A DEVICE FOR POSITION MEASUREMENT

Abstract: A device for position measurement of several measurement locations, e.g angular position of synchros/resolvers, and with time multiplexing between the different input channels. Two coating multiplexers (1, 2) have their respective outputs connected to a subtraction unit (5), which communicates with a position calculating microprocessor (8) via a demodulator (6) and an analogue/digital converter (7). The microprocessor communicates with the subtraction unit (5) via data bus (9).

Description: A DEVICE FOR POSITION MEASUREMENT The present invention relates to a device for position measure¬ ment or angular measurement of the kind indicated in the pre¬ amble of the attached patent claims. Synchros are used as measuring devices for measuring angles, inter alia for remote read-off (i.e. electrical transfer of angle from the measured location to the indicating instrument, transfer of moments, usually small, and remote control of servo mechanisms. When the angular measurement is desired to be converted into digital information, there is used the resolver which is a particular type of synchro. The resolver consists of two sta- tor windings which are shifted by 0o and one or two rotor windings. , ^ The resolver is supplied with A.C. voltage (e.g. 400 Hz) and this can be carried out in one of two ways : a. Feeding the two stator windings with E.sinOJt and E.cosωt, respectively. There is then induced in the rotor windings: k.E. (sin cυt . sinfc + cos ) t.cos 6 ) = k.E . sin ( ∞ t - θ ) , k being a constant, i.e. phase modulated signal with constant amplitude. The phase modulated signal may be converted into digital form by means of a demodulator. b. Feeding the rotor winding with E.sin^t. There is then in the sine winding induced: k.E sintot . ■ sin 8 , and in the cosine winding: k.E sinωt . cos θ . This latter method is the most frequently used one today -and conversion into digital signals. Use of resolvers for angular measurement competes in digital systems with code disks giving direct digital angular read-off. Parallel to the development of the code disks, the resolver-di ital-converters have developed in such a direction that the re¬ solvers still are to be preferred where great resolution (e.g. greater than 10 bits per revolution) is necessary. Most of the available resolver-digital converters are based on the resolver making use of solution b above, and the most freq¬ uently used converter is the so-called "tracking"-converter. The operation thereof is as following: " The analogous sine and cosine signals from the resolver are multiplied by cosine and sine, respectively, to the contents of an up/down counter. The output signals from the multipliers are subtracted and a modula ted signal is derived representing sine of the difference bet¬ ween the turning angle of the resolver and the contents of the counter. The difference signal is demodulated and through a voltage controlled oscillator provides counting pulses to the counter so that the loop is closed and the ^ contents of the counter fol¬ lows the resolver angle. Tracking converters provide high static accuracy, are insensitive to frequency, phase-and ampli¬ tude variations, as well as high frequency noise on the resolver signals provides good tracking of rapidly rotating resolver (NB velocity τ constant) , and enables direct angular read-off. Another widely used construction is the so-called Successive- approximation-converter. It operates almost like the tracking converter, but " instead of continuous read-off of the resolver signals, these are sampled once or twice per period. A DC vol¬ tage signal representing sine of β - 0 is formed by a sine and cosine multiplication and subtraction and said signal is ap¬ plied to a quick operating analogue/digital converter which includes a Successive-approximation register.. Successive- approximation converters enable rapid conversion, . τ hich permits relative rapid changes of the resolver speed, and direct angula read-off. A more recent construction converting several channels is the mi ¬ cro-processor-based system M 7200 (Micro Networks) . The opera ¬ tion (for one channel) is as following: Sine and cosine signals are demodulated (AC/DC) and converted into digital form separat¬ ely. The micro-processor thereby receives sine and cosine of the resolver angle. In the processor there is calculated tanθ = cos ^ Q— and there- after Θ is derived by means of an arc tan table and by inter¬ polation. Said system is somewhat expensive, since there is required one AC/DC converter for each channel. In addition very small speed of the resolver (10°/sec.) is acceptable before ac¬ curacy is reduced. The said system MN2700 provides great static accuracy, is insensitive to noise and is multiplexed for several channels. The necessity for position measurement on several axes on e.g a robot arm, where the requirements to accuracy, speed and low cost are great, has made it necessary to develop a new system where the advantages of the different said constructions are used. The device according to the ' invention is characterized by the features appearing from the attached patent claims, and from the description hereinafter with reference to the accompaning drawings. Fig. 1 is a block diagram of the device according to the in¬ vention. • Fig. 2 is a ' solid-state " control transformer which may constitu¬ te the subtraction unit incorporated in the device. Fig. 3 is a demodulator incorporated in the device. Fig. 4 illustrates as an example output signals from the cont¬ rol transformer during a measuring cycle. Fig. 5 is an alternative solution of the subtraction unit. In fig. 1 there is shown input multiplexers 1, 2, for the input 3 and 4, respectively. The input 3 has in the example the form sin Θ . sinύϋt and 4 has in the example the form cos 5. sin"Jt, where 6 is the turning angle of the resolver and j ±.s the angula frequency of the supplied signal to the resolver. The multi¬ plexers are fed with a multiplex address MPX.ADR in order to be able to alternate between a number of channels, eight in the chosen example. There may of course be more or less, dependent on the number of resolvers to be monitored. A subtraction unit 5 (SUB) has its two inputs connected to the respective multi¬ plexers 1, 2. The subtraction unit 5 may be a so-called solid state control transformer corresponding to a part of a tracking-converter, viz. quadrant selector 11, multipliers 12 and 13 for cosine and sine of , respectively, and difference amplifier 14, see fig. 2. "~ The said multipliers are special sineand cosine multipliers. Alternatively, these may be conventional linear multiplying digital- to analogue converters 18, 19 which must be fed with sine^and cosine 0 , respectively, through latches 20, 21 which are connected to a data -bus and.thereby the micro-processor. In this case, it is the micro-processor which has to calculate sine φ and cosine$.5. When the micro-processor 8 estimates correct angular position, the theoretical error signal from the difference amplifier should be equal to zero. The output from the subtraction unit 5 is connected to a de¬ modulator 6, which is shownin further detail in fig. 3. The demodulator consists of a commutator based on electronic switches 17, an ' : operational amplifier 15, and an integrator 16 which may be zero set. The output from the demodulator 6 is connected to an analogue/ digital converter 7, the output of which feeds a micro-processor 8 with data representing sin (Q-p) of e.g. format 12 bits. The system may according to a preferred embodiment operate with up to 8 resolvers. Each of these are connected during a comp¬ lete period of the carrier frequency by means of the input multi¬ plexers. The carrier frequency is e.g. 1 kHz. The subtraction unit 5 operates thereby one millisecond for each resolver. The digital input to subtraction unit also obtains a new value for each period, such that the output signal from the subtraction circuit may look like that shown in fig. 4. If we look on the conversion for one channel, the operation is as follows. The sampling points are indicated by (k-1) , (k) , (k+1) as the former, the present and the next sample, respectively. The angle of the resolver is θ and the estimated angle of the micro processor is 0. After a period (1 ms) where the estimated value is 0 (k-1) , sin£ - 0 (k-l)J is measured, denoted as The micro processor 8 calculates 0(k) = f £ø " (k-l}] , sin[Q -0) (k and the said function may in its simplest form be summation, since sine of small angles is approximately equal to the angle itself. 0 (k) will represent the resolver angle (in digital form) when the system has "moved into position". Thus, the micro-processor provides a continuous calculation of assumed correctposition based on the regular data which are supplied. An output unit 10 is connected to micro-processor 8, from which unit the correct positional data may be further used. Absolute measurement accuracy is limited by the subtraction unit, whereas the measurement resolution is not limited there¬ of, since the error signal sin -φ) is an analogue signal. The subtraction unit may e.g. have 14 bits. Digital input and the A/D-converter for ' the error signal may provide 12 bits. Dependent on the amplification in the demodulator, these 12 bits may represent the angular difference with high or snail re¬ solution. The limiting factor in this respect is the maximum speed of the resolver relative to the sampling frequency. Hig resolution is very important for stabilization of the servo sy tems. In the present system the resolution is e.g. one quarte of the resolution of the data word which the subtraction unit fed with (here: 0 ) . The system can compensate for velocity errors in the resolver and eliminate tracking errors at relative large velocities of the resolver. This is based on the forming of the said functi 0 (k). Dynamic accuracy is limited by the number of resolvers which a connected, but time delays in the measurement of each single r solver due to the multiplexing does not influence the measure-- ent. This is due to the fact that sin {Q-0) (k) is measured during the last period (of the carrier frequency) before calcu lations of (k) are made, even though^ (k-1) is e.g. "10 perio old". The immunity to noise from the tracking converter is almost maintained r since the measurement takes place as an average over the complete period of the signals. The system is cheap inter alia because, there is required one subtraction unit (price approximately equal that of a tracking converter) up to 8 channels, the A/D-converter capacity is onl used in an amount of 2.5%, -enabling other analogue measuremen to be made (e.g. accelleration, power, velocity, pressure, temp rature,position transmitters (DC) or the like) , the micro-pro¬ cessor may be the same which controls e.g. the servo system. In addition there may be used cheap conventional components. O 4 A ,

Claims: C l a i m s 1. A device for position measurement in servo controlled mecha¬ nisms, e.g. manipulators, industri robots, where the angular position of synchros or resolvers at several measurements locat¬ ions are converted into digital signals by means of time multi¬ plexing, characterized in that two coacting multiplexers (1, 2) have their respective outputs connected to a subtraction module (5) , which communicates with a position calculating micro-pro¬ cessors (8) via a demodulator (6) and an analogue-digital conver¬ ter (7) , the micro-processor (8) communicating with said sub¬ traction module (5) via a data bus (9) , that the subtraction unit (5) is a so-called "solid state control transformer" in the form of an electronic circuit measuring the difference between two angles, the resolver angle (θ) and the angle .φ) provided in digital form from the micro-processor (8) , respectively, and that the control transformer either is constituted by a quad¬ rant selector (11) , the two outputs of which are respectively connected to two multiplying units (12, 13) , which in turn are connected to the two inputs of a difference amplifier (14) , each of said multiplying units (12, 13) having a further input con¬ nected to said data bus -(9), or two linear digital-to analogue converters (18, 19) for the sine- and cosine signals, respecti¬ vely, the digital inputs of which are connected to a respec¬ tive latch cirquit (20, 21) being alternately fed with data from the micro-processor (8) and where these data represent sine and cosine, respectively, of the angle (0) calculated by the micro-processor. 2. A device as claimed in claim 1, characterized in that the demodulator (6) is constituted by commutating switches, prefer- rably electronic switches (17) connected to an integrator (16) via an amplifier (15) . ■ BU HE ALT OMPI , tfNATlO

Inventors: KJOSAVIK D
Applicants: KJOSAVIK D, TRALLFA NILS UNDERHAUG AS
Application year: 1981
Main classification: G01B7
IPC codes: B25J19, G01B7, G08C15, G08C19, H03M1