05-10-2016, 11:46 AM
1457856968-loadcellpaper1.docx (Size: 373.11 KB / Downloads: 6)
ABSTRACT:
The aim of the paper is to form a signal conditioning circuit with the elements such as load cell and instrumentation amplifier. The Load cell is a transducer which is used to measure force or weight applied on it. The load cell is used for Original Equipment Manufacture(OEM) applications which include electronic scales and weighing machines. The strain gauge load cell may have a difficulty in providing a stable weight display. The value will move up and down continuously for the same weight. The change in weight is mainly due to the presence of moisture in the load cell or load cell cable. To prevent the difficulty in changing values, the condition of the load cell junction box and trimming circuit must be checked whether it is damaged or contaminated with moisture. The load cell produces its actual output in milliVolt (mV). The variation of voltage when load is applied is also very low. In order to do further processing, an instrumentation amplifier is added. The output of the load cell is given to the instrumentation amplifier. The instrumentation amplifier is a high gain differential amplifier which amplifies the output of the load cell. The output of the amplifier is then used for further processing. Here the further processing includes acquisition of the output voltage from INA125P to LabVIEW through myRIO. The gain of the instrumentation amplifier can be varied depending upon the gain resistance Rg.
INTRODUCTION:
LOAD CELL:
Load cell is a transducer which is normally used to measure the force or weight. It produces an electrical signal corresponding to the force applied on it. As the load is applied to the correct spot on the load cell, a proportional electric signal is provided by the strain gauge.[2]
Here the load cell used is CZL601.It is a strain gauge type single point load cell. In a strain gauge type load cell the strain gauges in a fashion similar to the resistor arrangement in a whetstone’s bridge. The excitation source voltage of load cell is 9V-12V.
When load is applied on the load cell two of these strain gauges undergo compression and the other two undergo tension, therefore change in resistance undergoes. These change in resistance is based on the fact that when length of a conductor changes its resistance changes.
To provide the highest possible output, strain is distributed uniform with gauges in the gauge area at the maximum strain location. For different measurand, the load cell response can change. In a system as the mass contributes to inertial parameters, when a load is applied the characteristics of the load cell changes.[3]
It is noticed that when the load is applied on one side of the load cell, the output increases and on the other side it decreases. In order to take to have a linear value of output voltage the load is applied on the increasing voltage side making the other side fixed. This arrangement is similar to a cantilever beam arrangement.
The output of the load cell is in the range of very few mV. So to acquire it using myRIO an instrumentation amplifier is used.
INSTRUMENTATION AMPLIFIER:
In analog signal processing applications, the instrumentation amplifier is used for the purpose of voltage mode measurement. The output of the load cell is in the range of very few millivolts. In order to do further process such as acquisition the output voltage of the load cell has to be amplified. For amplifying such small voltages we normally use an instrumentation amplifier. An instrumentation amplifier is a high gain dc coupled differential amplifier which is used in circuits where high accuracy and stability are required. The instrumentation amplifier used here is INA125 P by Texas instruments.
The small differential signals provided by the transducers in which there may be a large common mode signals or levels are amplified using instrumentation amplifier. The bridge arrangement of resistors i.e. strain gauge is an example where the strain is converted into resistance which provides the differential output voltage in mV. The instrumentation amplifier has high Common Mode Rejection Ratio (CMRR) and it amplifies the differential output voltage signals to volts.[4]
The instrumentation amplifier is powered up with the single supply (+15V ).The gain of the amplifier depends on the formula
G=4+ (60KΩ/Rg)
The output voltage of the amplifier is given by the formula
Vo= (VIN+- VIN-) G
It can be seen that the gain of the amplifier depends upon the resistance Rg. Therefore by changing the resistance Rg the desired gain for the amplifier can be obtained.
SIMULATION USING MULTISIM:
The above diagram shows the simulation of load cell using INA125P.
The load cell is given as a bridge circuit with four resistances on all the sides. The instrumentation amplifier is provided with a supply voltage of 15V. The gain resistance Rg is connected between the terminals 8 and 9 and all other necessary terminals are given corresponding to the circuit diagram.
The output of the amplifier is taken from the pin 10. The SENSE (pin 11) and OUT (pin 10 ) are shorted and the output is measured with the help of a multimeter
CONCLUSION:
Thus the signal conditioning,calibration and measurement of load cell using labview serves to be a method which finds great industrial applications such as force and torque measurement, load measurement in food processing industries and in measuring instruments such as weight or force measuring scales. Thus this method provides accurate measurement and control for industrial applications.