Sample Problem Set 1

Weigh Scales Project

Answers

 

Return to Question Set 1

 

1. Sketch of device;  Long thin wire bent back and forth and placed on small pad, about the size of a plaster; Attach to device that is to be stressed, i.e. beam; As beam bends, so does gauge; Length increases to give an increase in resistance if in tension; for compression, length decreases and so does resistance.

 

2. Strain = change in length over original length = ∆l/l

 

Gauge factor =

Where ∆R = change in resistance

   R = unstrained resistance

   ∆l/l = strain

 

3.  Higher output voltage with half bridge versus one gauge and higher again for a full bridge.  Vout for half bridge = twice that of single gauge.  Vout for full bridge = four times that of single gauge.

 

4. Plot results:

 

Sensitivity = Output/Input = slope of above line = 0.0078 mV/gram

Offset = value of output when input is zero = intercept on y-axis = 2.425 mV

Max non linearity = max deviation from best fit straight line = occurs at first point above where mV should be 2.425 but is actually 2.75 so difference is 2.75-2.425 = 0.325mV.  Max non lin = 0.325mV

 

5. (i) Step 1.  Determine change in resistance for applied strain.

Gauge factor =

Therefore, when strain is applied strain gauge resistance = 120.252Ω

 

Step 2.  Determine equation for output from the bridge:

 

Enter value for Rgauge when strain is applied and enter value of 10 V for bridge supply:

= 0.0052

 

I.e. Vout = 5.2 mV

 

(ii) To remove any offset by balancing the bridge when no mass is applied.