JPH0351721Y2 - - Google Patents

Description

[Detailed explanation of the idea]

(a) Industrial application field The present invention relates to an infrared detection device. (b) Prior Art Figure 1 shows the principle circuit of this type of device. By passing the infrared ray 1 to be measured through the optical chopper 2, the infrared ray 1 to be measured and the infrared 4 emitted by the chopper 2 are alternately incident on the infrared sensor 3 in synchronization with the chopping period of the chopper 2. The infrared sensor 3 is made of, for example, a pyroelectric type, and outputs a signal corresponding to the difference in intensity between the two alternately incident infrared rays. This signal, of course, changes in synchronization with the chopper period, and is then amplified by an AC amplifier 5, and then passes through a bandpass filter 6 whose center pass frequency is the chopper frequency of the chopper 2, as shown in FIG. The result is a substantially sinusoidal signal A as shown in FIG. In the same figure, the solid line shows the case where the intensity of the infrared ray 1 to be measured is greater than that of the chopper infrared 4, and the broken line shows the case where the opposite relationship exists, and furthermore, the amplitude of each waveform is It corresponds to the strength difference of 4. The synchronous rectifier 7 rectifies the signal A using a synchronous pulse B synchronized with the chop cycle of the chopper 2, and as a result generates a signal C consisting of a half-wave rectified waveform of the signal A. These waveforms are shown in FIGS. 2B and 2C.
The synchronization pulse B is generated by a synchronization signal pulse generator 8 and has its waveform shaped by a waveform shaper 9. Now, assuming that the optical chopper 2 is a perforated disc chopper rotating at a constant speed, the synchronizing signal pulse generator 8 is constituted by a photointerrupter whose optical path is interrupted by such a chopper. The output signal C of the synchronous rectifier 7 is then converted into DC by an integrator 10, and further passes through a DC amplifier 11 to become a final DC signal D (FIG. 2D). The level of this signal D corresponds to the difference in intensity between the infrared ray 1 to be measured and the infrared ray 2 to be measured. Measured infrared ray 1
The intensity of signal D corresponds to the temperature of the object to be measured, and that of the chopper infrared 4 corresponds to room temperature.Therefore, the level of signal D represents the difference between the temperature of the object to be measured and the room temperature. Therefore, by correcting the room temperature from the level of the signal D, the temperature of the object to be measured can be determined. Now, in the above device, the circuit constants of the previous stage change due to temperature changes, especially due to the synchronous rectifier 7.
When the input to the synchronous rectifier 7 drifts in direct current, the effect appears as a change in the level of the final signal D, resulting in a measurement error. In order to improve this point,
By inputting the above signal A to the synchronous rectifier 7 through a coupling capacitor, it is possible to cut off the DC component that causes drift with such a capacitor.
This is shown in Publication No. 97612. However, in the case of such an improved device, it has been found that although the influence of the above-mentioned temperature drift is eliminated, the influence of other direct current components appears. This will be explained with reference to the main part specific circuit shown in FIG. In FIG. 3, the same parts as in FIG. 1 are denoted by the same numbers, and the capacitor 12 is a coupling capacitor inserted to eliminate the above-mentioned temperature drift. The FET 13 of the synchronous rectifier 7 receives a synchronous pulse B that takes two levels of O volts and -E volts.
is input and turns on when the same pulse B is O volts. As a result, the synchronous rectifier 7 outputs the waveform shown in FIG. 4 as its output signal C. In the figure, the solid line indicates the case where the intensity of the measured infrared ray 1 is greater than that of the tipper infrared 4 (that is, the case where the temperature T _B of the measured object is greater than the tipper temperature T _C ), and the dashed line indicates the opposite case. The following cases are shown. It should be noted that
The output signal C includes DC components V _DH and V _DL . The magnitude of the DC component V _DH or V _DL is proportional to the fourth power of the temperature difference T=|T _B −T _C |. The generation of such a DC component is due to the fact that in the synchronous rectifier 7, when the FET 13 is turned on, the potential at the midpoint P between the resistors 14 and 15 is forced to become O volts, and a charging current flows into the capacitor 12. considered to be a thing. Since the output signal C includes the DC component as described above, the relationship between the voltage V _D of the signal D finally generated from the DC amplifier 11 and the temperature difference T is as follows: V _D = αT ⁴ + βT ⁴ = (α+β)T Represented by ⁴ . Here, α and β are proportional coefficients for the sine wave component and DC component of the signal C, respectively. Therefore, as long as the coefficients of α and β are constant, the influence of β can be corrected by adjusting the sensitivity of the DC amplifier 11.
If the capacitance of D changes with temperature, β will change depending on the temperature, and such a change can no longer be corrected, which causes an error in the signal D. (c) Purpose of the invention The present invention uses a coupling capacitor as described above and aims to eliminate the conventional drawbacks caused by the presence of such a capacitor. (d) Structure of the invention The feature of the invention lies in the provision of a discharging resistor for discharging the charge accumulated in the coupling capacitor. (E) Example Figure 5 shows the main parts of this example, and Figures 1 and 3 show the main parts of this example.
The same parts as in the figure are given the same numbers. The feature of this embodiment is that a discharge resistor 16 is connected between one end of the coupling capacitor 12 and the ground. That is, due to the above-mentioned reason, charge is accumulated in the coupling capacitor 12, and the DC component V _DH
However, the accumulated charge is discharged through the resistor 16, and the DC component V _DH and V _DL are generated.
This allows the value of V _DL to be kept small. And this small value of the DC component means that the capacitor 1
Even if the capacitance of the DC amplifier 11 (first
The signal D outputted from FIG. 1 is accurate and contains almost no errors. Incidentally, since the resistor 16 also passes an alternating current component, if its value is made too small, the original alternating current signal becomes small, causing a problem of a decrease in the S/N ratio. Therefore, the value R of the resistor 16 is determined by the relationship between the discharge time constant τ expressed as the product of the capacitance C of the capacitor 12 and the chopper frequency of the chopper 2 (FIG. 1). That is, the value of R may be selected so that the value of signal C is maximized by a combination of C and R where τ is smaller than the half cycle of the chopper. An example of the circuit constants of this embodiment is shown in the table below.

[Table] (F) Effects of the invention According to the invention, infrared rays to be measured are input to an infrared sensor at a predetermined period, and an alternating current signal based on the output of the sensor is rectified by a synchronous pulse synchronized with the above period. In a device that converts the AC signal into a DC signal after passing through a rectifier, a coupling capacitor and a discharging resistor for discharging the accumulated charge of the capacitor are provided at a stage before the synchronous rectifier, and the AC signal is passed through the coupling capacitor. Since the circuit constant is inputted to the synchronous rectifier by the coupling capacitor, the drift due to temperature change in the circuit constant at the stage before the synchronous rectifier is removed by the coupling capacitor.
Moreover, the accumulated charge on the coupling capacitor, which would otherwise be significant as a result of synchronous rectification, is suppressed, so that an accurate DC signal can be obtained.

[Brief explanation of the drawing]

Figures 1 to 4 show conventional examples.
3 are circuit diagrams, FIGS. 2 and 4 are signal waveform diagrams, and FIG. 5 is a circuit diagram showing an embodiment of the present invention. 3...Infrared sensor, 5...AC amplifier, 6...
...Band pass filter, 7...Synchronous rectifier, 10
...Integrator, 11...DC amplifier.

Claims (1)

[Scope of utility model registration request] In a device that inputs infrared rays to be measured into an infrared sensor at a predetermined period, passes an alternating current signal based on the output of the sensor through a synchronous rectifier that rectifies it with a synchronous pulse that has the periodicity, and then converts it into a direct current signal,
Infrared detection characterized in that a coupling capacitor and a discharging resistor for discharging accumulated charge in the capacitor are provided at a stage upstream of the synchronous rectifier, and the alternating current signal is inputted to the synchronous rectifier via the coupling capacitor. Device.