Mechanical Encoders with Extended Cycle Rotation Lives Now Rival Optical Encoders at Much Lower Cost
As the electronic products industry grows more competitive, manufacturers are seeking greater economy in the manner in which they manufacture, as well as cost effective alternatives to expensive components. For example, the decision whether to utilize mechanical or optical encoders for various applications provides a compelling case in point. Until recently, incremental optical encoders have been the only viable choice for electronics applications requiring cycle rotation life exceeding 10,000 rotations. New developments in mechanical encoders, which include enhanced features, improved performance, as well as cycle rotation life up to 100,000 rotations, now provide a low cost alternative to their pricey optical counterparts.
InControl Systems, Inc., an electronic merchandising company based in Lewisville, Texas, designs custom point of purchase kiosks for suppliers to a wide variety of businesses including auto parts, electronics and office supply stores. These "catalog look-up" systems are specifically conceived to replace paper parts catalogs, which are cumbersome and tend to wear poorly with use. In developing one of their most recent products, the MidiGuide Electronic Catalog, InControl Systems sought to provide end-user retail stores with an informative, entertaining and easy-to-use selling tool that would allow customers to research and locate particular parts or products.
In developing the MidiGuide, a medium size device with a 320 x 240 screen, InControl Systems required a catalog system which was interesting to purvey and very easy to navigate. Accomplishing the second specification initiated a search by the company for a manufacturer who could provide such a device in order to prototype the product. Specifically, they were looking for a single button electronic device, which would provide a simple, intuitive way to scroll through the catalog menu. The key features sought were simplicity of design with a comfortable human machine interface as compared to complex touch screens or keypads which tend to be confusing for the user.
The search was concluded when InControl Systems chose the Type E27 incremental encoder from Elma Electronic Inc. After determining that an optical interface exceeded the necessary requirements for their MidiGuide device, InControl Systems opted for a mechanical interface, which was not only sufficient in capability but low cost. The company discovered that Elma’s E27 mechanical encoder comprised the specific component features they required without paying for unnecessary engineering. Providing a simple, intuitive interface the E27 encoder allows the user to scroll up or down on the catalog screen just by turning a single knob one direction or the other (right scrolls up the screen, while left scrolls down). Delivering a good price performance ratio, the Type E27 was also chosen for its ability to stand up well in a rugged retail environment.
In general, incremental encoders are utilized in a wide variety of common applications including: menu selection, automotive/truck radios, dimmer switches, remote controls, heating system controls and two-way communication devices. They are also used extensively in the manufacture of higher technology products, which include medical and biomedical monitoring instruments, robotic controls, test and laboratory equipment, aviation/GPS navigational systems and numerous other products. Almost all of these areas face tremendous downward price pressure. In many cases, reduced cost is the single factor on which competition is based.
Encoders are electronic components, which generate a signal in specific units of measurement and a scanning system such as a transducer or sensor. Encoders may be categorized into two basic types: absolute rotary encoders and incremental rotary encoders. Incremental encoders may be further categorized as either that which operates mechanically or opto-electrically. Absolute and incremental encoders differ in that the absolute type functions using exact position recognition. Absolute encoders employ a measuring system that determines the measuring value by obtaining information from a scale not based on counting. These
encoders contain a rotary disk separated into a system of coded tracks, which are read by a sensor to establish position information. Digital information on the disk delineates exact angular position and these values may then be accessed as coded outputs. Each position of an absolute encoder is unique, therefore, these particular encoders do not lose position when power is lost or removed. Depending on the application, this factor may or may not be a critical issue to consider.
Incremental encoders are designed specifically for angle of rotation measurements and determining angular speed. The measuring value of these encoders is deciphered by a summation of increments registered by counting the actual number of pulses on the rotary disk from a reference mark. The phase-differential between two channels determines the direction of rotation, which then indicates whether the registered pulses should be added or subtracted. A third channel reference pulse indicates an absolute point for each rotation and checks the accuracy of the position readings, which have been calculated. The number of pulses per revolution (PPR) determines the resolution or measuring step, the smallest digital unit of the measuring value.
Incremental optical encoders determine resolution through grid divisions on the rotating disk. The rotary disk is patterned with a single track of uniform lines around its periphery. These lines interrupt the beam of a light source, typically an incandescent lamp or a light emitting diode (LED). A second track is generally added which produces an index signal once per revolution. The light, which passes through a stationary-scanning mask positioned behind the rotary disk, is received by two photodetectors. These photovoltaic cells produce signals that are converted by the encoder electronics into square pulses shifted 90° or a quarter of a cycle out of phase from each other (the quadrature A and B signals). (To view animated GIF go to www.Elma.com.) Single channel incremental encoders (channel A only) generate a single train of pulses which tend to produce a less accurate readout than the two-channel encoders. While rotation life cycles are high, optical encoders are difficult to miniaturize. Another important drawback is cost, as optical encoders tend to be far more expensive than mechanical encoders owing to their costly fiber optic and photovoltaic components.
Incremental mechanical encoders are utilized as digital set point generators. Each unit contains a circuit component of conductive material and a multi-finger system of collection contacts. Some mechanical encoders are engineered with spring loaded selectors to achieve contact resistance within the evaluation circuit. An absolute code is utilized with these encoders for the default setting of rotational direction and interval. These encoders are well suited to fairly low rotational applications and are extremely cost effective. Until now, mechanical encoders have had limited cycle rotation life and resolution making them cost effective when utilized in appropriate applications.
In the past, low cycle rotation life of mechanical encoders has been a significant limiting factor. Recently, a dramatic increase from 10,000 rotations to 100,000 rotations has been made possible. The Type E27 incremental encoder from Elma Electronic Inc., for example, can now achieve a mechanical lifetime of 100,000 rotations as compared to 10,000 rotations in comparable switches.
The E27 consists of a rotational contact generator, which produces a series of indexing pulses at each of two output lines as the shaft is rotated. Each pulse is interpreted by the system as an increment or a decrement. The direction of change is determined by the phase difference between the output lines. The digital signal may be interpreted in a variety of ways including simulating a potentiometer or switch with an infinite choice of characteristics. This subminiature encoder features 30 pulses per 360° rotation for highly accurate control. The super durable Elma Type E27 delivers accurate digital output with an excellent mechanical detent feel. These mechanical encoders feature spring-loaded ball position indexing, customized actuators, vertical and horizontal mounting configurations, and an integrated, tactile push-button switch. An aluminum shaft allows easy-to-modify-customized versions for specific applications.
Reflowable mechanical design makes these encoders highly resistant to extreme temperatures. Unlike similar switches, Elma’s Type E27 incremental encoder is completely sealed in thermoplastic housing offering IP65 levels of resistance to solid and fluid material ingress.
With so many diverse applications, this cost effective alternative ($6 per switch) to the incremental optical encoder ($30-$100 per switch) will provide an excellent opportunity for manufacturers to enjoy significant savings on this component part. This latest generation of extended rotation life mechanical encoders has overcome one of the most often cited factors that prohibit their use. These mechanical encoders provide an extremely viable low cost alternative to their optical equivalents. With almost all of the positive attributes of an optical encoder, these new mechanical encoders deliver an exceptional price performance ratio.
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