INTRODUCTION
PLC’s can be used for almost anything nowadays. The majority are used in
industry to run machines or operations. They are very advantageous because of their
ability to execute numerous commands in any order a programmer wishes them to.
Although not the most economical household device, they can also be used in a home,
and that is the heart of this project.
The PLC is set up to turn on two fans in the house when the temperature gets high
enough and turn them off when it cools down. The fans will turn on when the
temperature in the room goes above 80oF, and they will turn off when the temperature in
the room goes down below 77oF.
MATERIALS
The materials used in this project range from advanced to simple and they will be
discussed here. The main part of this project is the PLC, a PLC-500. This consists of the
CPU, an input simulator card, a relay card, and an output card. There are also two
thermocouple wires to be used for measuring temperature, a power strip with a relays
attached to two of the outlets, and two fans. These parts will all be explained in detail
later on.
SETUP
The setup for this is nice and clean, but also very effective. The PLC is set in the
house with two type J thermocouples run into two separate rooms. The tips of the
thermocouples will read the temperature surrounding them ( in a way discussed later),
and when the temperature in either room goes above 80oF, the PLC is programmed to
send a signal to the power strip to turn it on. When the temperature in either room is
brought down below 77oF or lower, the PLC sends a signal to turn off power to the strip.
One stipulation that is there is if the fans are on and one room goes below 77oF but the
other room is above 80oF still the fans will stay on. This was designed this way on
purpose. If one room is at 80oF and the other is at 79oF, and the room that was 80oF drops
below 77oF, and the other room stays at 79oF the fans will turn off.
HOW IT WORKS
Since the thermocouples are the endpoints they will be discussed first. Type J
thermocouples are made of two wires (each coated) of different metals, that are stripped
at the end. When the ends are joined they produce a tiny voltage (a few millivolts) that is
dependent on temperature. As they are heated the voltage between them increases. The
voltage in these is set up with the GE ladder logic so that the millivolts can be divided by
two to reach oF. So the thermocouples sense the “temperature”, and send the signal to the
PLC. The thermocouples are wired into a thermocouple card in the PLC. There is a input
simulator card in the PLC which must have A1 and A2 turned on for the signal to go to
the output card. Once the signal goes to the output card there is a signal sent to the relays
which are placed in the power strip. There is one relay for each of the two outlets used in
the power strip. These relays are used so the current can be contained in the power strip,
the wall, and the fans. The reason the fans aren't connected directly to the PLC is because
the PLC can't handle more than one amp. The relays used are 10 amp relays, so they are
easily sufficient. So once the relays receive the signal they make the connection in the
power strip to give the fans power.
LADDER LOGIC
The first page of the ladder logic is just a cover page and the second page
describes the references. The next page starts with the variable declaration and then goes
to the ladder logic. For the variables there are two inputs for fan off switches, two outputs
(one for each fan), two internal inputs, two registers, and two analog inputs. Rung four is
the divisor for the first thermocouple. This is what changes the millivolts to degrees
Fahrenheit. It takes the millivolts and divides them by two for degrees. This is a setup GE
has for this exact application. Rung five is for the second thermocouple and it has the
same function. Rung seven is the greater than statement for the thermocouple in the first
room, and rung eleven is the greater than statement for the second room. The purpose of
these is to send a signal to the relays when the "temperature" goes above 80oF. Rungs
eight and twelve are the less than statements for the rooms. When the thermocouples
signal the temperature is less than 77oF the fans turn off. Finally, rung nine is the OR
statement for when one of the internal inputs goes on or off and the input simulators are
on, the fans turn either on or off.
RESULTS
The results for this project are as good as they possibly could be. When either
thermocouple is warmed up the fans go on, and when they are cooled off the fans go off.
If one is hot and the other is cold the fans stay on and this is just what was expected. All
in all the project went very smoothly and was a great learning experience.