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Rotary Encoders: Precision Positioning One Rotation at a Time

Often, some of the most important automation sensors are the ones people know the least about or may not even realize they need for their applications. One of those sensors is the rotary encoder, which every industry uses to provide mechanical values for rotation angles and convert them into electrical signals to help you position all critical machine parts with high precision. The robustness of rotary encoders enables many uses even under extreme conditions, such as in food processing, machining processes, automotive component assembly, conveyor systems, and packaging equipment. Many vision systems in use today for inspection also require an encoder input for inspecting objects on the move.

The two main types of rotary encoders are incremental encoders and absolute encoders. Here’s a quick look at why you would choose one over another and some key characteristics.

Incremental encoder features

    • High-resolution angle measurement
    • High shaft load, up to 500 N
    • High protection classes for harsh environments, up to IP69K
    • Corrosion-resistant designs with stainless steel housings are available
    • Available interfaces, such as ABZ, sin/cos, TTL, and HTL
    • Optical technology for precise, high-resolution measurements or magnetic technology for use in harsh environmental conditions is available

Incremental encoders for speed monitoring and position determination

Incremental encoders compare machine data to the last data collection point and record it. Each time the machines and the encoder are switched on, they define reference values. Incremental encoders output a precisely defined number of pulses per revolution. The signal serves as a measure of the angle or distance covered. The more signals are output per revolution, the higher the resolution of the incremental encoders and the more precise the measurement and control of your system. With incremental encoders, you can monitor and reliably control the rotation and belt speed of your machines and systems. Some incremental encoder interfaces include ABZ, sin/cos, TTL (Transistor-Transistor Logic), and HTL (High Threshold Logic), which meet common and established industry interface requirements.

Absolute encoder features

    • Needs no homing run (they always know where they are)
    • No data loss in case of power failures
    • Corrosion-resistant designs with stainless steel housings are available
    • Robust design for harsh environmental conditions
    • Available interfaces, such as RS485, SSI, and CANopen
    • Available as single-turn or multi-turn encoder
    • Optical for precise, high-resolution measurements or magnetic for use in harsh environments

Absolute encoders for high-precision positioning and data acquisition

Absolute encoders detect positions and, unlike incremental encoders, assign a unique value to each signal. You know where your machines are at any moment, even during a power failure. Absolute encoders do not require a reference value or a homing run each time the machine is switched on, so your machine data is not lost to the encoder when switched off. You can monitor and reliably control the rotation and belt speed of your machines and systems. Some interfaces offered for absolute encoders include RS485, SSI, and CAN open, which meet common and established industry interface requirements.

Consideration for precision, robustness, and compatibility

When you start to configure either your incremental or absolute encoder for your application needs, there are some other characteristics to consider, such as single-turn or multi-turn for more precision. Optical or magnetic encoder for either high precision or robustness for harsh environments. The encoder housing needed standard or stainless steel for wash-down areas. For your mechanical connection point, you will need to determine your shaft diameter and flange style. These may seem like a lot of options, but encoder manufacturers offer simple selection guides to ensure you choose exactly what you need for your application.

As you become more aware of the many different types of automation products, don’t forget the rotary encoder if you need some precise positioning or speed measurement on your next application.

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Choosing the Right Sensor for Measuring Distance

Distance-measuring devices help with positioning, material flow control, and level detection. However, there are several options to consider when it comes to choosing the correct sensor technology to measure distance. Here I’ll cover the three most commonly used types in the industrial automation world today, including photoelectric, ultrasonic, and inductive.

Photoelectric sensors

Photoelectric sensors use a light source, such as a laser or light-emitting diode, to reflect the light off an object’s surface to calculate the distance between the face of the sensor and the object itself. The two basic principles for how the sensor calculates the distances are the time of flight (TOF) and triangulation.

    • Time of flight photoelectric distance measurement sensors derive the distance measurement based on the time it takes the light to travel from the sensor to the object and return. These sensors are used to measure over long distances, generally in the range between 500 millimeters and up to 5 meters, with a resolution between 1 to 5 millimeters, depending on the sensor specifications. Keep in mind that this sensor technology is also used in range-finding equipment with a much greater sensing range than traditional industrial automation sensors.

    • In the triangulation measurement sensor, the sensor housing, light source, and light reflection form a triangle. The distance measurement is based on the light reflection angle within its sensing range with high accuracy and resolution. These sensors have a much smaller distance measurement range that is limited to between 20 and 300 millimeters, depending on the sensor specifications.

The pros of using photoelectric distance measurement sensors are the range, accuracy, repeatability, options, and cost. The main con for using photoelectric sensors for distance measurement is that they are affected by dust and water, so it is not recommended to use them in a dirty environment. The object’s material, surface reflection, and color also affect its performance.

Photoelectric distance measurement sensors are used in part contouring, roll diameter measurement, the position of assemblies, thickness detection, and bin-level detection applications.

Ultrasonic sensors

Ultrasonic distance sensors work on a similar principle as photoelectric distance sensors but instead of emitting light, they emit sound waves that are too high for humans to hear, and they use the time of flight of reflecting sound wave to calculate the distance between the object and the sensor face. They are insensitive to the object’s material, color, and surface finish. They don’t require the object or target to be made of metal like inductive position sensors (see below). They can also detect transparent objects, such as clear bottles or different colored objects, that photoelectric sensors would have trouble with since not enough light would be reflected back to reliably determine the distance of an object. The ultrasonic sensors have a limited sensing range of approximately 8 meters.

A few things to keep in mind that negatively affect the ultrasonic sensor is when the object or target is made of sound-absorbing material, such as foam or fabric, where the object absorbs enough soundwave emitted from the sensor making the output unreliable. Also, the sensing field gets progressively larger the further away it gets from the sensing face, thus making the measurement inaccurate if there are multiple objects in the sensing field of the sensor or if the object has a contoured surface. However, there are sound-focusing attachments that are available to limit the sensing field at longer distances making the measurements more accurate.

Inductive sensors

Inductive distance measurement sensors work on the same principle as inductive proximity sensors, where a metal object penetrating the electromagnetic field will change its characteristics based on the object size, material, and distance away from the sensing face. The change of the electromagnetic field detected by the sensor is converted into a proportional output signal or distance measurement. They have a quick response time, high repeatability, and linearity, and they operate well in harsh environments as they are not affected by dust or water. The downside to using inductive distance sensors is that the object or target must be made of metal. They also have a relatively short measurement range that is limited to approximately 50 millimeters.

Several variables exist to consider when choosing the correct sensor technology for your application solution, such as color, material, finish, size, measurement range, and environment. Any one of these can have a negative effect on the performance or success of your solution, so you must take all of them into account.