Some sensor applications are interesting not only for the transducer and signal-processing technology they contain, but also for the human-factor considerations embodied in how information is displayed or presented to users. This is often true of sensors associated with demanding, high-workload environments like heavy machinery, automobile operation, or in this case, aircraft operation.
One of the key real-time variables of obvious interest to a pilot is the rate of change of altitude, also known as vertical speed or simply “VS.” This is especially true for glider pilots, whose constant search for liftproducing atmospheric conditions is largely guided by sensitive, fast-responding VS indicators (VSIs).
The usual method used for sensing VS is by measuring the rate of change of barometric pressure; this is the technique illustrated in this VSI circuit (see the figure). Also of note is the method of presenting the resulting information.
Rather than add to the complexity of the aircraft control panel with another distracting visual display, this VSI circuit generates an audible tone that commences whenever the VS exceeds a minimum threshold of approximately ±25 feet per minute. Then it rises or falls in pitch according to the direction and magnitude of climb or sink rate. The tone is passively mixed with the pilot’s communication headset audio signal to become part of the pilot’s acoustic, rather than visual, environment.
VSI operation requires a sensitive barometric pressure sensor whose quantitative accuracy is of less importance than its sensitivity.
The stratagem used in this circuit is to adapt a sensitive (5-psig full scale) differential-pressure sensor (T1 = Fujikura FPM-05) to pseudo-absolutepressure operation by sealing one of its ports with epoxy glue. This ploy converts the tiny reservoir of air trapped in the sealed port into a pressure reference, against which variation in atmospheric pressure at the unsealed port will cause the sensor to output a highly resolved output signal.
The resulting pressure-proportional signal is differentiated by ac amplifier A1 with a 500-ms RC time-constant defined by the 5k output impedance of T1 combined with the 100-µF capacitor, C1. This combination is fast enough to give prompt feedback to the pilot (considerably faster than most mechanical VSIs). It is also long enough to provide a usable signal (about 20 mV/psi) to A1 and the voltage-to-frequency converter (VFC) circuitry (about 200 V/psi/sec = ~2 mV/fpm) that follows.
The VFC section consists of bipolar VSI window detectors A2 and A3. These enable the VFC multivibrator (A4) only when VS exceeds ±25 fpm, as indicated when A1’s output moves more than 50 mV from the null voltage at T1 pin 3. If −25 fpm < VS < +25 fpm, then both A2 and A3 will output approximately 0 V, causing A4’s pin 12 to be less than one diode drop above ground. This also will cause A4 to saturate with a zero output and thus disable the VFC.
VS > +25 fpm causes A2 to output approximately +9 V, pulling pin 12 to a center value of around 1.5 V. This causes A4 to output a tone of several kilohertz, which declines in frequency as VS continues to increase toward a full-scale value of about 2500 fpm.
VS < −25 fpm causes A3 to output +9 V, pulling pin 12 to a center value of approximately 6 V. It also causes A4 to output a distinctly different frequency of around 1 kHz, which gradually increases for larger negative VS as great as ~2500 fpm.
A4’s about 9 Vp-p output is attenuated and summed with the pilot’s headset audio (normally used to provide radio and cockpit intercom signals) by the R2/R3 passive voltage divider. The resulting aural VS indication, while obviously highly qualitative, is a useful indicator of aircraft behavior in many modes of flight, including soaring, steep turns, and landing.
Reprints
