Allen-Bradley 1756-OB16IS Benutzerhandbuch

Application Technique

Position-based Output Control with the MAOC Instruction

Catalog Numbers

1756-OB16IS, 1756-OB16IEFS, 1732E-OB8M8SR

This application technique describes applications that use the Motion Arm

Output Cam (MAOC) instruction with and without the use of scheduled

output modules.

Topic Page

Description 2

Operation 3

Configure Output Modules 9

Configure the Axis 11

Program the Instruction 12

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

2Position-based Output Control with the MAOC Instruction

Description The MAOC instruction enables you to synchronize digital outputs with axis

motion. At low speeds, you can use the instruction with a standard output

module to ease the axis and output synchronization. As performance

requirements increase, a scheduled output module is necessary to operate with

tight time synchronization.

This document describes three application examples that benefit from an

MAOC-based solution.

Strobe Trigger

A strobe trigger initiates systems, such as cameras, bar code scanners, or other

self-contained processes, that detect a rising edge input.

In this application, use the instruction to follow the position of the conveyor axis:

•The instruction triggers the strobe each time the conveyor moves a preset

distance.

•The instruction simplifies programming and the mechanical system.

Because the instruction controls the monitoring and trigger, it eliminates

the need to monitor limit switches or axis position in application code.

This frees up scan time and logic to focus on other aspects of the machine.

This also enables the strobe to have a very short and repeatable duration.

Applicator Control

The MAOC instruction is useful with glue applicators or other applications that

turn an output On and Off as the master line shaft changes position and forces

the output On if needed.

In this application, use the instruction to follow the master line shaft virtual axis:

•Program the compensation data to account for the mechanical delays of

turning the solenoid valve On and Off.

•Use enable bits to allow operator-based manual control of the outputs.

When used with gluing applications, the instruction reduces complex

compensation math. The Purge function is especially useful for glue applicators

that need to clear the lines.

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

Position-based Output Control with the MAOC Instruction 3

Product Diverting

In a product diverting application, product moves from one high-speed conveyor

to another via pneumatic or electrical drivers.

In this application, use the instruction to trigger the output when the product is

directly in front of the diverter.

This application is particularly useful for any kind of product quality rejection

system and also works well for material handling.

Operation By itself, the MAOC instruction enables position-based output control in these

ways:

•Uses the position of any motion axis in a ControlLogix® system as the

position reference

•Uses any output or DINT

•Updates outputs based on the motion axis position at the motion group

coarse update rate, typically 1…32 ms

The instruction can update standard digital output modules at the coarse update

rate. However, some high speed applications require a higher degree of accuracy.

The 1756-OB16IS, 1756-OB16IEFS, and 1732E-OB8M8SR scheduled output

modules improve the accuracy of the MAOC instruction by supporting the

ability to schedule output On and Off times. All of the scheduling configuration

for an output’s On and Off times is completed through the MAOC instruction.

The instruction then updates values in the module’s output tags that define the

scheduled output behavior.

Table 1 - Scheduled Output Module Features

Feature 1756-OB16IS 1756-OB16IEFS 1732E-OB8M8SR

Number of schedules 16 32 16

Output points available

for scheduling

8 (points 0…7) 16 (points 0…15) 8 (points 0…7)

Remote operation None Remote chassis On-Machine™ over the

EtherNet/IP network

Minimum schedule interval(1)

(1) The MAOC limits the minimum schedule interval (minimum pulse width) to 1/16 of the coarse update period.

100 μs 5 μs 100 μs

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

4Position-based Output Control with the MAOC Instruction

Factors that Impact Schedule Accuracy

The following sections describe factors that can contribute to a loss of accuracy

on the scheduled output modules.

Minimum Pulse Width

A schedule interval is the course update period (CUP) divided by 16. Each

schedule interval can have only one scheduled event within it. If multiple

schedules fall within a single schedule interval, only the last event is evaluated.

The width of a pulse must be longer than a schedule interval. If the pulse width is

shorter than or equal to the schedule interval, it is less than the minimum pulse

width, and a loss of accuracy can occur.

Figure 1 on page 5 shows a minimum repeatable pulse width for a

1756-OB16IEFS module that supports as many as 16 schedules per CUP. The

summary of events is as follows:

•A—The On and Off events are spaced far enough apart that they cannot

fall within the same schedule interval. This meets the minimum pulse

width criteria.

•B—Even though the On event occurs on the edge of the schedule interval,

the Off event is longer than the minimum pulse width and falls into a

different schedule interval.

•C—The On and Off events fall within the same schedule interval and

cancel each other. The output is never energized.

•D—The pulse has the same width as example C, but the On and Off

events fall into separate schedule intervals. In this case, the output is

energized. Because it is not possible to predict where the schedule interval

starts or ends, the output can turn On at times, but not at others—even on

the same machine.

•E—The pulse width is exactly equal to the schedule interval, and the

output is energized as scheduled in most cases. However, in some cases,

output behavior can be unpredictable. Because it is not possible to predict

where the schedule interval starts or ends, the output can turn On at times,

but not at others—even on the same machine.

Topic Page

Minimum Pulse Width 4

Schedule Limitations 5

Position Reference: Actual versus Command 7

Constant versus Changing Velocity 8

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

Position-based Output Control with the MAOC Instruction 5

Figure 1 - Schedule Intervals

Refer to the table below to determine the minimum pulse width for your

application.

Schedule Limitations

Each module has a limited number of total schedules it can process, and a limited

number of schedules it can process per CUP, as shown below.

Schedules can be allocated to multiple points, or even a single point, as long as

you maintain the minimum pulse width and maximum schedule count.

Next CUP

Current CUP

Previous CUP

Off

Output 1

Output 2

16 123456789

10 11 12 13 14 15 16 1

C D E

16 Schedule Intervals per CUP

Table 2 - Minimum Pulse Width

Discrete Output

CUP (µs)

MAOC Time Slot (µs) 1756-OB16IS (µs) 1756-OB16IEFS (µs) 1732E-OB8M8SR (µs)

500 31 100 31 100

1000 63 100 63 100

1500 94 100 94 100

≥2000 CUP/16 CUP/16 CUP/16 CUP/16

Table 3 - Schedule Limits by Module

Count 1756-OB16IS 1756-OB16IEFS 1732E-OB8M8SR

Total Schedule Count 16 32 16

Schedules per CUP 8 16 8

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

6Position-based Output Control with the MAOC Instruction

Figure 2 shows an example of the 1756-OB16IS module processing eight

schedules in the normal order (illustrated alphabetically). The module processes

the last schedule on output 0, then the last schedule on output 1, followed by the

last schedule on output 2, and so on. The module then processes the second to

last output for each output point. After the module processes the last schedule,

the processing completes. Any unprocessed schedules are not acted upon.

In the figure, the dashed lines signify the intended behavior, and the solid lines

show the actual behavior for these schedules:

•G—The schedule is processed; however, there is no change of state because

of the previous state of the output.

•H—The schedule turns output 1 Off, but much later than expected.

•I—This transition and all of the unmarked transitions are never processed.

If the 1756-OB16IS module in this example is replaced with a 1756-OB16IEFS

module with a higher schedule count, all of the schedules can be processed and

executed. Slowing down the master axis can also help because fewer schedules are

executed within each CUP.

Figure 2 - Schedule Limitations

Schedules Per CUP for 1756-OB16IS = 8

Next CUP

Current CUP

Previous CUP

Off

Output 0

Output 1

Off

Output 2

GDA

HE B

F C

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

Position-based Output Control with the MAOC Instruction 7

Position Reference: Actual versus Command

The MAOC instruction provides the option to use the actual feedback or the

command position as the master reference for the output cam:

•For axes configured as Feedback Only, the command position is equal to

the actual position.

•For axes configured as Virtual, the actual position is equal to the command

position.

Typically, the actual position for an axis has more variability than the command

position. For axes that have the option for either position, use the command

position to reduce possible error.

Figure 3 shows an extremely exaggerated case of actual position variability.

Filtering occurs when the position reference uses the actual position. This

increases the difficulty in determining when outputs change state, as shown by

labels A and B. In the example below, the MAOC instruction is bi-directional. If

the position reference is actual, the output energizes in a potentially troubling

manner. Setting the position reference to the command position improves the

output signal and the accuracy of the MAOC instruction.

Figure 3 - Command versus Actual Position

CUP

Actual

Command

Command On Time

Actual On Time

Time

Position

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

8Position-based Output Control with the MAOC Instruction

Constant versus Changing Velocity

The MAOC instruction calculates schedules without acceleration compensation.

Between each CUP, the instruction assumes a linear velocity. However, physical

systems behave in a non-linear fashion due to mechanics and physics. This can

lead to slight deviations between the scheduled position and physical position of

an event.

During times of high acceleration or deceleration, outputs can change state

sooner or later than planned. For example, in Figure 4, the intended result is to

turn the output On where position A intersects the physical position graph at

time D. The velocity compensation estimates that this will happen where

position A crosses the schedule position graph at time C. Because the physical

position lags behind the schedule position, the output turns On where position B

crosses the physical position graph. This demonstrates an output changing state

earlier than expected during acceleration.

This example is graphical only, and numerical difference is dependent on the

acceleration profile, the mechanical system, the available dynamic response, and

the duration of the coarse update period.

Figure 4 - Example of Constant versus Changing Velocity

osition

Time

Schedule Position

Physical Position

CUP 1 CUP 2

CUP 3

CUP 4

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

Position-based Output Control with the MAOC Instruction 9

Configure Output Modules For use with the MAOC instruction, output modules require minimal

configuration:

•When using a scheduled output module, the outputs associated with the

MAOC instruction operate independently of the requested packet

interval (RPI). Standard output modules are typically updated at the RPI,

but can be updated faster in certain circumstances.

•To associate an output module with an MAOC instruction, assign the

module’s scheduled data tag to the output tag in the instruction. Using any

tag other than the scheduled data tag can cause problems with the

execution.

•For the 1756-OB16IS, 1756-OB16IEFS, and 1732E-OB8M8SR

scheduled output modules, use the default settings shown in the figures

below.

Figure 5 - 1756-OB16IS Communication Format

Rockwell Automation Publication 1756-AT017A-EN-P - January 2013

10 Position-based Output Control with the MAOC Instruction

Figure 6 - 1756-OB16IEFS Output Data

Figure 7 - 1732E-OB8M8SR Timestamp

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