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4 The incremental encoder creates a series of square waves. The number of squar

4 The incremental encoder creates a series of square waves. The number of square waves can be made to correspond to the mechanical increment required. For example, to divide a shaft revolution into 1000 parts, an encoder could be selected to supply 1000 square wave cycles per revolution. By using a counter to count those cycles we could tell how far the shaft rotated. 100 counts would equal 36 degrees, 150 counts 54 degrees, etc. The number of cycles per revolution is limited by physical line spacing and quality of light transmission. We provide incremental resolutions up to 288,000 counts per turn through a combina- tion of direct read on the code disc and various multiplication techniques (see count multiplication on next page). Generally, incremental encoders provide more resolution at a lower cost than their absolute encoder cousins. They also have a simpler interface because they have fewer output lines. Typically, an incremental encoder would have 4 lines: 2 quadrature (A & B) sig- nals, and power and ground lines. A 12 bit absolute encoder, by contrast, would require 12 output wires plus a power and ground line. Incremental Encoders Photodetector Assembly Mask Light Source Code Disc Electronics Board Bearing Housing Assembly Optical Encoder Encoder Design Guide Any Questions? Call 1-800-ENCODER and ask for “Applications Assistance” Count Multiplication 5 An absolute encoder provides a “whole word” output with a unique code pattern representing each position. This code is derived from independent tracks on the encoder disc corresponding to individual photodetectors. The output from these detectors would then be HI or LO depending on the code disc pattern for that particular position. Absolute encoders are used in applica- tions where a device is inactive for long periods of time or moves at a slow rate, such as flood gate control, telescopes, cranes, valves, etc. Incremental encoders are often supplied with two channels (A & B) that are offset from one another by 1/4 of a cycle (90 degrees). This type of signal is referred to as quadrature and allows the user to determine not only the speed of rotation but its direction as well. By examining the phase relationship between the A and B channels, one can determine if the encoder is turning clockwise (B leads A) or counterclockwise (A leads B). Many counter and controller manufacturers include a quadrature detection circuit as part of their electronics. This allows the use of a two-channel quadrature input without further c o n d i t i o n i n g . With quadrature detection we have the ability to derive 1X, 2X or 4X the basic code disc resolution. 10,000 counts per turn can be generated from a 2500 cycle, two-channel encoder by detecting the Up and Down transitions on both the A and B channels. With a quality disc and properly phased e n c o d e r, this 4X signal will be accurate to better than 1/2 count. Another type of count multiplication, inter- polation, can be used to electronically subdi- vide the base resolution. Interpolation is achieved through the use of internal electron- ics and results in improved resolution. This interpolated signal can be further multiplied through the quadrature detection method mentioned above. Interpolative multipliers of 2, 4, 5,10 and 20 are readily available. More detail is available on pages 34 and 35. Count Multiplication Absolute Encoders Disc Pattern Light Source Photodetectors Output Design and Operation 8 Bit Absolute Disc 6 Typical Applications Measuring Wheel Linear Position with N/C Display Purpose To measure distance travelled for a cut-to-length operation Parameters Speed of Travel: 25 feet per minute Measuring Wheel Circumference:12 inches Desired Resolution: 0.005 inches Uni-directional measurement only Manufacturing plant environment, very dusty 50 foot electrical cable run to controller Integrate to programmable controller 12V power supply available Resolution Required: 12/0.005 = 2400 cycles per turn Output Frequency: 25 rpm x 2400/60 = 1000Hz Encoder Specifications Heavy Duty H25 Square flange mount D Shaft Seal SS Cycles per Turn 2400 Channels A Output IC 4469 (operates from 5-15 Volts) Termination SM16 (7 pin, side exit) Model Number H25D-SS-2400-A-4469-SM16 Purpose To encode the position of a work table through a ball screw Parameters Rotational Speed: 500 RPM Pitch: 1/4 Total travel: 20 inches Desired resolution: 0.0005 inches 20 foot cable run to counter Oil mist environment Overtravel protection required 5V power supply available Resolution required = Pitch/resolution = (0.25/0.0005) = 500 cycles per turn Output Frequency = 500 X 500 / 60 = 4167 Hz Encoder Specifications Heavy Duty H20 Square flange mount D Pilot (to accept seal) B Shaft Diameter 25 (0.25” nominal) Shaft Seal SS (protection from oil mist) Cycles per Turn 500 Channels AB Index Z (generates home pulse with microswitch at end of travel) Output IC 7272 (operates from 5-24 Vo l t s ) Termination SM16 (7 pin, side exit) Input Voltage 5 - 2 4 V Model Number H20DB-25-SS-500-ABZ-7272-SM16-24V 7 Encoder Design Guide Belt or Conveyor Linear Actuator Purpose To determine relative position, direction and speed of travel in a bi-directional conveyor belt Parameters Conveyor Speed: 100 feet per minute maximum Desired resolution: 0.002 inches Diameter of Conveyor belt drum: 4 inches Manufacturing plant: Dust and dirt 100 foot cable run to controller Programmable controller with high speed counter module requiring 12 volt differential line drivers. 12 Volt power supply Drum speed = (12 in/ft)(feet/min)/(PI X Diam) = (12 X 100)/(PI X 4) = 95.5 RPM Resolution required = (4 X PI)/(0.002) = 6283 cycles per turn Use the T5 interpolate feature: 6283/5=1256.6 base resolution, use 1257 Frequency output = 6285 X 95.5 / 60 = 10,004 Hz Encoder Specifications Heavy Duty H25 Square flange mount D Shaft Seal SS Cycles per Turn 6285-T5 Channels AB Complements C (for differential line driver) Output IC 4469 (operates from 5-15 Volts) Termination SM18 (10 pin, side mount) Model Number H25D-SS-6285-T5-ABC-4469-SM18 Purpose To encode the position and velocity of a rack and pinion Parameters 40 Tooth 1/20 pitch = 2 inches per turn 20 inch stroke Maximum linear velocity = 10 inches per second 0.0002 inch resolution Oil spray 10 foot cable length 24 Volt power supply Resolution required = 2 inches per turn/0.0002 inches = 10,000 cycles per turn Use 2500 base cycles per turn with T4 interpolate for 10,000 cycles per turn Maximum frequency output = 10,000 cycles per turn X 10 inches/sec X 1 turn/2inches = 50,000 Hz Encoder Specifications Heavy Duty H25 Square flange mount D Shaft Seal SS (protection from oil mist) Cycles per Turn 10,000-T4 Channels AB Output IC 7272 (operates 5-24 Volts) Termination SCS120 (side exit with cable seal, 120 inches long–uses shielded/ jacketed cable) Model Number H25D-SS-10,000-T4-AB-7272-SCS120 8 Encoders and Extreme Environments Encoder Quality Industrial Encoders are available for use over a wide range of envi- ronmental conditions. A large variety of designs allows the user to customize an encoder to his requirements. This also allows the spec- ifying engineer to select only the options needed without incurring unnecessary additional costs. There are a number of factors that must be considered to ensure reliable, consistent encoder operation in industrial applications. In particular, the encoder must have a high degree of mechanical and electrical stability. In order to achieve this stability the encoder must have a solid foundation. The encoder disc, shaft and bearings must be of the highest quality to assure the ultimate accuracy of the device. The encoder disc interrupts the light as the encoder shaft is rotated, and it is the code pattern etched on the disc which is primarily responsible for the accuracy of the electrical signal generated by the e n c o d e r. Should the disc pattern be inaccurate, the resulting signal will reflect that inaccuracy. BEI has been a world leader in the development of sophisticated, accurate divided circle machines. These machines are capable of accuracies in the sub arc second range. Originally intended for the military and aerospace industries, this quality is automatically incorporated into the industrial products. The shaft and bearings maintain accurate rotation of the disc and help to eliminate such errors as wobble and eccentricity which would be translated into position errors. The encoder disc must be carefully mounted to avoid eccentricity as the pattern is read. Such eccentrici- ty can cause inaccuracies in the encoder output that will not be apparent to the user during electrical testing but will cause false position information. In order to eliminate eccentricity errors, BEI has developed electronic centering fixtures capable of centering accuracies up to 40 millionths of an inch. When selecting an optical encoder for the industrial environment, the following areas may be considered: Heavy Loads In applications utilizing gears or drive belts, excessive radial (side) loading on the shaft can shorten bearing life. Therefore, encoders should be specified in accordance with the anticipated side loading. Typical maximum loads for industrial encoders are 5, 40, and 100 lbs. Ultra heavy duty encoders are available to withstand heavier loads as well as shocks of up to 200g ’s uploads/S4/ design-guide.pdf