HAPiGYM

Two Rapid Prototyping Environments for Wave Energy Control

Abstract

The HAPiGYM is a collection of numerical and experimental modelling environments for testing control of wave energy converters (WECs). It has two applications: rapid prototyping of the control policies themselves, and co-design of control and other WEC subsystems. This collection will grow over time. Initially two environments (‘GYM machines’) will be offered. These will be described in this paper.

The HAPiGYM addresses several technical and resourcing challenges surrounding control prototyping and co-design. Time, money, and cross-disciplinary knowledge are common barriers. Tank time can be prohibitively expensive. Hence many control researchers rely on numerical simulations, and many WEC developers use non-representative control models when designing the hydrodynamic absorbers. The state of the art in hydrodynamic models are not suitable for rapid control prototyping: they are either too slow or insufficiently accurate, leading to a ‘Sim2Tank’ gap, where simulation and tank trial results disagree. Hardware-in-the-Loop removes those
modelling uncertainties associated with the drive train and control hardware. However, uncertainties associated with the hydrodynamic model remain, and unrepresentative artefacts associated with the rig could be added. Two notable schemes that included tank testing for comparative evaluation were WECCCOMP and the Wave Energy Prize (WEP). WECCCOMP allowed participants to compare the performance of their controllers on the same tank test model. The WEP allowed participants to test their own WECs and controllers.
Both had metrics that were proxies for levelised cost of energy. The single metric and prescribed methodology focused the efforts and objectives of participants. As a consequence, simplifications inherent in metrics (e.g., the formula for electrical power) and methodology (e.g., spectral parameters of the sea state available as inputs to the control policy) led to control approaches that could not be replicated in real seas, i.e., certain types of control were able to exploit the ‘Tank2Sea’ gap. The HAPiGYM approach acknowledges the
issues surrounding Sim2Tank and Tank2Sea gaps. Rather than attempting to eliminate these, participants will be invited to contribute to a discussion of how testing methodology interacts with control. Participants will be able to suggest methods, metrics, and even future GYM machines. The HAPiGYM will offer a selection of settings for each GYM machine, including the resource (waves), type of PTO, and metrics. Participants will be able to rate their controllers against a suite of metrics and experimental set-ups. This will allow a more nuanced comparison between controllers. It will also facilitate more basic research on co-design, e.g. how PTO operational range
impacts control and hydrodynamic performance.

Stakeholder engagement identified the need for a simple environment to get started on (a small Sim2Tank gap), and a more challenging environment that reflected the control problems of commercial devices (a small Tank2Sea gap). The first two GYM machines offered will use the same buoy with different constraints: constrained to heave only, and unconstrained (6 DoF). Participants will be able to run Processor-in-the-Loop tests using a tank-calibrated rig simulation running on Open-Hardware controllers. The most promising projects
will be given free remote-access to the HAPiGYM, running in the FloWave tank.