Page 726 - Mechatronics with Experiments

712 MECHATRONICS
(PCB) assembly machines, and others. This example shows the components of a PLC based
system to control the position of a linear stage using an electric motor (i.e., stepper motor,
DC brushless motor, DC brush motor) and a rotary incremental encoder. Other real-world
components are also shown, such as the limit sensors to indicate the mechanical limits of
motion, a proximity sensor to establish a reference position (home position) after power-up,
since an incremental encoder can not measure absolute positions, and operator command
buttons (START and STOP). The components of such a closed loop translational position
control system include:
1. Mechanical motion mechanism: lead-screw or ball-screw and moving table.
2. DC servo motor with integrally built incremental encoder on the same mechanical
assembly.
3. DC motor drive: amplifier and DC power supply package.
4. PLC with a servo control module, where the encoder signal is feedback to the module
and the output signal from the module is the command signal to the amplifier.
5. “Home” reference sensor (ON/OFF type) and CW and CCW limit switches (ON/OFF
type) on the translational stage to define the motion limits and starting reference as
an absolute position.
6. START and STOP bush buttons for operator interface.
The sequence of operations, from start up to automated operation is then coded in the
ladder logic diagram. High performance motion profiles are defined in the servo control
module.

9.5 PLC APPLICATION EXAMPLE: CONVEYOR AND
FURNACE CONTROL

Consider the conveyor and heat furnace shown in Figure 9.13. The conveyor moves parts into
a heating chamber. The part is kept inside the temperature controlled furnace for a certain
amount of time (programmable) and at a certain temperature range (also programmable).
Then the conveyor moves to bring in the next part. In the process, the PLC controls the
temperature of the furnace and the speed of the conveyor. In addition, it controls the opening
and closing of the heater doors as well as position of the part inside the furnace using a
presence switch.

The system inputs are:
1. START switch (ON/OFF).
2. STOP switch (ON/OFF).
3. Part presence sensor (ON/OFF).
4. Door open switch (ON/OFF).
5. Temperature range sensor (analog).
The outputs are:

1. Analog output to heater element proportional amplifier.
2. ON/OFF output to motor starter.
3. ON/OFF output to door actuator relay control.