1. Field of the Invention
This invention relates to a method of electronically attenuating the torque command based on a polar grid modeled on the torque profile of a positive displacement pump in order to produce a constant pump pressure regardless of pump radial crankshaft/camshaft/crankarm location and the velocity of the fluid being pumped. In the method, an electronic processor compares the shaft displacement angle of the pump input shaft to a reference polar grid of the torque profile and varies the electrical power applied to the pump motor. The processor can also take into account the response time of the pump drive, the motor inductive reactance, system inertia, application characteristics of the pump, and regenerative energy during deceleration of the pump.
This invention also relates to a precision hydraulic energy delivery system. Direct coupling of the pump to a primary mover (motor) and related motor control allows for complete motion control of a hydraulically driven machine without the use of any downstream devices. By employing motion control algorithms in the motor control, the hydraulic output at the pump head is controlled in a feed forward method.
2. Description of the Prior Art
In the prior art, it is well known that in situations where higher pressures of fluid movement are desired, a positive displacement pump is commonly used. A positive displacement pump is usually a variation of a reciprocating piston and a cylinder, of which the flow is controlled by some sort of valving. Reciprocal machinery, however can be less attractive to use than rotary machinery because the output of a reciprocal machine is cyclic, where the cylinder alternatively pumps or fills, therefore there are breaks in the output. This disadvantage can be overcome to a certain extent by: using multiple cylinders; bypassing the pump output through flow accumulators, attenuators, dampers; or waste gating the excess pressure thereby removing the high pressure output of the flow.
In addition to uneven pressure and flow output, reciprocating pumps have the disadvantage of uneven power input proportional to their output. This causes excessive wear and tear on the apparatus, and is inefficient because the pump drive must be sized for the high torque required when the position of the pump connecting rod or cam, in the case of an axial (wobble plate) pump, is at an angular displacement versus the crankarm dimension during the compression stroke that would result in the highest required input shaft torque.
Moreover, if the demand of the application varies, complicated bypass, recirculation, or waste gate systems must be used to keep the system from xe2x80x9cdead-headingxe2x80x9d. That is, if flow output is blocked when the pump is in operation, the pump will either breakdown by the increased pressure or stall. If stalling occurs, a conventional induction electric motor will burn out as it assimilates a locked rotor condition with full rated voltage and amperage applied. Typically systems with fixed displacement pumps use a relief valve to control the maximum system pressure when under load. Therefore, the pump delivers full flow at full pressure regardless of the application thus wasting a large amount of power.
In this regard, certain prior art that attempts to correct the problems associated with torque output of a pump motor should be noted.
In U.S. Pat. No. 5,971,721, an eccentric transmission transmits a torque demand from a reciprocating pump, which varies with time, to the drive motor such that the torque demand on the drive motor is substantially constant. The result is the leveling of torque variation required to drive a positive displacement pump at the transmission input shaft with the effect of constant pump output pressure. This is accomplished by means of eccentric pitch circle sprocket sets with gear belts or eccentric pitch circle matched gear sets.
The use of an eccentric gear or sprocket set, has a significant effect on the overall torque requirement and the magnitude of the discharge pulse of the pump. But, because most pumps are of a multi-cylinder or are vane or gear types, the pump input shaft torque requirement would not be perfectly counter-acted (leveled) by using the reduction pattern developed by eccentrically matched transmission components.
In U.S. Pat. No. 5,947,693, a position sensor outputs a signal by sensing the position of a piston in a linear compressor. A controller receives the position signal and sends a control signal to control directional motion output from a linear motor.
In U.S. Pat. No. 4,726,738, eighteen or nineteen torque leads are measured along the main shaft in order to maintain constant shaft velocity revolution and are translated to a required motor torque for particular angles of the main shaft.
U.S. Pat. No. 4,971,522 uses a cyclic lead transducer input and tachometer signal input to a controller to signal varied cyclic motor input controls to provide the required motor torque output. A flywheel is coupled to the motor in order to maintain shaft velocity. However, the speed of the motor is widely varied and the torque is varied to a smaller extent.
U.S. Pat. No. 5,141,402 discloses an electrical current and frequency applied to the motor which are varied according to fluid pressure and flow signals from the pump.
U.S. Pat. No. 5,295,737 discloses a motor output which is varied by a current regulator according to a predetermined cyclic pressure output requirement. The motor speed is set to be proportional to the volume consumed and inversely proportional to the pressure.
It is seen from the foregoing that there is a need for electronic attenuation of the torque profile in a pump. When the torque profile is compared with the input shaft displacement and other known factors such as system inertia and response time of the pump drive etc . . . , a pump can produce constant pressure and therefore constant flow without the typically associated ripple common to power pumps for the full range of the designed volumetric delivery, by driving them in a feed forward method.
It should be noted that the foregoing hydraulic pumping systems control output pressure and flow in the micro sense. These concepts examine modulating the input shaft torque and speed to provide a constant hydraulic output, whether it be pressure or flow limited. See U.S. Pat. No. 5,971,721 and U.S. patent application Ser. No. 09/821,603, the contents of which are hereby incorporated by reference.
It should be further noted that attempts to provide a high dynamic range of hydraulic flow and pressure during operation of prior pumping systems, required placement of downstream devices in the liquid path to modulate the hydraulic output. With such systems, the pump provides the maximum hydraulic flow (as the prime mover) and the downstream devices adjust the output to match the application requirements.
The prime mover in such systems is typically a constant speed induction motor. In to order to control the hydraulic output, feedback devices, a processor (be it mechanically balanced or electronic) and hydraulic servo valves must be placed into the hydraulic stream for flow and pressure regulation. This treatment of hydraulic delivery places the xe2x80x9csmartsxe2x80x9d of the system in the hydraulic output portion of the system. Disadvantageously, these systems require many hydraulically driven devices, are mechanically (geometry) limited, are energy inefficient when total system performance is scrutinized and have a small range of dynamic response (typically 10-1).
Moving the xe2x80x9csmartsxe2x80x9d directly into the prime moverxe2x80x94by incorporating variable speed (VFC) controlled motorsxe2x80x94has been attempted. However, this provides limited torque delivery potential at low speeds, and many feedback devices are required for its operation. Further, the response of such a system is only generally higher than the 150 ms range and the energy savings potential is only in the 50% range.
These approaches addressxe2x80x94in the macro sensexe2x80x94the need for a prime mover coupled to a power pump that controls the energy, and therefore the flow (velocity) and pressure (torque) at the input shaft of the pump. Moreover, the desired system must replicate the motion control capabilities of existing systems without requiring the use of downstream flow control devices and feedback circuits.
It is therefore an object of the present invention to provide a method for electronic attenuation of pump torque variation requirements in order to produce a matched motor torque output that will result in constant output pressure from a pump.
It is therefore a further object of the present invention to provide control factors which vary the power and torque output of a pump motor based on calculated torque variation requirements.
It is therefore a still further object of the present invention to increase the energy efficiency of a pump system, by providing a force balanced relationship between the motor output and the application""s hydraulic requirement, thus allowing the use of energy saving torque drives without incurring the pressure variations associated with their use.
It is therefore a still further object of the present invention to decrease the wear and tear on the pump by providing a substantially constant force output from the motor of the pump and reduce the amount of cycles of the pump to the application""s requirement.
It is therefore a further object of the present invention to provide a method for electronic attenuation of pump torque variation by supplying information for design of an electronic transmission system that can achieve a modulated torque output from the motor to the pump.
To attain the objects described, there is provided a method for obtaining a polar map for process control within the electronic drive of a targeted pump. This polar map is calculated by a processor or is externally calculated then input into a processor. Once the torque profile of the pump is obtained and translated into a polar map, the processor can compare the shaft displacement angle of the pump input shaft to the reference polar map. The processor can also take into account selected factors such as the response time of the pump drive, the motor inductive reactance, system inertia, application characteristics of the pump, and regenerative energy during deceleration of the pump.
Using selected factors and the comparison results, the processor then signals the motor controller to vary the amperage, voltage, and frequency applied to the motor in order to regulate the torque output of the pump motor. With an accurately modulated motor torque output in concert with the established polar map (for the targeted pump), the pump output pressure will remain constant regardless of the pump""s crank arm location or the velocity of fluid flow.
It is also an object of the present invention to provide a hydraulic energy delivery system that allows for complete motion control of a hydraulically driven machine with the use of minimal or no downstream feedback devices.
It is therefore a further object of the present invention to provide control factors which vary the power and torque output of a pump motor by employing motion control algorithms.
To attain the objects described, there is provided direct coupling of a positive displacement pump to a pump drive motor and related controls. By employing motion control algorithms into the motor control, the hydraulic output at the pump head will simultaneously follow. Control features listed herein may be integrated into the system by developing algorithms and subroutines for the control system coupled to the pump.
The present invention will now be described in more complete detail with reference being made to the figures identified below.