Page 189 - Sensors and Control Systems in Manufacturing

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Cha p te r

• Shock and vibration resistance.
• Versatile connections. They can connect two or three wires with
an AC, AC/DC, or DC power supply and up to four-wire DC
connections.
• Wide operating temperature range. They operate between –
20 to +70°C, ±10 percent.
• Very fast response, particularly in the DC models. These
sensors can detect presence, send a signal, and reset in 50 μs
(2000 times per second) in DC models.
Inductive proximity sensors are generally limited by their sens-
ing distances and the material they sense. The effective range is lim-
ited to about 25 mm for most models and can be extended to only
about 100 mm with the large models.

3.5 Limit Switches—Traditional Reliability
Limit switches are mechanical position-sensing devices that offer
simplicity, robustness, and repeatability to processes. Mechanical
limit switches are the oldest and simplest of all presence- or position-
sensing devices: contact is made and a switch is engaged. This sim-
plicity contributes generally to the cost advantage of limit switches.
Yet, they can provide the control capabilities and versatility demanded
in today’s error-free manufacturing environment. The key to their
versatility is the various forms they can take in the switch, actuating
head, and lever operator. Two-step dual-pole limit switches can detect
and count two products of different sizes and can provide direct
power control to segregate or process the items differently. The lever
operator will rotate 10° to activate one set of contacts and 20° to acti-
vate another set. Because of the high amperage they can handle, limit
switches can control up to ten contacts from the movement of a single
lever.
They are easy to maintain because the operator can hear the oper-
ation of the switch and can align it easily to fit the application. They
are also robust. Limit switches are capable of handling an inrush
current ten times that of their steady state current rating. They have
rugged enclosures and also prewiring that uses suitable strain-relief
bushings to enable the limit switch to retain cables with 500 to 600 pounds
of force on them. Limit switches can also handle direct medium-
power switching for varying power factors and inrush stresses. For
example, they can control a multihorsepower motor without any
interposing starter, relay, or contactor.
Reliability is another benefit. Published claims for repeat accuracy
for standard limit switches vary from within 0.03 mm to within 0.001
mm over the temperature range of –4 to +200°F. Limit switches dissi-
pate energy spikes and rarely break down under normal mode surges.