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How does the oxygen measuring system detect faults?The oxygen measuring system outputs two measuring signals through two different channels:
- on channel K1 the measured value is available as an analog signal (0-10 V or 4-20 mA),
- on the multidirectional channel K2 it is available as a digital pulse duration modulated alternate signal (low/high phase: 0/5 V). The length of the low phase is the measure for the oxygen concentration.
| |  | | | The oxygen measuring system is working fault-free when the analog signal from channel K1 corresponds to the signal of digital channel K2 (maximum deviation 4%). In addition, the low phase of channel K2 has to last from 0.68 to 3.94 ms, which corresponds to an oxygen concentration of 0.1 or 25% by volume.
If the signal exceeds a time window of 0.5 to 4 ms or if the alternate signal changes into a constant fault signal of 5 V, the measured values lie outside the measuring range. If the hardware is defective, the output signal remains constant at 0 V.
Fig. 1: Output signals channel K2. | | | | | Since the measuring method is dynamic, the proper performance of the oxygen measuring system can be checked any time, also during operation; ideally such check should occur cyclically.
For this purpose, 24 V are applied to a separate test channel, causing the sensor current to be reduced 20% from outside. An oxygen concentration lower than the one actually prevailing is simulated in the measuring system.
Consequently, the measured signals on channel K1 and K2 have to drop equally, i.e. the measuring system has to correctly compute the virtual concentration of oxygen.
In this connection, a range of 4% is permissible, i.e. the measured value has to lie between 0.76 and 0.84 times the previous measured value during the self-test.
Rule: The measured signals have to decrease equally by at least 20%. | | | | | With this test layout it is possible for the first time not only to detect faults in the hardware of the measuring system but also on the sensor itself, i.e. on the zirconium dioxide chamber! | | | | | External monitoring unit of the userAn external device arranged downstream by the user has to take over the evaluation of the measured signals as well as handle and monitor the cyclical self-tests.
The reaction to a fault message has to be in accordance with the specifications of the user and is managed by his external monitoring unit as well.
For this reason, such device must meet certain requirements:
- The unit must be fault-proof, i.e. the processes described below must be carried out without any errors, the input signals have to be read-in without errors and the output signals have to be output without errors.
- The measured values of channels K1 and K2 must be compared permanently within the fault tolerance time permissible for the application.
- The plausibility of the time of the output signal K2 must be checked constantly. In this respect, static signals are to be considered internal errors.
- A self-test is to be initiated at cyclic intervals and its effects on the measured signal have to be determined and evaluated. The time interval between two test cycles must not exceed a certain value.
- When a fault message is output, the process must be transferred into a safe state.
| | | | | When is fault-free operation of the oxygen measuring system ensured?Fault-free operation of the oxygen measuring system is ensured when:
- the analog and digital output signals correspond to one another,
- the measured signal of channel K2 lies within a defined time window and is not static, and
- the self-test is carried out cyclically and correctly.
Accordingly, this is the first fault-proof oxygen measuring system that monitors its entire system on its own during operation and, in addition, requires only one oxygen sensor.
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