The HOLI 300 uses an Arduino microcontroller, a bike disc brake, an electrical brake and a furling sytem to limit rotor speed. In this section we explain how these components work together and how we measure rotor rotational speed.

Important Note

Preliminary content from design report

The content of this article is taken from the December 2013 preliminary design report. It represents intention of design at that stage but does not necessarily show the final version of the HOLI 300 turbine design.

Small Wind Turbine Safety Concept Flowchart
Small Wind Turbine Safety Concept Flowchart

Microcontroller and Emergency Disc Brake

The microcontroller in the safety system is independent from the turbine controller. This type of setup is only to be used in the presented prototype version of the turbine, because the team is limited to a preassembled off-the-shelf controller product. A commercial version of the turbine would include a customized controller which would be specified to include all safety features described here for the separate microcontroller. This would result in a significant reduction of cost.

As a microcontroller board for the project, the Arduino Fio is chosen. It is relatively cheap ([18.12 Euro as of December 18, 2013, [1]) and can be programmed via a USB-connected FTDI converter board (10.86 Euro as of December 18, 2012, [2]). In addition to several digital inputs which are needed for the measurement of the rotational speed, it also offers analog inputs which can be used to measure a voltage e.g. via a simple amplifier. The Arduino Fio also features a battery charge controller, an input channel for unregulated voltage and an interface to a battery. With these components, a redundancy to the turbine’s main battery is established as a power source for the microcontroller and as a consequence the safety system is protected from a fault either in its own battery or in the main battery.

The main task of the microcontroller is to actuate the emergency disc brake. Despite from stopping the rotor in case of an emergency as described below, the disc brake also serves as a device for blocking the rotor. The microcontroller furthermore lights a set of LEDs that are located at one of the lower edges of the nacelle so that they can be seen from a wide view angle to inform the operator about possible faults detected in the turbine. In order to satisfy the requirement of an independent safety system, the power source of the microcontroller also has to be independent from the turbine generator or controller power. Hence, two independent power sources are chosen for the microcontroller: Firstly, the microcontroller is connected to its own battery being able to maintain it in operational state for at least a day (uniterrupted power supply – UPS) and secondly the microcontroller is connected to the main battery that the controller is charging. The maintenance in operational state for one day results from the assumption that there is one day without proper operation (e.g. no wind) with empty main battery without the microcontroller being switched off externally. The microcontroller has its own load regulator to charge its own battery via the power from the main battery, converting the 24V from the main battery to a suitable charging voltage via a relatively cheap (5.78 Euro as of December 18, 2013, [3]) universal battery elimination circuit as used in RC applications.

The red column in the figure above reads as follows: When the microcontroller cannot supply voltage on the power output for the brake e.g. due to an electrical fault, a programming fault or because it is disconnected from the turbine or battery, the emergency disc brake is activated by its fail-safe mode that engages once the electrical power supply is cut off.

The orange column is related to the interrupt function of the microcontroller. An interrupt function is a function in the program flow of the microcontroller that activates once a certain input signal reaches a certain value, independent of the current step of the main program the controller is running. The aforementioned emergency stop button is implemented with an interrupt function in the microcontroller program. This ensures that whenever the emergency stop button is pressed, the emergency disc brake will be activated immediately.

In contrast to the interrupt function, the yellow column describes the main program flow of the program running on the microcontroller. By measuring the voltage at the battery and the controller, it can be determined if e.g. the battery is disconnected from the turbine system or the controller suffers an electrical fault. In either of these cases, the emergency disc brake gets activated immediately. In the next program step, the rotational speed of the rotor is measured and compared to the maximum allowable overspeed value which represents the design limit of the turbine (715 RPM). If this value is exceeded, the emergency brake is engaged. If none of the two criteria mentioned above is true, the emergency brake of the turbine can be disengaged and hence the rotor is unblocked, in case the brake has been engaged before.

Measurement of rotational speed

As mentioned in the previous paragraph, the rotational speed of the rotor has to be measured in order for the independent safety system to function properly. While the AC frequency of the generator could be used as an indicator for this speed, it is not independent of the drivetrain. To be precise, the rotor speed at the blades could not be measured from the generator frequency if the shaft broke. Hence, the measurement of the rotor speed is conducted as follows: The rotor with the blades has been determined as most critical for safety as blades might detach from the rotor hub at severe overspeed, which might e.g. occur at high wind speeds if the shaft breaks and the furling system fails to regulate the rotational speed. For this event, the detached blades could cause damage to the turbine environment, going as far as a life risk for e.g. operators around the turbine. Therefore a measurement and also a braking system has to be attached as close to the rotor as possible. As the braking system is located behind the first bearing, it makes also sense to locate the measurement of the rotational speed close to this point. Given structural integrity of the brake disc, the simplest and cheapest solution is to drill holes in the brake disc which are used to pulse a light signal that is emitted and received by an optical interrupter. Optical interrupters like the “Sharp GP1A57HRJ00F” are relatively cheap (1.43 EUR/piece as of December 7, 2013, [4]) and can be connected to the microcontroller via a basic set of resistors. The microcontroller then measures the pulse width and interpolates the rotor speed from this pulse width. With using two redundant optical interrupters for the critical rotational speed measurement on the brake disc, both the requirements for protection against faults in any component and simplicity of the product are fulfilled.

Controller

The controller possesses an electrical brake which will be engaged if the main battery is fully loaded. This is pictured by the green column in the figure above. This is a safety feature to enhance the lifetime of the turbine as it will only be operating when it is needed to charge the battery.

Furling System

The furling system, denoted by the blue column in the figure above, is the common overspeed protection system for the turbine and further described in the respective article on this website. It acts whenever the rotor enters rotational speeds which are higher than the rotational speeds defined for the cut-out wind speed (400.5 RPM). In case the Furling System fails e.g. due to blocking of the furling tail hinge by ice, the emergency brake is activated by the microcontroller at the maximum allowable rotor speed as described above.

[1] sparkfun, “Arduino fio,” , 2013.
[Bibtex]
@ELECTRONIC{sparkfun2013a,
author = {sparkfun},
year = {2013},
title = {Arduino Fio},
organization = {sparkfun},
url = {https://www.sparkfun.com/products/10116},
owner = {helgehamann},
timestamp = {2013.12.22}
}
[2] sparkfun, “Ftdi basic breakout – 3.3v,” , 2013.
[Bibtex]
@ELECTRONIC{sparkfun2013b,
author = {sparkfun},
year = {2013},
title = {FTDI Basic Breakout - 3.3V},
organization = {sparkfun},
url = {https://www.sparkfun.com/products/9873},
owner = {helgehamann},
timestamp = {2013.12.22}
}
[3] Heads Up RC, “3 amp universal battery elimination circuit (ubec),” , 2013.
[Bibtex]
@ELECTRONIC{HeadsUpRC2013,
author = {{Heads Up RC}},
year = {2013},
title = {3 Amp Universal Battery Elimination Circuit (UBEC)},
url = {http://www.headsuphobby.com/3-Amp-Universal-Battery-Elimination-Circuit-UBEC-G-150.htm},
owner = {helgehamann},
timestamp = {2013.12.22}
}
[4] sparkfun, “Photo interrupter gp1a57hrj00f,” , 2013.
[Bibtex]
@ELECTRONIC{sparkfun2013c,
author = {sparkfun},
year = {2013},
title = {Photo Interrupter GP1A57HRJ00F},
organization = {sparkfun},
url = {https://www.sparkfun.com/products/9299},
owner = {helgehamann},
timestamp = {2013.12.22}
}

Florian RoscheckAbout the author:
Florian Roscheck
Florian “Flo” Roscheck is a mechanical engineer who loves to play around with microcontrollers in his free time. As an industrial mechanic he also has some manufacturing experience. Being passionate about professional tinkering this is the third student contest project he participates in.

He took over project management and several mechanical and electrical engineering tasks in the project. Florian designed the furling system, the safety microcontroller, the hub connection, the frame and the website.

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