Allen-Bradley 1756-OB16IS User guide

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.

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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

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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

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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
On
Off
On
Off
Output 1
Output 2
16 123456789
10 11 12 13 14 15 16 1
A
C D E
B
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

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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
On
Off
On
Off
Output 0
Output 1
On
Off
Output 2
GDA
D
I
HE B
F C

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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
A
B
Time
On
Position

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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
P
osition
Time
CD
Schedule Position
Physical Position
CUP 1 CUP 2
CUP 3
CUP 4
A
B

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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

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10 Position-based Output Control with the MAOC Instruction
Figure 6 - 1756-OB16IEFS Output Data
Figure 7 - 1732E-OB8M8SR Timestamp