WO2013116585A1

WO2013116585A1 - Turbidity sensor with low signal amplification

Info

Publication number: WO2013116585A1
Application number: PCT/US2013/024253
Authority: WO
Inventors: Marco Sclip; Davide BORDIGNON
Original assignee: Illinois Tool Works Inc.
Priority date: 2012-02-03
Filing date: 2013-02-01
Publication date: 2013-08-08
Also published as: ITTO20120089A1

Abstract

Controlled optical sensor device (10; 100) comprising an optical emitter (21) generating optical radiation (23), in particular in the visible or infrared field, and an optical receiver (22) to detect said optical radiation (23) upon interaction with a medium to be measured (24), a driving current (CTU; Ip) of said optical emitter (21) setting the intensity of said radiation (23) being controlled by driving means (50, 40). According to the invention, this device comprises a circuit (30) for conditioning the output signal supplied by the optical receiver (22) including an amplifier (31) to which said output signal (52) of the optical receiver (22) is supplied as input voltage (Vin) of the amplifier (31), said amplifier (31) including a feedback network (310) configured to determine a gain (G) of the amplifier (31) and to supply a reference voltage (Vref), said amplifier (31) being configured together with said feedback network (31) to supply an output signal (Vo) of the amplifier (31) which is a function of the difference between the reference voltage and the difference, multiplied by the gain (G), between said reference voltage (Vref) and said input voltage (Vin) of the amplifier (31).

Description

TURBIDITY SENSOR WITH LOW SIGNAL AMPLIFICATION

Field of the invention

The present invention relates to a turbidity sensor with low signal amplification or a controlled optical sensor device comprising an optical emitter generating optical radiation, in particular in the visible or infrared field, and an optical receiver for detecting said optical radiation upon interaction with a medium to be measured, a driving current of said optical emitter setting the intensity of said radiation being controlled by driving means, in particular included in said optical sensor device .

Background of the invention

Optical sensors for detecting the turbidity of a fluid used in a washing machine or in a dishwasher are well known. These sensors typically comprise an optical emitter which emits light radiation through a medium such as the washing liquid and an optical receiver which receives the optical radiation emitted by the emitter after passing through said medium, namely the washing liquid. By means of a comparison between the measurements of the optical radiation emitted by the emitter and the optical radiation received by the receiver, it is possible to determine the degree of turbidity of the washing liquid. In particular, it is possible to compare the radiation received by the receiver in clean and dirty water conditions in order to establish the turbidity. The geometry of the sensor defines whether the radiation is reflected, refracted or attenuated by the medium to be measured. In addition to applications such as the turbidity sensor, this type of sensor is applicable to liquid level sensors, distance sensors and liquid presence sensors. As mentioned, different types of sensor may be analyzed by changing the relative position of the emitter and the receiver, for example arranged frontally or laterally so as to operate by means of reflection, and the types of lenses which may be used in conjunction with said emitter and receiver.

One of the main problems which is encountered with this type of sensor is the great difference in signals due to the tolerance variations between components. This is the case even if emitters and receivers from the same production batch are used. With reference to the diagram in Figure 1 which shows the output voltage Vo of an optical sensor, in particular a phototransistor of this sensor, as a function of the turbidity indicated in NTU (Nephelometric Turbidity Units), a typical turbidity sensor for household appliances has an optical emitter, for example an LED (Light Emitting Diode) , which emits an optical signal in a straight line towards a receiver, for example a phototransistor, while the suspended particles of the liquid which is the medium arranged between LED and phototransistor attenuate the signal absorbing, reflecting and diffusing the radiation emitted. The emitter and the receiver are contained in transparent plastic housings so as to prevent contact with the water. It is known to vary the value of the current which flows in the emitter in order to establish a reference level for the receiver when in the presence of fresh water, namely water which is assumed to be clean, and to therefore monitor the decrease at the receiver output with the increase in turbidity. The diagram in Figure 1 shows an example of a voltage output Vo of a typical turbidity sensor after adjustment to a given output voltage of 4V in clean water.

The extent of the variation in the output values at a certain turbidity level is influenced by a plurality of factors including the distance between the emitter and the receiver, the amplitude of the radiation cone emitted by the emitter, the amplitude of the receiver cone and the transparency of the plastic housings. Another major variation may arise as a result of the dependence, on temperature, of the behavior of these emitters and receivers .

Moreover, these types of sensors may have a sensitivity, namely the difference between the output signal corresponding to the value zero and the maximum value, which is low, and it is required to modify the geometry of the sensor, optimizing its optical design compared, for example, to the amplitude of the receiver and emitter cones, so as to increase it.

Object and summary of the invention

The object of the present invention is to provide an optical sensor device, in particular a turbidity sensor, which is able to adjust or set the output so as to ensure an increased sensitivity and also allow compensation of the different tolerances and behavior of the components.

According to the present invention this object is achieved by an optical sensor device having the characteristic features forming the subject of Claim 1.

The invention also relates to a method for adjusting the output of an optical sensor device.

The claims form an integral part of the teaching provided here in relation to the invention. Brief description of the drawings

The invention will now be described in detail with reference to the accompanying drawings which are provided purely by way of a non-limiting example, with reference to the accompanying drawings, in which:

Figure 1 is a diagram showing the output characteristics of known optical sensors;

- Figure 2 is a perspective view of an optical sensor device according to the present invention;

- Figure 3 is a circuit diagram of a first embodiment of the optical sensor device according to the present invention;

- Figure 4 is a detailed circuit diagram of a second embodiment of the optical sensor device according to Figure 3;

- Figures 5 and 6 are two equivalent illustrations of the optical sensor device module according to Figure 3;

Figure 7 is a diagram showing the output characteristic of the optical sensor device according to the invention;

- Figure 8 is a circuit diagram of a variation of an embodiment of the optical sensor device module according to Figure 3;

Figure 9 is a diagram showing the output characteristics which can be obtained with the optical sensor device module according to Figure 8.

Detailed description of embodiments of the invention In the description below, particular reference will be made to a turbidity sensor, in particular for washing machines, although the invention is not limited only to this type of sensor.

With reference to Figure 2, this shows a schematic view of an optical sensor device 10 for measuring the turbidity of the water, in particular the water of a washing machine, which comprises a printed circuit board 81 carrying an optical emitter 21 and an optical receiver 22. The printed circuit board 81 also carries the components 80 of an electronic control circuit associated with the emitter 21 and the receiver 22. As shown in detail in Figure 4, these components 80 may for example comprise a microcontroller 50, a current generator 40, an output signal conditioning circuit 30 and a temperature compensation circuit 60. The printed circuit board 81 also carries an electrical connector 70 for electrically connecting the optical sensor device 10 to an external circuit. In the circuit shown in Figure 3 the components 80 comprise essentially the output signal conditioning circuit 30.

An LED may be used for example as emitter 21, while a phototransistor , a photodiode or a solar cell may be used as receiver 22. With reference in particular to Figure 2, the emitter 21 and the receiver 22 are fixed to the printed circuit board 81 by means of respective metallic terminals 88 which are inserted and fixed inside respective holes of the printed circuit board 81. The printed circuit board 81 has two branches 86 which are spaced apart from each other so that one end of the board 81 has substantially a U- shaped form. The emitter 21 and the receiver 22 are fixed to the ends of the respective branches 86. The board 81 and the branches 86 are inserted, for example, inside a transparent plastic housing, not shown in Figure 1, and the branches in particular are inserted inside correspondingly shaped plastic arms between which a medium to be measured 24, for example washing water, may be arranged. In Figure 2 the emitting surfaces of the emitter 21 and receiving surfaces 22 are facing each other such that radiation 23 is transmitted directly via the medium 24, in particular the washing water, the turbidity of which is to be measured. In various embodiments, the positioning of emitter 21 and receiver 22 may instead be such that the radiation is propagated from the emitter 21 to the receiver 22 by means of reflection on the medium 24, for example having a radiation cone emitted by the emitter and a receiver cone, which are not aligned in the same direction.

Figure 3 shows a first embodiment of an optical sensor device 100.

The reference number 20 denotes an emitter/receiver circuit module comprising the LED emitter 21 which emits the light radiation 23 through the medium 24, for example the washing water. This light radiation 23 is received by a receiver 22, consisting for example of a phototransistor.

In this case the LED emitter 21 is connected between the power supply VCC and the terminal of an input output connector 170 which has a driving signal, in particular a driving current CTU.

A circuit module 30 for conditioning the output signal is arranged downstream of the emitter/receiver module 20, in particular in series with the emitter of the phototransistor which acts as optical receiver 22.

This circuit module 30 for conditioning the output signal operates in general as follows: the radiation detected by the phototransistor optical receiver 22 is converted into a voltage and is filtered by an RC circuit 32 which is connected to the emitter of the phototransistor . The emitter of the phototransistor is also connected to the input of an amplification circuit denoted overall by the reference number 31, the output signal Vo of which, in particular an output voltage, is sent to one of the output pins of the output connector 170. The output connector 70 also has pins for receiving externally and supplying to the circuits of the optical sensor device 10 the supply voltage VCC and the ground reference signal GND .

The amplification circuit 31 operates so as to amplify the signal. In this amplification circuit 31, in order to increase the sensitivity of the sensor, increasing the signal difference between a reference value, namely the value which is determined in a condition defined as a reference condition, preferably the value in clean water, and the maximum value is envisaged. This is obtained by means of a signal conditioning circuit 30 which uses a feedback circuit with a certain gain value.

In particular, therefore, an output signal 52 of the optical receiver 22 is supplied via the RC circuit 32 connected to the emitter of the phototransistor as input voltage V_in to the positive terminal of the operational amplifier 31a. The RC circuit 32 comprises in particular a load resistor 32a and a capacitor 32b. The amplification circuit 31 comprises a feedback network 310 configured to determine a given gain G of the amplifier 31 and provide a reference voltage V_ref . The amplification circuit 31 supplies, as a result of this feedback network 310, an output signal V₀ of the amplifier 31 as a function of the reference voltage V_ref and the gain G.

This feedback network 310 includes a resistor 312 and a resistor 315 which form a divider of the supply voltage VCC for providing said reference voltage V_ref . Moreover, the feedback network 310 comprises a feedback branch for bringing the output voltage back to the input, comprising a resistor 314 connected at one of its ends to the output of the amplifier 31 and at the other end to the negative, or inverting, input of the amplifier 31a. A further resistor 313 is connected to this inverting input and to the resistor 314, being connected at its other end to the node of the resistive divider formed by the resistors 312, 315 on which the reference voltage V_ref is formed.

The output signal Vo, in particular an output voltage, is sent to one of the output pins of the output connector 170.

With reference to Figure 4, this shows a circuit diagram of a second embodiment of the optical sensor device - denoted by 10 - which comprises the emitter/receiver circuit module 20 comprising the LED emitter 21 which emits the light radiation 23 through the medium 24, for example the washing water. This light radiation 23 is received by the receiver 22, consisting for example of a phototransistor.

The emitter/receiver module 20 is in this case operated by a current generator 40, operation of which is controlled by a microcontroller 50. This microcontroller 50, via a signal output 51 thereof, provides a PWM (pulse width modulation) signal Vm, which is filtered via a filter 42 in the current generator 40, in order to obtain an adjustable direct voltage, depending on the value of the duty cycle of the PWM signal Vm, on the output 51 which is used to adjust the base current of a transistor 41. This transistor 41 has a resistor 43 on its emitter electrode which converts the emitter current into a voltage which is read via a signal input 53 by the microcontroller 50 so as to form a closed-loop control system, the duty cycle of the PWM signal Vm from the output 51 being adjusted so as to obtain the desired current value in the emitter of the transistor 41. The transistor 41 is then connected via the collector electrode to the optical emitter 21, so as to adjust the driving current Ip thereof; namely, the collector current is the driving current Ip flowing in the LED. The circuit module 30 for conditioning the output signal is situated downstream of the emitter/receiver module 20, in particular in series with the collector of the phototransistor which operates as optical receiver 22.

This circuit module 30 for conditioning the output signal operates in this case in general as follows: the radiation detected by the phototransistor optical receiver 22 is converted into a voltage and is filtered by the RC circuit 32 connected to the collector of the phototransistor. This voltage on the collector of the phototransistor is read also by the microcontroller 50 via its signal input 52 which is connected to the input of the amplification circuit 31, the output signal Vo of which, in particular an output voltage, is sent to one of the output pins of the output connector 70. The output connector 70 also has pins for receiving externally and supplying to the circuits of the optical sensor device 10 and to its microcontroller 50 the supply voltage VCC and the ground reference signal GND and also comprises a further digital signal input pin for a calibration signal TU.

The optical sensor device 10 also comprises a temperature compensation circuit 60, which comprises a negative temperature coefficient (NTC) element 65; for example an NTC thermistor constitutes a particularly low- cost solution, even though other similar thermistors may be used. This negative temperature coefficient element 65 forms, with a resistor 64, a supply voltage divider VCC which provides a variable divider voltage depending on the temperature which corresponds to the voltage VT representing temperature information. This variable divider voltage depending on the temperature, or voltage VT, is supplied, via a filter 64, to an input 55 of the microcontroller 50 which is, for example, the input for a reference voltage, with which microcontrollers are conventionally provided, in particular the input of an analog-digital converter of the microcontroller 50.

The temperature compensation circuit 60 therefore supplies the microcontroller 50 with voltage values representing temperature information measured via the negative temperature coefficient device 65. This temperature information is used by the microcontroller 50 to vary the driving current Ip sent by the generator 40 to the emitter 21 so as to compensate for temperature effects, increasing or decreasing the intensity of the radiation emitted .

The methods for compensating the temperature may involve for example applying a predefined transfer function, in particular on the basis of the characteristics shown in the data sheets of the optical components. Alternatively it is possible to envisage performing calibration of each single device following measurement of the output signal Vo at at least two different temperatures. These compensation methods preferably employ a look-up table which, depending on the temperature read, applies a correction factor to the current in the emitter 21. An alternative to the look-up table may consist of a polynomial approximation function or a linear interpolation between values of a table.

In general, the microcontroller 50 is configured to perform also the necessary calibration settings of the emitter/receiver 20, as indicated in detail hereinbelow.

The functions of the microcontroller 50 therefore comprise :

- calibration of the emitter/receiver module 20 so as to supply the same output signal in the same operating conditions, using the reading of the output signal 52 and reference values of this signal, preferably values in clean water at a given temperature, stored in the memory of the microcontroller; by supplying the microcontroller 50 with a specific command signal, the latter performs adjustment of the current so as to obtain at the output the predefined reference value, for example 4V, and thereafter the current value necessary for obtaining this predefined reference value is saved in the memory of the microcontroller 50; Figure 4 shows for this purpose, in the connector, a digital input pin TU which activates the calibration procedure ;

- temperature compensation using the circuit 60 to detect the internal temperature of the sensor;

- compensation of variations between components due to mechanical assembly tolerances and thickness tolerances of plastic parts, with storage of the correction values during calibration at the production plant;

compensation for component ageing; this may be performed by calibrating again the sensor in clean water and compensates for ageing of the emitter and the receiver as well as a reduction in the transparency of the plastic housings of the optical module;

- storage, in an associated memory, of all the sensor calibration and compensation parameters in order to allow easier replacement thereof.

Figures 5 and 6 show schematically two different embodiments of the conditioning circuit 30, Figure 6 showing in particular the circuit with a Thevenin equivalent for the divider obtained by means of the resistors 312, 315.

Therefore, with reference to that shown in Figures 3 to 6, the radiation 23 detected by the optical receiver 22 generates a photocurrent which is converted by means of the load resistor 32a of the RC circuit 32 into an input voltage V_in, filtered by the RC circuit 32 itself. This input voltage V_in is then brought back to the positive input of the operational amplifier 31a. By applying Thevenin 's theory to the divider of the feedback network 310 formed by the resistors 312 and 315, the following is obtained :

V_ref = VCC *R5/(R2+R5)

Req =R2//R5=R2*R5/ (R2+R5)

and for the gain G :

G = 1+R4 / (R3+Req)

In the equations shown above, R2, R3, R4 and R5 denote, respectively, the values of the resistors 312, 313, 314, 315 of the resistive feedback network 310. G denotes the gain of the feedback circuit, while Req indicates the equivalent resistance of the divider consisting of the resistors 312 and 315 which fix the reference voltage V_ref .

The resultant feedback circuit 310 therefore performs together with the operational amplifier 31 the following transfer function with respect to the output voltage V_out : o = V_ref - ( V_ref - V_in ) * G .

By setting, by means of the values R2, R5 of the divider resistors 312, 315, the reference voltage V_ref as the voltage in a reference condition, for the turbidity sensor in the initial clean water condition, for example 4V in the example of Fig. 1 - this initial clean water being obtained by setting the calibration current, for example the calibration current CTU, in the emitter 21 - the characteristic of the measurement has the same output voltage V₀ as an amplifier without feedback 31a, but has a greater slope, since the slope is multiplied by the gain G, as shown in the diagram of Figure 7, which shows the output voltage of a turbidity sensor Vo without the amplifier 31 with network 310 (curve CI) and with the amplifier 31 with feedback network 310 (curve C2), in particular with reference voltage V_ref = 4V and gain G = 1.5. The circuit therefore achieves for an input voltage V_in equal to the reference voltage V_ref the same output voltage as a circuit without the amplifier 31, while the signal is amplified for values of the input voltage V_in different from the reference voltage V_ref .

Figure 8 also shows an alternative circuit 31' in which the reaction circuit 310 includes, instead of the resistor 314 with value R4, a variable resistor 320, for example a trimmer, which allows variation of the gain factor G and compensation of the deviation/dispersion of the signal of a turbidity sensor with respect to another, in this way reducing the tolerance variations or the 3- sigma value, as regards management of the quality based on control of the mean square deviation, for the output voltages of the sensors, as shown in the diagram of Figure 9. This diagram shows three characteristic curves Cll, C12, C13 relating to three different turbidity sensors. The three curves Cll, C12, C13 show output voltage values which are very different and increasing in each case. By applying amplification with decreasing gain values, i.e. G1<G2<G3, three characteristic curves substantially coinciding with each other are obtained.

Therefore, advantageously, the present invention, by means of an optical sensor device which associates with an optical emitter/receiver pair the use of amplification with a feedback network configured to determine an output voltage which can be adjusted or set by means of a gain of the amplifier and the value of a reference voltage, is able to provide a sensor with a much greater sensitivity.

Moreover, with the sensor device described it is possible to compensate for differences in the parameters and tolerances of components which are also derived from the same production batch, allowing both compensation and calibration of the sensor.

According to one embodiment, the same gain is adjustable so as to compensate easily for the variations in the values of the output voltages between different sensors in the same batch.

The device according to the invention also envisages the compensation of temperature effects. Owing to the arrangement of the emitter/receiver module, temperature sensor circuit and microcontroller in a single assembly integrated on the same board, the device is able to adjust autonomously its output in relation to the temperature variations. Moreover, by means of the microcontroller it is advantageously possible to store the calibration and compensation parameters.

The use of amplification with a feedback network and output voltage which can be adjusted or set by means of a gain of the amplifier and the value of a reference voltage result in an optical sensor device comprising an optical emitter generating optical radiation and an optical receiver for detecting this optical radiation upon interaction with a medium to be measured, in particular a turbidity sensor, which is especially efficient in obtaining an output signal which has a high sensitivity and can be easily compensated. These advantages are furthermore increased by the combined use of the microcontroller, in particular for controlling the current in the photodiode.

Obviously, without affecting the principle of the invention, the constructional details and the embodiments may be greatly modified with respect to that described and illustrated without thereby departing from the scope of the invention as defined in the accompanying claims.

The optical sensor device according to the invention, described here with particular reference to a turbidity sensor for washing machines, may also be used in applications such as liquid level sensors, distance sensors or liquid presence sensors.

The driving module may be included in the optical sensor or may be provided outside of it.

Claims

1. A controlled optical sensor device (10; 100) including an optical emitter (21) generating optical radiation (23), in particular in the visible or infrared field, and an optical receiver (22) to detect said optical radiation (23) upon interaction with a medium to be measured (24), a driving current (CTU; Ip) of said optical emitter (21) setting the intensity of said radiation (23) being controlled by driving means (50, 40), characterized in that it includes a circuit (30) for conditioning the output signal supplied by the optical receiver (22) including an amplifier (31) to which said output signal (52) of the optical receiver (22) is supplied as input voltage (V_in) of the amplifier (31), said amplifier (31) including a feedback network (310) configured to determine a gain (G) of the amplifier (31) and to supply a reference voltage (V_ref ) , said amplifier (31) being configured together with said feedback network (310) to supply an output signal (V₀) of the amplifier (31) which is a function of the difference between the reference voltage and the difference, multiplied by the gain (G) , between said reference voltage (V_ref) and said input voltage (V_in) of the amplifier (31) .

2. The sensor device as claimed in claim 1, characterized in that said feedback network (310) includes a resistive divider (312, 315) to set said reference voltage (V_ref) .

3. The sensor device as claimed in claim 2, characterized in that said feedback network (310) includes a feedback branch (314, 313) including a resistor (314) connected at one end at the output of the amplifier (31) and at the other end at the inverting input of the amplifier (31) and at an end of a further resistor (R3), connected in turn at the node of said resistive divider (312, 315) on which the reference voltage (V_ref) is formed.

4. The sensor device as claimed in claim 3, characterized in that the resistive divider (312, 315) to set said reference voltage (V_ref) is configured to obtain a reference voltage (V_ref) equal to a value supplied by the sensor (10; 100) in a reference condition, in particular in a clean water condition.

5. The sensor device as claimed in claim 4, characterized in that said feedback branch (314, 313) includes a variable resistor (320) .

6. The sensor device as claimed in one of the previous claims, characterized in that said optical sensor (10; 100) is configured to operate as turbidity sensor, in particular for use in washing machines.

7 . The sensor device as claimed in one of the previous claims, characterized in that said driving means (50, 40) include a microcontroller (50) configured to access calibration and/or compensation parameters and to adjust said driving current (Ip) as a function of said calibration and/or compensation parameters.

8 . The sensor device as claimed in claim 7, characterized in that it includes a temperature compensation circuit module (60) adapted to detect an operating temperature of said emitter (21) and receiver (22) and in that said microcontroller (50) is configured to adjust the driving current (Ip) of said optical emitter (21) as a function of temperature information (VT) supplied by said temperature compensation circuit module (60) .

9. The sensor device as claimed in claim 8, characterized in that said microcontroller (50) is configured to vary the driving current (Ip) of said optical emitter (21) to compensate for temperature effects increasing or decreasing the intensity of the emitted radiation ( 23 ) .

10. The sensor device as claimed in one of the previous claims 7 to 9, characterized in that said microcontroller (50) is configured to perform the adjustment of said current in said optical emitter (21) supplying a pulse width modulated signal (Vm) determining as a function of a value of its duty cycle the value of said driving current (Ip) of the optical emitter (21), the value of said duty cycle being controlled by a closed loop regulation as a function of a signal (53) which is taken proportionally to said driving current (Ip) .

11. The sensor device as claimed in one of the previous claims, characterized in that said driving means (50, 40) are included in said optical sensor device (10; 100) .

12. A method for adjusting the output of an optical sensor device (10; 100) as claimed in at least one of claims 1 to 11, characterized in that it includes the operations of

providing a reference voltage (V_ref ) ,

amplifying (31) said output signal (52, Vi_n) of the optical receiver (22) in a feedback mode, with a given gain value (G) , which is a function of the difference between the reference voltage (V_ref) and the difference, multiplied by the gain (G) , between said reference voltage (V_ref ) and said input voltage (V_in) of the amplifier (31) .

13 . The method as claimed in claim 12, characterized in that it includes providing a reference voltage (V_ref) equal to a value supplied by the sensor (10; 100) in a reference condition, in particular in a clean water condition .

14 . The method as claimed in claim 12 or 13, characterized in that it includes providing a variable gain value (G) , and adjusting said variable gain value (G) to compensate for the deviation or dispersion of the signal of a sensor, in particular a turbidity sensor, with respect to another .