In order to allow a precise driving of the motor generator so as to apply heave motion compensating forces onto the load in synchronism with the heave motion cycle and in a 20 correct amount, the control unit may comprise a sensor for measuring a variable representative of a heave motion to be compensated, the control unit being arranged to drive the motor generator on the basis of the measured variable.
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The control unit may further control the motor generator so as to drive the load in the first part of the heave motion cycle and to regenerate energy in the second part of the heave 30 motion cycle. The control unit further comprises a reference REF a value of which expresses a desired average voltage level and a second controller CNT2 such as a proportional controller or a proportional integrating controller.
Also, or instead thereof, it may be the case that the load is held by a floating platform (such as a vessel), which is subjected to the wave motion. The wave motion will result in vertical forces, thereby providing a periodic vertical movement of the load L as well as the floating platform FP. As a result, the electrical storage element (also referred to as energy storage element, electrical 20 storage, energy storage or similar wording) may be connected to the power line without the intervention of a converter to convert a power line voltage into an operational voltage of the electrical storage element, as will be explained below. As a result, the output voltage of the power supply is cyclically changed substantially in synchronism with the heave motion, causing the electrical storage element to be cyclically charged and discharged substantially in synchronism with the power supply output voltage cycle, so that the cycle of consumption of energy by the motor generator when driving the 25 load and the regeneration of energy is substantially matched by a corresponding discharging and charging of the electrical storage element, so as to buffer at least part of the regenerated energy in the electrical storage element without requiring a converter to interposed between the electrical storage element and the motor generator (or more specifically the motor inverter unit of the motor generator).
As an example, a hydraulic cylinder may be provided which extends and compresses synchronously with the heave motion, thereby interacting with for example a cable holding the load. In particular, an air flow along electrical 15 contacts of the super capacitors may be provided as the electrical contacts provide for a low thermal resistance, allowing cooling of the super capacitors. In accordance with the invention, in order to make the electrical storage element store and release electrical energy in a heave motion cycle, the power supply is controlled substantially in synchronism with the heave 25 motion. 6B shows a block schematic view of a control unit of the heave compensation 20 system in accordance with figure 7A; and fig 7 shows a schematic cross section of a vessel with solid roll damping ballast.
As another example, there is provided a vessel comprising an active heave compensation system in accordance with the invention. It is to be understood that these measurement principles may not only be applied in the active heave compensation system as disclosed in this document, but in any active heave compensation system, such as a heave compensation system comprising the motor generator to interact 35 with the load and a capacitor, super capacitor or other electrical storage to store at least part of the electrical energy regenerated by the motor generator. The energy storage element C, the 20 motor generator MG and the power line PL may have functions substantially similar or identical as described above with reference to figure 4 and 5 above. The regenerated energy may then at least partly be stored in the electrical storage element. In the cycle, energy may be produced and stored in a first part the cycle and may be required in another part.
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The motor generator may comprise a suitable driver such as a so called motor inverter unit that converts a direct current (DC) supply voltage into a switched, alternating electric supply to the motor-generator and vice versa. As a suitable output parameter, the output parameter of the power supply may be an output current of the power supply, wherein the output of the control unit is connected to a power 15 supply voltage setpoint input of the power supply.
15 A suitable value of the control unit setpoint may be determined by the control unit as follows: The control unit may be arranged to determine the control unit set point from: time averaging an actual power supply output voltage of the power supply, comparing the time averaged actual power supply output voltage with a power supply output voltage setpoint, and deriving the control unit set point from a result of the comparison. The motor generator com comprise a suitable drive unit, such as a so called motor inverter unit that 30 comprises a plurality of switches in order to provide a correct polarity from the electrical power supply line to the motor generator, the motor inverter unit thereby effectively transferring the DC supply voltage at the power line into an AC voltage drive of the motor generator and vice versa during regeneration of energy.. Dificultad para respirar y dolor de espalda