Encoder users have traditionally been reluctant to change – and with good reason. A motor on the factory floor or in an industrial installation is not the place for innovations which lack a solid track record.
This explains why tried-and-tested optical and magnetic encoder technologies remain in widespread use today. A more recent invention, however, the capacitive encoder, is now exciting strong interest among designers of motor-control systems. Based on fully tested principles and proven after many successful years in the field, this new approach to motion sensing opens up a range of benefits and provides more intelligence to users of rotary commutation encoders.
The varieties of encoder technology
Rotary encoders play a crucial role in nearly all motion-control applications. Indeed, the need for them is growing because of the increased use of Brushless DC (BLDC) motors, which offer improved control, precision and efficiency.
The encoder’s task is simple, in principle: to indicate the position of the motor shaft to the system controller. Using this information, the controller can accurately and efficiently commutate the motor windings as well as determine speed, direction and acceleration – parameters that a motion-control loop needs to maintain desired motor performance.
Encoders can be based on a variety of technologies, all of which provide the standard digital outputs of A and B quadrature signals, plus an index output in some models, as shown in Figure 1. Commutation encoders also provide U, V and W commutation-phase channel outputs, see Figure 2.
The three best known encoder types use optical, magnetic or capacitive techniques. The optical encoder has a slotted disc with an LED on one side and phototransistors on the opposite side. As the disc rotates, the light path is interrupted, and the resultant pulses indicate shaft rotation and direction. Although an optical encoder is cheap and effective, its reliability is compromised by two factors: contaminants such as dirt, dust, and oil can interfere with the light path, and the LEDs have a limited lifetime, typically losing half their brightness in a few years and eventually burning out.
The magnetic encoder’s construction is similar to that of the optical encoder, except that it uses a magnetic field rather than a beam of light. In place of the slotted optical wheel, it has a magnet which spins over an array of magnetoresistive sensors. Any rotation of the wheel produces a response in these sensors, which goes to a signal-conditioning front-end to calculate the position of the shaft. While it offers a high level of durability, the magnetic encoder is typically not as accurate as the optical encoder. Some types are also susceptible to magnetic interference produced by the motor itself.
A third approach, capacitive encoding, offers all the benefits of optical and magnetic encoder designs, but without their weaknesses. This technique uses the same principle as the established digital vernier caliper. It has two patterns of bars or lines, with one set on the fixed element and the other set on the moving element, together forming a variable capacitor configured as a transmitter/receiver pairing, as shown in Figure 3.
As the encoder rotates, an integral ASIC counts these line changes and also interpolates to find the position of the shaft and direction of rotation. This produces the standard quadrature outputs, and also the commutation outputs used to control BLDC motors.
The benefits of this capacitive technology are that it is immune to contaminants such as dust, dirt and oil, and it contains no element such as an LED that can wear out, making it inherently more reliable than optical devices. Capacitive encoders also offer performance advantages derived from their digital control features. These include the ability to adjust the encoder’s resolution (the pulses-per-revolution or PPR count) without the need to change to a higher- or lower-resolution encoder.
Benefits in production and in the field
The fundamental digital design of the capacitive encoder also offers many system benefits in all phases of encoder use, from product development, to installation and even maintenance. By contrast, the output of the optical or magnetic encoder is typically functional but dumb, and offers users no flexibility, insight or operational advantages. The capacitive encoder, however, uses a built-in ASIC and microcontroller to provide additional features and enhanced performance.
An example of these features is simple and quick one-touch zeroing for aligning the encoder with a BLDC motor. The process is straightforward: lock the shaft to the desired position by energising the appropriate motor phases, and command the encoder to Zero at this position. This takes mere seconds and requires no special instruments.
By contrast, an optical or magnetic encoder must be zeroed by mechanically aligning the commutation signals with the motor windings. This is a complex and often frustrating process. It requires locking the rotor, physically aligning it, and then back-driving the motor while using an oscilloscope to observe the back-EMF and encoder waveforms for proper zero cross alignment. This is often an iterative process with the steps usually needing to be repeated for fine-tuning and verification, so the entire cycle can take as long as 20 minutes.
The digital features of the AMT series also greatly enhance the system-design process, offering flexibility, providing diagnostic information and supporting assessment of the motor’s performance. In particular, since a single capacitive encoder can support a wide range of resolution and pole-pair values, designers can use its programmable resolution capability to dynamically adjust the response and performance of the control loop during controller and algorithm development without having to purchase and install an entirely new encoder.
The intelligence built into a capacitive encoder, such as the AMT series from CUI Inc., also supports the provision of on-board diagnostics for quicker analysis of field failures. The encoder can be queried to indicate whether it is operating properly or whether there is a failure due to mechanical misalignment on the shaft. Therefore, the designer can quickly determine whether the encoder is at fault, and if it is not, look for the source of the problem elsewhere.
Furthermore, engineers can use this feature for preventive measures, for example, executing an ‘Encoder Good’ test sequence before running the application. These capabilities, not available in optical or mechanical encoders, allow designers to keep downtime to a minimum while anticipating problems that might occur with units in the field. Finally the digital interface also simplifies the bill of materials. Since the encoder can be tailored in software to the application’s specifications for PPR, pole pairs and commutation direction, there is no need to list and stock multiple versions for a multi-motor product, or for multiple products.
The Windows® PC-based AMT Viewpoint™ software for CUI capacitive encoders speeds development and also turns time-consuming mundane tasks, such as identifying model number and version, into simple operations. It requires just a USB cable to interface to the encoder, and implements a simple serial-data format. The GUI allows the user to tailor and customise the encoder to the application’s needs, as shown in Figure 4.
A settings screen in the GUI lets users see key encoder waveforms and timing, with automatic adjustments as the encoder options are changed. Programming an encoder through the GUI takes just a few keystrokes and about 30s for the cycle to complete. Most dramatic, the aligning and zeroing of an encoder for either A, B, index or commutation mode takes only seconds, a sharp contrast to the time taken to complete this task with a non-programmable encoder.
In demonstration mode, users can go through the GUI and also perform encoder-related operations as if an actual encoder were attached, a convenient way to become familiar with the encoders and tools prior to any purchase or hands-on use.
Finally, the GUI also enables the user to create orderable part numbers for specific encoder versions, which include options in output format, sleeve (bore) adapter, mounting base and others.
Encoders based on capacitive technology offer much more than improved performance and reliability. A device such as CUI’s AMT31, with its built-in ASIC and microcontroller, provides intelligent functionality to give insights into the device’s operation and to simplify inventory management. The AMT31 is shown in Figure 5.
When these features are coupled with the PC-based GUI, it can provide an easy-to-use yet sophisticated capability, which greatly simplifies all aspects of encoder use from prototype design-in, evaluation, and debug through installation and configuration, to diagnostics and inventory management. And all of this is at comparable cost to other encoders, maintaining compatibility with standard output types and formats, while also achieving lower power consumption.