BMEN 3840 Biomedical Engineering Laboratory

 

Calibration of Load Transducer

Section Instructor Professor Edward Guo

Teaching Assistants:   Mark Eichler and Chi Hyun Kim, Biomedical Engineering Graduate Students

 

Location Bone Bioengineering Laboratory, 365 Engineering Terrace

 

Introduction

Determining mechanical properties of biological tissues is important in characterizing their normal physiological functions, alteration in diseased states and design of artificial replacement. In order to perform a mechanical test, mechanical force is routinely measured using various load transducers. Most load transducers involve different combinations of strain gages fixed to certain components inside the transducer. When subject to mechanical forces, the component deforms and induces deformation of the strain gages. The elongation/shortening of strain gages produces changes in resistance of the strain gages. These changes in resistance can be detected using a resistance bridge inside the transducers. In general, a voltage (excitation voltage) is applied to the resistance bridge and the changes in strain gage resistance reflect as changes in output signal voltage. This signal voltage will be amplified to achieve measurable signals. Before any load transducer can be used to measure mechanical forces, it has to be calibrated such that a relationship between the output voltage signal and load magnitude can be established. In this experiment, the concept of load transducer calibration will be introduced. The similar calibration procedure has been used to calibrate the load transducer used in the biomechanical test of cortical bone specimen in this phase of laboratory rotation. The objectives of this experiment are

  1. to teach  the basic concepts of load transducer calibration;
  2. to establish the relationship between the voltage signal and gain factor of the signal amplifier;
  3. to provide familiarity with the conversion of voltage signal output (analog output) and mechanical load using a linear regression;

 

Materials and Methods

A 50 lbs. Load transducer (220 Newtons) will be used for this experiment (MTS 661-11A). The load transducer is connected to a strain gage conditioner (Micro Measurements, 2120A) where the excitation voltage and gain can be set for the load transducer. The output signal will be measured with a multimeter (Fluke 45). The students are required to calibrate this load transducer in two working ranges: full scale (220 N corresponding to 10 volts of the output signal) and 50% of full scale (110 N corresponding to 10 volts of the output signal). These two ranges can be established by adjusting the gain of the signal conditioner.

  1. First, the students are asked to set up 10 volts of excitation in the strain gage conditioner;
  2. Then, the students are asked to zero out any signal by adjusting the balance of the strain gage conditioner with a custom calibration frame connected to the load transducer;
  3. Then, an initial weight will be applied to the load transducer, and the gain of the signal conditioner will be adjusted to achieve the desired working range.
  4. A series of weighs will be added to the load transducer up to the full load of the working range and the output voltages and the corresponding load will be recorded (minimum well-spread 10 data points should be obtained);
  5. The procedure will be repeated for the other working range of the load transducer (back to step 3).

As a group, the students are required to perform a linear regression of the load magnitude and the voltage output and report their relationship as well as the goodness of the correlation (coefficient of correlation r2) and the gain settings for the respective working range. The laboratory report should be written by each group and submitted to the instructor a week after the experiments.