Table of Contents



PRODUCT SPOTLIGHT:
CAPACITIVE ABSOLUTE ENCODERS


Introduction

AMT20 Products

Welcome to the CUI Devices product spotlight on capacitive absolute encoders. This presentation will provide an overview of encoder technology and their function, including an introduction to the features and benefits of CUI Devices' AMT capacitive absolute encoder series.

Objectives

  • Describe the functional theory of encoders; specifically absolute encoders
  • Understand the benefits of AMT capacitive encoders
  • Explain the different digital serial interfaces of AMT encoders
  • Describe AMT encoder installation and assembly
  • Illustrate the flexible options available with each AMT absolute encoder series

What is an Encoder?

Encoder Examples

An encoder is a device that senses mechanical motion. It translates mechanical motion such as position, speed, distance, and direction into electrical signals.

  • Position and location data
  • Velocity information
  • Distance travelled
  • Direction of movement
  • Commutation outputs
  • Absolute position information

Applications

Encoders are used is a range of industries and applications where motion feedback is required. Industrial, medical devices, security, robotics, automation, and renewable energy are just a few of the market sectors that use rotary encoders within a motion control system.

  • Industrial
  • Medical
  • Security
  • Test & Measurement
  • Consumer
  • Robotics
  • Renewable Energy
  • Automation

Types of Rotary Encoders

There are many different technologies used in rotary encoders on the market today. Here's a quick look at some of the different types:



Optical

Optical Encoder

Optical encoders are the most common type of encoder available, generating output code using infrared light and phototransistors.

  • Generates output code using infrared light and phototransistor
  • The most common type of encoder available
  • Most often used in precision applications and built in to electronic devices to control motion



Magnetic

Magnetic Encoder

Magnetic encoders generate code by detecting changes in magnetic flux fields. These are usually used in environments that encounter adverse conditions.

  • Generates output code by detecting changes in magnetic flux fields
  • Most often used in adverse environments
  • Resistant to most airborne contaminants



Fiber Optic

Fiber Optic Encoder

Fiber optic encoders generate code using a laser and phototransistor and are typically high cost, high resolution devices. They are used in explosion-proof applications where extremely flammable gasses are present.

  • Generates output code by using a laser and phototransistor
  • Most often used in explosion-proof applications where extremely flammable gasses are present



Capacitive

Capacitive Encoder

Capacitive encoders generate output code through detecting changes in capacitance using a high frequency, reference signal. CUI Devices' AMT capacitive encoders are high accuracy devices that are rugged and able to operate in a wide range of temperatures and other extreme environments.

  • Absolute position is obtained using the proprietary AMT capacitive ASIC
  • A processor manages the ASIC and tracks the absolute position putting it into the proper format for export on the digital serial bus
  • Highly flexible design allows for configuration into wide range of communication protocols

Capacitive vs Optical Encoding

Capacitive vs Optical Encoding Disks

Optical encoders use slits or markings on a disk that allow the transmitting LED to interrupt light shown through the disk. Optical disks are often made of glass or plastic. Glass disks are susceptible to breaking due to intense vibrations, and the plastic versions often have lower operating temperature ranges. Since optical encoders are simple devices, the pattern on the disk directly represents the output resolution of that encoder.

The AMT rotor disk is made of a FR-4 PCB material making it suitable for use across a wide range of temperatures. The pattern used for disrupting the capacitive field is made of copper. Because of the digital nature of the AMT capacitive encoder and its use of a proprietary ASIC and processor, it is able to create a wide range of resolutions from the same rotor disk.

How a Capacitive Encoder Works

How a Capacitive Encoder Works

AMT absolute capacitive encoders consist of three basic parts as shown in the illustration above. The transmitter PCB emits an ac field that is distorted by the metal pattern on the rotor as it turns. A sinusoidal metal pattern on the rotor distorts the ac field in a way that is repeatable and predictable. This occurs as a result of varying capacitive reactance between the signal generated by the transmitter and the metal on the rotor. The field receiver uses the AMT ASIC to convert the modulated signal into digital position information, while the processor on the absolute encoder retrieves the position data from the ASIC and prepares it for export on the digital serial bus.

Benefits of Capacitive Technology

How a Capacitive Encoder Works

Thanks to its capacitive design, the AMT series is not susceptible to environmental contaminants such as dirt, dust and oil that would disable a typical optical encoder. Additional advantages include the lack of an LED which can eventually fail, a wider temperature range, higher vibration tolerances, and very low current consumption. The digital nature of the design also allows for increased flexibility through programmability of various features, ultimately reducing assembly time and cost compared to other encoders. And, compared to magnetic encoders typically valued for their rugged performance, the AMT series offers higher accuracy and stable performance under various temperature conditions.

Rugged

  • Not susceptible to airborne contaminants
  • No LEDs to fail
  • Far less susceptible to vibration due to rotor materials

Cost

  • Greatly reduced assembly time & cost
  • Lower price than most competitive offerings

Current Usage & Accuracy

  • Lower current consumption than competition
  • Higher accuracy than rugged counterparts

Incremental vs Absolute Encoders

Absolute Encoders

To understand the importance of absolute encoders it is good to first understand the limitations of incremental encoders. This image shows how an incremental encoder uses quadrature output signals to convey position information. In incremental encoders, there are 4 distinct states, and those 4 states are repeated over the rotation of the encoder for however many increments/counts/pulses the encoder has. Since there are only 4 states, the host cannot determine the encoder's exact radial position without a reference. Many incremental encoders include an index signal which occurs once per rotation and can be used as a home location to count from.

This output is useful for obtaining speed information, direction of travel, and can be used to count up or down from the index position. However, this type of encoder is not useful when the host system must know the current position immediately after power on. An incremental encoder can give precise radial position, but only after physically rotating to the index location.

How a Capacitive Encoder Works Diagram

Unlike quadrature encoders that repeat the same 4 states over a revolution, absolute encoders generate a unique digital ‘word' for each position in its stated resolution. Because many absolute encoders are digital devices, resolution is expressed as an exponent of 2, otherwise known as binary. The numbers on the right of the absolute output illustration represent the numeric value of the bit when it is ‘on' or ‘high'. A 6 bit (26) absolute encoder can generate 64 unique, digital ‘words' that represent 64 positions over one revolution. Five positions are illustrated above. At the blue line, only the 20 bit is high, so the output is 1. At the green line, the 20, 21 and 22 bits are high: 1+2+4=7.

Digital Serial Protocols

There are many different serial interfaces available to engineers. AMT absolute encoders currently offer three different serial protocols:

  • Serial Peripheral Interface (SPI)
  • Multi-drop RS-485
  • 3-Wire Synchronous Serial Interface (SSI)

Serial Peripheral Interface (SPI)

Example of SPI pulses

Serial Peripheral Interface (SPI) is a full duplex two-way master-slave communications protocol which allows for multiple devices to share the same bus. In this protocol, all devices share a clock signal, a Master-Out-Slave-In (MOSI) signal, and a Master-In-Slave-Out (MISO) signal. Each device gets a unique Chip Select signal controlled by the bus host. Devices on this bus wait until their specific chip select line is asserted by the master. The master sends out data on the MOSI line, while toggling the serial clock line (SCLK) at the point where data should be captured by the slave. This is what makes the communication synchronous, which allows for greater design flexibility as the host is not constrained by any specific frequency. In SPI, the data is transferred most commonly in full bytes. The illustration above shows a two byte transmission.

Full duplex means that the slave device responds at the same time as the host is sending data. For the AMT22 absolute encoder series, this means incredibly fast position responses. Observing the illustration above we can see that “TCLK” is the maximum time from when the Chip Select line is asserted that position data is ready to be received from the encoder. To obtain the position of the encoder the host sends blank commands because the encoder is always ready to respond with position. If extended commands need to be issued to the encoder, the encoder is always listening and is ready to execute those commands after sending the position up front.

Physical configuration of an SPI

The illustration above shows the physical configuration of an SPI network. In this network there is one host and three slave devices. The amount of signal wires in an SPI system is 3 + (n*number of slave devices). The clock signal, MOSI, and MISO are all shared, with only the Chip Select lines unique.

Benefits of SPI

SPI's master-slave protocol and synchronous capabilities make it a viable option for many applications. The SPI protocol is fast and simple to use as most processors and controllers have SPI capability built in. Its digital serial protocol supports the inclusion of an error checking code into the position response allowing for greater assurance that the host received exactly what the slave sent.

Master-Slave Protocol

  • Only the master controls the clock
  • No data is transferred unless the clock is present
  • All slaves are controlled by the master clock

Synchronous

  • Data is synced with the clock
  • Clock can be any frequency up to the maximum allowed by the slave allowing for greater design flexibility

Features

  • Very fast protocol
  • Simple to use. Most processors and controllers have SPI capability built in
  • Digital serial protocol supports the inclusion of an error checking code into the position response allowing for greater assurance that the host received exactly what the slave sent

AMT20 Series - SPI Encoder

AMT20 Series

The AMT20 series offers a number of key specifications and features that differentiates it from the competition. Mechanically, it is low profile and light-weight. The encoder is rugged, offering a broad temperature range and immunity to dust and particulates. Its current consumption is also much lower than optical and magnetic encoders. Finally, the AMT20 series is flexible, offering a programmable zero position and a multitude of mounting options.

  • High 12 bit resolution (4096 positions)
  • A/B/Z quadrature outputs up to 1024 PPR
  • Wide operating temperature range from -40 up to +125°C
  • Low profile depth of 11 mm
  • Light-weight mechanical design of 15 g
  • Low current usage less than 20 mA
  • Programmable zero position over SPI
  • Robust design
  • Adapts to 9 common shaft diameters
  • Incremental resolutions configurable with AMT Viewpoint™ PC Software

AMT22 Series - SPI Encoder

AMT22 Series

The AMT22 series is the newest SPI encoder in the AMT absolute encoder line. This encoder offers high resolution position at 12 or 14 bits, comes with radial or axial connector orientations, and has a very simple command response protocol.

  • 12 bit (4096 positions) or 14 bit (16384 positions)
  • Wide operating temperature range from -40 up to +105°C
  • Low profile depth of 11 mm
  • Light-weight mechanical design of 15 g
  • Low current usage less than 20 mA
  • Programmable zero position over SPI
  • Robust design
  • Adapts to 9 common shaft diameters
  • Firmware updates and configuration with AMT Viewpoint™ PC Software

RS-485

RS-485 Example 1

RS-485 is a half-duplex communication standard defining the electrical characteristics of drivers and receivers for use in serial communications systems. It is a differential protocol designed for use in environments requiring especially robust communication.

Differential signaling is the method where information is transmitted using two complementary signals. The host converts a fundamental signal into a pair, the original, and its inverse (or complement). RS-485 requires the use of twisted-pair wiring where two wires are intertwined a specific number of times over the cable's length. When noise couples onto a twisted-pair cable, it presents itself the same on each wire.

RS-485 Example 2

When the receiver gets the complementary pair, a subtractor is used to find the difference between the two wires (hence the name differential), and because the added noise is the same on both wires, it is not present in the result of the equation, yielding only the fundamental signal. Because the receiver cares only about the difference between the two cables, the voltage levels of the system no longer matter. Therefore, a 3.3 V system can work on the same system as 12 V devices, so long as they all follow RS-485 requirements.

RS-485 Example 3

RS-485 has a shared bus but differs from SPI. In an RS-485 network, all devices share the same 2 wires labeled A and B. Each device has an RS-485 transceiver that manages communication with the bus and can be controlled by the processor of the host system. In most cases the host system may use serial UART to talk to the RS-485 transceiver, where the host tells the transceiver when to talk and when to listen. Because all devices share the same bus, each device must have its own unique node address. The host will put its transceiver in transmit mode, and then send the node address. Immediately after this it will put the transceiver in receive mode and listen for the response. Only one device can control the bus at a time which is why the transceiver must not change modes.

RS-485 Example 4

This last image illustrates what a position request looks like with the AMT21 absolute RS-485 encoder series. A and B are the complementary pairs. The host controls the bus by sending the node address 0x54 to an encoder on the bus and within microseconds that encoder responds with its current position.

Benefits of RS-485

RS-485 is a rugged option originally designed for use in telecom and industrial applications. Its smaller host system only requires one communication bus and is well-suited for noisy environments or applications where the encoder must be far away from the host. The RS-485 protocol also allows the encoder to respond with position information in microseconds.

Smaller Host System

  • Only one communication bus is required in the host system
  • The host only needs to control its transceiver, and doesn't require unique chip select pins for each device

Rugged

  • RS-485 is well suited for when encoders must be far away from the host
  • Works well in electrically noisy environments
  • Originally designed for use in telecom and industrial applications

Features

  • Very fast protocol
  • Encoder responds with position within microseconds
  • Digital serial protocol supports the inclusion of an error checking code into the position response allowing for greater assurance that the host received exactly what the slave sent

AMT21 Series - RS-485 Encoder

AMT21 Series

The AMT21 series is the fastest encoder in the AMT absolute encoder line. This encoder offers high resolution position at 12 or 14 bits, comes with radial or axial connector orientations, and has a very simple command protocol. The AMT21 encoder offers multi-turn capability with the ability to track the number of rotations in either direction.

  • 12 bit (4096 positions) or 14 bit (16384 positions)
  • Single-turn or multi-turn output options
  • Wide operating temperature range from -40 up to +105°C
  • Low profile depth of 11 mm
  • Light-weight mechanical design of 15 g
  • Low current usage less than 20 mA
  • Programmable zero position over RS-485
  • Robust design
  • Adapts to 9 common shaft diameters
  • Firmware updates and configuration with AMT Viewpoint™ PC Software

Single-Turn vs. Multi-Turn Encoders

CUI Devices' AMT21 absolute encoder series comes in single-turn and multi-turn variations. Single-turn encoders provide positioning data over one full revolution, 360°, with the output repeating for every revolution of the shaft. Multi-turn encoders also provide positioning data over a single turn, but have an additional “turns” counter that measures the number of revolutions. In the AMT21 multi-turn version, this turns counter is a 14-bit signed number plus two checkbits for error detection with a battery backup required to prevent loss of the turns count. The turns count is read in the same format as position and allows for -8192 to +8191 turns.

3-Wire Synchronous Serial Interface (SSI)

SSI Protocol Diagram

Synchronous Serial Interface (SSI) is typically a synchronous simplex one-way master-slave communication protocol that uses differential signaling with only a clock and data line to communicate from the host to the slave with no use of a chip select. However, this section will describe the variation of SSI specifically used with the AMT23 absolute encoder series.

The 3-wire SSI allows for a system that looks a lot like an SPI bus. In this communication protocol, the host asserts a chip select line to let a slave know it is being accessed, and then controls the clock line, receiving data on the DATA line. The difference here from SPI is that there is no command being sent, the slave does not have the ability to accept commands, and it only ever responds with its current position. This information is unidirectional making it a simplex communication protocol. The signals in this variant of SSI are not differential. Differing yet again from SPI, the SSI transmission is not constrained to individual bytes. As you will see in the illustration above, data for the 14-bit encoder is sent as 16 bits (14 position bits and 2 check bits). A 12-bit encoder only sends a total of 14 bits (12 position bits and 2 check bits). Referencing the SPI waveform again one will also see that the idle state of the clock line differs from SSI. With SSI the clock is high while idle, goes low at the beginning of an access, and then data is captured on the rising edge of the clock. Because of this, the SSI encoder will not be compatible with most SPI hosts.


SSI Protocol Diagram

The illustration above shows the physical configuration of a 3-wire SSI network. In this network there is one host and three slave devices. The amount of signal wires in an SSI system like this is 2 + (n*number of slave devices). The clock and data lines are all shared, with only the Chip Select lines unique.

Benefits of SSI

SSI's slave is controlled only by clock and chip select. Its no command and response structure makes the protocol very simple and the position responses fast, while its digital serial protocol also supports the inclusion of an error checking code into the position response. This allows for greater assurance that the host received exactly what the slave sent.

Master-Slave Protocol

  • Only the master controls the clock
  • No data is transferred unless the clock is present

Synchronous

  • Data is synced with the clock
  • Clock can be any frequency up to the maximum allowed by the slave allowing for greater design flexibility

Features

  • Slave is controlled only by clock and chip select, no command and response structure makes the protocol very simple and position responses fast
  • Bus configuration reduces number of overall signal wires in a system
  • Digital serial protocol supports the inclusion of an error checking code into the position response allowing for greater assurance that the host received exactly what the slave sent

AMT23 Series - 3-Wire SSI Encoder

AMT23 Series

The AMT23 series encoder offers high resolution position at 12 or 14 bits, comes with radial or axial connector orientations, and does not require a command protocol.

  • 12 bit (4096 positions) or 14 bit (16384 positions)
  • Wide operating temperature range from -40 up to +105°C
  • Low profile depth of 11 mm
  • Light-weight mechanical design of 15 g
  • Robust design
  • Adapts to 9 common shaft diameters
  • Firmware updates, zero set, and configuration with AMT Viewpoint™ PC Software

AMT Viewpoint

AMT Viewppint Screenshot

All of the AMT absolute encoders can be configured with the AMT Viewpoint. This software allows the user to zero the encoder's position, update firmware to newer versions, and obtain diagnostics information. This powerful tool is available online for download.

Configure

  • Resolution
  • Zero position
  • Encoder address (AMT21)

Monitor

  • All configurable settings
  • Output waveform and commutation logic values
  • Encoder firmware & data code
  • Encoder diagnostics

Simple Assembly

With the encoder's disk built-in to the assembly, mounting an encoder is very quick and easy. The encoder's metal housing also adds strength and durability to the overall package. Take a moment to watch the video above for detailed instructions on mounting an AMT encoder. Assembly of AMT encoders requires minimal time and effort. With just a few durable pieces, the encoder snaps together in seconds without the risk of damaging components like with optical encoders.

Versatile Shaft and Mounting Options

AMT absolute encoder kits come with 9 color-coded sleeves that will adapt to 9 different motor shaft diameters. Typical encoders on the market today only fit one motor size per sku. For example, if a manufacturer is utilizing motors with 2 mm, 5 mm, and 8 mm shafts in their system, they must purchase three separate encoders. With multiple popular mounting patterns and nine shaft size options, the absolute encoders can fit all three applications under one sku. With the ability to adapt to almost any application, AMT encoders are the most flexible encoders on the market today.

Mounting Patterns

Hole Pattern
mm/in
Number of Holes Recommended Screw
Ø16/0.63 2 M1.6
Ø19.05/0.75 2 #4
Ø21.45/0.844 3 M1.6 or M2
Ø25.4/1.0 4 M1.6 or M2
Ø32.44/1.277 2 #4 or M2.5
Ø46.02/1.812 2 #4 or M2.5

Shaft Adapter & Sleeves

AMT Tools and Sleeves

Summary

AMT absolute encoders combine levels of accuracy and durability unrivaled in other encoder technologies. The AMT's unique platform also delivers an unparalleled level of flexibility and intelligence thanks to the digital nature of the design. Not to mention, the encoders are easy to install, greatly reducing assembly time and cost. Lastly, the AMT modular encoder kits with wide ranges of mounting options allow them to adapt to virtually any size motor, making them the most versatile encoder series on the market today.

 
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