JPH04164217A - Pyroelectric type infra-red radiation sensor - Google Patents

Abstract

PURPOSE:To improve the reliability of a sensor by damping the internal noise of a sensor element by means of a filter and by curtailing unnecessary frequency components by means of a band-pass filter after amplifying the signal. CONSTITUTION:While a simulated static human body exists, a composite waveform of internal noise and detected signal of a low frequency is output from a pyroelectric infra-red radiation sensor element 1, the internal noise component of the element 1 being damped in a filter 2. In the next step, the output signal is amplified by a high gain amplifying circuit 3, and the drift included in the output signal being eliminated by a band-pass filter 4. In addition, the signals whose signal strength is lower than a certain level are damped and the signals whose signal strength is higher than the level are amplified by a non-linear type amplifier 5. The signals thus processed are input in a window comparator 6 to be judged whether or not the absolute value of the input signal exceeds the positive/negative reference voltage of the comparator. The output of the comparator 6 is input in a timer 7, which is made to improve the responsibility of a titled pyroelectric infrared sensor to the variation of the infra-red radiation from a visual field of the sensor.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a passive infrared sensor for detecting a heat dissipating body such as a human body. Use as an electric infrared sensor.

[Conventional technology]

Pyroelectric infrared sensors have been known for a long time. For example, as shown in Figure 4, a sensor element (a) is installed at the focal point of a concave mirror (b), and when a human body is detected, the output signal is sent to an amplifier circuit (ha). ), voltage comparator circuit (d), and output drive circuit (e) to perform detection output through an intrusion detection device. It is known that the sensor elements used in such detection devices have a characteristic that only when there is a change in the amount of infrared radiation incident from the field of view, an output corresponding to that change appears. .

In other words, when a person enters or exits the field of view of the sensor, a large change occurs in the amount of incident infrared rays on the element, and a signal with a large level is output. However, when there is no change in the amount of incident infrared rays, this output becomes equal to the internal noise level of the element. It will drop to.

Therefore, a large output temporarily appears only when a person enters or exits the sensor field of view or when the sensor is moving. Due to these properties, pyroelectric infrared sensor elements are said to have differential output characteristics.

[Problem to be solved by the invention]

In the conventional example, the above-mentioned operation was performed, so an output always appears when a person crosses the field of view of the sensor. However, after a person enters the sensor field of view, breathing,
For a human body that is still and does not move, although it is performing life activities such as blinking and heartbeat (hereinafter referred to as a "pseudo-stationary human body"), the output of the sensor element is temporarily outputted after outputting a large signal. Eventually, the noise level will be reached, and after that, unless there is a large movement, it will not be possible to determine whether a person is present or not. Therefore, a negative result will be output even though there is a person within the field of view.

By the way, the output of a known infrared sensor element is always 0.
It has been confirmed that internal noise centered around a frequency of about 5 Hz to 2 Hz is included.

On the other hand, the pseudo-stationary human body within the field of view also has microscopic movement, and the amount of infrared rays it emits is not strictly constant.
Fluctuations occur, such as constantly changing minute amounts, but these slight signals are also superimposed on the above noise and output.The signal band is centered around 0.05Hz to 0.3Hz, and internal noise Although they are very close to each other, there is a slight difference. However, the detected signal level is buried in the level of internal noise and the S/N ratio is very low, so it cannot be detected as is. Conventionally, such signals Influenced by the aspects of low level and high noise level,
Although continuous output could actually be obtained, it was only possible to detect cases where the output signal was large, such as a moving human body. In this way, infrared sensors were conventionally thought to have the properties of differential sensors;
This device was limited to use in alarm devices that issue an alarm by driving a buzzer or the like based on the initial output, or to only detect when a person passes through the field of view, but it also has a function that can detect a pseudo-stationary human body. It is requested to have both.

The present invention solves the above-mentioned conventional problems, and provides a pyroelectric infrared sensor that reliably detects an infrared radiator such as a pseudo-stationary human body with a relatively simple circuit configuration. The purpose is to

[Means to solve the problem]

In order to achieve the above-mentioned purpose, the inventor discovered that the sensor element also responds to minute changes in the amount of incident infrared rays due to fluctuations from a pseudo-stationary human body, and that the sensor element responds to minute changes in the amount of incident infrared rays due to fluctuations from a pseudo-stationary human body. Focusing on the fact that the output signal actually exists and that there is a difference in frequency band, we used a means to extract this signal independently as a detection signal. In other words, it has a circuit configuration into which an output signal from a pyroelectric infrared sensor element is input, and includes a filter for passing a frequency lower than the internal noise of the sensor element to reduce the noise regardless of the presence or absence of a detection signal, and this filter. We decided to use a technical means that includes an amplifier circuit with an extremely high amplification factor that amplifies the output, and a bandpass filter that passes frequency components that are lower than the internal noise and above a certain lower limit.

[For production]

First, we focused on the fact that there is a frequency difference between the internal noise and the detection signal in the filter, and since both have extremely low frequencies and the detection signal is even lower, we developed a filter that functions as a low-pass filter to cut out the internal noise. to be carried out. Furthermore, since the filter output is at a very low level, it is amplified by a high gain amplifier circuit. That is, the amplifier circuit performs the function of low frequency amplification to obtain a signal level more suitable for subsequent signal processing. and,
The bandpass filter removes ultra-low frequency drift caused by low-frequency amplification, and also emphasizes the signal to be detected, creating a signal with a much improved signal-to-noise ratio compared to the output signal of the sensor element itself. will be generated.

〔Example〕

Hereinafter, one embodiment of the sensor of the present invention will be described in detail with reference to the attached drawings. In FIG. 1, 1 is a pyroelectric infrared sensor element;
3 is an amplifier circuit for amplifying the output signal from filter 2 to a signal level that is easy to process; 4 is a filter for attenuating internal noise components to a level of about 100%; 4 is an amplifier circuit for amplifying the output signal from filter 2 to a signal level that is easy to process; 5 is a nonlinear amplifier circuit, 6 is a window comparator composed of two comparators, and 7 is a timer driven by the window comparator output.

In order to extract the detection signal 6 of a pseudo-stationary human body with the above-described configuration, first, when a pseudo-stationary human body exists, the pyroelectric infrared sensor element 1 outputs a composite waveform of internal noise and a low-frequency detection signal. This is passed through filter 2. The circuit constants of the filter 2 are set so as to have attenuation characteristics that can attenuate internal noise components of the sensor element 1 as much as possible. This makes it possible to improve the S/N ratio between the necessary detection signal and internal noise.

Next, the output signal of the filter 2 is amplified by the amplifier circuit 3 to a signal level that can be easily processed in a subsequent stage.

The higher the amplification factor of this amplifier circuit 3, the better, and a high gain is required. However, since low frequency amplification is performed, although filter 2 greatly improves the S/N ratio in relation to internal noise, an output signal containing extremely low frequency drift due to amplification is generated. Therefore,
In order to remove these, the signals are passed through a bandpass filter 4 to remove the drift 7 included in the signals up to the previous stage, and then a nonlinear amplifier circuit 5 with an exponential transfer function attenuates signals below a certain signal strength. , by amplifying the signal beyond that level to make the discrimination between noise and drift components and signal components more noticeable. FIG. 2 shows waveform characteristics of input and output signals of the nonlinear amplifier circuit 5. The transfer function of the circuit 5 does not require exponential characteristics in the strict sense. What is important is to set a transfer function that further improves the S/N ratio of the output signal.

The signal processed in this manner is input to the window comparator 6, and it is determined whether the absolute value of the input signal exceeds the positive and negative reference voltages of the comparator. In other words, the signal led to the window comparator 6 is a signal that emphasizes the necessary signals, so the signal level when a pseudo-stationary human body is present is higher than the noise level when nothing is present. It's also expensive. Therefore, these are identified and output is performed only when a pseudo-stationary human body actually exists. In order to improve the responsiveness of the timer 7 at the subsequent stage, the reference voltage at the absolute value level is set so as to create a window between the two comparators 6a and 6b. When a pseudo-stationary human body exists in the field of view of the sensor element 1, a signal exceeding the window width of the reference voltage of the two comparators 5a and 5b is input in response to a minute change in the amount of incident infrared rays. Therefore, the output will be output accordingly.

On the other hand, when there is no human body in the field of view, the input signal does not exceed the window width, so the window comparator 6 does not respond and maintains a stable output voltage. Note that when a human body moves within the field of view, the output of the sensor element 1 has a signal level that far exceeds the internal noise of the element, so it can be detected regardless of the operation of the entire circuit described above.

Next, the output of the 6-indian comparator 6 is input to a timer 7, which performs an operation to improve the responsiveness of the device to changes in infrared light from the sensor field of view.

In other words, the trigger characteristic of timer 7 is that when the input is high (H), it does not react and maintains a low (L) output (see Figure 3C), and when the input becomes L, the output becomes H,
and continues to maintain H (see Figure 3). However, the timer operation does not start at this point, and when the input changes from L to H, the timer operation starts for the first time while the output remains at H (see FIG. 3C). If the input remains high for a period of time determined by the time constant, the output of the timer 7 recovers to low. On the other hand, if the input returns to L before the timer set time after the timer 7 starts its operation with the output set to H, the timer operation that has started is canceled while the output is kept at H. Then, the L of the subsequent input signal
It has a characteristic that the timer operation is restarted by the transition from to H (see FIG. 3C). By the way, as mentioned above, when a pseudo-stationary human body is detected, the output of the window comparator 6 becomes like noise that goes back and forth between H and L. However, when a pseudo-stationary human body exists in the visual field, the timer outputs H. In order to maintain the operating time,
It is necessary to set the time constant of the timer. This can be achieved by making the set time longer than the maximum period of the signal output from the window comparator 6.

Experimentally, stable output could be obtained by setting the operating time to about 10 to 20 seconds. In this way, by providing the timer characteristics as in this embodiment, the time constant of the timer can be made small, and a non-detection state in which the human body is not detected in a relatively short period of time after the human body has left the field of view can be achieved. It is possible to return.

Note that in this embodiment, due to the phase characteristics of the filter 2 and bandpass filter 4 for reducing internal noise of the sensor element 1, the initial response to a large input signal that appears when a human body enters the field of view is somewhat slow. turn into. In order to eliminate this inconvenience, the output of the sensor element 1 is divided into two, one is input into the circuit of this embodiment and the other is input into a conventionally known circuit in parallel, and the logical sum of the outputs of the respective timers is calculated. This can be easily resolved. The filters and timers described above can be implemented using conventional analog techniques or digital techniques, with the same effectiveness.

〔Effect of the invention〕

In the circuit of the present invention, the internal noise of the sensor element is attenuated by a filter, the signal is greatly amplified, and then the extra frequency components are canted by a bandpass filter, so the overall S/N ratio can be greatly improved. Therefore, even though the frequencies of the internal noise and the signal to be detected are very close to each other, highly reliable detection can be performed.

In addition, by using this sensor, it is possible to detect stationary objects such as non-living objects, but it is also possible to detect objects that are extremely difficult to detect, such as pseudo-stationary human bodies. For example, management of people's presence in the room,
The scope of use can be expanded to include office automation, home automation, etc.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the sensor of the present invention;
Figure 2 is an input/output characteristic diagram of the nonlinear amplifier circuit, Figure 3 a %
d is a time chart explaining the trigger characteristics of the timer;
FIG. 4 is a block diagram showing the application of a conventional sensor to an alarm device. In the figure, 1... pyroelectric infrared sensor element, 2... filter, 3... amplifier circuit, 4... bandpass filter, 5
...Nonlinear amplifier circuit, 6...Window comparator, 7...Timer. -1- Patent applicant: Manuma Co., Ltd. Agent: Kazuo Kohara, patent attorney, and 2 others 3 Figures

Claims (1)

[Claims] 1. A passive sensor using a pyroelectric infrared sensor element, which includes a filter that passes a frequency lower than the internal noise of the sensor element, a high-gain amplifier circuit that amplifies the output of this filter, and the internal noise A pyroelectric infrared sensor comprising a bandpass filter that passes frequency components at a lower frequency and above a certain lower limit.