Patent Publication Number: US-2022228983-A1

Title: Liquid sensor and hydraulic unit

Description:
TECHNICAL FIELD 
     The present disclosure relates to liquid sensors and hydraulic units. 
     BACKGROUND ART 
     As a conventional liquid sensor, there is a liquid sensor which is installed in a machine and detects deterioration of lubricating oil in the machine (see Patent Literature 1). 
     CITATIONS LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2015-232582 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problems 
     However, since the conventional liquid sensor is installed in a machine by screw connection, a sealing structure is required between the liquid sensor and the machine, and there is a problem that workability of attachment work is low. 
     The present disclosure proposes a liquid sensor and a hydraulic unit capable of improving workability of attachment work. 
     Solutions to Problems 
     A liquid sensor of the present disclosure includes a light receiver configured to receive light. The light receiver is attachable to an outside of an accommodation unit into which a liquid is introduced and which has translucency. The light receiver receives light that has passed through the liquid in the accommodation unit or light reflected by the liquid, and outputs a signal related to a physical quantity of the liquid. 
     According to the liquid sensor of the present disclosure, since the light receiver is attachable to the outside of the accommodation unit into which the liquid to be measured is introduced, a seal structure for preventing leakage of a hydraulic oil introduced into the accommodation unit to the outside is not required between the liquid sensor and the accommodation unit, and workability of attachment work of the liquid sensor to the accommodation unit can be improved. 
     The translucency according to the present disclosure is translucency with respect to light passing through the liquid. 
     In one embodiment, the signal related to a physical quantity of the liquid includes a signal related to the deterioration degree of the liquid. 
     In one embodiment, the signal related to a physical quantity of the liquid includes a signal related to a hue of light that has passed through the liquid and has been received by the light receiver or light that has been reflected by the liquid and has been received by the light receiver. The deterioration degree of the liquid is determined from the hue. 
     In one embodiment, the signal related to a physical quantity of the liquid includes a signal related to presence or absence of the liquid in the accommodation unit. 
     In one embodiment, the accommodation unit includes a float disposed inside the accommodation unit. 
     In the above embodiment, when the float is at a level at which the light receiver is attached to the accommodation unit, the float blocks light and significantly changes the intensity of light received by the light receiver. This allows the accuracy of detection of the presence or absence of liquid can be improved. 
     The liquid sensor according to one embodiment includes: 
     a rechargeable power source configured to supply electric power to the light receiver, and 
     a thermoelectric conversion element configured to generate electric power for charging the power source. 
     In the above embodiment, since the liquid sensor includes a rechargeable power source that supplies power to the light receiver and a thermoelectric conversion element that generates power for charging the power source, wiring for supplying power from the outside to the liquid sensor is not required, and the liquid sensor is allowed to have a simplified wiring structure. 
     The liquid sensor according to one embodiment includes a wireless transmitter configured to be supplied with electric power from the power source and to transmit the signal related to a physical quantity of the liquid of the light receiver to outside. 
     According to the above embodiment, since there is no need for wiring for outputting the signal output by the light receiver to outside, the wiring structure can be simplified. 
     The liquid sensor according to one embodiment includes a light emitter configured to emit light. The light emitter is attachable to the outside of the accommodation unit. The light receiver receives light emitted by the light emitter and having passed through the liquid in the accommodation unit or light emitted by the light emitter and having been reflected by the liquid in the accommodation unit. 
     In one embodiment, the light emitter and the light receiver are arranged to face each other across the accommodation unit. 
     In general, in a transmission type sensor, a surface of the accommodation unit has less influence on the detection accuracy than in other types of photoelectric sensors. In the above embodiment, since the liquid sensor is a transmission type sensor in which the light emitter and the light receiver are arranged to face each other across the accommodation unit, it is possible to perform reliable detection as compared with a case where a sensor of another type is used as the liquid sensor. 
     In one embodiment, the light emitter and the light receiver are arranged on the same side with respect to the accommodation unit. 
     In general, in a reflective sensor, since a light emitter and a light receiver are arranged on the same side of an accommodating unit, the reflective sensor is smaller than other types of photoelectric sensors. In the above embodiment, since the liquid sensor is a reflective sensor in which the light emitter and the light receiver are arranged on the same side of the accommodation unit, the liquid sensor can be made smaller than a case where a sensor of another type is used as the liquid sensor. 
     The liquid sensor according to one embodiment includes a casing attachable to the outside of the accommodation unit. The light emitter and the light receiver are integrally provided in the casing. 
     In the above embodiment, when the light emitter and the light receiver are integrally provided in the casing in a state where the optical axis of the light emitter and an optical axis of the light receiver coincide with each other, it is not necessary to adjust the optical axes of the light emitter and the light receiver to align the optical axes, when the liquid sensor is attached, so that workability of attachment work of the liquid sensor to the accommodation unit can be improved. 
     In one embodiment, in the light emitter, a light emission manner, that is, a manner in which the light emitter emits light, is determined according to the signal related to a physical quantity of the liquid of the light receiver. 
     In the above embodiment, the liquid sensor notifies a user of information on the physical quantity of the liquid by the light emission manner of the light emitter. For example, when the detection result of the liquid sensor indicates an abnormality, the light emitter of the liquid sensor emits light in a light emission manner corresponding to the abnormality. In this way, so that the liquid sensor notifies the user that the liquid sensor has detected the abnormality. Thus, the user can easily recognize the information (for example, abnormality) on the physical quantity of the liquid to be measured by visually observing the light emission manner of the liquid sensor. 
     In addition, since the light emitter of the liquid sensor has both a function as a projector of a photoelectric sensor and a function of notifying the user according to the signal related to the physical quantity of the liquid, the structure of the liquid sensor can be simplified as compared with a case where these functions are separately provided. 
     In one embodiment, the accommodation unit is a liquid level gauge. 
     In general, a liquid level gauge is installed at a place where a user can easily visually observe the liquid level gauge. Therefore, according to the above embodiment, since the liquid sensor is attached to the liquid level gauge, the user can easily visually observe the light emitter of the liquid sensor, so that the user can easily recognize the information (for example, abnormality) on the physical quantity of the liquid to be measured. 
     In one embodiment, the light emitter changes its state from off to blinking or blinking to off according to the signal related to a physical quantity of the liquid. 
     In one embodiment, in the light emitter, the emission color changes according to the signal related to a physical quantity of the liquid. 
     A hydraulic unit of one embodiment includes the liquid sensor described above. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a hydraulic unit according to a first embodiment of the present disclosure. 
         FIG. 2  is a front view of a liquid sensor according to the first embodiment. 
         FIG. 3  is a cross-sectional view taken along line III-Ill in  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken along line IV-IV in  FIG. 3 . 
         FIG. 5  is a block diagram of the liquid sensor according to the first embodiment. 
         FIG. 6  is a graph relating to a hue detected by the liquid sensor according to the first embodiment. 
         FIG. 7  is a cross-sectional view similar to  FIG. 3  according to a second embodiment of the present disclosure. 
         FIG. 8  is a cross-sectional view similar to  FIG. 3  according to a third embodiment of the present disclosure. 
         FIG. 9  is a schematic perspective view of a hydraulic unit according to a fifth embodiment of the present disclosure. 
         FIG. 10  is a perspective view of a hydraulic unit according to a sixth embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, liquid sensors and hydraulic units according to embodiments of the present disclosure will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a perspective view of a hydraulic unit  1  according to a first embodiment of the present disclosure. The hydraulic unit  1  according to the present embodiment supplies hydraulic oil to an external apparatus such as a machine tool. 
     Referring to  FIG. 1 , the hydraulic unit  1  includes a hydraulic oil tank  10  that accommodates hydraulic oil, and a mount  11  attached to an upper portion of the hydraulic oil tank  10 . The hydraulic unit  1  includes a hydraulic pump (not shown), a motor (not shown) that drives the hydraulic pump, an oil cooler  12  that cools hydraulic oil discharged from the hydraulic pump, and a controller  13  that controls the motor. The hydraulic pump, the motor, the oil cooler  12 , and the controller  13  are mounted on the mount  11 , and the hydraulic oil tank  10  is disposed below the mount  11 . In addition, the hydraulic unit  1  includes a cooling fan  14  that supplies cooling air to the motor and the oil cooler  12 . The hydraulic unit  1  is herein described by way of example only; and not limited to the above-described structure. 
     A liquid level gauge  20  for externally checking the amount of the hydraulic oil accommodated in the hydraulic oil tank  10  is attached to a metal side surface  10   a  of the hydraulic oil tank  10 . Since the liquid level gauge  20  is attached to the side surface  10   a  of the hydraulic oil tank  10 , a user can easily visually check the amount of the hydraulic oil accommodated in the hydraulic oil tank  10 . In addition, a liquid sensor  30  that measures a physical quantity (hue in the present embodiment) of liquid (hydraulic oil in the present embodiment) is attached to the outside of the liquid level gauge  20 . 
       FIG. 2  is a schematic front view showing the liquid level gauge  20  and the liquid sensor  30  according to the present embodiment.  FIG. 3  is a schematic cross-sectional view taken along line III-II in  FIG. 2 . In  FIG. 3 , illustration of a detailed structure of the inside of the liquid sensor  30  is omitted. In  FIGS. 2 and 3 , the same components as in  FIG. 1  are denoted by the same reference signs as in  FIG. 1 . 
     Referring to  FIGS. 2 and 3 , the liquid level gauge  20  of the present embodiment is an oil level gauge for checking the oil level of the hydraulic oil accommodated in the hydraulic oil tank  10 . The liquid level gauge  20  of the present embodiment includes a liquid level gauge main body  21  made of a transparent acrylic resin and two bolts  22 A and  22 B for fixing the liquid level gauge main body  21  to the hydraulic oil tank  10 . The liquid level gauge  20  according to the present embodiment is an example of a translucent accommodation unit into which the liquid (hydraulic oil in the present embodiment) according to the present disclosure is introduced. 
     The liquid level gauge main body  21  of the present embodiment has a substantially rectangular parallelepiped shape, and is attached to the hydraulic oil tank  10  such that the liquid level gauge main body  21  has a longitudinal direction that extends along the vertical direction. The liquid level gauge main body  21  is hollow. Specifically, the liquid level gauge main body  21  includes a hollow portion  21   a  being a space extending in the longitudinal direction inside the liquid level gauge main body  21 . The hydraulic oil is introduced into the hollow portion  21   a  of the liquid level gauge main body  21  according to the amount of the hydraulic oil accommodated in the hydraulic oil tank  10 . The user can check the amount of the hydraulic oil accommodated in the hydraulic oil tank  10  by visually observing the oil level of the hydraulic oil in the hollow portion  21   a  of the liquid level gauge main body  21 . The liquid level gauge main body  21  is provided with an upper limit line HL indicating the upper limit of an allowable range of the oil level of the hydraulic oil and a lower limit line LL indicating the lower limit of the allowable range of the oil level of the hydraulic oil. 
     The bolts  22 A and  22 B of the present embodiment are made of metal. As shown in  FIG. 3 , each of the bolts  22 A and  22 B includes a communication portion  22   c  extending in the axial direction from a tip portion  22   a  toward a head portion  22   b  of the bolt. The communication portion  22   c  is open in the radial direction at an axial end portions on the head portion  22   b  side of each of the bolts  22 A and  22 B. In other words, the bolts  22 A and  22 B are what are called perforated bolts. The communication portions  22   c  of the bolts  22 A and  22 B fluidly connect the internal space of the hydraulic oil tank  10  to the hollow portion  21   a  of the liquid level gauge main body  21  in a state where the bolts  22 A and  22 B fix the liquid level gauge main body  21  to the hydraulic oil tank  10 . In other words, the internal space of the hydraulic oil tank  10 , the hollow portion  21   a  of the liquid level gauge main body  21 , and the communication portion  22   c  of the bolts  22 A and  22 B communicate with each other. 
     In the state shown in  FIG. 3 , the communication portion  22   c  of the bolt  22 B opens at the tip portion  22   a  into an area where the hydraulic oil accumulated in the hydraulic oil tank  10  exists. At this time, the hydraulic oil is introduced from the hydraulic oil tank  10  into the hollow portion  21   a  of the liquid level gauge main body  21  through the communication portion  22   c  of the bolt  22 B. In addition, the communication portion  22   c  of the bolt  22 B is filled with hydraulic oil. 
     As shown in  FIG. 3 , the oil level of the hydraulic oil introduced into the liquid level gauge main body  21  coincides with the oil level of the hydraulic oil accommodated in the hydraulic oil tank  10 . Thus, the user can check the amount of the hydraulic oil accommodated in the hydraulic oil tank  10  by visually observing the oil level of the hydraulic oil in the hollow portion  21   a  of the liquid level gauge main body  21 . 
     The liquid sensor  30  of the present embodiment is a photoelectric sensor for detecting the degree of deterioration of the hydraulic oil accommodated in the hydraulic oil tank  10  and the presence or absence of the hydraulic oil. More specifically, the liquid sensor  30  is a transmissive color sensor. As shown in  FIG. 2 , the liquid sensor  30  of the present embodiment includes a U-shaped casing  31 , a light emitter  32  that emits light, and a light receiver  33  that receives the light emitted from the light emitter  32 . The light emitter  32  and the light receiver  33  are provided integrally with the casing  31  so as to face each other. Specifically, the light emitter  32  and the light receiver  33  are provided integrally with the casing  31  so that the optical axis of the light emitter  32  and the optical axis of the light receiver  33  coincide with each other. 
     The liquid sensor  30  is attached to the liquid level gauge main body  21  such that the light emitter  32  and the light receiver  33  face each other across the liquid level gauge main body  21 . In addition, the liquid sensor  30  is attached to the liquid level gauge  20  such that the height positions of the light emitter  32  and the light receiver  33  overlap with the height position of the lower limit line LL of the liquid level gauge main body  21 . The liquid sensor  30  detects the presence or absence of hydraulic oil below the lower limit line LL of the liquid level gauge main body  21 . 
     A packing (not shown) is provided between the casing  31  of the liquid sensor  30  and the liquid level gauge main body  21  of the present embodiment. This packing makes the casing  31  and the liquid level gauge main body  21  water-resistant and dust-resistant, prevents external light from entering the liquid level gauge main body  21 , and brings the casing  31  into close contact with the liquid level gauge main body  21  so that the casing  31  is held by the liquid level gauge main body  21 . 
     The light emitter  32  of the present embodiment includes a light emitting element for emitting light and a drive circuit for driving the light emitting element. The light emitting element of the present embodiment is a white light emitting diode. In other words, the light emitter  32  of the present embodiment emits white light. 
     The light receiver  33  of the present embodiment includes a light receiving element that converts the intensity of light into an electric signal, an amplifier circuit that amplifies the electric signal output from the light receiving element, and an A/D conversion circuit that converts the analog electric signal amplified by the amplifier circuit into a digital electric signal. The light receiving element of the present embodiment is an RGB color sensor, and can detect the electric signal converted from received visible light separately for each color of red, green, and blue. 
     As shown in  FIG. 3 , the liquid sensor  30  of the present embodiment further includes a thermoelectric conversion element  34  that converts heat into electric power. The thermoelectric conversion element  34  of the present embodiment is provided so as to be in contact with the side surface  10   a  of the hydraulic oil tank  10 . The side surface  10   a  of the hydraulic oil tank  10  is likely to have a high temperature because the hydraulic oil accommodated in the hydraulic oil tank  10  has a high temperature due to heat generation from a sliding portion of the machine tool or the like. The thermoelectric conversion element  34  generates power using a temperature difference between the temperature of the side surface  10   a  of the hydraulic oil tank  10  and the temperature of the air around the liquid sensor  30 . 
       FIG. 4  is a schematic cross-sectional view taken along line IV-IV in  FIG. 3 . In  FIG. 4 , illustration of a detailed structure of the inside of the liquid sensor  30  is omitted. In  FIG. 4 , the same components as in  FIG. 1 to 3  are denoted by the same reference signs as in  FIGS. 1 to 3 . 
     Referring to  FIG. 4 , the hollow portion  21   a  of the liquid level gauge main body  21  has a circular cross-sectional shape in the cross section shown in  FIG. 4 . 
     As shown in  FIG. 4 , the optical axis X 1  of the light emitter  32  and the optical axis X 2  of the light receiver  33  coincide with each other. In addition, the optical axis X 1  of the light emitter  32  and the optical axis X 2  of the light receiver  33  extend to pass through the center C of the hollow portion  21   a  of the liquid level gauge main body  21 . Here, the optical axis X 1  of the light emitter  32  and the optical axis X 2  of the light receiver  33  do not necessarily need to pass through the center C of the hollow portion  21   a . It is preferable that the shortest distance from the optical axis X 1  of the light emitter  32  and the optical axis X 2  of the light receiver  33  to the center C of the hollow portion  21   a  is 5 mm or less. 
       FIG. 5  is a block diagram of the liquid sensor  30  of the present embodiment. In  FIG. 5 , the same components as in  FIGS. 1 to 3  are denoted by the same reference signs as in  FIGS. 1 to 3 . 
     Referring to  FIG. 5 , the liquid sensor  30  of the present embodiment includes a wireless transmitter  35  that wirelessly transmits a signal to an external apparatus, a wireless receiver  36  that wirelessly receives a signal from an external apparatus, and a control device  37 . 
     The wireless transmitter  35  communicates with the controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1 , and transmits an electric signal output from the light receiver  33  to the controller  13  of the hydraulic unit  1 . The wireless receiver  36  communicates with the controller  13  of the hydraulic unit  1 , and receives an electric signal such as a control signal from the controller  13  of the hydraulic unit  1 . The control device  37  controls the light emitter  32 , the light receiver  33 , the wireless transmitter  35 , and the wireless receiver  36 . 
     The liquid sensor  30  further includes a power source  38  for supplying (see a two-dot chain line in  FIG. 5 ) power to each of the light emitter  32 , the light receiver  33 , the wireless transmitter  35 , the wireless receiver  36 , and the control device  37 . The power source  38  of the present embodiment includes a rechargeable battery. In addition, the power source  38  is electrically connected to the thermoelectric conversion element  34 . The thermoelectric conversion element  34  generates electric power for charging the power source  38  using a temperature difference between the hydraulic oil tank  10  (shown in  FIG. 1 ) and air around the liquid sensor  30 . 
     (Measurement Using Liquid Sensor) 
     The light emitter  32  of the liquid sensor  30  according to the present embodiment emits white light from the light emitting element with the power supplied from the power source  38 . The light receiver  33  of the liquid sensor  30  receives light emitted from the light emitter  32  and having passed through the liquid level gauge main body  21  and the hydraulic oil in the liquid level gauge main body  21 . The liquid sensor  30  transmits the light intensity of each of colors of red, green, and blue of light received by the light receiving element of the light receiver  33  to the controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1  via the wireless transmitter  35  as an electric signal. Since each of the red, green, and blue lights received by the light receiver  33  is light having a wavelength that was not absorbed by the hydraulic oil in the level gauge  20 , it is possible to detect the hue of the hydraulic oil in the liquid level gauge  20  from the intensity of those lights transmitted by the light sensor  30 . In other words, the light receiver  33  of the liquid sensor  30  according to the present embodiment converts the hue being an example of the physical quantity of the hydraulic oil into an electric signal and outputs the electric signal. In short, the liquid sensor  30  according to the present disclosure outputs a signal related to a physical quantity (hue in the present embodiment) of liquid (hydraulic oil in the present embodiment). Here, the translucency of the liquid level gauge  20  according to the present disclosure is translucency with respect to light (white light in the present embodiment) emitted by the light emitter  32 . 
     The controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1  of the present embodiment determines the degree of deterioration of the hydraulic oil and the presence or absence of the hydraulic oil based on the signal related to the hue of the hydraulic oil output from the liquid sensor  30 . In other words, the liquid sensor  30  of the present embodiment outputs a signal related to the degree of deterioration of the hydraulic oil and a signal related to the presence or absence of the hydraulic oil. The degree of deterioration of the hydraulic oil and the presence or absence of the hydraulic oil are examples of physical quantities of the liquid (hydraulic oil in the present embodiment) according to the present disclosure. 
       FIG. 6  is an example of a calibration curve related to the hue used to determine the degree of deterioration of the hydraulic oil and the presence or absence of the hydraulic oil in the present embodiment.  FIG. 6  shows the intensity [arbitrary scale] of light for each color of red, green, and blue for air, water, and the hydraulic oil having different hues. In the present embodiment, the hue of the hydraulic oil is represented using an ASTM color measured conforming to ASTM D1500. The ASTM color numerically indicates the hue of the hydraulic oil by a color scale ranging from 0.5 of a light color to 8.0 of a dark color in increments of 0.5. The ASTM color is an index of the degree of deterioration of the hydraulic oil, and indicates that the larger the value of the ASTM color, the more the hydraulic oil is deteriorated. In the present embodiment, the calibration curve (or calibration table) as shown in  FIG. 6  is stored in the controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1 . 
     In the present embodiment, the controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1  determines the degree of deterioration of the hydraulic oil and the presence or absence of the hydraulic oil using the electric signals for each color of the red, green, and blue output from the liquid sensor  30  and the calibration curve shown in  FIG. 6 . Specifically, the controller  13  of the hydraulic unit  1  compares the intensity (hereinafter, referred to as a measurement result of the liquid sensor  30 ) of light for each color of red, green, and blue for the hydraulic oil to be measured, which are calculated from the output voltage of the light receiver  33  of the liquid sensor  30 , with the calibration curve shown in  FIG. 6 . The degree of deterioration of the hydraulic oil is calculated from the measurement result of the liquid sensor  30  using the calibration curve for the ASTM color in  FIG. 6 . In addition, the presence or absence of the hydraulic oil is determined based on whether or not the measurement result of the liquid sensor  30  indicates air by comparing the measurement result of the liquid sensor  30  with the calibration curve in  FIG. 6 . If determining that the measurement result of the liquid sensor  30  indicates air, the controller  13  of the hydraulic unit  1  determines that there is no hydraulic oil at the detection position (in the present embodiment, below the lower limit line LL) of the liquid sensor  30 . 
     When the temperature in a factory where the hydraulic unit  1  is installed is low, dew condensation occurs on the inner surface of the liquid level gauge main body  21  that defines the hollow portion  21   a , and the liquid sensor  30  may make an erroneous determination due to irregular reflection caused by the dew condensation or moisture mixing into the hydraulic oil. Therefore, the measurement using the liquid sensor  30  described above is preferably performed after it is determined that the hydraulic unit  1  is in a stable operation state by referring to the operation time or the oil temperature rise after the start of the hydraulic unit  1 . 
     (Notification by Liquid Sensor) 
     The liquid sensor  30  notifies the user of information on the physical quantity of the liquid (for example, abnormality) based on the light emission manner of the light emitter  32 . Specifically, the liquid sensor  30  notifies the user of the abnormalities related to the deterioration degree of the hydraulic oil and the presence or absence of the hydraulic oil based on the light emission manner of the light emitter  32 . 
     In the present embodiment, when the controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1  determines that the hydraulic oil has deteriorated more than the predetermined deterioration degree, or determines that there is no hydraulic oil at the detection position, the controller  13  of the hydraulic unit  1  transmits a control signal for controlling the light emitter  32  to the liquid sensor  30 . When the wireless receiver  36  of the liquid sensor  30  receives the control signal from the controller  13  of the hydraulic unit  1 , the control device  37  controls the light emitter  32  so that the light emission manner of the light emitter  32  changes according to contents to be notified. Thus, the liquid sensor  30  notifies the user of the abnormality (deterioration in hydraulic oil or decrease in hydraulic oil amount). The predetermined deterioration degree is, for example, a deterioration degree when the ASTM color becomes dark to 2.5 or more. 
     The light emission manner of the light emitter  32  is determined according to contents to be notified (in the present embodiment, deterioration of hydraulic oil and decrease in hydraulic oil amount). In other words, in the light emitter  32  of the present embodiment, the light emission manner is determined according to the signal related to the physical quantity of the hydraulic oil. When the deterioration of the hydraulic oil is notified, the light emitter  32  of the present embodiment blinks. In other words, when the deterioration of the hydraulic oil is notified, the light emission manner of the light emitter  32  changes from off to blinking. In addition, when the decrease in the hydraulic oil amount is notified, the light emitter  32  of the present embodiment is turned on. In other words, when the decrease in the hydraulic oil amount is notified, the light emission manner of the light emitter  32  changes from off to on. For example, the light emission manner of the light emitter  32  may change a blinking pattern such as a blinking interval and a blinking speed according to contents to be notified. 
     The controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1  may display a numerical value (for example, ASTM color value) or a color in accordance with the level of the deterioration degree of the hydraulic oil on a display unit  13   a  provided in the controller  13 , a display unit (not shown) provided in an external apparatus to which the hydraulic unit  1  supplies the hydraulic oil, or the like. Thus, it is possible to alert the user to replace the hydraulic oil before the hydraulic oil needs to be replaced. 
     In addition, the controller  13  (shown in  FIG. 1 ) of the hydraulic unit  1  may notify the user of deterioration of the hydraulic oil before the hydraulic oil needs to be replaced. For example, the controller  13  of the hydraulic unit  1  may notify the user of the deterioration of the hydraulic oil by causing the light emitter  32  to emit light even before the ASTM color becomes dark to 2.5 or more, for example. In this case, as the deterioration of the hydraulic oil progresses, the frequency of light emission of the light emitter  32  may increase. 
     According to the liquid sensor  30  of the present disclosure, since the light emitter  32  and the light receiver  33  are attached to the outside of the liquid level gauge  20  into which the hydraulic oil to be measured is introduced, a seal structure for preventing leakage of the hydraulic oil introduced into the liquid level gauge  20  to the outside is not required between the liquid sensor  30  and the liquid level gauge  20 , and workability of attachment work of the liquid sensor  30  to the liquid level gauge  20  can be improved. 
     In general, in the transmission type sensor, the surface of the liquid level gauge  20  has less influence on the detection accuracy than in other types of photoelectric sensors. In the above embodiment, since the liquid sensor  30  is a transmission type sensor, it is possible to perform reliable detection as compared with a case where a sensor of another type is used as the liquid sensor  30 . 
     In the above embodiment, since the light emitter  32  and the light receiver  33  are integrally provided in the casing  31  in a state where the optical axis of the light emitter  32  and the optical axis of the light receiver  33  are made to coincide with each other, it is not necessary to adjust the optical axes of the light emitter and the light receiver when the liquid sensor  30  is attached, so that workability of attachment work of the liquid sensor  30  to the liquid level gauge  20  can be improved. 
     According to the liquid sensor  30  of the present disclosure, since the liquid sensor  30  includes the rechargeable power source  38  that supplies power to the light emitter  32  and the light receiver  33  and the thermoelectric conversion element  34  that generates power for charging the power source  38 , wiring for supplying power from the outside to the liquid sensor  30  is not required, and the wiring structure can be simplified. 
     According to the above embodiment, since the detection result of the liquid sensor  30  is transmitted to the controller  13  of the hydraulic unit  1  via the wireless transmitter  35 , there is no need for wiring for outputting the signal output from the light receiver  33  to the outside, and the wiring structure can be simplified. 
     In the above embodiment, the liquid sensor  30  notifies the user of information on the physical quantity of the hydraulic oil by the light emission manner of the light emitter  32 . For example, when the detection result of the liquid sensor  30  indicates an abnormality, the light emitter  32  of the liquid sensor  30  emits light in a light emission manner corresponding to the abnormality, whereby the liquid sensor  30  notifies the user that the liquid sensor  30  has detected the abnormality. Thus, the user can easily recognize the information (for example, abnormality) on the physical quantity of the hydraulic oil by visually observing the light emission manner of the liquid sensor  30 . 
     In addition, since the light emitter  32  of the liquid sensor  30  has both a function as a projector of a photoelectric sensor and a function of notifying the user according to the signal related to the physical quantity of the hydraulic oil, the structure of the liquid sensor  30  can be simplified as compared with a case where these functions are separately provided. 
     In general, a liquid level gauge is installed at a place where a user can easily visually observe. Therefore, according to the above embodiment, since the liquid sensor  30  is attached to the liquid level gauge, the user can easily visually observe the light emitter  32  of the liquid sensor  30 , so that the user can easily recognize the information (for example, abnormality) on the physical quantity of the liquid to be measured. 
     In the present embodiment, the controller  13  of the hydraulic unit  1  determines the degree of deterioration of the hydraulic oil and the presence or absence of the hydraulic oil. However, the control device  37  of the liquid sensor  30  may determine the degree of deterioration of the hydraulic oil and the presence or absence of the hydraulic oil. In this case, the liquid sensor  30  does not need to include the wireless transmitter  35  and the wireless receiver  36 . 
     Second Embodiment 
     A liquid level gauge  120  of the second embodiment has the same configuration as the liquid level gauge  20  of the first embodiment except for including a float  123 , and the description of the first embodiment referring to  FIGS. 1 and 5  is applied to the second embodiment. In the second embodiment, components similar to those of the first embodiment are denoted by the identical reference signs, and the detailed description thereof will be omitted. 
       FIG. 7  is a schematic cross-sectional view similar to  FIG. 3  of the liquid level gauge  120  and the liquid sensor  30  of a third embodiment. 
     Referring to  FIG. 7 , the liquid level gauge  120  of the second embodiment includes a float  123  disposed in the hollow portion  21   a  of the liquid level gauge main body  21 . The float  123  is configured to float on the hydraulic oil. Thus, the float  123  indicates the position of the oil level of the hydraulic oil. The float  123  is made of a material having low transmittance with respect to the light emitted from the light emitter  32  so as to block the light emitted from the light emitter  32 . 
     The second embodiment produces actions and effects similar to those of the first embodiment. 
     In addition, according to the second embodiment, when the float  123  is at a level at which the light emitter  32  and the light receiver  33  are attached to the liquid level gauge  120 , the float  123  blocks the light emitted from the light emitter  32  and significantly changes the intensity of the light received by the light receiver  33 , it is possible to improve the accuracy of detecting the presence or absence of the hydraulic oil. 
     Third Embodiment 
     A liquid sensor  230  of the third embodiment has the same configuration as the liquid sensor  30  of the first embodiment except for the configuration of a thermoelectric conversion element  234 , and the description of the first embodiment referring to  FIGS. 1 and 5  is applied to the third embodiment. In the third embodiment, components similar to those of the first embodiment are denoted by the identical reference signs, and the detailed description thereof will be omitted. 
       FIG. 8  is a schematic cross-sectional view similar to  FIG. 3  of the liquid level gauge  20  and the liquid sensor  230  of the third embodiment. 
     Referring to  FIG. 8 , the thermoelectric conversion element  234  of the present embodiment is provided so as to be in contact with the head portion  22   b  of the bolt  22 B. 
     The third embodiment produces actions and effects similar to those of the first embodiment. 
     In addition, since the communication portion  22   c  of the bolt  22 B is filled with the hydraulic oil, the head portion  22   b  of the bolt  22 B made of metal tends to reach a high temperature by the heat of the hydraulic oil. According to the third embodiment, since the thermoelectric conversion element  234  is provided to be in contact with the head portion  22   b  of the metal bolt  22 B, the thermoelectric conversion element  234  can effectively generate electric power. 
     Fourth Embodiment 
     A liquid sensor  30  of the fourth embodiment has the same configuration as the liquid sensor  30  of the first embodiment except for the configuration of the light emitter  32  and the light receiver  33 , and the description of the first embodiment referring to  FIGS. 1 and 5  is applied to the fourth embodiment. In the fourth embodiment, components similar to those of the first embodiment are denoted by the identical reference signs, and the detailed description thereof will be omitted. 
     The light emitter  32  of the present embodiment includes a red light emitting diode, a green light emitting diode, and a blue light emitting diode. The light emitter  32  of the present embodiment can change the emission color by adjusting the amount of light emission of each of the red light emitting diode, the green light emitting diode, and the blue light emitting diode. 
     The light emission manner of the light emitter  32  is determined according to contents to be notified (in the present embodiment, deterioration of hydraulic oil and decrease in hydraulic oil amount). In the light emitter  32  of the present embodiment, the emission color changes according to the signal related to the physical quantity of the hydraulic oil. When the deterioration of the hydraulic oil is notified, the light emitter  32  of the present embodiment alternately changes the emission color to, for example, green and red. In addition, when the decrease in the hydraulic oil amount is notified, the light emitter  32  of the present embodiment alternately changes the emission color to, for example, blue and red. 
     The fourth embodiment produces actions and effects similar to those of the first embodiment. 
     Fifth Embodiment 
     A liquid sensor  30  of the fifth embodiment has the same configuration as the liquid sensor  30  of the first embodiment except for not including the light emitter  32 , and a detailed description thereof will be omitted.  FIG. 9  is a schematic diagram showing a state in which the hydraulic unit  1  according to the present embodiment is disposed in a factory. 
     Referring to  FIG. 9 , the light receiver  33  of the present embodiment may receive, for example, light L 1  from a fluorescent lamp F in the factory where the hydraulic unit  1  is installed, which passes through the hydraulic oil introduced into the liquid level gauge  20 . Alternatively, the light receiver  33  of the present embodiment may receive, for example, light L 2  shining into the factory through a window W of the factory where the hydraulic unit  1  is installed, which passes through the hydraulic oil in the liquid level gauge  20 . In this case, the liquid sensor  30  is preferably configured such that the casing  31  has translucency, or configured in an L shape by eliminating a portion facing the light receiver  33 . 
     The fifth embodiment produces actions and effects similar to those of the first embodiment. 
     In addition, according to the fifth embodiment, since it is not necessary to provide the light emitter, the configuration of the liquid sensor  30  can be simplified. 
     Sixth Embodiment 
     A liquid sensor  330  of the sixth embodiment has the same configuration as the liquid sensor  30  of the first embodiment except for being a reflective sensor, and detailed description of the same configuration will be omitted.  FIG. 10  is a schematic front view showing the liquid level gauge  20  and the liquid sensor  330  according to the present embodiment. 
     The liquid sensor  330  of the present embodiment is a reflective color sensor. As shown in  FIG. 10 , the liquid sensor  330  of the present embodiment includes a casing  331 , a light emitter  32  that emits light, and a light receiver  33  that receives light emitted from the light emitter  32 , passing through a liquid level gauge main body  21  having translucency, and reflected by the hydraulic oil in the liquid level gauge main body  21 . In the present embodiment, the light emitter  32  and the light receiver  33  arc disposed on the same side with respect to the liquid level gauge main body  21 . 
     The sixth embodiment produces actions and effects similar to those of the first embodiment. 
     According to the liquid sensor  330 , since the light emitter  32  and the light receiver  33  are arranged on the same side with respect to the liquid level gauge main body  21 , the liquid sensor  330  can be miniaturized as compared with a case where the light emitter and the light receiver are arranged across the liquid level gauge main body  21 . 
     In addition, since the liquid sensor  330  is a reflective sensor, it is not necessary to adjust the optical axis of the light emitter  32  and the optical axis of the light receiver  33 , and it is possible to easily attach the liquid sensor  330  to the liquid level gauge  20 . 
     Although the embodiments have been described above, it will be understood that various changes in form and details can be made without departing from the spirit and scope of the claims. 
     For example, in the first to sixth embodiments, the measurement target of the liquid sensor  30  is the hydraulic oil of the hydraulic unit  1 , but the measurement target of the liquid sensor  30  is not limited thereto, and another liquid such as cutting fluid or coolant liquid may be the measurement target. In other words, in the first to sixth embodiments, the liquid according to the present disclosure is hydraulic oil, but is not limited thereto, and may be other liquid such as cutting fluid, coolant liquid, or water. 
     In the first to sixth embodiments, the liquid sensor  30  is attached to the hydraulic unit  1 , but t the liquid sensor  30  may be attached to another machine or apparatus. 
     In the first to sixth embodiments, the liquid sensor  30  or  230  is attached to the liquid level gauge  20 , but the place where the liquid sensor is to be attached is not limited thereto, and the liquid sensor  30  or  230  has only to be attached to an object into which liquid is introduced and which has translucency. 
     In the first to sixth embodiments, the light emitter  32  and the light receiver  33  are provided integrally with the casing  31 , but the light emitter  32  and the light receiver  33  may be separately provided. 
     The light emitting element of the liquid sensor  30  may be a monochromatic light source of red, green, blue, or another color. 
     In the first to sixth embodiments, the deterioration of the liquid (hydraulic oil in the embodiment) is determined based on the intensity of light of each color of red, green, and blue output from the liquid sensor, but the way of determining the deterioration of the liquid is not limited thereto. For example, electric signals for red, green, and blue colors output from the liquid sensor may be respectively converted into cyan, yellow, and magenta colors to be used for determination of deterioration of the liquid. 
     In addition, in the first to sixth embodiments, the liquid level gauge  20  being an example of the accommodation unit includes the liquid level gauge main body  21  having a substantially rectangular parallelepiped shape, but the accommodation unit is not limited thereto. For example, the liquid level gauge  20  or  230  being an example of the accommodation unit may include a liquid level gauge main body with a semicircular cross-sectional shape in a cross section orthogonal to the longitudinal direction. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Hydraulic unit 
               10  Hydraulic oil tank 
               10   a  Side surface 
               11  Stand 
               12  Oil cooler 
               13  Controller 
               14  Cooling fan 
               20  Liquid level gauge (accommodation unit) 
               21  Liquid level gauge main body 
               21   a  Hollow portion 
               22 A,  22 B Bolt 
               22   a  Tip portion 
               22   b  Head portion 
               22   c  Communication portion 
               30  Liquid sensor 
               31  Casing 
               32  Light emitter 
               33  Light receiver 
               34  Thermoelectric conversion element 
               35  Wireless transmitter 
               36  Wireless receiver 
               37  Control device 
               38  Power source 
               120  Liquid level gauge (accommodation unit) 
               123  Float 
               230  Liquid sensor 
               234  Thermoelectric conversion element 
             HL Upper limit line 
             LL Lower limit line