Summary:  This is an extended case study of Tilley, the community wind turbine on Tiree. There are no new features in this example, which instead demonstrates how the various aspects of the physigram notation can be used to describe a complex control panel.

Tilley is a community-owned 900kw Enercon E44 wind turbine installed on the Hebridean island of Tiree. Power from Tilley feeds into the grid, and income from this is used to fund various island community projects.

As well as the income provided, Tilley is a critical part of the island infrastructure. The island’s electricity supply is connected to its neighbouring island, Coll, via an undersea cable, which in turn is connected with the larger island of Mull and from there to the mainland and the National Electricity Grid.

The island typically consumes between 1Mw and 2Mw, including the power for the radar station that tracks North Atlantic civilian air traffic. As Tilley can provide such a large proportion of the island’s electricity, its correct functioning is particularly important, since malfunction could distort the island electricity supply.

Turbine control panels

Tilley can be controlled remotely from Germany using an SMS-based system, but there is also a small internal control area within the wind turbine tower for use during on-site maintenance or when the mobile signal fails (a common hazard at a remote location).

There are two main control panels (below).  The one on the left can be thought of as the ‘digital panel’, as it is all soft touch buttons and small screens. The one on the right has much more obviously physical controls (big buttons and knobs).  Both need to be capable of being used by service engineers who are cold and wearing protective clothing.


Photos: left © William Simm right © Maria Angela Ferrario

The physical design choices of the ‘digital’ panel are still important.  However, the membrane keys are effectively all forms of bounce-back with minimal movement and little tactile feedback, not interesting as physigrams. Of course, this lack of tactile feedback is a problem and makes semantic feedback more critical, but this will be driven by the need to ensure reliable operation in a potentially dirty environment.


Note that the analysis here is based partially on educated guesses; it is not possible to ‘unplug’ Tilley as it would be to, say, remove the batteries from a electronic device and play with its buttons.

At the top of the panel is a large red emergency ‘stop’ button.  It is especially important that this can be operated easily with gloved hands … but equally important that it will not be pressed by accident. Designing the precise pressure for this is clearly critical. It is a bounce-back button. Restoring power after an emergency stoppage is not simply a matter of flicking the switch back, so it is appropriate that it is not an exposed state device. This does, however, raise the issue of how you know it has actually worked; for this, semantic feedback from the control panel is needed, or possibly simply the change in sound.


The green reset button to the right of the emergency stop is also a bounce-back.  It is less prominent, since it does not have to be located as quickly in an emergency. Again, the hidden state means that you would need to look for semantic feedback that the reset had taken place. However, by its nature ‘reset’ is idempotent, so it is safe to press it twice ‘just to make sure’.

Below the emergency stop button are a number of twist switches that isolate power to various parts of the system. These controls are exposed state, making it easy to know that they are in the correct position, which is important as otherwise an engineer might touch live high-voltage wires.

To the right of the emergency stop are ‘+’ and ‘–’ buttons that control the pitch of Tilley’s turbine blades. Each is a bounce-back button that increments or decrements the turbine blade angle. The buttons on the panel seem to be laid out to be symmetric, so that the layout does not make clear the close connection between the two buttons. That is, even though they do the opposite thing, their physical layout means they do not constitute a natural inverse.

Although this constitutes a hidden state device, the choice of increment buttons does seem appropriate. One could imagine a dial that directly controlled angle of attack. While this would be more efficient in terms of rapidly controlling the blade angle, studies on medical devices have shown that increment/decrement buttons are typically far safer in avoiding accidental large changes. Given the engineer has probably taken a multi-day journey to get to Tiree, a few extra seconds of button pressing are probably not an issue.