Data used to drive paddles paddles is pre-computed or generated in real time in wave generation software. A commonly used technique is to sum individual sine waves to create complex seas. Frequency, amplitude, angle and phase define a wave front. Summing individual wave fronts generates multi-spectral seas. Built-in functions allow regular sine waves, long crested multi spectral waves and mixed seas to be defined.
Modern Drive Systems
Edinburgh Designs wavemaker controllers are extremely sophisticated and have specialised digital controllers able to correct for absorption of reflected waves and 2nd order harmonics [1]. Force feedback drive systems control paddle position, velocity and acceleration and measure the nonlinear wave effects as they are produced. Absorption is achieved by creating a digital filter network that matches the impedance of the paddle to the water.
Other technologies such as wave gauge absorption exist, although they are a poor substitute for force feedback as they introduce a control parameter with high errors and latency. Force feedback control systems are exclusive to Edinburgh Designs Wavemakers.
Early Drive Systems
Early paddles used a crank to produce sinusoidal motion. An adjustable mechanical arm altered the stroke and the motor speed controlled the frequency. Some tanks had segmented paddles and angled waves could be produced by setting the phase of the cranks on a common drive shaft. Such systems could not be used for random waves and was time consuming to adjust. In the 1950’s larger machines used hydraulic drives with servo valves and electrical control system that could be driven with an analogue voltage. Most of the big naval towing tanks had direct drive servo hydraulics capable of generating long crested random waves.
In the late sixties, transistor amplifiers meant that direct drive electrical servo systems became possible. The size and reliability of electronic drives improved dramatically in the 1990s so that they are now competitive with hydraulic machines for all except the largest wave paddles. With servo control it is possible to control the paddle motion from a signal generated in the control room. Single frequency waves were produced with a sine wave generator. Complex spectra were generated using a bank of adjustable filters to allow selected frequencies from a white noise source.
When Edinburgh Designs began, most academics were using function libraries to craft each wave. Though this enabled the design of complex waves it took skill and time. In contrast, the commercial world used forms-based inputs which produced a restricted set of wave specifications but were quick and easy to use. We wanted to combine the advantages of both and created a special wave definition language called Ocean. This language was then developed into our wave generation software with its integrated graphical user interface and analysis capabilities.
References:
- Spinneken J. & Swan C., (2009). Second-order wave maker theory using force-feedback control. Part I. A new theory for regular wave generation, Imperial College London