August 1, 1997

Use RS-232C port to measure pressure

W Stephen Woodward, University of North Carolina--Chapel Hill

A simple circuit (Figure 1) and accompanying software turn a pressure sensor into an accurate and cheap pressure digitizer that works with any PC's RS-232C COM port. Many manufacturers make micromachined silicon piezoresistive pressure sensors. Sensors are available with full-scale sensitivities ranging from inches of water to hundreds of pounds per square inch. A particularly useful breed of these sensors has temperature-compensated outputs proportional to the product of applied pressure and excitation current. This feature makes these sensors suitable for ratiometric operation and accurate functioning with a range of supply voltages. The figure uses a handy example from this group: the Lucas Novasensor (Fremont, CA) family. This series of sensors undergoes factory calibration to a range of differential or absolute pressures, and the sensors incorporate on-chip gain-setting resistors that promote trim-free interchangeability.

Electrically, the sensor is equivalent to a strain-gauge bridge biased by R₁, the on-chip gain-setting resistor. Pressure-induced mechanical strain in the silicon of the piezoresistive bridge produces a pressure-proportional differential signal V_P=PxSxI_B, in which S is the sensor's sensitivity, P is the applied pressure, and I_B is the bias current. IC_1A and IC_1B sense V_P. Integrator IC_1C accumulates the resulting Q₁ collector current, causing its output to ramp up. The collector current is equal to:

Comparator IC_1D combines with a dual CD4052B multi plexer, IC₂, to close the feedback loop around IC_1C by periodically connecting R₃ to 5V. When this connection happens, the current through R₃ into the integrator equals I_BxR₁/R₃. If d equals IC₂'s duty cycle for integrator balance, then

Factory calibration sets S/R₁ equal to 1/(1000xfull-scale psi), so

Seven-bit binary characters, including start/stop bits, with the pattern of 0000011, drive the conversion. The PC's COM port transmits these characters on TXD at 9600/7, or 1371 Hz. With the start of each character, IC₂ samples the state of IC_1D's output. If this output is low, which indicates integrator balance, IC₂ goes into State 1 of its four possible states. This state shorts R₄, which latches IC_1D low for the remainder of the character time. Meanwhile, State 1 holds RCD negative, so the COM port receives no characters.

If, however, IC_1D's output is high, which indicates integrator imbalance, IC₂ goes to State 3, which latches IC_1D high. This action connects R₃ to 5V, which nudges the integrator toward balance, and drives RCD to 5V, which echoes the 0000011 character to the COM port. Because TXD has a 5-out-of-7 high/low pattern, the maximum value of d is 5/7, and the best choice for R₃/R₂ equals S/R₁x5/7=1000x 5/7=714. This ratio causes the fraction of characters echoed back to the port to range from 0 to 100% as the pressure ranges from 0 to full-scale psi.

The sensor program computes this average frequency as a fraction of 9600 Hz/7x0.979 sec=1342 and converts the result into a pressure reading by multiplying it by the appropriate full-scale factor. (You can download the program from EDN's Web site. At the registered-user area, go into the "Software Center" to download the file from DI-SIG, #2061.) Conversion resolution is proportional to conversion time and is greater than 10 bits (1 part in 1342) in 1 sec, as illustrated in the listing, and 12 bits in 3 sec.

(DI #2061)

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