Device for operating passive infrared sensors

Abstract

Device for operating a passive infrared detector (PIR) wherein - such PIR detectors are two-terminal devices and can be symbolized in the equivalent circuit diagram by a current source that carries a current I PIR depending on the change in irradiation, and which has a capacity C PIR is connected in parallel, whereby - the passive infrared detector (PIR) is equipped with two electrical connections and - at least one output of the passive infrared detector (IP, IN) controls at least one current source (I2) and - this power source (I2) current into a first node (S b ) feeds in and - this node (S b ) via an integrating filter (C 1b ) with the input of a comparator (CP) b ) is connected and - the output of this comparator (CP) b ) at least one digital integrating filter (Int b) directly or indirectly controls and - the output of this digital integrating filter drives at least one digital-to-analog converter (DAC) and - the output of this digital-to-analog converter controls at least one other current source (I1) and - this additional power source (I1) also sends its current to the first nodes (S b ) feeds in.

Description

Introduction

[0001] Various methods are known for measuring infrared radiation. A key sensor principle is the use of passive infrared detectors (PIR detectors).

[0002] These are characterized by simple and cost-effective production.

[0003] Such PIR detectors are two-pole and can be symbolized in the equivalent circuit by a current source that supplies a current I PIR depending on the change in irradiation and thus the temperature and which has a capacity C PIR connected in parallel. (See also Fig. 2.)

[0004] When evaluating the signal from a PIR sensor, various problems arise: On the one hand, the operating point of the PIR detector is shifted due to self-charging. On the other hand, the current source I PIRusually only a very low current with a relatively high internal resistance. This internal resistance R PIR is in Fig. 2. These boundary conditions result in the requirement for a large dynamic range and a very high internal resistance for the subsequent amplifier and analog-to-digital converter circuit (evaluation circuit).

[0005] However, due to the high internal resistance of an optimal evaluation circuit, charges once generated can no longer flow away. This can lead to the circuit leaving the evaluation circuit's operating range, as it is overloaded.

[0006] An evaluation circuit for a photodiode array is known from DE 691 11 800 T2. An evaluation circuit for a photodiode array is known from EP 0 986 250 A1. An evaluation circuit for image sensor elements is known from US Pat. No. 6,977,601 B1. The technical teachings of these documents do not solve the problems of PIR detectors. Object of the invention

[0007] The object of the invention is to provide an evaluation device with a wide dynamic range for evaluating the signal of a PIR detector without the risk of overloading due to charging of the inputs of the PIR detector evaluation circuit. At the same time, power consumption should be minimized.

[0008] This object is achieved by a device according to claim 1. Description of the invention

[0009] A device is claimed for operating a passive infrared detector (PIR), such PIR detectors being two-pole and being symbolized in the equivalent circuit by a current source which supplies a current IPIR depending on the change in irradiation and with which a capacitance CPIR is connected in parallel, and wherein the passive infrared detector (PIR) is provided with two electrical connections and at least one output of the passive infrared detector (IP, IN) controls at least one current source (I2) and this current source (I2) feeds current into a first node (Sb) and this node (Sb) is connected via an integrating filter (C1b) to the input of a comparator (CP b ) and the output of this comparator (CPb) is at least one digital integrating filter (Int b) directly or indirectly and the output of this digital integrating filter controls at least one digital-to-analog converter (DAC) and the output of this digital-to-analog converter controls at least one further current source (I1) and this further current source (I1) also feeds its current into the first node (Sb). Further description

[0010] The proposed system is in Fig. 1. The passive infrared detector (PIR) is connected with its two connecting lines to a discharge network R GThis, in turn, is connected to an analog-to-digital converter (ADC). The output of the analog-to-digital converter is connected via a first bus (T) with a first bus bandwidth to a digital filter (DF), whose output Out typically has a larger second bus bandwidth than the first bus bandwidth. During the development of the proposed method for operating a passive infrared detector (PIR), it was recognized that the charging of the inputs represents a significant obstacle to correct operation of the system. As will be explained further later, the proposed ΔΣ converter (ADC) is sensitive to such operating point drift. However, this increased sensitivity of the proposed ΔΣ converter (ADC) enables particularly efficient suppression of quantization errors by the comparator of the proposed ΔΣ converter (ADC).Therefore, the proposed ΔΣ converter and the discharge of the passive infrared detector via the proposed discharge network (R. G ) is an inventive unit. Based on this said inventive finding of the faulty charging of the inputs, the simplest solution to this problem can be achieved by discharging the input nodes by means of a switch when the voltage at the detector reaches the dynamic range. In this case, no evaluation of the voltage at the detector can be made during the discharge and shortly thereafter. Alternatively, the discharge can be prevented by a bleeder resistor between the terminals of the sensor or from the terminals to reference ground (R dis_1 , R dis_2). The requirement for reliable discharge of the PIR detector conflicts with the need for the highest possible input resistance of the evaluation circuit. Within the scope of the proposal, it was therefore recognized that it is sensible to make the average equivalent resistance of at least the discharge resistors of the passive infrared detector dependent on whether or not a measurement of the infrared radiation potential is being carried out using the passive infrared sensor (PIR detector). Before a measurement, the discharge resistors are switched to a very high-resistance state (measurement state). After the measurement, the discharge resistors are switched to a state that is lower-resistance than the measurement state.

[0011] Of course, it makes sense to run essential parts of this process in a signal processor. Only the input stages would then be implemented in specially designed electronics. Such a device would then be capable of performing the process described above.

[0012] The proposal is explained below using the attached figures: Fig. Figure 1 shows the basic blocks of a proposed passive infrared detector device. The device consists of a passive infrared detector (PIR) connected to the discharge network R GThe task of this discharge network is to eliminate charges from the PIR detector and to keep the PIR detector at a favorable operating point for the downstream analog-to-digital converter without burdening the system's dynamics. The analog-to-digital converter converts the signal from the discharge network into a first digital signal on a bus T with a first bus width (number of bits). A downstream digital filter (DF) filters the signal on the first bus T and outputs the data with greater resolution via an output bus (Out). Therefore, the output bus Out typically has a larger bus width than the first bus T. Fig. Figure 2 shows the unclaimed equivalent circuit of a passive infrared detector (PIR) with an equivalent power source (I PIR ) and a series circuit of parasitic detector capacitance (C PIR ) and associated loss resistance R PIR_Cand the internal resistance of the backup power source (R PIR ). This internal resistance of the current source (R PIR ) is parallel to the current source (I PIR ) and is typically very high. Excessive loading of the detector will therefore cause the output voltage to collapse. Fig. 3 shows a one-handed version of the analog-to-digital converter (ADC) from

[0013] Fig. 1. A first controlled current source (I1) (also referred to as further current source) is controlled by the feedback path and feeds into the first node (Sb). A second controlled current source (I2) (also referred to as current source) is controlled by an output of the passive infrared detector and also feeds into the first node (Sb). The sum of the two current source currents charges a capacitor (C 1b) charges or discharges them. If the control loop is stable, the second current source (I2) supplies a current with a different sign but the same magnitude as the first current source (I1). The comparator (CP b ) is connected to its input with this capacitor (C 1b ) and compares the voltage value at this capacitor and thus at the first node (S b ) with an internal comparison value. The up / down counter (Int b ) now counts in this example with each system clock either by one upwards or by one downwards depending on whether the input of the comparator (CP b ) above or below the switching threshold of the comparator (CP b ) is located. 6 bits of the counter value of the up / down counter (Int b ) are used, for example, for feedback. In this example, these 6 bits are converted by a digital-to-analog converter (DAC) into an analog signal that controls the additional current source (I1).

[0014] A digital filter (DF) filters the counter reading of the up-down counter (Int b ) to the output signal (Out), which is the output bus of the digital filter DF. List of reference symbols ADC Analog to Digital Converter C 1b First integrating filter (third capacity) CP b Comparator C PIR Parasitic capacity of the PIR sensor PIR DAC digital-to-analog converter DF Downstream digital filter I1 Additional power source I2 current source IN First connection for the PIR sensor Int b Integrating filter (in the simplest case an up / down counter) IN Second connection for the PIR sensor I PIR Current source of the PIR sensor equivalent circuit Out Output bus of the digital filter DF (=downstream filter DF) PIR PIR sensor R dis_1This is the first unfavorable discharge resistance in the prior art. This leads to a load on the output Ip and a reduction in the output signal. In the proposed device, this resistance is modulated. R dis_2 A second disadvantageous discharge resistance in the prior art. This leads to a load on the IN output and a reduction in the output signal. In the proposed device, this resistance is modulated. R G Discharge network. This discharge network prevents the PIR sensor from becoming charged against ground, as well as between the PIR sensor terminals. R PIR Internal resistance of the PIR sensor PIR, which is parallel to the outputs of the PIR sensor (see Fig. 1) R PIR_C Series resistance of the parasitic capacitance CPIR. S b First node T ADC output with typically smaller bit width than the bus out Val control input of the multiplexer MUX

Claims

[1] Device for operating a passive infrared detector (PIR) wherein - such PIR detectors are two-pole and can be symbolized in the equivalent circuit by a current source that supplies a current I PIR depending on the change in irradiation and which has a capacity C PIR connected in parallel, whereby - the passive infrared detector (PIR) is provided with two electrical connections and - at least one output of the passive infrared detector (IP, IN) controls at least one current source (I2) and - this current source (I2) supplies current to a first node (S b ) and - this node (S b ) via an integrating filter (C 1b ) with the input of a comparator (CP b ) and - the output of this comparator (CP b ) at least one digital integrating filter (Int b ) directly or indirectly controls and - the output of this digital integrating filter drives at least one digital-to-analog converter (DAC) and - the output of this digital-to-analog converter controls at least one further current source (I1) and - this additional current source (I1) also supplies its current to the first node (S b ) feeds in. [2] Device according to claim 1 - where the digital integrating filter is an up / down counter (Int, Int b ) and - the comparator output signal (CPO) of a comparator (CP) controls at least this up / down counter (Int) in its counting direction and - where the up / down counter (Int) increments or decrements its numerical value at predetermined and / or programmable time intervals. [3] Device according to 1 or 2, wherein a further digital filter (DF) is arranged downstream of the digital integrating filter.

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