Part 1-3 of this guide covered what an encoder is and differentiated the types of encoders, as well as the different output configurations. The 4th and final section of the Easy Encoder Guide will focus on the different application of encoders and how to select the correct encoder for a particular use.
Encoders have become a vital source for many applications requiring feedback information. Whether an application is concerned with speed, direction or distance, an encoder's vast capability allows users to utilize this information for precise motion control. With the emergence of higher resolutions, increased ruggedness, and lower costs, encoders have become the preferred technology in more and more areas.
Today, encoder applications are all around us. They are utilized in printers, industrial automation, medical devices, and scientific equipment. Encoders have become an essential component to applications in many different industries. The following is a partial list of industries making use of encoders:
There are five important criteria involved in selecting the proper encoder for a particular application:
Output
An incremental output takes form of a squarewave output. For an application requiring an incremental encoder, the output signal is either zero or the supply voltage. For TTL encoders the Voltage is 0-5V and HTL encoders are typically 0-24V. The output of an incremental encoder is always a squarewave due to the switching of high (input voltage value) and low (zero) signal value.
Absolute encoders operate in the same manner as incremental encoders, but have different output methods. The resolution of an absolute encoder is described in bits. The output of absolute encoders is relative to its position in a form of a digital word, instead of a continuous flow of pulses as seen by incremental encoders.
Desired Resolution
The resolution of incremental encoders is frequently described in terms of Pulses per Revolution (PPR) or Cycles per Revolution (CPR). Pulses per revolution are the number of output pulses per complete revolution of the encoder disk. For example, an encoder with a resolution of 1,024 means that there are 1,024 pulses generated per complete revolution of the encoder. The resolution of absolute encoders is typically specified by the number of bits. As an example, the equivalent to 1,024 pulses per revolution, an absolute encoder is described to have 10 bits (210 = 1024).
For most common speed control applications an incremental encoder with 1024-2048 PPR should work fine to regulate the desired speed of the system. However, when it comes to positioning within a motion control, the higher the resolution the better. Positioning systems are becoming more and more precise which requires the feedback device to have as much resolution as it possibly can. In some machine tool applications it is not unheard of to have an encoder that produces a million pulses or more. In addition, the higher resolution of the encoder will allow for a more robust position control loop which in turn allows for faster and more dynamic motion systems.
Another thing that must be taken into account when specifying the resolution is the maximum output frequency of the encoder. The relationship between the encoder frequency and the speed of the measured shaft is as follows:
f = (PPR)*(rev/sec)m/1000 = kHz
Where PPR = the pulses per rev of the encoder and (rev/sec)m = the maximum revolutions/second of the measured shaft or motor. This is important because many older control systems or drive amplifiers have limitations on the input frequency allowed from the output of the encoder.
Noise and Cable Length
When selecting the proper encoder for any application, the user must also take into account noise and cable length. Longer cable lengths are more susceptible to noise. It is crucial to use proper cable lengths to ensure the system functions correctly. For incremental systems it is recommended to use shielded, twisted-pair cables with preferably low capacitance value. The rating for capacitance value is normally in capacitance per foot. The importance of this rating is for well-defined squarewave pulse outputs from the encoder rather than "jagged" or "saw-toothed" like pulses due to the interference of noise.
Index Channel
The index channel is an optional output channel which provides a once per revolution output pulse. This pulse allows for the user to keep track of position within one rotation of the encoder and establish a reference point.
This output channel is extremely valuable for incremental encoders when an interruption of power occurs. In instances with a power failure, the index channel can be used as a reference marker for a restarting point. Absolute encoders do not have an issue with losing track of position in power loss situations, because every position is assigned a unique bit configuration.
Mechanical Construction
Rotary encoders are available in either solid or hollow shaft configurations and can be ordered to the desired diameter or the application. Cover and base mounting options are also considerations for specific application requirements. Solid shaft encoders would use a coupling to fasten the encoder shaft to the rotating shaft that is to be monitored. Hollow shaft encoders would slide directly onto the shaft that is to be monitored.
Enclosed cover options help protect the encoder from dust and debris particles when the encoder is used in harsh environments. Base options play a significant role in large vibration environments. Such mounting options are transfer adhesives which stick directly on the back of the encoder to the mounting surface, molded ears for direct mounting or by using tether mounts.
Throughout this guide, you've learned about rotary and linear encoders, with the many components and best-fit applications for each. Encoders are an excellent addition to any motion control system for capturing feedback information.
If you have any questions about using encoders or you're seeking consultation for your next motion control project, please contact our experienced staff for a free consultation.