Page 718 - Mechatronics with Experiments

704 MECHATRONICS
that it makes or breaks the contact connections. The plunger may be connected to activate
multiple output contacts in the main line.
The performance ratings of a relay are as follows:

1. Contact ratings: maximum voltage and current the contacts can carry, (V max max ,
, i
i.e., 24 V to 600 V, 50 A) in the main circuit, the number of normally open (NO) and
normally closed (NC) contacts operated by the single coil relay (i.e., 6 NO, 6 NC).
2. Coil ratings: nominal voltage (i.e., 6 V to 120 V range) to operate the control circuit
coil.

A basic relay design requires the coil to be energized to keep its contacts engaged.
Variations in the designs include the so-called latching relay. In this design, the relay uses
two coils: one for latching and one for unlatching. The relay is energized to start the contacts
and move the plunger. Then, a mechanical latch mechanism locks it in place and keeps the
contacts connected. Then, the coil does not need to be kept energized in order to keep the
contacts connected. In order to disconnect the contacts and release the mechanical latching
mechanism, the other coil is energized.
Contactors operate on the same principle and have a similar design to the relays.
The main difference being in their mechanical components so that the main line voltage
and current capacity that is conducted by the contacts can be much larger than the contact
ratings of a relay (Figure 9.6).
Starters are similarly designed to the relay principle except that they may have
overcurrent protection as well as “smooth start” related current shaping and limiting mech-
anisms built in to the design. The overload protection is built in based on the current–
temperature relationship in the contact material. When the current is too high for an
extended period of time, a bimetallic material breaks the contact, just as is the case in a
circuit breaker. Starters are used in motor control applications.

9.2.4 Counters and Timers
When a control logic requires that an action be taken after a certain delay, we use counters
or timers. If the delay is based on counting something, a counter is used. If the delay is
based on time, a timer is used. It should be emphasized that the counter and timer functions
can be implemented either in hardware as PLC I/O modules (counter and timer modules)
or in software by using general purpose I/O. Dedicated counter and timer hardware offers
higher frequency counting and higher resolution timing functions than can be accomplished
in software implementation. Hardware counters and timers may also be used as part of a
hard-wired control circuit without any PLC software involvement.
Both counter and timer hardware modules have three major circuits: the power circuit,
control circuit, and output circuit. The power circuit is needed to power the counter and
timer modules. The control circuit is the signal used to trigger the module. For a counter,
it is the signal transition from one state to another state (OFF to ON or ON to OFF) that is
to be counted. For a timer, it is the signal that triggers the start of the process of timing a
period. The logic about when to keep timing may vary. Once it is triggered, a timer can run
until the present time value, or the timing operation continues only while the control signal
is ON and is suspended when it is OFF. Both counter and timer have preset values. When
the counter counts up to the preset value, the output circuit is turned ON. When the timer
measures that the preset value of time has passed since the control signal triggered it, the
output circuit is turned ON. The output circuit of a timer and counter is similar to the output
circuit of a relay. An electrical contact is made (ON) or broken (OFF) as output action.