METHOD AND DEVICE FOR MEASURING ANGULAR DISPLACEMENTS

Abstract: In the device an unknown angular displacement of a mobile member (3) with respect to a fixed member (2) is calculated by passing a constant speed index (7) before a pair of sensors respectively integral to said fixed and mobile members. The sensor output signals are used to open a reference and anknown time intervals, wherein the number of pulses coming from a known frequency source is counted. The number of these pulses is therefore proportional to the known reference angle and to the unknown angular displacement and it is easy to obtain from these values the measure of the unknown angular displacement. The device is particularly suitable to be installed as transducer in a balance particularly of the pendulum-weight or counterweight type, in view of its low cost, not complex structure and high precision.

Description: Method and device for measuring angular displacements. TECHNICAL FIELD The present invention relates to a device for measuring angular displacement of a mobile member with respect to a fixed member. The device is especially fit for use in the field of balances and in particular of the pendular weiσht on counter-weight balances. The invention also relates to a method for carrying out the abo- ve measurement. BACKGROUND ART Various transducer are known in the art for measuring an unknown angle. They can be generally grouped in two main families, those so-called " conventional optical type " and those so-called "phy¬ sical parameter type ". According to the first type transducer, the unknown angle is mea¬ sured by rotating a glass disc, having a plurality of equally spaced, transparent and opaque zones along its periphery, :of an angle equal or proportional to the unknown angle. These zones can respectively let pass or intercept a light beam and therefo¬ re can easily detected and counted by a photodicdedevice that tran sduces the number of detected zones into a suitable electric si- gnal corresponding to the angle to be measured. Devices of the type mentioned above are extensively used at the present time, but their cost is rapidly increasing with the increase of their definition.According to the second type of transducer, on vary¬ ing the angular position between a fixed and a mobile member a corresponding variation of a specific physical parameter occurs, such as capacitance, inductance, resistance, magnetism, that is generally converted into a voltage signal..These devices are re¬ latively unexpensive and of high resolution ; however, they are highly sensitive to room condition variations which in practice- limit their use possibilities. From the foregoing it is apparent that it would be helpful to make available a device for increa¬ sing an angular displacement, combining low cost with high reso- lution and independance from external physical parameters. Therefore, it is an object of the present invention to provide a devide for measuring an angular displacement, in particular for balances, characterized by low cost and high preci¬ sion regardless of external factors that might be prejudicial to its resolution. Another purpose of the present invention is to provide a device of the above mentioned type with no friction between its compo¬ nents, this feature being very important in view of a precise mea- sure. DISCLOSURE OF THE INVENTION According to the invention there is provided a device for measu¬ ring an angular displacement of a mobile member with respect to fixed member, characterized in that it comprises means for generating a first and a second signal having duration proportio¬ nal to said unknown angular displacement and respectively, to a predetermined reference angle ; means for generating correspon¬ ding numerical values from said first and second signal ; means for processing said numerical values and for obtaining the mea¬ sure of said angular displacement from them. Preferably, the means for generating a first and second signal comprises first light emitting means and first photodetecting means integral with said fixed member, as well as second light emitting means and second photo detecting means integral with said mobile member. A constant speed rotating member provided with a plurality of " slits for passing between said first emitting and first photodetecting means and between said second emitting_ and second photodetecting means is provided thus generating si¬ gnals once a slit passes in correspondance with said first and second photodetecting means. Said signals are received by logic means which generates said first signal having duration propor- tional to the unknown angular displacement and said second si¬ gnal having duration proportional to the predetermined reference angle. It is a further purpose of the present invention to provide a method for measuring an unknown angular displacement of a mobile member with respect to a fixed member, characterized by the fol¬ lowing steps ' : - generating a first signal having a duration proportional to the unknown angular displacement and a second signal having dura- tion proportional to a known reference angle ; - using said signals to enable a first and a second counter to count pulses of fixed, predetermined frequency sent to said coun¬ ters, thereby a counting value proportional to the duration of the corresponding signals is obtained ; - obtaining from said counting values a measure of the unknown angular displacement. Further characteristics and advantages of the device and of the method according to the invention will be more apparent from the following description of an exemplifying, not limiting embodiment, made with reference to the attached drawings. BRIEF DESCRIPTION OF DRAWINGS In the drawings : Figure 1 is an elevational side view of the mechanical assera bly of the device according to the invention ; Figure 2 is a front view of the mechanical assembly of the device according to the present invention ? • Figure 3 is a top plan view of the mechanical assembly of - the device according to the present invention ; Figure 4 is a block diagram of the device according to the present invention ; and Figure 5 is a detailed representation of some blocks shown in Figure 4. BEST MODE FOR CARRYING OUT THE INVENTION With reference to Figure 2,there is shewn a fixed rnember 2 supported by a fixed frame 5. andamobile member 3 that is displaceable of an angle lower than 90° with respect to its rest position, indicated at A ; said angle is generally referred to as Mobile member 3, as shown in Figure 1, is integral with a shaft 1, the angular position of which is unknown. Fixed member 2 and mo- bile member 3 are both constituted by a photodiode and a photo- detector and, as more clearly shown in Figures 1 and 3, an opa¬ que disc 7,. on which two diametrally opposed, radial slits 5 are formed, is interposed between said photodiode-photodetector assem bly. in Figure 1 there are also shown electric terminals 4 of the photodiode and the photodetector of the mobile member 3, as well as in Figure 3 there are shown electric terminals 8 of the photo¬ diode and photodetector of the fixed member 2. Disc 7,that in the present embodiment of the invention is coaxial to shaft l,is dri- ven in rotation by a motor 9 in the direction of arrow F shown in Figure 2. With reference to Figure 4, the signals generated by the photo¬ diode of the mobile member 3 and by the photodiode of the fixed member 2 and received by the corresponding photodetectors y when slits 5 pass through mobile member 3 and fixed member 2, are sepa¬ rately fed to block 11. The output of block 11 are two pulse train signals, the pulses being synchronous with the slit posi- 3- tiσning before the relevant photodetector. Both pulse train signals are sent to logic block 12 the output of which are two double .level signals. These signals can be considered as enabling signals for the counters contained in the subsequent block 13. In particular, when the signal is at its high level, the counter 5 is enable to count the pulses coming from a quartz oscillator schematically shown with block 14 in Figure 4. It has to be poin¬ ted out that quartz oscillator 14 also controls block 12, in such a way that both blocks 12 and 13 operates in synchronism. O As already said, block 13 is formed by two independent counters and the values present in said counters can be sent to a micro¬ processor 15, that is connected to both counters of block 13 in any known way. Microprocessor 15, also controls the logic of block 12 and drives motor 9 of disc 7 through block 16 in a known way. In this way the operation of the assembly is fully synchronous and the angular velocity of disc 7 is controlled by means of a feedback system in any known way by microprocessor 15. ° With reference to Figure 5, the signal coming from the photode¬ tector integral with fixed member 2 is fed through line 41 to the inverting input of the operational amplifier 43. Likewise, the signal coming from the photodetector integral with mobile member 3 is fed through line 42 to the inverting input of the operational amplifier 44. Both non-inverting inputs of operatio¬ nal amplifiers 43 and 44 are connected to a reference voltage made variable through rneostat 40. In this way, operational am¬ plifiers 43 and 44 are mounted as comparators and their output state will vary according to the voltage present at the inver- 0 ting input. Normally the comparator output is at high level,but when slit 5 is interposed between the photodiode and the photo¬ detector, the latter will produce a voltage and the comparators ■ output will go to low logic level and immediately back to high -logic level as soon as slit 5 has overcome the photodetector.The output of comparators 43 and 44 is inverted by corresponding NOT gates 4.5 and 46. NOT gate 45 output is connected to AND gate 47, 49 and 50, while NOT gate 46 output is connected to AND gate 48. in order to understand how the circuit operates, assume that the flip-flop JK 51,52,53 and 54 have been all reset and therefore their outputs Q are all at low logic level ; thus, output Q. of flip-flop 52 is at high logic level and activates AND gate 47, thereby the positive value coming from NOT gate 45 is allowed to proceed toward input J of flip-flop 51. As input K of flip-flop 51 is connected to the output Q of flip-flop 52, said flip-flop will change its state when receiving the timing input coming from the quartz oscillator 14 and will enable, through line 55, the counter 13A of block 13, to count the pulses coming from the quartz oscillator 14. The change of state of flip-flop 51 has a further consequence of activating AND gate 48 ; therefore when a positive value is present at the output of NOT gate 46, it passes toward the output J of flip-flop 52, and,as the output K is αrounded, will change the state of this flip-flop and, as a consequence, will switch flip-flop 51 again toward the low logic level. So, on line 55 there is a high level signal the duration of which is directly proportional to the time elapsed between the passage of the same slit before the photodetector integral with the fixed member and the subsequent passage before the photodetector integral with the mobile member. As a consequence, the number of counted pulses will be proportio¬ nal to said time duration and, as all the system is synchronous, will be proportional to the unknown angle to be measured. In par- ticular, having regard to the mechanical arrangement shown in Figures 1,' 2 and 3, said angle will be equal to the unknown an¬ gle ~ plus a TT/2 angle, which corresponds to the basic angu¬ lar spacing between mobile member 3 and fixed member 2. For the same reason, the positive value coming from NOT door 45 • can also pass through AND door 49 and raise the output Q of flip-flop 53 to its high value, thus enablinσ counter 13B through line 56. After a 180° rotation of disc 7 and after counter 13B is enabled (which occurs simultaneously to that of counter 13A) the other slit passes before the fixed photodetector, the output Q of flip-flop 54 being raised to its high level, whereby / having regard to the configuration shown in Figure 4, the AND gate 49 is disactivated and AND gate 50 -is activated with a fur¬ ther new change of state of flip-flop 53 and the high-low tran- sition on output Q is produced so as to disactivate counter 13B. It is therefore apparent that a counting value proportional to the unknown angle ( <-? + IT/ 2 ) is present in counter 13A, while a counting value proportional, in this case, to IT is present in counter 13B. To this point microprocessor 15 can draw the values stored in the counters, reset all the flip- lop 51-54 and obtain from these values the angle to be measured. The above cycle can be repeated at a frequency that is essential- iy function of the switching time of the logic ; anyway such a measurement can be repeated at a frenquency high enough to assure the requested measure precision on a certain number of samples. On the other hand, due to the absence of any type of friction and of parts that might notably vary their characteristics in conse- quence of external causes, the single measure is affected only by the photodetector answer and by the constant speed of rotation of motor 9. By using actually available photodetectors, the measu¬ rement error is about 10 T e measuring device according to the present invention can be advantageously installed in a balance of the aoove-ciced type,con¬ necting shaft 1 to the mechanism supporting the plate on which the article to be weighed is placed. The plate displacement, • due to the weight of the article placed thereon, causes a cor¬ responding rotation of shaft 1 and a corresponding angular dis¬ placement of the mobile member 3 from its rest position. The signals generated when the continuously rotating disc 7 passes through fixed member 2 and mobile member 3 are processed as described above and the actual weight immediately visualized on a balance display. It is otherwise obvious that, besides the above cited use, the device according to the invention can be advantageously used in any other case in which the measurement of an angular displace¬ ment of a mobile member with respect to a fixed member is reques¬ ted. It has to be pointed out that variations to the device according to the invention are possible to modify some characteristics ;for instance, the number of slits 5 on the disc 7 can be increased, or, even,disc 7 can be mounred in a. non-coaxial relation with respect to shaft 1, to the end of increasing the resolution. Furthermore, the signal generation can be obtained, instead of by means of the above described optical system, which is prefer¬ red in view of its great simplicity and low cost, also by means of a rotary magnetic field. This system of course has a higher reliability due to the absence of mobile mechanical members,but has the disadvantages of a higher cost and of the need of discri¬ minating between point signals on a sinusoidal type response cur¬ ve ; in this case problems concerning the point stability and, therefore, the measure precision may be expected. The rotary magnetic field, of the circular type, or even not fully circular but anyway repetitive, can be generated by one of the known me¬ thods. A, magnetic sensor (pick-up coil, Hall sensor, etc.) or, prefera-^ a differential sensor (paired coils, Hall differential sensor, etc..) is placed in the magnetic field ; on the passage through a known point, a reference signal is generated that, if a dif¬ ferential signal is used, is a " zero " signal. Such a signal can be used in the same way as to the optical signals previous- ly described. It is clear that variations and/or modification can be made to the measuring device according to the invention, without depar- . ting from the scope of the invention itself.

Claims: , CLAIMS 1 - Device for measuring an angular displacement of a mobile mem¬ ber with respect to a fixed member, characterized in that it comprises : means (7,11,12) for generating a first and a second signal having duration proportional to said unknown angular dis¬ placement and, respectively,to a predetermined reference angle ; means (13) for generating corresponding numerical values from said first and second signal ; means (15) for processing said numerical values and from obtaining the measure of said angular displacement from them. 2 - Device according to claim 1, wherein said means for genera¬ ting a first and a second signal comprises : - first light emitting means and first photodetecting means in- tegral with said fixed member (2) ; - second light emitting means and second photodetecting .means integral with said mobile member (3) - a constant speed rotating member (7) provided with a plurality of slits (5) formed in such a way to progressively pass between said first and second emitting means and the corresponding photo¬ detecting means, whereby said first and second photodetecting means each generates a signal once a slit (5) passes in correspon- dance with said first and second emitting means and correspon¬ ding photodetecting means ; - logic means (11, 12) for receiving said signals and generating said first signal having duration proportional to said unknown angular displacement and said second signal having duration pro¬ portional to said predetermined reference angle. 3 - Device according to claim 1, wherein said means for genera¬ ting respective numerical values from said first and second signal comprises oscillator means (14) for generating a known re¬ ference frequency signal and counting means (13A,13B) for coun- ting means for counting the periods of said oscillator means, said counting means being driven by said first or second si¬ gnal. 4 - Device according to claim 2, wherein said member provided with a plurality of slits is an opaque disc (7) rotated on its axis at a constant speed. 5 - Device according to claim 4, wherein two slits (5) are pro¬ vided at the ends of a diameter of said opaque disc (7). 6 - Device according to the claim 1, wherein said means for ge¬ nerating a first and a second signal comprises means for gene¬ rating a rotary magnetic field and sensing means placed in said field. 7 - Device according to the claim 1, wherein said mobile member is integral with a shaft (1) mechanically connected to a plate of a balance, so that a rotation of said shaft corresponds to a positive displacement of said plate due to the weight of an article placed thereon. 8 - Method for measuring an unknown angular displacement charac¬ terized by the fact to comprise the following steps : - generating a first signal having a duration proportional to the unknown angular displacement and a second signal having duration proportional to a known reference angle ; - using said signals to enable a first and a second counter to count pulses of fixed, predetermined frequency sent to said coun¬ ters,, thereby a counting value proportional to the duration of the corresponding signals is obtained ; - obtaining from said counting values a measure of the unknown angular displacement. ./.

Inventors: CHIAVACCINI ROBERTO, CHIAVACCINI, ROBERTO
Applicants: CHIAVACCINI ROBERTO, CHIAVACCINI, ROBERTO
Application year: 1986
Main classification: G01D5
IPC codes: G01D5, G01G1, G01G23