Power Take off Mechanism

The current Stingray power take off uses hydraulic rams to force high pressure oil through a hydraulic pump connected to a conventional rotary electrical machine. The simple nature of the hydraulic circuit, which contains no accumulators to smooth flow, means the electrical machine experiences bi-directional rotary motion of variable velocity.

The power take off configuration consists of four hydraulic cylinders, two mounted in antipose on each side of the device as shown in Figure 7.2. One end of each of the cylinders is fixed to the post whilst the other is allowed to follow the movement of the pivotal arm. The configuration is such that the net moment arm for each opposed pair, i.e. the perpendicular distance between the action of the rams and the pivotal point, remains roughly unaltered throughout the oscillation cycle of the device. Its actual variation is shown in Figure 7.3. In combination with the hydraulic configuration of the cylinders, namely the coupling of a full bore to the opposing piston's partial bore (the effective area in push connected to the effective area in pull), then nominally the pressure in the hydraulic system always represents a specific torque applied to the arm, allowing for simplifications to be used when specifying the hydraulic system.

Figure 7.2: Configuration of hydraulic cylinders
I 500 E

Icwer cylinder upper cylinder


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Figure 7.3: Moment length cylinders have on pivot arm over 2 cycles The variation in total length of one set of cylinders, the distance between its mounting points, is shown in Figure 7.4 and demonstrates that the topology proposed results in a slightly unsymmetrical waveform. Also shown in this figure is the cylinder velocity, which has a peak of just 0.1 m/s.

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Figure 7.4: Characteristics of cylinder cycle

Figure 7.5: Moment exerted by power take off

Using hydraulic pump torque vs. speed data from the Stingray device [94] operating in an optimised cycle, the equivalent force applied to the arm by the entire hydraulic system can be obtained. The use of this data, converted into a look up table, allows the linear generators to deliver the exact same response as the current power take off system. The total moment applied to the arm by all four cylinders is given in Figure 7.5. It should be noted that the discontinuities shown here and throughout this Chapter result from the assumption of instantaneous switching of hydroplane angle of attack. This affects the results at positions of direction change, 5.25, 15.75, 26.25 and 36.75 seconds. At the maxima of the curves there is a slight discontinuity, not present in the physical situation, resulting from the use of a look up table for the load speed characteristics of the current hydraulic drive, evident as the motion changes from accelerating to decelerating.

The dimensions of one cylinder and mass of the entire power take off system are given in Table 7-1 and Table 7-2 respectively

Table 7-1: Approximate dimensions of cylinder

Push area (m3)


Pull area (m )


Outer diameter (m)


Cylinder length (m)


Table 7-2: Estimated mass of hydraulic system

Cylinder (1 of 4)

120 kg

Hydraulic motor

100 kg

Electric Machine (300kW)

500 kg

Hydraulic oil (0.2 m3)

140 kg


25 kg


1.2 tonnes

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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