Factors that Affect Mechanical Property Measurement - Load cell Accuracy

  

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Mechanical Testing is that part of engineering design, development, and research that provides data about material properties. Testing is also required during manufacturing to ensure that a material or product meets some predefined specification. Critical to the process is the accuracy of the load cell used to take the load measurements.

• Hysteresis is the maximum difference in sensor output between measurements made from 0 to 100% full scale output (FSO) and from 100 to 0% FSO. Although hysteresis is easily measured, its mechanism is not fully understood.

• Linearity is the variation in the constant of proportionality between the sensor’s output signal and the measured physical quantity. It is often expressed in terms of a percentage of full scale output. No sensor is truly linear, so it is necessary to fit a straight line to the sensor’s output versus input graph. Aleastsquares fit is the most common method of fitting a straight line to the sensor’s output graph. The least-squares line is drawn through the sensor response curve, such that the sum of the squares of the deviations from the straight line is minimized. Before microprocessors, the accuracy of the system was largely dependent on the linearity of the sensor. However, it is now possible through multi-point calibrations to effectively map out the non-linearities.

• Noise is the magnitude of any part of the sensor’s output that is not directly related to the physical quantity being measured. Force and strain resolutions on most testing systems are user-programmable. The programmed resolution should always be greater than or equal to the noise.

• Sensor location is important because it must be where the required property can be accurately measured. One thing to consider is whether the sensor should be mounted on the input or output ends of a transmission. If the sensor is mounted on the input end of a transmission along with a motor, then the resolution of the system will be enhanced by a factor equal to the transmission ratio. However, backlash and compliance in the transmission, belts, ballscrews, test frame, grips, and fixtures will also affect the output of the sensor. On the other hand, if the sensor is mounted on the output end of the transmission, it will more accurately measure the process, but the resolution will be reduced.

For full article on Common Errors in Mechanical Testing click here