At first glance lighthouse structures may appear old and dignified, but the technology inside is modern and robust. Availability as well as low operating costs with long maintenance intervals are the decisive criteria when selecting lamps and the electromechanical design.
France is the land of giants. Nowhere else on earth are there so many lighthouses along the coast that stand as tall as 80 metres. The impressive structures are not necessarily due to the fact that the sections of France’s coastline are particularly dangerous. Rather, the large and rich decorations of the buildings stem from the self-confident national pride ― and create publicity for themselves.
The name Henry Lepaute comes to mind as soon as one mentions the technology used in these maritime navigation marks. Lepaute builds the optics that ensure that the light can be seen from distances as great as 40 kilometres. 23 nautical miles ― the beam from the lamp room must be visible at least this far away.
The distinctive lighthouses with buoys, satellite navigation and radar are part of a quartet that gives skippers the necessary orientation. Particularly in areas near the coast, visual navigation signs cannot be replaced by anything else. The density is especially high in France along the Channel coast due to the enormous tidal differences in the water level. Some 120 lighthouses with rotating light are operated, monitored and maintained in the Grand Nation by the governmental agency Cerema – and used very often in these lighthouses are the optics produced by Henry Lepaute, which in turn are based on the basic research conducted by Frenchman Jean Augustine Fresnel on the wave theory of light. The result is a lens comprising up to 20 ring-shaped sectors with a focal length of up to 700 millimetres. The lenses refract the light that is emitted in all directions on a horizontal plane, thereby making it visible from a great distance.
The typical flash of the lighthouses can be achieved with three technical methods: through constant lighting with a flashing light, a constant light source with rigidly arranged lenses and a circumferential aperture as well as ― the most common variant – the combination of continuous light and a circumferential lens system. In the course of ongoing modernisation, Cerema makes use of LED technology ― thereby benefiting from the advantages afforded by a long-lasting and robust lighting system. According to information provided by Cerema, an availability rate of approximately 99 percent is achieved. “We would like to reach this level of operation reliability for the drive of the rotating optics as well”, emphasises Laurent Bernicot from the department for navigation and positioning systems at Cerema. “Reliability was a very important criterion for the choice of drive – and led us to FAULHABER.”
The result of the design with manufacturer evaluation: DC-micromotors of the FAULHABER 2342 and 3257 series as well as brushless DC-servomotors of the 3268 series ― each combined with appropriately matched FAULHABER planetary gearheads. In addition to the general functionality of the drives, intensive test runs served to determine their behaviour in salty air. “We have lighthouses on land and at sea. The drives need to operate reliably there as well ― and not only in France itself, but also in French Guiana or the Overseas Collectivity of Saint Pierre and Miquelon off the coast of Newfoundland.”
Double drive for maximum operation reliability
With respect to design, the high demands on operation reliability are reflected in a drive unit with two motors ― an arrangement also found in German lighthouses. “Two motors are connected to one gearhead and run alternately”, explains Dirk Berger, spokesperson of the Stralsund Waterways and Shipping Office. The alternating operation harmonises the operating hours and ensures availability, as longer periods of downtime often lead to start-up difficulties. Should one motor fail, the system in the lighthouse automatically switches to the second motor and reports the malfunction to the control centre. “We use remote monitoring, particularly with the large lighthouses”, reports Berger. A two-stage escalation level is integrated in the control here: prewarning in the event of impending malfunctions and failure.
Reliable operation is monitored with a sensor that measures the time per revolution. “The lighthouses must turn at the exact speed specified in their identifier”, says Dirk Berger. Hidden behind the identifier is a special type of flash sequence that is listed in the nautical charts and, particularly when navigating at night, allows a lighthouse be unmistakeably identified. “Our goal is therefore to achieve a constant rotational speed on the optical level”, explains Berger’s French colleague, Laurent Bernicot from Plouzane in Brittany. “Thus, our motors must be precise.”
The French lighthouses use a control developed by the EMF department at Cerema. The abbreviation stands for eau, mer, fleuves ― or water, sea, river. The intelligence in the lamp room specifies how frequently and at what rate the light is to be visible within a given unit of time. As in Germany, a sensor measures the rotational speed of the optics and transmits the information to the control. After performing the calculation, the controller adjusts the motor speed via the analogue set value directly with the motor supply voltage. The speed is constantly monitored and adjusted. Should a problem be detected, the automation solution provided by the EMF department saves the error and starts the second motor.
During the design process, the EMF employees selected three different motor types as the standard drive solution. The DC-micromotor of the DC2342 series is tailored for small optics that require only low motor power. Medium-sized optics are put into motion by the graphite-commutated motors of the FAULHABER DC3257 series. For powerful systems with high light intensity and correspondingly heavier construction, the brushless, four-pole servomotors of the 3268BX4 series are used. When Laurent Bernicot speaks of ‘heavy systems’, he is referring to a lamp optics system with a mass of between 200 kilogrammes and one tonne.
During design, it is therefore important to ensure that, apart from normal operation, the motors deliver powerful peak torques in order to safely handle the high starting torques of between five and eight Nm. “This was also an important factor in selecting the FAULHABER motors”, explains Bernicot. If, however, a standstill occurs because the lighthouses are only operated at night, the optics are decoupled from the motor via a freewheel integrated in the rotary machine when the lamp is stopped.
The bottom line
With respect to the high requirements on availability with long maintenance intervals, the framework conditions in a lighthouse are demanding. This is why it is so important to maintain a close project partnership. Cerema and the FAULHABER’s French subsidiary have been working closely together since 2011.
From clock-tower drive to high-precision electric motor
Over a hundred years ago, lighthouse optics were still driven by a clock-tower drive. From a design standpoint, this was, in fact, a modified mechanical clockwork for churches that had to be regularly wound up with weights by the lighthouse keeper. “The first electric drives appeared in the 1930s”, reports Dirk Berger, spokesperson of the Stralsund Waterways and Shipping Office. Another modernisation wave started in the 1980s. The technology is still in operation today. The long service life is the result of regular maintenance and operation at well below the maximum available torques.