What's Going On Down There (a signal processing adventure)

A safety feature unique to the Hyball ROV and its descendants is vacuum leak detection.  This is a system that allows the operator to draw some air out of the hull before launch, creating a slight vacuum.  A barometric pressure sensor inside the hull lets the operator know if any air or water is leaking back into the vehicle through one or more of its thirty-two O-ring seals.  Ideally, leaks are identified topside, but if a seal fails during a dive, this system acts as the first line of defense for the vehicle and its operators.

Since the original vacuum sensor was missing, I replaced it this, a TDR-120 pressure transducer.

The tricky thing about this sensor is that it produces a differential voltage signal.  Instead of offering one signal terminal whose voltage relative to a 0-volt ground is proportional to the vacuum pressure, it has two terminals at very close positive voltages.  The difference (hence "differential" signal) between these voltages is proportional to vacuum pressure.

I used this little circuit from the LM124 application examples to ground-reference the signal and amplify it so that 0 to -700 torr gauge pressure maps to 0-5V, which can be fed to one of the Arduino's input pins.

It looks slightly different in real life.  The two 714 op-amps are inside the LM324 chip (at bottom):

Rather than narrating the two signal processing operations the circuit performs (ground referencing and amplification), I'll let the simulated oscilloscope and captions do most of the talking:

1) The raw signal coming out of the sensor
(each of the two traces represents a signal pin.)

2) The ground-referenced version of the signal in (1)

3) The signal in (2) amplified for a 5V maximum peak.

The traces above simulate what you'd see while drawing a 700-torr vacuum with the hand pump (below.)

 

I tested the output scale of the signal processing circuit by connecting it to a voltmeter, hooking the pump hose directly to the sensor, and comparing readings between the meter and the pump's mechanical gauge.

Geek on and happy holidays!

-Jacob