Downloaded from http://www.everyspec.com MIL-HDBK-231A 30 September 1991 SUPERSEDING MIL-HDBK-231(AS) 1 JULY 1970 MILITARY HANDBOOK ENCODERS SHAFT ANGLE TO DIGITAL AMSC N/A FSC 5990 DISTRIBUTION. STATEMENT A. Approved for public release; distribution is unlimited. Downloaded from http://www.everyspec.com MIL-HDBK-231A FOREWORD 1. This military handbook is approved for use by all Departments and Agencies of the Department of Defense and supersedes MIL-HDBK-231(AS) dated 1 July 1970. 2. Beneficial comments (recommendations, additions, deletions) and any pertinent data which may be of use in improving this document should be addressed to: Commanding Officer, Naval Air Engineering Center, Engineering Specifications and Standards Department, Code 53, Lakehurst, NJ 08733-5100, by using the self-addressed Standardization Document Improvement Proposal (DD Form 1426) appearing at the end of this document or by letter. 3. This handbook provides basic and fundamental information on encoder techniques, terminology, operation, and number systems. It will provide valuable information and guidance to personnel concerned with the preparation of specifications and the procurement of encoders. The handbook is not intended to be referenced in purchase specifications except for informational purposes, nor shall it supersede any specification requirements. 4. An encoder is a device for sensing the rotation or position of a shaft and providing a means for indicating that position. A rudimentary application of the basic encoder concept, which is still widely used today in the maintenance of automobile engines, employs a simple index mark on the flywheel and a fixed reference mark adjacent to the flywheel. Rotation of the flywheel results in movement of the index mark relative to the reference mark and is a function of engine operation and main shaft position. The reference mark and the index mark are placed so that alignment of the two indicates that a selected piston is sitting at top dead center. Another common example of the encoder principle is the familiar automobile speedometer and odometer. This device senses the rotation of the drive shaft of the vehicle, thereby providing the operator with an indication of shaft speed in analog form calibrated in miles per hour (speedometer needle), as well as a digital count of shaft revolutions calibrated in linear miles to an accuracy of 0.1 mile (odometer). In general, any variable which can be represented as a function of the rotation of a shaft (pressure, speed, flow rate, positioning, distance, time, and so on) can be sensed using an encoder and a representative indication or signal can be provided for a broad variety of uses including control, operation, maintenance, and sequencing. 5. The dramatic advancement in weapon systems during World War 11 emphasized the need for rapid and accurate transmission of shaft position data to remote devices. Fire control systems were developed for automatic, high-speed gun laying which required a means for transmitting radar posi- tional data to a remote gun position to aim and fire the weapon. The primary solution was, of necessity, an analog approach using synchro devices Although adequate for the applications of that time, the analog technique has inherent inaccuracies due to deviations from linear input-output characteristics. However, development in this area led to the subsequent development of fast, accurate, and compact digital shaft encoders. ii Downloaded from http://www.everyspec.com MIL-HDBK-231A CONTENTS PARAGRAPH PAGE 1.0 SCOPE. 1 1.1 Scope. 1 1.2 Applicability. 1 1.3 Purpose. 1 2.0 REFERENCED DOCUMENTS 1 2.1 Government documents 1 2.1.1 Specifications, standards, and handbooks 1 2.2 Non-Government publications. 2 2.3 Order of precedence. 2 3.0 DEFINITIONS. 3 4.0 GENERAL REQUIREMENTS . 9 4.1 Need for computer control. 9 4.2 Survey of encoders - general 9 5.0 DETAILED REQUIREMENTS. 9 5.1 Encoder description . 9 5.2 Encoder classification . 10 5.2.1 Contact encoder. 10 5.2.2 Non-contact encoder. 10 5*3 Functional systems of encoder. 11 5.3.1 Input function . 11 5.3.2 Detection function . 13 5.3*3 Logic processing function. 17 5.3.4 Output function. 20 5.3.5 Summary of interrelation of functions. 22 5.3.6 Conclusion - functional systems. 23 5.4 Encoder types. 23 5.4.1 Encoder techniques . 23 5.4.2 Encoders. 24 5.4.3 Contact encoders . 25 5.4.4 Magnetic encoders. 27 5.4.5 Capacitive encoders. 31 5.4.6 Optical encoder. 34 5.4.7 Special devices. 39 5.4.8 Comparisons and summaries - encoders 41 5.5 Construction of encoders . 44 5.5.1 Size, shape and performance. 44 5.5.2 Structural design and housing. 45 5.5.3 Weight of encoders . 50 5.5.4 Mountings and physical interfaces. 51 5.5.5 Environmental determinants . 51 iii Downloaded from http://www.everyspec.com MIL-HDBK-231A CONTENTS PARAGRAPH PAGE 5.5.6 Cpmparative evaluation of size and structure 53 5.6 Systems technology . 54 5.6.1 Input mechanics. 54 5.6.2 Detection mechanics. 59 5.6.3 Logic mechanics. 67 5.6.4 Output mechanics . 77 5.7 Testing and evaluation 81 5.7.1 Test requirements and criteria 81 5.7.2 Quality assurance methods. 82 5.7.3 Mechanical inspection. 83 5.7.4 Electrical tests . 84 5.7.5 Typical test sequence. 86 5.8 Research and development . 92 5.8.1 Performance problem areas. 92 5.8.2 Research - current programs. 94 5.8.3 Encoder of the future. 97 5.9 Number systems and codes . 99 5.9.1 Number systems . 99 5.9.2 Coding techniques. 105 5.9.3 Binary decimal system. 107 5.9.4 Nonweighted binary codes . 108 5.9.5 Cyclic decimal code. 111 5.9.6 Noncyclic codes - nonreflected codes 113 5.9.7 Combination codes. 114 5.9.8 Gray coded Excess-3 BCD. 116 5.9.9 Two-out-of-five-codes. 117 5.9.10 Special purpose codes. 118 5.9.11 Code selection criteria. 119 5.9.12 Encoder logic. 119 5.10 Applications, tradeoffs, and error . 125 5.10.1 Applications . 125 5.10.2 Tradeoffs. 130 5.10.3 Accuracy factors, encoder error, and minimization 136 5.11 Selection of an encoder. 144 5.11.1 Encoder characteristics - general. 144 5.11.2 Selection of encoders for military use . 147 iv I Downloaded from http://www.everyspec.com MIL-HDBK-231A CONTENTS TABLE PAGE I. Light pattern sequence for a two-bit cam switch encoder. 149 II. Typical characteristics of magnetic materials. 150 III. Semiconductor characteristics. 151 IV. Semiconductor properties, various materials. 152 V. Angular correlation. 153 VI . Typical encoder minimum sizes. 154 VII. Instrument bearing classes and tolerances. 155 VIII. Gear tooth manufacturing tolerances. 155 IX. Typical encoder weights. 156 X. Environmental resistance . 157 XI. Torque and inertia for typical encoders. 158 XII. Bearing requirements . 159 XIII. Excess-three to natural binary conversion truth table. 159 XIV. Set-Reset selector circuit truth table . 160 XV. Set-Reset selector circuit control functions . 160 XVI . Set-Reset inter-disc or inter-belt selector circuit - truth table. 161 XVII. Sequence of operations for high reliability logic system 162 XVIII. Typical operating specifications for an external driver circuit. 163 XIX. Encoder test procedures. 164 xx. Counting in other number systems . 165 XXI. Four-digit natural binary code and corresponding decimal system numbers. 166 XXII. Incremental code and corresponding decimal system numbers 166 XXIII. 8-4-2-1 binary coded decimal . 167 XXIV. Binary coded octal system. 167 XXV. 7-4-2-1 binary coded decimal . 168 XXVI. Two-out-of-seven code. 168 XXVII. Two-out-of-five code . 169 XXVIII. Five binary number systems . 170 XXIX. Gray code. 171 XXX. Nonweighted, nonreflected cyclic code. 172 XXXI. Excess-3 code and derivation . 173 XXXII. Partial ICAO code, selected sequential values. 174 XXXIII. ICAO code, forbidden bit changes--Bits C1, C2 and C4 . 177 XXXIV. Gray-coded Excess-3 Binary Coded Decimal . 178 XXXV. Two-out-of-five coding . 179 XXXVI. Unit distance code for minutes or seconds with one-bit complement. 179 XXXVII. Unit distance code of 0-23 hours . 180 XXXVIII. Boolean logic summary. 181 XXXIX. Truth table for AND logic equation . 182 XL. Set-Reset truth table. 182 . v Downloaded from http://www.everyspec.com MIL-HDBK-231A CONTENTS TABLE PAGE XLI. Encoder specification checklist. 183 XLII. Size comparison of encoders for various configurations . 186 XLIII. Lubricant and bearing material considerations for high temperature operation. 187 XLIV. Lubricant and bearing material considerations for high vacuum operation . 187 XLV. Lubricant and bearing material considerations for nuclear radiation environments. 188 XLVI. Lubricant considerations for low temperature operations. 188 XLVII. Designation of encoder type. 189 XLVIII. Designation of code and scan . 189 XLIX. Designation of logic and output. 190 L. Designation of function. 190 FIGURE 1 Digital shaft encoder system - basic functional block diagram. 191 2 One bit digital encoder. 191 3 Two bit cam switch encoder (A) . 192 4 Two bit cam switch encoder (B) . 192 5 Encoder disc pattern with natural binary code. 193 6 Timing diagram for two-bit lead-lag switching. 194 7 Switching sequence for two-bit cam switch encoder with anti-ambiguity logic. 195 8 Contact encoder disc . 197 9 Pattern eccentricity in an encoder disc. 197 10 Pin contact encoder. 198 11 Pin encoder, cutaway view. 199 12 Magnetic encoder, cutaway view . 200 13 Magnetic encoder operation . 200 14 Typical magnetic encoding. 201 15 Toroidal magnetic head . 202 16 Magnetic head, hysteresis loop and readout voltage . 203 17 Capacitive encoder, cutaway view . 204 18 Capacitive encoder.