Encoders

What is an Encoder?

An encoder is a sensor of mechanical motion that generates digital signals in response to motion. As an electro-mechanical device, an encoder is able to provide motion control system users with information concerning position, velocity and direction. There are two different types of encoders: linear and rotary. A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion. An encoder is generally categorized by the means of its output. An incremental encoder generates a train of pulses which can be used to determine position and speed. An absolute encoder generates unique bit configurations to track positions directly.

Block Diagram for Encoders

Basic Types of Encoders

Linear and rotary 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.

Incremental Encoder

An Incremental rotary encoder is also referred to as a quadrature encoder. This type of encoder utilizes sensors that use optical, mechanical or magnetic index counting for angular measurement.

How do Incremental Encoders Work? 

Incremental rotary encoders utilize a transparent disk which contains opaque sections that are equally spaced to determine movement. A light emitting diode is used to pass through the glass disk and is detected by a photo detector. This causes the encoder to generate a train of equally spaced pulses as it rotates. The output of incremental rotary encoders is measured in pulses per revolution which is used to keep track of position or determine speed.

A single-channel output is commonly implemented in applications in which direction of movement is not significant. Instances in which direction sensing is important, a 2-channel, quadrature, output is used. The two channels, A and B, are commonly 90 electrical degrees out of phase and the electronic components determine the direction based off the phase relationship between the two channels. The position of an incremental encoder is done by adding up all the pulses by a counter. 

A setback of the incremental encoder is count loss which occurs during power loss. When restarting, the equipment must be referenced to a home position to reinitialize the counter. However, there are some incremental encoders, like those sold at Anaheim Automation, which come equipped with a third channel called the index channel. The index channel produces a single signal pulse per revolution of the encoder shaft and is often used as a reference marker. The reference marker is then denoted as a starting position which can resume counting or position tracking. 

NOTE: Incremental rotary encoders are not as accurate as absolute rotary encoders due to the possibility of interference or a misread.

Absolute Encoder

An absolute encoder contains components also found in incremental encoders. They implement a photodetector and LED light source but instead of a disk with evenly spaced lines on a disc, an absolute encoder uses a disk with concentric circle patterns.

How do Absolute Encoders Work? 

Absolute encoders utilize stationary mask in between the photodetector and the encoder disk as shown below. The output signal generated from an absolute encoder is in digital bits which correspond to a unique position. The bit configuration is produced by the light which is received by the photodetector when the disk rotates. The light configuration received is translated into gray code. As a result, each position has its own unique bit configuration.

Linear Encoder

A linear encoder is 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 magnetic linear encoder types function using this type of method. However, it is their physical properties which make them different.

How do Optical Linear Encoders Work? 

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 encoder system then combines 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 does a Linear Encoder Work? 

A Linear Encoder system uses 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.

Magnetic Rotary Encoder

A magnetic encoder consists of two parts: a rotor and a sensor. The rotor turns with the shaft and contains alternating evenly spaced north and south poles around its circumference. The sensor detects these small shifts in the position N>>S and S>>N. There many methods of detecting magnetic field changes, but the two primary types used in encoders are: Hall Effect and Magneto resistive. Hall Effect sensors work by detecting a change in voltage by magnetic deflection of electrons. Magneto resistive sensors detect a change in resistance caused by a magnetic field.

Hall-Effect sensing
The Sensor produces and processes Hall-Effect signals producing a quadrature signal as is common with optical encoders. The output is generated by measuring magnetic flux distributions across the surface of the chip. The output accuracy is dependent on the radial placement of the IC with respect to the target magnet. The chip face should be parallel to the magnet so the magnet to sensor air gap is consistent across the sensor face. 

Magnetic encoders avoid the three vulnerabilities that optical encoders face:
      • Seal failures which permit the entry of contaminents
      • The optical disk may shatter during vibration or impact
      • Bearing failures

Magnetic devices designed effectively eliminate the first two failure modes and offer an opportunity to reduce bearing failures as well. Magnetic encoders do not make errors due to contamination because their sensors detect variations in magnetic fields imbedded in the rotor and oil, dirt and water do not affect these magnetic fields.

Hall-Effect sensors generally have lower cost and are less precise than magnetic resistive sensors. This means that Hall-Effect sensors, when used in an encoder produce more "jitter", or error in the signal caused by sensor variations.

Commutation Encoders

A commutation encoder contains the same fundamental components as incremental encoders but with the addition of commutation tracks alongside the outer edge of the disk for U/V/W output.

How do Commutation Encoders Work?

Commutation encoders utilize a transparent disk which includes opaque sections that are equally spaced to determine movement. A light emitting diode is used to pass through the glass disk and is detected by a photo detector. This causes the encoder to generate a train of equally spaced pulses as it rotates. The output of incremental rotary encoders is measured in pulses per revolution which is used to keep track of position or determine speed. 

The outer part of the encoder disk includes commutation tracks which provide a controller with information on the exact position of the motor poles, so that the proper controller input can be supplied to the motor. The commutation tracks of the encoder read the motor position and instruct the controller as to how to provide efficient and proper current to the motor to cause rotation. Commutation output for U/V/W can be in the form of differential output or open-collector (manufacturer dependent).

How are Encoders Controlled?

Encoders are controlled through the rotation the shaft it is mounted to. The shaft comes into contact with a hub which is in internal to the encoder. As the shaft rotates, it causes the disc, with both transparent and solid lines, to rotate across the circuitry of the encoder. The circuitry of the encoder contains an LED which is captured by a photoelectric diode and outputs pulses to the user. The speed at which the disc rotates will be dependent on the speed of the shaft the encoder is connected to. Anaheim Automation's optical and magnetic encoder lines are powered from a single +5VDC power source, and is able to sing and source 8mA each.

Physical Properties

Linear Encoders

The key components of a linear encoder are a scanning unit, sensor, transducer or readhead, paired with a transmissive or reflective scale, which encodes position. The scale of a linear encoder 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 encoder is able to convert motion into digital or analog signals to determine the change in position over time.

Rotary Encoders

The key components of a rotary encoder are the disk, light sources and detectors, and electronics. The disk contains a unique pattern of concentric etched circles and alternates between opaque and transparent segments. This pattern provides unique bit configurations and is used to assign specific positions. For every concentric ring in a rotary encoder, there is a light source and light detector which identify lines etched on the disk. The electronics consist of an output device which takes the signal obtained from the sensor (light/detector source) to provide feedback of position and/or velocity. All of these components are enclosed in a single housing unit.

Incremental Encoders

The key components of an incremental encoder are a glass disk, LED (light emitting diode), and a photo detector. The transparent disk contains opaque sections which are equally spaced to deflect light while the transparent sections allow light to be passed through shown in Figure 2 below. An optical encoder utilizes a light emitting diode which shines light through the transparent portions of the disk. The light that shines through is received by the photo detector which produces an electrical signal output.

Where are Encoders Used?

Encoders have become a vital source for many applications requiring feedback information. Whether an application is concerned with speed, direction or distance, an encoders vast capability allow users to utilize this information for precise control. With the emergence of higher resolutions, 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, automation, medical scanners, and scientific equipment. 

Anaheim Automation’s cost-effective encoder product line is a wise choice for applications requiring feedback control. Anaheim Automation’s customers for the encoder product line is diverse: industrial companies operating or designing automated machinery involving food processing, labeling, cut-to-length applications, conveyor, material handling, robotics, medical diagnostics, and CNC machinery.

Encoders are used in Many Industries 

Encoders have become an essential component to applications in many different industries. The following is a partial list of industries making use of encoders:

• Automotive – The automotive industry utilize encoders as sensors of mechanical motion may be applied to controlling speed.
• Consumer Electronics and Office Equipment – In the consumer electronics industry, encoders are widely used office equipment such as PC-based scanning equipment, printers, and scanners.
• Industrial – In the industrial industry, encoders are used in labeling machines, packaging and machine tooling with single and multi-axis motor controllers. Encoders can also be found in CNC machine control.
• Medical – In the medical industry, encoders are utilized in medical scanners, microscopic or nanoscopic motion control of automated devices and dispensing pumps.
• Military - The military also utilizes encoders in their application of positioning antennas. 
• Scientific Instruments – Scientific equipment implement encoders in the positioning of an observatory telescope.

Applications for Encoders

An encoder can be used in applications requiring feedback of position, velocity, distance, etc. The examples listed below illustrate the vast capabilities and implementations of an encoder:

How to Select an Encoder

There are several important criteria involved in selecting the proper encoder:
      1. Output
      2. Desired Resolution (CPR)
      3. Noise and Cable Length
      4. Index Channel
      5. Cover/Base

Output 

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.

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 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. 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.

Cover/Base 

Cover and base options are considerations for specific application requirements. 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.

Cover Options:

How to Install an Encoder

After selecting the appropriate motor, it is important to know how to properly install it. The installation of each encoder is dependent upon its mounting or base option. If an encoder is to be mounted to a motor shaft, then a centering tool can be used to align the hole of the encoder to the shaft. The different mounting options have varying functionalities. An R-option allows for a +/- 15 degree play of motion in which the encoder can rotate back and forth. A T-Option however, uses adhesive to stick to the back of a motor. 

Anaheim Automation also provides the option of an encoder adder, where we mount the encoder for you, hassle free!

Advantages of an Encoder

- Highly reliable and accurate
- Low-cost feedback
- High resolution
- Integrated electronics
- Fuses optical and digital technology
- Can be incorporated into existing applications
- Compact size

Disadvantages of an Encoder

- Subject to magnetic or radio interference (Magnetic Encoders)
- Direct light source interference (Optical Encoders)
- Susceptible to dirt, oil and dust contaminates

Troubleshooting

PLEASE NOTE: Technical assistance regarding its Encoder line, as well as all the products manufactured or distributed by Anaheim Automation, is available at no charge. This assistance is offered to help the customer in choosing Anaheim Automation products for a specific application. However, any selection, quotation, or application suggestion for an Encoder, or any other product, offered from Anaheim Automation’s staff, its' representatives or distributors, are only to assist the customer. In all cases, determination of fitness of the custom Encoder in a specific system design is solely the customers' responsibility. While every effort is made to offer solid advice regarding the Encoder product line, as well as other motion control products, and to produce technical data and illustrations accurately, such advice and documents are for reference only, and subject to change without notice. 

Problem: No output
Solution: No output may be a result of various factors. Steps can be taken to ensure the proper functionality of the encoder. No mechanical movement results in any signal being output from the encoder. To correct this issue, observe if the encoder is rotating. Verify all wring between the encoder and the driver/controller is correct and the appropriate voltage supply is used. Having loose connections or improper voltage supply may not allow the encoder to function properly. Finally, ensure the correct signal type (e.g. open collector, pull-up, line driver or push-pull) is being used for your application. If the problem persists, swap encoders, if possible, to determine if the encoder is the issue. 

Problem: Unable to find index pulse
Solution: The index pulse, or reference marker, is a once per revolution output of an encoder and is best found using an oscilloscope. Verify all the wiring between the encoder and the driver/controller is correct and the appropriate voltage supply is used. If that does not solve the issue, try lowering the RPM of the motor, as the driver/controller may not be able to identify the index pulse at very high RPM values. 

Problem: Count output indicates incorrect direction
Solution: If the count output indicates an incorrect direction then check for the wire configuration. See if any wires are reversed. If they are reversed, simply swap wires. 

NOTE: If your application is using index, reversing the wire configuration causes the reference alignment to also change. If so, please make the appropriate changes to your application. 

Problem: Encoder is not rotating
Solution: When encoders are exposed in open environments, dust and debris particles may accumulate around the shaft. Simply clean the exposed area and ensure that there are not objects obstructing the encoder from rotating. 

Problem: Noise Interference
Solution: To improve the noise immunity of encoders it is strongly advised that no other electrical equipment be nearby or be kept at a fair distance. Encoder cables should also be shielded and proper wires should be grounded to minimize electrical noise. 

Problem: Distorted or incorrect output
Solution: Distorted or incorrect output can be any combination of loose wiring connections, encoder output not compatible with driver/controller, electrical noise or improper alignment. Check for wire connections, compatibility issues with the encoder and the driver/controller, alignment of the encoder and the shaft to solve this issue.

Formulas

The relationship between the encoder CPR frequency and the speed of the motor (RPM) is given by the following equation: 

f = (cycles/rev)*(rev/sec)/1000 = kHz 

RPM = Revolution per Minute
CPR = Cycles per Revolution 

Distance Conversion: 

(PPR) / (2*pi*radius of shaft) = pulses per inch
(Pulses per inch)^-1 = inch per pulse

Glossary

Absolute Encoder - provides the shaft position in a bit configuration and is able to maintain or provide absolute position even after instances of power loss/failure. 

Accuracy – difference in distance between the theoretical and the actual position. 

Cycles Per Revolution (CPR) - Cycles per revolution are the number of output pulses per complete revolution of the encoder disk 

Encoder - is a sensor of mechanical motion that generates digital signals in response to motion. 

Incremental Encoder - device that provides a train of pulses due in response to mechanical motion. The output of this encoder is in form of a squarewave. 

Index - a separate output channel which provides a single pulse per shaft revolution. It can be used to establish a reference or marker for a starting position. 

Interpolation - is the method of increasing the resolution of an encoder. This method allows for the encoder to produce a higher resolution output without increasing the overall size of the disk and encoder. 

Line Driver - is a sourcing output. This means that when in ‘ON’ state the line driver will supply Vcc and in the ‘OFF’ state the driver will float. A sinking input is required for line driver applications. 

Open Collector - is a sinking output. In the ‘OFF’ state, an open collector will be grounded and in the ‘ON’ state, the open collector will float. A sourcing input is required for open collector applications. 

Pulses Per Revolution (PPR) - the total number of pulses produced per full revolution of the encoder shaft. 

Push-pull - is a combination between a line driver and an open collector. In the ‘OFF’ state it will be grounded and in the on ‘ON’ state it will supply Vcc. 

Quadrature Encoder - two output channels which are out of phase by 90 electrical degrees. From the phase difference, the direction of rotation can also be determined. 

Resolution – number of line increments on a disk. Resolution for incremental encoders is often referred to as cycles per resolution and for absolute encoders it is in terms of bits. 

Single Channel Encoder – has only one output channel and is used in speed applications. 

Squarewave - a repetitive waveform corresponding to high and low signals.

Encoder Quiz

1. What are single output channel incremental encoders used for?

A. Sense Direction
B. Sense Speed (Tachometers)
C. Position Feedback

2. Which of the following is a NOT difference between absolute and an incremental encoder?

A. Absolute encoders provide a unique position.
B. Absolute utilize concentric circles on a transparent disc while incremental encoders utilize evenly spaced opaque sections to determine movement.
C. Both absolute encoders and incremental encoders lose position due to power loss/failure.

3. Which of the following applies to an Index Channel?

A. Position Tracker
B. Reference/ Homing Point
C. Determining Distance
D. All of the Above

4. What does an Encoder do?

A. Senses mechanical motion.
B. Provides information concerning position, velocity and direction.
C. Converts analog into digital information.
D. None of the above.
E. All of the above.

5. What does CPR stand for?

A. Cycles per Revolution
B. Counts per Revolution
C. Both A and B
D. None of the above.

6. Describe the different types of encoder outputs below.

TTL - are logic gate circuits designed to input and output two types of signal states: high (1) and low (0). The transition between high and low signals generates TTL squarewave outputs. 

Open Collector - is a sinking output. In the ‘OFF’ state, an open collector will be grounded and in the ‘ON’ state, the open collector will float. A sourcing input is required for open collector applications. 

Line Driver - is a sourcing output. This means that when in ‘ON’ state the line driver will supply Vcc and in the ‘OFF’ state the driver will float. A sinking input is required for line driver applications. 

Push-Pull - is a combination between a line driver and an open collector. In the ‘OFF’ state it will be grounded and in the on ‘ON’ state it will supply Vcc.

7. Which of the following are encoder advantages?

A. Low cost
B. High resolution
C. High reliability and accuracy
D. Compact size
E. Integration between optical and digital technology
F. All of the Above

8. Quadrature channels are out of phase by how many electrical degrees?

A. 45
B. 120
C. 60
D. 90

9. List the criteria for selecting an encoder:

1. Output
2. Desired Resolution (CPR)
3. Noise and Cable Length
4. Index Channel
5. Cover/Base

10. Calculation: If an encoder has a resolution of 1024 and is mounted to a shaft of diameter 1”, what will be the pulses per inch and inch per pulse with this combination?

Encoder FAQs

Q: What is an encoder?
A: An encoder is a sensor of mechanical motion that generates digital signals in response to motion. 

Q: How do you install an Encoder?
A: For step by step tutorials on how to install Anaheim Automation’s encoders, click here. 

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.

Required Maintenance

Encoders require very little maintenance due to their ruggedness and reliability. However, it is recommended to minimize an encoders exposure to dust particles or debris and also, unless designed for exposure to water or moisture. Also, under duress of shock and vibrations, encoder discs may become scratched resulting in encoder failure. If such an event occurs, the disc may need to be replaced to provide accurate readings.

Environmental Considerations for an Encoder

The following environmental and safety considerations must be observed during all phases of operation, service and repair of an encoder. Failure to comply with these precautions violates safety standards of design, manufacture and intended use of an encoder. Please note that even with a well‐built encoder, products operated and installed improperly can be hazardous. Precaution must be observed by the user with respect to the load and operating environment. The customer is ultimately responsible for the proper selection, installation, and operation of the encoder. 

The atmosphere in which an encoder is used must be conducive to good general practices of electrical/electronic equipment. Do not operate the encoder in the presence of flammable gases, dust, oil, vapor or moisture. For outdoor use, the encoder must be protected from the elements by an adequate cover, while still providing adequate air flow and cooling. Moisture may cause an electrical shock hazard and/or induce system breakdown. Due consideration should be given to the avoidance of liquids and vapors of any kind. Contact the factory should your application require specific IP ratings. It is wise to install the encoder in an environment which is free from condensation, dust, electrical noise, vibration and shock. 

Additionally, it is preferable to work with encoders in a non‐static, protective environment. Exposed circuitry should always be properly guarded and/or enclosed to prevent unauthorized human contact with live circuitry. No work should be performed while power is applied. Don’t plug in or unplug the connectors when power is ON. Wait for at least 5 minutes before doing inspection work on the encoder after turning power OFF, because even after the power is turned off, there will still be some electrical energy remaining in the internal circuit of the encoder circuitry. 

Plan the installation of the encoder in a system design that is free from debris, such as metal debris from cutting, drilling, tapping, and welding, or any other foreign material that could come in contact with the circuitry. Failure to prevent debris from entering the encoder can result in damage and/or shock.

Lifetime of an Encoder

The lifetime of an encoder is dependent on various factors such as environmental exposure and application use. By limiting the exposure of the encoder to electrical equipment, temperatures above recommended values, condensation, and vibration and shock, and using the encoder as directed by the manufacturer can extend the lifetime of an encoder.

Accessories

Along with the encoder line, 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 servo motors which can implement encoders for your application needs.