Patent Application
20090317266
The present invention relates to a hydraulic pump or motor having control of individual working chambers driving a load and relates particularly, but not exclusively, to an arrangement where the torque provided to that load may be varied rapidly.
One example of the above-mentioned pump/motor arrangement resides in the hydraulic drive of vehicle wheels in both on-road and off-road applications. Such arrangements have been the subject of much prior activity and the transmissions employed therewith comprise one or more fixed or variable-displacement hydraulic motors at individual wheels or alternatively motors positioned to drive groups of wheels.
In the most common positive displacement hydraulic machines the fluid chambers undergo cyclical variations in volume following a roughly sinusoidal function. It is known from EP0361927 that a chamber can be left to idle by holding an electromagnetically actuated valve, between the working chamber and the low-pressure source, in the open condition. Thus the output is varied through the action of first filling each working chamber with liquid, then deciding whether to reject the liquid back to the low-pressure source or to pump it at pressure to the output manifold. Pumping the liquid back to the low-pressure source means that a very small amount of power needs to be expended, during the time that a working chamber is idle, whilst still allowing the working chambers to become productive with a minimum latency period.
EP0494236 introduces an additional operating mode which allows the use of the hydraulic machine in a motoring cycle where torque is applied to the rotating shaft, thus allowing a controllable bi-directional energy flow. This type of motor has until now been limited to a control bandwidth and latency of half a shaft revolution, which for example is 17 Hz at 500 RPM, because whole cylinders are selected. However, there are a number of applications where high frequency torque adjustments could offer new and desirable capabilities.
This present invention provides a way of controlling hydraulic motors so as to modulate the output torque at frequencies of up to around 200 Hz and may lend itself to application in a number of fields, some of which are described in detail below by way of background information:
An increasingly common requirement in automotive drivelines is that the torque at individual wheels or groups of wheels (for example, rear axle, front axle) must be able to be modulated in both the braking and accelerating modes in order to limit wheel slip. This requirement is due to two factors. Firstly, slipping wheels do not allow the driver to maintain directional control of the vehicle and generally provide less decelerating or accelerating force than wheels that are not slipping. Secondly, the point at which individual wheels or groups of wheels start to slip is different from one another even contemporaneously on an individual vehicle. Slippage is determined by wheel or axle loading, weight transfer during braking and cornering, and the road surface conditions that may be different for different wheels.
Typically in vehicles with a conventional mechanical transmission between engine and wheels the torque modulation is achieved through the momentary application of the friction brakes to one or more wheels, under the control of a central vehicle stability controller. The controller typically takes as its inputs individual wheel speeds, vehicle angular acceleration rates, accelerator pedal position and steering wheel angle, and uses that to modulate the brakes. This system is typically called Antilock Braking System (ABS) when it operates only when braking, or Electronic Stability Control (ESC) when it operates in both braking and accelerating and includes sensors for vehicle movement and acceleration so as to control yaw. As well as applying the friction brakes, the engine torque may be reduced through the use of ignition retarding or interruption, reducing the fuelling rate or adjusting the throttle position.
Other systems exist to vary the distribution of torque to several driveshafts under electronic control. One such system is the E-Diff or Electronic Differential that uses electrohydraulically-actuated friction clutches to distribute torque between two or more driveshafts.
It is essential in all systems that the torque modulation is rapid so as to maintain the optimum slip rate for traction as much as possible. Typical bandwidths are around 10 Hz. ABS brakes, for example, are known to operate at up to 13 Hz. This is not generally high enough to maintain the wheels in the optimal slip condition (generally considered to be about 5-10% slip), but high enough to keep them oscillating between slipping and not slipping.
Another application where high torque control bandwidth is desirable is in the driving of electric generators. In this application one or more hydraulic motor(s) may drive one or more synchronous generator(s) for electrical supply to the distribution grid or an isolated power network. The shaft speed of these machines is linked to the AC voltage of the network. The modulation of shaft torque causes a near-instantaneous modulation of the generator current
Harmonic distortion of the current taken by loads on electrical grid, commonly caused by loads such as electronic equipment power supplies, is a high frequency intra-cycle variation from the intended sinusoidal wave, causing a corresponding high frequency intra-cycle variation of required shaft torque. By modulating the torque applied by the hydraulic motor to the generator at high enough frequencies, the current supplied to the grid can be modulated (without requiring complex power electronics) thereby helping to restore the voltage waveform to the required state. This capability also requires accurate control of the phase of the corrections with respect to the generator (and therefore hydraulic motor) shaft.
A separate problem is that the frequency of the AC voltage may start to deviate above or below the desired frequency due to a short term or sudden mismatch between the grid load and grid supply, for example when a new load is turned on or off. In the case that the a load is turned off, the frequency increases above the desired frequency. Reducing the generator torque reduces the power output and restores the correct frequency. If the generator torque can be modulated quickly enough then even very sudden load changes can be accommodated without a change to the grid frequency.
A further application where high torque control bandwidth is desirable is when a shaft is driving or being driven from a structure with a large mass that may suffer from vibratory resonances, for example a wind or tidal turbine. If the structure is excited with a torque load having a frequency matching a resonant frequency of the structure, damage to the shaft, attached machines, and the structure may result. By the correct control of the shaft torque at high enough frequency the resonances can be avoided, or eliminated if they have already begun.
From the above it will be appreciated that there are numerous applications in which rapid changes in output torque or power delivery are essential control requirements and many of these fail to employ hydraulic pump/motor arrangements due to their inability to provide adequate control. It is, therefore, an object of the present invention to provide a method of and apparatus for modulating the fluid output from a hydraulic pump/motor which is able to respond rapidly to changes in demand and which may also allow hydraulic pump/motors to be employed in control applications that they have hitherto been excluded from.
According to the present invention there is provided a hydraulic pump/motor with a plurality of cylinders each with a low pressure and high pressure actuated poppet valve under the control of a controller, where the controller is able to provide a sequence of signals to the valves in a phased relationship with the pump/motor shaft so as to effect either a pumping or motoring cycle but has the added capability to modify individual high pressure valve signals in the sequence to lengthen, shorten or adjust the time that the valves remain open, and so to provide for modulation of the pump/motor's torque output.
According to one aspect, the present invention provides: a method of controlling a fluid working machine having: one or more working chambers of cyclically varying volume; one or more inlet valves; one or more outlet valves; a rotating shaft driven by or driving a load; and a controller for receiving data on a first changeable parameter and controlling the operating and closing sequence of said valves to selectively enable said working chamber separately on each of the expansion and contraction strokes of said chamber, so as to supply or accept fluid in accordance with said first changeable parameter, the method comprising the steps of: monitoring a second changeable parameter requiring control at a higher frequency or having a lower latency than said first changeable parameter; and modifying the valve actuation to supply or accept fluid demand in accordance with a combination of said first and said second changeable parameter.
According to a further aspect of the present invention there is provided a fluid working machine having: one or more working chambers of cyclically varying volume; one or more inlet valves; one or more outlet valves; a rotating shaft driven by or driving a load; a controller, for receiving data on a first changeable parameter and controlling the opening and closing sequence of said valves able to selectively enable said individual working chambers separately on each of the expansion and contraction strokes of said chambers, so as to supply or accept fluid in accordance with said first changeable parameter, a monitor, for monitoring a second changeable parameter requiring control at a higher frequency or having a lower latency than said first changeable parameter; wherein said controller monitors said second changeable parameter and modifies the valve actuation to supply or accept fluid demand in accordance with a combination of said first and said second changeable parameter.
The present invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:
Referring now to the drawings in general but particularly to
The above pump/motor or fluid working machine 10 has three modes of operation namely: idling, motoring and pumping. With valve 12 in the open position, and valve 24 in the closed position the pressure in the high-pressure manifold 26 is at or above the pressure of the low-pressure manifold 14 and the pressure in the working chamber 16 equals the pressure of the low-pressure manifold. Pumping the fluid quanta taken into the working chamber back to the low-pressure manifold 14 defines the idle mode in which the fluid quanta is not employed but is merely recycled for possible future use. This stroke has a low parasitic loss as a very small amount of power needs to be expended and no useful work is done.
A motoring cycle starts by the closing of valve 12 at some point in the contraction stroke of the piston 20. With valve 12 in the closed position and valve 24 in the closed position, the pressure in the high-pressure manifold 26 and the pumping chamber 16 will equalise. For optimum motoring operation, i.e. the largest net fluid intake from the high-pressure supply, the timing of the closing of valve 12 is determined so that the equalisation happens at or shortly before Top Dead Centre of the piston motion (TDC) of the piston 20 movement. Once the pressure has equalised, valve 24 can be commanded to open such that the pumping chamber 16 is connected to the high-pressure manifold 26, and disconnected from the low-pressure manifold 14 by valve 12 so that the high-pressure fluid can act on the working chamber 16 to drive the piston 20 down and thereby producing torque on a crankshaft to which the piston is coupled (best seen in
A pumping cycle starts by the closing of valve 12 at some point in the contraction stroke of the piston 20 or at the beginning of said stroke. With valve 12 in the closed position and valve 24 in the closed position, the pressure in the high-pressure manifold 26 and the pumping chamber 16 will equalise. Once the pressure has equalised, valve 24 can be commanded to open or opens passively such that the pumping chamber 16 is connected to the high-pressure manifold 26, and disconnected from the low-pressure manifold 14 by valve 12. Valve 24 closes as the flow rate through it reaches zero at top-dead-centre (TDC), once again isolating the chamber 16 from the high-pressure manifold. The subsequent expansion of the working volume 22, as the pumping chamber 16 passes its limit condition, will depressurise the fluid in it and allow valve 12 to open. With valve 12 in the open position and valve 24 in the closed position it is now possible to intake fluid into the chamber 16 from the low-pressure manifold 14 as the pumping chamber 16 moves toward its bottom-dead-centre (BDC) position.
It will be appreciated that because the location within the cycle where the valve 12 and valve 24 open and close is under control of and therefore known by the controller, that the volume of quanta displaced into or received from the high-pressure supply in any cycle will be known.
It will further be appreciated that the above mentioned fluid quanta are equivalent to quanta of energy that can be added or subtracted to the kinetic energy of a load to which the pump/motor is connected. The amount of energy in each fluid quanta is directly related to the pressure and volume of the quanta by the equation
E= 1/10PV
where E is the energy in Joules, P is the pressure in Bar, and V is the volume in cubic centimetres. The speed change of an inertial load attached to the pump/motor 10 is easily worked out from the change in kinetic energy if the inertia I is known:
where RPM1 and RPM2 are respectively the initial and final shaft speeds, ΔE is the change in energy in the shaft, and/is the rotational inertia in kg m2.
The controller is provided with a means of monitoring a first changeable parameter related to the demanded displacement of fluid from the working chambers aggregated as a whole. By means of a control strategy employing the appropriate selection of pumping, motoring or idle strokes on several chambers, the controller can achieve the demand according to the first changeable parameter by the time-averaged volume of quanta output or input from or into said chambers actually being used.
In addition to the general or first control strategy discussed above in relation to
The system of
The further degree of monitoring or control is facilitated by the provision of one or more application-specific sensors shown generally at 90 and being connected to a modulation planner 92 through sensor connections shown generally at 94. The application sensors 90 convey to the modulation planner 92 information about the load behaviour (or another physical response that is under the influence of the load) so that the modulation planner 92 can determine whether the behaviour is as was intended by the general controller 32. Where the behaviour is not as intended, the modulation planner 92 provides a modulation request to a sequence modulator 96 though a modulation request channel 98. The modulation request would normally be arranged so as to bring the load behaviour back towards the desired behaviour. The sequence modulator 96 takes the modulation request, the shaft position and speed from the shaft sensor channel 58 and the valve operating sequence from the sequence channel 74, and provides a new valve sequence according to one of the nine sequence modifying methods described below herein. The new sequence is generated in phase lock with the shaft. A modulated valve sequence channel 100 conveys the new sequence to an amplifier 102 which provides the valve signals to the solenoids 12, 24 of the pump/motor 10 through the valve wiring 34, 36.
It will be appreciated that a number of secondary sensors 90 may be provided in order to improve the control aspects of the presently described system, the sensors provided varying with the application. For example the sensors required for traction control include one or more of: steering wheel angle; yaw rate; acceleration; wheel velocity; wheel angular acceleration; wheel slip; vehicle lateral acceleration; vehicle velocity and brake line pressure. Optional additional sensors could be employed for monitoring one or more of: vehicle roll rate and acceleration; vehicle pitch rate and acceleration; braking force applied at each wheel; tyre air pressure; vehicle acceleration and deceleration; payload mass and distribution thereof. For the purposes of brevity these are shown collectively at 90.
The sensors required for generator drive include one or more of: power factor, shaft speed and torque, grid frequency, and current and voltage harmonic frequency content.
Sensors required for a machine attached to a large structure such as a wind or tidal turbine include one or more of: accelerometers for detecting blade vibration; shaft torque and speed; blade pitch; blade velocity; blade tip relative position and velocity. It will be appreciated that the various electronic control components described above may be provided in any combination within the same physical unit 120, within groups of units or as individual components, and related communication channels and connections may be provided in software.
The following describe nine ways in which the pump/motor 10 valve operations may be altered by the modulation of controller 32, 120 so as to modify the sequence of fluid quanta accepted into or rejected from the pump/motor 10. These are each shown in
It will now be appreciated that by using the above techniques according to the need, the modulation planner may alter the torque or flow of the pump/motor between 100% motoring and 100% pumping, regardless of the original time-averaged torque or flow commanded by the sequence generator. In the case of a response to a sudden, unexpected, condition, modulation can be achieved with a delay of typically as little as 2 milliseconds.
Because the changes to the volume of fluid quanta accepted into or rejected from the cylinder and torque applied to the shaft are predictable in each case, the sequence modulator is able to communicate electronically the effect of the sequence alteration on either fluid displacement or shaft torque to the general controller, though the modulation reporting channel. The general controller may report a different requirement to the fluid supply using the modulation reporting channel.
An important property of the modulation is that the expected size of the quantum or change in shaft torque can be determined by the sequence modulator 96 at the moment the decision to alter the sequence is made, whereas the effect itself is generally felt a small time later. This advanced knowledge would be useful because the general controller 32 and fluid supply could have enough time to change their behaviour to suit the new conditions. For example, if the sequence modulator 96 momentarily reduces the pump/motor's fluid intake, the supply 60 could momentarily reduce its fluid output so as to avoid raising the pressure in the high pressure manifold 14. The fluid supply could in fact use one of the flow modulating methods described above herein, or alternative methods.
Alternatively, the fluid supply 60 may use its own sensors to determine the fluid flow rate required to maintain a zero net flow into or out of the high pressure manifold 14. In this way the fluid supply 60 is enslaved to the pump/motor 10. The fluid supply 60 may also utilise its own sensors shown schematically at 122 to correct for small, slowly introduced, inaccuracies in the quanta information or its own inaccuracy, for example leakage of hydraulic fluid to the low pressure side 64.
Referring now to
Systems with multiple pump/motors 10 such as the one shown in
The fluid supply 60 could adjust its flow to the fluid summing junction 130 by determining the total volume of fluid required from the quanta size and number, and providing the appropriate number and size of quanta itself. Where the fluid supply 60 is not able to or it is not desired to supply the fluid in quanta, the supply 60 could average the net volume of the quanta accepted and rejected over a suitable time period to determine a fluid flow rate to the summing junction 130.
Where the total energy ejected by the loads exceeds the total energy accepted, there will be a net outflow of energy (and fluid) into the fluid supply 60. The fluid supply 60 may be able to store this energy for later use for example in an accumulator or flywheel, or may dissipate the energy for example in a pressure relief valve or throttle valve arrangement.
Depending on the application, there are several possible events that could trigger the sequence modulation, some of which are described below:
In a vehicle traction control application, when any one or more of the sensors indicate an undesirable vehicle movement has occurred or is about to occur, the sequence modulator can initiate sequence modification according to one of the methods mentioned above herein. This will almost instantaneously alter the volume of the fluid quanta accepted or rejected by the pump/motor. The additional or missing quanta impart greater or lesser energy to each wheel, causing them to provide greater or lesser torque than they would have done had the controlling pulse sequence not been adjusted. A specific example of this occurs when a wheel speed sensor detects that the velocity of a wheel providing forward tractive torque increases too quickly during acceleration, possibly because the friction limit of the road surface and tyre combination has been exceeded. The sequence modulator would use one of the above methods to reduce the wheel torque, which will reduce the wheel speed and regain traction. The fluid supply could be informed of the alteration of the number and size of the fluid quanta accepted thereby to allow it to modify its output appropriately.
A second specific example occurs when the vehicle stability control computer finds a difference between the driver's intended travel direction and velocity as indicated by the steering yaw sensor, brake pressure input, accelerator pedal position and wheel speed sensors, and the vehicle's response sensed via lateral acceleration, yaw and individual wheel speed sensors. When the system detects that the measured intention is different to the measured response, modulation can be initiated in specific wheels so as to create traction forces acting on the vehicle to correct the difference. The modulation may also be accompanied by the reduction or increase of prime mover power and/or the operation of the mechanical brakes on individual driven or non-driven wheels. The system may transfer quanta of energy from left to right wheels or front to back wheels or a combination of them such that a restoring moment can be created rapidly without requiring the fluid pressure source to respond rapidly.
Because fluid quanta can be transferred between machines, it is possible for the system to provide control of the vehicle trajectory by transferring energy from left to right or front to back even when the pressure supply is temporarily not capable of providing fluid, for instance because the prime mover of the fluid supply is stopped.
A sophisticated vehicle stability control computer could examine the trend in one or more sensors and predict when an undesirable situation is about to happen, and initiate the sequence modification before that occurs.
In the application of the invention to the driving of an electric generator itself connected to an electrical distribution network or ‘grid’, a voltage, current or frequency sensor may detect the presence of grid voltages or frequencies not conforming to the required specification. In a specific example, a sensor may detect that the normally sinusoidally varying voltage output from the generator is not sinusoidal, having instead flattened peaks caused by the drawing of high currents at said peaks as is often the case where rectifying electrical loads are connected to the grid. By means of the invention, the modulating controller can increase the torque within a motoring stroke at the time of the peaks so as to maintain the required rotational speed profile, and therefore the required sinusoidally varying voltage profile. The modulating controller would typically employ an adaptive element that adjusts over several cycles to the amount of peak flattening, and adjusts the corrective modulation until the peak is properly sinusoidal.
In the application of the invention to the conversion of fluid kinetic energy to hydraulic energy through the means of blades such as those of a wind or water turbine, propeller or impeller, sequence modulation may be initiated as a result of characteristics of signals received from accelerometers measuring blade vibrations, the torque on the machine shafts or blade roots, the pitch of the blade, the position of points on the blade relative to other parts of the blade or device, the fluid velocity and blade velocity. A first specific example is where the specific resonant frequencies of the blades are known a priori. The sequence of cylinder actuations generated by the sequence generator can be monitored so as to infer the frequency characteristics of the torque applied to the blades, shortly before said torque is actually applied. Where the sequence is predicted to excite a specific undesirable resonance in the blades, the modulating controller can introduce an anti-phase torque modulation of the same amplitude to prevent or reduce the amplitude of the resonance. A second specific example of this occurs when accelerometers attached to the blades at certain points, said points being anti-nodes of undesirable vibratory modes of the blade, detect and quantify a vibratory resonance present or developing on the blade. The modulating controller may then add a high frequency torque component in anti-phase with the resonance and of the correct amplitude on top of the shaft torque being applied due to the normal operation, utilising the new high frequency and low latency capabilities of the invention for the most rapid vibration cancellation. A third specific example of this is the same as the second, but that the detection and quantification of the undesirable vibration can be detected by measuring the relative position of the anti-nodes using a sensor indicating the relative position of the anti-nodes compared to a reference point such as the shaft or blade attachment point.
From the above, it will be appreciated that the present invention may be employed in a number of control situations and may also be employed in control situations from which hitherto such hydraulic pump/motors have been excluded.
It will also be appreciated that invention has particular reference to machines where the at least one working chamber comprises a cylinder in which a piston is arranged to reciprocate, but its use with at least one chamber delimited by a flexible diaphragm or a rotary piston is also possible.
Still further, it will be appreciated that the present invention provides a method and apparatus for controlling controllable parameters at a higher frequency or lower latency than might be possible with the arrangements of the prior art.
Still further, it will be appreciated that the present invention may allow a pump/motor that is already being employed to drive a transmission system to be modified in accordance with the present invention and replace other more expensive components which are presently added to an already existing hydraulic pump/motor system.
It will also be appreciated that the source of pressurised fluid 60 may be a pump of the type described herein and, indeed, in some applications of the present invention may be a pump/motor 10 provided as an additional pump/motor as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 0614940.5 | Jul 2006 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2007/050457 | 7/27/2007 | WO | 00 | 1/27/2009 |
