5.1 PRESSURE SENSORS
Maintenance for piezoelectric sensors
COMPONENTS OF SENSOR MAINTENANCE
The maintenance of sensors contains typically a visual
inspection, a cleaning, once again a visual inspection after
cleaning, an insulation check and a calibration. In some
cases it might be necessary to exchange single components
as piezoelectric cables or cooling nipples.
Calibration means the process of checking precisely the
status of the sensor. Piezoelectric sensors are stable over
their lifetime if used under regular combustion conditions.
All piezoelectric sensors go through a lifetime cycle, see
1) It starts with an improvement of linearity. The performance
improvement based on linearity happens already
during the run-in procedure on the test engine at the
2) A further minor improvement occurs during operation
in the field. Now the sensor operates with highest
performance. It lasts for a certain number of cycles of
operation depending on the intensity of use.
3) Calibration in regular time intervals is recommended
to ensure that the measurements are always performed
with the correct sensitivity of the sensor.
4) At the end a sensor falls below the required accuracy
level. Calibration is absolutely essential.
1 2 3 4 1
OPTIONS OF SENSOR MAINTENANCE
Additionally a dynamic test on the engine might be
performed in order to control the dynamic behaviour
of the sensor. Spark plug sensor maintenance contains
as well a verification of the spark function.
Figure a) Figure b)
number of load cycles number of maintenances
ABOUT THE IMPORTANCE OF SENSOR CALIBRATION
Pressure sensors can be maintained upon request directly
at AVL (factory service). Another possibility is the in-house
maintenance based on above-mentioned steps with an
own calibration device. The main criterion for an internal
or external maintenance is the expected number of maintenances
per year. A maintenance directly at AVL offers
the advantage that the customer does not have to spend
any manpower and investment for equipment. On the
other hand the in-house solution allows the customer
ndependent maintenance at any time, instantly.
Regarding costs, Figure (b) shows the total costs over
the number of maintenances. (1) For every single maintenance
a fixed price will be charged (excluded exchange
parts) in case of factory service. (2) The inertial costs
are high due to the investment for maintenance devices
(incl. calibration unit). After this the costs per maintenance
are much less. The point where these two lines
cross each other represents an orientation when an
in-house maintenance becomes more economical than
a factory service.
THREE MAIN QUESTIONS FOR THE DECISION OF A MAINTENANCE PROCESS:
• How long is the maintenance allowed to take?
• How much does it cost per year to calibrate all sensors on a scheduled basis externally?
• How accurate is the system and how reliable are the resulting data?