Patent Publication Number: US-2002003484-A1

Title: PLC device having variable resolution of temperature measurement

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to a PLC device that is connected to a temperature sensor, in particular to a PLC device that has a variable resolution of temperature measurement.  
       [0003] 2. Description of the Related Art  
       [0004] A thermocouple sensor and a thermistor sensor are typical of temperature sensors for measuring a temperature of a target object.  
       [0005] The thermocouple sensor can measure the temperature very precisely. Thus, the thermocouple sensor is widely used as a sensor achieving a high-precision temperature control in a medical field, for example for an intensive care unit or the like. Alternatively, the thermocouple sensor is also widely used as a sensor achieving a high-precision temperature control in an industrial field, for example for a machine tool such as an injection molding unit.  
       [0006] The thermistor sensor has an advantage that it is cheaper than the thermocouple sensor, although it is inferior to the latter in measurement precision. Thus, the thermistor sensor is widely used as a temperature sensor in cases wherein a temperature control need not be conducted high precisely, for example for an electric rice-cooker, a vending machine and so on.  
       [0007] In other words, it is preferable that the thermocouple sensor is used in view of measurement precision, but it is preferable that the thermistor sensor is used in view of cost.  
       SUMMARY OF THE INVENTION  
       [0008] The inventor has studied and studied how to achieve higher-precision temperature measurement by means of the cheaper thermistor sensor.  
       [0009] In general, among a variable temperature range of a target object, a temperature range whose temperature should be measured precisely is restricted narrowly. For example, in the case of an electric rice-cooker, the temperature range whose temperature should be measured precisely is about 80 to 150° C. Alternatively, in the case of an air conditioner, the temperature range whose temperature should be measured precisely is about 0 to 40° C. Alternatively, in the case of a refrigerator, the temperature range whose temperature should be measured precisely is around 10° C. Other parts of the respective variable temperature ranges need not to be measured so precisely.  
       [0010] Therefore, the object of this invention is to provide a PLC device having a variable resolution of temperature measurement for a part of a variable temperature range of a target object.  
       [0011] The invention is a PLC device comprising: an input part that can be connected to a temperature sensor that can output an voltage value depending on a temperature of an target object; an voltage amplifier connected to the input part; a switch that can be selectively connected to one of the input part and the voltage amplifier to be switched to one of a voltage value from the input part and a voltage value from the voltage amplifier; an A-D converter connected to the switch, the one of the voltage value from the input part and the voltage value from the voltage amplifier being inputted to the A-D converter and being converted into a digital value by the A-D converter; and a temperature obtaining part connected to the A-D converter, that can obtain a temperature of the target object from the converted digital value.  
       [0012] According to the invention, the temperature of the target object can be obtained (measured) with two resolutions that are by the voltage value from the input part and by the voltage value from the voltage amplifier.  
       [0013] It is very effective to improve measurement precision by using the voltage value from the voltage amplifier, especially if the temperature sensor has a thermistor whose resistance depends on the temperature of the target object.  
       [0014] Alternatively, this invention is a PLC device comprising: an input part that can be connected to a temperature sensor that can output an voltage value depending on a temperature of an target object; a first voltage amplifier connected to the input part; a second voltage amplifier connected to the input part; a switch that can be selectively connected to one of the first voltage amplifier and the second voltage amplifier to be switched to one of a voltage value from the first voltage amplifier and a voltage value from the second voltage amplifier; an A-D converter connected to the switch, the one of the voltage value from the first voltage amplifier and the voltage value from the second voltage amplifier being inputted to the A-D converter and being converted into a digital value by the A-D converter; and a temperature obtaining part connected to the A-D converter, that can obtain a temperature of the target object from the converted digital value.  
       [0015] According to the invention, the temperature of the target object can be obtained (measured) with two resolutions that are by the voltage value from the first voltage amplifier and by the voltage value from the second voltage amplifier. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a schematic block diagram of a PLC device according to an embodiment of the invention;  
     [0017]FIG. 2 is a graph showing a relationship between a temperature of the target object and an output of the temperature sensor; and  
     [0018]FIG. 3 is a graph showing a relationship between a temperature of the target object and an output of the voltage amplifier. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     [0019] Embodiments of the invention are explained in more detail with reference to the drawings.  
     [0020]FIG. 1 is a schematic block diagram of a PLC device according to an embodiment of the invention. As shown in FIG. 1, the PLC (Programmable Logic Circuit) device  10  of the embodiment comprises an input part  11  that can be connected to a temperature sensor  30  that outputs an voltage value depending on a temperature of an target object.  
     [0021] In the case, the temperature sensor  30  is formed as a sensor that outputs a voltage value based on a ratio of a resistance of a thermistor  31  to a voltage-divider resistance  32 . The resistance of the thermistor  31  depends on the temperature of the target object. There is a relationship as shown in FIG. 2 between the temperature of the target object and the output of the sensor (voltage value). That is, a temperature measurement range is about −30 to 130° C., which range corresponds to an output voltage range 0 to 5 V.  
     [0022] An voltage amplifier  12  is connected to the input part  11  of the PLC device  10 . In the case, as shown in FIGS. 2 and 3, the voltage amplifier  12  is adapted to amplify (4 times) the output voltage value from the sensor  30  (the input voltage value of the input part  11 ) of a range of 1 to 2 V and adjust an offset of the amplified voltage value into a range of 1 to 5 V. The amplified voltage range corresponds to a temperature range of 50 to 80° C.  
     [0023] An switching end of a switch  13  can be selectively connected to one of the input part  11  and the voltage amplifier  12  in order to be switched to one of the voltage value from the input part  11  and the voltage value from the voltage amplifier  12 . A common end of the switch  13  is connected to an A-D converter  14  into which the one of the voltage value from the input part  11  and the voltage value from the voltage amplifier  12  is inputted and by which the same is converted into a digital value.  
     [0024] In a normal state, the switch  13  is adapted to transfer the voltage value from the input part  11  to the A-D converter  14 . Only when the voltage value from the input part  11  comes within the range of 1 to 2 V, the switch  13  is switched to transfer the voltage value from the voltage amplifier  12  to the A-D converter  14 . In the case, the switch  13  is adapted to be automatically switched.  
     [0025] After that, if the voltage value from the input part  11  goes out from the range of 1 to 2 V, the switch  13  is switched back to transfer the voltage value from the input part  11  to the A-D converter  14 . In the case, the switch  13  is adapted to be automatically switched back.  
     [0026] To the A-D converter  14 , a temperature obtaining part  15  that can obtain a temperature of the target object from the converted digital value is connected.  
     [0027] The temperature obtaining part  15  has a first corresponding table  15   a  for obtaining the temperature of the target object from a digital value into which the voltage value from the input part  11  has been converted, and a second corresponding table  15   b  for obtaining the temperature of the target object from a digital value into which the voltage value from the voltage amplifier  12  has been converted. These tables  15   a  and  15   b  are made in advance based on various calibration experiments.  
     [0028] The temperature obtaining part  15  is adapted to use either of the first corresponding table  15   a  and the second corresponding table  15   b , based on a state of the switch  13 .  
     [0029] Then, an operation of the embodiment is explained as below.  
     [0030] In the embodiment, if the temperature of the target object changes, the resistance of the thermistor  31  also changes. Thus, the temperature sensor  30  outputs a voltage value based on the ratio of the resistance of the thermistor  31  to the voltage-divider resistance  32 .  
     [0031] While the temperature of the target object is not within the temperature range of 50 to 80° C., the switch  13  is adapted to transfer the voltage value from the input part  11  to the A-D converter  14 . Thus, the output value of the temperature sensor  30  is directly converted by the A-D converter  14 , and then transferred to the temperature obtaining part  15 .  
     [0032] The temperature obtaining part  15  is adapted to use the first corresponding table  15   a  in a state wherein the switch  13  is adapted to transfer the voltage value from the input part  11  to the A-D converter  14 . Thus, the temperature obtaining part  15  can obtain a temperature of the target object from the digital value converted by the A-D converter  14 , according to the first corresponding table  15   a.    
     [0033] When the temperature of the target object comes within the temperature range of 50 to 80° C., the switch  13  is automatically switched to transfer the voltage value from the voltage amplifier  12  to the A-D converter  14 . Thus, the value into which the output value of the temperature sensor  30  is amplified by the voltage amplifier  12  is converted by the A-D converter  14 , and then transferred to the temperature obtaining part  15 .  
     [0034] The temperature obtaining part  15  is adapted to use the second corresponding table  15   b  in a state wherein the switch  13  is adapted to transfer the voltage value from the voltage amplifier  12  to the A-D converter  14 . Thus, the temperature obtaining part  15  can obtain a temperature of the target object from the digital value converted by the A-D converter  14 , according to the second corresponding table  15   b , more precisely (4 times on the average).  
     [0035] When the temperature of the target object goes out from the temperature range of  50  to  80  ° C. again, the switch  13  is automatically switched to transfer the voltage value from the input part  11  to the A-D converter  14 . Thus, the output value of the temperature sensor  30  is converted by the A-D converter  14 , and then transferred to the temperature obtaining part  15 . Then, a temperature of the target object can be obtained, according to the first corresponding table  15   a  again.  
     [0036] As described above, according to the embodiment, the voltage value from the input part  11  and the voltage value from the voltage amplifier  12  are selectively converted by the A-D converter  14  so that the temperature of the target object can be obtained by means of two resolutions.  
     [0037] Especially, enhancement of the measurement precision while using the temperature sensor  30  formed by the cheaper thermistor is a very important advantage in cost.  
     [0038] In addition, in the embodiment, the switch  13  is automatically switched based on the voltage value from the input part  11 . However, the switch  13  may be manually switched. Alternatively, the switch  13  may be controlled based on the temperature value obtained by the temperature obtaining part  15  or the like.  
     [0039] In addition, in the embodiment, the temperature of the target object is obtained from the digital value by using the first corresponding table  15   a  or the second corresponding table  15   b . However, the temperature of the target object may be obtained from the digital value by using a formula through which a temperature value can be calculated from the digital value.  
     [0040] The temperature range for the higher-precision temperature measurement and the amplification can be freely changed. It is also possible to set a plurality of temperature ranges for the higher-precision temperature measurement. In addition, it is possible to achieve three resolutions of temperature measurement by further amplifying a part of the once amplified range. If a plurality of temperature ranges is set for the higher-precision temperature measurement, a plurality of offset-adjusting circuits and a plurality of corresponding tables may be necessary correspondingly to the respective temperature ranges. The plurality of temperature ranges set for the higher-precision temperature measurement may overlap with each other.  
     [0041] In addition, a second voltage amplifier  12 ′ may be arranged between the input part  11  and the switch  13 . Then, the voltage value from the (first) voltage amplifier  12  and a voltage value from the second voltage amplifier  12 ′ are selectively converted by the A-D converter  14 , so that two resolutions of temperature measurement can be achieved.  
     [0042] As described above, according to the invention, the temperature of the target object can be obtained (measured) by means of the two resolutions that are by the voltage value from the input part and by the voltage value from the voltage amplifier.  
     [0043] Alternatively, according to the invention, the temperature of the target object can be obtained (measured) by means of the two resolutions that are by the voltage value from the first voltage amplifier and by the voltage value from the second voltage amplifier.