Rotary encoder for determining the angular position between two rotating components

Abstract

A rotary encoder for detecting the angle of rotation of a first rotatable shaft, with a first mark which is coupled with the first shaft, a second mark which is coupled with the second rotatable shaft, a third mark which is coupled with the second shaft, a fixed fourth mark and means for detecting a passage of the third mark though a vicinity of the fourth mark and means for detecting a coincidence of the angles of rotation of the first and the second marks.

Claims

1. A rotary encoder (1) for detecting the angular position of a first shaft (24), with a first mark (5) which is rotatable with the first shaft (24), in relation to a second shaft (21) rotating in a fixed ratio to the first shaft (24), said second shaft (21) including a second mark (3) which is rotatable with the second shaft (21), said rotary encoder comprising: an angular position detector, configured for detecting the angular position of the second shaft (21) and a coincidence of angle of rotation detector, configured for detecting a coincidence of the angle of rotation of the first mark (5) in relation to the second mark (3), wherein said coincidence of angle of rotation detector includes a sensor, wherein one of the first mark (5) and the second mark (3) is said sensor, and wherein said sensor is configured for detecting a presence and/or proximity of the other of said first mark (5) or second mark (3) in relation to said sensor.

2. The rotary encoder according to claim 1, characterised in that the angular position detector for detecting the angular position of the second shaft (21) comprises a third mark (2) which is rotatable with the second shaft (21) and a fixed position fourth mark (4), and wherein the rotary encoder further includes an evaluator (25), responsive to at least said fixed position fourth mark (4), and configured for detecting a passage of the third mark (2) through a vicinity of the fourth mark (4).

3. The rotary encoder according to claim 2, characterised in that the fixed position fourth mark (4) is a sensor (8) and the third mark (2) is configured to be detected by the sensor (8), and wherein the output from the sensor (8) is a periodic function of an angle of rotation of the second shaft (21), and wherein a base period of the periodic function corresponds to one rotation of the second shaft (21).

4. The rotary encoder according to claim 1, characterised in that the angular position detector for detecting the angular position of the second shaft (21) comprises an absolute encoder.

5. The rotary encoder according to claim 1, characterised in that the first and second shafts (24, 21) are rotatably coupled by means of a gear (31).

6. The rotary encoder according to claim 5, characterised in that the gear (31) is a form-fitting gear.

7. The rotary encoder according to claim 5, characterised in that the gear (31) is a reduction gear.

8. The rotary encoder according to claim 5, characterised in that the gear (31) is a ring gear.

9. The rotary encoder according to claim 5, characterised in that the first and second shafts (24, 21) are coaxially aligned and have end faces, arranged opposite one another, on which the first mark and the second mark (5, 3) are arranged respectively.

10. The rotary encoder according to claim 3, characterised in that the output from the sensor (8) is the periodic function of the angle of rotation (ω.sub.1) of the second shaft (21), and wherein one rotation of the second shaft (21) corresponds to |i−1|base periods of the function.

11. The rotary encoder according to claim 1, further including an evaluator (25) configured for coincidence of angle of rotation detector, and for receiving a passage signal (S.sub.D) from a passage detector, said passage detector configured for detecting a passage of the third mark (2) through a vicinity of the fourth mark (4), and wherein said evaluator (25) is configured for determining an angle position of said first shaft (24) and said second shaft (21) on the basis of an angle of rotation (α, β) covered between reception of the coincidence signal (S.sub.K) and reception of the passage signal (S.sub.D).

12. The rotary encoder according to claim 1, further including a sensor (11, 14) for detecting a relative angle of rotation of the first and/or the second shaft (21, 24).

13. A robot (100) with a rotary encoder (1) according to claim 5, characterised in that the robot (100) comprises at least two arm sections (101-109) which can be swivelled relative to one another, one arm section of which is connected in a rotationally fixed manner to the first shaft (24) and the other arm section being connected in a rotationally fixed manner to a housing (20) of the gear (31).

14. The rotary encoder according to claim 1, characterised in that, one of the first mark and the second mark (5, 3) is a magnet (7) and the other one of the first mark and the second mark (3, 5) is said sensor, said sensor comprising a magnetic field sensor (9).

15. A method for detecting the angular position of a first shaft (24) which is coupled via a gear (31) to a second shaft (21), so that one rotation of the first shaft (24) corresponds to i rotations of the second shaft (21), said method comprising the steps of: rotating the first and second shafts (21, 24); detecting a passage of a third mark (2) rotating with the second shaft (21) through a reference position; detecting a coincidence of angle of rotation between a second mark (3) rotating with the second shaft (21) and a first mark (5) rotating with the first shaft (24), wherein one of the first and second marks (5, 3) is a sensor configured for detecting a presence and/or proximity of the other of the first and second marks (5, 3) in relation to said sensor; determining an angle of rotation (α, β) covered between the passage though the reference position and the coincidence of angle of rotation; and determining the angular position on the basis of the angle of rotation (α, β) covered.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention are explained in the following description of exemplary embodiments with reference to the enclosed figures, in which:

(2) FIG. 1 shows a robot,

(3) FIG. 2 shows a schematic structure of a rotary encoder,

(4) FIG. 3 shows, in partial representation, conducting paths of a resolver,

(5) FIG. 4 shows a schematic structure of a rotary encoder in an alternative embodiment,

(6) FIG. 5 shows a schematic structure of a rotary encoder with a Harmonic Drive® gear,

(7) FIG. 6 shows a sketch illustrating the principle of the rotary encoder,

(8) FIG. 7 shows a diagram for determining the angle of rotation from a signal,

(9) FIG. 8 shows a flow chart of an operating method of the rotary encoder,

(10) FIG. 9 shows a diagram with coincidence signals and passage signals, entered over the angle of rotation of an input shaft, for a gear with identical direction of rotation of input and output shafts, and

(11) FIG. 10 shows a diagram with coincidence signals and passage signals, entered over the angle of rotation of an input shaft, for a gear with contrary direction of rotation of input and output shafts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(12) FIG. 1 shows a robot 100. The robot 100 comprises several arm sections connected swivelably with one another, in this case a base 101, a lower arm 103, an upper arm 105, an instrument 107 and three elbow pieces 102, 104, 106. Each elbow piece 102, 104, 106 is connected, so as to swivel around two axes 110 arranged at right angles to one another, with two adjacent arm sections 101 and 103, 103 and 105 or 105 and 107. The instrument 107 can in turn comprise an elbow piece 108 which connects a shaft and an end effector 109 of the instrument 107 in an articulated manner. In the configuration of the robot shown, three of the axes 110 are perpendicular to the drawing plane, further axes run in particular in the longitudinal direction of the arm sections 101, 103, 105.

(13) FIG. 2 shows a rotary encoder 1 and its surroundings in schematic form. Such a rotary encoder 1 can be provided on each individual axis 110 of the robot 100. In the following, by way of example, the case is considered that the rotary encoder 1 is arranged between the base 101 and the lower elbow piece 102 in order to monitor the azimuthal orientation of the lower arm 103.

(14) A motor 23 is firmly connected to a housing 20 of the elbow piece 102 in order to drive an input shaft 21 of the rotary encoder 1. A circular disc 22 is attached to the input shaft 21. The disc 22 carries two marks 2, 3, in this case in the form of two magnets 6, 7 or in the form of a single magnet, the magnetic field of which extends on two sides of the disc 22. A further mark 4 is formed by a sensor 8, for example a Hall effect sensor, which is mounted on the housing 20 in a fixed position in order to detect the passage of the magnet 6 once during the course of each rotation of the input shaft 21.

(15) An incremental encoder 10 comprises a sensor 11 fixed to the housing and a plurality of marks 12 which can be detected by the sensor 11, distributed evenly around the periphery of the disc 22.

(16) As soon as the sensor 8 detects a passage of the magnet 6, the angular position of the input shaft 21 is known, and its further rotation can be tracked quantitatively on the basis of the pulses supplied by the incremental encoder 10. For the same purpose, the sensor 8 and the incremental encoder 10 can also be replaced by a single absolute encoder. If the output signal of the latter assumes a predetermined value, for example passes though zero, this can be equated with the detection of the passage; the further rotation of the shaft 21 can be monitored on the basis of the further development of the output signal.

(17) A sun wheel 27 of a planetary gear 10 is located on an overhung end of the input shaft 21. A ring gear 30 of the planetary gear 10 is firmly connected to the housing 20. Planetary wheels 29 held by a support 28 mesh with the sun wheel 27 and the ring gear 30. The support 28 is firmly connected to an output shaft 24. i rotations of the input shaft 21 drive one rotation of the output shaft 24.

(18) The support 28 has several journals, on each of which one of the planetary wheels 29 is mounted. One of these journals carries on its tip facing the disc 22 a mark 5 in the form of a further sensor 9, which is configured to detect the magnet 7 when this is positioned opposite it at a short distance.

(19) Marks 13 on the support 28, distributed evenly around the periphery, together with a sensor 14 fixed to the housing, form a further incremental encoder 15.

(20) The outputs from the sensors 8, 9, 11, 14 are received by an evaluation unit 25. While the evaluation unit 25 can be permanently wired to the sensors 8, 11, 14 fixed to the housing 20, a slip ring or a wireless interface 26, for example an RFID interface, can be provided for the communication with the sensor 9. The RFID interface makes it possible not only to transmit an output from the sensor 9 to the evaluation unit 25 but also to supply operating energy to the sensor 9.

(21) In the present case the evaluation unit 25 is also connected to the motor 23 and controls its rotation. Since the evaluation unit 25 thus “knows” the direction of rotation of the motor 23, it can determine an angle of rotation passed through since an arbitrarily defined starting point in time in that it counts the signal pulses supplied by one of the sensors 11, 14 since this starting point, upwards or downwards depending on the direction of rotation. In this case one of the two incremental encoders 15, 10 is redundant and can be dispensed with without this affecting the function of the robot 100.

(22) According to an advantageous embodiment of the invention, the incremental encoder 15 and/or 10 is replaced with a resolver. Such a resolver is indicated schematically in FIG. 3; it comprises a conducting path 16 which is rotatable with the shaft which is to be monitored, for example on the disc 22, and oscillates about a circular path coaxial with the shaft 21 with a whole number n of periods, and two fixed conducting paths 17, 18 which oscillate with the same number n of periods as the conducting path 16, but phase-shifted by a quarter period relative to one another. An alternating current on the conducting path 16 induces an alternating voltage in the conducting paths 17, 18, the amplitude of which depends on the phase shift between the conducting paths 16, 17 and 18. Each individual period 19 of the conducting path 16 can in this case be equated with one of the aforementioned marks 13 or 12 which, if it lies exactly in phase with one of the conducting paths 17, 18 functioning as sensors 11 or 14, induces in this a maximum voltage. With each 360° rotation of the conducting path 16, such a phase coincidence occurs n-times, so that, as with a conventional incremental encoder, n pulses can be derived per rotation. However, since the induced amplitudes are continuously changeable with the angular position, the angular position can also be estimated in quantitative terms between two phase coincidence positions.

(23) FIG. 4 shows a schematic representation of a rotary encoder 1 with an alternative arrangement of first and second marks 3, 5. This variant is particularly suitable if the first and second shafts 21, 24 are so arranged that their end faces are opposite one another. In this case the mark 5 can be mounted on the end face of the first shaft 24 and the mark 3 can be mounted on the end face of the second shaft 21, which allows a more compact design to be achieved. Which of the marks 3, 5 is in this case the sensor and which is detected by the sensor is immaterial. The rest of the structure can be designed analogously to FIG. 2.

(24) FIG. 5 shows an advantageous embodiment of the rotary encoder 1 shown in FIG. 4 in which the planetary gear has been replaced with a Harmonic Drive® gear. At the end of the second shaft 21 there is a “wave generator” 32, which drives a so-called “flexspline” 33. An outer gearing 33′ of the “flexspline” 33 meshes with the inner gearing of a ring gear 30. A support 28 establishes a rotationally fixed connection between the “flexspline” 33 and the first shaft 24. Analogously to FIG. 4, the sensor 7 can again be arranged on the end face of the first shaft 24 and the magnet 6 can be arranged on the end face of the second shaft 21 or on the support 28 (or vice versa).

(25) FIG. 6 shows the rotary encoder 1 in a schematic diagram in order to explain in greater detail the functional principle involved in the detection of a passage as well as a coincidence of the angle of rotation. The figure shows an axial view of the coaxially arranged first and second shafts 21, 24. The two shafts 21, 24 each assume an angular position in which the marks 2 and 4 coincide and at the same time the marks 3 and 5 coincide (see marks drawn in solid lines), i.e. a passage and a coincidence of angle of rotation occur simultaneously. This position defines a reference position of the first shaft 24; an angle of rotation ω.sub.1 of the first shaft 24 can be stated as a deviation from this reference position. In the same way, an angle of rotation ω.sub.2 of the second shaft 21 can be stated as a deviation from the reference position.

(26) The choice of the position shown as a reference position is purely arbitrary and is made for the purpose of ease of recognition; in principle, any other angular position of the shafts 21, 24 can also be defined as reference position.

(27) If the second shaft 21 rotates around the axis of rotation 110, then the first shaft 24 rotates with it according to the gear ratio i. Depending on the construction type of the gear 10, the first shaft 24 can rotate in the same direction as the shaft 21 (i.e. i is positive) or in the opposite direction (i.e. i is negative). If both shafts rotate in the same direction, for example in the clockwise direction, then the third mark 2 lines up again with the fourth mark 4 when the second shaft 21 has completed a full rotation. The passage of the third mark 2 though the vicinity of the fourth mark 4 is detected though suitable means, for example the fourth mark 4 is designed in the form of a sensor 8, and a passage signal S.sub.D is transmitted to the evaluation unit 25. On each further full rotation, a passage signal S.sub.D is again detected.

(28) Following a full rotation of the second shaft 21, the first mark 5 located on the first shaft 24 has rotated further, according to ω.sub.2=i.Math.ω.sub.1, by the i-th part of a full rotation. Consequently, the next coincidence signal S.sub.K is detected when the second mark 3 located on the second shaft 21 catches up with the first mark 5 located on the first shaft 24 (see marks drawn in broken lines in FIG. 6). The coincidence of the marks 3, 5 is detected through suitable means, for example the first mark 5 designed in the form of a sensor 9, and a coincidence signal S.sub.K is transmitted to the evaluation unit 25. This principle continues accordingly with further rotation of the two shafts until a reference position of the first shaft 24 is reached again.

(29) If both shafts 21, 24 rotate in opposite directions, for example the first shaft 24 rotates in a clockwise direction and the second shaft 21 rotates in an anticlockwise direction, then starting out from the reference position a coincidence of the angle of rotation occurs first (marks drawn in broken lines) before the third mark 2 lines up with the fourth mark 4.

(30) Ideally, the signals S.sub.K, S.sub.D are pulse-formed, i.e. the angular range within which they indicate a passage or a coincidence through deviation from a quiescent value is so small that it can be regarded as point-formed. In practice, this is often not the case; instead, the angular ranges within which they deviate from the quiescent value can be several times greater than the angular distances between the marks 12 or 13, so that several pulses can come from the sensors 11 or 14 of the incremental encoder 11 or 15 during a passage or a coincidence.

(31) FIG. 7 shows by way of example such a curve of the signal S.sub.K as a function of the angle of rotation ω of the shaft 21 or 24, whereby the pulses of the incremental encoder 11 or 15 are symbolised though narrow vertical lines on each side of a maximum S.sub.K at ω.sub.s.

(32) In the simplest case, the evaluation unit 25 defines this angle of the signal maximum as the angle of coincidence.

(33) Alternatively, the evaluation unit 25 can determine the centroid of the area which lies below the curve S.sub.K and above a straight line S.sub.0 which can be defined arbitrarily. In this case the angle ω.sub.s is unambiguously derived from the angular value of the centroid; this can be calculated generally as

(34) ${\omega }_S=\frac{\int S_K\left(\omega \right)\omega d\omega }{\int S_K\left(\omega \right)d\omega }{\omega }_S=\frac{\int S_K\left(\omega \right)\omega d\omega }{\int S_K\left(\omega \right)d\omega },$
whereby the integrals in each case run from one point of intersection of the curve S.sub.K with the straight line S.sub.0 to the other. Thus, an angle ω.sub.s can be determined unambiguously for each coincidence signal S.sub.K and for each passage signal S.sub.D.

(35) The following part of the description now explains the method for determining the angular position of the first shaft 24 in relation to the reference position. The individual steps of the method are shown in FIG. 8. FIG. 9 shows a diagram in which the passages and coincidences of angle of rotation derived from the angle of rotation ω are represented for a rotary encoder with a gear ratio, selected by way of example, of i=15. Starting out from the reference position 47, the number j of full rotations of the second shaft 21 are entered on the ordinates of the diagram. The angular range ω of a full rotation is entered on the abscissa and ranges from 0° to 360°. For the reference position, j=0, ω=0. If, starting out from the reference position 47, the second shaft 21 has completed a full rotation, at j=1, ω=0, a passage signal is detected again. The shafts rotate further, whereby the second shaft 21 now catches up with the first shaft 24 and a coincidence of angle of rotation occurs at j=1, ω=360°/(i−1)=approx. 25.7°. Passage signals always occur at ω=0; further coincidences of angle of rotation occur at j=n, ω=n*360°/(i−1).

(36) The method is carried out by the evaluation unit 25. At the beginning of the method, the angular position ω.sub.1 of the first shaft 24 is unknown, it can for example correspond to the point 40 or 44 in FIG. 9; the angular position ω.sub.2 of the second shaft 21 is also unknown. The method starts with step S1. In step S1 it is checked whether a passage signal S.sub.D is present. If no passage signal S.sub.D is present, in step S2 it is checked whether a coincidence signal S.sub.K is present. If no coincidence signal S.sub.K is present either, the first shaft 24 is rotated (step S3), and the queries in steps S1 and S2 are repeated until a passage signal S.sub.D (step S1) or a coincidence signal S.sub.K (step S2) is received.

(37) If a passage signal Sp is received in step S1, in step S4 it is checked whether a coincidence signal S.sub.K is simultaneously present.

(38) If this is the case, then the angular position ω.sub.1 of the first mark 5 on the first shaft 24 coincides with the fourth mark 4. The first shaft 24 is thus in its reference position 47. The evaluation unit 25 therefore determines, in step S5, the angle of rotation of the first shaft 24 as zero (ω.sub.1=0), and the method ends.

(39) If a passage signal S.sub.D, 41, was received in step S1 but no simultaneous coincidence signal S.sub.K was received in step S4, then in step S6 a counter is initialised. The evaluation unit 25 thereupon measures the angle of rotation of the rotating second shaft 21 (step S7) by incrementing the counter each time a pulse from the incremental encoder 10 is received. The evaluation unit ends the measurement as soon as a coincidence signal S.sub.K, 42 is received (step 8). The value α of the counter then corresponds to the angle of rotation which the second shaft 21 has covered, starting out from the reception of the passage signal S.sub.D, 41, up until reception of the coincidence signal S.sub.K, 42.

(40) Based on this counter value α, the evaluation unit 25 now determines the angle of rotation ω.sub.1 of the first shaft 24 (step S9). Surprisingly, this is equal to the counter value, i.e. ω.sub.1=α. The method then ends.

(41) If, in step S1, no passage signal S.sub.D is initially received, but a coincidence signal S.sub.K, 45 is received in step S2, then the evaluation unit 25 also initiates the counter in step S10. The evaluation unit 25 picks up the pulses transmitted by the incremental encoder 10 while the second shaft 21 rotates (step S11), and increments the counter on each reception of a pulse. The evaluation unit 25 interrupts the measurement as soon as a passage signal S.sub.D, 46 is received (step 12). The counter value β of the counter reached at this point states the angle of rotation which the second shaft 21 has covered, starting out from the reception of the coincidence signal S.sub.K, 45 up until reception of the passage signal S.sub.D, 46.

(42) Based on this counter value β, the evaluation unit 25 now determines the angle of rotation ω.sub.1 of the first shaft 24 (step S13). To do so, the evaluation unit 25 first determines the angle of rotation which the first shaft 24 has assumed at the moment of coincidence of angle of rotation S.sub.K, 45. This angle of rotation is 360° minus the counter value β.

(43) However, starting out from the position of the coincidence of angle of rotation S.sub.K, 45, the second shaft 21 was rotated further by the angle β. Since the transmission ratio of the gear is i, consequently the first shaft 24 is rotated further from the coincidence of angle of rotation by β/i. Expressed mathematically, the angle of rotation is thus:

(44) ${\omega }_1=360°-\beta +\frac{\beta }{i}{\omega }_1=360°-\beta +\frac{\beta }{i}.$
The method then ends.

(45) Once the angle of rotation ω.sub.1 of the first shaft has been determined once using the above method, it can be kept continuously updated on the basis of the pulses from the incremental encoder 15, 10.

(46) The evaluation unit 25 can also determine the angle which the second shaft 21 has covered, measured from the last reference position 47 of the first shaft 24, since the relationship ω.sub.2=i.Math.ω.sub.1 remains unchanged.

(47) The method can be carried out with gears 10 with shafts 21, 24 rotating in the same direction (see FIG. 9) and with shafts 21, 24 rotating in opposite directions (see FIG. 10). The method explained above related to a gear with shafts rotating in the same direction (i=15). Since the method can be applied analogously to rotary encoders with shafts rotating in opposite directions, in this case reference is only made to FIG. 10, which shows the correspondingly valid signals with a gear ratio of i=−15.

(48) Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the allowed claims and their legal equivalents.

REFERENCE NUMBERS

(49) 1 rotary encoder 2 mark 3 mark 4 mark 5 mark 6 magnet 7 magnet 8 sensor 9 sensor 10 incremental encoder 11 sensor 12 mark 13 mark 14 sensor 15 incremental encoder 16 conducting path 17 conducting path 18 conducting path 19 period 20 housing 21 shaft 22 disc 23 motor 24 shaft 25 evaluation unit 26 receiver 27 sun wheel 28 support 29 planetary wheel 30 ring gear 31 gear 32 wave generator 33 flexspline 40 unknown angular position 41 passage signal 42 coincidence signal 44 unknown angular position 45 coincidence signal 46 passage signal 47 reference position 100 robot 101 base 102 elbow piece 103 lower arm 104 elbow piece 105 upper arm 106 elbow piece 107 instrument 108 elbow piece 109 end effector 110 axis