JoeGuardian Automation

17. High-Speed Counters and Encoders

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In the previous post, we covered standard PLC counters such as:

CTU = Count Up
CTD = Count Down
CTUD = Count Up / Count Down

Those counters work well for many basic counting applications.

But in real industrial automation, some signals are too fast for a normal PLC scan.

Examples:

Encoder pulses
High-speed product counting
Motor rotation feedback
Registration marks
Fast photoeye pulses
Indexing machine feedback
Position tracking
Speed measurement

For these applications, the PLC may need a high-speed counter.

In simple words:

A high-speed counter counts fast pulses that may be too quick for normal ladder logic to detect reliably.

Encoders are one of the most common devices connected to high-speed counters. An encoder electronically monitors the position of a rotating shaft and can provide information such as position, distance, and speed.

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Why Normal PLC Counters May Not Be Enough

A normal PLC counter depends on the PLC scan.

The PLC must see the input transition from OFF to ON before it can count it.

But if the pulse is very fast, this can happen:

Pulse turns ON
Pulse turns OFF
PLC scan never sees it
Counter misses the event

This is especially possible with:

Very short pulses
High-speed conveyors
Fast rotating shafts
High-resolution encoders
Small products moving quickly
Input filters set too long
Long scan times

A regular CTU instruction is great when the PLC scan can reliably detect the signal. But when pulses arrive faster than the scan or input module can handle, you need high-speed hardware.

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What Is a High-Speed Counter?

A high-speed counter, often called HSC, is a special counter designed to count fast input pulses independently of normal ladder scan timing.

Depending on the PLC platform, a high-speed counter may be:

Built into the PLC
Built into a compact controller
Part of a special input module
Part of a motion module
Part of a servo drive
Part of an encoder interface module

The key idea is:

The hardware captures fast pulses directly, then the PLC reads the accumulated count.

This is different from normal ladder logic counting, where the counter depends heavily on scan timing.

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High-Speed Counter vs Standard Counter

Feature	Standard Counter	High-Speed Counter
Counts using	PLC scan / ladder logic	Dedicated hardware
Best for	Slow or moderate signals	Fast pulses
Example input	Box photoeye	Encoder A/B pulses
Risk	Can miss fast pulses	Captures pulses more reliably
Common use	Production count	Position, speed, distance
Hardware needed	Standard input module	HSC input/module or motion hardware

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What Is an Encoder?

An encoder is a feedback device used to measure motion.

It can measure:

Position
Distance
Speed
Direction
Rotation
Linear movement

Encoders are used in:

Conveyors
Servo systems
Indexing tables
Packaging machines
Robotics
Printing machines
Labeling systems
Filling machines
Cut-to-length systems
Material handling

An optical rotary encoder commonly uses a disk with slots or marks and a light source/receiver. As the shaft rotates, the light beam is interrupted and converted into electrical pulses.

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Encoder Pulse Concept

Imagine an encoder connected to a conveyor shaft.

Every time the shaft rotates, the encoder produces pulses.

Example:

Encoder resolution = 1000 pulses per revolution
Shaft rotates 1 revolution
PLC/HSC counts 1000 pulses

If you know the conveyor roller circumference, you can convert pulses into distance.

Example:

1000 pulses = 1 roller revolution
1 roller revolution = 12 inches of belt travel
Therefore:
1000 pulses = 12 inches

This allows the PLC to track product movement more accurately than using only timers.

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Why Encoders Are Better Than Timers for Position Tracking

A timer estimates position based on time.

Example:

Photoeye detects box
Wait 2 seconds
Fire reject solenoid

This may work only if conveyor speed is constant.

But if conveyor speed changes, the product will not be in the same position after 2 seconds.

An encoder tracks actual movement.

Example:

Photoeye detects box
Wait 1500 encoder pulses
Fire reject solenoid

This is usually better because the reject timing is based on real conveyor travel, not time.

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Incremental Encoders

An incremental encoder generates pulses as it moves.

It provides relative movement, not absolute position by itself.

Simple concept:

More movement = more pulses

Incremental encoders provide relative position of the shaft. External electronics such as counters are required to accumulate the pulses in order to determine position, speed, and direction. They are commonly used for velocity control, point-to-point applications, and sequencing when position retention after power loss is not required.

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Incremental Encoder Signals: A, B, and Z

Many incremental encoders provide three main signals:

A pulse
B pulse
Z pulse / Index pulse

A and B Pulses

The A and B signals are offset from each other.

This offset allows the controller to determine direction.

If A leads B → one direction
If B leads A → opposite direction

This is called quadrature.

By placing two sets of slots 90 degrees out of phase, direction can be determined; these two signals are known as A and B pulses. A single index or Z pulse may also be used to identify a home/reference position.

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Z Pulse / Index Pulse

The Z pulse normally occurs once per revolution.

It is used for:

Home reference
Index reference
Position reset point
One pulse per revolution marker

Example:

Machine homes to sensor
Then uses Z pulse for precise reference

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Quadrature Encoder Concept

Quadrature uses two signals:

Channel A
Channel B

These signals are shifted 90 degrees apart.

This lets the counter know both:

How many pulses occurred
Which direction the shaft moved

Example:

A before B = Forward
B before A = Reverse

This is very useful for applications where the shaft can move in both directions.

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Absolute Encoders

An absolute encoder provides actual position information.

Unlike an incremental encoder, it does not only send pulses that must be accumulated.

Simple concept:

Absolute encoder = reports its actual position

Absolute encoders provide the absolute angular position of the shaft. They are useful when position data must be retained after power loss and are used in applications such as packaging machines, robotics, lead/ball screw positioning, rotary tables, and pick-and-place systems.

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Incremental vs Absolute Encoders

Feature	Incremental Encoder	Absolute Encoder
Output	Pulses	Unique position value
Position after power loss	Usually must re-home	Position retained/known
Common signals	A, B, Z	Parallel, serial, networked data
Cost/complexity	Usually simpler	Usually more advanced
Common use	Speed, distance, relative position	Absolute position, robotics, axes
Needs counter?	Yes, for pulse accumulation	Depends on interface

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What Happens After Power Loss?

This is an important practical concept.

Incremental Encoder

After power loss:

The controller may lose position count.
The machine may need to home again.

Why?

Because the incremental encoder provides pulses relative to movement. If the controller loses the count, it no longer knows the actual position.

Absolute Encoder

After power loss:

The encoder can report its actual position again when power returns.

This is why absolute encoders are preferred when position retention is important.

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Encoder Resolution

Resolution means how many counts or pulses the encoder provides per revolution or per unit of travel.

Examples:

100 pulses per revolution
1024 pulses per revolution
2500 pulses per revolution
10000 pulses per revolution

Higher resolution means more pulses per movement.

That can improve position detail, but it also increases the pulse frequency that the counter must handle.

Example:

Higher resolution encoder
+ faster shaft speed
= higher pulse frequency

This is why you must verify that the input module or high-speed counter can handle the frequency.

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Simple Encoder Math

Example:

Encoder = 1000 pulses per revolution
Roller circumference = 10 inches

Then:

1000 pulses = 10 inches
100 pulses = 1 inch
10 pulses = 0.1 inch

If the PLC counts 2500 pulses:

2500 pulses ÷ 100 pulses per inch = 25 inches

So the conveyor moved 25 inches.

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Speed Measurement Using Encoder Pulses

An encoder can also be used to measure speed.

Example:

Count pulses over a fixed time window.
Calculate pulses per second.
Convert to RPM or conveyor speed.

Example:

Encoder = 1000 pulses/rev
Measured pulses = 2000 pulses in 1 second
Speed = 2 revolutions/second
Speed = 120 RPM

This is a common method for speed feedback.

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High-Speed Counter Applications

High-speed counters are used for:

Encoder position counting
RPM measurement
Conveyor tracking
Cut-to-length systems
Fast product counting
Registration mark detection
Packaging machine timing
Machine indexing
Length measurement
Speed feedback
Zero-speed detection

In motion systems, feedback sensors such as optical encoders, resolvers, and Hall effect devices return position or velocity information so the controller or drive can close position and velocity control loops.

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Practical Example 1 — Conveyor Tracking

Requirement

Reject a bottle after it travels from inspection sensor to reject station.

Instead of:

Wait 2 seconds

Use:

Wait 1800 encoder counts

Logic concept:

Inspection_Fail detected
Store current encoder count
Target_Reject_Count = Current_Count + 1800
When Encoder_Count >= Target_Reject_Count
Fire Reject_Solenoid

This keeps the reject timing tied to actual conveyor travel.

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Practical Example 2 — Cut-To-Length

Requirement

Cut material every 24 inches.

Encoder setup:

100 pulses = 1 inch
24 inches = 2400 pulses

Logic concept:

Start measuring material
When encoder count reaches 2400 pulses
Trigger cut
Reset count
Repeat

This is common in:

Tubing
Wire
Film
Labels
Paper
Metal strip
Packaging material

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Practical Example 3 — Motor RPM Feedback

Requirement

Measure motor speed using encoder pulses.

Count pulses per second
Convert to revolutions per minute
Display RPM on HMI

Example:

Encoder = 1024 pulses/rev
Measured = 2048 pulses/sec
2048 / 1024 = 2 rev/sec
2 × 60 = 120 RPM

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Practical Example 4 — Homing

A machine axis may need to establish a known position.

Typical homing sequence:

Move slowly toward home sensor.
When home sensor turns ON, continue until Z pulse.
Reset encoder count to 0.
Machine position is now referenced.

The index or Z pulse is used as a home/reference marker, especially with incremental encoders.

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Practical Example 5 — Zero-Speed Detection

A high-speed counter can detect whether a shaft is moving.

Example:

If encoder count does not change for 2 seconds while motor command is ON,
generate zero-speed fault.

Possible causes:

Broken belt
Motor not turning
Encoder coupling loose
Encoder cable fault
Mechanical jam
Drive fault
Shaft slipping

This is a very useful diagnostic.

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Wiring and Signal Types

Encoder wiring depends on the encoder type.

Common incremental encoder signals:

A
B
Z
A-not
B-not
Z-not
+V
0V
Shield

Many industrial encoders use differential line driver outputs for better noise immunity.

The encoder application summary recommends differential line driver outputs as the most reliable signal type for incremental encoders, and low-capacitance cable for longer cable runs to reduce signal distortion.

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Encoder Troubleshooting

When an encoder or high-speed counter is not working, check:

Encoder power
0V common/reference
A/B signal wiring
Shield grounding
Cable damage
Loose connector
Encoder coupling
HSC module configuration
Counts increasing/decreasing
Direction bit
Input frequency limits
Noise from VFD/motor cables
Mechanical slipping
Homing sequence
Scaling math

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Common Problem 1 — Count Does Not Change

Possible causes:

Encoder has no power
A/B wires disconnected
Wrong HSC channel
Bad module configuration
Broken encoder coupling
Input type mismatch
Cable damage
No common reference
Encoder shaft not rotating

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Common Problem 2 — Count Changes in Wrong Direction

Possible causes:

A and B channels swapped
Direction setting inverted
Encoder mounted opposite direction
Mechanical direction changed
Scaling sign wrong

Fix may be:

Swap A/B channels if allowed by design
Change direction configuration
Invert direction in logic
Correct mechanical installation

Always follow machine documentation and site standards.

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Common Problem 3 — Count Is Noisy or Jumping

Possible causes:

Electrical noise
Poor shield termination
Encoder cable near VFD motor leads
Loose connection
Wrong input type
Long cable with wrong cable type
Bad grounding
Damaged encoder

VFD motor wiring can generate electrical noise, and low-level control or feedback wiring should not be routed near high-power drive conductors because induced noise can disturb signals.

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Common Problem 4 — Position Lost After Power Cycle

If the encoder is incremental, this may be expected.

Possible solutions:

Home the machine after power-up
Use Z index pulse
Use absolute encoder
Use battery-backed position system
Use drive-based position retention

If the application cannot tolerate losing position, consider absolute feedback.

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Common Problem 5 — PLC Misses Pulses

Possible causes:

Using standard input instead of HSC
Input filter too slow
Pulse frequency too high
PLC scan too slow
Encoder resolution too high for hardware
Wrong voltage/output type

Fix:

Use high-speed counter input/module
Check max input frequency
Use correct encoder output type
Adjust input filtering
Verify wiring and shielding

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High-Speed Counter and Ladder Logic

In many systems, the HSC hardware counts pulses independently.

The ladder logic then uses values such as:

Current_Count
Target_Count
Direction
Speed
Preset_Reached
Position_Error
Home_Complete

Example logic:

If Current_Count >= Target_Count
Then trigger output or move to next step

For high-speed control, avoid relying only on slow ladder logic when the action must happen with very precise timing. Some applications need hardware compare outputs or motion control hardware.

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Automation Technician Notes

When troubleshooting encoder/HSC systems, ask:

Is this a standard counter or high-speed counter?
Is the pulse frequency within the module rating?
Is the encoder incremental or absolute?
Are A and B wired correctly?
Is the count increasing in the expected direction?
Is the Z pulse used for home?
Does position reset after power loss?
Is shielded cable used correctly?
Are encoder cables separated from VFD/motor power cables?
Is scaling from pulses to units correct?
Is the HMI displaying raw counts or engineering units?

A good technician does not only ask:

Is the encoder working?

A good technician asks:

Is the controller receiving clean pulses, counting them correctly, and scaling them correctly?

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Best Practices

Use these habits:

Use high-speed inputs/modules for fast pulse signals.
Verify encoder resolution and max frequency.
Use shielded encoder cable.
Separate encoder wiring from VFD and motor power wiring.
Use differential line driver outputs when possible.
Document pulse-per-revolution and scaling math.
Use homing routines for incremental encoders.
Use absolute encoders when position must survive power loss.
Monitor raw counts and scaled position.
Use feedback faults for no-count or zero-speed conditions.

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Key Terms

Term	Meaning
High-Speed Counter	Hardware counter for fast pulses
Encoder	Feedback device for position, speed, or distance
Incremental Encoder	Encoder that generates pulses relative to movement
Absolute Encoder	Encoder that reports actual position
A/B Channels	Quadrature signals used for count and direction
Z Pulse	Index pulse used for home/reference
Quadrature	A/B signals 90 degrees out of phase
Resolution	Pulses per revolution or counts per unit
PPR	Pulses per revolution
HSC	High-speed counter
Homing	Referencing an axis to a known position
Scaling	Converting pulses into engineering units
Zero-Speed Detection	Detecting no motion when motion is expected

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Final Thoughts

High-speed counters and encoders are used when normal PLC counting is not fast or accurate enough.

A standard PLC counter is useful for basic product counting, but encoder pulses and fast position signals often require dedicated high-speed hardware.

Incremental encoders provide pulses that must be accumulated. Absolute encoders provide actual position. Quadrature A/B signals allow the controller to determine direction. The Z pulse can be used as a home reference.

For an Automation Technician, the key lesson is:

Standard counters count slow events.
High-speed counters count fast pulses.
Encoders provide motion feedback.

Once you understand high-speed counters and encoders, conveyor tracking, cut-to-length systems, speed feedback, indexing machines, and motion-related troubleshooting become much easier.