The circular hole window 3 is usually covered with a silicon plate 4. A pyroelectric infrared detection element 5 made of lithium tantalate crystal is arranged on the substrate 1 directly under the circular hole window 3, and an internal shield case 6 surrounds this element. Infrared rays incident through the circular hole window 3 reach the element 5 through a small hole 6a provided in the upper surface of the internal shield case 6.

Such an infrared incident path is interrupted within the case 2 by an optical chopper. This chopper consists of a shutter section 7 disposed close to the small hole 6a of the internal shield case 6, and first and second bimorphs 8a and 8b that move the shutter section. The first and second bimorphs 8a and 8b are
Their proximal ends are fixed by an insulating support 9 fixed to the substrate 1, and they face each other in parallel. The shutter section 7 has a plurality of slits 10 and is composed of first and second slit plates 7a and 7b that face each other in close proximity. The first and second slit plates 7a and 7b are fixed to the free ends of the first and second bimorphs 8a and 8b, respectively, and are moved in planes parallel to each other by displacement of each bimorph.

The slits of the first slit plate 7a and the slits of the second slit plate 7b are in a substantially overlapping alignment relationship,
Its details are shown in FIG. That is, the slits are all arranged at the same pitch P, but the slits 10a of the first slit plate 7a and the slits 10a of the second slit plate 7a are arranged at the same pitch P.
The slit 10b of the slit plate 7b is offset from each other by 1/4P when the first and second bimorphs 8a, 8b are not driven. Therefore, in the figure, when the first slit plate 7a moves to the right and the second slit plate 7b moves to the left by 1/8P, the first,
Each slit 10a of the second slit plate 7a, 7b,
10b are perfectly aligned, and the shutter portion 7 is in a fully open state, and by moving in the opposite direction by 1/8P, the shutter portion 7 is in a fully closed state.

The shutter section 7 is periodically opened and closed by a drive circuit to be described later, and as a result, as is well known, the element 5 outputs a signal corresponding to the relative temperature difference between the shutter section 7 and the object to be measured. do. 1st, 2nd
Diode 1 placed between bimorphs 8a and 8b
1 approximately detects the temperature of the shutter section 7 by utilizing its temperature characteristics. This detected temperature is used to correct the relative temperature difference outputted from the element 5.

In order to achieve the above-mentioned periodic chopping action in the shutter section 7, the first and second bimorphs 8
a and 8b must have an appropriate structure and be driven with an appropriate signal waveform. FIG. 3A shows the structure of the first and second bimorphs 8a and 8b, and FIG. 3B shows the signal waveforms for driving them. Each bimorph has the same structure, with a central electrode 20 made of a thin metal support plate such as Kovar or phosphor bronze.
and the first layer of PZT etc. pasted on both sides.
Terminal a consisting of second piezoelectric bodies 21 and 22 and first and second electrodes 23 and 24 provided in a film shape on the surface of these piezoelectric bodies, and connected to the center electrode 20 of the first and second bimorphs 8a and 8b. and the first and second electrodes 23,
When a voltage is applied between terminal b connected to 24,
Each bimorph 8a, 8b is polarized so as to bend in opposite directions.

If the amount of displacement of the bimorph with respect to the applied voltage is y, then there is the following relationship: y=3·l ² /(2t) ² ·d ₃₁ ·V.

Here, t: Thickness of the piezoelectric body l: Free length d ₃₁ : Piezoelectric constant V: Applied voltage Therefore, if you want to obtain a large amount of displacement with a bimorph with a certain piezoelectric constant, do you have to increase the free length? However, the only option is to make the piezoelectric material thinner or to increase the applied voltage. Making the piezoelectric material extremely thin reduces its mechanical strength and is difficult to manufacture. Furthermore, if the free length is increased, the size becomes large, which goes against the goal of miniaturization. Therefore, the applied voltage must be increased.

In this example, the free length l of the bimorph =
1.25 cm, piezoelectric constant d ₃₁ = 260×10 ^-10 cm/V, thickness t of piezoelectric body = 0.02 cm, and pitch P of each slit in the first and second slit plates 7a and 7b = 400 μ.
Assume that the free end of each bimorph must be displaced with an amplitude of 50 μm on one side, that is, a total of 100 μm on both sides, as described above, in order for the shutter section 7 to assume the fully open and fully closed states. , an applied voltage (V _PP ) with an amplitude of approximately 130V is required.
Figure 4 shows the applied voltage at a constant amount of incident infrared rays.
The relationship between V _PP and infrared detection output voltage is shown using actual measured values. Note that the driving frequency of the bimorph in this case is 3 Hz.

In this way, it is possible to determine the applied voltage V _PP so that the shutter section 7 is fully open and fully closed, but if we take into account the deformation error due to the temperature of each bimorph, it is possible to set the applied voltage V _PP to 60V. It is preferable to vibrate each bimorph with a relatively small amplitude. The details of this reason are described in the invention of Japanese Patent Application No. 58-218235. Therefore, in this embodiment as well, the applied voltage V _PP is determined to be 60V.

The drive waveform shown in FIG. 3B is the voltage waveform applied to terminal b in FIG. 3A in consideration of the above.
Note that terminal a is used as a reference potential. As shown in the figure, the driving waveform is an alternating square wave, but in this embodiment, consideration is given to rounding the shoulders of the positive rising portion and the negative rising portion of the alternating square wave. ing. As a result of adopting this rising waveform, the abnormal bimorph noise that conventionally occurs when driving with a complete square wave as shown by the dotted line in the figure is eliminated. If the time constant of the rising waveform is too long, the infrared detection output will decrease, so it is preferably set shorter than the thermal time constant τ _t of the infrared detection element 5.

FIG. 5 shows a circuit of a microwave oven of this embodiment incorporating the above-mentioned infrared detector. Power terminals 30, 31
Commercial power applied to the microwave oven is supplied to a microwave oscillation circuit 34 via a door switch 32 and a control switch 33 of the microwave oven. The microwave oscillation circuit 34 includes a high voltage transformer 35, a voltage doubler rectifier 36, and a magnetron 37. The commercial power is also supplied to the cooling fan motor 38 of the magnetron 37 in the same way.

The control switch 33 is composed of a bidirectional thyristor, and its gate signal is given from the microprocessor 40 via the interface 41. The microprocessor 40 performs timer operation and temperature operation in response to commands input from the keyboard 43 via the interface 42 . That is, in the case of timer operation, the control switch 33 is turned on for the timer time input from the keyboard 43, while in the case of temperature operation, the infrared detection device 50 is operated, and the infrared rays from the food that enter the infrared detector 51 are turned on. The food temperature is determined based on the amount, and it is determined by the keyboard 4.
The control switch 33 is turned on until the finishing temperature input in advance from step 3 is reached.

A DC voltage generator 45 connected to the power supply terminals 30 and 31 via a step-down transformer 44 supplies operating voltage sources to the microprocessor 40 and the infrared detection device 50, respectively.

The configuration and operation of the infrared detection device 50 will be explained below. When driving the device 50, the microprocessor 4
0 through the interface 41, the signal A,
Signal B is given. The signal A is a 3 Hz pulse train as shown in FIG. 6A, which causes the first transistor 53 to repeatedly turn on and off at 3 Hz. The signal B is at a constant level as shown in FIG. 6B, so that the second transistor 54 is turned on continuously, and the DC-DC converter 55 is activated by its emitter output. The DC-DC converter 55 is -
E Outputs a DC voltage of ₀ V (-60V). Therefore, the volume 56 is connected to the terminal a of the first and second bimorphs.
A constant reference potential -ESV (approximately -30V) is applied to the terminal b, as shown in FIG. 6c, when the first transistor 53 is off, -E ₀ V, and when it is on, 0V is added to each, and as a result, as mentioned above,
Each bimorph vibrates at a frequency of 3Hz.

Now, the capacitance of the first and second bimorphs are both
C _BM , and considering it from an AC perspective, when the first transistor 53 is off, the equivalent circuit shown in FIG. 7A is as follows.
When the CR time constant circuit of resistors R1, r2, R3 and _CBM is turned on again, the first transistor 53 is turned on, as shown in FIG.
As shown in the equivalent circuit shown in _FIG .
As shown in FIG. 6C, the shoulders of the positive and negative rising portions of the drive waveform are rounded. In this example, C _BM =
10nF, R1=330KΩ, R2=R3=100KΩ, r
1+r2=500KΩ, so that the rise time constant of the above waveform is 5 to 8 msec. Note that the thermal time constant τ _t of the infrared sensing element 5 is 15 to 30 msec.

Referring again to FIG. 5, terminal a is connected to volume 56.
As a result, the reference potential -EsV can be adjusted. This is advantageous because it means that the misalignment of the first and second slit plates 7a, 7b constituting the shutter section 7 during assembly can be substantially compensated for electrically. As compensation work, the fourth
While performing measurements as shown in the figure, the volume 56 may be adjusted so that the infrared detection output is maximized.

The output of the infrared sensing element 5 and the output of the diode 11 enter a processing circuit 57. In this circuit, the output of element 5 is AC amplified, passed through a bandpass filter with a center frequency of 3 Hz, synchronously rectified with signal A, and then DC amplified to obtain a DC value, which is then passed through a diode 11. After correction based on the output, it passes through an A/D converter 58 to obtain a digital quantity corresponding to the food temperature. This digital quantity enters the microprocessor 40 via an interface 41.

(F) Effects of the Invention According to the present invention, since the optical chopper is composed of a bimorph and a shutter driven by the bimorph, it is possible to reduce the size of the infrared detection device, and to improve the mounting accuracy of the shutter. can be electrically compensated for, making it easy to improve infrared detection accuracy.

[Brief explanation of drawings]

The figures show an embodiment of the present invention; FIG. 1A is an exploded perspective view of an infrared detector, FIG. 1B is a cross-sectional view of the same, FIG. Diagram A
are side views of the first and second bimorphs, Fig. 3B is a drive waveform diagram, Fig. 4 is an infrared detection output characteristic diagram, and Fig. 5 is a side view of the first and second bimorphs.
The figure is a microwave oven circuit diagram, FIG. 6 is a signal waveform diagram, and FIG. 7 is an equivalent circuit diagram. 5... Infrared detection element, 7... Shutter part, 8a,
8b...first and second bimorph.

Claims (1)

[Claims] 1 An infrared detecting element, an optical chopper for intermittently injecting infrared rays to be measured into the element, and a drive unit for driving the chopper, the chopper being attached to a bimorph and the bimorph, and the chopper being attached to the bimorph and causing the chopper to be moved by displacement of the bimorph. and a shutter section provided with a slit that acts when the driving section applies a driving voltage for displacing the bimorph to one electrode terminal of the bimorph, and when the driving voltage is applied to the other electrode terminal of the bimorph. An infrared detection device characterized in that a reference voltage is applied to adjust the position of the shutter section to a position where the amount of the measured infrared rays passes through a predetermined amount.