An application with zero safety relevance is pretty straightforward. If we take the example of the potentiometer which controls the volume of a domestic radio then its failure typically results in only a minor inconvenience and there is no need for the potentiometer’s performance to be monitored.
As safety relevance increases, the first step in the engineer’s armoury is to employ a sensor which can carry out some self-diagnostics often referred to as Built-In–Test or BIT. If the sensor fails one or more of its internal diagnostic tests, the sensor outputs an error flag instead of or as well as its output signal. Such error flags can take a variety of forms. For example, with an analogue sensor with a 0.5 to 10V output then the output can be reduced to <0.5V as an error signal. Similarly, devices such as modern inductive encoders (or ‘IncOders’) with digital outputs like SSI or SPI, can be configured so their communication protocol carries an error flag if necessary. Examples of built-in-tests include internal watchdog timer, internal flash data memory check or a timeout for receipt of a clock signal. Such sensors can continue to operate but the output contains a caveat which tells the host system “I’m giving you this data but watch out – it may be wrong”. The receipt of such a flag by the host system should then be used to trigger going to a fail-safe state. A sensor which outputs its own error flag is said to be internally referenced.
As safety relevance increases further, sensors should be referenced externally and, in turn, both internally and externally. We can illustrate with an example of a microwave satellite communications antenna on a ship. Such antennas are typically required to move within a (software) defined arc so that on-board personnel or other equipment are not affected by the microwave energy. The failure of a position sensor on one of the antenna’s axes can potentially lead to unsafe conditions. Such antennas are typically driven in azimuth and elevation axes by electric motors driving through a gearbox. The angle of the gearbox output shaft is typically measured by an absolute angle encoder whose failure can be internally monitored by the sensor itself and referenced by an internally generated error flag. Additionally, the output from a resolver or encoder on the motor’s shaft (input to the gearbox) can be counted by the host system and used as a rough guide to the approximate angle of the antenna axis. Should the two measurements differ outside of expected bands then the microwave energy may be halted as the fail-safe condition.
The next step along the safety spectrum is to use redundant or duplex arrangements whereby two sensors are used to measure the same parameter – such as the rotation angle of a shaft. The safety of such arrangements can be increased further by using different types or constructions of sensor so that their failure modes differ.
An example of a duplex (electrically redundant) sensor is shown below in which the first sensor is shown on the inner ring and the second is shown on the outer ring. Whilst both sensors have a common mechanical housing, each operates electrically independently. Each has its own set of 10 built-in-tests and corresponding error flagging functionality. The inner and outer devices differ by virtue of different numbers of winding pitches on inner and outer rings and their electronics may also be chosen to be different – for example
- with an inner device outputting 0-10V and the outer device outputting a digital signal in SSI or similar format.
- with the inner device set with its zero position at 12 o’clock and the outer device with its zero position at 6 o’clock
- with an inner device outputting an incremental measurement (such as A/B pulses) and the outer device outputting an absolute digital signal such as SSI so that the inner may be used to check against the outer and vice versa.
Such differentiation in the sensor’s design further helps mitigate against common failure modes and is one of the reasons why such devices are increasingly chosen for demanding, hi-rel applications.