Method and apparatus for monitoring a signal

A signal monitoring system comprising:

• an input interface configured to receive an analogue signal from a sensor;
• a first comparator configured to:
• compare an amplitude of the analogue signal to a first threshold pair comprising a first high threshold and a first low threshold;
• set a value of a primary monitoring signal to a first value when the amplitude of the analogue signal exceeds the first high threshold;
• and
• set the value of the primary monitoring signal to a second value when the amplitude of the analogue signal decreases below the first low threshold;
• a second comparator configured to:
• compare the amplitude of the analogue signal to a second threshold pair comprising a second high threshold and a second low threshold, wherein the second high threshold is greater than the first high threshold and the second low threshold is less than the first low threshold;
• set a value of a secondary monitoring signal to a first value when the amplitude of the analogue signal exceeds the second high threshold;
• and
• set the value of the secondary monitoring signal to a second value when the amplitude of the analogue signal decreases below the second low threshold;
• and
• a controller configured to:
• receive the primary monitoring signal and the secondary monitoring signal;
• compare the primary monitoring signal and the secondary monitoring signal;
• and
• determine from the comparison whether an error exists with the signal monitoring system.

A method of monitoring an analogue signal output from a sensor comprising:

• comparing an amplitude of the analogue signal to a first threshold pair, wherein the first threshold pair comprises a first high threshold and a first low threshold;
• setting a primary monitoring signal to a first value when the amplitude of the analogue signal exceeds the first high threshold and setting the primary monitoring signal to a second value when the amplitude of the analogue signal decreases below the first low threshold, wherein the value of the primary monitoring signal is set by a first comparator and wherein the primary monitoring signal is communicated on a first monitoring channel;
• comparing the amplitude of the analogue signal to a second threshold pair, wherein the second threshold pair comprises a second high threshold and a second low threshold and wherein the second high threshold is greater than the first high threshold and the second low threshold is less than the first low threshold;
• setting a secondary monitoring signal to a first value when the amplitude of the analogue signal exceeds the second high threshold and setting the secondary monitoring signal to a second value when the amplitude of the analogue signal decreases below the second low threshold, wherein the value of the secondary monitoring signal is set by a second comparator and wherein the secondary monitoring signal is communicated on a second monitoring channel;
• comparing the primary and secondary monitoring signals;
• determining from the comparison of the primary and secondary monitoring signals whether an error exists with the monitoring of the analogue signal;
• switching the role of the second comparator such that the second comparator sets the value of the primary monitoring signal and communicates the primary monitoring signal on the second monitoring channel;
• and
• switching the role of the first comparator such that the first comparator sets the value of the secondary monitoring signal and communicates the secondary monitoring signal on the first monitoring channel.