Preheat Oven
A software-based PLC will be used to control a preheat
oven used in a powdercoat paint process. The oven will have two sections,
with each section having the top and bottom independently controlled.
This results in four heating elements that can be controlled separately.
Under normal operation, three part temperature sensors and eight thermocouples
(two per heating element) will be used to maintain the temperature of the
part in each section at a user-defined setpoint. Numerous safety controls
will be implemented, along with two operating modes. These modes will
be detailed below.
Manual Mode
In manual mode, a user interface screen is displayed
that gives the temperature of each heating element, the temperature of each
part temperature sensor, and buttons to control each element as described
below. When the program enters manual mode, the heaters will be turned
to face the part by actuating a pneumatic cylinder and all of the heaters
will be turned off. The user can then turn each heating element on
or off independently. Buttons will also be provided to turn all of
the elements on or all of the elements off. A safety check will also
be performed continuously in manual mode as detailed below. A Shutdown
button will be provided to place the system into idle state as described
below and display the auto mode screen.
Auto Mode
In auto mode, a new user interface screen is displayed
that gives the temperature of each heating element and of each part temperature
sensor. The user will be able to enter setpoint temperatures and ranges
for each heating element. A user override will allow each heating element
to be turned on or off individually. The status of each heating element
will also be displayed. A start button will turn the heaters toward
the part and bring the temperature of the part to the desired setpoint, and
then maintain this temperature. While the oven is heating the part to
the desired temperature, a Preheat message will be displayed, and an Oven
Ready message will be displayed once the part has reached the desired temperature.
Once the line begins to move as indicated by an encoder, a System Running
message will be turned on and the system will continue to control to the temperature
setpoints. If the line does not begin moving within 30 minutes of when
the Start button is pushed, the system will return to idle state as described
below. If the Shutdown button is pushed at any time in auto mode, the
system will return to idle state. If a Manual button is pushed, the
system will go to manual mode as described above. If the line stops
while the system is running, the heaters will maintain the setpoint temperatures
for 30 minutes as before. The Oven Ready and Preheat messages will
also be displayed when appropriate. A safety and input check will also
be performed continuously in auto mode as detailed below.
Idle State
When the program is in idle state, all of the heating
elements will be off and the heaters will be turned away from the part.
A message will be displayed on the user interface screen to indicate that
the program is in idle state. A safety and input check will also be
performed continuously, as detailed below.
Safety Check
As indicated above, the safety check will be run continuously
in all modes. Several tests will be performed. First, the temperature
differential between the two thermocouples on a given heating element will
be checked. A large differential could indicate a faulty thermocouple
or a fire inside the oven. Next, the temperature of each sensor will
be compared to a given maximum temperature. This safety check will prevent
the oven temperature from becoming dangerously high. Finally, the incoming
temperature as detected by the first part temperature sensor will be compared
to a given maximum temperature to ensure that the part entering the oven
is not too hot. A different warning message will be generated for each
error and displayed to the screen, and the system will return to idle state.
The system can then be restarted and will operate normally; however, the
error messages will not be cleared until the problem has been resolved.
Input Check
As indicated above, the input check will be run in auto
mode. The user defined setpoints and ranges for each heating element
will be checked to see if they are in a valid range. If the input value
is not valid, a warning message will appear for three seconds and the new
value will not be accepted.
Control to Setpoint
When maintaining the oven temperatures, the part temperature
sensor will be compared to the bottom setpoints. If the part temperature
is greater than the upper limit, the appropriate element will be turned off.
If the part temperature is less than the lower limit, the appropriate element
will be turned on. The top thermocouple temperatures will be compared
to the bottom thermocouple temperatures and the top elements will be turned
on or off accordingly. If a user override has been set for any element,
that element will be controlled on or off as desired.
Physical Inputs: (3) Infrared part temperature
sensors – analog signal
(8) Type J thermocouples
(1) Line moving encoder – digital signal
Screen Inputs: Auto Mode
(4)
Part temperature setpoints (ºF)
(4)
Part temperature ranges (+/- ºF)
(4) Override off buttons
(4) Override on buttons
(4) Automatic
control buttons
(1) Start Oven button
(1) Shutdown Oven button
(1) Switch to Manual Mode button
(1) Return to Main Screen button
(1) Return to Powdercoat Screen button
Manual Mode
(4) Manual On buttons
(4)
Manual Off buttons
(1) Turn All Heaters On button
(1) Turn All Heaters Off button
(1) Shutdown Oven button
(1) Switch to Auto Mode button
(1) Return to Main Screen button
(1) Return to Powdercoat Screen button
Physical Outputs: (4) Heating Element relays
(1) Pneumatic Cylinder actuator
Screen Outputs: Auto Mode
(3) Part temperature sensor readings
(4) Thermocouple average temperatures for each heating element
(4) Heater status messages
(1) Program status message
(1) Invalid Input warning
(1) Oven Too Hot warning
(1) Incoming Parts Too Hot warning
(4) Heater Problem warning
Manual Mode
(3) Part temperature sensor readings
(4) Thermocouple average temperatures for each heating element
(4) Heater status messages
(1) Program status message
(1) Oven Too Hot warning
(1) Incoming Parts Too Hot warning
(4) Heater Problem warning
IV. SAMPLE OVEN CONTROL PROGRAM
V. ADDITIONAL DOCUMENTATION
- A 13-page user manual was provided to the sponsoring
company to explain exactly how the oven control program operates.
- A documentation pack was presented to the instructor
that included the following items:
- The complete oven control program (141 lines of code)
- Memory locations for all data items used in the control
program
- User interface screen displays for both auto and
manual mode
- A video showing the oven in normal operation
- A poster that highlights the main features of the
oven control program and displays several images of the various aspects of
the oven