Edge triggering
After the triggering edge, provided by
Time offset of trigger and output
signal
an initiator switch or PLC, the analogue
The sensor subdivides the processing and
output is updated, or if a digital output has
output of the measurements into cycles.
been selected, only a digital measurement
Assuming a maximum measuring rate of
is out put via an RS422 interface (see Fig 1).
2.5kHz, 400µs is taken up with cycles for
exposure, reading in, computation and
controlling.
In this example, only measurements for
a particular range of positions are to
be examined more closely. The trigger
signal (green) initiates the measurement
in the sensor. The time offset of the
trigger signal and output signal of the
sensor (blue) can be seen clearly on the
oscilloscope. The sensor needs this time for
the internal processing and output of the
A total period of approx. 1.6ms passes
measurement results. Encoders are also
before the first measurement is available
suitable as trigger signal sources.
at the output. As the processing occurs
Level Triggering
Level triggering, sometimes referred to
as gating, causes the sensor to output
measurements until the trigger condition
is satisfied (see Fig 2).
sequentially in time and parallel in space,
Rotary transducers can be found in all
the next measurement is available at
numerically controlled industrial systems.
the output after a further 400µs. The
The rotational speed or the closeness of
sensor time pattern therefore requires
the pulses from the encoder is a direct
a chronological interval between two
measure of a linear change, e.g. of a
consecutive trigger signals of at least four
conveyor belt. If an encoder, for example,
cycles with edge triggering, or five cycles
supplies 1,000 pulses per revolution and
for level triggering. Thus a measurement
if the internal processing time of 1.6ms in
is not output that was valid four cycles
the sensor is taken into account, then the
ago, but instead the measurement object
encoder may revolve at a maximum of
position is acquired exactly at the time of
0.625 revolutions per second (or 37.5rpm),
the trigger.
so that the maximum trigger frequency
for the measuring rate of 2.5kHz is not
Trigger pulse values
As a result, a maximum trigger frequency
The trigger pulse duration ti must be at
kHz. There remains a time uncertainty of a
Summary
least one cycle period (= 1/measuring
maximum of one cycle because the sensor
rate). With a slower measuring rate, the
continues operating in its cycle sequence
Edge triggering supplies single
trigger pulse duration must therefore
and the trigger edge can be located within
also be extended (e.g. from ti = 400µs at a
the first cycle, i.e. from the start to the
measuring rate of 2.5 kHz, to ti = 3.2ms at a
end of the first cycle. The trigger input is
measuring rate of 312.5 Hz).
interrogated within this period. The leads
The necessary level adaptation to the
to a minimum length of the trigger pulse
LVDS specification (see Fig 3) of the sensor
of at least one cycle.
occurs via a controller that permits trigger
levels from 2.4V to 24V.
of 625Hz occurs for a measuring rate of 2.5
Application
exceeded.
measurements on the output of a sensor
and so replaces sampling operation. Also,
the typical memory overflow associated
with continuous measurement is also
prevented, which ensures an exact
alignment between the measurement
and the time. If the sensor is combined
with a controller, taking into account
the maximum trigger frequencies, an
A typical measurement application with
economically practical measurement unit
triggering is the bridging of gaps with
is produced for production and process
single parts on a conveyor belt (see Fig 5).
automation applications.
For more information, please call the
Micro-Epsilon sales department on
0151 355 6070 or
email: [email protected]
www.micro-epsilon.com
Issue 21 PECM
39