Mark to space ratio is the relationship between the time a target object is
present in front of the sensor and the time it is absent.
To detect small targets at high speeds a sensor must feature a fast
Any delay in activation, detection or excitation may result in
missed targets. Each sensor will have a maximum switching frequency determined
by its design.
Maximum switching frequency is often calculated or established by testing
with regular target objects for example a toothed gear wheel with a 1:1 or 1:2
mark to space ratio. The critical issue is the absolute time period during
which the target is present in the sensor's active field. This time period will
suggest the maximum switching speed the sensor can attain with other shapes of
targets whose geometry and size will cause further variation.
Example of application of a Sensatech small target high-speed sensor:
1mm wide "tip sensor" for measuring the deflection of turbine rotor blades.
The specification included:
16 - 20 turbine blades of 1mm wide
Turbine operating at 60,000 - 120,000 rpm
The target tip starts at
This capacitive sensor was developed to measure the rpm of the turbine blade
and it's deflection or expansion due to centifugal force and or temperature
The tip of the turbine blade being detected was present for half a
microsecond and travelling at just below the speed of sound, Capacitive sensing
can then be seen as a suitable method of high speed small object sensing in a
high temperature environment.