1 2
PHASE SENSITIVE DEMODULATOR WHICH
AUTOMATICALLY COMPENSATES FOR FIGURE DESCRIPTIONS
VARIATIONS IN THE AC REFERENCE VOLTAGE FIG. 1 is a schematic diagram of a preferred embodi
BACKGROUND OF THE INVENTION , me"<°f demof lat°r; K , f , . A
5 FIG. 2 is a waveform diagram helpful in understand
This invention relates to phase sensitive voltage de- ing the operation of the circuit of FIG. 1. modulators in general and more particularly to a demodulator which automatically compensates for varia- DETAILED DESCRIPTION OF THE PREFERRED tions in line voltage. In general a phase sensitive de- EMBODIMENT modulator is a device which will provide a DC voltage 10 The. demodulator circuit is shown on FIG. 1. A reoutput having a magnitude which is proportional to the solver 11 is included as an example of an input device, magnitude of an input AC voltage and a polarity depen- The resolver will be excited by a AC reference voltage dent on the phase of that input with respect to a refer- across coil 13 which, for example, will be mounted on ence AC voltage. Demodulators are commonly used the resolver shaft. As the coil 13 rotates the voltages with devices such as resolvers in which the voltage and 15 from coils ls and 1?5 which wiu be fixed in the stator> phase of the output are related to a mechanical posi- wjn respectively provide outputs proportional to the tion. Typically in a resolver two outputs are provided, sjne and cosine of the shaft angle one an AC voltage proportional to the sine of the shaft Operation of the demodulator without the automatic angle and the other an AC voltage proportional to the compensation will first be described. Assume that the cosine of the shaft angle. The sign of the output will be 20 emitters of transistors 21 and 23 are connected to determined by the phase of the AC voltage with respect ground buss 25- Each transistor has its base connected to a reference, e.g., in phase will be positive and out of through a resistor 26 to one side of the secondary a phase will be negative. transformer 27 which has as its primary input the referIn most applications m which the sine and cosine ence voUage The collectors of transistors 21 and 23 voltages are to be used they must first be converted to 25 ^ ^ ... throu h a ... 29 and a resistQr DC voltages. This is the function of the demodulator. 31 tQ a itiye y0, , The ... of ^ Most prior art demodulators have simply rectified the second of transformer 27 is connected to ground AC voltage and provided means to sense the phase re- buss 25> ,fa ^ arr a ernent transistor 21 will have a lat.onship and provide a positive or negative DC output vb, Qn itg base fm on& ha,f ^ reference accordinslv
„, . ' , , , • , ■ . , . ■ cycle, e.g., the positive half, during which time it will
The major drawback to a simple demodulator of this ,' . .. ~~ •„ , ' i*
. . iU Ji . .. . iU *, . ,. , conduct and transistor 23 will have a positive voltage
sort is that variations in the reference or line voltage , . . ^ u ir i J • I.- i_
•li u « * J »u L * *u \ * ir .1. i on lts oase during the other half cycle, during which it
will be reflected through to the output. If the resolver ... , . „ &, -■ , „ . ,' ■ . ,
. c . . »u *u * i. Mi wl" conduct. Each of these transistors has associated
has a reference input of Efi then the output voltages will . ^, . . - _ .• . , lJe 35 wltn it another transistor. Transistor 33 has its base
connected between resistors 29 and 31 in transistor KE/i SIN 0 21 's collector circuit and transistor 35 is similarly con
and nected into the collector circuit of transistor 23. Each
of transistors 33 and 35 has its emitter connected to a KEfl COS 0, 40 positive voltage and its collector connected through a
where 6 is the resolver shaft angle. resistor 37 to a negative voltage.
If E« varies, the outputs will also vary. This problem Thus> when transistor 21 is on during its half of the may be solved by constructing a highly regulated refer- cycle> transistor 33 will also be on. and will have a posience ER. This, however, is a costly solution. Problems tive voltage at its collector output. Similarly, when of providing low harmonic distortion, low output impe- 45 transistor 23 is on, transistor 35 will provide a positive dance and reliability are also involved. In control sys- voltage output. When off, each of transistors 33 and 35 terns where the demodulator is used in a closed loop it will have a negative voltage output, is also possible to avoid this problem by using ratios of These voltages are used to control transistors 39, 40, output to input voltage. But where the demodulator is 41» 42, 43 and 44. Transistors 39 and 40 operate in required to provide digital or analog outputs for use in 50 conjunction with the sine output of resolver 11 and will a device such as a computer, such ratio techniques can- be explained in detail. Transistors 41 and 42 perform not be used. In these cases a voltage output which bears a similar function for the cosine output and transistors the same relationship to the quantity which it repre- 43 and 44 for the reference output to be described besents, e.g., sine and cos of shaft position, for all values low. These transistors are shown as Field Effect Tranof reference voltage is required. 55 sistors (FETS). However, other types of high speed
switching devices may also be used. SUMMARY OF THE INVENTION xhe one side of the sine output is provjded through
The present circuit provides a phase sensitive demod- resistors 45 and 47 to the input of an operational ampli
ulator which automatically provides compensation for fier 49. The other side is provided through resistors 53
variations in line voltage. The AC reference voltage is and 55 to the same input of amplifier 49. At the junc
sensed, rectified and summed with a DC reference volt- tion of resistors 45 and 47 a connection is made to the
age representing a nominal reference value. Variations source of FET 39. The junction of resistors 53 and 55
of the AC reference from the nominal value result in an is connected to the source of FET 40. Each of the output which is proportional to the difference between 65 FETS 39-44 has its gate connected to the junction of
the two. This output is then used to control the demod- a diode 57 and resistor 59 in series between the output
ulator causing greater or lessor portions of the input to of transistor 33 or 35 and the ground buss 25, and its
be rectified. drain connected to buss 25.
