New methods open up new opportunities
Encoder users have historically been reluctant to change because some innovative motor control technologies claiming superior performance and reliability require an excellent track record to support their application in workplaces or industrial installations. While optical and magnetic encoders have a long history and are based on seemingly "more concrete" physical concepts, capacitive encoders are also based on thoroughly tested principles and have been proven through years of successful field applications. This digital alternation approach, unlike motion sensing, offers numerous benefits, providing designers utilizing rotary commutation encoders with a whole new level of intelligence.
Rotary encoders are critical for almost all motion control applications. The increasing use of brushless DC motors (BLDC) has further expanded the demand for rotary encoders, offering numerous benefits in control, accuracy, and efficiency. The encoder's task is simple, in principle: to indicate the position of the motor shaft to the system controller (see Figure 1). The controller can use this information to accurately and efficiently direct the motor windings to rotate and determine speed, direction, and acceleration—parameters required by the motion control loop to maintain the motor's performance requirements.
Figure 1 shows the rotary encoder providing information on motor shaft direction, position, speed, and acceleration.
Encoders can be based on various technologies that provide standard A and B quadrature signal digital outputs, and some models also provide indexed outputs, see Figure 2a. Commutation encoders (which will be described in more detail below) also provide U, V, and W commutation channel outputs, see Figure 2b.
Figure 2a shows the standard A and B quadrature signals and index signal of the optical encoder.
Figure 2b shows the U, V, and W waveforms generated by the commutation encoder.
Encoder technology
The three most well-known encoder methods are based on optical, magnetic, or capacitive technologies, respectively. Simply put, optical encoders use a slotted disk with an LED on one side and a phototransistor on the opposite side. As the disk rotates, the optical path is blocked, and the resulting pulses indicate the rotation and direction of the shaft. While optical methods are low-cost and efficient, two factors reduce the reliability of optical encoders: contaminants such as dirt, dust, and grease interfere with the optical path, and LEDs have a limited lifespan, typically losing more than half their brightness within a few years and eventually burning out.
Aside from utilizing a magnetic field instead of a light beam, the structure of a magnetic encoder is similar to that of an optical encoder. A magnetic encoder uses a magnetic disk instead of a slotted optical wheel. This disk rotates on a set of magnetoresistive sensors, generating responses that are transmitted to a signal that regulates the front-end circuitry to determine the shaft's position. While this type of encoder is more robust, it is susceptible to electromagnetic interference generated by motors and is less accurate than an optical encoder.
The third method, capacitive encoding, possesses all the advantages of optical and magnetic encoders without their disadvantages. This technology utilizes the same principle as mature, low-cost, and precise digital vernier calipers. It has two cylindrical or linear elements, one on a fixed element and the other on a moving element, which together form a variable capacitor configured as a transmitter/receiver pair (see Figure 3). As the encoder rotates, an integrated ASIC counts the changes in these lines and uses interpolation to find the shaft position and direction of rotation, establishing a standard quadrature output, along with other commutation outputs provided by the encoder, for controlling the brushless DC (BLDC) motor.
The advantage of this capacitive technology is that it does not wear out and is unaffected by contaminants such as dust, dirt, and grease common in industrial environments, making it inherently more reliable than optical encoders. Capacitive encoders also offer performance advantages due to their digital control features, including the ability to adjust encoder resolution (pulses/revolutions) without needing to replace them with higher or lower resolution encoders.
Figure 3 shows the capacitive encoder counting the signal modulation pulses received from the rotor connected to the motor shaft.
Best choice
The new CUI AMT31 series is a paradigm of advanced capacitive encoders, providing A and B quadrature signals, index signals, and U, V, and W commutation signals. It features 20 selectable incremental resolutions between 48-4096 pulses per revolution (PPR) and 7 motor pole pairs between 2 and 20. The AMT31 series also features a locking hub for easy installation. It operates from a 5V power rail, requiring only 16mA of supply current.
However, the benefits of capacitive encoders go far beyond superior performance, flexibility, and short- and long-term stability. Unlike optical and magnetic encoders, the digital output of capacitive encoders brings system design into the 21st century, offering many unique system benefits in encoder use, covering all stages from product development to installation and even maintenance.
Why? Because the output of an optical or magnetic encoder can only get the job done, but it's "inflexible," failing to provide users with flexibility, insight, or operational advantages. In contrast, a capacitive encoder is digital, utilizing a built-in ASIC and microcontroller to provide additional features and enhanced performance. This intelligent output transforms user and performance scenarios in many ways, while remaining fully compatible with standard encoder outputs.
Significant and favorable changes are about to occur.
Let's take a closer look at the improvements that the ASIC and microcontroller, which are part of the CUIAMT31 series of isocapacitive encoders, can achieve:
● The digital characteristics of CUI capacitive encoders enable simple and quick "one-touch" zeroing. The process is very simple: by exciting the appropriate motor phase, the motor shaft is locked to the desired position, and the encoder is instructed to "zero" at this position. The total time is only one to two minutes, and no special instruments are required.
Conversely, using optical or magnetic encoders, zeroing involves mechanically adjusting the commutation signal of the motor windings—a multi-step, complex, and often frustrating process. It requires locking the rotor, performing physical alignment, and back-driving the motor, while simultaneously using an oscilloscope to observe the rear EMF and encoder waveforms to achieve correct zero-crossing alignment. This typically requires repetitive steps, necessitating repeated fine-tuning and verification, and thus the entire process can take 15 to 20 minutes.
● The digital features of the AMT series also greatly enhance the system design process, providing flexibility, diagnostics, and the ability to evaluate motor and motor-controller performance. In particular, the ability of a single capacitive encoder to support a wide range of resolutions and pole values allows designers to use this programmable resolution capability to dynamically adjust the response and performance of the PID control loop during controller and algorithm development without the need to purchase and install new encoders.
The intelligence built into the AMT series also allows for onboard diagnostics, enabling faster field fault analysis—an industry first. Encoders can be queried to indicate whether they are functioning correctly or if there is a fault due to mechanical misalignment of the shaft or other issues. Therefore, designers can quickly determine if an encoder has malfunctioned, identify the source of the problem, and eliminate potential issues with the encoder itself. Furthermore, engineers can use this functionality for preventative measures, such as performing test sequences before running programs to ensure the encoder is in good condition. These features, not available on optical or mechanical encoders, allow designers to minimize downtime while anticipating potential problems in the field.
● Finally, the digital interface can also simplify the bill of materials (BOM). Since the encoder can be customized using software according to the specific changes required (PPR, pole pairs, and direction of change), there is no need to list and store multi-motor products, or different models required for multiple products or at different installation sites.
Smart encoder combined with GUI: powerful pairing
The Windows PC-based AMTViewpoint software for CUI capacitive encoders accelerates development and simplifies time-consuming tasks such as verifying model and type. It requires only a USB cable to connect to the encoder and implements a simple serial data format.
Figure 4 shows that CUI's AMT viewpoint software provides an easy-to-use development interface.
The CUI's setup screen allows users to view key encoder waveforms and timings, which automatically adjust as encoder options change. Programming the encoder via the GUI requires only a few keystrokes, with each cycle taking approximately 30 seconds to complete. Most notably, alignment or zeroing the encoder for A, B, indexing, or commutation takes only a few seconds, a stark contrast to performing this task using a non-programmable encoder.
In demo mode, users can operate the GUI and perform encoder-related tasks just like connecting an actual encoder. This is a convenient way to familiarize themselves with the encoder and tools before purchasing or actually using them. Finally, the GUI also supports creating customizable part numbers for specific encoder models, including options such as output format, socket adapters, and mounting bases.
Summarize
Capacitive encoders offer numerous advantages, providing not only improved performance and reliability, but also intelligent features such as the integrated ASIC/microcontroller in CUI's AMT31 device, supporting programmable settings and installation characteristics, enabling intelligent operation and simplified inventory management. When these features are combined with a PC-based GUI, complex functions can be easily accessed, greatly simplifying all aspects of encoder use, from sample design, evaluation, and debugging to installation and configuration, diagnostics, and inventory minimization. All of this is achieved at a similar price point to other encoders, maintaining compatibility with standard output types and formats while achieving low power consumption. The AMT31 features easy-to-install adapters for different motor shaft sizes (see Figure 5), representing a further step in leveraging intelligent interfaces to provide a wide range of advantages not available in other encoder technologies.
Figure 5 shows CUI's AMT31 encoder, which offers a unique combination of durability and flexibility.
