Large grain elevators typically are controlled
electronically from a control room. GIPSA
personnel in the elevator also rely heavily on
electronic monitoring for official weights and
for verifying that the grain which has been
weighed and inspected is the same grain being
loaded on the ship. Components of a monitoring or
control system can be classified as follows:
Input Devices: These provide
the monitoring/control system with raw information
about the operation of the elevator. Examples are
limit switches, level switches, load cells, and
potentiometers.
Output Devices: These operate
the motors and hydraulic or pneumatic cylinders that
power the elevator. Examples are power relays and
solenoid Valves. Input and ouput devices are often
lumped together as "I/O" devices.
Logic Devices: These draw conclusions
from the information provided by input devices,
activate output devices, and alert human operators
of conditions requiring their attention. Examples
are relay logic circuits, programmable logic
controllers, and computers.
Indicating Devices: These
convey information to human operators. Examples are
indicator lights, dials, video displays, and audible
alarms.
Controls: These allow input from
human operators. Examples are buttons, keyboards,
mice, trackballs, touch screens, and thumbwheels.
Limit Switches: Limit switches are
the most numerous input devices in an elevator. They
tell the system the position of gates, doors,
turnheads, or distributors, and the alignment of
belts. A switch can only tell whether an object is
present or absent at a certain location. A pair of
switches can tell if a gate is completely closed or
fully open, but not where the gate is if it is
somewhere between those two limits. A limit switch
can be operated by mechanical contact, by breaking a
beam of light, or by detecting the disturbance of a
magnetic field caused by a metal object.
The following diagram shows a mechanical limit
switch wired to a pair of indicator lights. It has
an actuator arm which operates the switch contacts
when it is moved slightly by contact with a piece of
equipment such as a slide gate. The arm is
spring-loaded, so it returns from its actuated
position to its normal position when the equipment
moves away.
A limit switch has at least two sets of contacts:
"normally closed" (NC) and "normally open" (NO) .
When the switch is in the normal position (nothing
touching the arm), the normally closed contacts pass
electricity, so the diagram shows the normally
closed indicator light as lit. When the switch is in
the actuated position, the normally open (NO)
contacts pass electricity, so the normally open
indicator light is lit. The "common" wire carries
electricity to both sets of contacts. The
terminology of switches is the reverse of the
terminology of gates: Switch contacts let
electricity pass when they are closed. A gate lets
grain pass when it is open.
