Design a computer-based DAS that measures pressure. The pressure transducer is built with four resistive strain gages placed in a DC bridge. When the pressure is zero, each gage has a 120 Ω resistance making the bridge output y zero. When pressure is applied to the transducer, two gages are compressed (which lowers their resistance) and two are expanded (which increases their resistance). At full-scale pressure (p = 100 dynes/cm2), the bridge output is y = 10 mV. The transducer/bridge output impedance is therefore 120 Ω. You may assume the transducer is linear. The desired pressure resolution is 1 dyne/cm2. The frequencies of interest are 0 to 100 Hz, and the two-pole Butterworth analog low-pass filter will have a cutoff (gain = 0.707) frequency of 100 Hz (you will design it in part b). In terms of choosing a sampling rate, you may assume the low-pass filter removes all signals above 100 Hz.
a) Show the interface of the ADC to your computer. Justify your ADC precision.
b) Design the analog interface between the transducer/bridge and the your ADC. Use the full-scale ADC range even though it will complicate the conversion software. For example, if the ADC has a range of 0 to +5 V, then a pressure of p = 0 maps to a voltage at the ADC input of 0, and p = 100 dynes/cm2 maps to a voltage at the ADC input of +5 V. Include the Butterworth low-pass filter. (Figure 12.54)
Figure 12.54
c) Show the ritual that initializes any global variables, the ADC, and an appropriate interrupt.
d) Show the interrupt handler(s) that samples the ADC, calculates pressure in dynes/cm2, and stores the value in global variable Pressure. The software conversion maps a 0 ADC result into Pressure=0, and a 255 ADC result into Pressure=100.
Optimize the interrupt handler so that the number of execution cycles is minimized.