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Absolute
- 1. © 2012 CUIInc AMT203 Absolute Encoder
- 2. © 2012 CUIInc Contents Purpose The purpose of this training module is to familiarize you with rotary absolute encoders and show the benefits of the AMT203. Objectives Describe the functional theory of encoders; specifically absolute encoders Understand what makes the AMT203 revolutionary Explain the different components that make up the AMT203 Describe the installation and assembly of the AMT203 Illustrate the flexible options available with the AMT203 Content: 23 pages Learning time: 15 minutes
- 3. © 2012 CUIInc What Is An Encoder?
- 4. © 2012 CUIInc How An Encoder Functions
- 5. © 2012 CUIInc Encoders Provide Directional Information In this example, Signal A leads B, i.e., Signal A outputs a rising edge before Signal B. This indicates the shaft is rotating counter-clockwise. In this example, Signal B leads A. This indicates the shaft is rotating clockwise. Signal A Signal B Signal A Signal B Signal A leads signal B Signal B leads signal A
- 6. © 2012 CUIInc Encoders Provide Position Information
- 7. © 2012 CUIInc Encoders Provide Speed Information Encoders can detect speed when the number of output pulses is counted in a specified time span. The time element is typically provided by an internal oscillator or clock. The number of pulses in one revolution must also be known. The equation for calculating speed is: C PPR S = t 60 ÷ Where "S" is speed in rpm, "C" is the number of pulses counted in a "t" time interval. If 60 pulses were counted in 10 seconds from a 360PPR encoder, the speed can be calculated: S = = 0.1666 ÷ 0.1666 = 1 rpm60 360 10 60 ÷ All of the counting, timing and calculations can be done electronically in real time and used to monitor or control speed.
- 8. © 2012 CUIInc Encoders Provide Distance Information C = L ÷ (p * D) * PPR C = 12 ÷ (3.142 * 8) * 2000 = 955 Pulse count to achieve desired linear travel can be calculated in a similar fashion for devices that use ball screws, gears or pulleys to convert rotary motion to linear travel.
- 9. © 2012 CUIInc Quadrature Decoding Quadrature decoder circuit Channel A Channel B Quad A pulses Quad B pulses One cycle 1 42 3
- 10. © 2012 CUIInc What Is An Absolute Encoder? Absolute encoder output 1 7 15 31 63 20 21 22 23 24 25 1 0 0 0 0 0 1 1 1 0 0 0 1 1 1 1 0 0 1 1 1 1 1 0 1 1 1 1 1 1 Incremental encoder output 1 2 4 8 16 32 0 0 1 0 1 1 0 1 Bit Bit Value
- 11. © 2012 CUIInc Where Are Encoders Used? elevators factory aircraft automobiles satellites mobile equipment medical devices machine tools pick and place process automation packaging automation industrial robots
- 12. © 2012 CUIInc Types of Rotary Encoders Mechanical 24 (16) – 28 (256) Optical 28 (256) – 219 (524,287) Magnetic 28 (256) – 217 (131,071) 28 (256) – 213 (8,192) 212 (4,096) Fiber Optic Capacitive
- 13. © 2012 CUIInc How a Capacitive Encoder Works Receiver Rotor AC Field Transmitter AC field transmitter sends a signal to the metal rotor 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
- 14. © 2012 CUIInc Benefits of Capacitive vs Optical Technology capacitive optical Greatly reduced assembly time & cost Rugged code disc Not susceptible to airborne contaminants Higher gap tolerance Higher operating temperature range Lower mass, almost zero backlash No LEDs to fail
- 15. © 2012 CUIInc AMT203 Specification and Feature Highlights High resolution- 12-bit (4,096 PPR) Broad temperature range- -25 – +85° 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 than optical- <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
- 16. © 2012 CUIInc Ideal For Direct Motor Mounting The AMT Series can be used on any rotating shaft, however, it is ideal for mounting directly to motors: Zero position set by SPI interface – no mechanical adjustment! Mounting patterns for popular AC & DC motors 9 shaft diameter options Extremely low mass reduces potential backlash Small size fits in tight spaces Quick and easy mounting process
- 17. © 2012 CUIInc Easy Assembly And Installation Assembly of the AMT203 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.
- 18. © 2012 CUIInc AMT203 Assembly Double-click on the illustration below to view a short AMT assembly video.
- 19. © 2012 CUIInc Versatile Shaft And Mounting Options mounting patterns Using the shaft adapter and the 9 color-coded sleeves, both the AMT203 can be adapted to 9 different motor shaft sizes. This is done by snapping one of the sleeves into the shaft adapter. Shaft adapter & sleeves hole pattern mm/in # 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 Adapter Sleeves shaft sleeves Ø2 mm Ø3 mm Ø1/8 in Ø3/16 in Ø4mm Ø5 mm Ø6 mm Ø1/4 in Ø8 mm
- 20. © 2012 CUIInc AMT203 Demo Board With the AMT203 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
- 21. © 2012 CUIInc Serial Peripheral Interface SCLK MOSI MISO SS Master Slave SCLK MISO MOSI Example of SPI Mode 1,1 Note that the data only changes on the falling edge of SCK and is only read on the rising edge of SCK. 0 0 0 01 1 1 1 Advantages of SPI: Very fast >10 MHz Simple protocol (easy to program) Simple interface (no bidirectional pins) Supports full duplex data streaming 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 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) 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 Features of SPI:
- 22. © 2012 CUIInc AMT203 Purchasing AMT203 encoder Shaft adaptor and 9 sleeves Centering tool Spacing tool AMT203-V kit includes: AMT203-DMK includes: AMT203 demo board AMT203-V encoder kit USB cable Board to encoder interface cable Power supply Flash drive with drivers View AMT203-DMK View AMT203-V
- 23. © 2012 CUIInc www.cui.com/AMT_Modular_Encoders
Editor's Notes
- #3 Welcome to the CUI Inc training module for the AMT203 absolute encoder. This module will discuss how encoders function, what makes the AMT203 unique and the various parts that make up this revolutionary modular encoder.
- #4 An encoder is a device that senses mechanical motion. It translates motion such as speed, direction, and shaft angle into electrical signals. There are many different shapes and kinds of encoders. Most encoders generate square waves, making them ideal for use in digital circuits. For this training module we will consider only rotary encoders although encoders are also available in linear configurations.
- #5 Inside an encoder there is a disc fixed to a shaft that is free to rotate. On one side of the disc is 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, the signal transmitted in Channel A passes through the disc generating an output pulse. At the same time the signal transmitted in Channel B is blocked and no output pulse is generated. The dotted line represents the position of the disc relative to the output pulses.
- #6 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. information can be obtained.
- #7 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 by one for each pulse from the encoder 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. 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.
- #8 Encoders can detect speed when output pulses are counted in a specified time span. The time element is typically provided by an oscillator or clock in a microcontroller. The number of pulses in one revolution must also be known. In the equation above, S represents speed, C represents the number of pulses counted, PPR the resolution of the encoder, 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 360 CPR encoder, the shaft speed is 1 RPM.
- #9 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 CPR of the encoder are known, linear travel can be calculated. In the equation, C = encoder pulse count, L = desired cut length in inches, D = friction wheel diameter in inches, and PPR = total pulses per one revolution of the encoder. The second equation is based desired cut length of 12". Assuming the friction wheel diameter is 8" and encoder PPR is 2,000 we can calculate that 955 pulses must be counted to achieve a cut length of 12".
- #10 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. The AMT203 has an option for incremental output, a feature not available on other absolute encoders in the industry. When the AMT203 is programmed by the user to generate incremental output, it can be quadrature decoded by the same receiving electronics that are used for other types of position encoders.
- #11 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.
- #12 These are just a few of the many devices that absolute encoders are used in. Absolute encoders are typically used for repetitive operations, in applications where exact position must be known even if power is lost and regained, in life-critical devices and devices that aren’t often used. Aerospace, medical devices, material handling, robotics, heavy machinery, and manufacturing are just a few of the market sectors that use absolute encoders.
- #13 There are a handful of technology platforms currently in use today to provide absolute positioning information. Mechanical encoders are very low cost, low resolution encoders that generate output code by making and breaking circuits. They are most often used as panel controls such as the volume control on a stereo. Optical absolute encoders are mid-range to high cost and use infrared LEDs and phototransistors to generate output code. Magnetic absolutes can be low to high cost, low to medium resolution and generate output code by detecting changes in magnetic flux fields. Fiber optic absolutes are typically used in explosion-proof applications and are extremely expensive. They use a laser and phototransistor to generate output code. The AMT203 is a low cost, high resolution absolute that generates output code by detecting changes in the frequency of a signal modulated by capacitive reactance. Although capacitive encoders are relatively new, the code generation technology has been used reliably in digital calipers for over two decades.
- #14 The revolutionary AMT203 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, you are already familiar with capacitive encoding. The code generation used in digital calipers for decades is the same technology built into the AMT203. This capacitive code-generation technology has been shown to be reliable, accurate, economical and rugged enough to outlast other types of absolute encoders.
- #15 The AMT203 is unique among modular, absolute encoders because it utilizes capacitive instead of optical technology. This eliminates handling of the sensitive optical disk and issues related to LED burnout or lens contamination. It also simplifies the assembly process leading to reduced manufacturing cost. Lower current consumption, programmability, higher gap tolerance, expanded temperature range, reduced time to implement and low acquisition cost add to capacitive technology’s competitive advantage vs. optical.
- #16 The AMT203 offers a number of key specifications and features that differentiates it vs. 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 AMT203 is flexible, offering a programmable zero position and a multitude of mounting options.
- #17 With 4 mounting options and 9 shaft bushings the AMT203 encoder can easily mount to almost any motor. Its low mass disc means 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 as 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 AMT203 Demo Board. In either case, no mechanical positioning, which can be tricky and time-consuming, is necessary.
- #18 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 dc 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 AMT203 houses the circuitry that detects the motor shaft rotation. These top covers are metal, adding durability. The circuit boards that are responsible for detecting rotation are mounted into the top cover casing to make assembly even easier.
- #19 With the disk built-in to the top cover, the assembly is very quick and easy. Just snap the shaft adapter over a selected sleeve on the back shaft of a dc 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 difference between the AMT102 and 103 is that the 102 has a straight output connector while the 103 has a right angle output connector.
- #20 The AMT203-V kit comes with 9 color-coded sleeves that will adapt to 9 different motor shaft diameters. Typical absolute 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 AMT203- V can fit all three applications under one sku. With the ability to adapt to almost any application, the AMT203 is the most flexible absolute encoder on the market today.
- #21 The AMT203 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 AMT203 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 AMT203 absolute encoder.
- #22 The AMT203 uses SPI (Serial Parallel Interface) protocol to communicate position information. SPI is 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 AMT203 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.
- #23 The AMT203-V kit and the AMT203-DMK demo kit are available immediately through Digi-Key. Click on the links to go directly to the Digi-Key product page.