How are magnetic rotary encoders used in industrial automation?
This article is part two of our 'Match Made in Automation' Series. You can find the other installments of this series below:
- Part 1: Factory automation realizes boost from new technologies
- Part 3: Explore options for choosing an optical rotary encoder for motion control and position sensing
- Part 4: AI takes on growing role in HVAC system efficiencies
- Part 5: Extending operational lifetime for battery-powered devices is crucial
Rotary encoders are an electrical solution to measuring angular position in mechanical systems. They are typically used in control loops for motors or actuators but are also used in dials found in human interfaces.
Before magnetic and optical rotary encoders were developed, angular position used to be measured with variable resistors. The contacts would move across the resistive element, resulting in wear and a source of electrical noise. Modern encoders remove the physical electrical contact, overcoming these two restrictions.
The precision and reliability offered by modern rotary encoders are essential in many product categories. These include robotics, machine tools, printing presses, motion control systems, medical equipment, aerospace, gaming and entertainment, and automotive.
Renewable energy systems are also using rotary encoders. They provide pitch control on the blades of wind turbines to maximize turbine efficiency. Controlling the position of solar panels helps optimize the way they capture sunlight.
How to interpret encoder functionality
Rotary encoder functionality is classified as absolute or incremental. They can often be configured as either, or with a constantly variable analog output. The flexibility comes from having the electronic interface built into the encoder assembly.
Absolute rotary encoders produce a unique code pattern for each angle of rotation. Their resolution—the smallest change that can be measured—is defined as the number of code patterns per revolution, each code pattern representing a position within a 360-degree rotation. Code processing can generate more information, such as rotational speed or diagnostics.
Incremental rotary encoders measure angles and count revolutions, producing a pulse chain as the encoder shaft rotates. The resolution is defined by the number of pulses per 360-degree revolution.
Key rotary encoder terminology explained
Before looking at the differences between magnetic encoders and other types, it helps to understand the terminology found in encoder data sheets. In particular, we look at the differences between resolution, accuracy and precision.
What is encoder resolution?
As mentioned earlier, the resolution is the smallest angular change that can be measured. Incremental optical encoders will often use two LEDs and two photoelectric sensors separated by 90 electrical degrees. This twin-channel technique enables the direction of travel to be determined and facilitates resolution multiplication in the signal processing.
This gives either 2X or 4X the resolution achieved using a single LED and sensor combination. The 4X figure is obtained by counting both the leading and trailing edge of each signal pulse. Absolute optical encoders employ an LED array and multitrack rotating disk. The more tracks—or lines on the disc—the higher the resolution. The maximum resolution is related to the maximum size of the disc.
What is encoder accuracy and precision?
Encoder accuracy is the difference between the reported encoder position and its actual position. It is measured in degrees or fractions of degrees.
An encoder’s precision refers to the repeatability of measurement. The readings from an encoder can vary between revolutions. There may also be differences in precision for clockwise and counter-clockwise rotations.
When selecting an encoder it is important to consider the whole system. Engineers do not want to over-specify the performance of an encoder. When affixing a rotary encoder within a system, any potential mechanical variation could introduce higher error than the accuracy limitations of the encoder itself.
How do rotary encoders work?
Optical rotary encoders are by far the most common type in use. They feature a pattern, a light source and a photoelectric sensor. Resolution is expressed as Cycles Per Revolution (CPR). See our related article on optical rotary encoders for more details.
Magnetic rotary encoders substitute a magnetic field for the light source, and a Hall-effect sensor for the photoelectric detector.
A Hall-effect rotary encoder comprises a rotating wheel with several magnets around its circumference. Each magnet has two poles, north and south. An array of discrete Hall-effect sensors is positioned around the device. These are normally integrated alongside the signal conditioning, analog-to-digital conversion, and other processing elements.
Rotation generates a Hall-effect element, caused by the moving magnetic fields. Pairs of sensors may be positioned 90 degrees apart to produce a quadrature output for incremental mode operation. The resolution is determined by the number of magnetic poles and the number of sensors.
Comparing Hall-effect with optical encoders
Higher resolution and greater accuracy are traditional benefits of optical encoders. These advantages are being eroded as magnetic encoder technology advances.
The drawbacks of optical encoders include contamination from dust, dirt, moisture or chemicals. Also, they use bearings to keep the rotating and static parts of a rotary encoder concentric. These can be prone to wear. If optical rotary encoders are used in harsh industrial environments, they may need to be sealed and protected. Aging of the LEDs can result in output reduction. This can be a limiting factor for the operating lifetime of optical encoders.
By contrast, magnetic rotary encoders do not need bearings, and the Hall-effect sensors and associated electronics are hermetically sealed in a standard integrated circuit package. Subject to choosing the most appropriate package for the operating temperature range, these devices are extremely robust and do not degrade with age.
Perhaps the main design consideration when using magnetic encoders is their susceptibility to magnetic fields, which are commonplace around electric motors. Electromagnetic shielding can be incorporated into the encoder design, but this adds size and cost to the component.
The performance of magnetic sensors can also be limited or compromised by planar Hall effects, hysteresis and temperature changes. Compensating for these effects is possible but executing the complex algorithms demands more processing power.
Magnetic encoders are better suited in environments where an extreme range of temperatures and harsh conditions like moisture, particles, grease and oil are present.
Are Hall-effect encoders improving?
The continued scaling in semiconductor processing means Hall-effect encoders follow the trend of smaller size, lower power consumption, higher processing power and greater functional integration.
The benefits include:
- Higher resolution: Higher resolution leads to more precise position sensing. When functioning as an absolute encoder, the serial data stream produced by a device can be up to 22 bits wide for a resolution of 4,194,304 discrete positions. That said, encoders with 8-bit, 10-bit or 12-bit (4,096 discrete positions) suit most applications.
- Enhanced accuracy: Reducing signal noise and optimizing signal processing algorithms for advanced error compensation increases measurement accuracy.
- Increased speed and dynamic response: Faster measurements lead to applications that can accurately track fast-moving objects or machinery. A better dynamic response supports real-time feedback and control.
- Improved environmental resistance: Withstanding harsh operating conditions requires a robust underlying technology. Hall-effect encoders are engineered to be resistant to dust, moisture, temperature variations, vibration and other environmental factors that could otherwise affect sensor performance.
- Higher durability: Reliability has improved over time. Advancements in materials, sealing techniques and overall design have enhanced resistance to wear, extending operational lifespan.
- Integrated digital interfaces: Standard ports such as RS-485, SSI or SPI make it easier to interface encoders with control systems and digital devices. This can also facilitate faster data transfer and compatibility with various communication protocols to simplify system design.
An electric motor will feature a rotary encoder to provide input to the control circuitry. Magnetic Hall-effect rotary encoders can perform better than optical encoders in some industrial applications.
What’s next for Hall-effect rotary encoder performance?
Although incremental improvements in magnetic encoder performance will continue for the reasons given earlier, an opportunity to make a step-change improvement could be on the horizon.
Graphene, a material that comprises a single layer of atoms and that exhibits extreme electrical conductivity, is now being used to produce Hall-effect sensors with high sensitivity for the most demanding applications.
Although not yet available in Hall-effect magnetic encoders, high-performance graphene sensors could eliminate many of the limitations of today’s Hall effect magnetic encoders. Graphene can overcome the planar Hall effects. This would lead to better resolution, accuracy and precision.
The current generated in regular Hall sensors may move vertically and horizontally, along one arm of a Hall cross. Stray magnetic fields around the sensor will be at 90 degrees to the current. The Hall signal will be influenced by the field being measured and stray fields. This limits the resolution, accuracy and precision of magnetic Hall-effect encoders that use traditional Hall sensors.
Graphene sensors will have a single layer of carbon atoms. This two-dimensional material means that the current travels in one plane with no option for vertical movement. Fields at any angle other than perpendicular to the element will not contribute to the Hall voltage.
Conventional sensors also remember the fields previously applied. The sensor will behave differently, depending on whether the previous field was higher or lower. The historical influence of the field is known as hysteresis.
In a graphene Hall sensor, there will be no magnetic or magnetizable components, which means that the system will have no “memory” of the previous field. The sensor will produce the same response for the field present, regardless of whether the previous field was higher or lower.
Magnetic fields get weaker with distance from the source. Detecting weak fields requires an array of sensors. The high mobility of graphene leads to a high-sensitivity sensor. More sensitivity translates to detecting smaller fields. As a result, fewer sensors are required in the array.
Device calibration is often required. It can account for linear or non-linear changes in response behavior due to age or temperature. Adding calibration into a system is often expensive; recalibration is time-consuming and leads to downtime of the equipment.
The physical properties of graphene, combined with the simplicity of it being a single element compared with more complex sensor types, lead to stability with temperature and linear response with the field, reducing the need for complex calibration.
Choosing the right encoder
Rotary encoders of similar performance may still vary in price. It is important that the cost of the encoder is unlikely to be as significant as the role they play in an industrial control system. If they prove unreliable the cost of maintenance and equipment downtime will far exceed the cost of a rugged and robust part. Magnetic encoders based on the Hall effect can deliver a long-term cost savings over optical encoders.
