In the world of motor encoders, there are two main types – incremental and absolute. Incremental encoders do not retain their actual position upon power-up, whereas absolute encoders do. They both give position feedback, which the motion controller can use to calculate velocity, acceleration and jerk. These parameters are necessary for a fully capable motion control system.
Of the two, incremental encoders have been the go-to in the motion control universe. The reason for this is simple: they are robust and inexpensive. But they also have an Achilles heel – they require re-homing on power-up or startup of the machine for each axis to register a correct position.
Depending upon the complexity of the system, re-homing can be a very time-consuming process. I once worked on a fairly simple 12-axis system that, because of the number of interferences between different axes or axes and stationary objects, took nearly 20 minutes to initialize. Not only is this additional downtime, but if a home sensor is misaligned or moved because of an equipment issue, this can impact the specs or quality of the finished product from the line, or worse, lead to interference and damage of axes hitting each other in a multi-axis assembly.
Any of these conditions are costly and lead to lower overall equipment effectiveness (OEE). Absolute encoders are an answer to the issues that incremental encoders present, and there are now fewer reasons not to choose absolute encoders.
Several barriers to the use of absolute encoders have limited their adoption in factory motion control applications until recently. These barriers include their high cost, inherent integration complexity, lack of interoperability of communication, their size and form factor and lack of higher environmental certifications. Times are changing, however, and, in the past few years, innovations in absolute encoder technology have given the automation engineer fewer reasons to avoid them.
Absolute encoders have traditionally been significantly more expensive due to more complex onboard electronics and communication protocols used. This made them palatable only for higher-performance applications where the cost could be justified by better quality of product or improved system uptime. Recent developments in magnetic and capacitive sensing technologies and system-on-chip (SoC) designs, however, have significantly reduced their cost. As a result, entry-level absolute encoders are now closer in price to high-resolution incremental encoders.
Additionally, whereas absolute encoders were once thought to be fragile and hard to troubleshoot, ruggedization—partially due to the aforementioned SoC design—and additional smart encoder features have enabled the integration of self-diagnostics and predictive maintenance data into the system. These added capabilities justify any higher initial cost by reducing downtime and long-term maintenance costs.
Absolute encoders historically required more sophisticated communication interfaces compared to the simpler A/B/Z outputs of incremental encoders, requiring additional hardware or complex programming for the motion application. Engineers often avoided the added development time and controller requirements, especially for retrofits or simpler machines. As of late, the adoption of standardized open protocols such as BiSS-C, IO-Link and EtherCAT has enabled easier integration in motion software packages, and across systems. This diminishes interoperability issues, making absolute encoders more plug-and-play.
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Early absolute encoders were bulky due to additional onboard processing and memory requirements. This made them less viable for compact or integrated motion solutions. Recent advances in electronics miniaturization and packaging have led to compact absolute encoders suitable for embedded and space-constrained applications. They can now be used not only in compact factory automation systems, but also in robotics, medical devices and embedded motion systems.
The availability of suitable absolute encoders for harsh conditions, such as dust, temperature extremes and vibration, was limited in the recent past. Incremental encoders with proven ruggedized designs were more trusted.
In the past few years, expanded selections of IP67/IP69K-rated encoders and encoders with robust housings and bearings have been introduced by multiple automation vendors. This has increased the use of absolute encoders in heavy-duty and outdoor applications, such as material handling and food processing.
Notably, even the servo manufacturers are increasingly offering motors with standard integrated absolute encoders as part of their product lines. Each motor has a pre-installed absolute encoder with advanced features, including single-cable technology for both power and feedback, auto configuration and smart commissioning and real-time diagnostics.
Integrated absolute encoders also eliminate the need to select, mount and align an external encoder to the motor. This saves additional commissioning time on initial startup and avoids additional downtime when a motor does need to be replaced.
It is worth taking another look at absolute encoders for all axes in your motion control systems, whether new designs or upgrades and retrofits. Separate or integrated absolute encoders are redefining servo-drive architecture by simplifying system design, reducing points of failure and opening the door to smarter, safer and more maintainable automation.
