Patent Publication Number: US-2019170170-A1

Title: Hydraulic systems and components including wireless control technology

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is being filed on Aug. 9, 2017 as a PCT International Patent Application and claims the benefit of U.S. Patent Application No. 62/372,597 filed on Aug. 9, 2016, and claims the benefit of U.S. Patent Application No. 62/372,665 filed on Aug. 9, 2016, and claims the benefit of U.S. Patent Application No. 62/372,645 filed on Aug. 9, 2016, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to hydraulic systems and components. More particularly, the present disclosure relates to wireless control technology for electrohydraulic systems and components. 
     BACKGROUND 
     Hydraulic systems traditionally include electrohydraulic components that are electronically controlled. The electrohydraulic components are traditionally connected to corresponding control electronics through wired connections. Wired connections are also made to various hydraulic sensors incorporated within the system. Recently, there has been increased use of distributed sensing and control modules. As the number of sensors and control modules increases, the installation and maintenance burden will increase. 
     Wireless communication can effectively support bi-directional signal communication. However, wireless hydraulic systems still require power. In certain examples, the power for supporting a wireless hydraulic system may be derived from a battery or from a power line. Ultra low power modules can extend battery life, but the use of batteries to power a large number of nodes in a wireless hydraulic system can be cost prohibitive. Furthermore, for control modules where power is needed for controlling additional structures such as solenoids, the current and power requirements may extend beyond those available by batteries. In case of using a power line such as a traditional power cord, the hydraulic system would not be 100% wireless. 
     SUMMARY 
     The present disclosure relates generally to the use of wireless technology to lower installation, commission, and maintenance costs associated with increased intelligence within hydraulic systems. In one possible configuration and by non-limiting example, wireless technologies are used to provide communication between various active electrohydraulic components, sensors, and controllers of a hydraulic system. 
     In one aspect, the disclosed technology relates to a hydraulic system including a harvesting device for converting hydraulic energy into electrical energy, an electrohydraulic component powered by the electrical energy from the harvesting device, and a wireless transceiver powered by the electrical energy from the harvesting device. In one example, the harvesting device includes a hydraulic motor that drives an electric generator. In another example, the harvesting device includes a turbine that drives an electric generator. In one example, the electric generator is a 3-phase brush-less direct current generator. 
     In another aspect, the disclosed technology relates to a hydraulic system including a harvesting device for converting hydraulic energy into electrical energy, a flow control device powered by the harvesting device for actively controlling the hydraulic flow through the harvesting device, an electrohydraulic component powered by the electrical energy from the harvesting device, and a wireless transceiver powered by the electrical energy from the harvesting device. In one example, the harvesting device includes a hydraulic motor that drives an electric generator. In another example, the harvesting device includes a turbine that drives an electric generator. In one example, the electric generator is a 3-phase brush-less direct current generator. In another example, the flow control device includes a solenoid controlled variable sized orifice. In one example, the hydraulic system also includes a controller that varies a size of the variable sized orifice based on the electrical load require to be met by the harvesting device so that the harvesting device self-compensates to reduce voltage level fluctuations caused by variations in electrical loads. 
     In another aspect, the disclosed technology relates to an electrohydraulic assembly including an electrohydraulic component having electronic control circuitry, and a wireless transceiver that interfaces with the electronic control circuitry, wherein the wireless transceiver is configured to receive control commands for the electrohydraulic component, and the wireless transceiver is configured to transmit operational information corresponding to the electrohydraulic component. In one example, the electrohydraulic component includes a valve, motor, actuator, or pump. 
     In another aspect, the disclosed technology relates to a wireless device including a wireless transceiver module configured to interface with a standard wired electrohydraulic component to convert and retrofit the standard wired electrohydraulic component into a wirelessly controlled electrohydraulic component. In one example, the wireless transceiver module includes a plug that interfaces with the standard electrohydraulic component. 
     In another aspect, an electrohydraulic package includes at least one electrohydraulic component, at least one sensor, and a wireless transceiver device that interfaces with the electrohydraulic component and the sensor. 
     In another aspect, the disclosed technology relates to a hydraulic component including a hydraulic sensor, and a wireless device that interfaces with the hydraulic sensor. 
     In another aspect, the disclosed technology an electrohydraulic system including an electrohydraulic component having electronic control circuitry and a first wireless transceiver that interfaces with the electronic control circuitry, the first wireless transceiver is configured to wirelessly receive control commands for the electrohydraulic component, and to wirelessly transmit operational information corresponding to the electrohydraulic component, and a human machine interface including a second wireless transceiver configured to wirelessly transmit the control commands from the human machine interface to the electrohydraulic component, and to wirelessly receive the operation information from the first wireless transceiver. 
     A variety of additional inventive aspects will be set forth in the description that follows. The inventive aspects can relate to individual features and to combinations of features. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad inventive concepts upon which the examples disclosed herein are based. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example wireless electrohydraulic system in accordance with the principles of the present disclosure. 
         FIG. 2  shows a wireless transceiver device such as a wireless plug coupled to an electrohydraulic component. 
         FIG. 3  shows a wireless transceiver device such as a wireless plug coupled to a local electrohydraulic sensor. 
         FIG. 4  shows an electrohydraulic package arrangement including a plurality of electrohydraulic components and a plurality of local sensors that all interface with a common wireless transceiver. 
         FIG. 5  shows a wireless transceiver device interfacing with an electrohydraulic control device such as an electrohydraulic controller, a human machine interface, a touchscreen, a computer, an active display or other device suitable for controlling and interfacing with one or more electrohydraulic components. 
         FIG. 6  shows an electrohydraulic system in which a plurality of wireless transceivers have been coupled to corresponding sensors and active electrohydraulic components to create an interconnected wireless network. 
         FIG. 7  shows an example wireless electrohydraulic system including an electrohydraulic package having an industrial valve and a plurality of sensors that interfaces wirelessly with a human machine interface (HMI). 
         FIG. 8  illustrates a wireless electrohydraulic system in accordance with the principles of the present disclosure that can harvest hydraulic power and convert the harvested hydraulic power into electrical power for powering electrical components such as a wireless transceiver, sensors, actuators, and controllers. 
         FIG. 9  shows a first example of a self-powered wireless hydraulic system in accordance with the principles of the present disclosure that uses a motor and a generator to harvest hydraulic energy for conversion into electrical power for powering various electronic components. 
         FIG. 10  shows another example of a self-powered wireless hydraulic system in accordance with the principles of the present disclosure that uses a turbine to harvest hydraulic energy for conversion into electrical power for powering various electronic components. 
         FIG. 11  illustrates another example of a self-powered wireless hydraulic system in accordance with the principles of the present disclosure that includes a self-compensation mechanism for controlling a voltage output generated by a harvesting device of the self-powered wireless hydraulic system. 
         FIGS. 12A and 12B  show example phase voltage curves corresponding to an example brushless DC generator that can be used in an energy harvesting device in accordance with the principles of the present disclosure. 
         FIG. 13  is a graph showing motor or generator speed versus time for a system that does not include self-compensation. 
         FIG. 14  is a graph showing output voltage versus time for a system that does not include self-compensation. 
         FIG. 15  is a graph showing pressure differential versus time for a system that does not include self-compensation. 
         FIG. 16  is a graph that shows motor/generator speed versus time for a system that includes self-compensation. 
         FIG. 17  is a graph that shows output voltage versus time for a system that includes self-compensation. 
         FIG. 18  is a graph that shows pressure differential versus time for a system that includes self-compensation. 
         FIG. 19  is a flow chart illustrating a method for self-compensating a self-powered wireless hydraulic system. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure, relate to wireless hydraulic systems. In certain examples, wireless transceivers and electrohydraulic components within such systems can be powered without the use of batteries or a power cord. 
       FIG. 1  shows a first aspect of the present disclosure which relates to a wireless network  2  that can be used to wirelessly connect various components of a hydraulic system  14 . The various components of the hydraulic system  14  can include pumps  4 , fluid conveyance devices  6  (e.g., couplers, etc.), valves  8  such as industrial valves (e.g., proportional valves, solenoid driven valves, etc.), motors and actuators  10  (e.g., hydraulic cylinders), and human machine interfaces (e.g., touchscreens, computers, displays, controllers, joysticks or other structures), electronics, and sensors  12 . It will be appreciated that the wireless network  2  can be arranged in a star topology in which the various components interface with a central control location. It will be appreciated that the wireless network  2  can also be arranged in a mesh topology where the various electrohydraulic components communicate directly with one another. 
       FIG. 2  illustrates an electrohydraulic assembly in accordance with the principles of the present disclosure. The electrohydraulic assembly can include an electrohydraulic component  26  that includes electronic control circuitry. The electrohydraulic assembly can also include a wireless transceiver  30  configured to receive wireless control commands for controlling operation of the electrohydraulic component  26 . The wireless transceiver  30  also can be configured to wirelessly transmit operational information corresponding to the hydraulic component  26 . In certain examples, the hydraulic component  26  can be a valve (e.g., a proportional valve, solenoid valve, etc.), a motor, an actuator or a pump. In certain examples, the wireless transceiver  30  can include a wireless plug  34  that plugs into the electrohydraulic component  26 . In certain examples, the wireless plug  34  is an M8 industrial connector. In certain examples, the electrohydraulic component  26  is a standard wired electrohydraulic component and the wireless transceiver  30  is configured to convert or retrofit the standard wired electrohydraulic component  26  such that the standard wired electrohydraulic component  26  can send and receive information (e.g., control commands, diagnostic information, operational information, positional information, pressure information, temperature information, etc.) wirelessly. 
       FIG. 3  shows a hydraulic component  16  including a local hydraulic sensor  18  and a wireless transceiver  30  that interfaces with the hydraulic sensor  18 . In certain examples, the wireless transceiver  30  can be a wireless plug that converts the local hydraulic sensor  18  from a wired sensor to a wireless sensor. 
       FIG. 4  illustrates a hydraulic package  40  having a plurality of components that are co-located at one location. In certain examples, the hydraulic package  40  includes a plurality of local hydraulic sensors  18  (e.g., pressures sensors, temperature sensors, position sensors, speed sensors, flow sensors, etc.) that interface with a wireless transceiver device  30 . The hydraulic package  40  also includes a plurality of active hydraulic components  26  (e.g., hydraulic pumps, hydraulic motors, hydraulic valves, hydraulic actuators, etc.) that also interface with the wireless transceiver  30 . The wireless transceiver  30  interfaces with all the electrohydraulic components such that all the electronic components can be wirelessly controlled by a remote wireless control unit. The wireless transceiver  30  can also allow the electrohydraulic package to interface directly or indirectly with other electrohydraulic components or electrohydraulic packages that are integrated as part of the wireless network. 
       FIG. 5  shows a wireless transceiver  30  such as a wireless plug interfacing with a standard wired controller  36  which may include a human machine interface  38  such as a touchscreen, programmable display, computer, joystick or other structures. As such, the controller  36  is converted from a standard wired controller to a wireless controller. 
       FIG. 6  shows a hydraulic system  60  including the wireless network  2  for interconnecting various components of the hydraulic system  60  wirelessly. In certain examples, the various components of the hydraulic system  60  can include sensors and active components. The wireless transceivers  30  can be assigned to a single sensor or component (e.g., a local sensor  18  or an active electrohydraulic component  26 ) or one wireless transceiver  30  can be assigned to a plurality of different sensors and components that are generally co-located with respect to one another. 
       FIG. 7  shows an example wireless hydraulic system  80 . The hydraulic system  80  includes an electrohydraulic component such as an industrial valve  82  having electronic control circuitry  84 . The system also includes a transceiver device  86  having a microprocessor  88  that interfaces with the control circuitry  84 . The microprocessor  88  also interfaces with a wireless transceiver unit  90  that may include an antenna. The microprocessor  88  also interfaces with a plurality of sensors such as a position sensor  92  and a pressure sensor  94 . The wireless hydraulic system  80  further includes a wireless gateway  100  including a human machine interface  102  such as a programmable display, joystick, computer, touchscreen, etc. The wireless gateway  100  also includes a wireless device  104  including a CAN shield  106  that interfaces with the human machine interface  102 . The wireless device  104  also includes a microprocessor  108  that interfaces with the CAN shield  106  and also interfaces with a wireless transceiver unit  110  (e.g., a radio frequency module). The wireless gateway  100  can control the wireless hydraulic system  80  wirelessly using open loop pulse width modulation (pwm) with modulation control protocol or closed looped feedback. The wireless gateway  100  can also provide wireless monitoring and diagnostics with respect to the wireless hydraulic system  80 . The monitoring can include solenoid current corresponding to a solenoid used to drive movement of the industrial valve  82 , pressure sensor information, valve position information, cylinder sensor information, etc. 
       FIG. 8  shows a second aspect of the present disclosure which relates to a self-powered wireless hydraulic system  20  in accordance with the principles of the present disclosure. In certain examples, the system  20  can support wireless sensing and control devices within the system such as sensing and control devices for a hydraulic valve, a hydraulic cylinder, a hydraulic pump, a hydraulic motor, a hydraulic sensor, etc. The system  20  can be self-powered to the extent that the system  20  can convert hydraulic power to electrical power for powering the system  20 . Thus, the system  20  does not require a battery and does not require external power to be supplied by a wire such as a power cord. 
     Referring still to  FIG. 8 , the self-powered wireless hydraulic system  20  includes a wireless module  22  and a self-powering module  24 . The wireless module  22  can interface an electrohydraulic component  26  with a wireless transceiver  30  as discussed above with regard to  FIGS. 1-7 . The wireless transceiver  30  can communicate with a remote control unit or with other electrohydraulic components. Thus, the wireless transceiver  30  allows the electrohydraulic component  26  to be wirelessly controlled by a remote control station or other device. The wireless transceiver  30  also allows feedback data, sensed data, or other data to be transmitted wirelessly from the electrohydraulic component  26  to remote locations for diagnosis, control, or other purposes. 
     The electrohydraulic component  26  receives hydraulic fluid that is pressurized from a hydraulic power source such as a hydraulic pump  28 . The pressurized hydraulic fluid from the pump  28  passes through the self-powering module  24  before being conveyed to the electrohydraulic component  26 . At the self-powering module  24 , hydraulic energy from the pressurized hydraulic fluid can be harvested and converted to electrical power by a harvesting device  32 . The electrical power can then be used to power the wireless transceiver  30  and the electrohydraulic component  26  as well as other devices connected to the electrohydraulic component  26 . For example, the electrohydraulic component  26  can include a solenoid driven valve. The electrical power can be used to power the solenoid and can also be used to power active control circuitry within the electrohydraulic component  26 . The electrical power can also be used to power various sensors (e.g., pressure sensors, temperature sensors, speed sensors, position sensors or other sensors) provided at the wireless module  22  and that are interfaced with the electrohydraulic component  26 . 
       FIG. 9  is a more detailed view of the self-powered wireless hydraulic system  20 . As shown at  FIG. 9 , the self-powering module  24  includes a pump line  50  (e.g., a high pressure line) and a tank line  52  (e.g., a low pressure line). The pump line  50  connects to the pump  28  and also connects to a pressure point of the electrohydraulic component  26  which is depicted as a solenoid driven 3-position proportional valve  26   a  which controls movement of a hydraulic cylinder  29 . The tank line  52  connects to tank  54  and also connects to a return part of the valve  26   a . The harvesting device  32  is shown including a hydraulic motor  56 , a reducing valve  58 , and an electric generator  60 . The electric generator  60  can be configured to generate a dc voltage output. In one example, the electric generator  60  can include a brushless 3-phase dc generator. As depicted at  FIG. 9 , line  62  connects the upstream side of the motor  56  to the high pressure line  50  and line  64  connects the downstream side of the motor  56  to the tank line  52 . The reducing valve  58  is provided along the line  62 . The motor  56  is configured to drive the generator  60 . Energy is harvested by tapping a small amount of the pressurized hydraulic fluid from the high pressure line  50  and diverting the small amount of pressurized hydraulic fluid through the reducing valve  58  to the motor  56  such that the motor  56  is turned to drive the generator  60 . The hydraulic fluid is then returned through the line  64  to the tank  54 . The hydraulic motor  56  rotates the generator  60 . The generator  60  can include a rectifier and can provide dc voltage as an output. The dc voltage can be further regulated to meet constant voltage requirements for one or more electronic power inputs. 
     The electrical power generated by the generator  60  can be used to power various sensors, actuators, and communication and control devices in the wireless hydraulic system  20  including the self-powering module  24  itself. For example, the dc voltage output from the generator  60  can be directed to a power management module  61  that selectively directs electrical power as needed to the wireless transceiver  30 , to actuators  63  such as the solenoid of the valve  26   a  for actuation of the valve  26   a , and to sensors  65  for sensing various parameters of the hydraulic system. Thus, the generator  60  converts hydraulic energy into electrical energy for powering the wireless hydraulic system  20  including the electrohydraulic component  26 , the wireless transceiver  30 , and the self-powering module  24 . In this way, the need for cables or wires for supplying supplemental power to the wireless hydraulic system  20  is eliminated. 
     In certain examples, the harvesting device  32  is positioned in parallel with other hydraulic components such as the electrohydraulic component  26 . The electronics used in the wireless hydraulic system  20  generally require small amounts of power thus only a small amount of motor displacement is needed and the motor  56  can be relatively small in size. Additionally, only small flow rates are needed to power the electric motor  56 . In certain examples, the harvesting device  32  operates as a high pressured low flow device. Disturbance of electrical loading can be addressed by voltage regulation, and disturbances related to hydraulic fluctuation can be handled by the reducing valve  58 . 
       FIG. 10  shows another version of the self-powered wireless hydraulic system  20  in which the motor  56  has been replaced with a turbine  70  positioned along the low pressure line  52 . The reducing valve  58  is also positioned along the low pressure line  52 . The turbine  70  is configured to drive the generator  60 . The output from the generator  60  can be used in the same way as described below with respect to the example of  FIG. 9 . The example of  FIG. 10  includes a valve  26   b  that has been modified, as compared to the valve  26   a  of  FIG. 9 , to include an open center feature  72 . The hydraulic turbine  70  connects downstream of the valve  26   b , and the open center feature  72  allows for flow through the valve  26   b  to the turbine  70  even if the valve  26   b  is in a neutral position. The valve  26   b  can be solenoid controlled and the solenoid can be powered by the electrical power generated by the generator  60 . Hydraulic fluid flow through the hydraulic turbine  70  generates a mechanical rotation. The shaft of the hydraulic turbine  70  spins the generator  60  with the assistance of a rectifier, and dc voltage is outputted from the generator  60 . The voltage can be further regulated to meet the constant voltage required for various electronic power inputs. As with the example of  FIG. 9 , the electrical power from the generator  60  can be used to power various sensors, actuators, and communication and control devices in the wireless hydraulic system  20  as well as the self-powering module  24 . For example, the dc voltage output from the generator  60  can be directed to the power management module  61  that selectively directs electrical power as needed to the wireless transceiver  30 , to actuators  63  such as the solenoid of the valve  26   b  for actuation of the valve  26   b , or to sensors  65  for sensing various parameters of the wireless hydraulic system  20 . 
     In the example of  FIG. 10 , the turbine  70  is used to convert hydraulic energy into electrical energy for powering the wireless hydraulic system  20  including the electrohydraulic component  26 , the wireless transceiver  30 , and the self-powering module  24 . In this way, the turbine  70  eliminates the need for supplemental power to be wired to the wireless hydraulic system  20 . For turbines, high flow and low pressure drop is needed to generate efficient mechanical power. For this reason, the turbine  70  can be placed along the low pressure line  52  routed from the downstream side of the valve  26   b  to the tank  54 . Like in the example of  FIG. 9 , disturbances related to electrical loading can be handled voltage regulation, and disturbances related to hydraulic fluctuation can be handled by the reducing valve  58 . 
     The electrical load required by an electrohydraulic component in a hydraulic system, such as the electrohydraulic component  26  in the self-powered wireless hydraulic system  20 , can vary significantly from standby current to full actuation current. Such variations in electrical load can cause challenges with respect to providing stable power supply from the harvesting device  32 . In certain examples, various components are up-sized to ensure that sufficient voltages are provided to meet the highest load scenario. In such a case, the generator and motor may run at high speed to produce higher voltage than would typically be necessary. A voltage regulator can be used to provide the required value (e.g., bring the voltage from 48 volts to 24 volts). When the electrical load of the system increases, the generator speed is lowered. This reduction in generator speed results in a voltage drop. However, since the system has been up-sized, the dropped voltage is still higher than the minimum voltage requirement. For many applications, such an over design may be acceptable to provide adequate power. However, such systems do not operate at maximum efficiency and may result in wasted energy. If the various components are not up-sized, during high load situations, the voltage may drop below the minimum requirement. In this case, the active electronics associated with the various electrohydraulic components may not operate properly. 
       FIG. 11  shows a third aspect of the present disclosure which relates to a self-powered wireless hydraulic system  200  that has been modified to include a self-compensation mechanism for maintaining a relatively constant voltage output regardless of variations in the electrical load required to support the system. The wireless hydraulic system  200  has many of the same components as the wireless hydraulic system  20  of  FIG. 9 . For example, the wireless hydraulic system  200  includes a high pressure line  50  connected to a pump  28  and a low pressure line  52  connected to a tank  54 . The wireless hydraulic system  200  is connected to an electrohydraulic component  26  such as the solenoid driven 3-position proportional valve  26   a  of  FIG. 9 . It is noted that the wireless hydraulic system  200  can be fluidly connected to various other types of electrohydraulic components such as other types of motors, actuators, and valves. The wireless hydraulic system  200  includes a line  62  that connects an upstream side of a motor  56  to the high pressure line  50  and a line  64  that connects a downstream side of the motor  56  to a low pressure line  52 . A self-compensation valve  58   a  is positioned along the line  62 . The motor  56  drives a generator  60  which can be of the type described above. Additionally, the output of the generator  60  can be used in the same way described above with respect to the earlier embodiments to power the electrohydraulic component  26 , as well as the wireless transceiver  30 , the self-powering module  24 , various actuators  63  and sensors  65 , and other components in need of electrical power in the system  200 . 
     Additionally, power from the generator  60  can be used to control operation of the self-compensation valve  58   a . For example, the self-compensation valve  58   a  can be a variable orifice valve that includes a solenoid that is actuated using power generated by the generator  60  to control the size of a variable orifice. Under high electric load conditions, the orifice size of the self-compensation valve  58   a  can be enlarged so that more flow passes through the motor  56  and hence, more torque is generated by the motor  56  for powering the generator  60  in order to meet higher voltage demands. Under low electric load conditions, the orifice size of the self-compensation valve  58   a  can be reduced to provide less torque sufficient to support reduced voltage demands. 
     In certain examples, the speed of the generator  60  can be measured directly using a sensor  82 . In other examples, the speed of the generator  60  is measured indirectly using a sensing algorithm that detects the speed of the generator  60  based on a parameter such as phase voltage signature. The system  200  can include a power management module  61  that determines an electrical power demand and computes the necessary speed of the generator  60  to match or satisfy the demand. The power management module  61  can interface with the self-compensation valve  58   a  to vary the size of the orifice of the self-compensation valve  58   a  in response to variations in electrical power demand. It will be appreciated that a map or other control logic can be used to correlate the voltage output of the generator  60  with respect to the speed of the generator  60 . A power management circuitry or a controller (e.g., an amplifier) can drive or otherwise control the position of the solenoid of the self-compensation valve  58   a  such that when the electrical power demand is high, the orifice of the self-compensation valve  58   a  can be opened wider with less pressure drop. This causes an increased pressure drop in the hydraulic motor  56  which results in the motor  56  generating more torque. When the electrical load is low, the orifice of the self-compensation valve  58   a  can be closed to a smaller size with a corresponding higher pressure drop. This reduces the pressure drop that occurs across the motor  56 . Thus, the motor  56  generates less torque. However, since the electrical load is lower, the lower torque results in the generator  60  being operated at a maintained constant speed. 
     It will be appreciated that the speed of the generator  60  can be measured using the standard speed sensor  82 . In other examples, the speed sensor  82  can be eliminated by self-sensing the speed of the generator  60 . Generators such as brushless 3-phase DC generators have a signature phase voltage curve based on the brushless DC generator wiring. As shown in  FIGS. 12A and 12B , this signature curve can be generated based on simulation or measurement. A crossing zero method can be used to detect the edge of the phase voltage curve and thus determine the electrical rotation period or frequency F. Given the poles (P) of the brushless DC generator, the synchronous speed can be calculated as being 120 multiplied by frequency divided by the poles. Accordingly, based on the speed of the generator  60 , the flow rate through the hydraulic motor  56  can also be estimated since the hydraulic motor  56  has a known displacement for each rotation of the hydraulic motor  56 . 
       FIGS. 13-15  show the effect of an increase in electrical load from 0.5 amps to 1.25 amps that occurs at the five second point in a system that does not have self-compensation.  FIG. 13  shows rotation speed of the hydraulic motor or generator in relation to time.  FIG. 14  shows output voltage of the generator in relation to time.  FIG. 15  shows pressure differential across the motor in relation to time. As shown at  FIG. 13 , the motor speed drops from 3000 rpm to 1500 rpm due to the increase in electrical load. When the motor speed drops, the output voltage drops from 50 volts to 10 volts as shown at  FIG. 14 . Due to the speed slowdown, there is actually more pressure drop across the hydraulic motor. However, the increased hydraulic torque is not high enough to compensate for the electrical load change. In order to improve operation of this type of system, the system likely should be overdesigned to make sure that sufficient voltage is provided at the worst case scenario. 
       FIGS. 16-18  show a system that includes the self-compensation valve  58   a  of  FIG. 11  for maintaining a relatively constant voltage output despite changes in electrical load demand.  FIG. 16  shows the rotation speed of the hydraulic motor or generator in relation to time.  FIG. 17  shows the voltage output of the generator in relation to time.  FIG. 18  shows the pressure differential across the motor in relation to time. Similar to the example of  FIGS. 13-15 , the electrical load on the system has been increased at the five second mark. However, the speed of the hydraulic motor and thus the generator was controlled to a higher speed in response to the increase in electrical load encountered at the five second point. By raising the speed of the hydraulic motor in response to the increase in electrical load, the output voltage is generally maintained at a constant level despite the change in electrical load (see  FIG. 17 ). Thus, the compensation at the variable self-compensation valve  58   a  ensures that the pressure differential across the hydraulic motor is high enough to overcome an increase in electrical load. 
       FIG. 19  illustrates a method  400  for self-compensating a self-powered wireless hydraulic system. The method  400  includes an initial step  402  of measuring an output voltage. Next, the method  400  includes a step  404  of setting a desired output voltage. A step  406  includes calculating the difference between the desired voltage and the actual voltage applying the difference to a control algorithm (e.g., PID control) for generating a speed set point for the generator and the hydraulic motor. In one example, the speed set point can be added (e.g., like a baseline speed rpm). A next step  408  includes measuring the speed of the generator directly using an external sensor or measuring the speed of the generator indirectly using a sensing algorithm. A further step  410  includes applying the speed set point and the actual speed value through a control algorithm to create a compensation valve command. Next, a step  412  includes varying the size of an orifice of a self-compensation valve so that the hydraulic flow through the motor generates a torque at the motor that is sufficient to drive the generator at the speed set point that corresponds to the desired voltage. The steps  402 - 412  may be repeated at will for varying the size of the orifice of the self-compensation valve. 
     Various modifications and alterations of this disclosure will become apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that the scope of this disclosure is not to be unduly limited to the illustrated examples set forth herein.