US3806780A

US3806780A - Damping circuit for excavator multi-motor load sharing swing drive

Info

Publication number: US3806780A
Application number: US00233633A
Authority: US
Inventors: B Jones
Original assignee: Bucyrus Erie Co
Current assignee: Caterpillar Global Mining LLC
Priority date: 1972-03-10
Filing date: 1972-03-10
Publication date: 1974-04-23
Anticipated expiration: 1991-04-23
Also published as: AU5283973A; GB1403931A; AU472430B2; CA968404A

Abstract

The swing drive motor armatures on an excavator are connected in alternate fashion with the armatures of their associated generators to form a sandwiched loop. Sets of nodes which are at the same potential under steady state conditions are thus formed in the loop and a damping resistor is connected between a pair of these equal-potential nodes to conduct current when oscillations occur in the mechanical portion of the swing drive. As a result, the swing drive motors generate output torques which oppose and damp these oscillations. Alternative arrangements are shown in which four motor armatures and four generator armatures are connected in a single sandwiched loop, or in which a pair of sandwiched loops are formed with two motor armatures and two generator armatures in each loop.

Description

United States Patent [1 1 Jones DAMPING CIRCUIT FOR EXCAVATOR 1 MULTI-MOTOR LOAD SHARING SWING DRIVE [75] Inventor:

[73] Assignee: Bucyrus-Erie Company, South Milwaukee, Wis.

22 Filed: Mar. 10, 1972 21 Appl. No.: 233,633

Byron M. Jones, New Berlin, Wis.

[ Apr. 23, 1974 4/1924 Santuari 318/87 9/1919 Ferris 318/88 X 1 ABSTRACT The swing drive motor armatures on an excavator are connected in alternate fashion with the armatures of their associated generators to form a sandwiched loop. Sets of nodes which are at the same potential under steady state conditions are thus formed in the loop and a damping resistor is connected between a pair of these equal-potential nodes to conduct current when oscillations occur in the mechanical portion of the swing drive. As a result, the swing drive motors generate output torques which oppose and damp these oscillations. Alternative arrangements are shown in which four motor armatures and four generator armatures are connected in a single sandwiched loop, or in which a pair of sandwiched loops are formed with two motor armatures and two generator armatures in each loop.

1 Claim, 4 Drawing Figures SPEED AND DIRECTION c o N TROL 55 'PATENTEUAPR 23 mm SHEET 2 [IF 3 AND PATENTEDAPR 2 m 3.806; 780

SHEET 3 [1F 3 CONTROL SPEED AND ' DIRECTION DAMPING CIRCUIT FOR EXCAVATOR MULTI-MOTOR LOAD SHARING SWING DRIVE BACKGROUND OF THE INVENTION The field of the invention is electro-mechanical drive units and particularly for swing drive machinery for large excavators.

Any mechanical drive system has a natural resonant frequency or frequencies at which it will oscillate when excited. These mechanical oscillations subject the components of the system to cyclic loading which repeatedly stress the components and which may result in their premature failure.

This problem is particularly acute in the swing drive units of large excavators. Due to backlash in the gear train linking the swing drive motor and the swing gear, oscillations occur each time the system is started, stopped or reversed. These mechanical oscillations subject the shafts and gears in the swing drive to particularly high and repeated transient torques which considerably limit their useful life. In addition, stress oscillations occur when the swing drive is operated at or driven through certain swing speeds. Such oscillations occur when natural mechanical resonances of the system are excited by the rate at which gear teeth mesh or engage at various points in the gear train.

Mechanical damping means have been devised and installed in swing drive systems to reduce the number and magnitude of stress cycles. However, these known damping means are quite large, expensive and are themselves subject to premature fatigue failures. The latter problem is particulary prevalent when attempts are made to cut the cost of mechanical damping means or reduce its size to meet specific space limitations. As a result, in large excavating machines the life of the drive unit is often more effectively and economically extended by increasing the size, or capacity of those components which have, through experience, been found to be particularly subject to fatigue failure.

SUMMARY OF THE INVENTION The present invention relates to means for damping the oscillations which occur in the mechanical portion of an electro-mechanical drive unit, which means comprises a modification in the electrical portion of the drive unit. Specifically, the invention includes the connection of the drive motors and their associated generators in a loop configuration and connecting a damping resistor, or resistors, between equal-potential nodes in the loop. It has been discovered that the speed of each drive motor oscillates in response to mechanical vibrations, and that such speed oscillations in turn cause corresponding variations in motor armature voltage and current. The phase relationship of these voltage and current variations depends on numerous factors such as the physical location of the drive units with respect to each other and the nature of the mechanical oscillations which are occurring. Regardless of the phase relationship of these voltage and current variations, however, the present invention operates in response to the mechanical oscillations to vary the torque output of each drive motor in such a manner that the drive motors respond to oppose torque variations in the mechanical portion of the drive units to thereby damp the mechanical oscillations.

A general object of the invention is to provide a means for damping oscillations in an electromechanical drive system, which means does not require extensive mechanical alterations to present structures. No modifications are made to the mechanical portion of the drive system; instead, the arrnatures of the drive motors and their associated generators are connected in a sandwiched loop; that is, each motor armature is connected in series between two generator armatures and vice versa. Sets of equal-potential nodes are thus formed around the loop at the connection points of the armatures, and a damping resistor is connected between any two equal-potential nodes of a set. Under steady state operating conditions, the voltage at the equal-potential nodes of each set is the same and no current flows through the damping resistor. However, when transient torque demands are made on the drive motors, such as when the motor shafts are oscillating, a considerable ripple voltage is developed across the damping resistor. Current is thus caused to flow through the damping resistor. This current flow is in such a direction that more current is supplied to the swing motors which are operating at lower speeds with the resulting increase in their output torque. On the other hand the torque output of those swing motors which are speeding up is decreased with the result that the electrical portion of the drive unit damps, or opposes rapid fluctuations in torque.

Another object of the invention is to provide a means for damping oscillations in swing drive units presently in use. The alterations and equipment additions necessary to implement the present invention require a minimal amount of labor and space. Generally, a reconnection of the motors and their associated generators, along with the addition of one or more resistors is all that is required.

The foregoing and other objects and advantages of the invention will appear from the following description. In the description reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration preferred embodiments of the invention. Such embodiments do not necessarily represent the full scope of the invention and reference is made to the claims herein for interpreting the breadth of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view, with parts cut away, of a drag line excavator which incorporates the present invention,

FIG. 2 is a view in elevation, with parts cut away, of a drive unit which incorporates the present invention,

FIG. 3 is an electrical schematic diagram of a first embodiment of the invention, and

FIG. 4 is an electrical schematic diagram of a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An excavator 1 of the type to which the present invention has been successfully applied is shown (somewhat schematically) in FIG. 1. The excavator 1 is of the drag line type and has a lower works to which a pair of feet 2 and 3 are attached for walking. A circular shaped swing gear 4 is mounted on the lower works with its central axis 5 located on the machine center line. A revolving frame 6 is rotatably attached and mounted atop the lower works, and a boom 7 is pivotally attached to the front of the revolving frame 6 to support a bucket (not shown in the drawings) which is manipulated to perform excavating functions.

Mounted on the revolving frame 6 and securely fastened thereto, are four swing drive units 8, 9, 10 and 11. The swing drive units 8-11 are disposed above the swing gear 4 and around an approximately 120 sector of its circumference. Referring to FIG. 2, each drive unit 8-11 includes a direct-current, stabilized, shunt wound swing motor 12 mounted to the top of a gear case 13. Contained within the gear case 13 is a double reduction divided train transmission 14 which links the swing motor 12 with a vertical swing shaft 15 that extends downward through the bottom of the gear case 13 to the vicinity of the swing gear 4 on the lower works. A pinion gear 16 is attached to the lower end of the swing shaft 15 and engages the gear rack 4. The motors 12 on the respective drive units 8-11 are operated simultaneously to rotate their respective pinion gears 16 and to thereby revolve, or swing the frame 6 and attached boom 7 with respect to the lower works. The swing motors 12 are matched to divide the load equally between them under normal steady state operating conditions, and as an example of one embodiment each motor 12 is rated 620 horsepower at 1,000 rpm and 460 volts.

Although the mechanical portion of each drive unit 8-11 is not part of the present invention, the mechanism described above generates the problem which is solved by the present invention. To appreciate the magnitude of this problem, the physical size of the mechanical elements involved is illuminating. For example, the vertical swing shaft 15 is approximately 12 feet long and 16 inches in diameter. The pinion gear 16 is approximately 2 feet in diameter, and 8 barrels of oil are required to lubricate the transmission 14 of each drive unit 8-1 1.

The mechanical resonances which occur in this mechanical drive are particularly severe when the swing motor speed passes through the 500 rpm range. It has been calculated that the frequency at which this resonance occurs (3.4 cycles per second) corresponds to the natural torsional vibration frequency of the swing shaft 15 and its connected inertias. It has also been discovered in connection with making this invention that this frequency corresponds to the frequency with which the teeth on the pinion gear 16 engage the teeth on the swing gear 4 when the swing motors 12 are rotating at about 500 rpm. The result of these oscillations is the cyclic stressing of all the mechanical components in the swing drive units 8-11. This can eventually lead to fatigue failure in any one of these components.

Referring now to FIG. 3, the electrical connection of the swing drive units 8-11 is shown schematically. In the description of the electrical circuits to follow, the swing motor 12 on each drive unit 8-11 will be identified separately. The swing motor 12 of the first drive unit 8 is designated generally by the dashed line 19 and includes an armature M connected in series with an interpole, or commutating field winding 17. A separately excited motor field winding 18 is also magnetically coupled to the armature M Connected in series with the armature M, is the armature G of a first generator, designated generally by the dashed lines 20. The first generator 20 includes a series field winding 21 and a commutating field winding 22 both connected in series circuit with the armature 0,. Connected in series with the first generator 20 is a second swing motor 12 attached to the second drive unit 9 and designated generally by the dashed lines 23. Second swing motor 23 includes an armature M, a separately excited field winding 24 and a commutating field winding 25 connected between the armature M and the first generator 20. The other lead of the armature M connects to an armature G of a second generator, designated generally by the dashed lines 26. The second generator 26 includes a series field winding 27 and a commutating field winding 28, both connected in circuit between the armature G and commutating field winding 17 of the first swing motor 19. A first loop 29 is thus formed and includes, in order, the first motor armature M the first generator armature G the second swing motor armature M and the second generator armature G The loop 29 formed by the alternate connection of motor armatures and generator armatures is termed herein a sandwiched loop. The

generators

20 and 26 also include shunt windings, the connection and function of which is described hereafter.

The first sandwiched loop 29 includes two sets of equal-potential nodes. The first set includes

equalpotential nodes

30 and 31 located respectively at the connection between the first motor 19 and first generator 20, and the connection between the second motor 23 and second generator 26. This second set includes equal-

potential nodes

32 and 33 located respectively at the connection between the first generator 20 and second motor 23, and the connection between the second generator 26 and first motor 19. The term equalpotential nodes refers to points in the sandwiched loop 29 which assume the same voltage or potential level under steady state operating conditions. In other words, under steady state conditions, the voltage developed across the

nodes

30 and 32 equals the voltage developed across the

nodes

31 and 33, and therefore, according to Kirchoffs law the voltage level at node 30 equals that at node 31 and the voltage level at node 32 equals that at node 33.

A second sandwiched loop 34 is formed with the remaining two swing motors and their associated generators. Specifically, a third swing motor 12 attached to the third drive unit 10 and designated generally by the dashed lines 35, has an armature M connected in series with the armature G, of a third generator designated generally by the dashed lines 36. The third swing motor 35 also includes a separately excited field winding 37 and a commutating field winding 38 connected in series with the armature M The third generator 36 includes a series field winding 39 and a commutating field winding 40, both connected in series with the generator armature G A fourth swing motor attached to the fourth drive unit 1 1 and designated generally by the dashed lines 41 is connected between the third generator 36 and the armature G of a fourth generator designated generally by the dashed lines 42. The fourth motor 41 includes an armature M, connected in the loop 34 in series with a commutating field winding 43. The motor 41 also has a separately excited field winding 44. The fourth generator 42 has a series field winding 45 connected in series with a commutating field winding 46 between the generator armature G, and the third motor 35. Two sets of equal-potential nodes are thus formed. The first set includes

nodes

47 and 48 located respectively between the third motor 35 and fourth generator 42 and between the third generator 36 and fourth motor 41. The second set includes

nodes

49 and 50 located respectively between the third motor 35 and third generator 36 and between the fourth motor 41 and fourth generator 42.

The steady state speed and torque produced by the four

swing motors

19, 23, 35, and 41 is controlled by a speed and direction control circuit 51 connected to the first loop 29. The speed of a d-c motor is proportionate to the voltage developed across its armature and the torque output of the motor is proportionate to its armature current. Speed and torque information is fed to the speed and directional control circuit 51 by a first set of leads 52 connected across the first generator armature G, and its series field winding 21, and a second set of leads 53 connected across the commutating

field windings

22 and 25 of the first generator 20 and second swing motor 23. The voltage generated across the leads 53 is proportionate to the current flowing in the first sandwiched loop 29 and is, therefore, proportionate to the torque generated by the first and second motors l9 and 23. The voltage generated across the leads 52 is proportionate to the voltage drop across the first and second motor armatures M and M and is, therefore, proportionate to the speed of their shafts.

There are numerous commercially available speed and torque control circuits. in the preferred embodiment, a control circuit such as that disclosed in US. Pat. No. 3,518,444 issued to D. E. Barber on June 30, 1970, is used. The control circuit 51 operates to regulate the level of the current flowing in both the first and second sandwiched

loops

29 and 34, and to limit the voltage developed by the four

generators

20, 26, 36 and 42. The generators are stabilized shunt wound d-c machines rated 560 kilowatts at 480 volts and 1,200 rpm. Control of the generators is accomplished by regulating the current through both a first set of generator

shunt field windings

54, 55, 56 and 57 and a second set of generator

shunt field windings

58, 59, 60 and 61. The two sets of field windings are connected to the output of the control circuit 51 and a winding in each set is magnetically coupled to one of the generator armatures G G G or 6,. To drive the

motors

19, 23, 35 and 41 in one direction, current is supplied to the first set of generator field windings 54-57. The magnitude of this current determines the voltage developed by the

generators

20, 26, 36 and 42, which in turn determines the speed and torque output of the swing motors for any given load. Similarly, when the direction of swing is to be reversed, current is supplied to the second set of generator field windings 58-61 to reverse current flow in the sandwiched

loops

29 and 34. Because the drive units 8-11l are the same and the mechanical load is shared equally by them, the current flowing in each sandwiched

loop

29 and 34 and the voltage levels developed at corresponding nodes are substantially the same under steady state conditions. Thus, even though speed and torque feedback information is obtained from the first loop 29, control is effectively maintained over both sandwiched

loops

29 and 34 with a single speed and direction control circuit 51.. This economy of using a single control circuit is magnified further when larger numbers of drive units and loops are required.

When oscillations occur in the swing drive mechanisms, the load is no longer shared equally and a certain amount of control is lost over the current flowing in the second sandwiched loop 34. Also, the speed and direction control circuit 51 regulates average total current flow in the first sandwiched loop 29, and it does not respond instantaneously to transient voltage and current variations caused by mechanical oscillations. However, when such transient, or rapidly varying, currents flow in the first and second sandwiched

loops

29 and 34, the voltage pattern at the equal-potential nodes is substantially altered. Specifically, the voltage levels at the equal-potential nodes are no longer equal, but instead, vary with respect to one another in response to the imposed mechanical oscillations. The mechanical oscillations imposed on the swing drive motor shafts 15 cause, at any point in time, some of the motors to speed up and others to slow down. The speed variations are reflected as voltage variations across each motor armature. And herein lies an important discovery which the present invention implements. By connecting damping resistor(s) between pairs of equal-potential nodes, the voltage differentials which are generated between equal-potential nodes by oscillations in the mechanical portions of the drive units, cause current to flow through the damping resistor(s) to effectively damp the oscillations. Specifically, a first damping resistor 62 is connected between the equal-

potential nodes

32 and 33 in the first sandwiched loop 29 and a second damping resistor 63 is connected between the

equalpotential nodes

47 and 48 in the second sandwiched loop 34. During oscillation, the damping

resistors

62 and 63 each operate nearly instantaneously to divert armature current away from one swing motor and provide more armature current to the swing motor which is operating at the slower speed. As a result, oscillatory driving torques are generated by the

swing motors

19, 23, 35 and 41, which torques oppose the oscillatory load torques imposed by the mechanical system. The implementation of this discovery can be improved by the connection of additional damping resistors between other pairs of equal-potential nodes. For example, a third damping resistor 64 (shown in phantom lines) can be added to the first sandwiched loop 29 between the equal-

potential nodes

30 and 31, and a fourth damping resistor 65 (shown in phantom lines) can be added to the second sandwiched loop 34 between the

equalpotential nodes

49 and 50. In systems having additional equal-potential nodes, additional damping resistors can be added. Also, additional sandwiched loops similar to the

loops

29 and 34 can be formed where additional drive units are required.

The damping resistors are of relatively low value, preferably being comparable in resistance to the equivalent impedance of the circuit to which the damping resistor is connected. In the preferred embodiment, the damping resistors have values of 0.03 ohms and power ratings of 5 kilowatts.

The improvement which is obtained by the addition of the present invention to existing excavators is substantial. When the swing drive on a Bucyrus-Erie Model 1500 W dragline was connected as described above, the following results were obtained during normal digging operations.

Swing Shaft Torque (Ft. Lbs.)

350,000 400,000 450,000 500,000 Prior Art Numerous l20 26 4 7 With Invented Damper 24 2 0 The table indicates the number of instances in which the swing shaft torque exceeded the levels indicated during the operating interval. These results clearly illustrate the reduction in cycle loading of the swing machinery provided by the present invention.

An alternative arrangement of the invention is shown in FIG. 4. This second arrangement includes the same elements contained in the two sandwiched

loops

29 and 34 described above, and accordingly, these elements are identified with the same name and number. The distinction from the first arrangement is the formation of only a single sandwiched loop 66 which contains the four motor armatures M M M and M and the four generator armatures G G G and G Specifically, first motor 19 connects to the first generator 20 at a first node 67, first generator 20 connects the second motor 23 at a second node 68, second motor 23 connects the second generator 26 at a third node 69, second generator 26 connects to third motor 35 at a fourth node 70, third motor 35 connects to third generator 36 at a fifth node 71, third generator 36 connects to fourth motor 41 at a sixth node 72, fourth motor 41 connects to fourth generator 42 at a seventh node 73, and fourth generator 42 connects to first motor 19 at an eighth node 74 to complete the loop 66. The speed and direction control circuit 51 is connected by a first pair of leads 76 across the first generator armature 6,, and its series field coil 21 to sense the voltage thereacross, and by a pair of leads 77 across the commutator field windings 22 and to sense the current therethrough. As in the first arrangement described above the speed and direction control 51 operates in response to the voltage and current feedback information to control current flow through one of two sets of generator field windings. The first set includes

forward field windings

54, 55, 56 and 57 associated with the

respective generators

20, 26, 36 and 42; and the second set includes

reverse field windings

58, 59, 60 and 61 also associated with the

respective generators

20, 26, 36 and 42. By energizing the first field winding set, current flows in one direction in the sandwiched loop 66, while energization of the second field winding set results in reverse current flow and a consequent reversal of motor rotation.

The sandwiched loop 66 includes two sets of equalpotential nodes. The first set includes the four odd numbered

nodes

67, 69, 71 and 73; and the second set includes the four even numbered

nodes

68, 70, 72 and 74. The four swing drive units 8-11 are damped by connecting damping resistors between a pair of nodes of the first set and between a pair of nodes of the second set. Specifically in the embodiment shown, a first damping resistor 78 connects the second node 68 to the fourth node 70 and a second damping resistor 79 connects the first node 67 to the fifth node 71. Other damping resistors can be connected between pairs of equal-potential nodes in one of the two identified sets.

It should be pointed out, however, that there are countless sets of equal-potential nodes in a sandwiched loop. Specific sets have been identified herein because the equal-potential nodes in them are easily accessible for attachment of the damping resistor. In addition to the two sets already identified, the loop 66 includes, for example, a third easily accessible set of equal-potential nodes at the junction of the series field winding and commutating field winding of each

generator

20, 26, 36 and 42. Damping resistors can be attached between any pair of these nodes to practice the invention.

In should be apparent to those skilled in the art that the present invention applies to drive units using motors and energy converting machines other than the d-c motor and d-c generator specifically described herein. For example, the teaching of the present invention can improve the performance of drive units which use hydraulic motors driven by hydraulic pumps. The hydraulic motors are connected together serially with the hydraulic pumps, in alternate arrangement, to form one or more sandwiched loops. Points in each loop having equal hydraulic pressure under steady state conditions are thus established and an energy dissipating means such as a line with an orifice is connected between any two of these points to damp mechanical oscillations. Other means for successfully applying the present, invention to damp mechanical oscillations in systems driven by a plurality of motors should be apparent from the above description, and reference is therefore made to the following claims which specifically define the scope of the invention.

I claim:

1. In the swing drive of an excavating machine having a swing gear and a plurality of swing drive motors, each rigidly connected to said swing gear to impart a swing motion to the excavator when supplied with energy, and each being subject to oscillations which may occur in said swing drive by virtue of its rigid connection to said swing gear, the improvement comprising:

a plurality of generators connected in a sandwiched loop with said swing drive motors to supply energy thereto and to control said swing drive motors;

a plurality of equal-potential node sets formed in said sandwiched loop by the interconnection of said swing drive motors and said generators; and damping resistor connected between a pair of equal-potential nodes in one of said sets, said damping resistor having a value chosen to provide optimal damping which is substantially equal to the equivalent electrical impedance present across said pair of equal-potential nodes, said damping resistor being responsive to variations in potential between said pair of equal-potential nodes caused by said oscillations in said swing drive to dissipate energy and thereby damp said oscillations.

Claims

1. In the swing drive of an excavating machine having a swing gear and a plurality of swing drive motors, each rigidly connected to said swing gear to impart a swing motion to the excavator when supplied with energy, and each being subject to oscillations which may occur in said swing drive by virtue of its rigid connection to said swing gear, the improvement comprising: a plurality of generators connected in a sandwiched loop with said swing drive motors to supply energy thereto and to control said swing drive motors; a plurality of equal-potential node sets formed in said sandwiched loop by the interconnection of said swing drive motors and said generators; and a damping resistor connected between a pair of equal-potential nodes in one of said sets, said damping resistor having a value chosen to provide optimal damping which is substantially equal to the equivalent electrical impedance present across said pair of equal-potential nodes, said damping resistor being responsive to variations in potential between said pair of eQual-potential nodes caused by said oscillations in said swing drive to dissipate energy and thereby damp said oscillations.