The Mocean wave energy converter (WEC) is a hinged raft – two bodies connected by a hinge. Wave forces cause relative motion between the bodies about the hinge. In a full scale device, the hinge would contain a power-take-off (PTO) system, which converts the mechanical rotation into electricity. The hinge design is important in the full-scale machine, but also is critical to any scale models that we build. The picture above is of the hinge of the 1/50th scale model that we built for the Wave Energy Prize.
For the 1/50th scale model, we are not trying to generate electricity in the hinge, but instead we want to:
To replicate the PTO damping, we use a small motor and gearbox. When the shaft of the motor is turned, it generates an electromagnetic force that opposes the rotation, which is proportional to the amount of current that flows through the motor. When the terminals of the motor are shorted, the maximum current flows, generating the maximum damping torque. The mechanical energy from the rotation is dissipated in the resistance of the motor coils. So in our model, we take energy out of the waves and dissipate it through the motor.
We need to measure how much power we are absorbing (power = energy/time). Mechanical rotational power is:
Power = torque*omega
where the torque is the angular force applied through the rotation, and omega is the angular velocity (i.e. how fast the shaft is rotating). If we can measure these two values we can record the power absorption of our WEC as a function of time, and then get the average power for a give wave condition.
To make these measurements we used:
The torque sensor is connected to the motor with a coupling. Rotation of the hinge applies a torque through the torque sensor to the motor shaft, which turns.
The position sensor was placed externally to the hinge assembly; the sensor is connected to one side of the hinge, while the activator is connected to the other in order to measure the relative angle of the hinge. (We don’t have a good picture of the position sensor, but there is lots of information on the Gill website). The angular velocity is computed by differentiating the position.
The real challenge in the hinge assembly is that it’s all going in water. The position sensor is waterproof – great. But neither the torque sensor nor the motor are very happy in water. We designed custom housings for both, which used O-rings and lip seals to keep water out. It worked, but it was nerve-racking process putting it in the water the first time.
Of course, there are things that we would change if we did it again, but overall the hinge assembly was a success!
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