Page 133 - Sensors and Control Systems in Manufacturing

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Operate Cha p te r T w o
Release
Target

In
Target
Out
Response Release
time time
On
Sensor
Out
Operate Release
point point

FIGURE 2.88 Response/release times.

the sensing field, there is a slight delay before the switch restores to
the OFF state. This is the release time. Release time for an AC device
is typically one cycle (16.66 ms). The DC device release time is
typically 3 ms.

2.9.7.2 High-Speed Operation
Mechanical devices such as limit switches and relays do not operate
at speeds suitable for high-speed counting or other fast-operating-
circuit needs. Solid-state devices, however, can operate at speeds of
10, 15, or more operations per second. DC devices can operate at
speeds of 500, 1000, or more operations per second.
In order to properly achieve high-speed operation, some basic
principles must be applied.

2.9.7.3 Maximum Target Length
A response delay occurs when a sensor has a target entering the sens-
ing field, as previously stated. A similar delay results in order for the
respective load to operate. The time between when the sensor con-
ducts and the load operates is the load response time. Together these
delays make up the system response time T.
o
Similarly, delays occur when the target reaches the release point
in the sensing field caused by the sensor release time and the corre-
sponding load release time. In order to ensure that the sensor will
operate reliably and repeatedly, the target must stay in the field
long enough to allow the load to respond. This is called dwell time.
Figure 2.89 illustrates the time functions for reliable repeatable sensor
operation. Figure 2.90 illustrates the dwell range.