WO1987007010A1

WO1987007010A1 - Method and apparatus for accurate measurement of a torsional angle

Info

Publication number: WO1987007010A1
Application number: PCT/SE1987/000229
Authority: WO
Inventors: Kurt-Ove Stenberg
Original assignee: ELOPTRICON AB
Current assignee: ELOPTRICON AB
Priority date: 1986-05-09
Filing date: 1987-05-07
Publication date: 1987-11-19
Anticipated expiration: 1988-11-09
Also published as: JPH01502359A; SE453225B; SE8602111L; AU7359387A; SE8602111D0; EP0412960A1

Abstract

A process for accurate measuring of the rotational angle of a rotating object comprises the following procedural steps: a pulse emitting unit (3) produces a signal S1 consisting of pulses at a frequency f1 dependent upon the rotation of the object, an oscillator (4) emits a signal S2 consisting of pulses at a frequency f2 substantially larger than f1 and a measuring signal transmitting unit (5) produces a signal S3 indicating instants of beginning and of ending, respectively, of a rotational angle measurement. According to the invention, a counter (R1) in a calculator unit is caused to count pulses of the S1 signal, that is, periods for each revolution of the object, a counter (R2) counts pulses of the S2 signal for every period in the signal S1. At a first and a second impulse, respectively, of the signal S3 a first memory (M1) stores the number of S1 pulses = M1a and M1b, respectively, a second memory (M2) stores the sum of the S2 pulses, M2a and M2b, respectively, while a third memory (M3), at the end of the first period of S1 after the respective first and second impulses of S3, stores the sum of the S2 pulses during an S1 period = M3a and M3b, respectively, whereupon a calculating unit B1 calculates a value V1 related to the measured rotational angle from M1b + M2b/M3b - M1a - M2a/M3a.

Description

Method and Apparatus for Accurate Measurement of a Torsional Angle

The present invention relates to a method of accurately measuring an object that rotates around a first symmetry axis at a sub¬ stantially constant rotational speed, comprising the steps of:

— causing a pulse emitting unit that is fixed in a defined rotational angle relationship to the object to emit a first signal S.. consisting of pulses at a first frequency f₁ that is dependent upon the rotation of the object;

— causing an oscillator to emit a second signal S- consisting of pulses at a second frequency f_? that is substantially greater than f₁;

— causing a measuring signal transmitting unit to emit a third signal S, indicative of the respective instants of beginning and of ending of at least one rotational angle measurement during the rotation of the object.

The invention also relates to apparatus for carrying out such a method. A relatively good accuracy in the measurement of a rotational angle can be obtained by this method, with a corre¬ sponding apparatus. The difficulty lies in devising a pulse emit¬ ting unit that will emit pulses, each of which corresponds exact- ly to a like rotational angle magnitude, in practice on the order of 2 ιx10 —4 radians or less per pulse.

In a known form of apparatus for practising the method, the pulse emitting unit comprises a rotating plate with a transparent circular ring provided with a large number (on the order of 10,000) opaque radial lines, which are intended to be evenly distributed around the circular ring. A radiation source, for example for infra-red radiation, directs its radiation around the circle ring which, during rotation of the plate, produces pulses that are detected by a detector located at the other side of the circle ring, arranged to produce a signal consisting of corresponding pulses.

The problem is that it can be difficult to obtain an absolutely uniform division of the circle ring, so that the lengthhs of the pulses can vary somewhat. This can be compensated for, in and of itself, by effecting a corresponding calibration, in which an object's rotational angle is measured with some reference method, for example by means of laser interference under static conditions. This applies, however, only to the calibration of the rotational angle in relation to the number of whole pulses, or rather the corresponding whole periods. Every such period can be divided up according to a known method into a fixed number of im¬ pulses by means of an oscillator. But this method does not take into account the fact that those periods can differ somewhat in duration. The object of the invention is to provide a method of the type mentioned in the introduction which is distinguished by especially great accuracy. According to the invention, such a method is characterized by, in a computer unit:

— causing a first counter R₁ to count the pulses S. of the first signal, that is, the periods for each revolution of the object;

— causing a second counter R„ to count the pulses of the second signal S_ for every period in the first signal S.. , at a first impulse (a) and a second impulse (b) of the signal S,, causing a first memory M.. to store the number of S₁ pulses (= M.a or M.b), ^* causing a second memory to store the sum of the number of S„ pulses (= M«a or ₂b), causing a third memory M,, at the end of the first period of the signal S₁ after said first and second im¬ pulse of the signal S,, to store the sum of the S_? pulses during the S_. period (= M,a or M,b);

causing a calculating unit B₁ to calculate from M M.._jbb ++ MM₂-bb/M₃-bb -— f-.a — M~a/M,a a value V. related to the measured rotational angle. Such a method takes account of the fact that the pulses from the pulse emitting unit can have some varying period length, so that the fine division of periods obtained from the oscillator will be truly correct. In a preferred embodiment of apparatus for practising the method the pulse emitter unit comprises

— a disc rotatable in its plane that is provided in a transparent circular ring with a large number of radial, substantially uniformly distributed opaque surface elements;

— a radiation source so directing its radiation toward said circular ring that a detector instrumentality receives radia¬ tion pulses resulting from the radiation passing the circular ring, the detector instrumentality being so arranged as to convert these radiation pulses into said first signal S₁.

Such a pulse emitting unit can be further improved by arranging a first and a second mirror instrumentality to return radiation that passes through said circular ring back through the same to meet said detector instrumentality. The pulse period is thus re¬ duced to half, that is, the pulse frequency is doubled.

The invention will now be more particularly described with reference to the accompanying drawings wherein, by way of ex¬ ample:

Fig. 1 is a schematic block diagram of an apparatus according to the invention:

Fig. 2 is a diagram of the relationship between amplitude and time for the signals S₁ , S_ and S,;

Fig. 3 is a schematic block diagram of a calculator unit accord¬ ing to the invention;

Fig. 4 schematically shows a part of a pulse emitting unit ac- cording to the invention, seen from above; and

Fig. 5 shows the same unit as Fig. 4, but as seen from the side.

As can be seen from Fig. 1, a rotating object 1 is coupled by means of a shaft 2 to a pulse emitting unit 3. This involves 5 that the latter stand in a defined rotational angle relationship to the object and generate pulses with a first frequency f_. in dependence upon the rotation of the object, which are emitted in the form of a first signal S_, that is preferably converted to a square-wave pulse train.

10 An. oscillator 4 generates pulses at a frequency f which is sub¬ stantially larger than f.. and emits these in the form of a second signal S_?, preferably in the form of a square-wave pulse train.

A measuring signal transmitting unit 5 finds the position of rotation of the object, for example by optical means, and pro- 15 duces a third signal S, indicating the instants of beginning and of ending of a rotational angle measurement.

These three signals S.., S„ and S, are fed to a computer unit 6, which comprises a first and a second counter R₁ and R_?, a first, second and third memory M. , M_ and M.,, together with a cal- 20^'. culating unit B₁ , as can be seen in Fig. 3. The first counter R₁ is arranged to count the pulses of the first signal, that is, the period of each revolution of rotation of the object 1. The second counter R-, is arranged to count the pulses of the second signal S„ for every period in the first signal S₁.

25._^" The memory M_. , upon a first impulse (a) and a second impulse (b), respectively, of the signal S,, stores the total of the number of S. pulses (= M.a or M..b, respectively), while the memory M„ stores the total of the number of S„ pulses (= M_a or M„b, re- spectively). At the end of the first period of the signal S₁ ,

30 following the said first and second impulses of the signal S-., the memory M_. stores the total of the S_ pulses through an S, period (= M,a and M,b, respectively).

A calculator unit B₁ calculates with the aid of

M₁b + M_?b/M,b - M.a — M_a/M,a a value V₁ that is related to the measured angle of rotation.

The measuring method will be more clearly understood from Fig. 2. The signal S. is shown for two whole periods, n + 1 and n + 2. It will be understood that the latter period is somewhat longer than the first, so that the period n + 1 corresponds to 8 periods of the signal S₉ and the period n + 2 corresponds to 9 periods of the signal S_.

A first impulse (a) is obtained from the measuring signal trans¬ mitter unit at instant t₁. Thereupon there is transferred from R₁ the total of the number of S₁ pulses, = n, to the memory M., so that M.a = n. From R„ the total number of S_? pulses is trans- ferred to M„, so that M_a = 3. At the end of the first period of the signal S. , that is, when the period n + 1 is completed, the total of the S„ pulses through the period is transferred to the memory M,, so that M,a = 8.

In a corresponding manner, at the instant t_?, upon a second im- pulse (b) from the measuring signal transmitting unit, there is transferred a total from R. to M. , so that M₁b = n + 1 , and a total from R_ to M„, so that M_b = 7. At the end of the period of S₁ the total of the S-, pulses is conducted to the memory M,, so that M-b = 9.

V., related to the measured angle of rotation, obtained with the aid of the calculator unit, is thus

B₁ = M.b + M_b/M,b - M.a — M₇a/M,a, so that V. in this case = 1 29/72. The units are obviously the periods in the signal S... This depends upon the design of the pulse emitting unit. Figs. 4 and 5 show a preferred embodiment of such a pulse emitting unit. A circular disc 7 is rotatable about a shaft 2 which in this case is directly connected with a rotating object 1. It should be ob- served that the rotational angle relationship between a rotating object and the pulse emitting unit can be of many different types. Thus, more or less sophisticated optical designs can be found, according to the degree of precision desired. The circular disc 7 is made with a transparent circular ring 8 at its periphery that is provided with a large number of opaque radial surface elements, that is, lines 9, of which only a few have been indi¬ cated in Fig. 4. A radiation source 10 directs its radiation, which can, for example, lie in the infra-red region, at right 1 angles^ through the circular ring 8. The pulsing radiation that passes the circular ring is reflected back by first and second mirror instrumentalities 11a, 11b through another part of the circular ring and reaches a detector that comprises a pulse emit¬ ting unit. . From the latter there is thus given off a signal S₁.

The radiation that reaches the detector appears in approximately sine wave form, and with the aid of electronic means the signal S. emitted from the pulse emitting unit to the calculator unit 6 is converted to a square wave with many short rise and fall periods. The signal S. will in this manner take the form that is _ shown in Figs. 1 and 2.

The frequency of the signal S. obviously depends upon the rate of rotation of the disc 7 and the number of divisions, that is, the number- of radial surface elements in its circular ring 8. If theⁿrate of rotation is, for example, 10 revolutions/second and there are 10 divisions, the frequency in that case will be

^"2 10 x 10 = 2 x 10 /sec = *% = 200 kHz. (The factor 2 arises by reason of the double passage of the radiation through the disc 7.) In this measuring example, which is represented in

Fig. 2, it is calculated that V. = 1 29/72, with the period of ; the signal S... as the unit. If a pulse emitting unit of the type just described is used, an S₁ period will thus correspond to 5 x 10 —5 x 2 tr radians. In this example, which is shown in Fig. 2, the frequency f~ of the oscillator is f. , which is chosen for the sake of clarity. In practice f„ ought to be selected to be substantially larger, for example = 200 f.. In this way the S. periods (on the average) will be divided into 200 parts and, through the invention, which takes account of the ir¬ regularity of the S. periods, measuring precision is substantially improved as compared with the heretofore available technique.

The process according to the invention is applicable in situa¬ tions where an accurate measuring of the angle of rotation of a rotating object is required. By way of example, mention can be made of a process and apparatus for contactless and accurate gauging of machine parts that is the subject matter of a con¬ currently filed Swedish patent application. This process relates particularly to accurate measuring of shafts with finished bearing surfaces, such as crankshafts for vehicle engines.

In a corresponding apparatus the position of a laser beam in a measuring plane is measured by means of an optical system that comprises a polygonal mirror fixedly coupled to a pulse emitting unit of the type that is described in the present patent appli¬ cation. In addition, the angular position of the machine part — in that case the crankshaft — is determined with the use of a process and an apparatus according to the present invention, which is obviously applicable in numerous fields of measuring tech¬ nology.

Claims

1. Process for accurate measuring of a rotational angle of a rotating object that rotates around a first axis of symmetry with a substantially constant speed of rotation, comprising the procedural steps of

— causing a pulse emitting unit that stands in a determined rotational angle relationship to the object to emit a first signal S. consisting of pulses at a first frequency f. in dependence upon the rotation of the object,

— causing an oscillator to emit a second signal S_ consisting of pulses at a second frequency f« that is substantially larger than f. ,

— causing a measuring signal transmitting unit to emit a third signal S, indicating instants of the beginning and the ending, respectively, of at least one rotational angle measurement during the rotation of the object, c h a r a c¬ t e r i z e d by, in a computer unit:

causing a first counter (R.) to count the pulses of the first signal S. , that is, the periods for each revolution of the object,

— causing a second counter (R„) to count the pulses of the second signal S„ for every period of the first signal S. at a first impulse (a) and a second impulse (b), re¬ spectively, of the signal S,, causing a first memory (M.) to store the number of S. pulses = M.a and M.b, respective- ly, causing a second memory (M„) to store the sum of the number of S_ pulses = M„a and M_b, respectively, causing a third memory (M_), at the end of the first period of the signal S. after said first and second impulses of the signal S-., to store the sum of the S_? pulses through an S. period = M,a and M-,b, respectively,

causing a calculator unit B. to calculate from M.b + M M₂_bb//MM₃,bb -- MM._jβa -- MM₂_aa//MM-,.aa a. value V. related to the measured rotational angle.

Apparatus for accurate measurement of the rotational angle of an object (1) rotating around a first axis of symmetry at a substantially constant rotational speed, according to claim 1, comprising

— a pulse emitting unit (3) that stands in a determined rotational angle relationship to the object (1), which is arranged to emit a first signal S.. consisting of pulses at a first frequency f₁ dependent upon the rotation of the object,

— an oscillator (4) arranged to emit a second signal S„ con- sisting of pulses at a second frequency f_ that is sub¬ stantially larger than f. ,

— a measuring signal transmitting unit (5) arranged to pro¬ duce a third signal S, indicating instants of beginning and ending of at least one rotational angle measurement during rotation of the object, c h a r a c t e r i z e d by a computer unit (6) comprising

— a first counter (R.) arranged to count the pulses of the first signal S₁ , that is, periods for each revolution of the object,

— a second counter (R_) arranged to count the pulses of the second signal S„ for each period in the first signal S.. ,

— a first memory (M.), a second memory (M_) and a third memory (M,) so arranged that at a first impulse (a) and a second impulse (b), respectively, of the signal S, the first memory stores the sum of the number of pulses = M.a and M.b, respectively, the second memory (M_) stores the sum of the number of S_ pulses = M_?a and M„b, respectively, the third memory, at the end of the first period of the ■ signal S. , after said first and second impulses, re¬ spectively, of the signal S, stores the sum of the S_? pulses through an S. period = M,a and M,b, respectively,

— a calculator unit B. , arranged to calculate a value V. re- lated to the measured rotational angle from

M b + M₂b/M_.b - M..a - M₂a/M₃a.

3. Apparatus according to claim 2, c h a r a c t e r i z e d in that the said pulse emitting unit comprises

— a disc (7) rotatable in its plane that is provided in a transparent circular ring with a large number of radial, substantially uniformly distributed opaque surface ele¬ ments (9) ,

— a radiation source (10) which so directs its radiation towards said circular ring (8) that a detector instru- mentality (3) receives radiation pulses arising at the passage of the circular ring by the radiation, the detec¬ tor instrumentality being arranged to convert these radia¬ tion pulses into said first signal S..

4. Apparatus according to claim 3, c h a r a c t e r i z e d in that a first and a second mirror instrumentalities (11a, 11b) are arranged to reflect the radiation that passes said circular ring (8) back through the same to meet said detec¬ tor instrumentality (3).