Patent Application
20120283919
This patent disclosure relates generally to control of electronically driven systems and, more particularly, to an electronically driven swing drive for an upper structure that is rotatably connected to an under carriage of a machine.
Machines having rotatable upper structures are known. Examples of such machines include excavators, telescopic cranes and boom trucks, battle tanks, and others. Such machines may use different types of drive mechanisms to cause the selective rotation of an upper structure relative to an undercarriage. Known drive mechanisms include use of electric, hydraulic, or mechanical actuators. The type of drive mechanism used for each application will typically also dictate, in a fashion that is consistent for various machine types, the behavior of the machine. Electric drive systems, for example, may tend to drift when reaching a desired upper carriage position, while hydraulic drive systems may exhibit quicker stopping.
Based on the substantial similarity in the way machines operate depending on the type of drive that is used, experienced operators have developed skills within the framework of their expectations about how a machine will behave during operation. As can be appreciated, changes in the way a machine behaves during operation will not only decrease the productivity of an experienced operator, but may also pose safety considerations in that the various machine portions may not move exactly in a way that their operator expects them to.
Such issues of machine behavior during operation become especially pronounced when the drive systems are upgraded or otherwise redesigned, for example, when redesigning machine systems to improve their efficiency and to reduce overall machine fuel consumption. One example of a machine redesign along these lines is the conversion of a swing drive system from a hydraulic drive to an electric or hybrid-electric drive. It has been noted, for example, by operators testing an electrically-driven swing mechanism in excavators that the deceleration of the upper structure of the machine is inconsistent and sluggish when approaching zero speed or rest and can cause the upper structure's final rest position to have gone further than the operator's expectations and this is sometimes referred to as “machine drift.”
This issue has been recognized and at least one solution has been proposed in the past to address the different machine behavior when changing from a hydraulic to an electric swing drive system. An example of a control system for an electric swing motor that attempts to mimic machine operation with a hydraulic swing motor can be seen in U.S. Pat. No. 7,772,792, which was granted to Kawaguchi et al. on Aug. 10, 2010 (the '792 patent). The '792 patent describes a rotation control device for the rotary body of an excavator. The control device provides a small first torque command when rotating the rotary body, but can also provide a second, larger torque command when an acceleration is commanded. More specifically, in one embodiment of the '792 patent, a control system is described that includes numerous tables, which are populated with acceleration values that are determined based on a velocity command. The various tables are dedicated to emulating various effects of a hydraulic and mechanical systems of the excavator, such as hydraulic fluid pressure, rotating inertia of the rotary body depending on the position and loading of the bucket, and others. In this way, the device described in the '792 patent attempts to emulate the operation of a hydraulic system by use of the electric system by, for example, providing a slight deceleration when the rotary body is rotating to emulate the operation of a hydraulically motivated actuator when fluid is diverted from the swing actuator to actuate the boom and/or bucket actuators.
Even though the devices described in the '792 patent are at least partially effective in emulating the behavior of a hydraulically activated swing mechanism with an electric motor, the extensive tabulation of data is labor intensive and cannot automatically adapt to different operating conditions and loads. Moreover, the '792 patent discloses use of a mechanical brake to arrest rotation of the rotary body at the end of a swing, which has been found in other machines to be an unacceptable behavioral characteristic for operators besides presenting issues with the reliability and longevity of the braking mechanism. Also, the addition of a mechanical brake reduces the overall operating efficiency of the machine thus increasing fuel consumption.
The disclosure describes, in one aspect, a swing drive system for a machine. The machine may include an upper structure rotatably associated with an under carriage. The swing drive system can be adapted to selectively rotate the upper structure relative to the under carriage in response to a command signal from an operator of the machine. The swing drive system includes an electric power generator adapted for connection to an engine of the machine for receiving mechanical power therefrom to drive the generator. An electric swing motor is disposed to selectively receive electrical power. In addition, the motor receives a torque command signal to drive a sprocket that is adapted to mesh with a ring gear connected to the under carriage of the machine. A sensor is associated with the swing drive system and adapted to provide a sensor signal indicative of a swing speed of the upper structure relative to the under carriage. An electronic controller is associated with the sensor and the electric swing motor. The electronic controller is operable to provide the torque command signal to the electric swing motor in response to the command signal. The electronic controller is configured to receive the command signal and the sensor signal. The electronic controller can provide the torque command signal to maintain a swing motion of the upper structure in response to the command signal and based on the sensor signal during a normal operating state. The electronic controller can further provide the torque command signal based on the sensor signal to brake the swing motion at a substantially constant rate when the command signal is indicative of a zero desired swing speed during a braking operating state.
In another aspect, the disclosure describes a method for operating an electrically driven swing mechanism disposed to selectively swing an upper structure of a machine relative to an under carriage of the machine. The method includes providing an electronic controller operably associated with an electric swing drive motor and configured to provide a torque command to the electric swing drive motor. A driving torque command is provided to the electric swing drive motor that is sufficient to maintain a desired swing speed of the upper structure based on a command signal from an operator and based on an estimated speed of the upper structure relative to the under carriage. A braking torque command is provided to the electric swing drive motor that is sufficient to reduce a speed of the upper structure at a substantially constant rate based on the estimated speed of the upper structure relative to the under carriage when the command signal is indicative of a zero desired swing speed.
In yet another aspect, the disclosure describes a machine having an upper structure rotatably associated with an under carriage and a swing drive system configured to selectively rotate the upper structure relative to the under carriage in response to a command signal from an operator of the machine. The machine includes an electric power generator connected to an engine of the machine for receiving mechanical power therefrom to drive the generator and an electric swing motor disposed to selectively receive electrical power. In addition, the motor receives a torque command signal to drive a sprocket meshed with a ring gear connected to the under carriage of the machine. A sensor is associated with the swing drive system and disposed to provide a sensor signal indicative of a swing speed of the upper structure relative to the under carriage. An electronic controller is associated with the sensor and the electric swing motor. The electronic controller is operable to provide the torque command signal to the electric swing motor in response to the command signal. The electronic controller is configured to receive the command signal and the sensor signal. The electronic controller provides the torque command signal to maintain a swing motion of the upper structure in response to the command signal and based on the sensor signal during a normal operating state. The electronic controller further provides the torque command signal based on the sensor signal to brake the swing motion at a substantially constant rate when the command signal is indicative of a zero desired swing speed during a braking operating state.
This disclosure relates to a machine that is configured to operate in a fashion consistent with a hydraulically-driven swing drive mechanism but that instead uses an electrically driven swing-drive mechanism. The electrically driven swing-drive mechanism can also provide advantages that are not available for the hydraulically-drive swing drive mechanism. In one aspect, the systems and methods disclosed herein are applicable to not only newly manufactured machines, but also to machines that are reconditioned, refurbished and/or retrofitted with electric systems to replace hydraulic systems.
As used herein, the term “machine” may refer to any machine that performs some type of operation associated with an industry such as mining, construction, fanning, transportation, marine or any other industry known in the art. For example, although an excavator is shown in certain figures, the machine may generally be an earth-moving machine, such as a wheel loader, excavator, dump truck, backhoe, motor grader, or may alternatively be any other type of machine, such as a material handler, a locomotive, paving machine or the like. Similarly, although an exemplary bucket is illustrated as the attached implement of the illustrated excavator, any implements may be utilized and employed for a variety of tasks, including, for example, loading, compacting, lifting, brushing, and include, for example, buckets, compactors, forked lifting devices, brushes, grapples, cutters, shears, blades, breakers/hammers, augers, and others.
With the foregoing in mind, an excavator 100 is shown for purpose of illustration in
In reference now to
As shown, the swing drive system 200 is an electrical drive or hybrid-electric drive system that includes an internal combustion engine 202 connected to an electric power generator 204. The engine 202 may be a compression ignition or diesel engine having an output shaft that is connected to a rotating group of the generator 204 such that electrical power can be generated during operation of the engine 202. In the illustrated embodiment, the generator 204 is configured to provide three-phase alternating current (AC) at about 480V, but any other electrical parameters or type of electrical power may be used. The engine 202 is further connected to a hydraulic pump 206, which provides pressurized hydraulic fluid to various components and systems of the machine 100 during operation. In the illustrated embodiment, the pump 206 is a reciprocating-piston type pump having a rotary input shaft that is connected to the engine 202 via the rotating group of the generator 204. In this way, mechanical power produced by the engine 202 is converted to electrical power at the generator 204 and to hydraulic power at the pump 206. Although a direct connection is shown between the engine 202, generator 204 and pump 206, any appropriate method of connection may be used. For example, gear sets, transmissions and any other type of mechanical transmission or transformation device may be used. Moreover, a switchable gear or isolation device may be used in transmitting power from the engine 202 to the generator 204 and/or the pump 206.
During operation, electrical power is transferred from the generator 204 to an inverter 208 by conduits 210 that electrically interconnect the inverter 208 with the generator 204. The inverter 208 is a device configured to transform the AC power provided on the conduits 210 to direct current (DC) power and vice versa. In the illustrated embodiment, DC electrical power from the inverter 208 can selectively be provided to a power storage device 212, which can include any appropriate type of electrical power storage such as batteries or an array of electrical capacitors. A bi-directional DC/DC converter 214 that is configured to manage or control the DC power flowing to and from the storage device 212 is electrically connected between the inverter 208 and the storage device 212.
Returning now to the inverter 208, a main power conduit 216 carrying AC power is connected to the electrical swing motor 118, which is also shown in
A block diagram for one embodiment of a swing control 300 that may be operating within the swing controller 218 is shown in
The control 300 is further associated with a sensor 306, which is configured to provide information indicative of the position, direction, speed and/or acceleration of the upper structure 104 relative to the undercarriage 102 (
In the illustrated embodiment, the signal 304 from the control lever 302 and the signal 308 from the sensor are provided to a state determinator 314. The state determinator 314 is configured to determine the desired speed, acceleration and/or position of the upper structure 104 when the control lever 302 is moved. The state determinator 314 may be further configured to determine the current speed, acceleration and/or position of the upper structure 104 based on signals received from the sensor 306. Parameters relating to the desired or actual position of the upper carriage, although potentially unnecessary for accomplishing the motion of the upper carriage, may be further determined in the state determinator 314 by appropriately calculating changes in position relative to a known position. The desired torque to be applied and the actual speed of the upper carriage, which are related parameters based on the moment of inertia of the upper carriage, may be determined based on a derivative-type or rate of change over time calculation of desired and actual position information. Similarly, the desired and actual acceleration of the upper carriage may be determined based on a derivative-type or rate of change over time calculation of desired and actual speeds, respectively. The state determinator 314 is further configured to determine changes in the direction of swing by, for example, monitoring speed, acceleration and/or position parameters as available and applicable.
In the embodiment illustrated in
The signal indicative of the torque request to be applied by the swing motor 118 (
In the illustrated embodiment, the table 321 is activated to receive signals 304 during rotation of the upper structure by the operator. The estimated or measured speed magnitude 322 is determined at a calculation block 324 based on the signal 308 from the sensor 306 or by any other method, for example, by interrogation of the electrical current being provided to the swing motor 118 (
When the control lever 302 is placed at a neutral position or a position indicating that the operators wishes the upper structure to stop swinging (hereinafter, zero position) while the upper structure is in motion, the state determinator 314 ceases to provide the signal 304 to the speed map 321 and stops outputting signal 320 and instead switches to provide the signal 304 to a braking map 323. The braking map 323 is populated with braking torque values that are required to provide an initial deceleration to the upper structure 104. In the illustrated embodiment, the signal 304 is provided to the braking map 323 to allow for operator adjustments during braking. Nevertheless, use of the signal 304 in this operating state, i.e., during braking of the upper structure, is optional and may be omitted such that the braking torque values 328 provided by the braking map 323 may be solely based on the measured speed magnitude 322. As can be appreciated, the braking torque value 328 is provided to replace the output of the PID controller 316 instead of being provided directly to the motor 118 through a junction 330, as shown in
The PID controller 316 is further configured to operate using variable gains. As is known, a PID controller can typically use proportional, derivative and integral gains during operation, which are selected based on various structural aspects of a particular system as well as based on the desired dynamic response of the system. In the illustrated embodiment, the PID controller 316 is configured to receive its operating proportional, integral and/or derivative gains 332 from a gain schedule or gain table 334. The gain table 334 is populated with variable gain values that are selected based on a state input 336 provided by the state determinator 314, which in the illustrated embodiment is determined based on the signals 304 and 308.
More particularly, at least a first set of gains is provided in the gains table 334 for use during a first operating state at which the upper structure is swinging while the operator maintains the control lever 302 displaced to a certain degree. As can be appreciated, the first set of gains may include a single set of gain values 332 provided to the PID controller 316, or it may alternatively include ranges of gain values that are tabulated against the input signal 304 from the control lever 302.
The gains table 334 further includes at least a second set of gains for use during a second operating state at which the swing of the upper structure is undergoing braking after the operator restores the control lever 302 to the zero position. As in the first set of gains, the second set of gains may include a single set of gain values, which are more aggressive than the first set of values. Alternatively, the second set of gains may include a range of values that are selected based on the speed of the upper structure or, as shown in the illustrated embodiment, based on the signal 308 from the sensor 306. The signal 308 can be provided to the gains table 334 via the state determinator 314 when the control lever 302 is set to the zero position.
To better illustrate the operation of the swing control under various operating states, a state-flow diagram for a method of operating the swing control mechanism for an upper structure of a machine is shown in
The normal operating state 402 is active while the swing lever is at a non-zero position indicating that the operator is commanding a swing motion. When the swing lever is placed at the zero position, an initial deceleration or stopping operating state 404 is activated. With reference to
The magnitude of the braking torque applied when braking the upper structure may be determined by use of a relatively aggressive torque lookup table, which determines braking torque based on the swing speed magnitude of the upper structure, as previously described. In an alternative method, the braking torque may be determined by use of a PID controller operating with aggressive gains that are intended to emulate operation of the upper structure under a hydraulic swing drive system. The braking torque is applied while the swing lever is maintained at the zero position, which indicates that the operator still expects the upper structure to stop swinging. In the event that, during the stopping operating state 404 and before the speed of the upper structure reaches zero, the swing lever is moved from the zero position, a transition 408 switches back to the normal operating state 402.
The stopping operating state 404 is maintained until the speed of the upper structure reaches zero. Because of the rotational inertia of the upper structure and, in general, due to general overall system delays, even with the application of a zero torque request when speed reaches zero, the speed of the upper structure will cross zero speed and begin tending to rotate in the opposite (and undesired) direction. In other words, if swinging in the CW direction, the upper structure will stop and begin to slightly rotate in the CCW direction. When the swing of the upper structure changes direction, and while the swing lever is maintained at a zero position, a transition 410 will occur to a third or settling to zero speed operating state 412. In the illustrated embodiment, the settling operating state 412 is shown as separate from the normal operating state 402 for purpose of illustration but the two states may be integrated into a single operating state.
During operation in the settling to zero speed operating state 412, a controller provides less aggressive but appropriate torque commands to the swing motor such that the upper structure settles to a zero speed in a smooth and non-aggressive manner. Torque response during the settling to zero speed operating state 412 can be less aggressive than torque response in the stopping operating state 404 and the normal operating state 402. In one embodiment, for example, torque control at the stopping state 404 may emphasize the proportional component by having an aggressive proportional gain of a PID controller, while torque control at the settling to zero speed operating state 412 may de-emphasize the proportional component. Once speed has settled at zero, the controller may remain in the settling to zero speed operating state 412 until the swing lever is moved from the zero position. At that time, a transition 414 may switch operation back to the normal operating state 402 so that the appropriate swing may be carried out. The switching between the normal, braking and settling operating states 402, 404 and 412 may repeat each time an operator commands a swing.
Certain qualitative graphs illustrating various operating parameters during an exemplary braking operation encompassing all three operating states previously discussed are shown in
In the illustrated exemplary event, the command 502 is non-zero in the first state 510 until the first line 508 is reached. During the first state 510, the substantially constant command 502 causes a substantially constant swing speed magnitude 504, which may be in the CW or CCW direction. A maintenance torque 506 is provided during the first state 510 to overcome friction and maintain the swing speed magnitude 504. In this operating state the proportional gain 507 is at a normal operating level for the PID control. When the command crosses the first line 508 it drops to zero and, in the illustrated example, remains at zero through the second and third states 512 and 516. The speed 504 drops to zero over the duration of the second state 512 at a substantially constant downward slope. This type of aggressive and constant deceleration rate of the upper structure during the second state 512 is desired for controllability of the machine and is consistent with machines having hydraulic swing mechanisms. In this operating state the proportional gain 507 is high for the PID control to keep torque constant and aggressive all the way to zero speed. The speed 504 reaches zero at the second line 514, and undershoots the zero speed before settling to zero during the third state 516.
The torque 506 switches direction to oppose the swing motion of the upper structure when the second state 512 is initiated. As can be seen from the graph, the magnitude of the braking torque 506 during the second state 512 is larger than the driving torque 506 applied during the first state 510. The torque 506 non-aggressively reduces to zero during the third state 516 as the speed 504 settles to zero. In this operating state the proportional gain 507 is small for the PID control to prevent machine oscillations around zero speed. As shown, the value of the proportional gain 507 in this operating condition is lower than that used during the first state 510 but in an alternative embodiment a proportional gain that is the same as that of the first state 510 may be used.
The present disclosure is applicable to machines having electrically driven swing mechanisms for upper structures rotatably associated with under carriages. In the past, certain applications that extensively used hydraulic systems, especially for operating their work implements, such as excavators and cranes, included hydraulically driven swing control mechanisms. In a typical hydraulically powered swing mechanism, a hydraulic pump provided pressurized fluid to a hydraulic swing motor to effect swing motion of the upper structure. When stopping the swinging motion of the upper structure with a hydraulically powered drive, the rotational momentum of the upper structure is primarily dissipated by use of relief valves that circulate fluid around a closed loop that includes the hydraulic swing motor. Such relief valves are configured to maintain a pressure at the outlet of the hydraulic swing motor, which provides a braking torque that aids to stop the swinging motion of the upper structure. When converting to an electrically powered swing control mechanism, however, a drift was noticed as the electrical controller associated with the electric swing motor settled to zero speed when braking the swing motion of the upper structure.
The functionality and system behavior that emulates a hydraulically operated swing control mechanism with an electrically driven swing motor is desirable for various reasons such as the avoidance for the need to re-train operators. To effectively provide the braking torque that is desired when the upper structure's swing is stopped, while still providing fine torque control during a swing, the disclosed systems and methods use an operating state-based control scheme. In a first state of operation, fine torque control can be provided by a PID controller during the swing. When a zero swing speed is desired, in one embodiment, the PID controller's gains are made aggressive to provide a more proportional-term-driven response in a second state of operation. Stated differently, control is taken away from the fine-control PID controller to provide sufficient braking torque. When most of the rotational momentum of the upper structure has been absorbed, fine control is once again provided in a third state of operation to help the upper structure settle to a zero speed.
In general, operation of the machine with an electrically powered swing control is more efficient and can provide consistent and reliable operation of the machine under various operating conditions. For example, in reference to
It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated.
Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.
