Your System Has Powered Up – But Does It Know Where It Is? Absolute Position Sensors vs. Homing

Position Sensors

Homing is designed to find the position.
An Absolute Position Sensor already knows it.

🟩 Absolute Sensor

⚡ Power ON

📍 Position = 137°

✅ Ready

🟥 Encoder + Homing

⚡ Power ON

❓ Position Unknown

🔄 Homing

⏱ 12.5 s

✅ Ready

🧩 Further Reading – Measurement as a System

This article is part of an engineering series exploring how reliable measurement depends on proper system design rather than on a single sensor component.

Before diving deeper into industrial temperature sensing, you may also find the following articles in the series useful:

Absolute Position Sensors vs. Homing in Motion Control Systems

Many mechanical systems are capable of highly accurate motion, executing complex commands and operating within fast, closed-loop control systems.

However, the moment power is lost and the system is restarted, one fundamental question arises:

Does the controller actually know the mechanism’s current mechanical position?

In many cases, the answer is no.

The motor may have stopped at a specific angle, a robotic arm may be left halfway through its travel, a process valve may remain partially open, or an EO/IR positioning system may have stopped while pointing in a particular direction.

When power is restored, the controller does not necessarily know where the mechanism actually is.

To solve this problem, many systems perform a homing sequence—a controlled movement toward a known mechanical reference position.

However, homing is not always the simplest, safest, or most efficient solution.

In applications where the controller must know the mechanism’s position immediately after power-up, an Absolute Rotary Position Sensor can fundamentally change the way the system is designed.

What Is Homing?

Homing is the process of moving a mechanism after power-up until it reaches a known reference position.

That reference can be established using various methods, including:

A limit switch
A proximity sensor
An optical marker
A mechanical stop
An encoder index pulse
Any dedicated reference sensor used to identify the zero position

Once the reference point has been detected, the controller resets its position counter and begins tracking all subsequent movement relative to that point.

This approach is widely used in systems based on incremental encoders. While incremental encoders accurately measure movement, they do not necessarily provide the mechanism’s absolute position immediately after startup.

For example, an incremental encoder may report that the shaft has rotated 20° clockwise since power-up.

What it cannot determine is whether the shaft originally started at:

10°
90°
180°
Or any other angular position.

Without a reference point, the controller knows how far the mechanism has moved, but not necessarily where it actually is.

Why Can Homing Become a Problem?

In many simple machines, homing is an effective and perfectly acceptable solution.

The motor slowly moves toward a limit switch, the controller establishes the zero position, and normal operation begins.

In more advanced systems, however, homing introduces additional mechanical, electrical, software and safety considerations.

Unwanted Motion During Startup

To locate the home position, the mechanism must move.

In many applications, that movement itself becomes the problem.

For example:

A robotic arm may collide with nearby equipment.
A medical mechanism may already be in contact with a patient.
A valve may unintentionally change the state of a running process.
An EO/IR system may lose its line of sight.
An unmanned platform may move at a critical moment.
An actuator may apply force to an already-loaded structure.
A door, cover or moving assembly may encounter an unexpected obstruction.

In these situations, requiring the mechanism to move simply to determine its position becomes a system-level design challenge rather than a positioning problem alone.

Longer Startup Time

A homing sequence takes time.

The system cannot begin normal operation immediately after power-up. Instead, it must first:

Initialize the controller
Verify the sensors
Move the mechanism
Search for the home position
Detect the reference point
Reset the position counter
Return to the required operating position

In many industrial machines, this additional time may be negligible.

In systems that must recover immediately after a power interruption, however, the delay can become significant.

Additional Hardware

A homing system typically requires an additional reference device.

This may be a limit switch, proximity sensor, optical sensor, or mechanical stop.

Each additional component adds:

Wiring
Connectors
Installation space
Assembly cost
Another potential point of failure
Additional testing
Maintenance requirements
Software complexity

The true cost of homing is therefore not limited to the price of a single sensor.

It includes the total system cost required to support the homing process throughout the product’s lifecycle.

Dependence on the Reference Position

If the home switch becomes worn, damaged, misaligned or fails to be detected correctly, the system may:

Travel beyond its intended limits
Fail to locate the home position
Enter a fault condition
Establish an incorrect position reference
Damage the mechanism
Require operator intervention

The harsher the operating environment, the more critical the reliability of the homing mechanism becomes.

What Is an Absolute Position Sensor?

An Absolute Position Sensor provides an output that directly represents the mechanism’s current mechanical position.

Each angular position corresponds to a unique output value.

For example, over a measurement range of 0° to 360°:

0° corresponds to one output value
90° corresponds to another
180° to another
360° to the end of the measurement range

Depending on the sensor model, the output may be:

Analog voltage
Ratiometric voltage
PWM
Dual redundant outputs
Digital interface

As soon as power is restored, the controller reads the sensor output and immediately knows the mechanism’s actual position.

There is no need to move the shaft to a reference point simply to determine where it is.

Relative vs. Absolute Position Measurement

The key difference is not the electrical output.

It is how the system behaves after power-up.

Relative Position Measurement

A relative sensor measures how much movement has occurred from a starting point.

For example, it may report that:

The shaft has rotated 30°
The motor has generated 500 encoder pulses
The actuator has moved 12 mm

However, if the starting position is unknown, the final position cannot be determined with certainty.

Absolute Position Measurement

An absolute position sensor reports the mechanism’s actual position.

For example:

The shaft is at 42°
The valve is 65% open
The actuator is halfway through its travel
The mechanism is approaching its end stop

This information is available immediately after power-up, regardless of where the mechanism was before power was removed.

What Happens After a Power Loss?

Imagine a rotary mechanism operating at 137° when a power failure occurs.

The mechanism stops exactly where it is.

When power is restored, two very different scenarios are possible.

System Using Only an Incremental Encoder

The controller restarts and the encoder count begins again from zero.

From the controller’s perspective, the mechanism may appear to be at 0°, even though its actual mechanical position is 137°.

To establish the correct position, the system must perform a homing sequence.

System Using an Absolute Position Sensor

Immediately after power-up, the controller reads the sensor output.

The sensor reports that the mechanism is already at 137°, allowing the controller to continue operating according to the programmed control logic.

No movement toward a reference position is required.

When Does Homing Become a Real Challenge?

Robotics

Each joint of a robotic arm may stop at a different angle before power is removed.

After restart, a homing sequence may require multiple axes to move simultaneously or sequentially.

If the workspace is occupied, this movement may introduce a risk of collision.

An absolute position sensor on each joint allows the controller to determine the arm’s exact pose immediately after startup.

EO/IR Systems and Positioning Mechanisms

Stabilized EO/IR systems often need to maintain their pointing direction even after a temporary power interruption.

Moving to a home position can:

Lose the current line of sight
Delay mission recovery
Cause unnecessary mechanical movement

Knowing the actual position immediately after power-up allows the system to resume operation far more efficiently.

Actuators and Process Valves

In process control applications, the valve position at startup is often critical.

The controller must know whether the valve is:

Fully closed
Fully open
Partially open
Or in an unexpected position

Moving the valve simply to establish a reference position may disrupt the process itself.

Medical Equipment

Medical systems frequently include mechanisms that remain in direct contact with patients, examination tables, support arms, or imaging equipment.

Automatic movement during startup may not always be acceptable—or safe.

An absolute position sensor enables the controller to determine the mechanism’s position before initiating any movement.

Unmanned Systems

Unmanned vehicles often incorporate steering mechanisms, antennas, cameras, gimbals, hatches and actuators.

After power-up, the controller must immediately know the position of these mechanisms.

In many mobile platforms, performing a full homing sequence may simply not be practical.

Industrial Automation

An automated machine may stop while a workpiece, fixture or tool remains inside the working envelope.

A homing sequence performed immediately after restart could result in:

Damage to the workpiece
Damage to the tooling
Mechanical interference
Unexpected machine movement

Knowing the actual position before commanding any motion significantly reduces these risks.

Does an Absolute Position Sensor Always Eliminate Homing?

Not necessarily.

An absolute position sensor can eliminate the need for homing to establish the mechanism’s position, but certain systems may still perform controlled verification procedures after startup.

Examples include:

Mechanical integrity checks
End-of-travel verification
Detection of shaft slippage
Functional testing of safety limit switches
Periodic calibration
Validation following maintenance

The important distinction is that the controller already knows the mechanism’s position before these procedures begin.

Verification becomes an optional diagnostic process-not a prerequisite for normal operation.

Engineering Case Study – An EO/IR Positioning System That Must Know Its Position Immediately After Power-Up

An EO/IR payload installed on an unmanned ground vehicle incorporates a rotary positioning mechanism that controls both a thermal camera and a daylight imaging sensor.

The mechanism has a mechanical travel range of 0° to 160° and may stop at any angular position due to:

Normal system shutdown
Power loss
Emergency Stop activation
Switching between power sources
Controller restart

In the original design, the mechanism’s position was measured using an incremental encoder mounted on the motor.

Following every restart, the controller had no knowledge of the camera’s actual pointing angle and therefore initiated a homing sequence toward a reference sensor located at the end of travel.

The Challenge

Assume the system loses power while the camera is positioned at 125°.

When power is restored, the mechanism must travel back toward its home position.

If the homing speed is intentionally limited to 10°/s to prevent mechanical shock or collision, the theoretical travel time to the reference position becomes:

125° ÷ 10°/s = 12.5 seconds

Once the home position has been reached, the controller must still:

Detect the reference sensor
Stop the motor
Reset the position counter
Move back to the required mission angle
Verify correct operation before resuming normal service

If the previous mission angle was also 125°, the mechanism may travel a total of 250°:

125° back to the home position
125° back to the target position

At 10°/s, this represents approximately 25 seconds of mechanical movement, excluding sensor validation, settling time and software verification.

In mission-critical applications, this delay can be significant.

In addition, the homing sequence requires:

A limit switch or proximity sensor
A mechanical mounting bracket
Additional wiring and connectors
An extra controller input
Dedicated software logic
Fault handling if the home sensor is not detected
Physical movement of the camera after every restart

The Solution – Measuring the Output Shaft with the Euro-CMRK

Rather than relying solely on the motor encoder, the design was upgraded by adding a Euro-CMRK Absolute Rotary Position Sensor directly to the gearbox output shaft.

The Euro-CMRK is a non-contact Hall-effect sensor that uses a compact puck-and-magnet arrangement.

Because the sensor can be programmed for measurement ranges between 30° and 360°, the full electrical output can be mapped directly to the mechanism’s actual travel of 0° to 160°.

Immediately after power-up, the sensor measures the magnet’s position and provides an output corresponding directly to the mechanism’s current angular position.

If the mechanism stopped at 125°, the controller already knows its position.

There is no need to move the mechanism toward a home position simply to determine where it is.

The controller reads the sensor output during startup and immediately resumes operation using the mechanism’s actual position.

Making Better Use of the Output Signal

The Euro-CMRK operates from a 5 V supply and provides a programmable ratiometric output, typically configured between 5% and 95% of the supply voltage.

This results in an effective signal range of approximately:

0.25 V at the beginning of travel
4.75 V at the end of travel
4.5 V total usable output span

When this entire voltage range represents only 160° of mechanical travel, the theoretical sensitivity becomes:

4.5 V ÷ 160° = 28.1 mV/degree

If the same voltage range were spread across a full 360°, the sensitivity would be:

4.5 V ÷ 360° = 12.5 mV/degree

In other words, programming the sensor to match the mechanism’s actual travel increases the available signal change per degree by approximately 2.25×.

This does not improve the sensor’s intrinsic accuracy.

However, it allows much better utilization of the controller’s ADC input range and available measurement resolution.

Dual Outputs for Diagnostics

The Euro-CMRK is available with dual-output configurations.

In a typical setup:

Channel 1 increases clockwise.
Channel 2 decreases in the opposite direction.

The controller can continuously compare both outputs to verify signal plausibility.

For example, if one channel is reading approximately 70% of its programmed range, the second channel should produce the complementary output defined by the programmed transfer function.

A mismatch between the two signals may help detect:

Open-circuit wiring
Short circuits
Failure of one output channel
Signals outside the valid operating range
Supply voltage faults
Wiring errors

Although dual outputs alone do not make the system functionally safe, they provide an additional diagnostic layer that is unavailable with a single-output sensor.

Designed for Compact Mechanical Integration

The Euro-CMRK has a compact housing measuring approximately 21.5 mm in diameter and only 5.2 mm thick, making it suitable for installation directly adjacent to the output shaft, even in space-constrained assemblies.

The sensor measures the position of a rotating magnet without any mechanical connection between the sensor and the moving shaft.

According to the installation guidelines, the recommended air gap between the sensor face and the magnet is typically 0.5 to 3 mm, with a maximum allowable radial offset of ±3 mm.

This configuration is particularly attractive when:

There is no room for a coupling
Mechanical loads should not be transferred to the sensor
The electronics and mechanism are physically separated
A low-profile sensor is required
Small shaft misalignment must be accommodated

As with any magnetic position sensor, proper magnet selection, alignment and spacing are essential for achieving the specified measurement performance.

Frequently Asked Questions

Does an Absolute Position Sensor retain position information when power is removed?

In many applications, it doesn’t need to.

An Absolute Hall-Effect Position Sensor measures the current position of the shaft or magnet immediately after power-up and outputs a signal corresponding to its actual angular position.

Because it measures the existing mechanical position directly, the sensor does not need to remember where it was before power was removed.

What is the difference between an Absolute Encoder and an Absolute Hall-Effect Position Sensor?

Both technologies can provide position information immediately after startup, but they are designed for different application requirements.

An Absolute Encoder typically offers higher resolution and advanced digital communication interfaces.

An Absolute Hall-Effect Position Sensor generally provides a compact, cost-effective solution for angular position measurement, with outputs such as analog voltage or PWM.

The best choice depends on factors such as required resolution, response speed, communication interface, operating environment, and overall system cost.

Can an Absolute Position Sensor be used together with an Encoder?

Yes.

This is a common architecture in high-performance motion control systems.

The encoder provides motor feedback and speed information, while the Absolute Position Sensor measures the actual position of the load and reports it immediately after power-up.

Does an Absolute Position Sensor eliminate the need for Limit Switches?

Not always.

A position sensor can often eliminate a limit switch used solely for homing.

However, many systems still use independent limit switches as dedicated safety devices.

Position measurement and mechanical safety should be considered as two separate functions.

Which is better: a Shaft Sensor or a Puck Sensor?

A shaft-mounted sensor is ideal for direct mechanical coupling and straightforward installation.

A puck-style sensor is often preferred when installation space is limited or when direct mechanical coupling should be avoided.

The best solution depends on the mechanical design, available installation space, shaft alignment, loading conditions, and environmental requirements.

Is a 360° sensor suitable for every rotary application?

Not necessarily.

The designer should consider whether continuous rotation is required, how the sensor behaves at the 0°/360° transition, and how the controller handles wrap-around.

For mechanisms with limited angular travel, programming the sensor specifically for the application’s operating range often provides better performance.

Do Hall-Effect sensors wear out over time?

The sensing element itself operates without mechanical contact and therefore does not suffer from track wear like a conventional potentiometer.

However, the overall lifetime of the sensor may still depend on components such as bearings, shafts, seals, connectors, and wiring.

Can the output signal be matched to the application’s angular travel?

Yes.

Many programmable sensors allow the start angle, end angle, output direction and transfer function to be configured.

This allows the electrical output to be optimized for the mechanism’s actual travel and the controller’s input range.

Glossary

Absolute Position Sensor

A sensor that reports the mechanism’s actual position immediately after power-up, without requiring a reference movement or previously stored position.

Homing

A controlled procedure in which the mechanism moves to a known reference position in order to establish the system’s zero point.

Incremental Encoder

A sensor that measures relative movement using pulses.

After power-up, it typically requires a reference position to determine the mechanism’s absolute location.

Limit Switch

A mechanical or electronic switch that detects when a mechanism reaches a predefined position.

It is commonly used for motion limits, homing, or safety functions.

Hall Effect

A sensing technology that detects magnetic fields to measure position without mechanical contact between the sensor and the moving component.

Ratiometric Output

An analog output whose voltage varies proportionally with the supply voltage.

Commonly used in 5 V automotive, industrial and OEM control systems.

PWM (Pulse Width Modulation)

A digital output in which the measured position is represented by the duty cycle of the output signal.

Redundancy

The use of two or more independent measurement channels to improve diagnostics, fault detection and overall system reliability.

Shaft Sensor

A rotary position sensor that is mechanically coupled directly to the rotating shaft.

Puck Sensor

A contactless rotary position sensor that measures the position of a rotating magnet without requiring a mechanical shaft connection.

Backlash

The free mechanical movement between mating transmission components before torque is transferred.

Backlash is commonly found in gearboxes and gear-driven systems.

Startup

The sequence of operations performed by the controller immediately after power-up and before normal system operation begins.

Wrap-Around

The mathematical handling of the transition between the end and the beginning of a circular measurement range, such as 359° to 1°, where the two positions are physically adjacent despite appearing numerically far apart.

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