01-08-2012, 12:47 PM
Timers and Counters for Sequential Control
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Counters
Counters are used to eliminate need to use internal relays
(latches) to represent the number of events that have occurred.
Mechanical or Digital
Digital Counters
increments or decrements based on input signals
output of a counter is in the form of relay contact
Components
Count Register- Holds the pre-assigned count value
Accumulator- Increments or decrements a count each time
an input pulse is received
Relay Contact- Activated when the accumulator values is
equal to the register values
Using Counter in the Conveyor
Part is placed on the conveyor. PD0 starts the motor and
PD1 stops it.
After 10 parts ( a full box), the conveyor will not start until
the box is picked up.
Need to pickup is signaled by Light L
Completion of the pickup is signaled by a push button PB.
Timers
The simple timer requires the specification of the time
delay and the input signal that activates the delay.
When the input signal is received, the timer waits the
specified delay time before switching on the output
signal.
The timer is reset (the output is set back to its initial
value) by turning off the input signal.