Frequently Asked Questions
What is the resolution for Linear Encoders?
The resolution of linear encoders is a measuring step (resolution) corresponding to the distance between two successive edges of Channel A and Channel B.
How do Linear Encoders work?
Magnetic linear encoders use a magnetic sensor readhead and a magnetic scale to produce TTL or analog output for Channel A and B. As the magnetic sensor passes along the magnetic scale, the sensor detects the change in magnetic field and outputs a signal. This output signal frequency is proportional to the measuring speed and the displacement of the sensor. Since a linear encoder detects change in the magnetic field, the interference of light, oil, dust, and debris have no effect on this type of system; therefore they offer high reliability in harsh environments.
How do I determine the linear distance traveled of my encoder?
To obtain the linear distance traveled for linear encoders, it is important to determine the number of counts per inch which then can be used to determine the number of full squarewave pulses per inch. You can use the formula for Rotary Encoders to determine this value. Refer to the example below:
Counts per Inch = (1/Resolution)*(1/1000)*25.4
Full Squarewave Pulses per Inch = (Counts per Inch) / 4
Example: Resolution = 5 microns
Counts per Inch = 1/(5*10^-6)*(1/1000)*25.4 = 5080
Full Squarewave Pulses per Inch = 5080/4 = 1270
Along with the line of linear encoders, Anaheim Automation carries a comprehensive line of single-ended and differential encoder cables with four, six, and eight leads, cable lengths up to 16 feet, and encoder centering tools. Additionally, Anaheim Automation offers an extended line of stepper, brushless and servomotors which can implement encoders for your application needs.
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
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 do Linear Encoders Work
Rotary and Linear encoders are broken down into two main types: the absolute encoder and the incremental encoder. The construction of these two types of encoders is quite similar; however they differ in physical properties and the interpretation of movement.
The light source and lens produce a parallel beam of light which pass through four windows of the scanning reticle. The four scanning windows are shifted 90 degrees apart. The light then passes through the glass scale and is detected by photosensors. The scale then transforms the detected light beam when the scanning unit moves. The detection of the light by the photosensor produces sinusoidal wave outputs. The linear encoders then combine the shifted signals to create two sinusoidal outputs which are symmetrical but 90 degrees out of phase from each other. A reference signal is created when a fifth pattern on the scanning reticle becomes aligned with an identical pattern on the scale.
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.
Physical Properties of Linear Encoders
The key components of linear encoders are a scanning unit, sensor, transducer or readhead, paired with a transmissive or reflective scale, which encodes position. The scale of linear encoders is generally made of glass and mounted to a support and the scanning unit contains a light source, photocells, and a second glass piece called the scanning reticle. Collectively, the linear encoders are able to convert motion into digital or analog signals to determine the change in position over time.
How to Select Linear Encoders
There are several important criteria involved in selecting the proper linear encoder:
2. Desired Resolution
The output of linear encoders is dependent upon what is required by the application. There are two output forms which are incremental and absolute. Incremental output forms are present as either analog or squarewave outputs. The output circuits available for most linear encoders are Push-Pull and Line Driver configurations. The main advantage of using a line driver output rather than a push-pull output is signal strength over longer cable lengths.
The resolution of linear encodera is frequently described in terms of ?m. This is considered the measuring the step (resolution) corresponding to the distance between two successive edges of Channel A and B. It is important to note that the resolution is dependent upon the sensor readhead and not the scale.
The index channel is an optional output channel which provides an output pulse every pole pitch distance, or it can have a unique reference signal by using an external marker. The pole pitch is alternating magnetic north and south poles that are magnetized at a fixed distance. The ordinary index output is in accordance to that value. However, a reference signal can be used as a homing or position reference. It uses an external marker and as the sensor readhead passes this marker, an output signal is generated.
It is important to note the overall accuracy of linear encoders system before implementing it in a design. The overall accuracy of linear encoders system is additive. This means that the accuracy of the sensor readhead and the magnetic scale add together to determine the overall accuracy of the system. Most sensor readheads have a single value for their accuracy, but the accuracy of the scale may change due to its length. Consult the factory regarding these values.
What are Linear Encoders
Linear encoders are a sensor, transducer or reading-head linked to a scale that encodes position. The sensor reads the scale and converts position into an analog or digital signal that is transformed into a digital readout. Movement is determined from changes in position with time. Both optical and linear encoders function using this type of method. However, it is their physical properties which make them different.