May 2006 Articles & Features

Launch Tech

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Eye On Electronics

By Mike Dale | May 2006

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The increasing quantity and complexity of vehicle electronics, including driver convenience/safety features, promise to keep service and repair very interesting in the years to come.

Most drivers think they have good visibility from behind the wheel, but accident statistics prove otherwise. Blind spots cause accidents during lane changes. Poor visibility below the belt line of the vehicle results in parking lot mishaps. While visibility to the rear of the vehicle is especially poor, vision in front can be restricted by darkness, fog and a host of weather-related issues.

Camera systems are now being installed to help drivers see in places they couldn't before. The service industry is going to need to understand how these systems work and the advantages they offer to your customers.

Electronic camera systems were first designed with NASA and the machine controls industry as the primary customers. Over the last few years, advances in camera technology have improved the performance level of these cameras and lowered their cost into the range of automotive affordability.

The place to begin to understand camera technology is with the human eye. Light is brought into the eye through the lens, inverted, then focused on cone-shaped light-sensitive cells at the back of the eye. The amount of light allowed in is controlled by the iris and the amount of pupil it exposes. The cells at the back of the eye are sensitive to the primary colors of red, green and blue. Other shades are detected as combinations of these primary colors. Yellow, for example, is seen by the cells in the eyes as a mixture of blue and green. The absence of color is seen as black. The presence of all colors is seen as white.

Cameras work in ways similar to the human eye. A camera accepts the light that reflects off the object being viewed. This light is focused through a lens to form an image that is much smaller, and upside down, compared to the actual object. Three chemical layers in the light-sensitive film capture the image focused on it by the lens, each layer sensitive to one of the same three primary colors.

Since film is sensitive to color and light intensity and duration, the camera is equipped with a timed exposure device (to control shutter speed) and a lens diaphragm (f-stop) that can open and close. These two elements control the amount of light allowed into the camera and onto the film. Repeating the exposure at a fixed rate (24 to 30 frames per second) is what captures motion in a movie camera.

Today's digitally based photography takes this a step further. The image is brought into the camera through the same system of a lens, shutter and lens diaphragm. Instead of the image being captured by photo-sensitive film, it's captured by pixels . A pixel is a square or rectangular area of a silicon chip that's sensitive to light intensity and a single color. As the light passes through the pixels, elements sensitive to each of the primary colors detect those portions of the image that consist of those colors.

Pixels absorb light energy by collecting photons , which are the tiny energy packets that make up light. The photons collected in each pixel are converted to an electrical charge by a photodiode. This charge is then converted into a voltage, amplified, then converted into a digital value by an analog-to-digital circuit. This results in a digital output from each pixel that can be organized into a digital picture.

In automotive terms, there are two possible kinds of image sensors�charge coupled device (CCD) sensors and active pixel sensors (APS). In CCD sensors, the pixel charges are processed sequentially by the circuitry that surrounds and supports the sensor network. CCD sensors are essentially matrices of capacitors whose charge comes from the photons of light they receive. They're passive in that they receive the light information and store it as a charge in a given cell. External circuitry then registers and moves these charges, accumulating them as color information that can be reconstructed to make a picture.

In the case of active pixel sensors, pixel measurement information is processed simultaneously by circuitry within the sensor pixels and in the sensor itself. You could compare the two sensor methods to cathode ray tube (CRT) and liquid crystal diode (LCD) technology monitors. In the CRT and the CCD sensor, the light that affects the pixels is processed by exterior circuitry. In the LCD monitor and the APS, the processing is done on a pixel-by-pixel basis right at, or in fact beneath, the pixel.

Where the processing is done is a key issue. The CCD sensor is capable of producing a very high-quality, noise-immune photo because the amplification can be done by a single high-quality, low-noise amplifier that's located off the sensor chip. The downside to this is that packaging a separate amplifier adds to the overall cost of the device.

In the case of the APS sensors, the amplification is done right at the pixel using simple, less-noise-immune circuitry. This allows for reduced size and tighter packaging, at the cost of some additional noise.

Common circuits that are added include analog-to-digital conversion, noise reduction processing and timing and clock control. Digital logic functions include interface, timing, processing and output functions. The image sensor alone cannot provide useful digital imaging. Especially in the case of CCD sensors, this added signal processing is a significant user of both space and battery power.

For automotive applications, the APS sensor has more advantages. This sensor can be made using the same CMOS technology used to make 90% of today's computer chips. In an industry where it may cost literally billions to set up a new manufacturing technology, the APS sensor offers a huge cost advantage.

Since the APS sensor is a CMOS design, it has all the advantages of that technology, one being that all of the image sensor functions can be integrated onto a single chip. It's possible to make a complete camera-on-a-chip that can include everything but the lens on a chip of silicon 6mm square.

Another advantage of the CMOS process is low power consumption. APS sensors consume less than 1% of the power needed for CCD sensors. Typical APS sensors can run from a single 5-volt or 3.3-volt supply; CCD sensors may need multiple power supplies. Clock pulses of from 5 to 15 volts may be needed with CCDs to efficiently move the charge from the capacitors into the domain shift register and into digital form.

A huge advantage of both CCD and APS sensors over film is that these electronic sensors respond to about 70% of the light that reflects off the object being viewed. Photographic film, on the other hand, responds to only about 2% of the incident light focused on it by the lens. This means that these cameras are much more sensitive to light than film, and include a wider range of light wavelengths than are visible to the human eye.

The big challenge for automotive vision systems is what to do with the digital information once the camera captures it. The driver with his eye on the road ahead has little spare capacity to watch a monitor displaying the image. There are two basic concepts for presenting the information: One is to present the image to the driver for his interpretation by way of a monitor or vision screen. The other is to present the image to vision software for analysis.

Displaying the image on a viewing screen can be acceptable under some circumstances. For low-speed maneuvering, such as backing up, a driver can safely monitor the mirrors, a video monitor and the view through the windows. It's possible to use the same screen as the navigation system or even a head-up display projected onto the lower windshield.

One possible use for the combination of a camera and a head-up display is for detecting animals in the roadway. Vision systems can detect the heat of an animal�say, a deer�beyond the range of the headlights. This can give the driver more warning time without interfering with the need for normal awareness of the road ahead.

APS sensors have advantages when it comes to software analysis of the image. Both the photodetector and the readout amplifier are a part of each pixel. This allows the accumulated charge to be read out over a wire pair that is unique to the pixel. This makes it possible to look at each pixel or subset of pixels individually. For movie cameras, this means it's possible to have electronic zoom or tilt. For vision systems, it means that specific areas can be looked at more closely if the first scan indicates a problem.

For automotive systems, the individual addressability of pixels means that the analysis software can zoom in on an object of interest as soon as it's apparent that something may be coming that the driver needs to be aware of. Software-programmable steps can be initiated as the object nears and the issue becomes more urgent. At each step the level of warning can be increased so the driver may react.

Vision systems are currently being installed on a growing range of vehicles. The range of applications and the uses to which they'll be put depends on consumer acceptance. You can expect to see increased use of these systems especially on high-end vehicles where their utility and “techno-content” will appeal to those customers.