Cut Sheet

V8-T12-36 Volume 8—Sensing Solutions CA08100010E—November 2012 www.eaton.com
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12.1
Sensor Learning Course
Learning Module 23: Limit Switches, Proximity Sensors and Photoelectric Sensors
Switching Logic
Output Logic Functions
The outputs of two or more
sensors can be wired
together in series (and) and
parallel (or) to perform logic
functions. Factors that need
to be taken into
consideration, however, are
excessive leakage current or
voltage drop and inductive
feedback with line powered
sensors. For these reasons,
series and parallel
connections for logic
functions is not commonly
done. It is usually easier to
connect direct to a PLC’s
inputs and perform the logic
functions through the PLC
program.
Output Response Time and Speed of Operation
Photoelectric, inductive
and capacitive sensors can
operate considerably faster
than a limit switch, making
them better choices for high-
speed operations such as
counting or sorting. The time
it takes to respond, its speed
of operation, is based on
several factors. Let’s take a
moment to consider them.
When a system is first
powered up, the sensor
cannot operate until the
electronics are “powered
up.” This is referred to as
time before availability.
For AC sensors, this delay is
typically from 35 milliseconds
up to as high as 100
milliseconds. For a
DC sensor, the delay is
typically 10 milliseconds.
Time Delay Before Availability
Once the target enters the
sensing range and the
detector causes the output to
change state, a certain
amount of time elapses. This
is called response time. For
an AC sensor this is usually
less than 10 milliseconds.
DC devices respond in
1 millisecond or less.
Load
Sensor
Target
ON
ON
OFF
ON
OFF
Time Delay
Before Availability
Selector
Switch
Sensor
Response Time and Release Time for an
Inductive Proximity Sensor
Similarly, when the target
leaves the sensing field there
is a slight delay before the
switch is restored to the
OFF state. This is referred
to as the release time.
An AC sensor typically
releases in one cycle (16.66
milliseconds) and DC devices
usually in 1 millisecond,
or less.
In order to properly achieve
high-speed operations, there
are some basic principles
that need to be applied. In
addition to the sensor’s
response and release times,
there is a similar delay for the
load to operate. This is called
the load response time. The
total times combined are
referred to as system
response time. Similarly,
there are load release time
and system release time for
when the target exits the
sensing field. In order to
ensure reliability and
repeatability, the target must
stay in the sensing field long
enough to allow the load to
respond. This is referred to as
dwell time.
Release
Time
ON
Sensor
OFF
IN
Target
OUT
Response
Time
Operate
Point
Release
Point
Release
Operate