Capacitive Absolute Encoders
AMT20 Series



Introduction

Welcome to the CUI Product Spotlight on the AMT20 Series absolute modular encoder. This spotlight will discuss how encoders function, what makes the AMT20 Series unique and the various parts that make up this revolutionary absolute modular encoder.

Objectives

  • Describe the functional theory of encoders; specifically absolute encoders.
  • Understand what makes the AMT20 Series revolutionary.
  • Explain the different components that make up the AMT20 Series.
  • Describe the installation and assembly of the AMT20 Series.
  • Illustrate the flexible options available with the AMT20 Series.

What is an Encoder?

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

How An Encoder Functions

Inside a rotary encoder there is a disc fixed to a shaft that is free to rotate. On one side of the disc is a signal source, on the other side a receiver. As the disc turns, the signal source is alternately allowed to pass and be blocked. When the signal is passed through the disc, an output pulse is generated.

In the illustration, signal A passes through the disc generating an output pulse. At the same time signal B is blocked and no output pulse is generated. The dotted line represents the position of the disc relative to the output pulses.

Encoders Provide Directional Information

Detection of shaft direction is very useful and even critical to some applications. In a radio, the rotational direction of the volume knob tells the receiving circuit whether to increase or decrease the volume with each square wave. In automation equipment, the rotational direction is detected and other operations are initiated when a pre-set number of pulses for that direction has been achieved. An elaborate and sophisticated set of movements can be executed automatically to perform tasks like placing components on a pc board, welding seams in an automobile body, moving the flaps of a jumbo jet or just about anything that involves a set of precise motions. In the first illustration above, signal A leads B, i.e., signal A outputs a rising edge before signal B. This indicates the shaft is rotating counter-clockwise. In the second illustration, signal B leads A. This indicates the shaft is rotating clockwise.

  • In this example, Channel A leads B, i.e., Channel A outputs a signal before Channel B. This indicates the shaft is rotating counter-clockwise.
  • In this example, Channel B leads A. This indicates the shaft is rotating clockwise.

Encoders Provide Position Information

Each pulse from Channel A or B increases the counter in a user's system by one when the encoder is turning counter-clockwise and reduces it by one for each pulse when it is turning clockwise. The pulse count can be converted into distance based on the relationship between the shaft the encoder is coupled to and the mechanics that convert rotary encoder motion to linear travel.

The index channel pulse occurs only once per revolution. Often the index channel is used to initialize the position of the shaft the encoder is attached to. A motor turns the encoder until the index channel is detected as a zero or starting point and an automated process can begin. Then the number of complete revolutions the encoder shaft has moved can be read and recorded. The counter adds one revolution when the index occurs during counter-clockwise rotation and subtracts one turn when it occurs during clockwise rotation. By adding the turns count to the pulse count, complete and accurate rotation information can be maintained as long as the encoder is powered.

Encoders Provide Speed Information

Encoders can detect speed when output pulses are counted in a specified time span. The number of pulses in one revolution must also be known. In the equation below, S represents speed in revolutions per minute (RPM), C represents the number of pulses counted, PPR represents the encoders number of pulses per revolution, and T represents the time interval in seconds during which the pulses were counted. The second equation shows that if 60 pulses are counted in a time interval of 10 seconds using a 250 PPR encoder, the shaft speed is 1.44 RPM.

The equation for calculating speed is:

S = (C/PPR) / (T/60)

Therefore if 60 pulses were counted in 10 seconds from a 250 PPR encoder, the speed can be calculated as

S = (60/250) / (10/60) = (0.24) / (0.1667) = 1.44 RPM

All of the counting, timing and calculations can be done electronically in real time and used to monitor or control speed.

Encoders Provide Distance Information

Encoders can detect distance traveled based on the number of pulses counted. In most applications, rotary motion is converted to linear travel by mechanical components like pulleys, drive gears and friction wheels. In this illustration of a cutting table, if the diameter of the friction wheel and the PPR of the encoder are known, linear travel can be calculated. Pulse count to achieve desired linear travel can be calculated in a similar fashion to the diagram below for devices that use ball screws, gears or pulleys to convert rotary motion to linear travel.

In the equation, C is the number of encoder pulses counted, L is the desired cut length in inches, D is the friction wheel diameter in inches, and PPR is the total pulses per revolution of the encoder. The second equation is based on a desired cut length of 12". Assuming the friction wheel diameter is 8" and encoder PPR is 2000 we can calculate that 955 pulses must be counted to achieve a cut length of 12".

Quadrature Decoding

Quadrature decoding is a means of increasing the accuracy of the encoder by counting every state change from both channels in one cycle. Both channel A and channel B produce two state changes per square wave cycle. The quadrature decoder circuit detects both state changes in each cycle for both channels. You can see that two quad A pulses and two quad B pulses, i.e., 4 pulses are obtained from the encoder for every 1 square wave cycle.

What is an Absolute Encoder?

Unlike incremental encoders that generate a simple chain of square waves, the absolute encoder generates a unique, digital 'word' for each position in its stated resolution. Because it is a digital device, resolution is expressed as an exponent of 2, i.e., 28, 210, 212, etc. 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, as illustrated above, can generate 64 unique, digital 'words' that represent 64 positions in 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 & 22 bits are all high; 1+2+4 = 7. At the red line, the 20, 21 & 22 and 23 bits are high; 1+2+4+8 = 15.

A major advantage that absolute encoders have over standard incremental encoders is that they offer much higher resolutions such as 212 (4,096)~216 (65,536), allowing for extremely fine position information which is required for high-precision operations. You will notice the illustration of the incremental encoder shows a repetitive train of 0s and 1s. No absolute position can be obtained.

Types of Rotary Encoders

Just as there are many methods of commutating current to a motor, there are many types of encoders that can perform that task.

Optical

Optical encoders with commutation output currently dominate the market, most often used in precision applications and built in to electronic devices to control motion.

  • 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 encoders with commutation output are often used in applications where there are extreme temperatures, high humidity or exposure to particulates or liquids.

  • 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 commutation encoders are sometimes called 'explosion proof' and are used in applications where methane, propane, or other highly combustible gases 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

The AMT20 is not recommended for explosion-proof applications but can withstand similar environmental factors as magnetic encoders and generally outperform optical encoders thanks to its proprietary capacitive technology.

  • Generates output code through detecting changes in capacitance using a high frequency reference signal
  • Relatively new compared to the other types listed
  • Technology has been used for years in digital calipers and has proven to be highly reliable and accurate

How A Capacitive Encoder Works


The revolutionary AMT20 Series absolute modular encoder consists of three basic parts as shown in the photograph. The ac field transmitter emits a signal that is modulated by the metal pattern on the rotor as it turns. The sinusoidal metal pattern on the rotor creates a signal modulation that is repetitive 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 a proprietary ASIC to convert the modulated signal into output pulses that can be read by the same circuits used to receive optical encoder output.

If you have ever used digital calipers, then you are already familiar with capacitive encoding. The code generation used in digital calipers for decades is the same technology built into the AMT.

  • Ac field transmitter sends a signal to the receiver as it turns.
  • The metal pattern on the rotor creates a signal that is repetitive and predictable.
  • CUI's proprietary ASIC converts the modulated signal to output pulses.

Specification and Feature Highlights

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. It is green, with a current consumption much lower than optical encoders. Finally, the AMT20 Series is flexible, offering a programmable zero position and a multitude of mounting options.

  • High resolution - 12-bit (4,096 PPR)
  • Broad temperature range - -40° ~ +125° C
  • Incremental option - A/B quadrature option for >8,000 RPM
  • Low profile - 11 mm depth
  • Light-weight mechanical design - 15g net weight (0.53 oz.)
  • Low current consumption - <10 mA
  • Programmable zero position - saves time and money
  • Robust design - capacitive technology not susceptible to dust and particulates
  • Adapts to 9 common shaft diameters - allows for a high level of flexibility

Ideal For Direct Motor Mounting

With 4 mounting options and 9 shaft bushings the AMT20 Series encoders can easily mount to almost any standard motor. Its low mass disc means that there is virtually no additional backlash or increased moment of inertia making it a more reliable component for measuring and controlling the motor. Its small size allows for mounting in tight spaces and to small motors.

Zero position is often used in an application at the 'home' position, the point where all operations controlled by the encoder feedback begin. The AMT is different than other encoders in that the zero position can be easily set through the SPI interface or by using the AMT20 Demo Board. In either case, no mechanical positioning, which can be tricky and time-consuming, is necessary.

  • 9 shaft diameter options
  • Extremely low mass reduces potential backlash
  • Small size fits in tight spaces
  • Quick and easy mounting process
  • Zero position set by SPI interface — no mechanical adjustment

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 brushless dc motors mated with commutation encoders.

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

Product Benefits

The AMT20 Series offers an array of benefits among modular encoders because it is able to combine durability, accuracy and efficiency in one solution.

Rugged

  • Not susceptible to airborne contaminants
  • Higher operating temperature range
  • No LEDs to fail
  • Far less susceptible to vibration

Low Cost

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

Efficient & Accurate

  • Lower current consumption
  • Higher accuracy

Simple Assembly

With the disk built-in to the top cover, assembly is very quick and easy. Just snap the shaft adapter over a selected sleeve on the back shaft of a motor, align and mount the selected base unit with one of the mounting hole options, and snap the top cover into place in seconds. The top cover of the AMT20 houses the circuitry that detects the motor shaft rotation. These top covers are metal, adding durability.

Assembly of the AMT20 Series requires minimal time and effort.

With just a few durable pieces, it snaps together in seconds without risk of damaging a glass optical disk or other fragile components.

Versatile Shaft and Mounting Options

The AMT20 Series kits come with 9 color-coded sleeves that will adapt to 9 different motor shaft diameters. Typical commutation encoders on the market today fit only 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 four popular mounting patterns and nine shaft size options, the AMT20 Series kit can fit all three applications under one sku. With the ability to adapt to almost any application, the AMT20 is the most flexible commutation encoder on the market today.

Mounting Patterns

Hole Pattern
mm/in
Number of Holes Hole Size
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

Shaft Sleeves

Shaft Adapter

AMT20 Series

CUI's AMT20 Series is a rugged, high accuracy absolute encoder outputting 12 bits of absolute position information. The AMT's design also simplifies the assembly process, reducing the time consuming tasks of mounting and alignment to mere seconds with the "One Touch Zero" feature.

Absolute Encoder

  • 12-bit (4,096 PPR) resolution
  • -40°C to 125°C operating temperature range
  • Incremental option - A/B quadrature option for > 8,000 RPM
  • Low profile - 11 mm depth
  • "One Touch Zero" set
  • Axial mounting orientation

Demo Board

The AMT20 demo board can be interfaced with a PC via USB cable or used on a stand-alone basis. The demo board comes with a sample AMT20 Series encoder, thumb drive with drivers and TCL software, power supply, interconnect cables and user guide. Data is exchanged via SPI (Serial Parallel Interface) link when connected to a PC or direct using the three membrane switches. It's an excellent tool for evaluating the outstanding flexibility of the AMT20 absolute encoder.

With the AMT20 Demo Board you can:

  • Set zero position
  • Monitor shaft position
  • Set CW or CCW for count increase/decrease
  • Select HEX or DECIMAL position display
  • Select incremental (A/B) or counter (STB/UDN) output
  • Access/read/write 128 bytes of user EEPROM
  • Experiment with all encoder functions

Serial Peripheral Interface

The AMT20 Series uses SPI (Serial Parallel Interface) protocol to communicate position information. SPI is a very simple, synchronous protocol compatible with many other serial protocols like SSI, I2C, Microwire and others. It is a two-way communications protocol and data can be received by or sent from the master or slave device.

Unlike I2C there is no concept of transferring ownership of the bus i.e. changing bus master and there are no slave device addresses. SPI is a much simpler protocol and because of this you can operate it at speeds greater than 10MHz (compared with the 3.4MHz maximum for I2C). The AMT20 sends data to the master, usually a controller. In SPI, the controller can send data to the encoder on the MOSI line, but only the SCLK output from the master is required because the master does not typically create other data useful to the encoder.

Features of SPI:

SPI is a Master-Slave protocol
  • The Master device controls the clock (SCK)
  • No data is transferred unless a clock signal is present
  • All slaves are controlled by the master clock
  • The slave devices may not manipulate the clock
SPI is a Synchronous protocol
  • The data is clocked along with a clock signal (SCK)
  • The clock signal controls data I/O and read
  • Since SPI is synchronous, the clock rate can
  • Vary, unlike RS-232 style communications
SPI is a Data Exchange protocol
  • As data is being clocked out, new data is clocked in
  • Data is exchanged - no device can transmit only or receive only
  • The master controls the exchange through the clock line (SCK)

Advantages of SPI:

  • Very fast >10 MHz
  • Simple protocol (easy to program)
  • Simple interface (no bidirectional pins)
  • Supports full duplex data streaming

Summary

The AMT20 series delivers the best of both worlds, combining 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 AMT20-V absolute modular encoder kit with a range of mounting options allows it to adapt to virtually any size motor, making it the most versatile encoder series on the market today. For more information, please see our website.


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