Page 133 - Programmable Logic Controllers, Fifth Edition - Mobile version

without feedback, for example, when the controller tells
the stepper motor how many steps to move and how
fast to move, but does not verify where the motor is.
Closed-loop control compares speed or position feed-
back with the commanded speed or position and gen-
erates a modified command to make the error smaller.
The error is the difference between the required speed
or position and the actual speed or position. Motor/controller
Figure 6-45 illustrates a closed-loop servo motor sys-
tem. The motor controller directs operation of the servo
Servo
motor by sending speed or position command signals to Controller amplifier
the amplifier, which drives the servo motor. A feedback
device such as an encoder for position and a tachometer
for speed are either incorporated within the servo motor Position Speed
or are remotely mounted, often on the load itself. These feedback feedback
provide the servo motor’s position and speed feedback in- Servo
formation that the controller compares to its programmed motor Load
motion profile and uses to alter its position or speed.

6.8 Seal-In Circuits Feedback
device
Seal-in, or holding, circuits are very common in both relay Tachometer: speed
logic and PLC logic. Essentially, a seal-in circuit is a method Encoder: position
of maintaining current flow after a momentary switch has Figure 6-45 Closed-loop servo motor system.
been pressed and released. In these types of circuits, the seal- Source: Photos courtesy Omron Industrial Automation, www.ia.omron.com.
in contact is usually in parallel with the momentary device.
The motor stop/start circuit shown in Figure 6-46 is a
typical example of a seal-in circuit. The hardwired circuit
Wiring diagram
L1 L2 L3 consists of a normally closed stop button in series with a
normally open start button. The seal-in auxiliary contact
of the starter is connected in parallel with the start button
M to keep the starter coil energized when the start button is

Start
M
M Inputs Ladder logic program Output
L1 L2
Stop Motor
OL starter
Stop T1 T2 T3 Start Stop coil M
Start
Motor
starter coil
M
Motor

Programmed
Ladder control diagram
L1 Start Motor L2
Stop starter coil
M
M

Seal-in contact
Hardwired
Figure 6-46 Hardwired and programmed seal-in circuit.

114 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

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