Rotation controller for a differential actuator

Abstract: A rotation control apparatus controller for use with a differential actuator which includes two induction motors and a differential mechanism for producing a constant torque on its output shaft basing on the differential speed of the induction motors, comprises two inverters each located between one of the induction motor and a common power source for the induction motors and having indivisual voltage/frequency output characteristics. The inverters set separately the voltage/frequency output characteristics of power supplied to the induction motors so as to control the speed of the induction motors separately, thereby controlling the output torque on the output shaft of the differential actuator. And the output shaft of the differential actuator is controlled to constant speeds regardless of the load torque on the shaft basing on the difference of setting frequencies on both inverters.

Description: The invention relates to a drive arrangement having two individually controlled induction motors coupled by a gear mechanism to a common output shaft. Fig. 1 shows a conventional control circuit for an inverter which drives an induction motor used as a differential actuator. The control circuit includes an input terminal 101 in connection with an AC power source, an AC-to-DC converter 102 for transforming AC power into DC power, a smoothing circuit 103 for smoothing the transformed DC power, a DC-to-AC inverter 104 for transforming the smoothed DC power into AC power with arbitrary frequency, and an induction motor 105 drived by the AC power supplied by the inverter 104. The operation of the above-mentioned differential actuator is as follows. The AC-to-DC converter 102 receives AC power of three-phase, for example, supplied through the AC power source terminal 101 and transforms the AC power into DC power. The smoothing circuit 103 makes the produced DC power smooth. The inverter 104 which operates in a switching manner in a certain sequence transforms the smoothed DC power into AC power with a desired frequency, and supplies the AC power to the induction motor 105. The AC power produced by the inverter 104 has the voltage vs. frequency (v/f) characteristics as shown in Fig. 2, and the rotational speed of the induction motor 105 is controlled on the basis of the v/f characteristics shown. When the inverter 104 produces the output voltage v in proportion to the output frequency f as shown in part of the v/f characteristics of Fig. 2, the induction motor 105 is driven with a constant torque characteristics as shown in Fig. 3(a) and 3(b), or when the inverter 104 produces a constant output voltage v irrespective of variable output frequency f as shown in part of Fig. 2, the induction motor 105 produces a constant output characteristics in a wide speed range. Various types of differential actuator controller using the above-mentioned inverter have been proposed as disclosed, for example, in Japanese Patent Publication Laid-open No. 54-27660 (published in 1979). The differential actuator torque controller arranged as described above is capable of torque control in the normal operating condition except for the lowest speed range as shown in Fig. 3(a). However, the torque ripple increases and the output is lacking sufficient and stable torque in the lowest speed range and therefore the torque control for the induction motor is difficult in this speed range. When the induction motor 105 is rotated in the lowest speed range, the output frequency of the inverter 104 is under the limit so that the rotational speed control for the differential actuator is difficult, too. From DE-A-23 37 011 an arrangement is known which comprises two 3-phase induction motors, the first of which is controlled by a control circuit (not an inverter), whereas the second motor is only switched on or off. The output pinion of first motor works through reduction gears on an output shaft, whereas the output pinion of the second motor works through a reduction gear onto the output pinion of the first motor, so that both motors mechanical work parallel either in the same direction or against each other. It is an object of the present invention to provide a drive arrangement which provides a sufficient output power also at low output speeds. According to the invention, a drive arrangement having two individually controlled induction motors coupled by a gear mechanism to a common output shaft, is characterized in that the motors are coupled to a differential gear mechanism, the output shaft of which having an output speed according to the difference of the output speeds of the two motors to provide rotation control for a differential actuator, and comprising two inverters having individual voltage vs. frequency output characteristics and located respectively between said induction motors and a power source which supplies power to said induction motors. Preferable embodiments are defined in the dependent claims. Fig. 1 is a block diagram showing the induction motor control circuit used in the conventional differential actuator rotation controller; Fig. 2 is a graph showing the inverter output voltage vs. frequency characteristics employed in the conventional rotation controller; Fig. 3(a) and (b) is a graph showing the induction motor output torque vs. speed characteristics of the conventional rotation controller; Fig. 4 is a block diagram showing an embodiment of this invention; Fig. 5 is a brief cross-sectional diagram showing the differential actuator used in the above embodiment; Fig. 6 is a brief perspective diagram showing the principal portion of the mechanism shown in Fig. 5; Fig. 7 is a graph showing the induction motor output torque vs. speed characteristics of the above differential actuator; Fig. 8(a) is a graph showing the induction motor output torque vs. speed characteristics of the above differential actuator rotation controller plotted when the setting frequency of the inverter is varied; Fig. 8(b) is a graph showing the induction motor output torque vs. speed characteristics of the above differential actuator rotation controller plotted when the difference of both frequencies of the driving electric power is controlled by the inverter. Fig. 9 is a graph showing the induction motor output torque vs. speed characteristics of the differential actuator plotted when the output shaft is loaded; Fig. 10 is a side view of the gripper to which this invention may be applied; Fig. 11 is a side view of the workpiece pushing apparatus to which this invention may be applied; Fig. 12 is a perspective view of the clamper to which this invention may be applied; Fig. 13 is a perspective view of the spindle of the machine tool to which this invention may be applied; Fig. 14 is a perspective view of the joint section of the arm of the industrial robot to which this invention may be applied; and Fig. 15 is a perspective view of the conveyor mechanism to which this invention may be applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the inventive differential actuator rotation control apparatus will now be described in detail with reference to the drawings. In Fig. 4, a differential actuator 1 is made up of two induction motors 2 and 3 having the same structure and same output characteristics and a differential mechanism 4. The induction motors 2 and 3 supplied with power from a common 3 phase AC power source 5 through power lines 6 and 7, respectively, on which are provided two inverters 8 and 9 having the voltage vs. frequency characteristics as described previously using Fig. 1. Polyphase power meters 10 and 11 are inserted between the induction motor 2 and inverter 8 and between the motor 3 and inverter 9, respectively. The differential actuator 1 has one output shaft 12 coupled with a digital torque sensor 14 for measuring in digital manner the output torque of the shaft 12, and a torque meter 15 for reading out the measured output torque is attached to the digital torque sensor 14. The detailed structure of the differential actuator 1 includes as shown in Fig. 5 a virtually cylindrical case 16, the above-mentioned induction motors 2 and 3 and the differential mechanism 4 all integrated within the case 16, the bonnets 18 which function to close both end openings of the case 16 and also to support bearings 17 of the output shafts 12 and 13. The induction motors 2 and 3 produce the torque by the electromagnetic induction acting between stator 20 and 30 fixed on the interior wall of the case 16 and rotors 22 and 32 fixed on the external surface of bored axles 21 and 31. The axles 21 and 31 are supported rotably by bearings 19 secured to the inner flanges 16a of the case 16, and the output shafts 12 and 13 of the differential actuator 1 run through the bore of the axles 21 and 31, respectively, without intervention of rotation with each other. The axles 21 and 31 are coupled at their central end faces with a first and second bevel gears 42 and 43 of the differential mechanism 4. The first and second bevel gears 42 and 43 are in engagement with third bevel gears 44 and 45, which are coupled through bearings 46 with a small differential axis 47 intergrated with a cylindrical large differential axis 48, in Fig. 6. The large differential axis 48 has its external faces fixed to the central ends 12a and 13a of the output shafts 12 and 13, respectively. Next, the operation of the foregoing differential actuator 1 and its controller will be described. The differential actuator 1 operates such that the axles 21 and 31 of the induction motors 2 and 3 are rotated by the electromagnetic induction in the directions shown by the arrows P and Q, respectively, in Fig. 5. It is first assumed that the rotational speed ϑ1 of the axle 21 is higher than that ϑ2 of the axle 31, i.e., ϑ1 > ϑ2, and in this case the differential speed causes the third bevel gears 44 and 45 to rotate around the small differential axis 47 and the differential mechanism 4 rotates around the central axis of the first and second bevel gears 42 and 43 as shown by the arrow P (see Fig. 6). Accordingly, the output shafts 12 and 13 also rotate in the direction P, and their rotational speed is as follows. ϑo = ½(ϑ1 - ϑ2) (1) Next, it is assumed that the axle 21 has a rotational speed ϑ1 which is lower than that ϑ2 of the axle 31, i.e., ϑ1 < ϑ2, and in this case the third bevel gears 44 and 45 are rotated by the second bevel gear 43 and the differential mechanism 4 rotates in the direction shown by the arrow Q, as opposed to the previous case. Accordingly, the output shafts 12 and 13 also rotate in the direction Q. Further assumption is made that the axles 21 and 31 have an equal rotational speed, i.e., ϑ1 = ϑ2, and in this case ϑo = 0 as given by Equation (1), namely the differential mechanism 4 stops rotating, and the output shafts 12 and 13 also become stationary. On this account, when the axles 21 and 31 are operated in the normal speed range, the output shafts 12 and 13 can rotate smoothly and easily from a zero speed to a certain low speed, facilitating the speed change of the differential mechanism 4 in the low-speed range. Next, the operation with both axles 21 and 31 rotated in the same direction will be described. The first case is that the axle 31 of the induction motor 3 in connection with the second bevel gear 43 of the differential mechanism 4 (Figs. 5 and 6) is rotated in the same direction P as of the axle 21. The output shafts 12 and 13 have their rotational speed ϑo added by the speed of the first and second bevel gears 42 and 43 as follows. ϑo = ½(ϑ1 + ϑ2) (2) Next, when the axles 21 and 31 rotate in the same direction at an equal speed, i.e., ϑ1 = ϑ2, the third bevel gears 44 and 45 rotate together with the first and second bevel gears 42 and 43, and in this case the output shafts 12 and 13 rotate at the same speed as that of the axle 21 and 31 with the exertion of the output power which is a total of outputs of the induction motors 2 and 3. The output shafts 12 and 13 of the differential actuator 1 have such a wide speed range including a zero speed, and it is expressed as follows. {-½(ϑ1 + ϑ2)} < {+½(ϑ1 + ϑ2)} (3) On the other hand, there is the following steady relationship between the output torques T1 and T2 of the induction motors 2 and 3 and the output torque To of the output shafts 12 and 13. To = T1 + t2 (4) When the output torques T1 and T2 are in an equilibrium state, the shaft output torque becomes To = 2T1 or To = 2T2 , and a large torque is produced on the output shafts 12 and 13. The following describes the method of controlling the output torque of the differential actuator 1 using the circuit shown in Fig. 4. An experiment was conducted for evaluating the speed vs. torque relation of the output shafts 12 and 13 of the differential actuator 1 for the setting frequencies of the inverters 8 and 9 of 35 Hz and 30 Hz, respectively, and the rotational speed of the induction motors 2 and 3 of 937 r.p.m, with the result shown by the solid line in Fig. 7. The maximum output torque reached 29.6 kg/cm as shown. The dashed line in Fig. 7 shows the output torque produced by the conventional system where a single induction motor is driven at a setting frequency with the maximum output torque of 4.7 kg/cm. The maximum output torque indicated by the solid line is far greater than that indicated by the dashed line, proving that the inverter controller for the differential actuator 1 produces a sufficient torque even in the lower speed range and it favorably stands up to practical uses. With the setting frequency of the inverter 8 fixed to 35 Hz and that of another inverter 9 varied between 5 Hz and 30 Hz, the drooping torque-speed characteristics, which is identical to that of DC motors, is obtained as shown in Fig. 8(a). Namely, a wide dynamic range of speed control is attained. This achieves the output of sufficiently large torque by a small speed adjustment, and it corresponds to the operation of a differential actuator made up in combination of two DC motors in which the differences of voltages applied to both d.c. motors is adjusted. Fig. 8(b) shows the result of a performance test conducted using the Prony brake for the inventive differential actuator rotation controller. The graph shows that the output shafts 12 and 13 are maintained at a constant target speed against the variation of the load torque applied to the output shafts 12 and 13 by the Prony brake through the adjustment of the differential frequency made by changing concurrently the setting frequencies for the two inverters 8 and 9. When the output shafts 12 and 13 are loaded by braking, for example, with the setting frequency for the inverter 8 being maintained at 35 Hz and that for the inverter 9 being varied, the output torque vs. speed characteristics of the shafts 12 and 13 becomes as shown in Fig. 9, and in this case the ouput torque is made virtually constant independently of the rotational speed. Even at a low speed of the output shafts 12 and 13 of the differential actuator 1, the induction motors 2 and 3 constituting part of the actuator 1 are running at high speeds, causing the output shafts 12 and 13 to produce a sufficiently large torque with an extremely small torque ripple despite the slow speed. The following describes some example of mechanism to which the differential actuator 1 with associated torque controller described above is applied. Fig. 10 shows the application of this invention to a gripper 50. The differential actuator 1 has virtually same structure as described on Fig. 5, and its one output shaft 13 is extended to have a thread section 13a, on which a nut member 51 is engaged. The nut member 51 is provided with tabs 51a, and similar tabs 18a are provided on the bonnet 18 of the case 16, so that two pairs of arms 52 and 53 are pivoted by pins 18b and 51b on the respective tabs 18a and 51a. The arms 52 and 53 are linked by a pin 54, which also functions to pivots grip arms 55 together with the arms 52 and 53. The grip arms 55 are provided at their end with grip plates 56 confronting each other, and the grip plates 56 hold a workpiece 57 in response to the swing motion of the grip arms 55 caused by the linear motion of the nut member 51 due to the rotation of the output shaft 13 of the differential actuator 1. The grip mechanism 50 by application of the inventive rotation controller and differential actuator 1 enables gripping of the workpiece 57 with a desired force which is easily adjusted to meet various requirements such as the case of holding a soft material. Owing to the characteristics shown in Fig. 9, when the workpiece 57 is in movement, it can be moved from one position to another by being held with a constant grip force irrespective of its moving speed. Fig. 11 shows the application of this invention to a workpiece pushing mechanism 60 used in various automated machines. The differential actuator 1 is fixed on the bed 61 by means of a fixture 62, and its output shaft 12 has a thread section 12a on which is engaged a nut member 63. The nut member 63 has an L-shaped pushing attachment 64, whch pushes the workpiece 65 on the bed 61 in the linear motion caused by the rotation of the output shaft 12. The linear motion of the pushing member 64 can be adjusted by torque control for the output shaft 12 in such a manner of pushing the workpiece 65 slowly with a large torque and retrieving it to the home position quickly with a small torque. It is also possible to feed the workpiece at a constant speed by using the characteristics shown in Fig. 9. Fig. 12 shows the application of this invention to a clamp mechanism 70. The clamp mechanism 70 is coupled at its clamp arm 74 through a key member 75 to the end of one output shaft 12 of the differential actuator 1 which is secured to the base by a fixture 72, so that a workpiece 78 is clamped between a clamp member 76 attached at the end of the clamp arm 74 and a clamp pad 77 on the base. The clamp force is produced by the swing motion of the clamp arm 74 caused by the rotation of the output shaft 12, and it can readily be adjusted through the torque control described above. Fig. 13 is a perspective view of the machine tool drive unit to which the present invention may be applied. In the figure, the differential actuator 1 has its output shaft 12 coupled at the end with a spindle 80 by means of a fixture 81. Attached at the top of the spindle 80 is a cutting tool 82 for machining a workpiece 83, which is machined at a constant rotational speed by moving the spindle 80 toward the workpiece 83 at a constant speed through the inventive constant speed control for the output shaft 12 of the differential actuator regardless of the magnitude of load torque. Fig. 14 shows the application of this invention to an industrial robot. The differential actuator 1 is used as part of the joint section 87 which links an arm 86 with the robot main body 85. The joint section 87 includes a U-shape rotary flange 89 with its both ends coupled with the output shaft 12 of the differential actuator 1 which is fixed on the main body 85 by means of a fixture 88. In this arrangement, the output shaft 12 has its rotational speed controlled arbitrarily including stoppage regardless of the load torque applied to the arm 86, whereby the constant speed control for the arm 86 is accomplished. Fig. 15 shows the application of this invention to a conveyer mechanism. The differential actuator 1 has its output shaft 12 provided with a thread section 12a, which is in engagement with a hinge section 93a of a supporting board 93 standing at one end of a table 91 having a guide section 92, and the output shaft 12 has its end fixed on one end of a slide member 94 which slides on the guide section 92 along the axis of the output shaft 12. A slider section 94a is formed at the bottom of the slide member 94, and a workpiece 95 is placed on the slide member 94. In this arrangement, the slide member 94 can be fed at arbitrary speeds in the speed range from stoppage to the normal operating speed, and the conveyer mechanism 90 rendered constant speed control can readily be assembled with a machine tool or the like. The inventive rotation control apparatus incorporates two inverters having constant but separate voltage/frequency characteristics placed between two induction motors and the power source for the differential actuator producing the output torque by the differential speed between the two induction motors. On this account, the rotational speed of both induction motors can be controlled separately by the associated inverters, whereby the output torque on the shafts of the differential actuator can be controlled stably in the extended speed range from stoppage to the normal operating speed. As described above in detail, the inventive rotation control apparatus for the differential actuator having two induction motors and a differential mechanism incorporates a speed controller for controlling the rotational speed of both induction motors at individual frequencies so that the actuator output shaft is controlled to constant speeds ranging from stoppage to the normal operating speed basing on the difference of speeds of both induction motors, whereby the rotational speed of the output shaft is controlled accurately to desired speeds independently of the magnitude of the load torque applied to the output shaft.

Claims: Drive arrangement having two individually controlled induction motors coupled by a gear mechanism to a common output shaft, characterized in that the motors (2, 3) are coupled to a differential gear mechanism (4), the output shaft (12, 13) of which having an output speed according to the difference of the output speeds of the two motors (2, 3) to provide rotation control for a differential actuator, and comprising two inverters (8, 9) having individual voltage vs. frequency output characteristics and located respectively between said induction motors (2, 3) and a power source (5) which supplies power to said induction motors (2, 3). Drive arrangement according to claim 1, wherein one inverter (8) provides constant-frequency power to one of said induction motors (2) and the other inverter (9) provides variable-frequency power to the other induction motor (3). Drive arrangement according to claim 1, further comprising a digital torque sensor (14) attached to said output shaft (12, 13) of said differential actuator for measuring the output torque on said output shaft in a digital manner, said digital torque sensor (14) having a measured value read out on a torque meter (15), torque control being implemented by setting a frequency of said variable-frequency inverter (, 9). Drive arrangement according to claim 1, further comprising polyphase power meters (10, 11) each provided between each of said inverters (8, 9) and the induction motors (2, 3), said polyphase power meters (10, 11) measuring power supplied to said respective induction motors (2, 3). Drive arrangement according to claim 1, wherein said two induction motors (2, 3) and said differential mechanism (4) are accomodated in an integral manner within a common housing (16, 18).

Inventors: MIURA KUNIHARU MITSUBISHI DENK, MOHRI NAOTAKE, SAITO NAGAO MITSUBISHI DENKI K, YANAI YOSHIHIKO MITSUBISHI DEN, MIURA, KUNIHARU MITSUBISHI DENKI KABUSHIKI KAISHA, MOHRI, NAOTAKE, SAITO, NAGAO MITSUBISHI DENKI KABUSHIKI KAISHA, YANAI, YOSHIHIKO MITSUBISHI DENKI KABUSHIKI KAISHA
Applicants: MITSUBISHI ELECTRIC CORP, MOHRI NAOTAKE, MITSUBISHI DENKI KABUSHIKI KAISHA, MOHRI, NAOTAKE
Application year: 1986
Main classification: H02P7
IPC codes: H02P5