Page 727 - Mechatronics with Experiments

PROGRAMMABLE LOGIC CONTROLLERS 713
Controller : PLC

Relay: Door
START
close/open
STOP Presence
sensor Heater LS1
Product Door Temp sensor Door
1 2
LS2

Conveyor

Motor
& Drive
encoder

FIGURE 9.13: PLC control (logic and closed loop control) of a heater and a conveyor motor.

The desired control logic is expressed in pseudo-code as follows:
1. The operation is started when the START button is pressed. When the STOP button
is pressed the cycle is stopped.
2. PLC opens the doors of the furnace and waits until the doors open switch is ON.
3. Then the motor is started. The motor is stopped when the part presence sensor is ON.
4. The furnace doors are closed.
5. The PID loop which controls the furnace temperature is enabled. The PID controls the
heater output based on the preprogrammed desired temperature range and temperature
sensor feedback. The PID control output has a deadband. In other words, when
the temperature is within a desired range, the heater is turned OFF. These aspects of
the PID control loop are specified as part of the PID block setup.
6. After the furnace temperature is within the desired range, a timer is started and the
temperature control continues for a specified amount of time (i.e., 2 minutes).
7. When the heating time expires, the heater is turned OFF (PID loop is disabled).
8. Repeat starting with Step 2.

Figure 9.14 shows the ladder logic diagram program that implements the above
described control logic. Notice that for each I/O, there is a memory location assigned (see
the numbers under the input and output devices as illustration on left and right side of the
diagram). Then the basic cycle logic is implemented in the ladder diagram using the input–
output memory references, timer, and PID functions. Notice that in order to implement the
desired logic, internal memory (local data) is used in addition to the memory mapped to the
I/O channels. The desired temperature range and heating time data should be adjustable by
an appropriate operator interface device, such as a PC communicating with the PLC over a
serial communication bus.