sensors in industries

Thread Starter

Nathan Hale

Joined Oct 28, 2011
Hi folks , hope all is well.
I was reading how airplanes, radars, satellites and other expensive things like those have sensors / electronic devices that feed them data.
does any one know if designers usually put just one sensor and call it a day or do they usually have back up sensors that will kick in case the first on fails? if they do have back up sensors, would you know what the industry standard is? is it one back up or two or three or ore??

thank you

Glenn Holland

Joined Dec 26, 2014
I used to work in the rail transit business which has a very high standard for safety critical or "Vital" functions and here's what we use in train control systems.

Usually only one sensor is used for monitoring a given function, but they are ANDED or Exclusive ORED with other sensors that monitor other similar functions. This is called a "Checked Redundant" process.

For example, the alignment of the rails of a track switch is monitored by two proximity sensors, one must indicate the switch is aligned for curve route and the other must indicate the switch is NOT aligned for straight route. There is also a sensor for the switch locking mechanism. Indications of the three variables form a simple Boolean expression that must be "True" in order to permit a train to proceed over the switch.

There are no "back up" sensors so if any one fails, the process will default to HALT mode and we get a trouble call. Obviously, a train control system can be made fault tolerant by simply shutting off propulsion and applying the brake, but an aircraft cannot come to a stop in mid air.


Joined Aug 27, 2009
For things that I work on we normally don't have redundant fine position sensors because if something is not right we want it to stop. We do have hard stop, limit, home and fine position check sensors for things like a traveling profile faraday that measures critical beam density over several hundred mm to make sure the encoder reading are sane. The sensor flag would be something like a fine piano wire to insure the actual travel from home in counts to the fine position check sensors could only off by a few counts before it was detected. It always does an initialize from the hard stop sensor on the motor plate and fine sensor calibration check several times before repeated measurements.

I made a 1/4 resolution position tester using a junk one (with only the track and stop sensor) so we could design a system to bench test the good ones. There would be set of sensor down the length of the track on a real one to check movement accuracy. The encoder is directly on the motor shaft so if anything moves we should know it.
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DNA Robotics

Joined Jun 13, 2014
In the marine industry, they like to leave you stranded.

They monitor things that don't matter. A Volvo diesel had an intake air temperature sensor go bad and they wouldn't let the engine go over 1000 RPM. Who cares what the intake air temperature is? Luckily boats usually have 2 engines, so I measured the sensor on the other engine and bypassed the bad one with a resistor.

The Chevy gas engines had a chronic problem with their exhaust elbow temperature sensors. A family's vacation would have been ruined if I hadn't bypassed a bad one with a resistor.


Joined Apr 24, 2011
In critical systems, where “critical” is defined in either cost in dollars or literally human life, there is used a concept called “fail safe.” That just means when a sensor or some other device fails the output will tend towards a value that does not mask any dangers.

One example of this is a line of air in line (bubble) detectors I worked on that is often used in the medical industry to insure air bubbles are not pumped into a patient’s blood supply. The output was a single bit that would indicate the presence of air. The design was such that most single component failures would drive the line to indicate air. For those failures not covered by this fail safe output the output could be tested with an input signal which would also make the output tens to the “air” state; this did not use a logic gate on the output but instead disabled an internal driver to kill the received signal.

Next up on the safety list are “dual redundant” sensors. In these air detectors this single sensor has two independent control circuits and two outputs. If either or both outputs show the air state for bubbles. Additionally, every single component failure would drive either one or both outputs to the air output, or leave one output still functioning. Thus a test signal similar to the above would discover a fault in little time.


Joined Dec 13, 2013
the cnc machines here have a kind of redundance, on the dgital drive ones, there is a position sensor on the motor used to synch the drive with the motor shaft position, and a "scale" used to measure the position of the axis. in software, you can use the motor sensor to measure the position of the axis if the scale fails. usually not at the same accuracy. some older machines use this too.