S ERVO M OTOR I N T E R FA C E S
S ERVO M OTOR I N T E R FA C E S
Servo motor interfaces are used in applications requiring control of servo motors via servo drive controllers. A servo motor is a specially designed motor that contains a permanent magnet. The speed of a servo motor can be easily varied by changing the input voltage to the motor. A servo module provides the drive controller with a ± 10 VDC signal, which defines the forward and reverse speeds of the servo motor. These modules are generally used when axis motion control, either linear or rotational, is required. A common linear motion example is a leadscrew assembly, which translates rotational movements from a servo motor into linear displacement (see Figure 8-28).
Applications that once employed clutch-gear systems or other mechanical arrangements to perform motion control now use servo interfaces. The advantages of servo control are shorter positioning time, higher accuracy, better reliability, and improved repeatability in the coordination of axis motion. Typical applications of servo positioning include grinders, metal- forming machines, transfer lines, material-handling machines, and the precise control of servo driver valves in continuous process applications.
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Lead Screw
Servo Motor
Interface
PLC
Figure 8-28. Servo motor interface application.
Servo positioning controls operate in a closed-loop system, requiring feed- back information in the form of velocity or position. Servo control interfaces may receive velocity feedback in the form of a tachometer input, or position- ing feedback in the form of an encoder input, or both. The feedback signal provides the module with information about the actual speed of the motor and the position of the axis. This information is then compared with the desired velocity and the desired position of the axis. If the module detects a difference between the desired and actual values, it will correct its output until the error between the feedback data and the set point velocity and position values is zero.
Figure 8-29 shows a servo control configuration block diagram. PLCs that have positioning control capabilities require two modules—one to imple- ment the servo control task and one to receive feedback and close the loop. Some manufacturers, however, offer complete servo control for one axis in
a single module. Servo control, like stepper motor control, can occur in either single-step or
continuous positioning mode (see Figure 8-30). Depending on the manufac- turer, multiaxis control can also be synchronized in either single-step or continuous mode.
The PLC processor sends all of the move and position information, includ- ing acceleration, deceleration, and the final and feed velocities, to the servo module. In axis positioning applications, including those performed by servo
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Processor Data
Servo Motor
Servo Voltage ± Drive
10 VDC
Velocity Feedback
Motor
Position Feedback
Tachometer
Encoder
Figure 8-29. Servo control block diagram.
Accl. Decel.
Return Move 3 (a) Single-step mode
Accl. Decel.
Return Move 3 (b) Continuous mode
Figure 8-30. Servo control in (a) single-step and (b) continuous modes.
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systems, the term feed velocity indicates a period of constant velocity. When the module is operating, the processor monitors its status without interfering with the module’s complex, rapid calculations. The processor updates the module with a new move for an axis when the previous move has been completed and the module is ready for a new profile. The acceleration and deceleration parameters are given as speed in inches per minute per second (ipm/sec) at a specific resolution. Figure 8-31 illustrates a typical field connection diagram for a servo motor interface.
DC Power Supply
+V –V
Channel A Common
Channel B Encoder
Common Marker
Servo Motor
Common
Output ±
10 VDC Servo
Motor Drive
JOG FWD
Loss of
JOG REV
feedback detection
Stop
(VDC)
Limit Switch
Tach
Figure 8-31. Servo motor interface connection diagram.
When servo interfaces are used for positioning control, the feedback resolu- tion provided by the system is a key issue. For example, if an interface uses
a leadscrew (a rotational-to-linear motion translator) for axis displacement and an encoder to provide a feedback signal to the servo module, the user must know the leadscrew pitch, the number of encoder pulses per revolution, and the multiplier value in the encoder section of the interface. Some interfaces allow the user to select a multiplier, thus providing better feedback resolution without changing the encoder. The example at the end of this section will show you how some of these parameters are used.
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The feedback resolution of a servo positioning (linear) interface can be defined as:
Pitch of motion translator
Feedback resolution =
(Encoder pulses per revolution)(Feedback multiplier) Each servo interface has a predefined resolution, which varies from 0.001 to
0.0001 inches. A trade-off exists between axis velocity and feedback resolu- tion, since axis speed is directly proportional to feedback resolution. Typical axis positioning speeds range from 500 to 1000 inches per minute (ipm) and encoder feedback input frequencies range up to 250 kHz. Remember that resolution, or accuracy, diminishes as the speed increases (e.g., a resolution of 0.0001 inches at 450 ipm will be 0.001 inches at 900 ipm).
E X AM PLE 8 -3
A PLC system uses a servo interface to perform a one-axis position- ing of a metal part. This part will be machined at a defined profile, which will be stored in the processor’s memory. A leadscrew, which allows travel of 1/8th inch (0.125) per revolution, moves the part along an X-axis. A quadrature incremental encoder, which has a 200 kHz pulse frequency that provides 250 pulses per revolution, supplies position feedback information. The encoder is connected to an encoder feedback terminal in the servo interface that provides a software programmable multiplier of × 1, × 2, and × 4 increments per pulse ( × = times).
(a) Find the feedback resolution and the number of pulses that will be received if the part travels 12.5 inches. (b) Also, describe a way to double the feedback resolution without changing the encoder.
S OLU T I ON
(a) Feedback resolution is a function of the leadscrew pitch and the product of the number of pulses per revolution generated by the encoder and the feedback multiplier. The leadscrew’s pitch is 1/8th inch, which means that the part will travel 0.125 inches for every rotation (see Figure 8-32).
The feedback resolution is therefore: . 0 125 inch/rev =
. 0 0005 inches/pulse
250 pulses/rev 1 ×
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Thus, a metal part moving 12.5 inches will generate a position feedback of:
. 12 5 inches
= , 25 000 pulses
. 0 0005 inches/pulse
Axis Motion
Feedback
8 threads per inch (8 pitch) in this example
Pitch is 1/8 inch in this example
Figure 8-32. Leadscrew (linear) displacement system.
(b) Using a multiplier of × 2 would improve the 0.0005-inch resolution (movement per encoder pulse) to 0.00025 inches (0.0005 ÷ 2=
0.00025). This × 2 multiplier option allows both of the quadrature pulses (A and B) to be counted, yielding twice as many pulses in one rotation.
8 -5 A S C I I , C OMPUTER , AND N ETWORK I N T E R FA C E S
Some special I/O modules aid in the communication of information to the real world. These intelligent modules accept data from and transmit data to field devices, including computers and other PLCs. This data is transmitted in one of the following forms:
• ASCII characters •
a computer language, such as BASIC or C
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C HAPTER 8
Special Function I/O and Serial Communication Interfacing
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S ECTION 2
Components and Systems
Local and remote I/O processors fall into the proprietary category of communication interfaces, since they communicate information through a network to the PLC’s subsystems. However, they were discussed in the remote I/O section of Chapter 6, since these modules also fall under the discrete I/O category.
Figure 8-33. RS-232 ASCII interfaces from (a) Mitsubishi and (b) Allen-Bradley.
If an ASCII interface does not use a microprocessor, the main PLC processor handles all of the communications interfacing. This significantly slows down the communication process and the program scan, since the processor must handle each character or string of characters that is transmitted to or received from the module on a character-by-character (interrupt) basis. That