WO2016124274A1

WO2016124274A1 - A method to improve sensor accuracy using multiple shift resistors and a system thereof

Info

Publication number: WO2016124274A1
Application number: PCT/EP2015/078458
Authority: WO
Inventors: Shakkeer M T P MUHAMMED; Selvakumar BALASUBRAMANIAN; Rajesh Kumar SUDHAKAR; Balaji Arcot SUNDARAVADIVELU
Original assignee: Danfoss A/S
Priority date: 2015-02-06
Filing date: 2015-12-03
Publication date: 2016-08-11

Abstract

The invention discloses a method to improve sensor accuracy by using multiple pull-up/pull-down resistors as shift resistors. Generally, it is easy to choose a single value for pull up resistor to achieve sensor accuracy for using one resistor at one input port of microcontroller. This method is used where a single input port of microcontroller has to support measurement of physical variables of many sensors. The method uses multiple resistors to improve the sensor accuracy. Different shift resistance combinations used in the method give different voltage effects to same input voltage under same detection point of physical variable which pulls up or pulls down the input voltage at ADC pin. The method reduces hardware complexity, increases reliability, simple in operation and high in sensor accuracy. Further, the present invention also relates to a system for improving sensor accuracy to measure temperature, pressure and other physical variables by using multiple shift resistors.

Description

A METHOD TO IMPROVE SENSOR ACCURACY USING MULTIPLE SHIFT

RESISTORS AND A SYSTEM THEREOF

TECHNICAL FIELD

The invention relates to a method for improving sensor accuracy. More particularly, the invention aims at achieving an improved sensor accuracy to measure temperature, pressure and other physical variables by using multiple shift resistors in applications where single input port needs to support measurement of physical variables of multiple sensors. Further, the present invention also relates to a system for improving sensor accuracy to measure temperature, pressure and other physical variables by using multiple shift resistors.

BACKGROUND

Generally, sensors are used to provide input signals, for example, to an industrial controller or the like, such signals allowing the controller to respond to the machine or the process and to generate outputs to actuators affecting the operation of the machine or the process. Therefore, the measurements of sensors should be very accurate.

CN203785709 discloses a multi-sensor data concentration acquisition system comprising a signal acquisition module, a data acquisition card, and sensor data acquisition units. Further, the signal acquisition module comprises various types of sensors like temperature sensors, flow sensors, voltage sensors, current sensors, and pressure sensors. In patent CN203785709, each sensor data acquisition unit comprises a single-chip microcomputer integrated with an internal A/D converter which makes the utility model bulky, complex and costly. CN 103885379 discloses a rotating speed type multisensory signal collecting device. Accordingly, the collection device comprises a power supply module, an industrial personal computer, a PLC, a signal conditioning circuit and an AD collection card. Further, the PLC transmits control signals for switching on and off the signal conditioning circuit to receive the signals transmitted by the sensor. Further, CN 103885379 discloses a rotating speed type multisensory signal collecting device.

CN203657830 discloses a utility model applied to the field of a vehicle-mounted sensor, and provides a voltage output type multifunctional sensor. Further, a multifunctional sensor is simultaneously provided with a temperature detection function, a humidity detection function and a light intensity detection function.

FR2998062 discloses a measuring circuit which uses one or multiple sensor resistance terminals electrically connected with a sensor resistor element.

FR2998062 further discloses a computing unit which is provided to control the resistance value of the sensor resistance element at the selected sensor resistor terminal. However, FR2998062 uses a multiplexer of selection which connects one component of resistance of measurement and one connection of resistance of sensor EP2546984 discloses sensor with concurrent autosensing of output mode and manual selection. Further, EP2546984 uses a threshold circuit which receives sensor signal to produce a switched signal based on sensor signal. Also,

EP2546984 discloses using auto-detection circuit which routes switched signal to solid-state switching devices like p-n-p and n-p-n transistors. EP2546984 uses a micro-processor to monitor sensor output voltage. Furthermore, EP2546984 uses sensor to measure optical intensity, magnetic reluctance, temperature, pressure and capacitance.

CN202442727 discloses a utility model which relates to a multi-sensor collection data based monitoring device for a hot pressing process of a manmade board, and belongs to the technical field of the wood processing industry.

CN202442727 discloses a complex utility model which uses a human computer interaction interface and central processing unit (CPU) of a computer.

US 6494616 discloses multiplexed sensor array which includes a plurality of sensors connected to two or more groups of wires such that individual sensors can be addressed. Further, rectifiers are connected in series with the sensors to significantly reduce reverse current through a sensor that can lead to significant errors. Furthermore, the array includes a plurality of temperature sensors, especially for use in measuring temperature at different depths in a body of water.

US 6704679 discloses a system for acquisition of logic states including sensors operable in all-or-nothing mode, each sensor including a switch linked with a pulling resistor between ground and power-supply potential. US 6704679 uses a multiplexer to shift between different sensors used in the invention. Further, US 6704679 discloses that each sensor has its own resistor.

DE3327653 discloses a resistor structure on a multi-sensor silicon chip. Further, four different resistor structures are provided on a silicon chip having a monocrystalline membrane, and by integrating, in the membrane, a first, second and third resistor structure, each formed from four pressure-sensitive

semiconductor resistors as a coherently doped region and connecting the four resistors to form a Wheatstone bridge in each case. The objective of DE3327653 is also to measure physical variables like temperature using sensor elements.

Methods and systems to measure the physical variables of sensors like temperature, pressure, intensity etc. as known in the prior art have drawbacks associated therewith including :

1. Complexity in hardware.

2. Bulky apparatus

3. Expensive components are used.

4. Less accuracy of measurement when using a sensor from a plurality of sensors. OBJECTS

The main object of the present invention is to provide a method for improving sensor accuracy in the measurement of physical variables like temperature, pressure and light etc. Another object of the present invention is to provide a support to a plurality of sensors by using plurality of pull-up/ pull-down resistors.

Yet another object of the present invention is to provide a method where single input port can support a sensor for measurement of physical variables like temperature, pressure, light etc. from plurality of sensors. Another object of the present invention is to provide a system to improve sensor accuracy when measuring physical variable by using multiple shift resistors.

Still another object of the present invention is to provide a system to improve sensor accuracy in the measurement of physical variable which decreases the hardware complexity involved. Yet another object of the present invention is to provide a simple and cost- effective method for improving sensor accuracy in the measurement of physical variables.

Yet another object of the present invention is to provide a system to improve sensor accuracy when measuring physical variable by using multiple shift resistors useful for measuring temperature in a refrigeration system.

SUMMARY

Accordingly, the present invention provides a method to improve sensor accuracy when measuring a physical variable by using multiple shift resistors, said method comprising : a) accessing a single sensor which is pre-configured from a plurality of sensors which senses a physical variable to produce a signal based on the physical variable; b) using the signal of the sensor to select a resistance from plurality of combinations of resistances which are obtained by different combinations of plurality of resistors; c) producing a center tap voltage corresponding to the resistance; d) producing a shift in the center tap voltage to obtain a shift voltage and sending the shift voltage to an analog to digital converter (ADC) to produce an ADC data; e) receiving the shift voltages for all said combinations of shift resistors at the ADC to produce an average of the shift voltages ; and f) converting the ADC data to actual physical variable reading.

In another embodiment of the present invention, the physical variable used is selected from the group consisting of temperature, pressure and light etc.

In yet another embodiment of the present invention, the sensor used is wired in series with a resistor from a plurality of said resistors to form a voltage divider and resistor used is connected to a power supply at one end.

In still another embodiment of the present invention, the resistors used are connected between a center tap of the voltage divider and micro-controller input/output pins.

In another embodiment of the present invention, the sensors used are two wire sensors. In yet another embodiment of the present invention, the output of sensor used is converted into digital form by using an analog-to-digital converter (ADC).

In still another embodiment of the present invention, the resistors used are pull- up resistors or pull-down resistors. In yet another embodiment of the present invention, atleast one resistor is a pull-up resistor among the plurality of resistors.

The present invention also provides a system to improve sensor accuracy when measuring a physical variable by using multiple shift resistors, said system comprising : a pre-configured sensor selected from a plurality of sensors capable of sensing physical variable to produce a signal based on the physical variable; a plurality of resistors; and a micro-controller unit comprising of an analog to digital converter; wherein each resistor at one end being wired in series with the sensor to form a center tap to obtain center tap voltage, second end of atleast one resistor being connected to a power supply and second end of other resistors being connected to pins of said micro controller unit; wherein the analog to digital converter of said micro-controller unit adjusting value of voltage obtained from said center tap to improve the sensor accuracy to measure the physical variable.

In another embodiment of the present invention, the physical variable measured is selected from the group of temperature, pressure and light etc.

In yet another embodiment of the present invention, the resistors used in the system are pull-up resistors or pull-down resistors. In still another embodiment of the present invention, at least one resistor used in the system is a pull-up resistor amongst the plurality of resistors.

In yet another embodiment of the present invention provides a refrigeration system comprising : a system to improve sensor accuracy by using multiple shift resistors as provided by the present invention; and an electronic refrigeration controller; wherein the electronic refrigeration controller is integrated with the system to control the operation of compressor, fan, defrost heater based on temperature reading obtained from the system.

In still another embodiment of the present invention, the electronic refrigeration controller further comprises of plurality of switches to receive digital input to operate the compressor, the fan and the defrost heater.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings wherein :

Figure 1 shows a simple voltage divider circuit to measure the resistance change due to change in temperature where the resistance change is converted to a voltage change. Pull up Resistor, Sensor resistance are designated by Rland Rt respectively, capacitor is designated by C, and Power Supply is designated by Vcc.

Figure 2 shows a circuit diagram for obtaining different combination of shift in voltage by using different combinations of resistors. First resistor, second resistor, third resistor and sensor resistance are designated by Rl, R2, R3 and Rt respectively, capacitor is designated by C and Power Supply is designated by Vcc.

Figure 3 shows a diagram with different colours of lines depicting different shift resistor combinations. Figure 4 shows a diagram for analog to digital conversion result of input voltage difference as shown in figure 3.

Figure 5 shows a diagram for arrangement of shift resistors where one resistor is being pulled up. First resistor, second resistor and Sensor resistance are designated by Rl, R2 and Rt respectively. Figure 6 shows a diagram when micro-controller unit (MCU) or any other logic device configures the pin as open drain and Resistor (R2) is out of action.

Sensor resistance is designated by Rt.

Figure 7 shows a diagram for arrangement of shift resistors where one shift resistor is being pulled down. First resistor, second resistor and sensor resistance are designated by Rl, R2 and Rt respectively.

Figure 8 shows an electronic refrigeration controller using the method to improve sensor accuracy while measuring temperature by using multiple shift resistors.

Figure 9 shows a flowchart of transforming of analog voltage of ADC to a physical variable.

While the invention is described in conjunction with the illustrated embodiment, it is understood that it is not intended to limit the invention to such embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention disclosure as defined by the claims. DETAILED DESCRIPTION

A method to improve sensor accuracy when measuring a physical variable using multiple resistors as shift resistors will now be described in detail with reference to the accompanying drawings. Generally, to measure the change in resistance due to change in temperature, pressure or other physical variables, the change in resistance is converted to a voltage change with a simple voltage divider. Voltage dividers can be used to allow a microcontroller to measure the

resistance of a sensor. The sensor is wired in series with a known resistance to form a voltage divider and a known voltage is applied across the divider. The microcontroller's analog-to-digital converter (ADC) is connected to the center tap of the divider so that it can measure the tap voltage and, the sensor resistance is computed by using the measured voltage and the known resistance and voltage. The output voltage varies proportional to change in temperature or pressure or any other physical variable. The output voltage is measured by using an ADC (Analog to Digital Converter) and the read voltage is mapped to the physical variable like temperature value in ADC software application. (Reference: wikipedia)

Specifically, for the applications where temperature measurement and

monitoring are necessary, typically thermistors or resistive temperature detectors (RTD) are widely used. In such applications, change in temperature is detected by measuring change in resistance of sensors which varies proportional to temperature changes.

Figure 1 shows a simple voltage divider circuit to measure the resistance change due to change in physical variable where the change in resistance is converted to a voltage change. Accordingly, a pull-up resistor value which is designated by Rl has to be selected by considering the resolution of ADC used and resistor change of sensor as compared to physical variable. The capacitor (C)

compensates for the load capacitance of the circuit. The selected value of pull-up resistor should satisfy to meet the specification for accurately measuring the physical variable of the sensor which can be temperature, pressure or any other physical variable. However, for an application, where a single input port can support measurement of physical variable from plurality of sensors, in such cases, it is difficult to choose a single pull-up value resistor. For example, a single port can be used for measuring two types of sensors: sensor type 1 and sensor type 2. In such cases, it is difficult to choose a single pull-up value resistor Rl that can achieve sensor resolution. Generally, it is difficult to use a single pull up resistor for multiple sensors, as each type of sensor has its own range of resistance and curve. Accordingly, the sensor type-1 may require a higher value pull-up resistor, while the sensor type 2 may require a lower pull-up value for accurate measurement of the physical variable from the sensor. Thereafter, a moderate value has to be selected which further leads to losing accuracy specification in one type of sensor. Even if a resistance value is selected for a resistor such that it is a moderate value amongst various sensors, the accuracy of the physical variable read would be affected. The sensors used can be two wire sensors. In order to improve sensor accuracy, a method to improve sensor accuracy using multiple resistors as shift resistors is used as is described in figure 2. The basic idea in this method is that under same detection of physical variable, that is, same voltage input from the perspective of selected sensor resistance value which corresponds to the physical variable of selected sensor, the input voltage to the ADC pin is shifted up and down based on the combination of pull up resistors and the selected shift resistors. The shifted value of voltage could represent change in the physical variable to increase sensor accuracy by two times. Accordingly, the shift in voltage to ADC input can be achieved by using additional pull-up and pull-down resistor along with existing pull up resistor Rl. The capacitor (C) compensates for the load capacitance of the circuit. Pull up/pull-down resistors from circuit are enabled and disabled to get different combination of shifted voltage. This method can be applied to all types of sensors. Accordingly, the value of shift resistors will change based on the range of the resistances of the sensors used. The values of shift resistors are chosen such that the net effect of the shift is zero. Some of the values obtained from combination of shifted voltage have high quantization noise and some have low quantization noise. These values from all combinations of shifted voltage are averaged, thereby increasing the accuracy.

The number of shift resistors to be used depends on the hardware resource available. However, with increase in the number of shift resistors, the probability to reduce the quantization error increases, thereby increasing the accuracy to measure the physical variable.

Each shift resistor can be connected to a Micro-Controller Unit (MCU) port or a Logic device which can pull the resistor to +5V or pull down the resistor to GND (Ground) or make itself as an open drain output, so that it is out of action.

Figures 5 to 7 show different arrangements of shift resistors to be connected to micro-controller unit.

Using any two microcontroller General Purpose Input Output (GPIO) pins which are connected to one end of these pull-up/pull-down resistors, different combinations can be achieved. Further, the GPIO pin can be configured as an output pin by using three modes: a) writing high (to select as pull-up) b) writing low (to select as pull-down) c) high impedance mode (to isolate the resistor from the circuit).

Selection of shift resistors should be done in a manner, so that a deviation is not induced in the measured physical variable. The values of shift resistors are chosen such that the net effect of the shift is zero.

Multiple shift resistors can be combined with a pull-up resistor to detect the physical variable within the range of the sensors. The maximum number of combinations depend on the total number of shift resistors and one pull up resistor for detection of voltage of a single sensor which is pre-configured from multiple sensors. An individual pull-up resistor is required for each sensor. A pull-up resistor for one sensor can be used as a shift resistor for another sensor. The number of pull-up sensors can be increased based on the requirement.

Since different combinations of using multiple shift resistors and one pull up resistor will give different effect to the same supply voltage under the same detection of physical variable, the input voltage to the ADC pin will be pulled up or pulled down based on the combination of pull up resistor and selected shift resistors. ADC result to every single combination is step-changed at particular range of the measured physical variable. Since there are multiple lines at one point of physical variable, there is always a step change on one or more than one combination lines. By using such kind of method, it is possible to get fine resolution than original ADC. Analog voltage value is converted to digital voltage value by using the formula given below :

Voltage (Digital value) = Vadc / (Vdd / 2n) where n represents the number of bits of micro-controller used, Vadc is voltage at ADC, Vdd is power supply. A system incorporating the method of increasing sensor accuracy when measuring a physical variable by using multiple shift resistors is described now. A pre-configured sensor is selected from a plurality of sensors which is capable of sensing physical variable to produce a signal based on the physical variable. A micro-controller unit (MCU) comprises of an analog to digital converter (ADC). Each resistor at one end is wired in series with the sensor to form a center tap to obtain center tap voltage. The second end of atleast one resistor is connected to a power supply and second end of other resistors is connected to pins of said micro controller unit. The analog to digital converter of the micro-controller unit adjusts the value of voltage obtained from the center tap to improve the sensor accuracy to measure the physical variable. The resistors used are pull-up resistors or pull-down resistors. However, atleast one resistor is a pull-up resistor amongst the plurality of resistors.

A typical low cost electronic refrigeration system has the following functionalities as described in figure 8. The controller is responsible for maintaining the desired temperature inside the cabinet based on cooling action by the refrigeration cycle. The key elements of the systems are compressor, evaporator, condenser, expansion device, sensors and relays. The electronic refrigeration controller controls the operation of the key elements of the system depending on the temperature of cabin air, evaporator and condenser. The resistance of the temperature sensor varies based on the temperature. Accordingly, the temperature is sensed by the controller through a voltage divider network.

The compressor is switched on when the temperature inside the cabinet rises above the preconfigured level and switched off when the temperature reaches the desired level. For example, the controller can be configured to switch on the Compressor when the cabinet temperature reached 2 Degree Celsius and switch off when the cabinet reached - 1 Degree Celsius. Apart from cabinet sensor, the condenser temperature and evaporator temperatures are measured using temperature sensors like NTC5K, NTC10K, PT1000 or PTC. The resistance of the sensors depend on the temperature. Accordingly, the temperature is measured through a voltage divider connected to the ADC .The analog voltage measured by ADC is transformed to temperature as shown in a flowchart of figure 9.

The defrosting of evaporator coil is either time based or temperature based, according to the configuration done by the end user based on the system designed. Defrosting can be done by switching off the compressor, by switching on the electric heater or by hot gas defrost. The Fan must be switched on to make the best use of the cooling action of the compressor. Defrost could also be triggered by front button or push button on the controller. Defrost can exit based on time or based on the evaporator temperature. The measured evaporator temperature is also used to delay the fan switch ON, to avoid unwanted distribution of air at high temperature. Condenser temperature is also monitored to switch off the compressor if the condenser temperature rises above the desired level. Two digital inputs are also used to provide data for defrost switch and pull down switch to Electronic refrigeration controller.

The analog voltage measured by ADC is transformed to temperature as shown in a flowchart of figure 9. It starts when the power is ON. ADC channels are configured in the MCU. Sensor type is read from configuration file at port 1. If accuracy improvement in the sensor is not required, normal sampling process without shift resistance method is performed. Further, if accuracy improvement in the sensor is required, the shift resistors are selected by performing the required GPIO settings. Still further, the Port 1 ADC input voltage is read and stored in a variable. Then, combination count is incremented and GPIO setting is performed for next combination of shift resistors. Again, Port 1 ADC input voltage is read. This read value is added to the variable "Accumulated voltage". A check is performed that all the calculations for all shift resistor combinations are done or not to find the ADC input voltage. If the calculations for all shift resistor combinations are not done, then the step of incrementing combination count is performed and GPIO setting for next combination of shift resistors is done. When the calculations for all shift resistor combinations are done, then value of average ADC voltage is found by diving accumulated voltage by number of combinations. Thereby, physical value of temperature, pressure or any other physical variable corresponding to average ADC voltage is found This ADC voltage is transformed to temperature using lookup table which varies between each different sensor types. The same procedure is repeated on other port to read the temperature on other sensors.

EXAMPLE 1 (Comparative) : Accuracy of sensor without using multiple shift resistors

In a typical application where temperature can be measured, one port is taken to be compatible for measurement of both PT1000 and NTC sensors. The ADC used is 10-bit resolution and reference voltage for the ADC is 5V. The pull-up value is selected as 3.9KOhm (Rl) with trade-off that PT1000 sensor loses accuracy but NTC meets the specified accuracy with 3.9 kilo-Ohm.

Table 1 provides resistance value for PtlOOO sensor for a set of temperature between -3°C to +3°C. The circuit used is same as shown in figure 1. The value selected for Rl is 3.9 kilo Ohms. Temperature •R-ptlOOO (ohms)

-3 988.3

J2 992.2

-1 996.1

0 1000

1 1003.9

2 1007.8

3 1011.7

Table-1

The Change in resistance for 1°C must create a voltage difference greater than the ADC resolution. By using circuit as shown in figure 1, from calculation, it can be easily found that the 1°C temperature variation from table- 1 leads to less than 4mV variation.

Since ADC in the Micro-controller Unit (MCU) has 10 bit resolution, therefore, the ideal minimum voltage resolution is 5/1024 = 4.883mV. So, the basic resolution of ADC to the PT1000 sensor temperature is more than 1°C. EXAMPLE 2 : Enhancing the accuracy of sensor by using multiple shift resistors

The method of using multiple pull up/pull down resistors as shift resistors is used to improve PT1000 accuracy up to ± 0.5°C. Under same detection temperature, (i.e. same input voltage from the perspective of the sensor resistance value which corresponds to temperature of the sensor used) the input voltage to the ADC pin is shifted up and down based on the combination of pull up resistors and the selected shift resistors. Considering the input voltage to be 'Vo', the typical shifted value of voltage is +2/5VLSB (Voltage corresponding to 1 least significant bit (LSB)), + 1/5VLSB, -2/5VLSB and -1/5VLSB. Voltage

corresponding to 1 least significant bit can be 4.88mV. The shifted value represents the change in temperature corresponding to a range of ± 0.25°C to ± 0.5°C. The shift in voltage to ADC input is achieved by using additional pull-up and pull-down resistor along with existing pull-up resistor 3.9 kilo Ohms. These pull-up/pull-down resistors are enabled and disabled from the circuit to get different combination (as shown in table 2) for detection of voltage of a single sensor which is preconfigured. The circuit for the method used is shown in figure 2. Using any two microcontroller GPIO pin, which are connected to one end of these pull-up/pull-down resistors, nine combinations are obtained. GPIO pin can be configured as output pin with a) writing high (to select as pull-up) b) writing low (to select as pull-down) c) in high impedance mode (to isolate the resistor from the circuit).

510KOhm (R2) and 820KOhm (R3) are selected approximately and they are easily available in the market. Two shift resistors are combined with original pull-up resistor for any detection of temperature. Hence, maximum nine combinations are obtained for shifting sensor detection voltage as shown in Table-2.

Figure 3 shows a diagram with different colours of lines depicting different shift resistor combinations. X-axis is used for plotting the detection temperature and Y-axis represents the values of voltage at ADC input. Different colours of lines show the different shift resistor combinations used. Based on shift resistor combinations used, it can be noted how the voltages change.

Figure 4 shows a diagram for analog to digital conversion result of input voltage difference as shown in figure 3. Here, X-axis is used to plot the detection temperature and Y-axis represents digital values after conversion from analog values using ADC module. Here, the ADC result to every single combination of using shift resistors is step-changed about every 1.5°C. This value of 1.5°C can vary depending on the value and number of shift resistors used to get a net zero shift. However, since there are nine lines at one temperature, so at every 0.5°C points, the lines are always step changed on one or more than one combination lines. Accordingly, by using the method, it is possible to get fine resolution than by using original ADC. The experimental result for using this method shows that accuracy of sensor PT 1000 is ± 0.5°C.

Table-2

Since different shift resistance combination gives different effect to completely same input voltage under same detection temperature, the input voltage to the ADC pin is either pulled up or pulled down based on the combination of pull up resistor and the value of the shift resistors.

ADVANTAGES :

1. The method for improvement of sensor accuracy provides better accuracy for measurement of physical variables.

2. The method for improvement of sensor accuracy provides a high performance with less hardware complexity. 3. The method for improvement of sensor accuracy provides simple and cost-effective solution.

4. The method for improvement of sensor accuracy is reliable as the quantization error is reduced for different shift voltages. 5. The system with improvement in sensor accuracy gives better accuracy for measurement of temperature and other physical variables.

Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.

Claims

1. A method to improve sensor accuracy when measuring a physical variable by using multiple shift resistors, said method comprising : a) accessing a single sensor which is pre-configured from a plurality of sensors which senses a physical variable to produce a signal based on said physical variable; b) using said signal of said sensor to select a resistance from plurality of combinations of resistances which are obtained by different combinations of plurality of resistors; c) producing a center tap voltage corresponding to said resistance; d) producing a shift in said center tap voltage to obtain a shift voltage and sending said shift voltage to an analog to digital converter (ADC) to produce ADC data; e) receiving said shift voltages for all said combinations of shift resistors at said ADC to produce an average of said shift voltages ; and f) converting said ADC data to actual physical variable reading.

2. The method as claimed in claim 1, wherein said physical variable used is selected from the group consisting of temperature, pressure and light.

3. The method as claimed in claim 1, wherein sensor used is wired in series with a resistor from a plurality of said resistors to form a voltage divider and resistor used is connected to a power supply at one end.

4. The method as claimed in claim 1, wherein resistors used are connected between a center tap of the voltage divider and micro-controller input/output pins.

5. The method as claimed in claim 1, wherein sensors used are two wire sensors.

6. The method as claimed in claim 1, wherein output of sensor used is converted into digital form by using an analog-to-digital converter.

7. The method as claimed in claim 1, wherein resistors used are pull-up resistors or pull-down resistors.

8. The method as claimed in claim 1, wherein atleast one resistor is a pull-up resistor amongst the plurality of resistors.

9. A system to improve sensor accuracy when measuring a physical variable by using multiple shift resistors, said system comprising : a pre-configured sensor selected from a plurality of sensors capable of sensing physical variable to produce a signal based on said physical variable; a plurality of resistors; and a micro-controller unit comprising of an analog to digital converter; wherein each resistor at one end being wired in series with said sensor to form a center tap to obtain center tap voltage, second end of atleast one resistor being connected to a power supply and second end of other resistors being connected to pins of said micro controller unit; wherein said analog to digital converter adjusting value of voltage obtained from said center tap to improve the sensor accuracy to measure said physical variable.

10. The system as claimed in claim 9, wherein physical variable used is selected from the group of temperature, pressure and light.

11. The system as claimed in claim 9, wherein resistors used are pull-up resistors or pull-down resistors.

12. The system as claimed in claim 9, wherein atleast one resistor is a pull-up resistor amongst the plurality of resistors.

13. A refrigeration system comprising : a system to improve sensor accuracy by using multiple shift resistors as claimed in claim 9; and an electronic refrigeration controller; wherein said electronic refrigeration controller is integrated with said system to control the operation of compressor, fan, defrost heater based on temperature reading obtained from said system.

14. The system as claimed in claim 13, wherein said electronic refrigeration controller further comprises plurality of switches to receive digital input to operate on said compressor, said fan and said defrost heater.