Choosing a code wheel: A detailed look at how encoders work
Incremental rotary encoders have many applications in interfacing electronics to mechanical subsystems, but how do they really work? What should designers hone in on when selecting one? An encoder mechanics expert offers a perspective on how to sort through the different fabrication technologies, parameters, and choices.
Incremental rotary encoders are a cost-effective and reliable method of collecting and transmitting rotational data. As such, their use is becoming more common in a myriad of applications, including copiers, printers, factory automation, servo and stepper motors, robotics, pick-and-place machines, and office automation products, among many others.
Typically, these encoders are part of a closed-loop feedback system that collects and processes information, such as position, direction, and speed. The following discourse will focus primarily on transmissive encoder technology and one specific component of rotary encoders ‚Äì the code wheel ‚Äì that defines the resolution, accuracy, and general quality of the entire encoder product.
A transmissive encoder employs a light source (typically LED) and a sensor, separated by a code wheel mounted on a hub. This code wheel contains a data track, which is a series of alternating clear and opaque areas. When light is passed through the code wheel's data track, the sensor on the opposite side picks up its energy. As the code wheel rotates, the light creates a pulse at the sensor, creating an electrical signal subsequently converted to various streams of usable information.
Choice of materials
When specifying encoders, designers are often given the choice of material to be used for the code wheel. Options typically include Mylar film, glass, or metal. Each type of material has advantages as well as shortcomings.
Although metal code wheels have been employed for a number of years, they have been manufactured traditionally using primarily chemical etch technology, which has limitations in resolution and signal quality. Recently, electroformed nickel code wheels have begun to displace those manufactured utilizing chemical etching because the electroforming process allows many advantages, including higher resolution and more uniform window and bar spacing, which converts to a more "clean" signal from the encoder.
In addition, proprietary manufacturing methods developed by code wheel manufacturers can create further quality differentiation. For example, Thin Metal Parts in Colorado Springs, Colorado, has developed a process where code wheels can be manufactured devoid of grain direction lines, which can scatter light.
When choosing code wheel material, many factors must be considered, including the environment to which the equipment will be subjected, accuracy of the transmitted data, and cost.
The primary environmental considerations are temperature, humidity, shock, and cleanliness. Temperature and humidity are areas where Mylar film code wheels show their greatest weakness. The material, which is typically 0.007" thick, is highly unstable with fluctuations in temperature and humidity. For example, a 1" diameter film code wheel subjected to a 30 ¬∞F increase in temperature and a 50 percent increase in relative humidity may expand by 0.001".
In encoder applications where accuracy is critical, these types of fluctuations in size can cause signal anomalies. Furthermore, the material can become distorted and begin to curl when relative humidity drops below 35 percent. Repeated or continuous exposure to harsh conditions also may result in film discoloration, which will cause equipment failures.
Both glass and metal code wheels are highly resistant to temperature and humidity factors. Chemically etched code wheels, often manufactured with copper, have higher sensitivity to corrosion than electroformed nickel code wheels, which are highly corrosion resistant.
Glass code wheels (Figure 1) are very delicate and not recommended for applications where shock is a factor. Both Mylar and metal have low mass and can withstand high levels of shock without distortion.
Most optical encoder assemblies are enclosed and sealed, eliminating cleanliness concerns. In unprotected applications that exhibit a high level of dust and dirt contamination, the open spaces defining the data track in metal code wheels may become clogged and cause failures. Because high levels of contamination can obstruct the optics in the encoder, conditions such as these should be avoided for all optical encoders, regardless of code wheel material. Today, the cost of sealed assemblies is low enough that open systems should not be necessary.
Counts Per Revolution (CPR) is the number of pairs of clear and opaque areas around a code wheel's perimeter. This is most often the primary consideration when choosing encoders. Although the code wheel's native resolution is a primary factor, many OEMs now offer a combination of specialized code wheel design and computer logic to create virtual resolutions, which can be many times higher than the CPR on the code wheel itself.
- Transmissive code wheel: Code wheel used in optical encoders where light is transmitted through an alternating pattern of openings.
- Counts Per Revolution (CPR): The number of pairs of bars and openings around a code wheel's perimeter. Also called native resolution.
- Data track: The area of the code wheel containing the alternating pattern of bars and openings.
Typically, glass offers the highest native resolution, followed by Mylar film and metal. In the past, when chemical etching was the primary manufacturing method for metal code wheels, resolution capability and tolerances were limited. Today, electroforming technology has allowed metal code wheels' resolution capability to be much higher at extremely accurate tolerances. With these advances, many applications that were previously relegated to using glass or Mylar film due to resolution can now be manufactured in metal.
At the center of every code wheel is an opening that allows a metal hub to be attached (see Figure 2). The combination of hub and code wheel is a critical factor in the encoder's performance. First, the concentricity of the code wheel's hub openings, Inner Diameter (ID), and Outer Diameter (OD) must be extremely tight. If the OD and ID are not perfectly concentric with respect to each other, the code wheel's mass will not be evenly distributed and will cause vibration during high rotational speeds. This constant vibration will likely result in early encoder failure. Also, if the hub and ID sizing allow significant "slop" when mounting, vibration can become an issue. Furthermore, the ID's concentricity relative to the data track is critical for encoder accuracy.
During glass and Mylar film code wheel manufacturing, the code wheel's features are imprinted on a sheet of base material subsequently cut out utilizing a laser, punching, or machining to define the ID and OD. Because the code wheel's ID and OD are created in a separate process from data imprinting, extreme measures must be employed to ensure proper OD, ID, and encoder information alignment. To maintain necessary specifications, the procedure is quite costly. Furthermore, repeatability can be an issue. A significant advantage of electroformed nickel code wheels is that the entire code wheel design including ID, OD, and data track information is defined in one single photolithographic step. This ensures that each item's relative positioning with respect to the other is maintained at the highest level.
Typically, the most economical code wheels are manufactured with Mylar film. This is mainly due to the minimal number of manufacturing steps needed for production coupled with their limited desirability in many applications. Glass code wheels are generally the most expensive, often 10x or more the cost of similar Mylar film or metal.
The cost of precision machining and polishing these code wheels' IDs and ODs is the single largest factor influencing cost. Metal code wheels have historically been priced at a level between that of Mylar and glass. However, with recent advances in technology and increased production rates, the cost of metal, especially that of electroformed nickel, has declined. Currently, a typical metal code wheel costs slightly more than Mylar and significantly less than glass.
Code wheels turning to metal
The advances of electroforming technology have allowed OEM encoder manufacturers to expand the range of encoders available with metal code wheels. In many applications where glass has been specified due to the limitations of both metal and Mylar film, metal may now be an option at a fraction of the cost of glass. Also, in low-cost applications where Mylar has been specified, electroformed nickel code wheels can offer competitive pricing with many additional benefits.Thin Metal Parts