Rockwell Automation 1771-QC, D17716.5.25 User Manual

AllenBradley

Servo Positioning Assembly

User

(Cat.

No. 1771-QC Series B)

Manual

Using This Manual 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Manual's Purpose 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Audience 11 Vocabulary 11 Manual Organization 12

Introducing the Servo Positioning Assembly 21. . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter What is the Servo Positioning Assembly? 21 Its Its Function 22 Its Features 24 Summary 27

Objectives

Applications

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Positioning Concepts 31. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter ClosedLoop Leadscrew Encoder Feedback 37 Summary 311

Objectives

Positioning

Pitch

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Positioning With an AllenBradley Programmable Controller 41

Chapter Where the Servo Positioning Assembly Fits In 41 Independent Move/Moveset 42 In Synchronizing Axes 48 Specifying Summary 413

Objectives

Position

. . . . . . . . . . . . . . .

of I/O Scan

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Axis Position

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

410. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Hardware Description 51. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter Indicators 51 Inputs/Outputs 52 External Power Supplies 57 Compatible Processors 58 Fault Responses 59 Specifications 511 Summary 513

Objectives

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table of Contentsii

Installing the Assembly 61. . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter Configuring the Modules 61 Setting Switches and Jumpers 63 Keying 67 Inserting the Module 69 Connecting Connecting AB Encoder and Drive 627 Startup Sequence 631 Summary 632

Objectives

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

to T

erminals 610. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Formatting and Interpreting Data Blocks 71. . . . . . . . . . . . . .

Chapter Relationship of Data Blocks 71 Status Block 74 Parameter Block 717 Moveset Block 740 Command Block 760 Summary 779

Objectives

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

71. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Programming 81. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter Programming PLC2 Family Block Transfer Instructions 83 PLC2Family Block Transfer Timing 86 PLC3 Block Transfer Instructions 813 PLC3 Block Transfer Timing 814 Programming Example 821 Summary 840

Objectives

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Integrating Axes 91. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter OpenLoop Procedure 91 ClosedLoop Procedure 96 Tachometer Summary 911

Objectives

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Calibration

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

91. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

98. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Troubleshooting 101. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter Monitoring 1771M3 Controller Indicators 101 Monitoring 1771ES Expander Indicators 103 Monitoring the Status Block 104 Troubleshooting Flowchart 107 Summary 1012

Objectives

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table of Contents iii

Glossary A1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Status Block B1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Parameter Block C1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Moveset Block 61. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Command Block E1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Using This Manual

Chapter

Manual's Purpose

Audience

Vocabulary

This manual shows you how to use the series B Servo Positioning Assembly (cat. no. 1771-QC). If you have a series A Servo Positioning Assembly, refer to publication 1771-817.

To use the servo positioning assembly, you must be able to program and operate an Allen-Bradley PC processor. In particular, you must be able to program block transfer instructions.

In this manual, we assume that you know how to do this. If you don’t, refer to the appropriate manual for the PC processor you will be using. Consult our Publication Index (publication SD499) for a list of our publications.

Some inconsistency exists throughout industry in the nomenclature used for components of closed-loop servo positioning systems. Therefore, as you read this manual, you should be aware of the names we use for these components.

We refer to the Servo Controller Module (cat. no. 1771-M3) as the

1771-M3 controller.

We refer to the Servo Expander Module (cat. no. 1771-ES) as the

1771-ES expander.

We refer to the device that receives the velocity command signal from

the 1771-ES expander as the servo drive. The servo drive converts ac power to dc power for the servo motor in proportion to the velocity command signal. What we refer to here as the servo drive, others may refer to as a servo controller. So, if you refer to this device as a servo controller, be aware of our nomenclature as you read this manual.

PC refers to programmable controller.

For an extensive list of terms we use this publication, refer to the glossary in appendix A.

11

Chapter 1

Troubleshooting

Manual Organization

This manual is organized into the following chapters:

Chapter Title What's

Introducing the Servo Positioning Assembly

Positioning Concepts concepts of closedloop positioning,

Positioning with AllenBradley PC'

Describing Hardware

Installing the Assembly installing the servo positioning assembly

Formatting and Interpreting Data Blocks

an overview of the servo positioning assembly features

including velocity loop, positioning loop, and feed forward

the servo positioning assembly' a servo system, and the servo positioning assembly' processor

describing the servo positioning assembly its specifications, and its compatibility with other hardware components you will need for a closedloop positioning system

and interconnecting hardware

formatting parameter and control data for block transfer to the servo positioning assemblyinterpreting status and diagnostic data received in block transfer from the servo positioning assembly

, its applications, functions, and

s communication with the PC

Covered

, move description,

s position in

12

8 Programming

10 Troubleshooting

Integrating Axes

generating a ladderdiagram program to transfer data blocks between the PC data table and the servo positioning assembly

adjusting the servo positioning assembly for optimum operation with the machine axis it is to control

using indicator status and statusblock information to diagnose and correct problems

Chapter

Introducing the Servo Positioning Assembly

Chapter Objectives

What is the Servo Positioning Assembly?

This chapter gives you an overview of the servo positioning assembly, its applications, functions and features.

A servo positioning assembly controls the motion of one of your axes. It consists of:

one Servo Controller Module (cat. no.1771-M3) one Servo Expander Module (cat. no. 1771-ES) that includes two Field

Wiring Arms (cat. no. 1771-WB)

With a basic servo positioning assembly (plus a servo drive, motor, tachometer, and encoder) you can control the motion of one user-supplied machine axis. You can add a second 1771-ES expander to control a second axis and a third 1771-ES expander to control a third axis. A 1771 I/O chassis can accommodate one 1771-M3 controller and a maximum of three 1771-ES expanders.

The 1771-M3 controller requires one I/O chassis slot; it requires no wiring (figure 2.1a). You can install it at any I/O slot in the I/O chassis. The 1771-ES expander requires a pair of slots that make up an I/O module group (Figure 2.1b). You make all wiring connections to the 1771-ES expander.

21

Chapter 2

Introducing the Servo Positioning Assembly

Figure 2.1 Servo

Positioning Assembly

Its Applications

Its Function

(a)Servo

Controller Module

(cat.no.1771-M3)

(b)ServoexpanderModule

(cat.no.1771-ES)

17954

Typical applications for a servo positioning assembly include positioning for:

grinding transfer lines material handling drilling riveting rotary indexing v-belt cutting glass cutting

Figure 2.2 shows a servo system for closed-loop axis control. The 1771-M3 controller communicates with the 1771-ES expander through I/O chassis backplane connections.

22

Chapter 2

Introducing the Servo Positioning Assembly

Command Position Data

Status Block

Processor

Figure 2.2 Closedloop

Axis Servo System

Axis Motion

Velocity Command

Drive Disable

Motor Tach Encoder

Velocity Feedback

Servo

Drive

Tach Input for Loss-of-Feedback Detection

Position Feedback

Servo Controller (cat. no. 1771 -M3)

NOTE: A second and third Servo Expander Module

could be installed in this I/O chassis for control of a second and third axis.

Servo Expander (cat. no. 1771 -ES)

The PC processor sends commands and user-programmed data from the data table to the 1771-M3 controller as directed by a block-transfer write instruction. The 1771-M3 controller coordinates the block transfer automatically, keeping ladder diagram programming to a minimum.

Based on information it receives from the processor, the 1771-M3 controller sends axis motion commands to the 1771-ES expander.

The 1771-ES expander closes the servo positioning loop. It commands axis motion by generating an analog voltage for your servo drive. Every

2.4 milliseconds (ms) it updates this analog output voltage according to motion commands from the 1771-M3 controller, discrete inputs, and

Discrete Inputs:

Jog Forward

Jog Reverse

Home Limit Switch

Hardware Stop

Hardware Start

Feedrate Enable

Discrete Output:

Hardware Done

10998

23

Chapter 2

Introducing the Servo Positioning Assembly

feedback from your encoder. The 1771-ES expander is able to provide this fast servo sample rate because the update is independent of the I/O scan.

A drive-disable output provides a signal to disable the servo drive in conditions such as loss-of-feedback or a hardware-stop signal. A hardware-done output signals the completion of each single-step move. Discrete hardware inputs include:

hardware stop jog forward jog reverse home limit feedrate enable hardware start

Its Features

The 1771-M3 controller sends axis status and diagnostic data to the data table as directed by a block-transfer read instruction. Because axis-command and status data is stored in the data table, axis motion control can interact with other axes, discrete I/O, and report generation.

See the following table for a list of the many useful benefits you’ss derive from an A-B servo positioning assembly.

24

Chapter 2

Introducing the Servo Positioning Assembly

Feature Benefit

incremental encoder feedbac

absolute or incremental positioning commands

programmable gain break

programmable acceleration/deceleration

programmable inposition band

programmable jog rates

programmable dwell precise dwell times

digital

precise closedloop positioning

programming flexibility

precise positioning at low speed with stability at high speed

optimize the machine cycle time over varying loads

flexible positioning accuracy

flexible manual positioning

excessfollowingdetection

lossoffeedback detection

software travel limits guards against axis overtravel

backlash takeup

offset

preset

automatic drive shutdown if the axis following error becomes too large

allow automatic drive shutdown during a move if tachometer or encoder feedback is lost

compensates for mechanical backlash

compensates for a variation in tool length or fixture dimension

easy redefinition of axis coordinates

25

Chapter 2

Introducing the Servo Positioning Assembly

Feature Benefit

optically isolated guards against noise entering the analog output

[1]

backplane circuits and limits the potential for damage due to improper connection

external hardware start

encoder input selectable for hightrue or lowtrue

synchronized start of feedrate override

[1]

sensing of customer power supply loss

feed forwarding

[1]

synchronizes moves with other axes

compatibility with a wider range of encoders

activates a preloaded feedrate override value to change speed on several axes simultaneously

an orderly shutdown of the servo system and to provide you with this diagnostic information

to allow you to reduce following error by up to 99.9% without increasing instability

26

constantvelocity command

moveset override

[1]

diagnostic words in the status block

[1]

for

runs an axis continuously at a selected velocity (could apply to controlling a conveyor with no programmed end point)

Modifies a moveset while it is being executed

provide your ladderdiagram program with access to diagnostic information

hardware and program troubleshooting

Chapter 2

Introducing the Servo Positioning Assembly

[1]

These features are only available on the series B servo positioning

assembly.

Summary

This chapter was intended to be very general. Upcoming chapters cover these topics in greater detail. To prepare for those details, read about positioning concepts in chapter 3.

27

Positioning Concepts

Chapter

Chapter Objectives

ClosedLoop Positioning

This chapter presents positioning concepts and terminology. If you are thoroughly familiar with the concepts of closed-loop servo positioning, you can skip ahead to chapter 4.

Closed-loop positioning is a precise means of moving an object from one position to another. Typically, an electric motor supplies the mechanical power, and the needed motion is linear. Therefore, we must convert the rotary motion of the motor’s shaft to linear motion.

Axis Motion

One common method of converting rotary motion to linear motion is with a leadscrew (Figure 3.1)

Figure 3.1 Leadscrew

Converting Rotory Motor Motion Into Linear Axis Motion

Axis Motion

Slide

Motor

The leadscrew assembly is referred to as the axis. A leadscrew assembly consists of a long threaded shaft (the leadscrew) and slide having an internal thread that matches the leadscrew. When the motor rotates the leadscrew clockwise, the slide moves forward. When the motor rotates the leadscrew counterclockwise, the slide moves backward.

Shaft Rotation

11999

31

Chapter 3

Positioning Concepts

Velocity Loop

Most closed-loop servo positioning installations use a dc motor to power the leadscrew. To accurately control the velocity of the dc motor, we need a velocity loop (Figure 3.2).

The velocity loop contains a summing point, an amplifier, and a tachometer. A tachometer is a precision generator that produces a voltage signal directly proportional to the angular velocity of the motor shaft. The output of the tachometer is the velocity feedback signal which is subtracted from the velocity command signal. The difference is the velocity error signal that is amplified to provide power for the motor to run at the commanded velocity.

Figure 3.2 Velocity

Loop

Velocity Command

Axis Motion

Motor

Summing Point

Amplifier

Velocity Error

Velocity Feedback

Velocity Error = (Velocity Command  Velocity Feedback)

Tach

12000

32

Whenever the velocity deviates from the commanded velocity, the velocity feedback signal adjusts the velocity error signal until the velocity matches the velocity command signal.

Chapter 3

Positioning Concepts

Positioning Loop

When we want to move the slide a specific distance, we can turn the motor on at a specific velocity for a specific length of time. However, this could produce imprecise positioning. To accurately control the position of the slide, we need a positioning loop (Figure 3.3).

Axis Feedrate

Figure 3.3 Velocity

Following Error = (Position Command) - Position

Position Command

Following Error

Position

Loop and Positioning Loop

D/A

Velocity Command

Axis Motion

Velocity Feedback

Incremental Position Feedback

Motor

Amplifier

Encoder

Tach

12001

The positioning loop includes a summing point, an amplifier, a D/A converter, and an incremental digital encoder to produce a position feedback signal. The axis feedrate is integrated in a register to produce the position command value. Incremental position feedback is integrated in a register to produce the actual position value. The position value is subtracted from the position command value. The difference is the following error, which is amplified and converted to an analog velocity command signal. This signal directs the axis to move in the right direction; the position value moves closer to the position command value.

The following error is a function of the axis velocity divided by the positioning-loop gain (K1). The following error is multiplied by the gain

33

Chapter 3

Positioning Concepts

to generate the velocity command. Gain is expressed in ipm/mil (where 1 mil - 0.001 in) or mmpm/mil (where 1 mil = 0.001 mm).

For example, with a velocity of 100 ipm and a gain of 1 ipm/mil, the following error is:

velocity 2following error =gain = 1 ipm/mil = 100 mil

When you increase the gain, you decrease the following error and decrease the cycle time of the system. However, the gain that you can use is limited by the drive, the motor, and the machine; a gain that is too large causes instability.

Feed Forward

To decrease the following error without increasing the gain, we can add a feed forward component (Figure 3.4).

Figure 3.4 Velocity

Velocity Command = K (following Error) - K (Axis Feedrate)

Loop, Positioning Loop, and Feed Forwarding

Feed Forward

100 ipm

Axis Motion

Motor

Encoder

Tach

Axis Feedrate

34

Position Command

Following Error

Position

Velocity

Command

D/A

Incremental Position Feedback

Velocity Feedback

12002

Chapter 3

Positioning Concepts

Feed forwarding requires an additional summing point and an amplifier. The axis feedrate is multiplied by the feed-forward gain (K2) to produce the feed-forward value. The feed-forward value is added to the following error multiplied by the gain to generate the velocity command.

Without feed forward, the axis will not begin to move until the axis feedrate builds up enough following error to generate a sufficiently large velocity command to overcome friction and inertia to move the axis. However, the feed-forward value could generate a velocity command to move the axis almost immediately. This immediate response keeps the actual position closer to the position command, thereby reducing the following error.

35

Chapter 3

Positioning Concepts

Leadscrew Pitch

Pitch is 1/4 inch in this example

Leadscrew pitch is the linear distance from one peak of the screw thread to the next. A leadscrew with a pitch of 1/4 inch is shown in Figure 3.5.

Figure 3.5 Leadscrew

Example Showing Pitch

4 threads per inch (4 pitch) in this example

36

12003

If the leadscrew has only one thread, the pitch is also equal to the lead, which is the distance the axis travels each revolution of the leadscrew. You can see from Figure 3.5 that the axis will travel 1/4 inch per revolution if the pitch is 1/4 inch. Since leadscrews normally have only one thread, and pitch is a more common term than lead, in this publication we use the term pitch to refer to the distance the axis travels for each revolution of the leadscrew.

Do not confuse leadscrew pitch with its inverse, which is the number of pitch (threads) per inch. In the example of Figure 3.5, the leadscrew has 4 pitch (threads) per inch. A leadscrew with a pitch of 1/4 inch is often described as being a 4-pitch (per inch) leadscrew.

Chapter 3

Positioning Concepts

Encoder Feedback

Light Source

An incremental digital encoder provides feedback that indicates the magnitude and direction of any change of axis position. As shown in Figure 3.6, the encoder shaft is attached to a transparent disc marked with uniformly spaced lines. Strategically located photodiodes detect light. As the disc rotates, the lines break up the light reaching the photodiodes. As a result, the output (channel A, channel B, and marker) from each photodiode is a series of electrical pulses.

Figure 3.6 Incremental

Encoder  Showing How Signals Are Generated

Photodetectors

Channel A

Channel B

Marker

Disc

Marker

11000

37

Chapter 3

Positioning Concepts

Channel Phase Relationship

The photodetectors are placed so that the channel A and channel B output signals are out of phase by 90

(Figure 3.7). The lead/lag relationship of these signals indicates the direction of axis motion. Also, the phase relationship of these signals allow the decoding circuit to count either 1, 2, or 4 feedback pulses for each line of the encoder (Figure 3.7). This provides flexibility in establishing feedback resolution.

Channel A

Channel B

Marker

Figure 3.7 Encoder

Forward

Signals  Showing Phase Relationship

Channel A

Channel B

Marker

Note: For the servo positioning assembly, the encoder marker must be high when both channel A and channel

B are high, or the marker is not recognized unless you set the marker logic jumper to the notgated position.

Reverse

11001

38

Feedback Resolution

The following discussion of feedback resolution assumes that you are using a leadscrew, and that the encoder is coupled directly to the leadscrew with no intermediate gearing. These assumptions apply to many applications. If your application differs, be sure to account for the differences.

Feedback resolution is the smallest axis movement the servo positioning system can detect. It is determined by:

leadscrew pitch - axis displacement per revolution encoder lines - number of lines per revolution feedback multiplier - selected as x 1, x2, or x4

Chapter 3

Positioning Concepts

The following equation shows how these factors determine feedback resolution:

leadscrew pitch feedback resolution = (encoder lines) (feedback multiplier)

You must select the leadscrew pitch, encoder lines, and feedback multiplier to provide desired feedback resolution and meet other requirements of your application.

The programming resolution of the servo positioning system is 0.0001 inch or 0.001 millimeter. If you select a feedback resolution coarser than that, round off your position commands so that the effective programming resolution is no finer than the feedback resolution you chose.

If you select a feedback resolution finer than the programming resolution, positioning can be smoother. However, the maximum axis speed is directly proportional to the feedback resolution. There is always a trade-off between feedback resolution and maximum axis speed. The maximum encoder input frequency for the servo positioning assembly is 250kHz. Therefore, to avoid a programming error, you must limit the axis speed to conform to this formula:

programmed 1.5 x 10

axis speed < 1.28 x feedback res x feedback mult

The 1.28 factor allows for a 127% feedrate override value.

Each encoder line represents a fraction of a revolution of the leadscrew. For example, consider a 250 line encoder. Each line represents 1/250 of a revolution of the leadscrew.

Also, consider a 4-pitch (per inch) leadscrew for this example. The slide moves 1/4 inch for each revolution. With an x1 multiplier, each feedback increment represents 1/250 of 1/4 inch or 0.001 inch slide movement. This is the feedback resolution.

0.25 in/rev feedback resolution = 250 lines/rev x 1 increment/line

= 0.001 in/increment

39

Chapter 3

Positioning Concepts

Therefore, if we cause the leadscrew to move the slide 2 inches, we will get 2,000 feedback pulses.

Now, consider replacing the 250-line encoder with a 500-line encoder. By doubling the number of feedback pulses per revolution of the leadscrew, we improve the feedback resolution from 0.001 inch to 0.0005 inch.

Another way to improve feedback resolution is to use a higher feedback multiplier. You can select a multiplier of x1, x2, or x4. For example, with the 4-pitch (per inch) leadscrew and the 250-line encoder, if you select an x2 multiplier you get the same feedback resolution improvement of from

0.001 inch to 0.0005 inch. With an x4 multiplier, you improve the feedback resolution to 0.00025 inch.

Marker

Besides the channel A and B output, an incremental encoder has a marker output (Figure 3.6 and Figure 3.7). The marker pulse occurs once every revolution. With a 4-pitch leadscrew, the marker pulse occurs at each 1/4 inch interval of slide travel.

We can use a market pulse to establish a home position somewhere along the slide travel. For example, we can place a limit switch near the end of the slide travel. The first market pulse after the limit switch is activated could then designate the home position (Figure 3.8).

310

Figure 3.8

Pulse  Establishing a Home Position

Marker

Limit Switch

Switch

Marker Pulse

Chapter 3

Positioning Concepts

Summary

Axis Motion

Home Position

12004

Once we establish a home position, we can use it as an absolute reference point for all moves.

In this chapter we described concepts of closed-loop positioning. Now you are ready for concepts of position with an Allen-Bradley PC. This material is covered in chapter 4.

311

Chapter

Positioning With an AllenBradley Programmable Controller

Chapter Objectives

Where the Servo Positioning Assembly Fits In

Servo Positioning Assembly

The previous chapter described concepts of closed-loop positioning. This chapter describes where the servo positioning assembly fits into a positioning system, and how the servo positioning assembly communicates with the PC processor.

Figure 4.1 shows where the servo positioning assembly and a servo drive fit in the positioning system we described in the previous chapter. The servo drive contains the velocity loop summing point and amplifier. The servo positioning assembly contains the positioning loop summing point and the feed forward summing point. The servo positioning assembly sends the analog velocity command signal to the servo drive.

Figure 4.1

the Servo Positioning Assembly Fits in a Positioning System

Where

Axis Motion

Feed

Forward

Motor

Tach

Encoder

Axis Feedrate

Position Command

Following Error

Position

Velocity

Command

D/A

Servo Drive

Velocity Feedback

Incremental Position Feedback

Figure 4.2 shows where the servo positioning assembly fits in a PC system. The PC processor constantly communicates with the servo

12005

41

Chapter 4

Positioning with Allen-Bradley PC

positioning assembly through the I/O scan. The PC processor acts on a block transfer read instruction to receive status blocks. Based on the status information received, the PC processor acts on a block transfer write instruction to send either parameter blocks, move blocks, or control blocks.

Figure 4.2

the Servo Positioning Assembly Fits in a PC System

Where

Processor

Independent of I/O Scan

Move/Moveset

Output Scan

Parameter, Moveset, and Command Blocks

Input Scan

Status Blocks

Servo

Positioning

Assembly

Outputs

Inputs

12006

Although the servo positioning assembly sends data to and receives data from the data table through the I/O scan, the positioning loop is closed on the 1771-ES expander (at the positioning loop summing point). This allows the 1771-ES expander to provide a servo sample period of 2.4ms, independent of I/O scan.

You must describe the axis motion you want in moveset blocks in the data table. You can enter a maximum of 21 separate move blocks in a moveset block (Figure 4.3).

42

Chapter 4

Positioning with Allen-Bradley PC

Figure 4.3

Moveset Block is Sent to the 1771M3 Controller That Sends the Move Blocks Sequentially

A to the 1771ES Expander

TwoMoveBlock

1771-ES expander

Current

Move

Next Move

Move blocks sent in sequence as each current move is started.

Moveset block in

the PC Processor

data table

Move 1

Move 2

Move 3

Move 4

Move 21

complete moveset (21 moves max) is sent in a single block transfer

Move 1

Move 2

Move 3

Move 4

Move 21

Moveset register

in the 1771-M3

controller

12007

The PC processor sends a complete moveset block to the 1771-M3 controller in a single block transfer. The 1771-M3 controller can hold a moveset block for each of the three possible axes.

The 1771-ES expander can hold two move blocks, the current move block available for execution and the next move block. After the current move is completed and the next move is to be executed, the next move block becomes the current move block (Figure 4.4).

43

Chapter 4

Positioning with Allen-Bradley PC

Figure 4.4

the 1771ES Expander, as Each Current Move is Completed, the Next Move Block is

In Ready to T

ake its Place

Current Move Block

Next Move Block

Start

Move

Move 1 Move 1 Move 2 Move 3 Move 20 Move 21

Move Move 2 Move 3 Move 4 Move 21 Move

Time

Start

Move

Start

Move

Start

Move

Start

Move

Start

Move

Initially, the 1771-M3 controller sends the first move block to the 1771-ES expander. Then, as each move is started the 1771-M3 controller sequentially sends each of the remaining move blocks to the 1771-ES expander.

A move block for a move to position defines motion of the axis from one position to another. Figure 4.5 shows the profile of an axis move. The horizontal axis in the figure represents axis position. The vertical axis represents axis velocity. Moves plotted above the position axis are in the positive direction (from left to right), moves plotted below the position axis are in the negative direction (right to left).

12008

44

Rate +

Final Velocity

or Feedrate

Figure 4.5 Onemove

Startpoint

Profile for an Axis

Acceleration

Move

Constant Velocity

Deceleration

Endpoint

Position

11010

In the move shown in Figure 4.5, the axis:

starts from a resting position accelerates to a final velocity

Chapter 4

Positioning with Allen-Bradley PC

moves at the final velocity some distance decelerates to zero velocity (at which time it has reached the

programmed endpoint)

Move Values

Each move block can specify several values. The servo positioning assembly executes the move based on these items you enter:

endpoint acceleration final feedrate deceleration

When you select a deceleration value, the 1771-ES expander automatically calculates the point at which the deceleration must begin.

You can combine several single moves like that of Figure 4.5 to form a moveset. Figure 4.6 shows an example that consists of four moves. Move 1 starts at position coordinate 0 and ends at position coordinate 2. Move 2 continues axis motion to position coordinate 5. Move 3 continues to position coordinate 7. Move 4 then causes the axis to reverse direction and move back to position 0. The axis stops after it returns to its initial starting position. A drawing like that of Figure 4.6 is a moveset profile. You can use such profiles as an aid in programming axis motion.

Figure 4.6 Moveset

Rate +

Profile with All Singlestep Moves

Move 1 Move 2 Move 3

12345 678

Position

Rate 

Move 4

11011

45

Chapter 4

Positioning with Allen-Bradley PC

You can program multiple movesets for a given axis.

Move Selection

For each move, you have each of the following selections:

Absolute or incremental positioning - In an absolute move, the

endpoint value specifies a position coordinate relative to the current axis zero position. In an incremental move, the endpoint value specifies a position coordinate relative to the last programmed endpoint achieved by the axis.

Global or local values - You enter a global final feedrate value and a

global accel/decel rate value. These global rates apply to all moves except those for which you select to specify local rates. A local rate applies only to a single move.

Halt or run - After completing a move for which you have selected

halt, the 1771-ES expander will not execute the next move until it receives a begin or start command. After completing a move for which you have selected run, the 1771-ES expander will immediately execute the next move without waiting for a start command. With halt selected, the module executes a single-step move. With run selected, you can select moves to be either single-step moves or continuous moves.

Single-step or continuous - When the 1771-ES expander executes a

single-step move, it decelerates the axis to zero velocity at the programmed endpoint. When it executes a continuous move, it attempts to blend the move smoothly with the final feedrate of the next move (if the next move is in the same direction). The moves in Figure 4.6 are all programmed as single-step moves. Figure 4.7 shows the same moveset with all moves programmed as continuous. A moveset can contain a mix of single-step and continuous moves.

46

+ 217 hidden pages