WorldWide Drilling Resource

41 OCTOBER 2021 WorldWide Drilling Resource ® Sensible Sensors by Britt Storkson Owner, P2FlowLLC Electronic sensors are everywhere. We must have sensors to have computer controls. A sensor is defined as a device which detects or measures a physical property and converts it to an electrical volt- age which is representative of and proportional to whatever they are designed to sense. For example, if we use a pressure sensor, almost always the output is a voltage which is linearly proportional to the pressure inside the sensing “chamber”. So if we have a 0-100 psi full-scale pressure sensor and there is 50 psi in the sensing chamber and it has a 0-10 volt output, the output under these conditions will be 5 volts (½ of the full scale). Not only should one “read” and interpret the sensor voltage properly, but one has to check to see if the sensor is actually there and working properly. If it isn’t there and/or working properly, all systems impacted by this sensor output should be shut down immediately. With the water pump controls we make, we use a 4-20 mA (milliampere) pressure sensor with current flowing through a 75 ohm resistor for a voltage range of 0.3 to 1.5 volts. So if we “read” (measure) at least 0.3 volts from this sensor, we know the sensor is there and almost certainly working properly. If we “read” less than 0.3 volts or more than 1.5 volts, we know the sensor either isn’t there or isn’t working properly. Either reading is unacceptable and the system is programmed to shut down immediately and display an error code indicating what failed. When most sensors fail, they go out of “regulation”. This means the failed sensor voltage output exceeds the highest acceptable output voltage. So if you read 12 volts coming out of a sensor which is designed to output a maximum of 10 volts, the sensor has failed. Likewise, if the sensor is not there or the wiring has been cut or damaged, one would read (and should read) zero (0) volts. Often, a resistor to ground is provided so the input reading the sensor is not left “floating” with the voltage randomly drifting up and down. The resistor “leaks” a small amount of electrical current to ground, so it pulls it to ground (logic 0) if the sensor is not pres- ent but has sufficient resistance so as not to load the sensor output and alter its readings. The input resistance on almost all semiconductors is extremely high . . . often 10 gig ohms (billion ohms) or more and they will oscillate if left unconnected, which makes them useless. For a practical application, Malaysian Airlines flight MH370 disappeared somewhere over the Indian Ocean. All commer- cial airplanes have items called transponders that periodically broadcasts the aircraft location. It has been reported the transponders were turned off shortly after takeoff. That should have been a huge red flag, but it wasn’t. How they were turned off - whether the crew turned them off deliberately or something failed (like electric power going to the transponder used for operation) that caused the transponders to turn off wasn’t a big issue at the time. The big issue is the sensor “was not there” and therefore the plane could not be tracked accurately. The fact it was not reported and responded to immediately raised suspicions. Finally, not only does one need to have a sensor in place and functioning properly, one must tell the computer what to do if it doesn’t “see” the sensor. With water pump controls, it’s pretty simple. We just turn off the pump and indicate on the display what went wrong. With other computer controls like autopilots or self-driving cars, it gets a lot tougher. The “industry standard” solution to this has been to hand the control back to the operator, but what if the operator isn’t present, unattentive, incapacitated or impaired? Sounds like a recipe for disaster to me. With computer controls, there should be no undefined functions - ever. That’s another subject for another day. Suffice it to say, if you want your computer con- trol system to work right, everything must be defined and decided. Computers are wonderful things, but unlike their human “masters”, they lack judgment skills - and should be treated accordingly. Britt Britt Storkson may be contacted via e-mail to michele@worldwid-