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Hardware for an electrical machines laboratory computer data acquisition system 1970

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HARDWARE FOR AN ELECTRICAL MACHINES LABORATORY COMPUTER DATA ACQUISITION SYSTEM by JAMES ELLWOOD JORDAN B.Sc, U n i v e r s i t y of Manitoba, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of E l e c t r i c a l Engineering We accept t h i s thesis as conforming to the required standard Research Supervisor.. Members of Committee. Head of Department... < Members of the Department of E l e c t r i c a l Engineering THE UNIVERSITY OF BRITISH COLUMBIA December, 1970 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree a t t he U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . 1 f u r t h e r a g r ee t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t he Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f E-L^CRtCAU E*)6lAiE£S>06 The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada p a t e DEC. J4/7Q ABSTRACT Hardware for an el e c t r i c a l machines laboratory computer data acquisition system i s considered. A survey of existing equipment and a study of the role of a computer system in laboratory instruction i s made for the UBC e l e c t r i c a l machines laboratory. From this, specifications for the hardware required for a data acquisition and processing system are , studied and a system configuration proposed. Transducers for measuring voltage and current waveforms on a machine are considered and designed. The performance of the transducers constructed i s evaluated in two sets of measurements. In the f i r s t set, measurement error, offset d r i f t , common-mode rejection ratio, and frequency cutoff are measured for the trans- ducer set (by i t s e l f ) . Measurement errors are found to be less than 1% F.S. In the second set of measurements, a system similar to the one proposed for the machines laboratory i s tested. Results from this set of measurements indicate that the system design proposed i s workable. ( i i ) TABLE OF CONTENTS Page ABSTRACT • ••• ' i i LIST OF ILLUSTRATIONS - i v LIST OF TABLES • • v ACKNOWLEDGEMENT v i 1. INTRODUCTION . .' 1 1.1 D e s c r i p t i o n of System Desired 1 1.2 Scope of Thesis Project 2 2. SYSTEM CONFIGURATION • • 3 2.1 System Study.... 3 2.2 Input/Output Device - 9 2.3 Transmission Link 9 2.4 Analog to D i g i t a l Interface 10 2.5 Computers •• 12 2.6 F i n a l System Configuration.. 13 3. TRANSDUCERS 16 3.1 Proposed S p e c i f i c a t i o n s 16 3.2 Transducer Design 16 3.2.1 Current Transducers 17 3.2.1.1 H a l l M u l t i p l i e r 17 3.2.1.2 Constant-Current Source . . . . 20 3.2.1.3 Level A m p l i f i e r . 20 3.2.2 Voltage Transducers 24 4. RESULTS 27 4.1 F i r s t Set of Measurements 27 4.2 Second Set of Measurements 29 5. CONCLUSIONS AND SUGGESTIONS 30 APPENDIX A Current Transducer Amplifier Design.. 33 APPENDIX B Voltage Transducer Amplifier Design 35 APPENDIX C D e s c r i p t i o n of Tests 37 C l Transducers Only 37 C . l . l Measurement Erro r 37 C.1.2 Of f s e t D r i f t 39 C.1.3 Common-Mode Rejection Ratio 39 C.1.4 Frequency Response 39 C.2 System Test . 39 REFERENCES ' 4 5 ( i i i ) LIST OF ILLUSTRATIONS Figure Page 2.1 Block Diagram of System ..... 8 3.1 Current Transducer Configuration 18 3.2 H a l l M u l t i p l i e r 18 3.3 Current Regulator Configurations . 21 3.4 Current Regulator 22 3.5 . D i f f e r e n t i a l A m p l i f i e r Design 22 3.6 Voltage Transducer Design 26 5.1 A Superior D i f f e r e n t i a l A m p l i f i e r 31 C . l Measurement Error Test 38 C.2 O f f s e t D r i f t Test 38 C. 3 CMRR Measurement 38 C.4 Frequency Response Test 40 C.5 System Test 40 C.6 Flow Chart 42 C.7 Data A c q u i s i t i o n Program L i s t i n g 43 C.8 Interface C h a r a c t e r i s t i c s Measurements 44 (iv) LIST OF TABLES Table • Page 2.1 C h a r a c t e r i s t i c s of Tamper E l e c t r i c a l Machine Sets ^ 2.2 Conventional Measuring Instruments 5 2.3 Interface S p e c i f i c a t i o n s ' 11 2.4 Computer S p e c i f i c a t i o n s 14 3.1 H a l l M u l t i p l i e r S p e c i f i c a t i o n s 19 3.2 S p e c i f i c a t i o n s of F a i r c h i l d UA723C 23 3.3 S p e c i f i c a t i o n s of Motorola MC1741CG ••• 2 5 4.1 Measured Transducer C h a r a c t e r i s t i c s 28 (v) ACKNOWLEDGEMENT The author would l i k e to express h i s appreciation to h i s supervisor, Dr. A. D. Moore, for guidance and assistance during t h i s p r o j e c t , and to Dr. B. J . K a b r i e l for reading the manuscript. Appreciation i s also expressed to the s t a f f of the E l e c t r i c a l Engineering Department, UBC, f o r h e l p f u l assistance, to fellow students f o r proof-reading, and e s p e c i a l l y to Miss Veronica Komczynski f o r typing the th e s i s . The work described i n t h i s t hesis was c a r r i e d out under National Research Council of Canada grant A-3357. The author g r a t e f u l l y acknowledges f i n a n c i a l support received i n the form of N.R.C. Postgraduate Scholarships. (vi) 1 1. INTRODUCTION This thesis i s concerned with a proposed on-line computer data a c q u i s i t i o n and processing system f o r an undergraduate e l e c t r i c a l machines laboratory. The long-term o b j e c t i v e of th i s project i s to introduce a d i g - i t a l computer system i n t o the educational environment of a laboratory as an i n s t r u c t i o n a l a id as w e l l as a measurement and computation f a c i l i t y . 1.1 Description of System Desired The system envisaged f o r the e l e c t r i c a l machines laboratory w i l l acquire measurement data from the e l e c t r i c a l machines and compute experimental r e s u l t s f o r the users. Such a system could conceivably consist of a small d i g i t a l computer, a set of transducers f o r each machine, and some form of in t e r f a c e between the transducers and computer. The system developed w i l l not replace the e x i s t i n g measuring instruments. Instead, i t w i l l allow the machines and meters to be connected normally to r e t a i n the educational value of s e t t i n g up the experiment and to permit v i s u a l monitoring of the machine's v a r i a b l e s . In this way, the f a c i l i t y can be used i n an i n s t r u c t i o n a l r o l e as w e l l . The students w i l l commence the experiment by making measurements with the normal instruments and c a l c u l a t i n g experimental r e s u l t s as usual. Simultaneously, the computer w i l l compute the correct values f o r the experiment and the student's r e s u l t s can be checked at this point. Once the students have demonstrated an understanding of the p r i n c i p l e s involved, the computer can perform the tedious task of c a l c u l a t i n g r e s u l t s f o r the remainder of the experiment. I f desired, tutoring of the students could be incorporated as w e l l . The transducers must be capable of making measurements which are as accurate as those obtained from conventional instruments. As w e l l , i t would be desirable to measure inputs with frequencies up to the ninth 2 harmonic of 60 Hz. to permit transients and harmonic waveforms on the machines to be studied. Such a system w i l l hence be useful f o r general-purpose experimental work as w e l l as laboratory i n s t r u c t i o n . 1.2 Scope of Thesis P r o j e c t The f i r s t part of t h i s thesis project i s the study and s p e c i f i c a t i o n of the system j u s t described. This work i s presented i n Chapter 2, System Configuration. The second part of t h i s project i s the development, construction, and checkout of a set of transducers f o r the e l e c t r i c a l machines laboratory. The design of these transducers i s described i n Chapter 3, Transducers. In the l a s t part, tests were conducted on the transducers developed and on a system simulating the one proposed for the machines laboratory to determine t h e i r actual performance. Results O f these tests are summarized i n Chapter 4, Results. 3 2. SYSTEM CONFIGURATION 2.1 System Study The UBC e l e c t r i c a l machines laboratory consists p r i m a r i l y of s i x e l e c t r i c a l machine sets, each comprising an induction machine, a synchronous machine, and a d.c. machine, a l l on a common sh a f t . The laboratory i s sup- p l i e d with power from a 230 v o l t d.c. bus and a 208 v o l t (r.m.s.), 3-phase, 60 Hz. a.c. bus. The machines may be connected i n various configurations using a patchboard arrangement by the experimenter. The d e t a i l s of t h e i r e l e c t r i c a l c h a r a c t e r i s t i c s are summarized i n Table 2.1. A number of instruments are normally used for making measurements on the e l e c t r i c a l machines. P r i m a r i l y , a multimeter i s used for measuring resistance, voltage, and current (up to 10 amperes). A number of ammeters are a v a i l a b l e f o r measuring a.c. currents up to 300 amperes and d.c. currents up to 15 amperes. As w e l l , current shunts and m i l l i v o l t m e t e r s are a v a i l a b l e for measuring currents up to 1000 amperes. Wattmeters are used for measuring e l e c t r i c a l power i n a number of ranges up to 15 kilowatts. The s p e c i f i c a t i o n s of the instruments a v a i l a b l e are b r i e f l y o u t l i n e d i n Table 2.2. Of p a r t i c u l a r i n t e r e s t are the accuracy s p e c i f i c a t i o n s , most of which are of the order of -1% F . S V For mechanical measurements, a stroboscope i s used to measure shaft angular v e l o c i t y and a mechanical arm and spring balance are used to measure shaft torque. The majority of experiments i n the e l e c t r i c a l machines laboratory involve the measurement of q u a n t i t i e s on only one machine. However, a s i g n i f - i c a n t number of experiments use a second machine connected to the machine under t e s t . By measuring the e l e c t r i c a l power consumed or generated by the second machine, the mechanical power produced or absorbed by the machine under test can be c a l c u l a t e d . By providing extra transducers to measure 4 Table 2.1: CHARACTERISTICS OF TAMPER-ELECTRICAL MACHINE SETS (a) Steady State: Wound Rotor Induction Motor RPM - 1690 H.P. - 2.5 Stator Volts-208, 30, 60 Hz. Stator F.L. Amps.-8.0 Rotor Volts - 274 Rotor Amps. - 4.3 D.C. Motor RPM - 1760 H.P. - 2.5 Arm. Volts-230 VDC. Arm. F.L. Amps-10.3 Shunt F i e l d 2 3 Q Volts Shunt Field _ Amps. ,58/.44 Synchronous Motor RPM - 1800 K.W. - 1.6 KVA - 2 . 0 . Arm. Volts-120/208, 30, 60 Hz. Arm. F.L. Amps-5.5 @ p.f.= 0.8 F i e l d Volts - .115 D.C. F i e l d Amps. - 1.2 (b) Transients: Des c r i p t i o n of Condition (1) Induction Motor-Stator current for start-ups with no-load and shorted rotor (2) Induction Motor-Rotor current f o r start-ups with no-load and shorted rotor (3) Synchronous Motor-Stator current for i n d u c t i o n - l i k e start-ups; I f = 0. (4) Induction Motor-connected as a synchronous motor for 10, 20, & 30 f a u l t condition experiment (5) D.C. Motor Max. Measured Value ( u n i d i r e c t i o n a l ) 20 A. 24 Ar . 47 A.. - * 20 A. • . 20 A., cutout 5 Table 2.2: CONVENTIONAL MEASURING INSTRUMENTS (a) AVO-Meter: Model 8: (i) Ranges: D.C. Voltage: 1000, 500, 250, 100, 25, 10, 2.5 v o l t s A.C. Voltage: 1000, 250, 100, 25, 10, 2.5 v o l t s D.C. Current: 10, 1, .1; .01, .001, .000250, .000050 A. A.C. Current: 10, 2.5, 1. .1, A. Resistance: 0-2000, 0-200 K, 0-20 M Q. (ii) Accuracy: D.C. Voltage: 2% F.S. D.C. Current: 1% F.S. A.C. : 2 1/4 %F.S. ( i i i ) Input Requirements: A l l D.C. Voltage Ranges: 20,000 Q/T. (50uA.for f u l l d e f l e c t i o n ) D.C. Current Ranges: P o t e n t i a l drop = 0.5 V. at f u l l load except 50 yA. range which absorbs. 125 mV. A.C. Voltage Ranges: Above 100 V.+ 1000 QjV- (1 mA. for f u l l d e f l e c t i o n ) ; 25 V, consumes 4 mA,; 10 V. consumes 10 mA; 2.5 V. consumes 40 mA. A.C. Current Ranges: 0.2 V. drop across terminals on a l l ranges. (b) A.C. Ammeters: ( i ) Input Ranges and (number of instruments): 300 A.(2); 100 A. (1) ; 50 A.(7) ; 25 A. (2) ; 15 A. (2); 5/10 A. (8) ; 5 A . ( l ) ; 2.5/5 A . ( l ) ; 2 A.(3); 1.5/3 A.(1); 1 A . ( l ) ; . .25/.5 A.(2) ( i i ) T ypical Accuracy: 2% F.S. (c) A.C. Voltmeters: ( i ) Input Ranges and (number of instruments): 150/300/600 V. (2); 150/300 V. (14); 60/120 V. (1); 30/60 V. (3); 30 V.(2); 2.5/15/30/ 75 V. (1); 15/30 V. (1) 6 Table 2.2 (cont'd) (d) Clamp-on Ammeters: ( i ) Input Ranges and (number of instruments): 15/60/150/600 A.(1); 10/25/50/100/250/500 A.(1) (e) D.C. Ammeters: ( i ) Input Ranges and (number of instruments): 5 A. (1) ; 15 A. (1) (f) Shunts; ( i ) Input Ranges and (number of each type): [50 mV. output] 1000 A.(2); 500 A.(1); 300 A.(3); 200 A.(1); 150 A.(2); 100 A.(4); 80 A.(5); 75 A.(1); 50 A. (9); 25 A.(1); 24 A.(1); 15 A. (1); 5 A.(3); 1.5 A.(2). (g) D.C. M i l l i v o l t m e t e r s ; ( i ) Input Ranges and (number of instruments): 50 mV.. (6) ; 50 mV. (2) ; 50 mV. (2) ( i i ) T y p i c a l Accuracy: 1/4 to 1/2 of 1% F.S. (h) D.C. Voltmeters: • " ( i ) Input Ranges and (number of instruments): 6000 V. (1); 300 V. (1); 150/300 V. (1); 15/150 V. (2); 10/20 V. (1); 3/15/150 V. (4); 1.5/15/150 V. (3) ( i ) Wattmeters: ( i ) Input Ranges and (number of instruments): 7.5/15 KW.(5); 3/6/12 KW.(1); 1.5 KW. (1); 1.5/3/6 KW.(l); 1.0 KW. (1) ; . 750 KW. (2) ; .750 W. (1) ; 750 W. (3) ; 375/750/1500 W, (1) ; 150 W. (1); 100 W.(1); 37.5/75/150 W. (2) ( i i ) T y p i c a l Accuracy: 1/2 to 3/4 of 1% F.S. 7 e l e c t r i c a l quantities on the additional machine, a transducer to measure torque should not be required for most of the undergraduate experiments. Since the actual measurement of mechanical power would be desirable, a tor- quemeter might eventually be added or the load angle transducer developed by the Power Group''" adapted for use with this system. Angular shaft velocity may be measured using a tachometer and voltage transducer. Hence, only transducers to measure e l e c t r i c a l inputs are required. Since power may be calculated from voltage and current inputs using the computer, transducers for voltage and current are a l l that are required. For experiments using two a.c. machines, as many as eleven voltage transducers and ten current transducers could be used (one d.c. c i r c u i t , three 3-phase circuits, and tachometer output). However, a saving in the number of transducers can be realized by using only one a.c. machine in two-motor experiments or by measuring only two of three phases in two-a.c. machine experiments. A minimum of six current transducers and seven voltage transducers are required with this constraint. Since only one undergraduate experiment involves the use of two a.c.. machines, this limitation appears to be j u s t i f i e d . A generalized block diagram for the laboratory computer system i s shown in figure 2.1. It consists of six subsystems: a set of transducers, an analog link, an analog to d i g i t a l interface, a d i g i t a l link, a d i g i t a l computer, and an input/output device for user communication. This diagram is generalized in the sense that either the d i g i t a l or analog link may be short in length depending on the type and location of computer and the trans- mission scheme chosen. Specifications for the input/output device, the main transmission link, the analog to d i g i t a l interface, and the computer are studied in the following sections. Suitable devices are chosen in each case and a system for the e l e c t r i c a l machines laboratory is proposed. I I I U U I III /c inputs TRANSDUCER SET analog link , ANALOG D/G/ ML 1 INTERFACE j INPUT/ buTPur DEVICEl digital link timing COMPUTER data —p*— BLOCK DIAGRAM OF SYSTEM : FIGURE 2.1 00 9 2.2 Input/Output Device An input/output device for user communication i s required for several purposes. F i r s t of a l l , i t w i l l allow the experimenter to issue commands to the computer to govern the data a c q u i s i t i o n and processing from the experiment. Secondly, i t w i l l permit the input of data concerning the experiment (e.g.: conversion units, transducer ranges, channel inputs, e t c . ) . F i n a l l y , i t w i l l be required for the output of results from the computer. The device required must be able to accept inputs from the user ( t y p i c a l l y alphanumeric characters at rates up to 120 five-character words/ minute). Also, i t must be able to produce outputs of a s i m i l a r nature, preferably making permanent copies of this information. F i n a l l y , the device selected must be suitable for use i n the e l e c t r i c a l machines laboratory environment. The most suitable input/output device for this project i s the standard KSR Model 33 teletype. This device can be used to type i n or p r i n t out information at a rate of up to 10 characters/second. Although a CRT display device would have a faster output response and would allow the output of graphic information as w e l l , the teletype was chosen on the basis of cost. 2.3 Transmission Link The transmission l i n k i s required to transmit measurement data from the e l e c t r i c a l machines laboratory to the computer. Since the trans- ducers are required to produce data which i s accurate to ^1%, the transmission l i n k must be capable of conveying this information without a substantial degradation i n accuracy. Also, the transmission l i n k chosen must be able to handle data from inputs up to 540 Hz.in frequency (since the transducers accept inputs up to the ninth harmonic of 60 Hz.). F i n a l l y , i t i s important that the cost and complexity of the data l i n k be minimized. 10 The two a l t e r n a t i v e s considered were analog, - and d i g i t a l transmission The p r i n c i p l e advantage of d i g i t a l transmission over analog transmission i s i t s higher immunity to noise i n t e r f e r e n c e . However, the hardware required to implement d i g i t a l transmission i s generally more co s t l y and more complex than that required f o r analog transmission. As w e l l , the use of d i g i t a l transmission would require the design of a dedicated e l e c t r i c a l machines laboratory analog to d i g i t a l i n t e r f a c e . Since a l l the computers a v a i l a b l e i n t h i s department f o r t h i s p r o j e c t are equipped with analog i n t e r f a c e s , i t would be d e s i r a b l e to use analog transmission to take advantage of these e x i s t i n g f a c i l i t i e s . Since analog transmission at l e v e l s i n the order of i 5 to *10 V.F.S. was expected to y i e l d adequate performance for our purposes, t h i s means of transmission was selected for use i n the laboratory/computer transmission l i n k . 2.4 Analog to D i g i t a l Interface The analog to d i g i t a l i n t e r f a c e must meet several requirements. F i r s t of a l l , i t must be able to handle at l e a s t t h i r t e e n analog inputs and preferably more with provisions for simultaneous sampling of a l l inputs (to permit c a l c u l a t i o n of power and phase r e l a t i o n s h i p s ) . Secondly, i t must be able to handle input, s i g n a l s up to 540 Hz. (ninth harmonic of 60 Hz.) i n frequency. Next, the quantization e r r o r which adds to the t o t a l uncertainty of the measurement must be an order of magnitude less than ^1% F.S. Hence, 2 the d i g i t a l representation of the input should be at l e a s t 10 b i t s ( f o r 9 b i t s , e r r o r i s * 1/2 l e v e l i n 2 = 512 l e v e l s for unipolar input; hence, 10 b i t s y i e l d s an e r r o r of -1 part i n 1024 parts for a b i p o l a r i n p u t ) . The s p e c i f i c a t i o n s of the a v a i l a b l e i n t e r f a c e s are shown i n Table 2.3. As these devices are now designed, the hybrid interface i s the only s u i t a b l e choice f o r t h i s p roject since only i t has i n d i v i d u a l sample-and-hold 11 Table 2.3: INTERFACE SPECIFICATIONS Parameter 1. M u l t i p l e x Channels 2. M u l t i p l e x Input Level 3. Sample and Hold Units 4. Operating Frequency 5. A/D Conversion Time 6. A/D Word Length 7. A d d i t i o n a l Features Hybrid Interface 16 ±100' Y. ±10 V. ±1 V. 1 per channel 3 20 KHz. 20 ysec. 12 b i t s -Analog Computer -D/A Converters -Sampling Frequency Control Sys terns Lab. Interface 8 ±5 V. < 10 KHz, 10 b i t s -D/A Converters 12 units and enough input multiplexer channels. Upon modification to include these features, the systems laboratory i n t e r f a c e s f o r the PDP-8/L and NOVA computers would be as s u i t a b l e as the hybrid i n t e r f a c e . 2.5 Computers The computer chosen f o r th i s p r o j e c t must meetseveral requirements. F i r s t of a l l , the word length of the computer should be s u f f i c i e n t to permit an accurate d i g i t a l representation of the input data to be stored i n one word. For ±1% accuracy, a word length of at l e a s t 7 b i t s (max. erro r i s ±..1/2 l e v e l i n 2^ l e v e l s f o r unipolar input) i s required. ' Secondly, the memory required to store the programs to operate the system must be estimated. Although i t i s f e a s i b l e to segment the programs and store them on an external storage device, at l e a s t a por t i o n of the programs must be stored i n the computer's memory at any given time. An e x i s t i n g 3 implementation of software f o r t h i s system divides the software into three portions, one of which occupies approximately 6 K words of memory. This implementation was made using the PDP-9 computer and i t s associated Monitor System Software. I t may be po s s i b l e to reduce the number of program i n s t r u c t i o n s by w r i t i n g a more s p e c i a l i z e d routine since the PDP-9 Resident Monitor occupies 4 1635 words of memory alone. However, th i s i s balanced by the need for a d d i t i o n a l software to permit time-sharing or p o l l i n g of the various machine; sets. As w e l l , an a d d i t i o n a l software segment w i l l be required f o r s p e c t r a l analysis and transient a n a l y s i s . A t y p i c a l Fast Fourier Transform routine requires 6400 words of memory storage (both data and program) to handle 2048 time samples on the PDP-8/L computer.~* Hence, i t would be desirable to s e l e c t a computer with a memory s u f f i c i e n t l y l a rge to handle at l e a s t 6 K words of program i n s t r u c t i o n s . 13 A t h i r d consideration i s the amount of data to be stored i n memory. An upper l i m i t on the amount of data which i s produced from an experiment may be estimated by considering how much information i s required f o r transient and s p e c t r a l a n a l y s i s . Since input waveforms w i l l include frequency components up to the ninth harmonic of 60 Hz. and since i t i s d e s i r a b l e to observe these waveforms f o r i n t e r v a l s of time up to one second, as many as 1080 samples of data (Nyquist rate sampling) w i l l be produced per channel per second. However, f o r steady-state measurements, much l e s s data i s required so that such a large data storage requirement may not be j u s t i f i e d . As w e l l , i t may be po s s i b l e to b u f f e r the data i n the computer and output i t v i a the data t r a n s f e r to an external bulk storage device between input samples. Thus, a memory of 8 K words or l a r g e r i s recommended. Other factors governing the d e c i s i o n of computer are speed, bulk storage capacity, existence of data a c q u i s i t i o n software, and a v a i l a b i l i t y of h i g h - l e v e l languages and compilers. The computers a v a i l a b l e were the DEC PDP-9, the DEC PDP-8/L, and the DATA GENERAL NOVA. The s p e c i f i c a t i o n s are compared i n Table 2.4. Although the PDP-8/L and NOVA computers are portable and could be used d i r e c t l y i n the laboratory, the PDP-9 was selected because of i t s 16 K word memory, i t s magnetic tape transports, and i t s e x i s t i n g data a c q u i s i t i o n software. An important f a c t o r i n t h i s choice was the a v a i l a b i l i t y of a s u i t a b l e analog i n t e r f a c e . 2.6 F i n a l System Configuration The system s p e c i f i e d uses a PDP-9 computer, the h y b r i d i n t e r f a c e , a multi-wire -10 V.F.S. analog transmission l i n k , a standard KSR Model 33 teletype, and a t h i r t e e n to s i x t e e n input transducer set (to be discussed i n Chapter 3). 14 TABLE 2.4: COMPUTER SPECIFICATIONS Parameters PDP-9 PDP-8/L NOVA 1. Memory s i z e 16 K words 4 K words 4 K words 2. Word Length 18 b i t s 12 b i t s 16 b i t s 3. Cycle Time 1 ysec. 1.6 usec. 2.6 usec. 4. Add Time 2 usee. 3.2 ysec. 5.9 ysec. 5. Peripherals -Teletype -Display -Hybrid I n t e r - face -Magnetic Tape Drives -Paper Tape -Teletype -Paper Tape -Analog Inter- face -Teletype -Paper Tape -Analog Interface 6. Bulk Storage Capacity 7. Features 160 K words -D i r e c t Memory Access -Data Channel -Program Interrupt -Extended Arithmetic Element -Monitor Software System -Data Channel -Program I n t e r - rupt -4 Accumulators -Data Channel -Program Interrupt 15 . The system configuration s p e c i f i e d i s by no means absolute. Event- u a l l y , i t might be desi r a b l e to use the PDP-8/L or NOVA because of the demands on the PDP-9 computer. The ±10 V .F.S..analog outputs of the transducers for s i g n a l transmission to the hybrid i n t e r f a c e can e a s i l y be modified to a ±5 V.F.S. l e v e l s u i t a b l e f o r use with the PDP-8/L and NOVA i n t e r f a c e s . However, use of the PDP-8/L or NOVA computers w i l l require a d d i t i o n a l hardware such as an a d d i t i o n a l 4 K of memory, a bulk storage device such as a d i s c , and mod i f i c a t i o n of the systems laboratory i n t e r f a c e to handle at l e a s t t h i r t e e n channels of input with simultaneous sampling of a l l channels using i n d i v i d u a l sample-and-hold devices. As w e l l , a s p e c i a l laboratory i n t e r f a c e and d i g i t a l . transmission l i n k could be added should noise interference prove to be a serious problem. 16 3. TRANSDUCERS 3.1 Proposed S p e c i f i c a t i o n s The following s p e c i f i c a t i o n s are proposed as guidelines f o r the design of the transducers. The transducer set w i l l consist of 13 to. 16 units with at l e a s t 6 current transducers and 7 voltage transducers. Both types of transducers must be able to operate with inputs with frequencies up to 540 Hz. with ar> accuracy better than ^1% F.S. They must also be able to operate over a t y p i c a l indoor temperature range of 21° * 10°C. The current transducers must be able to handle currents up to 50 amperes, the l a r g e s t current observed on the machine sets (see Table 2.1). As w e l l , the design of the current transducers must permit current measurement i n conductors at voltages as high as 500 v o l t s abbve ground. The voltage transducers must be able to measure voltages up to 500 v o l t s with common-mode voltages up to 500 v o l t s ( l a r g e s t steady-state voltage a n t i c i p a t e d was approximately 150% of the peak value of rated machine output voltage, i . e . : 1.5 x x 208 V.). A number of input ranges should be provided with each transducer to permit accurate measurements at lower input l e v e l s . F i n a l l y , the design of the transducers must allow the conventional instruments to be connected normally. 3.2 Transducer Design Six current transducers and ten voltage transducers were provided i n t h i s design. Though only s i x current transducers and seven voltage trans- ducers were required, three a d d i t i o n a l voltage transducers were included because of the a v a i l a b i l i t y of 16 multiplexer channels and the low cost of the voltage transducers. 17 3.2.1. Current Transducers A H a l l M u l t i p l i e r was chosen f o r use as a current transducer. The design used incorporates a constant-current source to provide e x c i t a t i o n for the H a l l m u l t i p l i e r , and a d i f f e r e n t i a l a m p l i f i e r to boost the output s i g n a l f o r transmission to the computer. A block diagram of t h i s configuration i s - shown i n f i g u r e 3.1. The H a l l m u l t i p l i e r was selected i n preference-to a number of other devices, i n c l u d i n g s e r i e s resistances, current transformers, and magneto- r e s t r i c t i v e devices. Current transformers and magnetorestrictive devices were considered unsuitable because of d.c. cutoff and high cost, r e s p e c t i v e l y . The H a l l m u l t i p l i e r was chosen over ser i e s resistance p r i m a r i l y because of i t s e l e c t r i c a l i s o l a t i o n between input and output and i t s a v a i l a b i l i t y i n a number of input current ranges. 3.2.1.1 H a l l M u l t i p l i e r A simple diagram of a H a l l - E f f e c t m u l t i p l i e r i s shown i n f i g u r e 3.2. The. current to be measured, i ^ , flows through the winding e n c i r c l i n g the i r o n core causing a p r o p o r t i o n a l magnetic f i e l d to be set up i n the a i r gap. The voltage at the output of the H a l l - E f f e c t device, v^, i s equal to K ^ i ^ i ^ , where i s a constant, and i ^ i s the e x c i t a t i o n current. I f i ^ i s constant, v, i s equal to Ki„, where K i s a constant, h f Two d i f f e r e n t m u l t i p l i e r s were used i n the current transducer design to provide s e v e r a l input current ranges. Six current transducers with input ranges, 0-40 amperes and 0-20 amperes, and s i x current trans- ducers with input ranges, 0-3 amperes and 0-1.5 amperes, are provided for the s i x current inputs allowed. A l i s t of s p e c i f i c a t i o n s of the H a l l multip- l i e r s u t i l i z e d i n the design i s presented i n Table 3.1. 1 8 0 CURRENT TRANSDUCER CONFIGURATION: FIGURE 3.1 19 Table 3.1: HALL MULTIPLIER SPECIFICATIONS (BELL INC.) Parameter Model No. HM-3500 Model No. HM-3010 (for p a r a l l e l connected f i e l d c o i l s ) A. Magnetic F i e l d Input Resistance Temp. Coeff. of above Reactance Current Rating Frequency Range 1 mft. 0.39%/°C. 20 mft/KHz. 40 A." , < 1 KHz. 0.06ft. 0.39%/°C. 4 ft/KHz. 3 A. < 1 KHz. H a l l Input Resistance ( i f = 0) Temp. Coeff. of above Magnetoresistance Current Rating Frequency Range .02%/°C. 20 fi < + < +2.5% of Rin 330 mA. < 100 KHz. 2.5 -ft. < +.15%/°C. < 25% of Rin 330 mA. <L 500 KHz. C. H a l l Output Resistance Load Resistance Maximum Output Temp. Coeff. of V j 0°C.to 50°C. -25°C. to 75°C. io fi. 50 fi. 200 mV. <; ±.5% < ±1% 10 fi. 50 ft. 200 mV. < ±.5% < ±1% D. H a l l Output vs. F i e l d Input L i n e a r i t y E r r o r (ij=K) Remanent Residual Phase S h i f t Frequency Response Inductive Er r o r Voltage < .5% F.S. < .5% F.S. 3.°/KHz. -.3 dB./KHz. < .2 mV./KHz. < .5% F.S. < .5% F.S. 3P/KHz. -.3 dB7KHz. < .1 mV./KHz. H a l l Output vs. H a l l Input L i n e a r i t y E r r o r (if=K) R e s i s t i v e Error Voltage Thermal Error Voltage < .25% F.S. < .3 mV;. < .3 mV. d.c. < .25% F, < .1 mV. < 12 mV. d.c. 20 3.2.1.2 Constant-Current Source A constant-current source was implemented using a voltage source and a current regulator. Both discrete-component "voltage/current cross- 6 over supplies and i n t e g r a t e d - c i r c u i t regulators were considered f o r use as current regulators. Several possible configurations, as shown i n f i g u r e 3.3, were considered for supplying e x c i t a t i o n current to the 12 H a l l m u l t i p l i e r s In f i g u r e 3.3(i) and 3.3(H), one main discrete-component regulator i s used while i n f i g u r e 3 . 3 ( i i i ) i n d i v i d u a l i n t e g r a t e d - c i r c u i t regulators are used. Configuration 3.3(i) i s superior to 3 . 3 ( i i ) and 3 . 3 ( i i i ) f or current r e g u l - a t i o n ; however, a f a i r l y large current regulator i s needed to maintain the voltage drop across the m u l t i p l i e r s (approx. 50 v o l t s @ 300 mA.) . On the basis of lower cost, i t was decided to use i n d i v i d u a l regulators as shown i n f i g u r e 3.3( i i i ) . The design of the i n d i v i d u a l regulators was straightforward. F a i r - c h i l d UA723C voltage regulators were used i n the c i r c u i t shown i n f i g u r e 3.4 to regulate the voltage drop across a current-sensing r e s i s t a n c e . The s p e c i f i c a t i o n s of the yA723C regulator are l i s t e d i n Table 3.2. The current-, handling c a p a b i l i t i e s of the regulators were extended by using power t r a n s i s t o r s at the outputs. This design provides a measured current r e g u l a t i o n of 0.05% for load v a r i a t i o n s (0£L to 20ft.) under t y p i c a l laboratory conditions. 3.2.1.3 Level Amplifier A . l e v e l a m p l i f i e r i s required to boost the ±200 mV.F.S. output of each H a l l m u l t i p l i e r up to the ±10 V.F.S. transmission l e v e l . Since the H a l l - E f f e c t device has a common-mode voltage superimposed on i t s output due to the voltage drop i n the r e s i s t i v e semiconductor material from the 300 mA. e x c i t a t i o n current, a d i f f e r e n t i a l a m p l i f i e r i s required. This a m p l i f i e r must have a d i f f e r e n t i a l gain of 50 and a common-mode r e j e c t i o n r a t i o better Current Regulator CM I c 21 Hall Multiplier Current Regulator 6 6 i O (it) (.tu) CURRENT REGULATOR CONFIGURATIONS : FIGURE 3.3 R, R. CURRENT REGULATOR: FIGURE 3.4 Rp (current sensing resistance) RL (LOAD) 2000-3000JT. 49.911 200 mv. ma.%. 49.9J7. DIFFERENTIAL AMPLIFIER DESIGN FIGURE 35 23 Table 3.2: SPECIFICATIONS OF FAIRCHILD yA723C Parameter L i n e Regulation Load Regulation Ripple Rejection Average Temp. Coeff. of V 0 Reference Voltage Long Term S t a b i l i t y ^ Input Voltage Range Output Voltage Range Input-Output Voltage D i f f e r e n t i a l ' Condition Vsupply=12 V. to 15 V. Vsupply=12 V. to 40 V. vsupply=12 V- to 15 V. (0° < T A <70° C) Load Current=l to 50 mA. Load Current=l to 50 mA." (0 < T A < 70°C) f=50 Hz. to 10 KHz. f=50 Hz. to 10 KHz. 0 £ T A < 70°C. Min. 6.80 9.5 2.0 3.0 Typ. 0.01 0.1 0.03 74 86 0.003 7.15 0.1 Max. 0.1 0.5 0.3 0.2 0.6 Unit 0.015 7.50 40 37 38 %V %V %V out out Out P o u t ^ v o u t dB. dB. %/°C. V". %/1000 hrs, V . v'. V .. ABSOLUTE MAXIMUM RATINGS Input-Output Difference Voltage 40 V. Maximum Output Current 150 m A . Internal Power D i s s i p a t i o n 800 mW. Operating Temperature Range 0°C to 70°C. Current from V,,™ 15 mA. , 24 than 250:1. F i n a l l y , the output impedance of the a m p l i f i e r should be as low as p o s s i b l e to f a c i l i t a t e proper s i g n a l transmission. The a m p l i f i e r designed f o r t h i s purpose i s shown i n f i g u r e 3.5, using a Motorola MC1741CG operational a m p l i f i e r with s p e c i f i c a t i o n s as l i s t e d i n Table 3.3. The d e t a i l s of the design of t h i s a m p l i f i e r are sum- marized i n APPENDIX A. 3.2.2 Voltage Transducers N A voltage d i v i d e r followed by a unity-gain buffer a m p l i f i e r as shown i n f i g u r e 3.6 were used i n the design of the voltage transducer. This arrangement allows the reduction of the ±500 V.F.S. laboratory inputs to the i l O V.F.S. transmission l e v e l s . To permit voltage measurements with respect to an ungrounded reference, a balanced d i v i d e r and d i f f e r e n t i a l a m p l i f i e r were used. Two measurement ranges, 0-500 v o l t s and 0-10 v o l t s , were provided. The d e t a i l s of the design of these transducers are summarized i n APPENDIX B. 25 TABLE 3.3: SPECIFICATIONS OF MOTOROLA MC1741CG Parameter Input O f f s e t Voltage Input O f f s e t Current Input Bias Current Input Resistance Large Signal Voltage Gain Output Voltage Swing Input Voltage Range CMRR ^ Supply Voltage Rejection Ratio Power Consumption Transient Response (unity gain) - r i s e t i m e -overshoot Slew Rate (unity gain) * The following apply Input Offset Voltage Input Offset Current Input Bias Current Large Signal Voltage Gain Output Voltage Swing Condition R < 10 K ft. Min. > 2Kft., V 0 U T = 1 0 V - P^ > 10 Kft. R^ > 2 Kft. R < 10 Kft. R < 10 Kft. V T „ = 20 mV. IN R^ = 2 Kft.; C < 100 pf. > 2 Kft. 0.3 20,000 ±12 ±10 ±12 70 for 0°C < T < 70°C. - A - R g ^ 10 Kft. Typ. 2.0 30. 200. 1.0 100,000 ±14 ±13 ±13 90 30 50 > 2 Kft. 15,000 V = ±10 V, OUT R > 2 Kft. 0.3 5.0 0.5 Max. 6.0 200. 500. 150 85 7.5 300. 800. Units mV. - nA. nA. V. V. V. dB. yV./V. mW. ysec. % V. /ysec. mV. nA. nA. + 10 —0 4 2K-Q. (vA-Ve,!^ Zoo* VOLTAGE TRANSDUCER DESIGN: FIGURE ' 3.6 27 4. RESULTS Two s e r i e s of measurements were made on the transducer set to determine i t s performance. 4.1 F i r s t Set of Measurements The f i r s t set was conducted on the transducer set by i t s e l f . Measurements were made to determine: (1) measurement error, (2) o f f s e t d r i f t , (3) common-mode r e j e c t i o n r a t i o , and (4) frequency response. A d e s c r i p t i o n of the tests performed i s included i n APPENDIX C. The r e s u l t s of these tests are presented i n Table 4.1. To begin with, the performance of the current transducers under t y p i c a l laboratory conditions was checked. The largest magnitude of measure- ment error observed was 0.82% F.S. for both the 0-40 and 0-3 ampere input ranges. The t y p i c a l output o f f s e t d r i f t over a period of three hours was found to be 0.12% F.S. for a l l input ranges. F i n a l l y , the t y p i c a l frequency cutoff was measured to be approximately 30 KHz.(for the 0-3 ampere input range) . A s i m i l a r set of measurements was made on the voltage transducers to check t h e i r performance. For the 0-500 v o l t input range, the la r g e s t measurement error observed was 0.39% F.S. The output o f f s e t d r i f t i n t h i s case was n e g l i g i b l e f o r a three-hour measurement period. The worst common- mode r e j e c t i o n r a t i o was 104:1. As w e l l , the frequency response was f l a t to w i t h i n ±3 dB. up to a frequency of 40 KHz. Though the 0-500 v o l t input range was found to perform adequately, the 0-10 v o l t input range was un- s a t i s f a c t o r y because the 2:1 voltage d i v i d e r used i n the design did not provide f o r large common-mode voltages. Table 4.1: MEASURED TRANSDUCER CHARACTERISTICS Max. D.C. Typ. A.C. Output tfy , Measurement Measurement Offset D r i f t Transducer CMRR Error (%F.S.) Error (%F.S.) %F.S./3 hrs. @D.C. Voltage 0.39 0.66 n e g l i g i b l e 104:1 (0-500 V.) - Current (0-40A.) 0.82 ~ 0.115 Current (0-3A) 0.63 0.114 29 4.2 Second Set of Measurements To evaluate the design more f u l l y , a system similar to the one proposed for the e l e c t r i c a l machines laboratory was set up. This system was implemented in the hybrid computer laboratory using the PDP-9 computer, the hybrid interface, and the transducer set developed. System errors were measured using d.c. inputs only. A f u l l description of the tests conducted is included i n Appendix C. The worst measurement errors were found to be 2.2% F.S. for the current inputs and 1.4% F.S. for the voltage inputs. These values are some- what larger than anticipated. The main source of error appears to be the offset voltage in the analog to d i g i t a l interface. At the time of measure- ment, the input offset voltage on the hybrid interface was rather large (2£70 mV.). Subsequent measurements show that the normal input offset voltage is of the order of 20 mV. Hence, better results may be expected with this system than were obtained in this test. As well, i t should be feasible to reduce the overall error even more by compensating for the offset voltage either with hardware or software. 30 5. CONCLUSIONS AND SUGGESTIONS A set of transducers for measuring current and voltage waveforms on an e l e c t r i c a l machine set was designed, constructed, and tested. The results of this design have been presented and i t i s concluded that the hardware developed i s useful as a part of a data acquisition and processing system for the e l e c t r i c a l machines laboratory. A complete system i s proposed for use with the transducers developed. P a r t i a l tests were made on t h i s system. The results indicate that the system design proposed i s workable. Future development of this equipment w i l l require additional hardware and software before p r a c t i c a l computer-assisted laboratory i n s t r u c t i o n can commence. For hardware, i t i s recommended that additional ranges for input to the transducers be provided. In p a r t i c u l a r , the addition of lower voltage ranges to the voltage transducer w i l l require an improved d i f f e r e n t i a l l e v e l amplifier such as shown i n figure 5.1. As w e l l , transducers to measure shaft angle, v e l o c i t y , and torque might be added. Overload protection for the current transducers must be investigated. Though the voltage trans- ducers may be e a s i l y protected using back-to-back zener diodes at the inputs of the operational amplifiers, interruption of an overcurrent i s more d i f - f i c u l t . The use of fast-acting fuses designed for SCR protection i s a possible solution. In addition, low-pass f i l t e r s with lower cutoff frequencies than the ones now being used on the transducer inputs may be required. If the transducers are to be used with a PDP-8/L or NOVA computer, hardware such as an additional 4K of memory, an external bulk storage device such as a d i s c , and modifications to the multiplexer on the analog interface to handle 16 channels with i n d i v i d u a l sample-and-hold units w i l l be desirable. Alter n - a t i v e l y , use of the PDP-9 computer w i l l require the i n s t a l l a t i o n of 16 lines. 31 A SUPERIOR DIFFERENTIAL AMPLIFIER : FIGURE 5.1 : c 32 from the e l e c t r i c a l machines laboratory to the hybrid computer laboratory. Eventually, a display or x-y p l o t t e r would be us e f u l for the display of grap h i c a l information. To provide a u s e f u l working system, an e f f i c i e n t software package i s e s s e n t i a l . Software i s a v a i l a b l e f o r data a c q u i s i t i o n and processing 3 7 using the hybrid i n t e r f a c e and PDP-9. ' However, a d d i t i o n a l software w i l l be required, even i f these routines can be used, to c a l c u l a t e the proper values required for the experiment. Eventually, a dedicated executive routine may be required to permit time-sharing of a number of machines and to sequence the various routines i n the proper order. 33 ' APPENDIX A CURRENT TRANSDUCER AMPLIFIER DESIGN The output voltage, e^, from a d i f f e r e n t i a l a m p l i f i e r of the 9 c o n f i g u r a t i o n shown i n f i g u r e 3.5 i s R • s where R^ i s the feedback r e s i s t a n c e , R g i s the input r e s i s t a n c e , and e-^_e2 i s the input d i f f e r e n t i a l voltage. For t h i s design, Rf/R g w a s s e t a t 50 to boost the 1200 mV.F.S. output from the H a l l m u l t i p l i e r up to ±10 V.F.S. Several non-ideal aspects of the operational a m p l i f i e r must be considered to avoid e r r o r s . F i r s t of a l l , an o f f s e t i n the output voltage may e x i s t due to the input o f f s e t voltage, the input o f f s e t current, and the input bias current. If a balanced c i r c u i t i s used, the input bias current e f f e c t s w i l l tend to cancel each other out so that the output o f f - 9 set voltage i s e s s e n t i a l l y AV m i r r = [1 + R./R ] V + R.I OUT f s os f os where V i s the input o f f s e t voltage and I i s the input o f f s e t current. OS OS To reduce AV^^, R g was set at a f a i r l y low value, 49.9ft., which does not attenuate the H a l l m u l t i p l i e r output s u b s t a n t i a l l y . At 25°C, AV^TirT1 r r J OUT max. was c a l c u l a t e d to be 0.306 v o l t s . This voltage may be nu l l e d using the zero n u l l feature of the operational a m p l i f i e r . Over the temperature range, 0° to 70°C, AV should not exceed 0.383 v o l t s . Hence, the ° ' OUT max. net maximum o f f s e t voltage a f t e r n u l l i n g i s approximately 0.076 v o l t s (error = 0.76% F.S.). Though th i s c a l c u l a t i o n i s rather crude, the actu a l implementation has v e r i f i e d that the o f f s e t voltage i s not a serious problem. 34 S i n c e t h e H a l l m u l t i p l i e r i s r e s i s t i v e i n n a t u r e , t h e e x c i t a t i o n c u r r e n t g e n e r a t e s a common-mode v o l t a g e of a p p r o x i m a t e l y 0.5 v o l t s a t t h e o u t p u t t e r m i n a l s . To check t h e e r r o r due t o t h e common-mode v o l t a g e , t h e common-mode r e j e c t i o n r a t i o o f t h e d i f f e r e n t i a l a m p l i f i e r must be c o n s i d e r e d . The o v e r a l l common-mode r e j e c t i o n r a t i o f o r t h e r e s i s t i v e n e t w o r k and g o p e r a t i o n a l a m p l i f i e r c o n f i g u r a t i o n of f i g u r e 3.5 i s CMRR t + CMRR CMRR c op amp where t h e common-mode r e j e c t i o n r a t i o o f t h e r e s i s t i v e n e t w o r k i s g i v e n by 1 + R / R CMRR = - c . 4a where a r e p r e s e n t s t h e maximum f r a c t i o n a l d e v i a t i o n i n r e s i s t a n c e . U s i n g 1% t o l e r a n c e r e s i s t o r s , t h e o v e r a l l common-mode r e j e c t i o n r a t i o i s a p p r o x - i m a t e l y 1275 y i e l d i n g an o u t p u t e r r o r o f a p p r o x i m a t e l y 0.196% F.S. A t e m p e r a t u r e c o e f f i c i e n t s p e c i f i c a t i o n o f -50 ppm./°C. l i m i t s r e s i s t a n c e v a r i a t i o n s t o ±0.25% f r o m 0° t o 50°C. w h i c h i s more t h a n a d e q u a t e . 35 APPENDIX B VOLTAGE TRANSDUCER AMPLIFIER DESIGN The design of t h i s a m p l i f i e r i s s i m i l a r to that used f or the current transducers. The c i r c u i t designed i s shown i n f i g u r e 3.6. This c i r c u i t may be analyzed by taking the Thevenin equivalent c i r c u i t at the output of the r e s i s t i v e d i v i d e r . The equivalent c i r c u i t i s balanced with an equivalent voltage source of value ^s (V. - V_) v o l t s A B R + h s and two equivalent resistances of value R R_ s B n _ R + R^ s B. This reduces the r e s i s t i v e network to the standard d i f f e r e n t i a l a m p l i f i e r form as i n f i g u r e 3.5. The output voltage i s equal to , - "' <Y- V • \ A value of R^/R^ equal to 1/50 was chosen for t h i s design. To provide an input resistance of several thousand ohms, R was chosen to be 100 Kft. and R^ to be 2 Kfi. To d i s s i p a t e the power l o s s , 7 watt p r e c i s i o n power r e s i s t o r s were used for V Since R^/R^ i s close to unity, the input o f f s e t voltage i s not ampli f i e d . A net maximum output o f f s e t error of 0.08% F.S. was ca l c u l a t e d f o r t h i s design. 36 The output error due to common-mode voltages at the input i s a serious problem. For a 500 v o l t common-mode voltage, an o v e r a l l common- mode r e j e c t i o n r a t i o of 100 i s required to l i m i t the output error due to these voltages to 1% F.S.- Since the o v e r a l l CMRR i s approximately equal to 1 + R,/R' 4a a must not exceed 0.005. Hence, resistances with a tolerance of ±0.5% were s p e c i f i e d for t h i s design. Also r e s i s t o r temperature c o e f f i c i e n t s of ±50 ppm./°C. were used. 37 APPENDIX C DESCRIPTION OF TESTS C l Transducers Only C . l . l Measurement Error The configuration shown i n f i g u r e C l was used to measure the input/output c h a r a c t e r i s t i c s of the transducers. Inputs were applied and measured and from t h i s the correct outputs were c a l c u l a t e d based on the desired transducer r a t i o . Next, the act u a l outputs were measured and the di f f e r e n c e s between the act u a l and calculated outputs were used to compute the measurement e r r o r s . This measurement also gives some i n d i c a t i o n of the l i n e a r i t y of the transducers since a number of inputs were used i n the t e s t s . Measuring Equipment: D.C. Voltage Source: Heathkit 0-400 V.D..C. reg. supply D.C. Current Source: Lambda 1-4 V.D.C reg. supply and p r e c i s i o n resistances A . C Voltage Source: a.c. l i n e (120 V.(r.m.s!)@ 60 Hz) Dropping Resistances: p r e c i s i o n 1(1-.0012) ft. .:i(l+.0013) ft. .05(1+.0014) ft. Voltmeter: Fluke 4^/2 d i g i t D i g i t a l Voltmeter accuracy: s p e c i f i e d to be <.05% F. S. measured to be 1. .10% F.S. Measurement Conditions: -bench environment -approx. 70°F. temperature -1 hour warm-up period Inputs Used: D.C: 420,300,200,100,50,0 v o l t s A.C : 120 V. (r.m.s.) voltage source voltmeter —a voltage transducer voltage source precision current sensing resistance^ . current » (/« limiter J voltmeier -o-a- V ) volt me ter V j voltmeter FIGURE C.I: MEASUREMENT ERROR TEST transducer voltmeter ciirrent transducer {V)voltmeter FIGURE C.2: OFFSET DRIFT TEST vol tage } source © © V) voltmeter vol (meter FIGURE C.3: CMRR MEASUREMENT 39 C.1.2 Offset D r i f t With no inputs, the output voltage was monitored f or a period of approximately three hours using the FLUKE d i g i t a l voltmeter (as shown i n f i g u r e C.2). The measurement conditions were the same as i n C . l . l . C.1.3 Common-Mode Rejection Ratio In t h i s case, test inputs of 250 v o l t s d.c. and 120 v o l t s a.c. (60 Hz.) were used as shown i n f i g u r e C.3. From t h i s , the common-mode gain of the voltage transducers was computed. The common-mode r e j e c t i o n r a t i o was c a l c u l a t e d using G CMRR = — , cm assuming G^ equal to 1/50. In th i s set of measurements, the FLUKE d i g i t a l voltmeter was used to measure the various voltages and the test conditions were i d e n t i c a l to those of C . l . l . C.1.4 Frequency Response Inputs from a WAVETEK s i g n a l generator were used i n t h i s t e s t as i l l u s t r a t e d i n f i g u r e C.4. The gain of the transducers was estimated using a TEKTRONICS TYPE 581 o s c i l l o s c o p e . The cutoff frequency was considered to be the lowest frequency at which the gain was reduced by 3 dB. The values obtained are only approximate. C.2 System Test The transducers'developed were connected to the hybrid interface and PDP-9 computer i n the hybrid computer laboratory as shown i n f i g u r e C.5. Constant voltages and currents were used as tes t inputs and were measured using the FLUKE d i g i t a l voltmeter. The PDP-9 computer was programmed as 40 voltage source voltage ) source voltage transducer current transdi rcer oscilloscope FIGURE CA: FREQ. RESPONSE" TEST voltage •/ source * voltmeter precision resistance voltage source voltmeter FIGURE C.5 : SYSTEM TEST 41 shown i n the flow chart of f i g u r e C.6 and the program l i s t i n g of f i g u r e C.7. In a d d i t i o n , measurements were made on the hybrid i n t e r f a c e i t s e l f as shown i n f i g u r e C.8 to determine i t s input/output c h a r a c t e r i s t i c s . From t h i s , the gain of the i n t e r f a c e was computed assuming l i n e a r i t y and the measure- ment errors were ca l c u l a t e d using t h i s r a t i o . . Test Conditions: - approx. 66 F. - hybrid computer laboratory environment - gain of in t e r f a c e measured and used to compute measurement error - sampling period approx. 101 usee./channel - error calculated from worst error committed i n 10 successive samples. Inputs: D.C. Voltage: Heathkit 0-400 V.D.C. reg. supply; @ 420.4, 350.4, 300.4, 230.5, 160.5, 109.9, 20.18 v o l t s D.C. Current: Lambda 1-4 V.D.C. reg. supply; p r e c i s i o n dropping re s i s t a n c e s : 1(1-.0012), 0.1(1 + .0013), 0.05(1 + .0014)fi. @ 38.19, 33.31, 24.98, 20.02, 15.00, 10.92, 2.50 amperes CLfAR A/D B l F F E X RBGlSTKK | S E T Sg^H T o T^ ICK M Q O £ | PAUSE 1 j S er s^rJ T O r)o<-p MQPe^j I |f/JiT7ALfZE A/O UJA/T LOOP j t L O A D M O X C(+AAJAJ£L POO. /AOrti M U X A D D R E S S gee. START A/D CO«JO£KSIO/J ;O/SABCE |fAje^emeAJr mo* C^AO/OEL. MO.[[ FiOW c/Mr FIGURE C6 43 Figure C.7: DATA ACQUISITION PROGRAM LISTING 10 303 100 110 115 120 122 130 131 701104 140200 200200 740001 701607 740000 740000 200200 701607 200201 040300 200200 040301 200301 701407 701306 200202 040302 440302 600122 701117 060010 440301 440300 600115 600100 / CLEAR S I I # 11 (A/D BUFFER REG.) / DEPOSIT A ZERO IN 200 / LAC 200 / COMPLEMENT AC / LOAD S & H VIA SOI* 16 to TRACK / NOP / NOP / LAC 200 / LOAD S & / LAC 201 / DAC 300 / LAC 200 /DAC 301 : / LAC 301 / LOAD MUX ADDRESS DIRECTLY / START A/D CONV.; DISABLE SFC / LAC 202 : (202) = 777746 / DAC 302 : (302) = A/D COUNT / ISZ 302 / JMP . _ 1 / CLA-E.T.* 1, LOADBUFF, JAM AC / CLEAR BUFF /// DAC* 10 ./ ISZ 301 (MUXADD) / ISZ 300 (COUNT) / JMP 115 / JMP 100 H VIA SOI # 16 to HOLD (201) = 777760 (300) = COUNT (200) = 000000 (301) = MUXADD 200 201 202 000000 777760 777746 / -y COUNT / -> A/D COUNT 300 301 302 000000 000000 000000 44 7 7 volt Age souice 0 voltmeter UO.O No.l NO.N o— Hybrid Interface PDP-9 INTERFACE CHARACTERISTICS MEASUREMENTS: FIGURE C8 : 45 REFERENCES 1. K a b r i e l , B. J . , p r i v a t e communication, Power Group, Dept. of E l e c t . Engrg., U. of B r i t i s h Columbia, Vancouver. 2. Bekey, G. A. and Karplus, W. J . , Hybrid Computation, Wiley and Sons, New York, N. Y., U.S.A., 1968, 125-127. 3. H a s l i n , S., "An On-line Computer System f o r Processing Experimental Waveforms", Summer Essay, Dept. of E l e c t . Engrg., U. of B r i t i s h Columbia, Vancouver, 1968. 4. D i g i t a l Equipment Corp., Advanced Software System Monitors, D.E.C., Maynard, Mass., 1968, Chapter 5. 5. Rothman, James E., "Fast Fourier Transform Subroutine", Program L i b r a r y Catalog, DECUS., Maynard, Mass., 1969, page 16-Y. 6. Birman, P., Kepco Power Supply Handbook, Kepco Inc., Flushing, N. Y., 1967. 7. Crawley, B., M.A.Sc. Thesis, Dept. of E l e c t . Engrg., U. of B r i t i s h Columbia, Vancouver, 1969. 8. Burr-Brown Research Corp., Handbook and Catalog of Operational A m p l i f i e r s : LI-227, 1969. 9. F a i r c h i l d Semiconductor Inc., Linear Integrated C i r c u i t Handbook, Mountain View, C a l i f . , 1967.

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