Explore options for choosing an optical rotary encoder for motion control and position sensing
CNC machines require precise position detection. Optical rotary encoders are used on the motor control circuits to provide position and movement feedback.
This article is part three of our 'Match Made in Automation' Series. You can find the other installments of this series below:
Optical rotary encoders are a popular option for position sensing and motion control in applications using motor drives. The applications range from industrial automation and smart buildings to domestic appliances, automotive electrification and e-mobility. Rotary encoders are also used to sense the positions of knobs and dials in digital control panels.
Optical sensing offers several advantages compared to magnetic techniques, including immunity to electromagnetic interference and the potential for greater resolution. The rotating assembly will either be a sleeve or ball bearing. The ball bearing design can provide smoother movement and longevity, especially in applications that require high rotational speeds.
Suppliers offer installation and alignment support using specialized tools. These jigs are used to position the code wheel and mount the PCB in the correct orientation before using electronic calibration software to finalize the setup. The process is straightforward and can normally be completed in minutes.
Absolute and incremental are the two main encoding techniques used. Choosing between an absolute or incremental encoder will depend on the application.
What’s the difference between absolute and incremental encoding?
An absolute optical encoder provides the absolute position of an object directly and requires no reference point. The encoder assembly usually comprises a disc with evenly spaced optical lines or patterns and a sensor to read the patterns.
As each pattern is unique, each represents a unique position. When the disc or strip rotates, the sensor detects the pattern and translates it into a binary code, for immediate and precise position detection.
Absolute encoders can provide high-resolution measurements without additional processing or counting. This provides absolute position information directly, with no need for a reference point, calibration or initialization. They also provide high precision and accuracy. With these characteristics, absolute encoders are used in applications such as positioning and movement control in computer numerical control (CNC) machines and the joints and actuators of industrial robots.
Cobots and industrial robots use rotary encoders to control their movement. Optical rotary encoders can offer advantages when physical proximity is restricted.Incremental optical encoders measure relative motion or changes in position from a reference point. As the graduated encoding disc rotates, the sensor generates pulses based on the transitions between the graduations.
Counting the pulses determines the relative position or movement. To obtain an absolute position, the incremental encoder needs an initial reference point or a known starting position. Incremental encoders can still offer cost and complexity advantages over absolute encoders. They can be used for applications that call for monitoring and controlling speed, position and direction of rotation. Examples include material handling and conveyor systems.
What techniques improve accuracy or precision of optical encoders?
Greater integration has led to more output channels. A greater number of channels means smaller angular increments between each output, resulting in finer position feedback.
Encoders with more outputs providing higher resolution results in more precise position detection. This benefits motion control applications. In addition, finer position interpolation between output points with reduced interpolation errors helps increase accuracy.
Detecting position changes more frequently can improve high-speed tracking. Higher frequency position sensing is necessary for capturing rapidly changing movements. Increasing the number of output channels in encoders that use an index pulse can improve the accuracy. More accurate pulse detection assists sensor synchronization.
Many applications require extremely high positioning precision. Some optical encoders apply interpolation to increase resolution down to the nanometer level. The applications that need this level of precision include semiconductor wafer alignment in processes such as lithography, etching and inspection. Other high-precision applications include metrology and calibration equipment, and controllers for ultra-precise CNC machine tools.
Another option is to use Manchester, or M, coding to enhance resolution for fine position measurement. M coding supports relatively high data density with enhanced differentiation between data and noise when decoding the data to extract movement and position information. In addition, clock information is included in the encoded signal, which can be extracted directly and so eliminates the need for a separate clock signal.
How can optical encoders be made smaller?
There can often be tight space constraints in applications where motors are used. In some types of automotive applications, motors drive window winders or valve actuators. Here, the size of the encoder can become a restriction.
Integration can help. The light source needed to illuminate the optical code wheel, as well as the photodetectors, signal-processing circuitry and optical elements, are typically integrated into a single package. This saves space while also simplifying the circuit design and enhancing reliability.
In addition to this type of integration, minimizing the physical separation between these elements helps further reduce the overall size. Micro-scale optics such as microlenses or microprisms can be used in place of bulky traditional lenses to achieve further reduction in the overall size of the encoder.
What’s the difference between transmissive or reflective sensing in optical encoders?
Another criterion to consider when working with optical encoders is whether to choose a transmissive or reflective type. In the transmissive type, the light source and sensor are positioned on opposite sides of the code wheel, which has alternating transparent and opaque segments. In a reflective type, where the light source and sensor are positioned on the same side, the code wheel has alternating reflective and non-reflective segments.
Transmissive optical encoders tend to have higher resolution and accuracy and can provide precise position feedback. Alignment between the light source, code disk and sensor must be exact. Transmissive encoders can be compact because the source and sensor are on opposite sides of the code wheel. This makes transmissive optical encoders suitable where space is limited, such as small-scale machinery.
The reflective type has the advantage of being simpler to assemble and align since the light source and sensor are on the same side of the code wheel. Reflective encoders can be less susceptible to interference from ambient light sources compared to transmissive encoders.
What are programmable optical encoders?
Some optical encoders allow key features to be configured to suit the intended application. Look for features that can be user-programed, such as the encoder's resolution. This can be in the order of 20 bits per single turn, with incremental output up to 8,192 counts per revolution (CPR), and commutation output up to 32 pole pairs.
The commutation signal output can be used to provide critical timing information needed to control brushless DC (BLDC) motors. Some optical encoders also offer large spatial tolerance and code wheel gap tolerance, which can ease installation and lower lifetime maintenance costs.
Conclusion
Rotary encoders provide basic rotor position information needed for controlling motor drives and can be necessary for movement and position sensing in motor-driven mechanisms. They can also be used for contactless position sensing in knobs and dials of digital control panels.
Optical encoders provide immunity to electromagnetic interference and can offer high positional resolution. Manufacturers are leveraging techniques such as packaging, feature integration and microlenses to deliver highly miniaturized solutions. In addition to features such as the number of channels, which determine the output resolution, ease of installation and the support provided for setting up and calibrating the encoder, can simplify assembly and help maximize reliability in the field.
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Explore Optical Rotary Encoder for Motion Control and Position Sensing Options | Avnet Silica
