Original author: Alex Zaretsky, Renesas Electronics Europe.
The world is moving towards an eco-friendly environment where saving energy in any possible way is crucial. Manufacturers of electrically operated products migrate to switching mode power supplies and add intelligence to the systems to achieve this. Lighting applications are also migrating to LED based light sources mostly because of their low energy waste and high efficiency, and traffic and pedestrian lights are no exception. From January 2006, traffic signals, pedestrian crossing lights and illuminated EXIT signs have to meet Energy Star efficiency levels for input power of 5W and above. Although, in 2007 traffic and pedestrian signals were estimated to be the major application still using halogen lamps, this created a transition to and the following demand of LED lighting in these applications. To further support energy saving in LED lighting applications, as well as to comply with the European traffic lights regulations and be competitive in the market, more intelligence is required. This can be achieved by means of closed loop sensors, communications and efficient LED driving. Traffic signals suppliers tend to provide energy efficient and sophisticated traffic lights that can also combine good optical quality with attractive design.
There are different traffic lights systems available in the market today including traffic signals, stop signals, pedestrian crossing lights, traffic displays and street lights. Most of the basic systems are supplied from a distribution power block which is located in a central control unit. The control block provides DC power supply to the traffic signal heads using conventional power cables where a simple ON/OFF type of signal is used. The distributed power spans 12V, 24V, 40V and 48V DC. Since different countries have different regulations, the supplied voltage to the traffic signals depends on the country and its standard luminance requirements. For example, to comply with the UK market requirements for traffic lights, 48VDC has to be provided for each unit.
It is well known that basic traffic signals are equipped with the three colours red, green and amber. However, in many cases the red light contains some orange in its colour, and the green light has some blue colour. That is to provide some support for people with red-green colour blindness. This kind of system requires extra intelligence and multi-colour support where the light intensity is properly calibrated and then dimmed to a certain level to produce the right hue.
Today’s traffic lights are also equipped with intelligent sensors for detecting and counting passing vehicles. The sensors are used to improve the performance of traffic lights and adjust the time between different light colours accordingly. The sensors are used to detect crossing pedestrians or cyclists so the traffic lights can be changed accordingly. Another important use of the sensors is the release of traffic jams. So if a traffic jam occurs, the system can trigger the traffic lights to change earlier than usual, to help disperse the traffic jam.
There are also systems with sensors that are interruptible and are capable of giving priority to special traffic such as ambulances or police vehicles. This kind of system can detect an approaching emergency vehicle with sirens in operation and change the lights accordingly. The sensors which are used in detecting the pre-emption are usually based on wireless, infrared or optical transmitters that send a request to the lights controller to change the lights accordingly once an approaching relevant emergency vehicle is detected. Some systems also utilize audio sensors which are capable of reading a certain type of siren and used to detect approaching ambulances that way.
To optimise traffic flow, traffic lights manufacturers also implement sensors to detect vehicles and bicycles with metal rim approaching a pedestrian crossing or a junction. This allows sensing the presence of traffic before changing lights The sensors are usually built with inductive sensors and work by detecting changes to the electromagnetic field.
When a vehicle enters the sensors’ magnetic field, it causes the light to change. The crucial weakness of this type of detection is that a fault in the sensor device can cause the system to fail to detect cars waiting at a turn lane or cross-street when at a junction, thus causing considerable, indefinite delay to the traffic as well as resulting non-compliance with the signal safety. Another disadvantage of this type of technique is the limitation in the type of vehicles the sensor can detect. Only decent mass vehicles which include a metal such as steel usually affect the sensors magnetic field and trigger the system but it does not always work for small vehicles such as motorbikes without a proper sensitivity adjustment. Detection of a small car, a motorcycle or even a bicycle requires the detector system be adjusted more sensitive than what is typically required for normal vehicle detection. Modern detectors such as rack-mount and shelf-mount detectors offer multiple sensitivity levels for such purpose. Although inductive sensors is the most common technique, alternative solutions can be laser based sensors or rubber hoses filled with air.
Very popular today are variable message signs that integrate proximity sensors and respond to close proximity events by displaying driver’s speed or flashing a warning message as car approaches. These systems are cheap to install and many of them are solar powered making them ideal for accident reduction where mains electricity is not available. Sensors interface to the system controller in various ways. The output of analogue sensors can be read by an analog to digital converter (ADC). Digital sensors which are more common these days offer serial interfaces such as Inner-Integrated Circuit (I²C) or Serial Peripheral Interface (SPI) to communicate to microcontrollers. The advantage of the I²C bus is network capability where multiple I²C compatible sensors or other devices can be connected in a network. On the other hand SPI offers a higher communication speed.
Some traffic lights are equipped with speed cameras as well, where an additional signal head is used to display the speed on the 7’s of an approaching vehicle. There are many different types of traffic lights available today from different manufacturers. The combination of intelligence and high luminance in the systems helps increase traffic safety at intersections. The variety of choice allows customers to select the right systems to fit their application requirements as well as to comply with the regulations in their country.
Different manufacturers implement various ways to adopt LEDs in traffic lights systems, thus reducing power consumption by 20% compared to halogen lanterns. Some of them utilize only two to five LEDs in each signal head of a traffic lights unit to produce the lights. Thus saving system cost and energy. Many traffic lights today also implement symbols within the signal head. These include a green arrow, which indicates moving forward is allowed when active, and the walking man symbol in crossing signals. They are for non-motorised traffic such as pedestrians and cyclists and are activated using LED backlighting. In some North and East European countries this additional signal head is also dedicated for public transport such as trolleys and tramps. This signal head is usually built with a multi channel LED approach which allows implementation of special traffic symbols for public transport. The actual picture of an arrow, of a walking man or other signal is marked within the optical material at the time of manufacturing and is used as an optical enclosure for each signal head. When the LEDs on the back of the optics are lit, the light is only visible within the arrow, while the rest of the area within this signal head is not light conductive. Again, since less LEDs are required to produce the picture, the advantage of this type of approach is saving cost on LEDs and thus also reducing energy consumption. Other manufacturers utilize dozens of LEDs in each signal head and use individual LEDs to create images and shapes within the head. The advantage of this type of approach is allowing flexibility and programmability of the images. It also enables the inclusion of moving objects on the signal head, where the matrix of LEDs within the signal head switches the appropriate LEDs to create the required image. To provide the level of brightness that complies with the local traffic authority’s regulations, LED based traffic signals have to be dimmable. This allows the installers to set the required level of brightness.
As was previously mentioned, control systems are central control units that are used to provide power management and configuration services to traffic light systems. Most of the control systems can provide up to 900W, some of them even more. These control units operate all of the traffic lights in the location. So whether there are multiple traffic signals at a junction or a number of traffic signs fixed at a bridge, a single control system manages all of the traffic lights and traffic signs at that particular location.
Since different voltage levels have to be supported for different countries, these control systems include intelligent and configurable power supplies to provide a wide DC voltage range to fit variable voltage requirements.
Control system manufacturers implement a variety of HMI options for the configuration and programming of the traffic lights and signs. Many companies provide touch screens with a reliable operating system such as Linux, as well as several serial interfaces such as USB, Ethernet and RS485. These allow technicians to make configurations and upload bitmap data.
Traffic signs and variable message signs are used to display roadway information and are gaining wider adoption, thus improving driver information and safety and easing congestion as part of Intelligent Transportation System (ITS). Sign types vary from simple signs, based on just a few one to three channel LEDs, which are mostly used within cities and other built areas including flashing speed limit signs, car par space and entry signs and other warning signs. These signs have lower performance and LED lighting requirements but offer significantly higher growth in the market.
Large variable message signs for motorways such as motorway information and inner city congestion variable message signs and toll lane signs are built with a matrix of RGB LED pixels to provide colour variation and the effect of moving objects, depending on the requirements. Every manufacturer has a different approach to designing large size LED based traffic displays, but the most common method is to divide the display region into pixel segments and implement a number of small identical drivers for each segment to control a number of pixels in it. These segment controls are centrally controlled by a main control system within the unit which is programmed with the message or bit map to be displayed. RS485 is a common interface among the majority of the manufacturers. It is used as a communications protocol between the main controller and the segment controllers which are pre-programmed with unique addresses.
One of the fastest growing markets for LED replacement is street lights. In the early stages of this developing market, it was considered that a single colour white LED approach was sufficient to satisfy the main requirements. However, it is worth noting that this is not actually the best option for human visual perception, which varies depending on the light conditions and even the location. Interestingly, photopic vision in a well-lit environment and scotopic vision at night overlap somewhere on the green/blue spectrum. At this point, the human eye has the best visual perception in low light conditions. It is therefore more efficient to produce light colour in the right spectrum, allowing better visibility while saving energy. For example, human visual perception is different in the countryside to how it is in the city. In the countryside, the eye requires more of a green hue in the lights, but more red in the city where there is a more brightly-lit environment. Different developers take different approaches to resolve this, but it is common to develop a multi channel LED driver to satisfy the colour mix requirements. To further contribute to energy saving, producing the right mix of colours allows a higher level of brightness with lower energy consumption. Many street lamp manufacturers today also implement proximity sensors that automatically dim or brighten the lights, depending on whether a presence is detected within a certain radius of the lamp. There are also lamps with integrated ambient light sensors which can automatically adjust the brightness of the light according to the ambient light level. All of these features add intelligence to systems and, most importantly, contribute to significant energy savings.
Although LEDs offer significant advantages such as life longevity, power to light efficiency and low voltage operation, two of the inevitable disadvantages of LED technology today are their light degradation, which typically starts occurring with time, and colour shifting with temperature change. Unfortunately, all manufactured LEDs have different characteristics which LED driver designers have to take in to consideration when developing LED power supplies for traffic lights and other industrial LED based lighting products. In traffic light systems, street lamps and other LED lighting products, the ability to detect the start of any light intensity degradation is essential. This is to ensure that the light output level is compliant with the required European regulations for the minimum brightness of traffic signals, such as ECE65. The system can be configured to automatically compensate the light output level by increasing the current through the LEDs once degradation is detected. This can be achieved by constantly monitoring the current through each LED channel. Renesas lighting microcontrollers integrate up to 9 channels of 10-bit ADCs and up to 3 channel comparators that can be used to monitor currents and adjust the PWM outputs accordingly in multi-channel as well as in single channel LED systems. Another important aspect to look at is LED failure detection. This can be crucial in LED based traffic signals since the system has to be protected from accidentally lighting the wrong colours. Also, one of the main advantages of adopting LEDs for street lights is their life longevity which reduces maintenance costs. The system has to be able to detect the LED’s failure and act accordingly. Therefore it is important to be able to monitor the system for LED failure as well as for excessive current and temperature. Renesas lighting microcontrollers integrate dedicated peripherals to allow the required protection procedures in the system. Many manufacturers also utilize multi-channel LED drivers to support the traffic signal heads and street lamps with extra LEDs, should one of the channels fail.
Renesas Electronics provides industry leading microcontroller solutions for the LED lighting market, allowing system flexibility and programmability. Renesas lighting microcontrollers integrate LED driver dedicated peripherals to allow high integration and cost savings. As the world’s leading microcontroller supplier, Renesas Electronics is committed to producing market dedicated microcontroller solutions that support a wide range of connectivity options and integrated analogue peripherals that enable added intelligence and high integration in traffic lights and street light applications. In addition to supporting a wide input voltage range and providing guaranteed operation at high temperatures, Renesas lighting specific microcontrollers offer best in class power performance to support mobile traffic systems which operate from a rechargeable battery. They also provide a single chip solution for both power stage control and for driving a number of LED channels. They achieve this without involving the CPU, and only use the internal hardware architecture of the chip and its unique integrated peripheral set, leaving room for processing sensor readings and communications.
