Frequently Asked Questions
Why do contaminants such as dirt, oil and water cause optical encoder failure, but not magnetic encoder failure?
Optical encoders use LED lights that shine through slots on a disk and are captured by a photodetector on the opposite side of the disk. Any contaminants on the disk may cause improper readings, therefore causing errors in the output signal. Magnetic encoders are less susceptible to errors in the presence of contaminants such as dirt, oil and water because those components do not have any effect on the magnetic field.
When is an optical encoder superior to a magnetic encoder?
Generally, Optical Rotary Encoders can produce more pulses per revolution (PPR). For PPR values above 2048, optical encoders may be the only choice. In the past, optical encoders were more accurate than some magnetic encoders, even at the same PPR.
Advantages and Disadvantages
- Highly reliable and accurate
- Low cost
- High resolution
- Integrated electronics
- Fuses optical and digital technology
- Can be incorporated into existing applications
- Compact size
- Subject to magnetic or radio interference (Magnetic Encoders)
- Direct light source interference (Optical Encoders)
- Susceptible to dirt, oil and dust contaminates
Optical Encoders can be used in applications requiring feedback of position, velocity, distance and etc. The examples listed below illustrate the vast capabilities and implementations of optical encoders:
• Assembly Machines • Robotics
• Packaging • Labeling Machines
• X and Y Indication Systems • Medical Equipment
• Printers • Textiles
• Testing Machines • Drilling Machines
• CNC Machines • Motor Feedback
Q: What is an encoder?
A: An encoder is a sensor of mechanical motion that generates digital signals in response to motion.
Q: What is the difference between absolute and incremental encoders?
A: Absolute and incremental encoders are different in two ways:
- Every position of an absolute encoder is unique
- An absolute encoder never loses its position due to power loss or failure. Incremental encoders lose track of position upon power loss or failure
Q: What is a channel?
A: A channel is an electrical output signal from an encoder.
Q: What is a quadrature?
A: A quadrature has two output channels, with repeating squarewaves, which are out of phase by 90 electrical degrees. From the phase difference, the direction of rotation can also be determined.
Q: What is an index pulse?
A: The index pulse, also referred to as a reference or marker pulse, is a single output pulse produced once per revolution.
Q: What other types of encoder technologies are there?
A: There are two types of encoder technologies.
- Optical: This type of technology uses a light shining into a photodiode through slits in a metal/glass disk.
- Magnetic: Strips of magnetized material are placed on rotating discs and are sensed by Hall-Effect Sensors or magneto-resistive sensors.
Q: What types of applications are encoders implemented in?
A: They are frequently utilized in stepper motors, automation, robotics, medical devices, motion control and many other applications requiring position feedback.
Q: Does any encoder disk (codewheel) work with any encoder module?
A: No, each resolution and each disk diameter works with a different encoder module.
Q: What is PPM?
A: PPM stands for pulse per revolution in rotational motion for rotational motion and pulse per inch or millimeter for linear motion.
Q: When can a single output channel be used in an incremental encoder?
A: A single output channel for an incremental encoder can be used when it is not important to sense direction. Such applications make use of tachometers.
How to Select an Encoder
There are several important criteria involved in selecting the proper encoder:
2. Desired Resolution (CPR)
3. Noise and Cable Length
4. Index Channel
The output is dependent on what is required by the application. There are two output forms which are incremental and absolute. Incremental output forms take form of squarewave outputs. For an application requiring an incremental encoder, the output signal is either zero or the supply voltage. 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, absolute encoders output a unique word for each position in form of bits. Equivalent to 1,024 pulses per revolution, an absolute encoder is described to have 10 bits (210 = 1024).
Desired Resolution (CPR)
The resolution of incremental encoders is frequently described in terms of cycles per revolution (CPR). Cycles per revolution are the number of output pulses per complete revolution of the encoder disk. For example, an encoder with a resolution of 1,000 means that there are 1,000 pulses generated per complete revolution 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. 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.
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 and establishes 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 last sustained index channel can be used as a reference marker for a restarting point. Therefore, when such an occurrence takes place, an index channel can prove to be quite valuable in applications utilizing incremental encoders.
Note: Absolute encoders do not have an issue with losing track of position in power loss situations due to every position being assigned a unique bit configuration.
Cover and base options are considerations for application needs. Enclosed cover options help protect the encoder from dust particles. 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. Anaheim Automation also offers various base options for mounting purposes.
Anaheim Automation offers a selection of cover and base options to meet your application needs.