Modern control systems almost always use some position sensing device as part of their overall function. A position sensor detects linear or rotational movement of the object it is connected to and transmits an output signal to the machine. In most applications that use position sensors, operational failure does not result in compromised safety, but there are some applications where sensor failure is considered catastrophic. In such applications that emphasise the importance of safety, this guide will help engineers in ensuring a safe sensor design.
Essential terms used in safety sensor applications
A position sensor, in general, is an umbrella term pertaining to a range of devices such as a linear position sensor, encoder, transducer, and transmitter. All of these devices either detect or measure angular, rotary, or rectilinear movement of an object. Specific designs include potentiometers, Hall-effect sensors, LVDTs, and encoders. In applications where safety is crucial, failure needs to be defined as follows:
- The sensor has no output. This happens when the sensor intermittently or permanently stops sending signal output.
- The sensor transmits an incorrect output with a corresponding error flag. The sensor flags the wrong transmission automatically.
- The sensor transmits a false output signal, but there is no error flag. This occurs when the sensor produces a correct reading, but the position is incorrect.
The third scenario is considered as the most severe failure which may be encountered with a position sensor.
Designing for safety-related applications
Engineers need to specify a safety spectrum to create a position sensor assembly suited for safety-related applications effectively. As the safety degree increases, sensor assembly also changes to accommodate the need for increased safety. It also follows that improving the safety features of a sensor design also increases the overall acquisition cost.
If the application does not require safety relevance, the assembly is simple and straightforward. For example, a potentiometer used in controlling radio volume does not require performance monitoring because if the sensor fails, the inconvenience resulting from the failure is only minor.
When the relevance of safety increases as required in the application, engineers may employ self-diagnostic methods. Failing any of the diagnostic tests should lead to an error flag instead of an output. Most modern position sensors like inductive encoders allow for configuration so that the protocol for communication produces an error flag if needed.
Further along in the safety spectrum, design engineers may reinforce the parameter by using duplex sensor arrangements. A duplex sensor arrangement refers to an assembly where two sensors are measuring the same metric. Aside from using two sensors to measure identical parameters, engineers may reinforce the safety feature of these assemblies by constructing each sensor differently so that each sensor produces a different failure mode.
In many demanding applications like the military, aerospace, medical laboratories, and aeronautics, sensor failure needs to be avoided at all costs. Nevertheless, if the fault cannot be prevented, engineers may modify the arrangement so that the system continues to operate even when the sensor fails initially. In conclusion, sensor safety needs vary according to the application and engineers need to design a sensor assembly according to these needs.